THE GUATEMALAN EARTHQUAKE 

OF FEBRUARY 4, 1976, 

A PRELIMINARY REPORT

This study was conducted by the 

U.S. Geological Survey in 

cooperation with the Government 

of Guatemala and with partial 

support from the Organization 

of American States

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1002

wil|i^,;;:: ~a%^
THE GUATEMALAN EARTHQUAKE 

OF FEBRUARY 4, 1976, 

A PRELIMINARY REPORT

A. F. ESPINOSA, EDITOR

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1002

A preliminary report of a series of 

closely related studies of the 

destructive Guatemalan earthquake of 

February 4, 1976

This study was conducted by the 

U.S. Geological Survey in 

cooperation with the Government 

of Guatemala and with partial 

support from the Organization 

of American States

UNITED STATES GOVERNMENT PRINTING OFFICE : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

First printing 1976 

Second printing 1977

Library of Congress Cataloging in Publication Data

The Guatemalan earthquake of February 4, 1976.

(Professional paper - U.S. Geological Survey; 1002) "The study was conducted by the U.S. Geological Survey in 

cooperation with the Government of Guatemala and with partial support from the Organization of American 

States."

Bibliography: p.

1. Earthquakes-Guatemala. I. Espinosa, A. F. II. Series: United States. Geological Survey. Professional 

paper; 1002.

QE535.2.G9G8 551.2'2'097281 76-608166

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-02858-1
CONTENTS

	Page

Glossary __________________________________________— VII

Symbols and abbreviations __________________________—- IX

Abstract ___________________,___________—————— 1

Introduction _____________________________________——— 1

Acknowledgments ___-________________________-- 3

Tectonic setting and seismicity, by William Spence and Waverly Person —————— 4

Introduction _______________________________________ 4

Seismicity ___________________________________________ 4

Historical seismicity __________________________________ 5

Tectonic history of the Caribbean-North American plate boundary ————— 8

The Caribbean-Cocos-North American plate triple junction _____—__—_— 9

Instrumentally recorded seismic activity prior to the main event, by David H.

Harlow ____..______________________________________ 12

Introduction ____________________________________ 12

Seismically active areas in Guatemala _______________—_—__—_ 12

Distribution of S—P times ______________________________ 14

Seismic activity in the vicinity of Guatemala City _______—_________ 15

Summary ____________________________________ 16

Main event and principal aftershocks from teleseismic data, by Waverly Person,

William Spence, and James W. Dewey _____________________ 17 

Main event source parameters from teleseismic data, by James W. Dewey and

Bruce R. Julian ____________________________________ 19

Introduction ______________,_____________________ 19

Focal mechanism ________________________________ 19

Seismic moment _________________________________ 19

Source dimensions, displacement, stress drop, and direction of fault propagation ___.._..__________________________________ 20 

Future earthquakes on the Motagua fault system ______________— 22 

Strong-motion recordings of the main event and February 18 aftershock, by

Charles F. Knudson _________________________________ 24

Introduction ________________________________________ 24

Seismic instrumentation ________________________________ 24

Seismoscope records of main event ________________———————— 24

Strong-motion accelerogram of February 18 aftershock _____________ 29

San Salvador accelerograph of main event ____________________ 29 

Aftershocks from local data, by Charley J. Langer, Jean P. Whitcomb, and

Arturo Aburto Q _____________________________________ 30

Introduction ________________________________________ 30

Instrumentation and field procedure __________________________ 30

Data and analysis ___________________________________________ 30

Results and discussion _______________________________ 31

Geologic effects, by George Plafker, Manuel G. Bonilla, and Samuel B. Bonis___ 38

Introduction ___________________________________________________ 38

The main fault ______________________________________ 38

Relationship of faulting to damage _________________________ 41

Secondary faults __________________________________ 42

Mixco zone _..______________________________________________ 43

Villa Linda-Castanas zone __________________________ 44

Incienso-Santa Rosa zone _________________________ 45

Regional tectonic relations of earthquake faults ______________________ 46

Landslides _________________________________________ 47

Landspreading, fissuring, subsidence, and sand mounds _____________ 48

Volcanic activity __________________________________ 49

ill
IV CONTENTS

Page

Intensity distribution and source parameters from field observations, by Alvaro

F. Espinosa, Raul Husid, and Antonio Quesada _______________ 52

Introduction ________________________,_______________ 52

Casualties and damage ________________________________ 52

Intensity distribution in Guatemala __________________—____ 52

Distribution of adobe damage ________________________-___ 58

Intensities in Guatemala City _____________________________ 61

Intensities in neighboring countries _________________________ 61

Ground motion at intermediate distances __________-__-_-__— 62

Source parameters from field observations __________—___——————— 62

Damage and engineering implications, by Raul Husid, Alvaro F. Espinosa, and

Antonio Quesada ___________________-___---_— 67

Introduction _____________________________________— 67

Earthquake-resistant design practice in Guatemala _________—_——— 67

Types of structures _____________-___—_————————————— 67

Damage survey ___________________________ — __ — __— 67

General observations ________________________—__———————— 70

Structural engineering observations in Guatemala City, by Karl V. Steinbrugge__ 80

Introduction _____________________________—__—————_— 80

Life loss and construction __________——————-—-——- 80

Design and construction practices ___—__—_———————————-- 80

Specific damage observations _________-_—_————————————— 81

References cited _____________________________-___—-_—— 85

Chronological historical record of damaging earthquakes in Guatemala, 1526-

1976 ______________________________________________ 87

ILLUSTRATIONS

Page

FIGURES 1-4. Maps showing—

1. Guatemala, Central America ——————————————— 2

2. Tectonic setting of the Caribbean plate __————_— 5

3. Regional seismicity, 1902-74 ___________________ 6

4. Historical seismicity of Guatemala ——————————— 8

5. Diagram of tectonic evolution of the Caribbean plate —————— 10

6. Diagram of projected tectonic configuration of the Caribbean

plate in 50 m.y __————————————————————————— 10

7. Map of station locations of the 1973 Guatemalan seismograph

network _____________——_——_———_——— 12

8. Guatemalan seismicity map, 1945-75 ———————————————— 13

9. Histogram showing distribution of S—P time intervals ——— 14

10. Map showing epicenters and known faults within the 1973

seismic network ___—————————————————— 15

11. Location of epicenters of the main event and principal aftershocks _____________————_——————————— 18

12. P-wave focal mechanism solution —_——————————————— 19

13. Azimuthal variation of the G-wave displacement spectral density 21

14. Effect of source propagation on G-wave amplitudes —————— 22

15. Location of strong-motion field network _————_————— 25

16. Seismoscope plate, roof of University administration building,

Guatemala City ________————————————————— 26

17. Seismoscope plate, ground floor of University administration

building, Guatemala City _——_————————————— 27

18. Deconvolved seismoscope recording ————————————————— 28

19. Deconvolved seismoscope recording ————————————————— 28

20. Accelerogram of aftershock, IBM building, Guatemala City __ 28

21. Sample seismograms from the 1976 temporary seismic network 32

22. Map of aftershock epicenters from 1976 temporary seismic network ____________________——__————————— 37

23. Map showing regional setting of Motagua and Mixco faults — 39
CONTENTS

Page

FIGURES 24-28. Aerial photographs showing—

24. Displaced furrows on the Motagua fault trace, farmland west of Cabanas ___—————_————————— 41

25. Displaced furrows on Motagua fault trace, farmland

west of El Progreso ______——__————————— 42

26. En echelon offset, in a soccer field, of the Motagua

fault trace—Gualan ______________——__ 43

27. Most easterly, locally visited trace of the Motagua

fault _____________________________ 44

28. Subinal, Guatemala _____________—__——— 45 

29. Map showing zones of secondary faults and locations discussed

in text __________________________-_—_ 46 

30-32. Photographs showing—

30. Fault displacement in Guatemala City _———————— 46

31. Fault damage to house in Guatemala City _______ 47

32. Fault damage to house in Guatemala City, vertical displacement ______________________—___ 48

33. Map showing extent of earthquake-induced landslides _____ 49

34. Photograph of landslides in road cut, San Cristobal, Guatemala 50 

35-38. Aerial photographs showing—

35. Landslides north of Chimaltenango, Guatemala __—— 50

36. Landslide in gully, Guatemala City ________—__ 50

37. Landslide-dammed lake, Rio Pixcaya __________ 51

38. Ground cracks and sand mounds in unconsolidated alluvium near Quebradas _________________— 51

39. Photograph of linear sand mounds and craters, Motagua River

Valley ____________________________________ 51

40. Map showing location of intensity-questionnaire sampling distribution in Guatemala _________________ 57

41. Photograph of landslide obstructing road between Guatemala

City and El Progreso ______________________- 58

42. Photographs of damage to railroads at Gualan, El Jicaro, Puerto

Barrios wharf, and fault trace near Las Ovejas _____—— 59

43. Map showing Modified Mercalli intensity distribution in Guatemala _________________________________ 60

44. Schematic diagram of slip progression and effect on wave amplitudes due to a moving dislocation _______———__——, 61

45. Contour map of damage to adobe-type structures _________—_ 62

46-57. Photographs showing—

46. Intensively damaged towns of Joyabaj, Comalapa,

Tecpan, and San Martin Jilotepeque _________ 63

47. Wharf destroyed by earthquake at Puerto Barrios ___ 64

48. Collapsed chimney, Guatemala City ___________ 64

49. Head of a landslide, Guatemala City _____________ 65

50. Overthrown marble statues, Guatemala City _____ 65

51. Distortion and severe damage to wood construction,

Puerto Barrios ______________________ 68

52. Collapsed second story at Colegio San Javier, Guatemala City _________________________ 68

53. Partial collapse of three-story reinforced-concrete structure, Guatemala City ___________________ 69

54. Collapsed church, Guatemala City ____________ 70

55. Collapsed third story of Hotel Terminal, Guatemala

City _____________________________ 71

56. Collapsed gas-station structure, Guatemala City ____ 72

57. Collapsed water tower, Guatemala City _________ 73

58. Drawings of collapsed water tower shown in figure 59 _______ 73

59. View of collapsed water tower, Villanueva ___________ 74

60. View of collapsed grain silo, Villalobos ________________ 74

61. Drawing of cracked sedimentation tank __________________ 75
VI CONTENTS

Page

FIGURES 62-76. Photographs showing—

62. Damage to reinforced masonry, Colonia San Francisco,

Guatemala City _________ — __—__——_—— 75

63. Fault trace across San Jose Rosario subdivision, Guatemala City _________________-________ 76

64. Collapsed Agua Caliente Bridge _____——_—————— 76

65. Damaged Benque Viejo Bridge _—___—__—__ — __ 76

66. Column failure in reinforced-concrete framed structure,

Guatemala City _____________________ 76

67. Partial collapse of protruding structure on 11-story

Cruz Azul building, Guatemala City _—————— 77

68. Collapsed single-story reinforced-concrete framed

building, Licorera Mixco, Mixco _—_—_—_———— 77

69. Heavily damaged adobe houses, Antigua and Guatemala City _________—_—_——————— 78

70. Damage to flat beams and slab in a reinforced-concrete building, Guatemala City ——————_————— 79

71. Undamaged steel-frame Finance Ministry Building,

Guatemala City _—____——_——_——_——_—__ 81

72. Main terminal building of the International Airport,

Guatemala City _________________—__————— 82

73. Partially collapsed third story of Hotel Terminal,

Guatemala City ______________________ 82

74. Collapsed column in Hotel Terminal, Guatemala City__ 83

75. Collapsed second story, Catholic Boys School, Guatemala City ________________—____—__ 83

76. Column failure in second story, Catholic Boys School,

Guatemala City ____—_——_———_————— 84

TABLES

TABLE 1. Coordinates and magnifications of seismograph stations —__ 13

2. Number of events each month originating from the central fault

	system compared with the number of all regional events 16

3. Epicenter location of aftershocks ______—__—_—————— 18

4. Distance, azimuths, and spectral density for G-waves _—__ 20

5. List of seismograph stations occupied during this study ____ 31

6. Aftershocks of the main event located by the temporary seis­ 

	mograph network _______________—__ 34

7. Characteristics of earthquake fractures along the Motagua fault 40

8. Casualties and damage, Departmentwide ————————————— 53

9. Casualties and damage by municipality ___________—___ 54

10. Felt area for Modified Mercalli intensities VI and higher __ 58

11. Damage to adobe type structures _—_——_———_————— 60
GLOSSARY

Accelerogram. A record made by an acceleroineter, showing 

the amplitude of acceleration as a function of time.

Accelerometer. An instrument designed to record ground accelerations.

Aftershock. Secondary small magnitude earthquakes which 

follow the main event.

Amplification. Seismic-wave amplification is a relative measurement of the ratios of seismic waves recorded on alluvium to those recorded on crystalline rock. In the frequency domain is the ratio obtained from two stations 

for a selected spectral component.

Bar. 10" dyne cnr2.

Body-wave magnitude (mb ). A measure obtained from the 

amplitude and period of P- or S-waves at teleseismic 

distances.

Colegio. School.

Deconvolved. Division of a time series into a new time series 

plus a shorter wavelet. Usually the wavelet has a 

physical significance such as the impulse response of a 

system.

Departamento. A subdivision of the Republic of Guatemala 

geographical territories. Similar to States in the United 

States.

Department. See Departamento.

Energy (E). A measure of the seismic energy released by 

an earthquake. Dimensions are in ergs. This quantity E 

is often given in empirical relationships as functional 

dependent on magnitude.

Epicenter. The point on the Earth's surface vertically above 

the hypocenter or point of origin of an earthquake.

Fracture zone. An elongated complex of geologic features, 

mostly faults, thought to represent the contact between 

major geologic units which are moving relative to each 

other.

G-waves. Seismic surface waves impulsive and slightly 

dispersed are called G-waves (mantle Love waves). The 

suffix 2, 3, 4, and so on indentifies the great circle path 

of propagation. Odd numbers identify the short great 

circle path from epicenter to station Gl; G3 is that 

epicentral distance plus 360°. Even numbers identify 

waves that propagate in opposite azimuths.

Hazardous structure. An unsafe structure because of poor 

design, poor construction, defects in foundations, or 

damaged due to faulting or vibrational effects of an 

earthquake.

HYPO71. The identification of a software package which 

computes the hypocenter parameters of an earthquake.

Hypocenter. The point of origin of an earthquake, where rupture begins and from which seismic waves originate.

Intensity. A numerical subjective index describing the effects 

of an earthquake on man, on structures, and on the 

Earth's surface. The Modified Mercalli scale of 1931 with 

ratings from I to XII, as defined below, is used in the 

United States (modified from Richter, 1958):

Modified Mercalli intensity scale

I. Not felt, except by a very few under specially 

favorable circumstances.

II. Felt only by a few persons at rest, especially on 

upper floors of buildings. Delicately suspended 

objects may swing.

III. Felt quite noticeably indoors, especially on upper 

floors of buildings, but many people do not recognize it as an earthquake. Standing motorcars 

may rock slightly. Vibration like passing truck. 

Duration estimated.

IV. During the day, felt indoors by many; outdoors by 

few. At night some awakened. Dishes, windows, 

and doors disturbed; walls make creaking sound. 

Sensation like heavy truck striking building. 

Standing motorcars rocked noticeably.

V. Felt by nearly everyone, many awakened. Some 

dishes, windows, and so on broken; a few instances of cracked plaster; unstable objects 

overturned. Disturbance of trees, poles, and 

other tall objects sometimes noticed. Pendulum 

clocks may stop.

VI. Felt by all, many frightened and run outdoors. 

Some heavy furniture moved; a few instances 

of fallen plaster or damaged chimneys. Damage slight.

VII. Everyone runs outdoors. Damage negligible in 

buildings of good design and construction; considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motorcars.

VIII. Damage slight in specially designed structures; 

considerable in ordinary substantial buildings, 

with partial collapse; great in poorly built 

structures. Panel walls thrown out of frame 

structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. 

Changes in well water. Persons driving motorcars disturbed.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown 

out of plumb; great in substantial buildings, 

with partial collapse. Buildings shifted off 

foundations. Ground cracked conspicuously. Underground pipes broken.

X. Some well-built wooden structures destroyed; most 

masonry and frame structures destroyed with 

foundations; ground badly cracked. Rails bent. 

Landslides considerable from river banks and 

steep slopes. Shifted sand and mud. Water 

splashed (slopped) over banks.

XI. Few, if any, masonry structures remain standing. 

Bridges destroyed. Broad fissures in ground.

VII
VIII GLOSSARY

Underground pipelines completely out of service. 

Earth slumps and land slips in soft ground. 

Rails bent greatly.

XII. Damage total. Waves seen on ground surfaces. 

Lines of sight and level distorted. Objects 

thrown upward into the air. 

Isoseismals. Contour lines of equal intensity. 

Left-lateral movement. A horizontal movement in which the 

block across the fault from the observer has moved to 

the left.

Love waves. A type of surface wave.

Magnitude (ML, mb, Ms). A quantity characteristic of the 

total energy released by an earthquake. The "intensity" 

rating, as contrasted to magnitude, describes its effects 

at a particular place. Richter (1958) devised the logarithmic magnitude scale, which is in terms of the motion which would be measured by a standard type of 

seismograph located 100 km from the epicenter of an 

earthquake. ML is the magnitude determined from local 

recordings, nib is known as body-wave magnitude, and 

Ms as a surface-wave magnitude.

Main event. Refers to the main earthquake.

Meizoseismal area. The area within the isoseismals of higher 

Modified Mercalli intensity ratings.

Modified Mercalli. See Intensity.

Particle displacement. The difference between the initial position of a soil particle and any later position.

Particle velocity. The time rate of change of particle displacement.

Phase. Any change of frequency in the seismogram of an 

earthquake. The seismograph responds to the motion 

of the ground and makes a seismogram. The seismogram 

is a line that is related to the motion of the Earth in 

any one chosen direction. The principal phases are called 

P, S, and L phases.

Plate. One of the mechanically independent lithospheric 

segments comprising the outermost layers of the Earth.

Right-lateral movement. The contrary of left-lateral movement. See Left-lateral movement.

Sand mounds. The deposits resulting from turbid upward flow 

of water and siome sand to the ground surface from increased ground-water pressures when saturated cohesionless materials are liquefied by earthquake ground 

motions.

Sedimentation tank. A large reinforced concrete tank designed 

to house large amounts of water.

Seismogram. A record of ground motion caused by an earthquake.

Seismograph. A system for amplifying and recording the 

signals from a seismometer.

Seismometer. A device that detects vibrations of the Earth, 

and whose physical constants are known for calibration to 

allow calculation of the actual ground motion from the 

seismograph.

Seismoscope. An instrument that inscribes on a glass plate a 

permanent record of an earthquake. This instrument is 

an unconstrained pendulum that records strong ground 

motions.

Seismoscope plate. A record of ground motion caused by an 

earthquake.

Station code. Abbreviation of seismological stations: (1) of 

the WWNSS, (2) of stations in Guatemala prior to the 

February 4 earthquake, and (3) of field portable seismograph stations in Guatemala.

1. WWNSS stations mentioned in this report: 

ALQ, Albuquerque, New Mexico 

BRK, Berkeley, California 

COL, College, Alaska 

DUG, Dugway, Utah 

GEO, Georgetown University, Washington, D.C.

GIE, Galapagos Island

GOL, Golden, Colorado

GUA, Guam

KIP, Kipapa, Hawaii

LON, Longmarie, Washington

PAS, Pasadena, California 

PAL, Palisades, New York 

PTO, Porto, Portugal 

PRE, Pretoria, South Africa 

SJG, San Juan, Puerto Rico 

WES, Weston, Massachusetts

2. Stations in Guatemala:

ARC, Quirigua, Department of Izabal 

BVA, Buema Vista, Department of Guatemala 

CHI, Chimachoy, Department of Sacatepequez 

FGO, Volcan de Fuego, Department of Escuintla 

MMA, Magdalena Milpas Altas, Department of

Sacatepequez

REC, Recreo, Department of Guatemala 

TER, Terranova, Department of Escuintla

3. Field stations in Guatemala:

ARC, Quirigua, Department of Izabal 

CCO, Chichicastenango, Department of Quiche 

CHM, Chimaltenango, Department of Chimaltenango 

CML, Chiquimula, Department of Ghiquimula 

ELC, El Chol, Department of Baja Verapaz

FFF, La Pina, Department of Izabal

GCG, Guatemala City, Department of Guatemala

JAP, Jalapa, Department of Jalapa

JOY, Joyabaj, Department of Quiche

PAL, Palencia, Department of Guatemala

PTO, Puerto Barrios, Department of Izabal 

RIO, La Esmeralda, Department of Izabal 

SAN, Sanarate, Department of El Progreso 

SJE, San Jeronimo, Department of Baja Verapaz 

TEC, Tecpan, Department of Chimaltenango 

TEL, Teleman, Department of Alta Verapaz 

VIT, Vitalis, Department of Izabal

Stereographic projection. A coordinate system or network of 

meridians and parallels, projected from a sphere at suitable intervals, used to plot points whose coordinates are 

known and to study orientation and distribution of planes 

and points.

Strike. Angle between true north and the trace or projection 

of a geologic surface or lineation on the horizontal 

plane.

Strike-slip fault. Fault in which movement is principally horizontal.

Strong motion. Ground motion of sufficient amplitude to be 

of engineering interest in the evaluation of damage due 

to earthquakes. This type of motion is usually recorded 

on seismoscopes and accelerographs.
GLOSSARY IX

Surf ace-wave magnitude (Ms). A measure obtained from the

amplitude and period of surface waves at teleseismic

distances.

Teleseismic. Refers to distances in the far field. 

Thrust fault. A steeply or slightly inclined fault in which the

block above the fault has moved upward or over the

block below the fault.

Trench. Elongated bathymetric depression of the sea floor.

Thought to be areas where oceanic crust is subducted into

the upper mantle. 

Universidad. University. 

Wavelet. A short time series used in the deconvolution of a

longer time series. It refers also to a time pulse. 

Zone. Suburbs or districts in Guatemala City.

SYMBOLS AND ABBREVIATIONS

[See the Glossary also]

A _———————————_———__ area of dislocation.

°C —————————_————________ degrees Celsius, 

cm _ — — — — — ——___ —— ____________ centimetre.

cm/rad ___ ———— __ — — __ ———___ centimetres per radian, 

cm-s _—————__—_____—__________ centimetres-second.

cm/yr _——————_——________ centimetres per year.

D _————__————________ average fault displacement.

dyne-cm ————__—_——__________ dyne-centimetre.

dyne cm" 2 __—__—_____——__ dyne per square centimetre.

A _—__—————_—__________ epicentral distance.

ACT _ — — — — — — _ — — ——— _ — —— — ______ — _ stress drop.

E _ — — _ — — _ — — _ — _____________ energy. See Glossary.

Es _ ———— —— ——— __ ——— _ — _ — _________ seismic energy.

G2, G3, G4 _————_——_______ G-waves. See Glossary.

g ————————— acceleration due to gravity (980 cm-s"2 ).

H ______________________________________________ height.

Hz ————_ — _______ ——___ Hertz (cycles per second).

h _________________——_____________ focal depth.

Imm -——————___— Modified Mercalli intensity rating.

ICAITI _——— Institute Centroamericano de Investigacion y

Tecnologia Industrial, 

kg _.——____________________________ kilogram.

km _———_______—_—___________ kilometre.

km2 ——______________________ kilometre squared. 

km3 ————_—__________________ kilometre cubed.

km/s ____________________ kilometres per second. 

L ——____________________ fault length.

m __ — _——_______— — __________________ metre.

M ——_______________________ magnitude.

nit —————________________ body-wave magnitude.

ML —————————————————_______ local magnitude.

Mo -—------________-__-_____ seismic moment.

Ms ___________________ surface-wave magnitude, 

min ____________———————_____——— minute, 

mm/min ________________ millimetres per minute.

ms _______________________ millisecond.

ms/day __________________ milliseconds per day.

m.y. ____________________________ million years.

M _______________ rigidity around faulted medium. 

NEIS _______ National Earthquake Information Service. 

Q _______________________ seismic quality factor.

r _________________ radius of a circular dislocation.

RMS _______________________ root-mean-square. 

ROCAP —_ Regional Office for Central America Programs 

s ________________________________ second. 

S ————_—_______—————— sensitivity of seismoscope.

Sd -_-_______________ relative ground displacement.

SQ ____________ solution quality in computer program for

calculating hypocenters. 

a _______________________________—— effective shear stress.

71! _______________ natural period of seismoscope.

Tz ----__________-_ second harmonic of seismoscope.

6. _____________________________________________ azimuth.

u -_________ average horizontal fault displacement.

USAID _____ U.S. Agency for International Development. 

UTC ________________ Universal Coordinated Time. 

WWV _____ Call letters for National Bureau of Standards

time and frequency radio broadcast station,

Fort Collins, Colorado.

WWNSS __ Worldwide Network of Standard Seismographs. 

w ___________________________ fault width. 

X __________________ horizontal particle velocity.

THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976,

A PRELIMINARY REPORT

A. F. ESPINOSA, EDITOR

ABSTRACT

The Guatemalan earthquake of February 4, 1976, with 

a surface-wave magnitude of 7.5, was generated by left-lateral 

slippage on the Motagua fault and was felt over an area of 

at least 100,000 km2. This earthquake claimed more than 

22,700 lives and injured more than 76,000 people. The preliminary estimate of losses is about $1,100,000,000.

Ground breakage was observed and mapped for a distance 

of nearly 240 km along a segment of the fault. It is the 

longest surface rupture to have occurred in America since 

1906. The inferred total length of faulting is nearly 300 km, 

postulated on the basis of small aftershocks and high damage 

concentration west of Guatemala City. Maximum horizontal 

slippage as measured on the fault, about 25 km north of 

Guatemala City, was 325 cm. The average horizontal displacement is approximately 100 cm. Slippage also occurred on a 

number of secondary faults and caused damage to houses 

and other structures in the vicinity of Guatemala City.

The February 4 earthquake caused extensive landsliding 

along the highway that leads from Guatemala City to the 

Pacific and Atlantic Oceans. Landslides also blocked many 

railroads and destroyed many communication routes in the 

highlands of Guatemala. A large landslide near Tecpan covered two small villages and dammed a river. Additional landslides were triggered by aftershock activity.

Guatemala contains three major earthquake-generating 

zones. The first zone, which has the highest level of seismicity, 

is the Benioff zone that dips northeastward beneath Guatemala and is due to the Cocos plate being thrust beneath the 

Caribbean plate. The second lies at shallow depths along the 

chain of active volcanoes; the third is the fault system that 

bisects Guatemala from east to west. It is in this third zone 

that the February 4 earthquake occurred.

The preliminary fault plane solution is consistent with almost pure sinistral slip on a nearly vertical fault striking 

about N. 65° E. This solution agrees with the geologically 

mapped fault trace. The seismic moment was determined to be 

2.6xlOB7 dyne-cm from the amplitude of mantle Love waves 

(G-waves). A fault depth of 29 km was calculated from the 

seismic moment and the field observations of fault length and 

fault displacement. The earthquake was a low-stress-drop 

event, 3^Aff^18 bars. The energy release was l.lXlO23 ergs.

Thousands of aftershocks followed the main earthquake. 

Epicenters for 78 small aftershocks determined from locally recorded data delineate the Motagua fault and several 

secondary faults. A number of epicenters in the eastern part 

of Guatemala follow the general trend of the inferred extension of the Motagua fault. A large number of events that

occurred south of the Motagua fault and west of long 90.3°W. 

at depths of less than 14 km are associated with secondary 

faults, such as the north-south faults that bound the Guatemala City graben.

The maximum Modified Mercalli intensity was IX in the 

Mixco area, in some sections of Guatemala City, and in 

Gualan. The Modified Mercalli intensity reached VI over an 

area of 33,000 km2. The concentration of high intensities observed near the western end of the Motagua fault suggests 

the influence of a westward-propagating fault rupture.

Communities and small towns that suffered 100 percent 

damage covered an area of 1,700 km2 . At some localities, adobe 

structures near the causative fault (within 10 km) were 

essentially undamaged; at greater distances from the fault 

in the highlands, there was widespread collapse of adobe 

buildings. Modern earthquake-resistive structures in Guatemala City were damaged, including several hospitals. Several 

water tanks and corrugated-steel grain silos and numerous 

heavy parapets collapsed.

The information compiled in this study will be useful in 

assessing and delineating earthquake hazards in Guatemala 

to reduce the potential for loss of life and property in future 

earthquakes. The information in this study is also important 

in evaluating the effects to be expected from large strikeslip earthquakes elsewhere, such as those that occur in coastal 

California. A more detailed 'study of the distribution of intensities in Guatemala City could provide the basis for delineating variations in the hazards due to ground shaking and 

ground failure throughout the city and could assist in developing land-use policies.

INTRODUCTION

The Republic of Guatemala in Central America 

suffered extensive loss of life and severe damage to 

its economy from the February 4, 1976, earthquake. 

A request by the Guatemalan government to the 

Organization of American States prompted the U.S. 

Geological Survey to send a team of scientists and 

engineers to Guatemala to investigate the cause of 

the earthquake, the geologic, seismologic, and engineering effects, and the hazards resulting from 

the earthquake. U.S. Geological Survey Open-File 

Report 76-295, which was released in March 1976, 

provided a brief interim summary of the activities
GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

r i V ,^'Cuilapa ,-- jUT| ApA

92 00 W 9100' 90° 00' 89° 00' 88° 00'

FIGURE 1.—Guatemala and its Departments (States), including its general location in Central America. Base map modified

from Guatemala, Institute Geografico Nacional, 1974, 1: 500,000.

of the earthquake investigation team. The open-file 

report and this publication represent the immediate 

response by our agency to the request of the Guatemalan government.

Guatemala has a long history of damaging earthquakes. Historic chronicles date earthquake occurrences in Guatemala from the period of the Spanish 

conquest. The chronicles indicate that the cities of 

Antiqua and Guatemala City have been badly damaged by earthquakes more than 15 times since the 

early 16th century. The most damaging earthquakes 

to have occurred before the February 4 event were 

on December 25, 1917, and on January 3, 1918. 

These earthquakes and their aftershocks claimed 

numerous lives and partially destroyed Guatemala 

City.

The February 4 earthquake caused extensive 

damage, as the following statistics from a few of 

the cities and Departments (States) show: 88,404 

houses were destroyed and 434,934 people left homeless in the Department of Guatemala, about onehalf that number in the Department of Chimaltenango (fig. 1), and all houses were destroyed in 

the towns of San Pedro Sacatepequez, El Jicaro, 

Sumpango, Tecpan, and Gualan.

From the above, it is obvious that the principal 

hazard to life from earthquakes in the Republic of

Guatemala is the collapse of manmade structures 

because of ground shaking. There are, in addition, 

other .serious earthquake hazards, especially landslides in the highlands and surface breakage along 

active faults. Landslides can interrupt lifelines, in 

particular, roads, railroads, and communication networks, and they can dam streams and cause lakes 

that are subject to sudden release upon failure of 

the dam.

There is much to learn from this earthquake that 

is directly relevant to the problem of reducing earthquake hazards in the United States. The fault that 

produced the earthquake is the boundary between 

two crustal plates and is tectonically similar to the 

San Andreas fault in California and to the Fairweather fault in southeastern Alaska. Many of the 

modern buildings in Guatemala City are similar to 

structures in the United States and in particular 

to those in cities in coastal California.

The different papers of this report present (1) 

the tectonic setting and the seismicity of the region; 

(2) the local seismic activity before the February 

4 earthquake; (3) the main event and principal 

aftershocks as recorded at teleseismic distances; 

(4) the source parameters of the main event and 

possible future earthquakes on the fault that slipped 

on February 4; (5) estimation of strong ground
INTRODUCTION

motion due to the main event; (6) aftershocks from 

local data and their relation to primary and secondary faulting; (7) the geologic effects, including primary and secondary faults, landslides, and relation 

of faulting to damage; (8) the intensity of shaking 

and its distribution, casualties and damage, groundmotion parameters at intermediate distances, and 

source parameters from field observations; (9) 

damage and engineering implications and earthquake-resistant design code; and (10) design, construction practice, and general observations on damage to high-rise structures.

The results from this collective investigation will 

aid in the reduction of earthquake hazards in Guatemala and in the United States, but the material 

presented in this report should be considered as 

preliminary. Some of the details may change, and 

the conclusions may be modified upon further 

scrutiny of existing data and upon analysis of new 

data gathered in later geologic field studies.

ACKNOWLEDGMENTS

We gratefully acknowledge: Colonel J. Guillermo 

Echeverria Vielman, Coordinator of the Comite Nacional de Emergencia; Mr. Jose Asturias, Scientific 

Coordinator of the Comite Nacional de Emergencia; 

Dr. Eduardo Gonzalez Reyez, Executive Secretary 

for Education, Science, and Culture of the Organization of American States; Dr. Gabriel Dengo, Director of the Institute Centroamericano de Investigacion y Tecnologia Industrial; and Ing. F. Hernandez Cruz, Director General, Institute Geografico 

Nacional, for their full support in our investigation. 

Mr. M. Freundel, Assistant Director USAID/ 

ROCAP, provided ground transportation and an excellent chauffeur. We wish to acknowledge the support given to us, in the questionnaire canvassing of 

Guatemala City, by Ing. M. A. Castillo Barajas, 

Director, and Ing. H. E. Molina, Assistant Director, 

of the Civil Engineering Department of the Universidad de San Carlos. Also, we are grateful to the 

university students who participated in this effort

and to Dr. S. B. Bonis of the Institute Geografico 

Nacional, who also obtained a great number of 

intensity questionnaires for our program. We wish 

to thank the Camara de Construccion and the Institute Geografico Nacional for providing us with 

temporary office space. We also would like to thank 

the helicopter officers and staff of La Fuerza Aerea 

Guatemalteca for their efficient and enthusiastic 

support of our canvassing fieldwork.

We gratefully acknowledge the support given to 

us by members of the Guatemala Air Club and particularly Messrs. Rudy Weissenberg C., Antonio 

Delgado Wyld, and Francois Berger, who generously made their personal aircraft available for 

our reconnaissance studies of the Motagua fault. 

Mr. David Schwartz of the Woodward-Lundgren 

Company participated with us in some of the fieldwork along the Motagua fault and freely shared 

his personal knowledge of the geology in part of 

this area. We wish to thank Messrs. D. F. Miller 

and A. Sunderman, engineers with ROCAP, for 

their support while we were in Guatemala. Also, 

Dr. 0. Salazar of the Institute Geografico Nacional 

gave full support to our efforts in Guatemala.

We wish to thank Mr. A. Gregg of the InterAmerican Geodetic Survey; the U.S. Military 

Group; Mr. C. Urrutia, Director, and Mr. E. Sanchez, technician, of the Observatorio Nacional for 

their support to our seismic network task. The helicopter support for this program was provided by 

the U.S. Army, 114th Aviation Company, from 

Panama. Mr. J. Irriarte, a geologist with the Institute Nacional de Electrificacion, gave us field support and participated with us in the deployment of 

our seismic equipment. Our most sincere thanks are 

also due to the Guatemalans, both private citizens 

and public officials, who helped by relating their 

personal experiences about the earthquake, to those 

who provided support to our mission, and to those 

who provided information to our efforts. Without 

their support, this investigation would not have 

been as fruitful as it has been.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

TECTONIC SETTING AND SEISMICITY

By WILLIAM SPENCE and WAVERLY PERSON

INTRODUCTION

The February 4, 1976, main event occurred within 

the Motagua fault zone, the active boundary between 

the North American and Caribbean plates. The 

tectonic setting for the region of the Caribbean 

plate, with seismicity for the years 1962-69, is 

shown in figure 2.

The absolute motion of the Caribbean plate has 

been shown to be nearly fixed, assuming stationarity 

of the hot-spot reference frame (Jordan, 1975). 

Thus, the average relative plate motions—CocosCaribbean (7.47 cm/yr at 25.3° azimuth), North 

American-Caribbean (2.08 cm/yr at 252.4° azimuth), and Cocos-North American (9.01 cm/yr at 

35.0° azimuth)—directly cause the seismicity of the 

region of Guatemala. These plate motions were calculated at lat 15° N., long 90° W. from relations 

given by Minster and others (1974). The rupture of 

the February 4 earthquake progressed into the zone 

of the triple junction between the Cocos, Caribbean, 

and North American plates and thus assumes a 

particular significance.

SEISMICITY

The seismicity of the zone from lat 13°-18° N., 

long 87°-95° W. for the period 1902-75 is shown 

in figure 3A. The main event and its nine largest 

aftershocks, through March 7, 1976, are shown by 

x's (Person and others, this report). The Motagua 

fault zone is notable for its lack of major earthquakes from 1902 (or earlier) until the February 

1976 main event and is a clear example of a seismic 

gap on the Caribbean-North American plate boundary. The one large earthquake epicenter shown near 

the western end of the mapped Motagua fault 

(Ms -7.5, 1921) is assigned a focal depth of 120 

km (Gutenberg and Richter, 1954) and thus is associated with the subducting Cocos plate rather 

than with the Motagua fault. The eastward extension of the Motagua fault, the Swan fracture zone, 

has been seismically active during this period. It is

characterized by shallow, left-lateral, strike-slip 

faulting (Molnar and Sykes, 1969). The maximum 

seismicity in this figure occurs with the zone from 

lat 14°-15° N., long 91°-94° W. and suggests that a 

complex tectonic regime is associated with the Caribbean-Cocos-North American triple junction.

Five cross sections through this Middle America 

seismic zone, for the period 1962-72, are shown in 

figure 35 (A. C. Tarr, written commun., 1976). 

These show that the diffuse seismicity in figure 3A 

is largely attributable to the underthrusting of the 

Central American continental mass by the Cocos 

plate. Of the 1,331 earthquakes that occurred during the period 1902-75, shown in the map portion 

of figure 3A, the 17 events with Ms^7.0 apparently 

are associated with the subduction of the Cocos 

plate. The seismicity of these cross sections is more 

diffuse than that of seismic cross sections that are 

based on high-quality local data for island arcs (for 

example, Engdahl, 1971, 1973; Mitronovas and 

others, 1969). An increased resolution of the dip 

of the Cocos plate beneath Nicaragua has been obtained by a relocation of earthquakes in that area 

(Dewey and Algermissen, 1974). An improved picture of Guatemalan regional seismicity as a series 

of cross sections could provide insight into the 

nature of the associated triple junction.

A major seismic gap has been noted at the western coast of Central America between long 88° and 

91° W. (Kelleher and others, 1973). Now that the 

Motagua fault-lock zone has ruptured, the possibility that additional major earthquakes could occur deserves special study. South of the Motagua 

fault, an eastward relaxation of the Caribbean plate 

coupled with an accelerated local subduction of the 

Cocos plate could result in underthrust-type earthquakes that would fill this seismic gap. North of 

the Motagua fault, an accelerated westward motion 

of the North American plate could override the 

Cocos plate and lead to normal faulting off the 

southwestern coast of Mexico.
TECTONIC SETTING AND SEISMICITY

60°W

20°N

10°N

20°N

10°N

FIGURE 2.—Tectonic setting of the Caribbean plate. The Motagua fault zone, Swan fracture zone, Oriente fracture zone, 

and Puerto Rico Trench are elements of a major transform fault system on which the North American plate moves 

westward, at 2.1 cm/yr, relative to the nearly stationary Caribbean plate. Large arrows indicate direction of plate movement. The location of the February 4 Guatemalan earthquake is shown by the large dot. Small squares are previous earthquake locations. Serrated line indicates subduction. Modified from Jordan (1975) by Tarr and King (1976).

HISTORICAL SEISMICITY

Historical accounts of damaging earthquakes in 

Guatemala are numerous and date from the time of 

the Spanish conquest. A particularly destructive 

earthquake occurred in 1541 near Ciudad Vieja, the 

primary early Spanish settlement in Guatemala, 

and caused the deaths of approximately 150 Spaniards and at least 600 Indians and Negroes. The 

Spanish settlement subsequently was moved to 

nearby Antigua, and this city became the original 

capital of Guatemala. Antigua was extensively damaged by earthquakes 11 times between 1565 and 

1773. Accounts say that Antigua was destroyed in 

1586, 1717, 1773, and 1874. Following the 1773 

earthquake, Guatemala's capital was moved to 

Guatemala City.

Before 1976, Guatemala City had experienced 

damaging earthquakes in 1917-18, 1863, and 1862. 

The December 25, 1917, earthquake destroyed or 

seriously damaged about 40 percent of the houses 

in Guatemala City. This earthquake was followed 

by large earthquakes on December 29, 1917, January 

3, 1918, and January 24, 1918. The most damaging 

earthquake of the 1917-18 series was that on January 3, 1918.

A chronological historical record of important 

Guatemalan earthquakes, from 1526 to the time of 

the February 1976 main event, for noninstrumental 

and instrumental data is given on p. 87. Included 

are earthquakes occurring in or offshore from 

Guatemala that caused damage or are known to 

have had a magnitude greater than or equal to 6.0. 

The source of descriptive information for each 

earthquake is indicated in parentheses. At this 

writing, primary sources have not been checked for 

all earthquakes, and thus the information in the 

chronological listing may contain occasional omissions or errors.

In figure 4, the earthquakes in the chronological 

historical listing that have instrumentally determined epicenters are plotted as triangles, and the 

larger or better described earlier earthquakes are 

plotted at their maximum damage zone as squares. 

A notable recent earthquake series occurred near 

Mazatenango. The October 23, 1950, earthquake 

(Ms = 7.1) was followed, within 2 weeks, by six 

aftershocks of Ms^6.0. The pattern of historical 

seismicity in Guatemala is similar to that based on 

reliable epicenter estimates, such as shown in figure 

3A. Notably, there is a general lack of major his-
GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

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TECTONIC SETTING AND SEISMICITY

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FIGURE 3.—Continued.

torical earthquakes that can definitely be associated 

with the Motagua fault.

Because of scattered population centers and poor 

communication, the historical accounts of earthquakes provide only approximate estimates of an 

earthquake's epicenter. As an illustration, there 

have been two Ms = 8.3 earthquakes with epicenters 

at lat 14.0° N., long 91.0° W. in 1902 and 1942. The 

first of these is reported to have destroyed Quezaltenango and to have activated Santa Maria Volcano (Vassaux, 1969). No data are available, in 

conventional sources, on the cities of Antigua, 

Comalapa, or Mazatenango, which were nearer to 

the epicenter. The 1942 earthquake is described as 

"strongly felt throughout the central region of 

Guatemala" (Seismological Society of America 

Bulletin, 1942). The problem for earlier earthquakes 

sometimes is more severe.

However, based on both historical and modern 

records of Guatemalan seismicity, we conclude that 

large earthquakes on the Motagua fault occur only 

rarely. Although the historical record is incomplete 

and imprecise, we feel that, before the February 

4 main event, the most recent large earthquakes 

that may have occurred on the Motagua fault are 

those of April 1765 and 1773. The 1765 earthquake 

destroyed many towns in the Department of Chiquimula and is placed by us on the Motagua fault 

at about long 89.5° W. The major earthquake of 

July 29,1773, was preceded by numerous foreshocks, 

unlike the February 4 event. However, Montessus 

de Ballore's (1888) source reports that, although 

the 1773 event destroyed Antigua, it was felt even 

more strongly to the north, in Chimaltenango and 

Quezaltenango. Thus, this major earthquake may 

have occurred on the western Motagua fault. The 

moderate earthquake of January 1929 caused considerable damage at Puerto Barrios. It is reasonable to associate this earthquake with the western 

portion of the Swan fracture zone or possibly with 

the eastern extremity of the Motagua fault. A 

moderate earthquake occurred somewhat west of 

the 1929 event on August 10, 1945. The U.S. Coast 

and Geodetic Survey located it at lat 15.4° N., long 

88.8° W. (not shown in fig. 4), and this earthquake 

could reasonably be associated with the Motagua 

fault. This event caused significant damage at 

Quirigua and was felt in the Departments of Chiquimula, Zacapa, and part of Izabal (Seismological 

Society of America Bulletin, 1945).

We conclude that the return period for major 

earthquakes on the central and western Motagua 

fault is at least 200 years. The average relative
GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

D Location Based On 

Damage Reports

92'* 91°

FIGURE 4.—Historical seismicity of Guatemala. Epicenters based on P-wave arrival time data are shown by triangles. The 

closed dashed line is an inferred ring fault. The larger or better described earlier events (a chronological historical listing is on p. 88) are plotted as squares at the site of maximum reported intensity. The zone near Antigua apparently has particularly high seismicity. Except for the 1765 event, there is a marked absence of historical seismicity associated with the Motagua fault. Distribution of faults and volcanoes modified from Bonis, Bohnenberger, and Dengo (1970). 

Additional geological information is available from Dengo and Bohnenberger (1969) and Dengo (1969). Segmentation and 

offset of the Guatemalan volcanic linears (light parallel lines) into eastern, central, and western limbs are described by 

Stoiber and Carr (1973).

plate velocity between the North American and 

Caribbean plates of 2.1 cm/yr then implies that the 

maximum displacement of a major Motagua fault 

earthquake should be of the order of 4 m. The February 4 main event had a measured maximum displacement of about 1.5 m (Plafker and others, this 

report). This difference can be explained by the 

occurrence of creep and small-to-moderate earthquakes on the Motagua fault or by sudden differential displacement at the time of the February 4 

main event along buried parallel strands within the 

Motagua fault system.

TECTONIC HISTORY OF THE CARIBBEAN-NORTH 

AMERICAN PLATE BOUNDARY

Until recently, the tectonic history of the Caribbean region has been one of the more perplexing 

elements in plate-tectonics reconstructions. The 

Caribbean plate has existed for approximately 50

million years (Malfait and Dinkelman, 1972; Pinet, 

1972).

The Caribbean-North American plate boundary 

between Hispaniola and Guatemala consists of two 

fracture zones, the Oriente fracture zone to the east 

and the Swan fracture zone to the west (fig. 2). 

The Cayman Trough and Ridge are subparallel to 

these fracture zones. An offset in seismicity corresponds to the offset of these two fracture zones 

(Bowin, 1968). Holcombe and others (1973) have 

used morphological arguments to demonstrate that 

this offset corresponds to an active center of eastwest spreading, the Cayman Rise.

Centered at the Cayman Rise, a series of symmetrically spaced, north-south-trending ridges has been 

mapped out to 150 km east and west from the 

Cayman Rise (Holcombe and others, 1973). This 

evidence for east-west spreading conclusively supports the occurrence of extensive left-lateral dis-
TECTONIC SETTING AND SEISMICITY

placement at the Caribbean-North American plate 

boundary. Pinet (1972) argues that this left-lateral 

displacement is longer than 1,000 km, based on correlation of salt-piercement structures on opposite 

sides of the Motagua fault, in northern Honduras 

and in Guatemala. Hess and Maxwell (1953) estimated this left-lateral offset to be about 1,100 km.

The Caribbean plate is inferred to be approximately stationary in an absolute plate-velocity reference frame (Jordan, 1975). This conclusion is based 

on the hot-spot hypothesis as advanced by Wilson 

(1963) and Morgan (1971) and supported by 

Minster and others (1974). Thus, the North American plate is moving westward with respect to the 

middle-American portion of the Caribbean plate 

and is overriding the Cocos plate. The left-lateral 

offset associated with the Caribbean-North American plate boundary should persist through Guatemala; the Motagua fault is the continental extension of this active plate boundary.

This model has been dramatically confirmed by 

the occurrence of the 1976 Guatemalan earthquake. 

Heretofore the lack of field evidence for left-lateral 

displacement on the Motagua fault (McBirney, 

1963; McBirney and Bass, 1969; Falquist and 

Davies, 1971; Kesler, 1971) and the sphenochasm 

hypothesis for the opening of the Gulf of Honduras 

(Carey, 1958; Freeland and Dietz, 1971; Uchupi, 

1973) had left the character of the northwestern 

Caribbean in some doubt.

One model for the evolution of the Caribbean plate 

is illustrated in figure 5 (Malfait and Dinkelman, 

1972). This model begins with an eastward extension of the "East Pacific" plate into the present-day 

Caribbean zone and requires a progressive extension of subduction and island-arc volcanism from 

the position of southern Mexico towards the western 

side of the South American plate. This model includes a clockwise rotation of the Caribbean plate 

from the inferred north-northeast motion of the 

"East Pacific" plate to the present-day position as 

shown in figure 2.

Malfait and Dinkelman's model also includes 

large-scale left-lateral displacement at the Polochic 

or Motagua fault systems. A surprisingly well 

fitting reconstruction of pre-Late Cretaceous plate 

elements at this zone can be obtained by moving 

the North American plate eastward on the Polochic 

fault (for example, King, 1969) to a position where 

a match is obtained for the 500-fathom isobaths of 

eastern Yucatan and eastern Honduras. The required displacement is 1,000-1,100 km.

THE CARIBBEAN-COCOS-NORTH AMERICAN PLATE 

TRIPLE JUNCTION

Jordan (1975) has shown the southwestern Caribbean plate triple junction to be an exceedingly large 

zone of plate interaction. The northwestern Caribbean triple junction, which includes the Motagua 

fault system, is a smaller and probably less complex 

zone. We here attempt to set first-order bounds to 

the Caribbean-Cocos-North American triple junction zone. We use evidence of the occurrence of 

unusual tectonic activity, including very high levels 

of seismicity and volcanism, as indicative of the 

presence of this triple junction zone.

Although the eastern portion of the Motagua fault 

is approximately linear and parallel to the local 

direction of relative plate motion, this fault begins 

to curve, concave to the north, at about long 89.5° 

W. This change in fault direction may be tied to 

the complex stress field associated with this triple 

junction. The extreme western wedge of the Caribbean plate, west of about long 89.5° W. and between 

the mapped Motagua fault and the linear volcanic 

segments (shown in fig. 4), is characterized by 

north-south-striking faults and a diffuse zone of 

volcanism (Bonis and others, 1970).

Although the main-event rupture followed the 

curve of the Motagua fault, few (perhaps none) of 

the aftershocks and none of the extensive northtrending secondary faulting occurred to the north 

of the Motagua fault (Langer and others, this report; Plafker and others, this report). Plafker, 

Bonilla and Bonis (this report) found the occurrence of active secondary faulting (normal, down 

to the east) as far as 30 km from the main fault. 

This remarkable character of the aftershock distribution and secondary faulting is consistent both 

with: (1) the tendency of the Motagua fault to develop parallel to the relative motion of the North 

American and Caribbean plates and (2) an eastward relaxation of the Caribbean plate following 

the primary rupture at the Motagua fault.

Present-day seismicity, shown in figures 3 and 4, 

is concentrated in the zone from lat 14°-15° N., 

long 91°-94° W., the preponderance of large earthquakes occurs at shallow and intermediate depths 

at the smaller zone from lat 14°-15° N., long 91°- 

92.5° W. The linear extension of the eastern Motagua fault passes through the concentration of large 

shallow- and intermediate-depth earthquakes at lat 

14°-15° N., long 91°-92.5° W.

In recent geologic time, the central Guatemalan 

volcanic lineament has been the most active volcanic segment in Central America (Stoiber and
10 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

LATE CRETACEOUS

EARLY OLIGOCENE

FIGURE 5.—Model for the evolution of the Caribbean plate. (Used by permission from Malfaat and Dinkelman, 1972.) Arrows 

indicate motion relative to the North American plate. Solid triangles indicate volcanoes.

FIGURE 6.—Possible appearance of the Caribbean-Cocos-North 

American plate triple junction and nearby plate boundaries 50 m.y. from now, assuming that there will be no 

major changes in local plate motions. The tendency for 

this plate geometry to develop may explain the presentday east-west elongation of the Caribbean-Cocos-North 

American plate triple junction. Open arrows indicate motion relative to the Caribbean plate; serrated line indicates underthrusting or subduction.

NORTH AMERICAN PLATE
TECTONIC SETTING AND SEISMICITY 11

Carr, 1973). Fuego Volcano (lat 14.48° N., long 

90.88° W.) is midway in the central Guatemalan 

volcanic segment and is located about 45 km westsouthwest of Guatemala City. Fuego is near the linear extension of the eastern Motagua fault and is 

the most active volcano in Central America. Fuego 

produced major eruptions totaling 0.5 km3 during 

October 1974 (Bonis, 1976).

Assuming that the long, tabular concentration of 

extremely high seismicity and volcanism in southern 

Guatemala, coupled with the curve of the Motagua 

fault, reflects the complex tectonics of a triple junction zone, we suggest approximate bounds to this 

zone of lat 14.0°-15.0° N., long 89.5°-94.0° W. This 

triple junction of two trenches and a transform fault 

should be viewed as a three-dimensional zone of 

accommodation between three lithospheric plates.

Because of the relative westward motion of the 

North American plate, there should be a definite 

tendency for the Motagua fault to evolve westward. 

This should ultimately result in a new east-westtrending boundary between the Cocos and North 

American plates. This boundary should be characterized by both subduction of the Cocos plate and 

left-lateral transcurrent motion of the North American plate. The eastern terminator of the CaribbeanCocos-North American triple junction will tend to 

remain spatially and geometrically stable. Such a 

present-day evolution from the Guatemalan triple 

junction zone may explain the extended east-west 

character inferred for this triple junction. Figure 

6 is a schematic diagram showing the CaribbeanCocos-North American plate triple junction and 

nearby plate boundaries as they might appear 50 

m.y. from now, assuming that no major changes in 

local plate motions occur.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

INSTRUMENTALITY RECORDED SEISMIC ACTIVITY PRIOR TO THE MAIN EVENT

By DAVID H. HARLOW

INTRODUCTION

Instrumental recording of earthquakes in Guatemala began in 1925 with the installation of a threecomponent Wiechert seismograph at the Observatorio Nacional in Guatemala City (Vassaux, 1969). 

No other permanent seismographs were operated in 

Guatemala until early 1973, when three radio-telemetered high-gain seismic stations were installed 

near Guatemala City and Pacaya and Fuego Volcanoes as part of a cooperative project between the 

U.S. Geological Survey and government agencies of 

Guatemala, El Salvador, and Nicaragua to monitor 

seismic activity and ground tilt at active volcanoes 

in Central America. This project is described in 

detail by Ward and others (1974). The cooperating 

agency in Guatemala was originally the Institute 

Geografico Nacional and since early 1975 has been 

the Observatorio Nacional.

In March 1973, seismographs were temporarily 

installed at Chiantla and Chiquimula in cooperation 

with Dartmouth College, the Institute Geografico 

Nacional, and the U.S. Geological Survey. The purpose of these instruments was to monitor earthquake activity on an east-west system of faults that 

cuts across central Guatemala and consists mainly 

of the Polochic, Motagua, and Joctan faults (Quittmeyer, written commun., 1974).

Three additional radio-telemetered stations were 

added in February 1975, bringing the total to six. 

The purpose of this seismic network is to monitor 

earthquake activity associated with faults and active 

volcanoes.

The locations of the permanent and temporary 

seismic stations are plotted in figure 7, and station 

data are listed in table 1. Seismometers with a natural frequency of 1 Hz are employed at each station. The seismic signals from the permanent stations are relayed to the Observatorio Nacional and 

recorded on drum recorders at a paper speed of 60 

mm/min. A description of the instrumentation and

FIGURE 7.—Seismic station locations. Temporary seismic stations operated during 1973 are shown by open triangles, 

and the existing telemetering stations are shown by solid 

triangles. Volcanic cones are also shown.

the response curve can be found in Ward and others 

(1974). For the Chiantla and Chiquimula stations, 

recording was done on site.

SEISMICALLY ACTIVE AREAS IN GUATEMALA

Molnar and Sykes (1969) studied the regional 

tectonics of the Caribbean and Central America by 

using earthquake hypocenters and focal mechanisms for earthquakes larger than magnitude 4.0 

recorded by the WWNSS between 1954 and 1967. 

The most prominent seismic feature is a zone of 

earthquakes that dips northeastward beneath Central America and appears to be the result of the 

convergent plate motion and underthrusting of the 

Cocos plate beneath the Caribbean plate. Hypocenters for earthquakes within this zone range from 

near-surface depths at the Middle America Trench 

to depths of approximately 100 km beneath the 

line of active volcanoes and then to maximum depths 

of about 250 km (fig. SB). During this time inter12
INSTRUMENTALITY RECORDED SEISMIC ACTIVITY PRIOR TO MAIN EVENT 13

TABLE 1.—Coordinates and magnifications of seismograph stations

Stations 1

Present locations: 

FGO _ _ ________

CHI _____________

MMA _ ________

BVA _ __ ___ _.

EEC __ _._ _ __ _

TER

1973 locations: 

Chiquimula

Chiantla

Latitude

<°N.)

14°

14°

14°

14°

14°

14°

_ _ 14°

_ _ 15°

26.74' 

35.69' 

32.28' 

40.00' 

26.25' 

18.25'

47.4' 

21.6'

Longitude 

(°W.)

90°

90° 

90° 

90° 

90° 

90°

89° 

91°

50.43' 

51.62' 

40.89' 

38.24' 

31.36' 

41.01'

33.6' 

27.0'

Installation 

date

2/73 

2/75 

2/75 

2/73 

2/73 

2/75

3/73 

3/73

Magnification at 

25 Hz

120,000

60,000 

60,000 

120,000 

120,000 

240,000

120,000 

60,000

1 Station codes are listed in the Glossary.

val, however, only a few shallow-focus earthquakes 

locate along the fault system through Central Guatemala, which marks the boundary between the North 

American and Caribbean plates (Malfait and Dinkelman, 1972; Jordon, 1975). Included in this fault 

system is the Motagua fault (fig. 7) that was the 

source of the February 4 earthquake.

Figure 8 is a seismicity map of Guatemala based 

on 30 years of data recorded by the Wiechert seismograph at the Observatorio Nacional. The gain of 

this seismograph is 35, and thus it is sensitive to 

earthquakes larger than roughly magnitude 2.0 in 

the vicinity of Guatemala City and larger than magnitude 5.5 at a distance of 200 km. Earthquake epicenters are located only approximately by using the 

S —P-wave difference to calculate distance and Pwave amplitudes on the two horizontal components 

to determine azimuth. In addition, felt earthquake 

reports sent to the Observatorio Nacional aid in 

verifying the locations. Therefore, this group of 

earthquakes is approximately equivalent to what 

would have been located by the WWNSS. Even 

though the epicenters are subject to large errors, 

these long-term data provide important data on 

the features of seismically active zones in Guatemala and their relative level of seismic activity over 

three decades.

The greatest concentration of seismic activity 

shown in figure 8 is along the Pacific coast of Guatemala and is probably associated with the Benioff 

zone that dips northeastward beneath the country. 

Recent results, from a seismic network in Nicaragua 

(Aburto, 1975), show that, in addition to the very 

active dipping Benioff zone, a separate but seismically less active zone of shallow earthquakes with 

depths less than 20 km occurs along the Nicaraguan 

chain of active volcanoes. Data from the six-station 

seismic net suggest that a similar group of shallow 

earthquakes occurs in the vicinity of the volcanoes 

near Guatemala City (fig. 10). Historically this

FIGURE 8.—Seismicity map for Guatemala (1945-75) compiled 

by J. Vassaux, Observatorio Nacional, Guatemala. 

Hatched areas are regions of frequent seismic activity. 

Dots indicate the number of earthquake series or swarms 

at specific areas during this period: A, fewer than 5 times ; 

B, between 6 and 10 times; and C, more than 10 times.

shallow seismic zone is the source of moderatesized locally destructive earthquakes in Central 

America (Carr and Stoiber, 1976, unpub. data) including, most recently, the Managua earthquake of 

December 23, 1972, that damaged much of the capital city of Nicaragua. Therefore, the earthquakes in 

Guatemala that occur near the comparatively less 

seismically active northern boundary of the large 

hatched area shown in figure 8 may also be associated with shallow seismicity along the line of volcanoes.
14 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

Two other small areas of relatively low-level seismic activity are located in figure 8 in west-central 

and east-central Guatemala. These areas lie along 

the fault system that runs across central Guatemala, and the eastern area occurs on the Motagua 

fault. Previous data (Molnar and Sykes, 1969), the 

extensive ground breakage on the Motagua, and its 

tectonic similarity to the San Andreas fault indicate 

that the earthquakes in these areas occur at shallow 

depths.

Thus, there are three sources of magnitude 4 or 

larger earthquakes in Guatemala, and their relative 

levels of seismic activity appear to have been constant for the last 30 years. The main source of seismicity is the zone of earthquakes that dips northeastward beneath the country. The other two sources 

are shallow and seismically less active. One lies 

along the chain of active volcanoes, and the other 

along the fault system that crosses central Guatemala. Historically, however, these shallow, relatively less active zones are the sources of the majority of moderate-sized locally destructive earthquakes. Although the deep shocks are more numerous, their depths, which are in the range of 50-250 

km under the more heavily populated regions, lessen 

their threat by placing them at considerable distances from cities and towns.

DISTRIBUTION OF S-P TIMES

Distributions of S-P times from the station at 

Fuego Volcano and the two stations temporarily 

installed along the fault zone in central Guatemala 

are plotted in figure 9. Approximately 10 percent 

of the recorded earthquakes could not be used, either 

because the arrival times were unclear or because 

the earthquake waves were large enough to exceed 

the dynamic range of the instruments, thereby 

making the S-P time unreadable for S-P intervals 

of less than 15 or 20 s (a unit S —P interval is equivalent to a focal distance of about 8 km). For earthquakes within 5 km, the lowest detectable magnitudes are roughly estimated (Brune and Alien, 

1967) to be 0.5 for Fuego and Chiquimula and 0.8 

for Chiantla.

Events with S —P times of less than 5 s were recorded at all stations, indicating that, at each site, 

local faults are active. For microearthquakes within 

40 km of the stations, an average of 5.5 events per 

day were recorded at Fuego (Harlow and Ward, 

unpub. data, 1976), and about 1.0 event per day 

was recorded at Chiquimula and Chiantla. The 

higher seismicity at Fuego is probably related to 

its high level of recent volcanic activity. Local

150

100

40-i

20-

FUEGO VOLCANO 

64 DAYS

CHIQUIMULA 

213 DAYS

CHIANTLA 

58 DAYS

FIGURE 9.—Distribution of S—P times in seconds during 1973 

at seismic stations in Guatemala. Useful recording days 

are noted for each station.
INSTRUMENTALITY RECORDED SEISMIC ACTIVITY PRIOR TO MAIN EVENT 15

seismicity near Fuego originates both on local faults 

and at the volcano. The Chiquimula station is 8 

km north of the Jocotan fault and 25 km south of 

the Motagua fault. Thus, events with 1- to 2-s S-P 

times at Chiquimula likely occur on the Jocotan 

fault or on north-south-trending fractures in the 

Ipala graben just to the south of the Jocotan fault 

in this area (Carr, 1974). The numerous events 

with S-P times of 3 to 5 s could occur either on the 

above faults or on the Motagua fault. The data 

suggest that the Polochic fault is also seismically 

active in the vicinity of Chiantla. The events with 

S —P times of 7 and 8 s and greater at Chiantla 

could originate on the western end of the Motagua 

fault. Thus, the occurrence of microearthquakes 

within 40 km of these seismograph stations indicates that the Polochic, Motagua, and Jocotan faults 

are probably seismically active.

Intermediate-depth earthquakes with magnitudes 

greater than 1.5 to 2.0 originating in the Benioff 

zone beneath the stations would be expected to produce peaks in the histograms at S-P times of 10 to 

14 s. Peaks in this range occur at the Fuego and 

Chiquimula stations. There are no clear peaks in 

the distribution of S - P times at Chiantla, however, 

possibly because of the relatively short recording 

time or the diffuse seismic activity in southeastern 

Mexico (Molnar and Sykes, 1969; see also fig. 8).

91*00' W 90°I5'W

BVA

O

MIXCO FAULT 

/-\ j_Antigu(

AGUA Qfc) 

VOLCANO ? ,

O

TECUAMBURRO 

VOLCANO

I4«45' 

N

-I4«00'

90°I5'W

FIGURE 10.—Epicenter locations and known faults in the 

region of the six-station seismic network (solid triangles). 

The size of the circles reflects the estimated error in the 

calculated location. Station code is in the Glossary.

SEISMIC ACTIVITY IN THE VICINITY OF 

GUATEMALA CITY

Epicenters of shallow (less than 15 km deep) 

microearthquakes near Guatemala City are plotted 

in figure 10. These data include 29 events recorded 

by the three-station network during 65 useful recording days in 1973 and 75 events recorded by the 

six-station network from March to September 

1975. For earthquakes that occur inside or near the 

seismic network, magnitude 1 events are the smallest that can be located. The most intense seismicity 

occurs within 10 km of Fuego Volcano, which has 

had large eruptions in 1971, 1973, and 1974, plus 

minor eruptive activity in 1975. These events, then, 

are probably related to eruptive processes at Fuego. 

Epicenters show that the Jalpatagua and Mixco 

faults are seismically active (ground breakage on 

the Mixco fault zone was caused by the February 

4 earthquake). Other epicenters lie on or near other 

known faults in this area. Thus, the shallow seismicity indicates that there is a complex pattern of 

active faults near Guatemala City.

Events that may originate on the Motagua fault 

have not been systematically located because they 

are outside the network and their epicenters are 

therefore poorly determined. A check of all regional 

earthquakes recorded by the six-station network 

from March 1975 through January 1976 was made 

to determine how many events might have originated on the fault zone across Guatemala represented 

by the Polochic, Motagua, and Jocotan faults. Criteria used to identify regional events occurring at 

shallow focal depths north of the seismic net are: 

(1) relative arrival time at the seismic stations to 

roughly determine azimuth and (2) apparent velocities across the network of less than 10 km/s. The 

second criterion passes shallow earthquakes at distances up to approximately 175 km from the seismic 

net and discriminates against events that occur 

noHh of the seismic net but at depth on the Benioff 

zone. The lowest detectable magnitude for these 

earthquakes is about 1.5 for events on the Motagua 

fault nearest to Guatemala City and 4.0 for events 

at a distance of 150 km. The results, listed by month 

in table 2, show that, at most, only 11 percent of 

the regional earthquakes could have originated on 

the Motagua fault during this period. In addition, 

no unusual activity such as swarms was observed. 

This, together with previous data from other 

sources, suggests that, although the Motagua fault 

is an historic source of large and damaging earthquakes, it is not continuously the most seismically 

active tectonic feature in Guatemala.
16 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

TABLE 2.—Number of events each month originating from the 

central fault system, compared with the number of all regional events

Date

1975 March __ __ __ _

April __ _ _ _ _ _

May _ _ ___ __

July —— ——— __

August ____ _

September _

1976 January

Total _ _ _ _ _

Possible 

central 

fault 

system 

events

9

14

28

a 5

9

8

14

6

6

8

107

Total 

regional 

events

76

77

90

M3

49

104

119

127

123

124

932

1 The numbers of events for July are extrapolated from 22 useful recording days.

SUMMARY

Seismic data collected over an interval of about 

30 years before the earthquake of February 4 reveal 

three main sources of seismic activity in Guatemala. 

The most intense source of seismic activity is a 

zone of earthquakes that dips northeastward beneath Guatemala and results from the Cocos plate 

being thrust under the Caribbean plate. The danger 

of this seismic zone is greatly reduced by the depths, 

and therefore the distance, of these events, which

range from 50 to 250 km under the more heavily 

populated regions. During the interval of the last 

30 years, the second and third source regions have 

consistently generated fewer shocks. The second 

source of earthquakes occurs at shallow depths along 

the chain of active volcanoes. This zone is historically the source of many moderate-sized, locally 

damaging earthquakes and, at least in the vicinity 

of Guatemala City, consists of many complexly related active faults. The third source is the fault 

system that cuts across central Guatemala and includes the Motagua fault, which was the source of 

the February 4 earthquake. During 1973, about 1.0 

local (within 40 km) microearthquake per day was 

recorded at two high-gain seismograph stations installed on this fault system, suggesting that the 

Polochic, Motagua, and Jocotan faults are seismically active. During the 11 months preceding the 

February 4 earthquake, however, only 11 percent of 

all regional earthquakes recorded at a seismic network near Guatemala City could have originated 

from this fault system. Thus, the seismic zone that 

produced the most destructive earthquake in the recent history of Guatemala has exhibited a level of 

seismicity over the last 30 years that is lower than 

the prominent seismic activity that occurs on the 

deep seismic zone.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, 

A PRELIMINARY REPORT

MAIN EVENT AND PRINCIPAL AFTERSHOCKS 

FROM TELESEISMIC DATA

By WAVERLY PERSON, WILLIAM SPENCE, and JAMES W. DEWEY

The hypocenter of the main event of the Guatemala earthquake was 

determined by using stations available to the National Earthquake Information Service (NEIS) throughout the world. The preliminary hypocenter and origin time parameters are:

Origin time: 09 01 43.3 UTC (03 02 43.3 local time)

Latitude: 15.32° N.

Longitude: 89.08° W.

Depth: 5 km (constrained)

Magnitude: Ms = 7.5

The main event is located near Los Amates about 157 km northeast of 

Guatemala City on the Motagua fault. It should be emphasized that the 

hypocenter of the main event represents the point of the initial rupture. 

The fault break extends at least 160 km westward towards Guatemala 

City and 80 km towards the northeast (Plafker and others, this report).

Data from 90 stations were used in locating the main event, including 

readings from Guatemala. The wide distribution of these stations gives us 

reasonable confidence in the epicenter solution. Travel-time anomalies, 

however, could conceivably be producing a location bias of tens of kilometres; the preliminary location of the Managua, Nicaragua, earthquake 

of December 23, 1972, for example, was biased 25 km to the northeast 

of the true epicenter. In the case of the Guatemala earthquake, a component of location bias in the direction of the Motagua fault would be 

difficult to detect.

There were neither reliable teleseismic depth phases nor stations close 

enough to the epicenter of the Guatemala main event to enable us to estimate hypocentral depth with confidence. The hypocenter was restrained 

to a shallow depth, 5 km, because of the surface faulting that accompanied the earthquake and because the depths of aftershocks located by 

Langer, Whitcomb, and Aburto (this report) were in the range 0-12 km. 

Computation with no depth restraint of the hypocentral parameters of 

the main event always yielded focal depths in the shallow crust, but these 

results could be fortuitous.

The magnitude is based on the average of surface-wave data from 

several stations. Ms = 7.5 is consistent with amplitudes of 100-s G-waves 

measured by Dewey and Julian (this report).

Short-period P-wave arrivals of the February 4 main event and subsequent aftershocks are generally emergent. The teleseismic records strongly 

suggest that the main event was a multiple rupture.

The February 4 main event was followed by damaging aftershocks. The 

two largest aftershocks (both mb = 5.8), as of March 7, 1976, occurred 

soon after the main event. These aftershocks occurred near Guatemala 

City, possibly on the north-south-trending Mixco fault (see fig. 7). Other

17
18 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

MAIN EVENT 

O AFTERSHOCKS 

FAULT

92° W

FIGURE 11.—Epicenters of main event and the principal aftershocks through March 7, 1976. These locations are based on 

data available as of April 5, 1976, and are subject to slight revision. Light parallel lines represent volcanic linears. 

(Base map modified from Bonis and others, 1970, and Plafker and others, this report.)

TABLE 3.—Epicenter location of aftershocks

Date

Feb. 4

6

6

6

8

9

10

Mar. 7

7

Origin time

09 30 28.3

04 11 03.3

18 11 59.2

18 19 17.7

08 13 51.9

11 44 46.7

06 17 43.0

02 54 05.4

03 15 40.3

Latitude

14.7

14.6

14.3

14.7

15.7

15.3

15.0

14.9

14.7

Longitude

90.6

91.1

90.2

90.6

88.5

89.1

89.7

90.9

90.5

Depth * 

(km)

5

5

5

5

5

5

5

5

5

Magnitude

5.8 nib

4.8 nib

5.2 nib

5.8 nib

5.7 Ms

5.1 nib

4.7 nib

4.8 nib

4.9 nib

1 Constrained.

damaging aftershocks could have occurred immediately after the main 

event, and their seismic signatures could be buried in the coda of the 

main event. The Ms = 5.7 aftershock of February 8 is near the eastern end 

of the surface-fault rupture. The two aftershocks in table 3 that are 

not alined with the Motagua fault zone lie immediately north of the 

central Guatemalan volcanic lineament shown in figure 11.

Table 3 is a list of preliminary origin times, epicentral coordinates, 

and magnitudes of the principal aftershocks occurring through March 

7, 1976, as located by NEIS. Focal depth was restrained to 5 km in the 

aftershock hypocenter determination. Figure 11 shows epicenters of the 

main event and principal aftershocks. There were no foreshocks located 

by the NEIS, and we have no foreshocks recorded at high-gain stations 

at teleseismic distances.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

MAIN EVENT SOURCE PARAMETERS FROM TELESEISMIC DATA

By JAMES W. DEWEY and BRUCE R. JULIAN

INTRODUCTION

The Guatemala earthquake occurred on the Motaqua fault, a strike-slip fault thought to be part of 

the boundary between the Caribbean plate and the 

North American plate. The characteristics of the 

focal mechanism of the Guatemala earthquake are 

of considerable interest to U.S. seismologists, because there may be similarities between this fault 

and strike-slip faults in California that are also 

associated with plate boundaries. Conversely, the 

history of strike-slip earthquakes in California and 

other regions may help anticipate the future course 

of the aftershock sequence of the Guatemala earthquake.

FOCAL MECHANISM

The P-wave first-motion pattern of the main 

event is shown in figure 12. The east-northeaststriking nodal plane corresponds to the fault plane 

of the earthquake. Motion across this plane is leftlateral strike-slip. The strike of the fault plane is 

well determined and is about N. 65° E. Possible dips 

vary from 84° N. to 82° S. The motion is of almost 

pure strike-slip character. The fault plane in figure 

12 agrees well with the geologically mapped fault 

trace.

SEISMIC MOMENT

The Guatemala earthquake produced mantle Love 

waves that had periods of around 100 s and that 

were well recorded by many seismographs of the 

WWNSS, from which the seismic moment of the 

earthquake can be determined. For this preliminary 

report, we estimated the displacement spectral density of the ground motion by multiplying the measured pulse amplitudes by 70 s for those phases with 

periods near 100 s (Brune and Engen, 1969). Amplitudes were normalized to an epicentral distance of 

90° (see Brune and Engen, 1969) with Q, the seis-

• + COMPRESSION 

O - DILATATION

FIGURE 12.—Stereographic projection of P-wave first motions for the main event. Circles represent first motions 

especially measured for this study. Plus and minus represent first-motion data reported to the NEIS. The solution has been chosen to be consistent with all the readings made by us and, in addition, to be as consistent as 

possible with the first motions reported to the NEIS. The 

northeast-striking nodal plane corresponds in strike and 

sense of displacement to the Motagua fault.

mic quality factor, taken to be 107 and the group 

velocity of the G-wave taken to be 4.4 cm/s (table 

4 and fig. 13). These spectral densities were then 

adjusted for the orientation of the focal mechanism, 

assuming a vertical strike-slip fault as the earthquake source. More accurate determinations based 

on Fourier analysis of digitized records are planned.

19
20 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

TABLE 4.—Distance, azimuths, and spectral density for G-waves

Station 1

SJG

SJG

7 KIP 7 PRE

ALQ

DUG

LON

GOL

GIE 7 WES

7 KIP 7 PRE

7 GUA

ALQ

ALQ

DUG

DUG

LON

LON

GOL

COL

COL

GIE 'GEO

7 WES

PTO

PTO

Phase -

G2

G4

G3

G2

G3

G3

G3

G3

G2

G3

G2

G3

G2

G2

G4

G2

G4

G2

G4

G2

G2

G4

G3

G2

G2

G2

G4

A 

(degrees) 3

337.7

697.7

425.1

239.1

385.0

392.2

401.5

388.1

334.1

391.1

294.9

480.9

240.7

335.0

695.0

327.8

687.8

318.5

678.5

331.9

297.8

657.8

375.9

334.1

328.9

286.8

646.9

e

(degrees) 1

14.5

14.5

41.8

46.1

79.8

80.0

81.0

88.0

118.8

141.2

221.8

226.1

230.4

259.8

259.8

260.0

260.0

261.0

261.0

268.0

270.9

270.9

298.8

317.2

321.2

346.2

346.2

Spectral density3 at 90° 

(cm-s)

7.11

5.06

2.88

1.98

6.65

5.09

5.10

6.39

5.40

2.48

2.41

1.78

1.49

8.65

13.35

5.21

8.69

4.38

5.10

5.30

5.89

5.86

7.80

2.46

2.99

5.11

7.42

Spectral density B 

corrected for 

focal mechanism

5.18

3.68

4.52

3.45

3.41

4.08

6.42

5.88

9.07

3.53

5.89

2,93

3.41

3.38

3.75 (median)

3.73

9.27

3.67

5.33

1 Station code listed in Glossary.

- Phase: denned in Glossary.

3 Distance traveled by G-wave from source to station.

* Azimuth from source to station of the phase in question, measured clockwise from the direction 

of fault rupture propagation (S. 65° W.).

n Spectral density at 90°, estimated by multiplying the ground displacement by 70 s and applying the 

distance correction factor of Brune and Engen (1969).

6 The correction factor, assuming a vertical strike-slip fault, is fir/2 cos 26]- 1.

7 These stations were within 15° of a G-wave nodal plane and have not been used in the computation 

of moment.

Several of our stations lay near nodes of the theoretical Love-wave radiation pattern (fig. 13). Because the radiation-pattern correction for these stations is subject to large uncertainty, we used in the 

computation of seismic moment only those stations 

that were well removed (more than 15°) from the 

theoretical nodal planes (table 4). The mean of the 

adjusted displacement spectral densities is 4.77±0.43 

cm-s; the median is 3.75 cm-s. The mean seems 

unduly influenced by a few large values, and we take 

the median as more representative of the sample 

as a whole. The seismic moment corresponding to 

a displacement spectral density of 3.75 cm-s is 

2.6 xlO27 dyne-cm (Brune and Engen, 1969).

The provisional surface-wave magnitude assigned 

to the main event was Ms = 7.5 (Person and others, 

this report). The amplitudes of 100-s surface waves 

do not support a major adjustment of this magnitude; the moment computed from these amplitudes 

falls in the middle of the "cloud" of data points in 

Brune and Engen's (1969) graph of 20-s surfacewave magnitude versus moment.

SOURCE DIMENSIONS, DISPLACEMENT, STRESS 

DROP, AND DIRECTION OF FAULT PROPAGATION

The zone of the largest (M>5.0) best located 

aftershocks of the main event, occurring during the 

first week, is about 250 km long (Person and others, 

this report). This zone of the largest aftershocks 

coincides very closely with the zone of surface faulting mapped after the earthquake (Plafker and 

others, this report). Possibly an additional 50 km 

of fault rupture could be postulated on the basis 

of small aftershocks recorded after the earthquake 

(Langer and others, this report) and from high 

damage west of Guatemala City (Espinosa and 

others, this report). For the purpose of the analysis 

that follows, we shall take the fault length as 300 

km.

The relationship between seismic moment (M0 ), 

fault length (L), width (iv), and average displacement (D) is (Aki, 1966)

(D
MAIN EVENT SOURCE PARAMETERS FROM TELESEISMIC DATA 21

OI335 cm-s

\

FIGURE 13.—Azimuthal variation of the displacement spectral 

density normalized to an epicentral distance of 90°. The 

dashed lines are theoretical nodal lines of the Love-wave 

radiation pattern for a vertical strike-slip fault, striking N.65° E.

where ^ is the rigidity of the faulted medium, here 

taken to be_3xl0lx dynes cm~2 . The average displacement, D, is inferred from geologic observation 

to be 100 cm (Plafker and others, this report). 

Together with our estimate of seismic moment, a 

fault length of 300 km and an average displacement 

of 100 cm implies

w = 29 km.

This fault width is apparently greater than those of 

earthquakes on California's San Andreas fault; the 

seismogenic fault width associated with the California earthquake of 1906, for example, is thought 

to be 10 km (Thatcher, 1975).

If we had chosen M0 equal to the mean rather 

than the median of the moments observed at individual stations, if we had taken L = 250_km rather 

than L = 300 km, and if we had taken D less than 

100 cm, the discrepancy between the fault width of 

the Guatemala earthquake and that of the 1906 California earthquake would be even greater. There are 

alternative explanations for this discrepancy:

1. Displacement on the fault at depth may be 

larger than surface-fault displacement, so that 

100 cm would be significantly smaller than 

the actual D. The difficulty with this explanation is that the displacements observed at the

surface are quite uniform over long distances 

(Plafker and others, this report) ; it is hard 

to visualize a process acting uniformly over 

100 or more kilometres that would retard surface-fault slippage relative to slippage at 

depth. In fact, one might make the contrary 

argument—that seismic-fault displacement on 

a long strike-slip fault will have a tendency 

to decrease with depth from the free surface.

2. Seismic rupture on the Motagua fault may actually have extended to several tens of kilometres in depth. Such rupture would have to 

produce a large amount of long-period energy 

in order to significantly affect amplitudes of 

100-s G-waves. However, the fault rupture at 

depth need not necessarily have produced a 

large amount of short-period energy. Likewise, the shallow depths of aftershocks recorded by Langer, Whitcomb, and Aburto 

(this report) do not preclude fault rupture 

extending several tens of kilometres into the 

crust, since such rupture could occur comparatively slowly in a medium that is incapable 

of producing high-frequency strike-slip earthquakes.

The stress drop, A<r, for the main event may be 

estimated from

2 /D\ 

— M I — KKnopoff, 1958).

7T \ W I

(2)

With ^ 

km,

dynes cm- 2 , ZT=100 cm, and w =

r = 6.6 bars.

A stress drop of 6.6 bars is less than the worldwide average for interplate earthquakes ; Kanamori 

and Anderson (1975) find that 30 bars is typical for 

such earthquakes. If, as discussed above, w were 

less than 29 km, the stress drop would be correspondingly larger.

The epicenter of the main event lay about 90 km 

from the eastern end of the inferred 300-km-long 

zone of fault rupture, a position that suggests that 

the fault rupture propagated from northeast to 

southwest. The level of shaking near the western 

end of the fault might be expected, under such 

circumstances, to be higher than the level of shaking at the eastern end of the fault because of constructive interference of waves from the propagating source. The mantle-wave observations tend to 

support such a conclusion, the amplitudes being 

larger for waves leaving the event to the southwest
22 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

PTO 

G2

PTO 

G3

100s

FIGURE 14.—Effect of source propagation on G-wave amplitudes at station PTO. G2 and G4 left the source at an 

angle of about 15° from the direction of fault rupture. 

G3 left the source at an angle of 165° from the direction 

of fault rupture. Note that the amplitude of G4 is at 

least equal to that of G3, although G4 has traveled 213.6° 

farther than G3.

than for those leaving toward the northeast. Figure 

14 shows a striking case of this phenomenon in the 

records from Porto, Portugal. The high intensities 

observed near the western end of the Motagua fault 

(Espinosa and others, this report) may thus be, in 

part, an effect of source propagation.

FUTURE EARTHQUAKES ON THE MOTAGUA 

FAULT SYSTEM

At the time this paper was written (April 1976), 

Guatemala had experienced several moderate 

(5<M^6) aftershocks. In order to anticipate the 

future activity of the Motagua fault, we may study 

the seismic history of other major continental 

strike-slip faults. We shall consider the North Anatolian fault system, the seismicity of which one of 

us (Dewey, 1976) has recently studied. The following conclusions seem consistent also with the history 

of large earthquakes on California's San Andreas 

fault; they may therefore be valid for other major 

continental strike-slip faults like the Motagua fault;

1. Major earthquakes, involving hundreds of kilometres of fault rupture, do not tend to recur 

on the same segment of a strike-slip fault

within a short period of time. This conclusion 

is based on the three Anatolian earthquakes 

comparable in size to the Guatemala earthquake : those of December 26, 1939, November 

26, 1943, and February 1, 1944, none of which 

have yet been followed by earthquakes of 

comparable size on the same section of the 

fault. Likewise, neither of the great San Andreas earthquakes of January 9, 1857, or April 

18, 1906, has yet been followed by another 

great earthquake on the same section of the 

fault.

2. Large sections of a strike-slip fault ruptured in 

a large earthquake will not produce aftershocks of magnitude greater than 5. This conclusion is based on the characteristics of aftershocks of the Anatolian earthquakes; it is consistent with reports of aftershocks to the 

San Andreas earthquake of 1906 (Dewey, 

1976). To date, this conclusion seems to be 

valid for the Guatemala earthquake.

3. Those moderate aftershocks that do occur will 

tend to be concentrated near the ends of the 

fault rupture. This has thus far been the case 

with the Guatemala earthquake; the largest 

aftershocks (M^5.5) have occurred near 

Guatemala City, at the western end of the 

fault break, and near the eastern end of the 

fault break (Person and others, this report).

4. Regions near the ends of the fault rupture may 

continue to experience moderate earthquakes 

for some years following the main event. It 

seems apparent that occurrence of the large 

shock does not significantly reduce the level 

of tectonic strain in the regions near the ends 

of the fault rupture. In fact, on theoretical 

grounds, the occurrence of the major earthquake should produce high tectonic strain 

near the extremities of the fault rupture 

(Chinnery, 1963).

For Guatemala, conclusions 1-4 imply that most 

of the Motagua fault ruptured by the earthquake 

will be seismically inactive during the next decades. 

The regions near the ends of the main fault rupture, 

near Guatemala City and south of Puerto Barrios, 

may, however, have several moderate earthquakes 

(5<M<6) in the coming decade.

There is also the possibility that the occurrence 

of the main event could induce, in the next several 

decades, a major earthquake on a segment of the 

Motagua fault adjacent to the fault ruptured by the 

February 4 earthquake. Such a migration of seismic
MAIN EVENT SOURCE PARAMETERS FROM TELESEISMIC DATA 23

sources over a period of several decades seems to 

have occurred on the North Anatolian fault and 

has also been postulated for the San Andreas fault 

(Savage, 1971).

There seems to be ample fault length to generate 

a major earthquake east of the rupture of February 

4 where the Motagua fault trends into the tectonically similar Swan fracture zone in the Gulf of Honduras (Spence and Person, this report). In the west, 

the likelihood of a future major earthquake, similar 

to this earthquake but centered to the west of it, may 

depend on whether the Motagua fault persists for 

hundreds of kilometres as a continuous fault west 

of the fault rupture of February 4, 1976.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

STRONG-MOTION RECORDINGS OF THE MAIN EVENT 

AND FEBRUARY 18 AFTERSHOCK

By CHARLES F. KNUDSON

INTRODUCTION

The seismic history of Guatemala includes many 

destructive earthquakes (Person and others, this 

report). In 1773, the former capital, Antigua, was 

destroyed. A series of earthquakes beginning on 

November 17, 1917, and continuing with shocks on 

December 25 and 29 and also on January 3 and 24, 

1918, destroyed Guatemala City. In the last 50 

years, Guatemala has felt many strong earthquakes, 

but, until the series in February, none were of sufficient magnitude to cause the destruction experienced in the past.

Two strong-motion accelerographs and three seismoscopes were located in Guatemala City at the 

time of the February earthquakes. One accelerograph, an RFT-250, had not been reinstalled following repairs. The second accelerograph, a Montana 

type, was operational, but the lamp burned out at 

the time of the earthquake. The only records obtained were from the two seismoscopes, which were 

installed at the Universidad de San Carlos.

SEISMIC INSTRUMENTATION

The first useful seismograms from Guatemala 

were obtained in March 1919 from the Wiechert 

instruments donated to Guatemala by Georgetown 

University. These instruments, two horizontal and 

one vertical, have registered 20 to 39 shocks annually since initial installation. The seismic station 

of the Observatorio Nacional is a three-story 9.9 

x 13.7-m building. There is one story below ground 

level and two above. The portion of the building 

below ground level and the first story above ground 

level are of concrete frame with brick-filler walls. 

The third story is of wood frame construction. The 

first and second stories of the building serve to 

house a second building, or vault, that is built of 

concrete framing with brick-filler walls and is the 

seismograph and accelerograph vault. In May 1947,

the 30-cm Montana-type accelerograph was installed. Numerous records have been obtained since 

installation of this accelerograph, but none have 

been of any engineering significance.

The Centre de Investigaciones de Ingenieria of 

the Universidad de San Carlos purchased one RFT250 accelerograph and three seismoscopes in the 

late 1960's. Two of the seismoscopes were installed 

in the administration building of the university, 

one on the ground floor and the second on the roof. 

The other seismoscope was installed in the Engineering Laboratory Building but was not deployed at 

the time of the February earthquakes. It was removed early in 1975 and taken to the Observatorio 

Nacional for repair. Although repaired and operational in July of 1975, it had not been reinstalled 

when the February earthquakes occurred.

Four SMA-1 accelerographs were sent to Guatemala by the U.S. Geological Survey after the main 

event. In view of the distribution of the epicenter 

locations for the February 6, 8, 9, and 10 earthquakes, along with a fault length of more than 240 

km (Plafker and others, this report) on the Motagua fault and secondary faulting in the Mixco area, 

it was decided to deploy the SMA-1 accelerographs 

at Puerto Santo Tomas, Zacapa, and Chichicastenango. An additional accelerograph and a seismoscope were later deployed in the center of Guatemala City at the IBM building (fig. 15). These four 

accelerographs were installed as temporary aftershock strong-motion instruments, and they will be 

removed in 2 or 3 months.

SEISMOSCOPE RECORDS OF MAIN EVENT

The two seismoscope records were recovered on 

February 10, 1976. The administration building at 

the university is a four-story building with dimensions of 29.76x59.46 m. The seismoscope plate (fig. 

16) from the instrument on the roof of the building

24
STRONG-MOTION RECORDINGS OF MAIN EVENT AND FEBRUARY 18 AFTERSHOCK 25

9I°00' W 90°00' 89"00' 88°00'

6°00

— I5°00'

GULF OF 

'.HONDURAS

HONDURAS

® Chichicastenongo

6TITLAN 

\ LAKE -

Guotemola 

City

ft

FEB 6, 1976 

5.75 (MAG) 

14.6 90.4

Mam Event Epicenter 

Aftershock Epicenters

O Strong -Motion Seismograph 

Fault

EL SALVADOR I4°00'

9I°00 90°00' 89°00 88°00' W

FIGURE 15.—Strong-motion field network, surface fault breakage (Plafker and others, this report), and epicenters of main 

event and aftershocks. Date, magnitude, and location are given for each (Person and others, this report).

had been dislodged. The ground motion on this record exceeded the maximum radius of the plate before it was dislodged. The trace on the seismoscope 

plate of the ground floor (fig. 17) had a maximum 

relative displacement of £d = 5.3 cm for Ti^O.78 s, 

and a 10-percent damping.

Two sections of the maximum excursions of the 

recorded motion on the ground-floor seismoscope 

were analyzed in order to recover the levels of 

ground accelerations. Part of the trace between the 

two analyzed sections could not be followed, but it 

is certain that section one was first real time. The 

following constants were used in the analysis of 

the deconvolution of the seismoscope plate: 7\ = 0.78 

s (natural period of seismoscope), Tz = 0.055 s (second harmonic of seismoscope), and S = 5.8 cm/rad,

(sensitivity of seismoscope). The T2 and S values 

are average determinations obtained from similar 

seismoscopes, and the other constants were obtained 

in the field calibration of these instruments.

The results of the above analysis are shown in 

figures 18 and 19. These two figures show the acceleration as a function of time for the north and 

east direction of motion. The first section was followed for 2 s, and the second section for 5 s. Maximum accelerations as shown are about 200 gals. A 

600-gal acceleration appeared on the north directional component at approximately 1 s after the beginning of the second section. Unfortunately, there 

is insufficient recoverable trace length, and hence 

the data available do not warrant a spectral-analysis evaluation, since the time window is very short.
26 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 16.—Seismoscope plate of main event located on a rooftop instrument at the administration building of the Universidad de San Carlos, Guatemala City. Instrument was about 30 km south of Motagua fault surface breakage. Arrow 

indicates north. The plate is scratched all over. Recording of main event is shown in the middle part of plate and to 

the sides before dislodging.
STRONG-MOTION RECORDINGS OF MAIN EVENT AND FEBRUARY 18 AFTERSHOCK 27

FIGURE 17.—Seismoscope plate of main event located on a ground-floor instrument at the administration building of 

the Universidad de San Carlos, Guatemala City. Instrument was about 30 km south of Motagua fault surface breakage. Arrow indicates north. The plate is scratched all over. Recording of main event is shown in middle part of plate.
28

400

GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

A B A

600

400

-400

2 <- —m

FIGURE 18.—Deconvolved 2-s section of seismoscope of the 

main event. Acceleration as a function of time. A, North 

component; B, east component.

FIGURE 19.—Deconvolved 5-s section of seismoscope of the* 

main event. Acceleration as a function of time. These 5 

s of recordings are later than those shown in figure 18. 

A, North component; B, east component.

FIGURE 20.—Accelerogram of the February 18, 1976, aftershock recorded in Guatemala City by SMA-1, No. 1926, 

at station IBM building, lat 14.64° N., long 90.51° W. 

(Sens.=sensitivity; Per.=period; crit.=critical.)

SOUTH

Sens. =1.73 cm/g 

Per. .038 s 

Damp = 0.60 crit

DOWN

Sens. = 1.83 cm/g 

Per. = .038 s 

Damp. = 0.56 crit

EAST

Sens. = 1.76 cm/g 

Per. = .038 s 

Damp = 0.60 crit

rS^^j

5 CM
STRONG-MOTION RECORDINGS OF MAIN EVENT AND FEBRUARY 18 AFTERSHOCK 29

STRONG-MOTION ACCELEROGRAM OF FEBRUARY 

18 AFTERSHOCK

Several aftershocks have been recorded on the accelerographs located at Zacapa, the Observatorio 

Nacional, and at the IBM building stations. These 

aftershocks have a Richter magnitude less than 4.0 

and are not considered significant. An aftershock 

was recorded at the IBM building on February 18, 

1976, at 03:59 local time. The accelerogram in figure 

20 shows a vertical motion of approximately 0.1 g 

during the first second.

SAN SALVADOR ACCELEROGRAPH OF MAIN 

EVENT

The Observatorio de San Salvador reported that 

an AR-240 located in San Salvador at the Biblioteca 

station was triggered by the Guatemala earthquake. 

The readings obtained from the accelerograms were 

0.066 g on the north component; 0.25 g on the vertical and 0.053 g on the east component (M. Martinez, oral commun., 1976). The Modified Mercalli 

intensity rating in San Salvador was V (M. Martinez, oral commun., 1976),
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

AFTERSHOCKS FROM LOCAL DATA

By CHARLEY J. LANGER, JEAN P. WHITCOMB, and ARTURO ABURTO Q.

INTRODUCTION

Aftershocks of the main event were monitored in 

two phases by single-component portable seismographs from February 9 to February 27. This study 

represents a combined effort by the U.S. Geological 

Survey and the Nicaraguan Institute de Investigaciones Sismicas. Rapid deployment of portable instrumentation around the Motagua fault zone provides a data base for the first detailed aftershock 

investigation of a major earthquake (magnitude 

greater than 7.5) in Central America. Tectonic and 

seismic aspects of the main event and large aftershocks are discussed in other sections of the report 

(Spence and others, Person and others). The topic 

addressed here is hypocentral locations of a representative sample of locally recorded aftershocks and 

their relationship to primary and secondary faulting.

INSTRUMENTATION AND FIELD PROCEDURE

Aftershocks were recorded by portable, smokedpaper seismographs, each consisting of a vertical 

transducer, a high-gain amplifier, and a crystalcontrolled clock. The seismograph recorded at a 

speed of 60 mm/min, and the trace separation was 

1 mm, which allowed 48 hours of continuous operation. Precise time corrections were determined with 

an oscilloscope by comparing WWV radio time with 

recorder clock times during record changes. Clock 

drift did not exceed 20 ms/day. Seismograph magnifications generally ranged between 50,000 and 100,- 

000 at 10 Hz. Amplifier gains were limited by the 

background noise at the sites, most of which were 

on unconsolidated soils and close to cultural noise 

sources. Because of the intense aftershock activity 

at many of the station locations, the peak-to-peak 

deflection of the recorder pen was limited to 10 mm.

A two-phase aftershock recording program was 

required because of the great length of fault rupture 

(more than 240 km), constraints imposed by the 

available logistical support, and the limited amount 

of seismograph equipment available. The phase I,

or western, network (table 5) was installed on 

February 9 and 10 and extended approximately 95 

km east-west between Sanarate and Chichicastenango. Another portable seismograph was installed 

in Guatemala City after the main event by personnel at the Observatorio Nacional. This network surrounded the western end of the Motagua fault zone 

and also encompassed many of the northeast-trending secondary faults in the vicinity of Guatemala 

City, Chimaltenango, and Tecpan. The phase I operation was terminated on February 17 when all seismographs, except those in Guatemala City, were removed.

During phase II, a much broader seismograph 

network was installed to the east between Guatemala City and Puerto Barrios (table 5). It covered 

about 225 km of the central and eastern segments 

of the Motagua fault and adjacent regions. On 

February 18, 19, and 20, seismographs were located 

at eight sites (table 5). The Puerto Barrios station 

was relocated at a site near La Pina on February 

21 because of the high cultural background noise 

at Puerto Barrios. Phase II was completed on February 27 when all the instruments were retrieved.

DATA AND ANALYSIS

Several thousand aftershocks were recorded during the field investigation (fig. 21). The amount of 

seismic activity was greatest at the western end, 

near Tecpan and Chimaltenango, and did not noticeably diminish during the 8-day monitoring period 

of the western network. The unusually high level of 

observed seismicity in this area is not merely a function of station location or of time, that is, early in 

the aftershock sequence; the Tecpan-Chimaltenango 

region is unique to the total aftershock zone in terms 

of level of seismicity.

Arrival times were determined by using a lowpower magnifier and were corrected for variations 

in distance between minute marks. S-phases were 

easily identifiable in many cases, often at two or

30
AFTERSHOCKS FROM LOCAL DATA 31

TABLE 5.—List of seismograph stations occupied during this study

Name

Joyabaj

Chimaltenango

El Chol

Guatemala City

Guatemala City

Jalapa

Teleman

Chiquimula

Quirigua _ _ _ .

Vitalis _ _ .

La Pina

Puerto Barrios

Symbol

cco

TEC

JOY

CHM

ELC

GCG

PAT,

SAN

GCG

SJE

JAP

TEL

CML

ARC

RIO

VIT

FFF

PTO

Latitude 

<°N.)

Western

14.950

14.766

14 QQft

14.635

14.958

14.586

14.664

14.784

Eastern

14.586

15.065

14.638

15.339

14.801

15.273

15.656

15.312

15.600

15.712

Longitude 

<°W.)

network

91.110

on QQfi

on 804

90.818

90.487

90.533

90.361

QO 1Qfi

network

90.533

on 947

90.003

8Q 744

89.533

89.039

88.994

88.806

88.608

88.583

Elevation 

(metres)

1,990

2,320

1,400

1,760

995

1,497

1,310

770

1,497

1,005

1,370

65

360

70

10

120

40

40

Period of 

operation

Feb. 9-Feb. 17

Feb. 9 Feb. 17

Feb. 9-Feb. 17

Feb. 9-Feb. 17

Feb. 9-Feb. 17

Feb. 9-present

Feb. 10-Feb. 17

Feb. 10-Feb. 17

Feb. 6-present

Feb. 18-Feb. 27

Feb. 18 Feb. 27

Feb. 19-Feb. 27

Feb. 18-Feb. 27

Feb. 18 Feb. 27

Feb. 20-Feb. 27

Feb. 18-Feb. 27

Feb. 23-Feb. 27

Feb. 20-Feb. 22

more stations for the same earthquake. Accuracy 

of most P-wave times is thought to be within ±0.1 

s; the selected S-wave readings are believed accurate to ±0.20 s.

Seventy-eight hypocenters (table 6), most of 

which lie inside or very near to the margins of the 

temporary seismic networks, were determined by 

the HYP071 computer program (Lee and Lahr, 

1975). A measure of their solution quality is denoted by the symbol SQ and ranges between B 

(good) and D (poor). This SQ rating is dependent 

upon the number and accuracy of data, station distribution, and crustal velocities. All D-quality solutions are a few kilometres outside the network; 

otherwise they would be rated as B or C.

The average root-mean-square (RMS) errors of 

the travel-time residuals are 0.17 s, which implies 

that the random errors in reading the P- and S- arrivals account for most of the RMS errors. An average of the standard errors indicates hypocentral accuracies of about ±1.3 km in the horizontal plane 

and approximately ±2 km in the vertical plane. 

Although the standard errors may not represent 

actual error limits, particularly for hypocenters 

outside the seismograph net, S-phase data mitigate 

the possibility of gross mislocations. Any systematic 

location error or bias is most likely caused by the 

six-layer Managua velocity model of Brown, Ward, 

and Plafker (1973) used in the HYP071 program. 

This model was employed in this study because of 

the absence of velocity data for interior Guatemala. 

Although the model is an assumed velocity structure 

for the Managua area, it is representative of volcanic terrane and therefore may be generally applicable to the Motagua fault zone west of long 90.5° 

W. To the east, where crystalline and marine sedimentary rocks are predominant (Bonis and others, 

1970), increased velocities would be expected in the 

upper layers. The Managua model, however, is considered adequate for obtaining preliminary locations.

Because the peak-to-peak signal amplitudes were 

electronically clipped, local magnitudes, ML, are estimated from the aftershock coda lengths (Lee and 

others, 1972). The lower magnitude threshold for 

hypocentral determinations using either the western 

or eastern network data is about 2.2. None of the 

larger aftershocks reported by Person, Spence, and 

Dewey (this report) occurred within a temporary 

seismograph net. The largest located event (magnitude 3.8) is approximately one order of magnitude 

below the limit for teleseismically locatable earthquakes in Central America.

RESULTS AND DISCUSSION

Aftershock epicenters are distributed along the 

Motagua Valley from the lowlands near the Gulf of 

Honduras westward to the Guatemalan highlands 

northeast of Lake Atitlan, a distance of some 300 

km. A large number of located events occurred on 

secondary faults south of the Motagua fault and 

west of long 90.3° W. (fig. 22). Focal depths range 

from near surface to about 14 km. In particular, 

we note the following aspects:

1. The eastern terminus of the causal fault rupture 

is most likely defined by the cluster of 12 epicenters southeast of Puerto Barrios. The gen-
32 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

N N

X X

LO O

b •'-

cO

£_

(D

l

jQ 

T3

CD 

CD

L_ 

<D

Q.

E

cO

E 

o

U

C\J 

CM

c 

O

CD

E

CD I\ 

O)

O

(D

u_

U

U

& t-l

g,

O § C'i

•E 1

I?C o 

O o ^

si

02 t/2

iH 

^1^

si

M «H

88

5 H 

H SS*\

SI •a*

.£ e

ej "8

«H^ 0

bo 5 g c-i W _g

" *rH "^

o as

^

I. ra OS

-
AFTERSHOCKS FROM LOCAL DATA 33

o - 

_J I

0

il

_Q "D 

CD 

CD 

L

0

"5. 

E

LO

E

o

u

ID

c 

O

0

E

CD IsG)

CM

JD 

CD

•43
34 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

TABLE 6.—Aftershocks of the main event located by temporary seismograph network

[ J No. sta.--refers to the number of stations used to obtain hypocentral solutions; v 2 DMIN--distance 

to the closest seismograph station; 3 RMS--root mean square errors of travel time residuals; 

^Standard errors refers to the indices of precision relating to the values and distribution of 

the unknown errors in the hypocentral solution where DLAT = error in latitude, DLON = error in 

longitude, and DZ = error in depth; 5SQ--a measure that is intended to indicate the general 

reliability of the hypocentral solution where A = excellent epicenter, good focal depth; B = 

good epicenter, fair focal depth; C = fair epicenter, poor focal depth; D = poor epicenter, 

poor focal depth; 6M --local magnitude of shock.]

Western network

Date Origin 

(Feb. 1976) (UTC)

11

11

11

11

11

11

11

11

11

12

12

12

12

12

12

12

12

12

12

12

12

12

12

12

13

13

13

13

0636

0932

0953

1044

1051

1142

1636

2210

2253

0039

0144

0215

0333

0408

0439

0545

0702

0743

0744

1057

1203

1927

2211

2250

0627

0701

1344

2359

48.69

20.37

02.55

34.78

03.91

03.72

23.01

58.78

51.55

13.82

35.11

04.82

36.40

15.10

54.52

45.58

02.43

42.34

35.68

35.51

33.49

36.00

59.01

34.00

42.32

32.46

01.31

50.50

Lat. N 

(deg)

14.750

14.808

14.760

14.763

14.767

14.724

14.790

14.637

14.809

14.759

14.861

14.694

14.855

14.745

14.636

14.827

14.859

14.800

14.852

14.714

14.589

14.590

14.674

14.760

14.673

14.684

14.755

14.767

Long. W 

(deg)

91.126

90.510

90.980

91.024

90.975

90.998

90.984

90.680

90.596

90.501

90.343

90.468

90.710

90.499

90.668

90.513

90.340

90.544

90.619

90.796

90.625

91.037

90.482

90.355

90.483

90.477

90.987

91.025

Depth 

(km)

9.2

10.0

7.0

10.3

4.2

4.0

4.9

4.0

2.0

2.0

12.2

6.0

12.0

13.0

12.1

13.0

10.5

12.3

12.2

11.7

13.2

10.0

12.0

5.4

10.0

11.9

5.7

1.1

No. 1 

sta.

8

7

7

7

7

8

7

10

7

6

7

8

9

8

7

8

8

7

9

8

7

8

7

7

7

8

8

8

DM IN2 

(km)

14

17

2

3

2

5

3

6

20

18

18

12

18

17

16

15

14

18

18

9

21

9

13

11

13

13

1

3

RMS 3 

(sec)

0.18

0.14

0.11

0.09

0.19

0.22

0.04

0.16

0.24

0.25

0.20

0.11

0.21

0.22

0.11

0.19

0.11

0.19

0.25

0.10

0.09

0.13

0.20

0.28

0.16

0.18

0.16

0.20

Standard errors'* 

DLAT DLON DZ 

(km) (km) (km)

1.1

0.9

0.6

0.6

1.5

1.3

0.3

1.7

0.4

0.6

1.9

0.7

0.9

0.9

1.1

0.9

0.8

1.1

1.0

0.7

1.0

0.8

0.9

2.6

0.7

0.9

1.0

1.5

1.3

0.6

0.8

0.7

1.5

1.3

0.3

0.4

0.3

0.5

1.7

0.4

0.7

0.8

0.4

0.6

0.5

1.0

0.7

0.4

0.4

1.6

1.0

1.4

0.8

0.7

2.3

1.2

1.5

2.1

1.2

0.5

1.5

1.4

0.4

1.8

1.2

1.9

2.4

2.2

1.6

2.4

1.4

1.9

1.3

2.7

4.3

1.2

1.0

1.2

2.4

4.8

5.3

2.2

1.7

1.5

SQ5

C

B

B

C

B

C

B

C

C

C

C

C

B

B

C

B

B

B

B

B

C

C

C

C

C

C

C

C

ML 6

3.2

3.1

2.9

2.8

2.9

2.9

3.0

3.3

2.4

2.7

2.6

3.3

2.7

2.5

3.0

2.8

3.3

-

3.0

3.2

3.2

3.4

2.4

2.9

3.1

3.2

3.2

3.3
TABLE 6.—Aftershocks of the main event located by temporary seismograph network—Continued 35

Western network

Date Origin 

(Feb. 1976) (UTC)

14

14

14

14

14

14

14

14

14

14

14

14

14

15

15

15

15

15

15

16

16

17

17

17

20

21

21

21

22

22

22

22

23

24

24

0300

0315

0424

0916

1543

1757

1842

1912

2036

2044

2122

2219

2318

0034

0436

0650

1053

1308

2019

0758

0911

0345

0527

1549

0321

0205

0752

1503

0500

0642

1209

2138

0503

0417

0757

40978

59.79

53.89

58.15

57.80

55.16

41.94

55 .22

28.16

04.68

55.03

24.40

26.40

45.75

12.15

51.18

24.11

31.57

59.95

08.62

46.82

47.51

05.94

25.54

50.53

56.04

07.81

52.99

33.55

40.71

58.28

32.95

36.68

00.95

18.11

Lat. N 

(deg)

14.858

14.696

14.831

14.711

14.699

14.700

14.754

14.645

14.815

14.743

14.740

14.746

14.741

14.776

14.808

14.728

14.720

14.782

14.792

14.848

14.750

14.708

14.723

14.791

15.152

15.052

14.991

14.971

15.671

15.526

15.275

15.217

15.314

15.670

15.556

Long. W 

(deg)

90.636

90.545

90.319

90.737

90.481

90.514

90.312

90.950

90.583

90.377

91.007

90.355

90.323

90.965

90.551

90.359

90.748

90.980

90.982

90.678

90.998

91.008

90.801

90.974

89.228

89.452

89.627

89.676

88.445

88.520

89.007

89.003

88.906

88.457

88.519

Depth 

(km)

11.7

10.0

9.0

10.7

12.0

12.4

10.0

4.0

10.6

6.0

11.4

8.0

5.0

6.2

2.5

2.5

10.0

6.4

3.8

12.2

10.0

7.8

11.8

2.4

1.5

5.9

10.4

11.2

10.0

8.5

10.0

14.0

8.9

10.0

5.1

No. T 

sta.

9

8

7

9

8

8

7

8

8

8

8

7

6

9

10

8

8

9

6

11

7

6

10

6

8

7

8

6

5

6

6

7

6

5

6

DM IN 2 

(km)

20

20

14

12

13

17

11

14

19

9

5

9

9

4

18

7

12

2

3

21

2

6

10

4

24

29

23

24

16

22

3

7

11

20

11

RMS 3 

(sec)

0.10

0.20

0.25

0.17

0.19

0.32

0.29

0.14

0.14

0.24

0.10

0.24

0.21

0.15

0.27

0.24

0.25

0. 10

0.25

0.16

0.23

0.04

0.20

0.09

0.16

0.18

0.20

0.08

0.16

0.07

0.16

0.16

0.16

0.21

0.10

Standard errors 1* 

DLAT DLON DZ 

(km) (km) (km)

0.4

0.9

1.2

1.0

0.6

1.6

0.6

1.2

0.7

0.7

1.1

0.5

1.4

0.6

0.6

0.8

0.5

0.6

2.4

0.7

1.8

0.5

0.8

1.2

0.9

1.0

0.9

0.5

0.8

0.7

2.8

1.5

1.3

0.6

2 _ 2

0.3

0.8

1.5

0.7

0.5

1.6

0.7

0.8

0.6

0.8

1.0

0.6

0.5

0.6

0.5

0.9

0.5

0.5

2.3

0.4

1.9

0.5

0.6

0.8

0.7

1.0

0.7

0.5

2.3

1.5

1.0

0.8

0.9

1.4

2.1

1.2

2.8

3.7

2.1

1.5

3.7

2.8

0.8

1.9

3.5

0.5

1.6

2.0

1.6

1.1

2.7

3.3

0.9

4.0

1.1

1.2

0.5

2 . 2

1.4

1.1

4.0

2.0

1.2

1.6

5.5

1.8

0.9

1.6

1.1

2.8

SQ 5

B

C

C

B

C

C

B

C

B

B

C

B

B

B

C

B

B

B

C

B

n

c

B

B

C

C

c

B

C

D

D

C

C

C

D

V

3.1

3.2

2.4

3.1

2.7

3.2

2.2

5.8

3.5

3.1

3.2

2.8

2.8

3.4

3.4

2.4

3.2

3.4

3.2

2.9

3.1

2.8

2.9

2.9

2.4

3.1

3.2

2.9

2.6

2.9

3.7

3.8

3.2

2.7

2 . 2
36 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT 

TABLE 6.—Aftershocks of the main event located by temporary seismograph network—Continued

Eastern network

Date Origin 

(Feb. 1976) (UTC)

24

24

24

24

25

26

26

26

26

26

27

27

27

27

0807

0821

1316

1337

0128

0033

0510

1120

1903

2216

0120

0344

0458

1200

06.51

49.24

05.14

59.96

48.30

22.94

13.27

06.00

20.29

11.70

58.22

29.49

00.86

38.49

Lat. N 

(deg)

14.983

15.660

15.485

15.496

14.977

14.964

15.617

14.841

15.561

14.972

15.580

14.972

15.537

15.602

Long. W 

(deg)

89.635

88.438

88.601

88.599

89.674

89.690

88.437

89.641

88.515

89.612

88.451

89.662

88.565

88.621

Depth No. 1 

(km) sta.

8.5

1.6

10.0

12.4

5.9

7.6

7.0

8.6

8.0

8.0

9.0

10.0

2.8

5.7

8

6

5

6

7

8

5

7

6

8

5

8

5

7

DMIN2 

(km)

23

19

13

12

25

25

18

13

11

21

17

23

8

1

RMS 3 

(sec)

0.11

0.13

0.20

0.27

0.13

0.05

0.19

0.14

0.21

0.17

0.05

0.28

0.14

0.20

Standard errors'* 

DLAT DLON DZ 

(km) (km) (km)

0.3

0.6

0.6

2.6

0.3

0.2

1.0

1.1

0.5

0.5

0.9

0.5

0.4

2.0

0.4

0.5

0.9

2.5

0.4

0.2

0.8

0.6

1.1

0.7

1.0

0.7

3.3

4.0

1.6

0.5

2.6

4.2

1.2

1.0

1.1

2.0

2.1

2.0

1.0

1.8

2.8

2.8

SQ 5

B

C

C

D

B

B

C

B

C

C

C

C

D

C

V

2.7

3.2

2.5

3.0

3.3

2.7

2.4

2.9

2.3

2.8

3.1

2.7

2.5

2.4

eral trend of the southern group of eight 

aftershocks is in line with the inferred extension of the Motagua fault (Plafker and others, 

this report), whereas the four epicenters 

slightly to the north may be associated with 

induced movement at the eastern end of the 

San Agustin fault.

2. Epicenters associated with the western end of 

the Motagua fault do not extend beyond the 

mapped fault breakage. Consequently, with 

the data at hand, the aftershock pattern does 

not suggest a more precise limit to the primary fault rupture than the obvious diminution of seismicity west of long 90.45° W. Also, 

there are no located aftershocks that appear 

to be related to induced movement on the 

western segment of the San Agustin fault.

3. The distribution of energy release along the 

Motagua fault proper is roughly uniform, with 

exception of the concentration of activity west 

of Zacapa. The group of seven epicenters between long 89.6° W. and 89.7° W. may be a 

result of fracturing east of where the Motagua 

fault bends from a general east-west direction to a northeasterly direction. Three northeast-trending secondary faults (not shown in

fig. 22), which cut Paleozoic metamorphic 

rocks, are mapped in this area (Bonis and 

others, 1970).

4. The majority of aftershocks located west of 

long 90.3° W. are directly associated with secondary faulting. Four groups are considered 

to be of principal interest: 

a. Tecpan (long 91° W., lat 14.75° N.). The 

high level of activity observed at the 

Tecpan seismic station (fig. 21) is reflected by the dense cluster of epicenters 

located in this area. Plafker, Bonilla, 

and Bonis (this report) have defined a 

lineament that projects through Tecpan 

and the center of the northeasterly 

trending concentration of aftershocks. 

Therefore, on the basis of the epicentral 

locations, the lineament can be interpreted as a northeast-striking fault, 

b. Chimaltenango. Four epicenters occurring 

in the vicinity of a northeast-striking 

lineament that runs through Chimaltenango lend support to the existence of a 

secondary fault.

c. Guatemala City region. These aftershocks 

are very likely associated with faults
I6°00

9I'00'

AFTERSHOCKS FROM LOCAL DATA

90°00' 89'00'

37

I5'00'

I4'00'

GUAEMAL

EXPLANATION

*

event epicenter

o 

Aftershock epicenters (teleseismically located)

Aftershock epicenters(regionally located)

A 

Portable seismograph

Main event causal fault.

Arrows show sense of horizontal slip;

barbs on downthrown side;

dotted where inferred

Fault with known or suspected 

late Cenozoic displacement

Lineament possibly indicative of fault

FIGURE 22.—Aftershock epicenters and portable seismograph locations (geology from Plafker and others, this report). See 

table 5 for station names and their geographical locations and table 6 for aftershock-location parameters. Station code 

is in the Glossary. (Base map modified from Guatemala, Institute Geografico Nacional, 1974, 1:500,000.)

forming the Guatemala City graben. 

The Mixco fault, west of the city, ruptured the ground surface. Some epicenters appear to correlate with the 

Mixco fault and also with the northerly 

extension of the mapped fault bounding 

Guatemala City on the southeast.

d.

5.

Agua Caliente (long 90.35° W., lat 14.75° 

N). A group of epicenters 15 km north 

of Palencia (station PAL) surround the 

Agua Caliente Bridge site. Secondary 

faulting, although not mapped at this 

locale, is certainly indicated by the aftershock cluster and may have contributed, 

in part, to the collapse of the bridge. 

The preponderance of aftershocks lying off the 

Motagua fault west of long 90.3° W. suggests

that induced motion along secondary faults is 

rare east of long 90.3° W.

There is an apparent southerly bias of epicentral locations along the Motagua fault proper. The spatial distribution of aftershocks 

thought to be associated with the primary 

fault indicates a systematic offset of 2 to 3 

km. This offset would suggest that (1) the 

Motagua fault is dipping steeply to the south 

in accordance with the main-event focal mechanism of Dewey and Julian (this report) or 

(2) there is a large contrast in seismic velocities across the fault similar to that observed 

by Eaton, 0' Neill, and Murdock (1970) on the 

San Andreas rift zone near Parkfield, California.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

GEOLOGIC EFFECTS

By GEORGE PLAFKER, MANUEL G. BONILLA, and SAMUEL B. BONIS *

INTRODUCTION

This report is based on preliminary field studies 

of the geologic effects of the earthquake made during an 11-day period from February 5 to 16, 1976. 

During this period, we examined the main and 

secondary faults on the ground, using vehicles and 

helicopters for logistic support. In addition, several 

reconnaissance flights were made with fixed-wing 

aircraft over most of the area that was strongly 

affected by the earthquake to identify and map 

landslides, liquefaction phenomena, and damage to 

communities near the surface faults. Some of our 

interpretations may be modified by the more detailed field investigations that were being conducted 

by the U.S. Geological Survey and other organizations at the time this paper was written (MarchApril).

THE MAIN FAULT

The main fault along which the destructive main 

event (Ms = 7.5) occurred was identified for 240 km 

in the Motagua Valley and the mountainous area 

west of the valley 2 (fig. 23). This fault is of special 

interest because it is the most extensive surface 

rupture in the Northern Hemisphere since the 1906 

San Francisco earthquake. Identification of this 

fault permits evaluation of damage relative to the 

earthquake source and provides critical new information on the present mode of deformation to a major tectonic belt of Central America. The eastern 

part of this major fault, within the Motagua Valley, 

has been named the Motagua fault (Dengo and 

Bohnenberger, 1969; Institute Geografico Nacional, 

Chiquimula 1:250,000 sheet, 1969), and this name 

is herein applied to all the fault that slipped during 

the earthquake.

Ground breakage was observed in a discontinuous 

line extending 240 km from near Quebradas in the 

lower Motagua Valley on the east to about 10 km

east of Patzaj on the west. 2 At the closest point, the 

fault is 25 km north of the center of Guatemala 

City. The fault could not be identified farther to 

the west because the area is characterized by 

young volcanic deposits and rugged terrane in 

which numerous earthquake-triggered slope failures 

effectively mask the fault-related surface fractures. 

At the eastern end, the fault trace is obscured in 

the lower Motagua Valley by swamps and dense 

tropical vegetation. However, the occurrence of 

aftershocks southeast of Puerto Barrios (Langer 

and others, this report) near the eastern coast suggests that the faulting probably extends at least 

that far. If so, the main break is on the order of 300 

km long2 . Some of the characteristics of the faulting at localities where it was studied on the ground 

are given in table 7.

The fault trace is a well-defined linear zone with 

a gradual change in average strike from N. 65° E. 

at the eastern end to N. 80° W. at the western end. 

It consists of right-stepping en echelon fractures 

and connecting low compressional ridges that locally form the "mole tracks" that are characteristic 

of strike-slip faults (figs. 24-28). Individual fractures within the zone may be as much as 10 m long; 

most are tightly closed, but some have spread as 

much as 10 cm. The fractures are oriented at angles 

of as much as 35° to the fault trace and have the 

northeasterly azimuths that are to be expected for 

sinistral slip. The width of the fracture zone is 

mostly 1 to 3 m, with a maximum observed width 

of about 9 m. At one locality near El Progreso, 

where the fault surface is exposed in a highway cut, 

the zone of slip is 1 to 3 m wide, and the dip is essentially vertical.

Displacement across the fault in most places is almost entirely horizontal and sinistral. The strike1 Institute Geografico Nacional de Guatemala.

2 Later, more detailed studies indicate that the length of surface faulting is 230 km and that the main break from relocated epicenter data 

is 270 km.

38
GEOLOGIC EFFECTS

GUATEMALA

7 GRABENt 

HONDJJBAS V

EXPLANATION

(ft 

Main event epicenter

-~- T^5) © y~-———— p_/ i,_ ff'OM_0'

' • / •MIXCO . 

i FAULT ,

Aftershock epicenters

1976 earthquake faults

Arrows show sense of horizontal slip;

barbs on downthrown side;

..- ,PALA dotted where inferred * -*6R|BEN|.

Fault with known or suspected 

late Cenozoic displacement

Lineament possibly indicative of faull

EL SALVADOR

Major stratovolcano

FIGURE 23.—Relationship of the Motagua and Mixco faults to the main-event epicenter, the epicenters of large aftershocks, 

and major structural and volcanic features in northern Central America. Numerals along the Motagua fault refer to localities listed in table 7. Epicenters are from data of the NEIS and Person, Spence, and Dewey (this report); faults 

and volcanoes are modified from Dengo (1968) and Bonis, Bohnenberger, and Dengo (1970).

slip component of displacement appears to increase 

irregularly from about 73 cm near near Quebradas 

at the eastern end of the exposed trace to a measured maximum on a single trace of 142 cm in the 

area due north of Guatemala City. 3 At the extreme 

western end of the observed trace, a single measurement suggests that displacement there may decrease to 68 cm. However, since this locality is in 

an area of large-scale gravity sliding, the reliability 

of the measurement is uncertain. The vertical offsets that we observed along the fault are generally 

minor (less than 30 percent of the horizontal component) and down to either the north or the south. 

An exception is the 10-km-long segment near Quebradas at the eastern end of the observed surface

3 Later studies indicate that maximum sinistral displacement is as much 

as 325 cm in the area between El Progreso and Chuarrancho.

trace where the vertical displacement is consistently 

down to the north and locally as much as 50 percent 

of the sinistral component.

Subsidiary faults and splays appear to be relatively scarce along the Motagua fault; the only two 

occurrences noted in our preliminary reconnaissance are near El Progreso and Chuarrancho. Just1 

northeast of El Progreso, a subsidiary fault about 1 

km long with 20-cm sinistral displacement is oriented roughly parallel to, and 400 m south of, the 

main fault tracer Near Chuarrancho (north of Guatemala City), a| prominent surface break splays off 

the main trace in a northeasterly direction at an 

angle of about 15°. This splay has a sinistral offset 

of 28 cm and was estimated from the air to be about 

250 m long.
40 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

TABLE 7. Characteristics of earthquake fractures along the Motagua fault

[Measured aggregate displacement: ^, estimate; (?), measured displacement probably not true value; >, measured displacement probably 

minimum value; S, sinistral or left-lateral; V, vertical. Leaders 

indicate no data. Observations by George Plafker, S. B. Bonis, and 

M. G. Bonilla, February 6-13, 1976]

Approx. width

Station Trend of of fault Average trend 

(fig. 23) fault zone zone (m) of fractures

Measured

displacement Sense of 

(cm) displacement Ground surface Remarks

N70E 1.5 72 Pasture Down-to-north. Northfacing scarp as 

much as 5 m high 

along part of fault. 

See fig. 27.

N65E N32E >33 Dirt road

Railroad 

embankment

Minimum displacement, 

measured across the 

largest fracture 

in a zone containing 7 fractures.

Offset railroad tracks. 

Unreliable measurement due to gentle 

curve in tracks.

N65E

Nb5E

N50E

N46E

93

89

Concrete- Good displacement 

lined canal measurement.

Soccer field Good displacement 

measurement on 

offset sidelines. 

See fig. 26.

10

11

N61E

N70E

N75E 

N80E

N75E

N75E

N61E >60

3.25-9 

4 N40E 20

100

105

20

Asphalt 

highway

Dirt road

Plowed field

Pasture

Pasture

Cultivated 

field

Displacement may be 

minimum value if 

highway fill partially decoupled 

from ground.

Poor displacement 

measurement due to 

curve in road.

Subsidiary parallel 

fault 1 km long, 

and 200 m south of 

main break.

Fair displacement 

measured on offset cactus fence.

North side down. 

See fig. 25.

12 N75E N40E 142

13 (?)68

Pasture Fair displacement on 

offset path. Splay 

to north off main 

fault trends N60E 

with 28 cm sinistral, 

and 17 cm vertical 

displacement.

Dirt road Poor measurement.

Probably masked by 

landsliding.
GEOLOGIC EFFECTS 41

FIGURE 24.—Oblique aerial view looking south towards linear trace of the Motagua fault (arrows) in farmland west of 

Cabanas. Furrows with sinistral offset may be seen in the field at left. See figure 23 for location of Cabanas.

Faulting during the February 4 earthquake coincided closely with the previously recognized fault 

on the southern side of the Motagua Valley in the 

area east of El Progreso (Dengo and Bohnenberger, 

1969 ; Bonis and others, 1970). Locally, however, the 

faulting of February 4 was as far as 1 km from the 

Motagua fault as it was mapped before the earthquake. Moreover, faulting related to this earthquake 

has shown that the Motagua fault extends 85 km 

beyond its previously recognized western limits.

Much of the Motagua fault trace is marked by 

linear stream valleys, minor scarps, shutter ridges, 

and sag ponds that are suggestive of repeated geologically youthful tectonic activity along this fault. 

Earthquakes that destroyed Omoa, Honduras, in 

1859 (Montessus de Ballore, 1888) and caused damage at Quirigua (near Las Amates) in 1945 (Seismological Society of America Bulletin, v. 35, p. 194) 

and at Puerto Barrios in 1929 (Seismological Society of America Bulletin, v. 19, p. 55) may have

been generated along the Motagua fault or its offshore extension. However, because surface breaks 

were not observed and because the epicentral locations are not well constrained by the seismological 

data, it is not possible to preclude the alternative 

that these earthquakes were caused by movement 

on other faults in the area.

RELATIONSHIP OF FAULTING TO DAMAGE

The Motagua fault break caused extensive damage to buildings, roads, and the railroad. In Gualan, 

Cabanas, Subinal, and several smaller communities, 

structures that were astride the fault were damaged 

by the tectonic displacements. The most intense 

damage from shaking is within 40 km of the Motagua fault trace (Espinosa and others, this report) 

and is predominantly in areas of thick pumiceous 

ash-flow deposits of Pleistocene age. These poorly 

consolidated deposits may have amplified ground 

motions. However, other factors, such as lateral
42 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 25.—View towards the north showing rows in a cultivated field west of El Progreso (station 11, table 7) that are 

offset 105 cm in a sinistral sense by the Motagua fault. See figure 23 for location of El Progreso.

variations in energy release along the fault, construction practices, topography, and movement on 

subsidiary faults, undoubtedly influence the distribution of damage resulting from seismic shaking.

SECONDARY FAULTS

Secondary faults (faults which underwent surface displacement approximately concurrent with 

that on the main fault but which at the surface do 

not join the main fault) ruptured the ground surface in the Mixco area, in the western part of Guatemala City, and in the area between those cities. 

Data presently available indicate that the secondary 

faults occurred as much as 30 km from the main 

fault. This distance from the main fault is one of 

the longest ever documented for secondary faults 

associated with historic strike-slip faults, and this 

fact alone makes the study of these faults of international interest. The faults are particularly important to Guatemala for at least two reasons. 

First, they traverse an urban area, and their future

behavior should be considered with regard to present and future land use adjacent to them. Second, 

the generally north-trending faults near Guatemala 

City, on some of which the 1976 secondary ruptures 

occurred, may themselves be capable of producing 

damaging earthquakes. Even though such earthquakes probably would be of smaller magnitude 

than those produced by the Motagua system of 

faults, they could damage Guatemala City because 

of their proximity.

The following description of the distribution and 

trends of the secondary faults relies on data gathered by the Sociedad Geologica de Guatemala, 

which in a remarkably short time prepared a map 

of the faults. The description of details of the faulting at particular places is based on field examinations by the writers. The secondary faults can be 

grouped into the three zones indicated in figure 29; 

individual faults are not shown, primarily because 

of the scale of the figure. The description of the 

faults will proceed from west to east.
GEOLOGIC EFFECTS 43

FIGURE 26.—Oblique aerial view of Motagua fault trace crossing a soccer field at Gualan (station 5, table 7). Note characteristic right-stepping en echelon fractures and sinistral offset (89 cm) of white sideline stripe at right. See figure 23 

for location of Gualan.

MIXCO ZONE

The 1976 surface faults in the Mixco area have 

been mapped only in reconnaissance, and the general trend, length, and width of the zone of ruptures are uncertain at present. Faults are well developed northeast of La Brigada at locality 1 (fig. 

29), and a rupture crossed Highway CA1 just south 

of Mixco (loc. 2, fig. 29). The bearing between 

these two points is about N. 27° E.; however, en 

echelon faults occur in a broad band that extends to 

the area east of Ciudad Satellite (loc. 3, fig. 29). 

If the band is all considered part of the Mixco fault 

zone, then the trend may be more northerly than 

N. 27° E. For the present, the whole band (A, fig. 

29) will be considered part of the Mixco zone, but 

further mapping may require subdivision into two 

or more zones. Individual faults in the zone commonly strike between N. 10° E. and N. 30° E.; their 

lengths range from about 100 m to 3.5 km. The total

length of the Mixco zone of faults is greater than 

10 km.

Three of the faults were examined at and northeast of locality 1 (fig. 29). One of these, traceable 

for 1.2 km, cut several paved roads and ruptured 

the curbs and pavement at each crossing (fig. 30). 

Maximum measured displacement consisted of 

about 12 cm vertical slip (down to east) combined 

with about 5 cm of right slip, or about 13 cm of oblique slip. The vertical component was conspicuous 

at each street crossing, and the right-lateral component, though not conspicuous, could be seen at 

nearly all the crossings. The fault was essentially 

vertical, and rifting (displacement of the walls of 

a fracture perpendicular to the walls (Gill, 1972)) 

was very minor. In bare ground the fault appeared 

as a zone of discontinuous cracks, locally en echelon 

(stepping left), vertical displacement being distributed over a zone of variable width ranging up
44 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 27.—View looking east along fault trace at the most easterly locality visited on the ground (station 1, table 7). Fault 

trace trends along base of 5-m-high scarp in foreground and through the fallen tree in the distance, which has a base 

diameter of more than 5 m. The tree was split and toppled by fault movement of about 72-cm sinistral displacement and 

37-cm displacement down to the north. The north-facing steep scarp was probably formed by many repeated earlier movements along this same trace.

to about 2 m. Along part of its length the fault 

coincided with moderately inclined slopes that may 

be degraded fault scarps.

A second fault, nearly parallel to the first, could 

be followed for about 1 km. This fault intersected 

and displaced a high garden wall, passing about 4 

m from a house without damaging it. The wall, 

made of brick with a reinforced concrete beam at 

midheight, was displaced vertically 13 cm, down on 

the east; slight right-lateral separation was noted. 

Over most of its length, this rupture was near the 

base of a moderately inclined east-facing slope that 

may be a degraded fault scarp.

A third fault in this general area could be traced 

for 1.7 km. Its principal displacement was also vertical, down to the east. The vertical displacement 

measured at a severed garden wall was 12 cm; no 

strike-slip was noted. The fault passed through a

group of concrete-block houses (Husid and others, 

this report), where its course was marked by vertically displaced roofs and foundations (fig. 31), 

broken windows, and severely damaged interior and 

exterior walls (fig. 32). Aerial photographs taken 

in 1966 before construction of the houses show an 

east-facing break in slope, probably a degraded 

fault scarp, that the 1976 rupture followed in part.

VILLA LINDA-CASTA5JAS ZONE

A zone of faults (zone B, fig. 29) trending N. 20° 

E. extends more than 8 km from Colonia Villa 

Linda (loc. 4, fig. 29) to Colonia Castanas (loc. 5, 

fig. 29). Individual faults in the zone range in 

length from perhaps 100 m to 3 km and commonly 

strike between N. 18° E. and N. 31° E. One of the 

faults (near loc. 6, fig. 29) was examined briefly 

from the air and on the ground. It vertically dis-
GEOLOGIC EFFECTS 45

FIGURE 28.—Part of the village of Subinal, 7 km west of El Progreso, showing the destruction of adobe structures near the 

Motagua fault trace. The fault is a broad zone of ground cracks that cuts diagonally across the lower right corner of the 

photograph (arrows).

placed the highway called Anillo Periferico (not 

shown on fig. 29) by as much as several centimetres, crossed an open field, cracked a masonry 

wall, and damaged at least two houses to the extent 

that they had to be vacated. In the open field the 

fault followed a small steepening in a gentle slope, 

which suggests that displacements of the same 

sense (down to the southeast) had occurred along 

the same line before 1976. In the same area were 

some deceptive artificial "scarps" resulting from 

shallow excavations, apparently to obtain topsoil. 

This fault was followed for about 0.5 km, but others 

nearby and parallel to it that we did not examine 

are much longer.

INCIENSO-SANTA ROSA ZONE

A zone of discontinuous faults (C, fig. 29) extends from the area north of Incienso Bridge (loc.

7, fig. 29) to the vicinity of Colonia Santa Rosa 

(loc. 8, fig. 29), a distance of more than 7 km. Individual ruptures in the zone are generally less than 

1 km long; strikes generally cluster around N. 19° 

E. but vary widely. One fault in the zone displaced 

the highway northwest of Incienso Bridge about 4 

cm, relatively up on the east side. An excellent exposure in the roadcut there shows that the 1976 

displacement occurred on a preexisting fault zone 

that had earlier displacement in the same sense. 

The earlier displacement, as well as 1976 displacement, was apparent reverse movement, up on the 

side toward the deep canyon spanned by the bridge. 

The cumulative displacement was not measured but 

is clearly several times larger than the 1976 displacement. A paleosol is cut by the fault, and some 

evidence suggests that the topsoil may have been 

faulted before 1976 also; thus, it suggests geologically young, pre-1976 movement on the fault.
46 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

90°35' mm m ,«»„•»

14° 35'

FIGURE 29.—Zones A, B, and C (hatched areas) of secondary 

faults, and numbered localities discussed in text. A, Mixco 

zone; B, Villa Linda-Castanas zone; C, Incienso-Santa 

Rosa zone.

REGIONAL TECTONIC RELATIONS OF EARTHQUAKE 

FAULTS

The Motagua fault is part of a complex zone consisting of four major subparallel arcuate fault zones 

that trend in a general east-west direction across 

Guatemala and northern Honduras. As used in this 

paper, these are the Motagua and San Agustin 

faults in the Motagua Valley; the Polochic zone to 

the north, comprised of the Polochic and Chixoy 

(not labeled in fig. 23) faults; and the Jocotan 

Jocotan and Chamelecon faults (fig. 23). For convenience, this broad group of faults is referred to 

zone to the south, which consists primarily of the 

herein as the Motagua fault system.

The nature of the faults in this system and their 

relationship to the Cayman Trough (also referred 

to as Bartlett Trough) and the tectonics of the Caribbean region have been the subject of much study 

and speculation. Most workers agree that the faults 

in the Motagua system are old fundamental breaks 

that have undergone recurrent displacement at least 

since the late Paleozoic. Some have postulated large 

sinistral displacements on the faults in this zone 

during the Cenozoic, although significant vertical 

movements occurred during the earlier history of

FIGURE 30.—Fault displacement of road at locality 1 (fig. 29). 

Note right-lateral component of displacement.

the zone. Excellent recent comprehensive summaries of the onshore geologic data relevant to the 

tectonic development of the region, including extensive bibliographies, have been given by Dengo

(1968), Dengo and Bohnenberger (1969), Malfait 

and Dinkelman (1972), and McBirney and Bass

(1969). Data on seismicity and marine geology and 

geophysics in the Caribbean and their relationship 

to plate-tectonics models have been presented by 

Molnar and Sykes (1969) and Jordan (1975).

The secondary faults of the Guatemala CityMixco area are part of a system of predominantly 

dip-slip faults in Guatemala, western Honduras, 

and El Salvador that lie between the Motagua fault 

and the chain of stratovolcanoes that passes 

through the highlands of Guatemala and El Salvador (Dengo, 1968; Williams and others, 1964; Williams and McBirney, 1969). As shown in figure 23, 

these secondary faults group roughly into three sets 

that may in part reflect reactivated fractures in the 

crystalline basement rocks. The dominant set trends 

generally north to north-northeast; in a number of 

places, faults in this set bound prominent structural 

depressions such as the graben in which Guatemala 

City is located, the Ipala Graben of eastern Guatemala and western El Salvador, the Ulua Graben in 

western Honduras, and a series of grabens along 

the Chamelecon-Jocotan fault zone. A second important set of faults is located along, and approximately parallel to, the northwest-trending chain of 

stratovolcanoes (fig. 23) that comprise the Middle 

America volcanic arc. This set of faults becomes
GEOLOGIC EFFECTS 47

FIGURE 31.—Fault damage to a house in Guatemala City. The roof, foundation, and sidewalk have been displaced vertically.

increasingly prominent towards the southeast, 

where it bounds the central trench of El Salvador 

and the broad Nicaragua Depression (Williams and 

others, 1964, fig. 5; Dengo, 1968, fig. 9). A third 

set of oblique faults, not shown in figure 23, strikes 

northeast; it is locally well developed in the southeastern part of Guatemala and is present in much 

of the adjacent area to the southeast (Williams and 

others, 1964). Although detailed studies of the displacement histories of these faults have not been 

published, there can be little doubt that many of 

them are geologically youthful features. Most of 

them offset upper Tertiary or Quaternary deposits, 

in places they are marked by prominent scarps that 

border topographic depressions, and some of them 

serve as conduits for Quaternary volcanic eruptions. 

Available data on the 1917-18 series of moderatesized earthquakes that heavily damaged Guatemala 

City raise the possibility that those earthquakes 

may have originated on faults south or southwest 

of the city. The description by Vassaux (19B9, p. 

18-22) shows that Amatitlan and Villa de Guadalupe, both south of Guatemala City, sustained more

damage than the city proper in the November 17, 

1917, earthquake, which initiated the destructive 

series. Vassaux (1969, p. 21) concludes that there 

were at least two centers of activity during the 

series, including Petapa (about 15 km south-southwest of Guatemala City) and Escuintla (45 km 

southwest of Guatemala City). However, we suggest 

that the most likely cause of these earthquakes was 

a series of fault displacements on the Mixco system 

and, perhaps, on the extension faults that bound 

the graben in which Lake Amatitlan is situated 

(about 16 km south of Guatemala City).

LANDSLIDES

The main event and some of the large aftershocks 

triggered numerous landslides throughout a broad 

region of central Guatemala parallel to the main 

fault and extending as far westward as long 91°30' 

W. (fig. 33). The landslides, numbering in the 

thousands, were mainly falls, slides, and flows involving thick pumiceous pyroclastic rocks, but they 

also included slides of consolidated bedrock (figs. 

34 and 35). The overwhelming majority of the
48 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 32.—Fault damage to the rear exterior wall and roof of a house in Guatemala City. Vertical displacement near the

front of the house was 12 cm.

slides occurred along the steeper slopes of the 

deeply incised drainages in the Guatemalan highlands and at the larger road and railroad cuts and 

fills. They blocked many transportation routes, interrupted surface communication lines, and in places 

damaged structures built in their paths (fig. 36).

Some of the larger slump blocks and rotational 

slumps observed contain several million cubic 

metres of material. A number of these larger slides 

(indicated in fig. 33) have formed natural dams 

behind which lakes are developing (fig. 37). Such 

lakes are potentially hazardous because, when the 

dams are overtopped, rapid erosion and relatively 

sudden breakout could cause catastrophic flooding 

of inhabited areas and communication routes downstream.

Many highland drainages are choked with landslide debris, particularly those areas of intense 

landsliding shown by a distinctive pattern in figure 

33. This debris could become sufficiently water saturated during the rainy season (June through October) to become mobilized and move as debris flows

either naturally or as a result of aftershock activity. 

Such flows could pose a major hazard to communities and transportation routes situated downstream.

LANDSPREADING, FISSURING, SUBSIDENCE, AND 

SAND MOUNDS

Landspreading, involving near-horizontal movement of mobilized or liquefied water-saturated 

granular deposits toward free faces, occurred at a 

number of localities in the Motagua Valley, along 

the Atlantic coast in Guatemala and Honduras, and 

along some lake shores in the highlands (fig. 33). 

Extension cracking and subsidence that accompanied the spreading damaged structures in many 

of these areas.

Throughout the lower Motagua Valley and part 

of the Chamelecon Valley in Honduras, widespread 

fissuring and settling of sediments in areas of high 

water table have caused damage to irrigation facilities, roads, levees, railroads, buildings, and pasture 

lands. At many localities, the compaction of satu-
GEOLOGIC EFFECTS 49

GUATEMALA

HONDURAS

EXPLANATION

Approximate limits of earthquake

induced landslides. Pattern indicates

areas of intense landslide activity

Solid triangle indicates 

londslide~dammed drainage

Area of abundant ground cracks

related to liquefaction, "x" indicates

small area of cracking EL SALVADOR

FIGURE 33.—Areas of earthquake-induced landslides and of ground cracks probably related to liquefaction of unconsolidated 

deposits. Landslide distribution is from a preliminary study of post-earthquake aerial photographs by Edward Harp, 

Ray C. Wilson, and Gerry Wieczorek of the U.S. Geological Survey.

rated materials was accompanied by ejection of 

water or water-sediment mixtures and the formation of sand mounds (figs. 38 and 39). Similar 

effects of sediment liquefaction were observed in the 

delta on the northern side of Lake Amatitlan near 

Guatemala City and along the shore of Lake Atitlan; they reportedly occurred as far away as Lake 

Ilopango in El Salvador.

VOLCANIC ACTIVITY

There is no indication that the main event or any 

of its aftershocks were related to volcanic activity. 

One of the writers (Bonis), who studies the active 

volcanoes of Guatemala on a continuing basis, believes that the amount of ash erupted from the

Volcano Pacaya, located south of Guatemala City, 

may have increased slightly but that the apparent 

increase is well within the limits of the prequake 

variations in the volcano's activity.

Numerous reports have been received of steam 

suddenly venting from the ground after the earthquake or of changes in hot-spring activity. Because 

this area is characterized by widespread and abundant thermal activity related to the volcanoes or 

their deposits, such reports are to be expected. 

Those reported anomalies that have been checked 

by geologists, however, suggest that there is no 

evidence of dramatic new volcanic activity initiated 

by the earthquake that could be a hazard to life or 

property.
50 

iGUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 34.—Landslides in steep road cut in stratified pumice and ash deposits at San Cristobal, west of Guatemala City.

FIGURE 35.—Aerial view looking1 northeastward along Rio 

Pixcaya, due north of Chimaltenango, showing numerous 

landslides in pyroclastic deposits. The river was partially 

dammed by a major landslide, shown by arrow in the 

middle distance. (Also see fig. 37.)

FIGURE 36.—Aerial view of landslide in pyroclastic deposits 

near the edge of a steep-sided gully (barranco) in Guatemala City. Slides such as this (as shown by arrow) and 

their associated headwall cracking caused extensive damage to homes, roads, and other facilities in the northern 

part of the city.
GEOLOGIC EFFECTS 51

FIGURE 37.—Landslide-dammed lake along Rio Pixcaya. The 

toe of the dam had been breached by the river by the 

time this photograph was taken on February 13.

V -*a

•*

<*-

- j FIGURE 39.—View of linear sand mounds and circular to ellip-

"f tical craters in area of water-saturated unconsolidated

'. deposits along the Motagua River.

FIGURE 38.—Aerial view of ground cracks and sand mounds 

(white patches) in unconsolidated alluvial deposits along 

the Motagua River north of Quebradas in the lower Motagua Valley. Ground cracks are believed to result from 

liquefaction of water-saturated sediments and spreading 

towards the river channel.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD

OBSERVATIONS

By ALVARO F. ESPINOSA, RAUL HUSID, and ANTONIO QUESADA 4

INTRODUCTION

The earthquake of February 4 in Guatemala was 

felt over an area of at least 100,000 km2 . It originated on the Motagua River Valley to the east of 

Los Amates and propagated west along the Motagua fault through Gualan and El Progreso to 

Chuarrancho (details of the surface faulting mapping are given by Plafker and others, this report). 

The sense of motion from field observations as well 

as from instrumental seismic determination (Dewey and Julian, this report) is a left-lateral strikeslip fault.

This paper is a preliminary report on the earthquake-damaged area studied during the period February 6-22. The purpose was to obtain information 

in Guatemala City and along the Motagua fault 

area to deMneate the distribution of intensities 

(Modified Mercalli), damage to adobe-type structure, strong motions, and other related phenomena.

The ground movement in the fault zone was very 

severe, and numerous estimates of the time duration 

of strong shaking range between 30 and 40 s. The 

first movement was vertical and was followed by a 

strong horizontal ground motion, which was so 

strong that it hindered people from getting out of 

bed, and in many instances people were thrown 

down or were unable to walk. In many areas of the 

country, a second intense horizontal motion was reported nearly a minute after the main disturbance. 

In one particular case illustrating the last report, a 

man tried to get out of bed and failed. He waited 

several seconds and tried again, failing for the 

second time. He stayed in bed for about 30 s, and 

then he was able to get up, pick up a child from a 

crib, and go out. As he was going out, he felt the 

second severe horizontal ground motion, which collapsed his house.

Organization of American States, Washington, D.C.

CASUALTIES AND DAMAGE

The statistics for casualties and damage are 

given in table 8 by Departments and in table 9 by 

municipalities. These figures were provided by the 

Comite Nacional de Emergencia, Presidencia de la 

Republica de Guatemala. The total number of 

houses destroyed, as of February 15, 1976, was 

254,750, and 1.07 million people were left homeless. 

From a total population of 3,213,962, there were 

22,868 deaths and 77,190 injuries as of March 3, 

1976. The total loss in Guatemala is $1,100 million 

(from Ministry of Finance statistics).

INTENSITY DISTRIBUTION IN GUATEMALA

The areas of maximum Modified Mercalli intensity are concentrated in and near the town of Gualan, Department of Zacapa, and to the west in the 

town of Mixco, Department of Guatemala. Maximum intensity in the meizoseismal area was IX. In 

the Gualan area, however, much of the damage could 

be classed as VIII. On the Modified Mercalli scale 

(Richter, 1958), large landslides, such as those that 

developed between Guatemala City and El Progreso 

and also between Guatemala City and Antigua, suggest an intensity greater than IX. Another factor 

that yields higher intensities is surface faulting, 

examples of which were observed in Gualan (see 

cover photograph) and along the Motagua fault. 

The authors visited, by car and helicopter, villages 

in areas of high, intermediate, and low damage a'nd 

by using questionnaires gathered data (fig. 40) 

used to assess the Modified Mercalli intensity ratings throughout the nation.

These intensities are rated by using the abridged 

version of the Modified Mercalli intensity scale 

(Richter, 1958), with the following exceptions: 

landslides are not rated in this report as intensity 

X; rails bent greatly are not rated as intensity XI; 

and destroyed bridges are not rated as intensity XL

52
INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD OBSERVATIONS

TABLE 8.—Casualties and damage, by Departments

53

Department (state)

Guatemala

El Progreso

Sacatepdquez

Chimaltenango

Santa Rosa

Solola

Totonicapdn

Quezaltenango

Huehuetcnango

Quiche"

Baja Verapdz

Alta Verapa"z

Izabal

Zacapa

Chiquimula

Jalapa

Jutiapa

Population

1,681,756

78,564

105,210

214,290

20,591

50,707

162,678

79,241

54,562

150,075

49,820

59,664

185,570

107,148

76,605

88,802

91,505

Deaths

5,570

2,028

1,582

15,754

40

110

27

14

10

845

152

18

75

695

50

91

15

Injuries

16,549

7,767

8,855

52,592

291

500

89

228

50

5,722

718

955

579

1,998

578

475

48

Percent 

damage

68.82

90.45

71.00

88.00

1.60

10.00

54.00

1.00

N.A. 1

75.00

82.50

67.50

40.00

72.86

50.00

51.67

10.00

!.A. - Information not available.

These exceptions have been made because the Modified Mercalli intensity scale is used to represent the 

intensity of an earthquake based on purely vibrational effects as well as on the damage sustained 

by structures from the earthquake. The above 

effects are of a secondary nature to the seismic 

energy release. In the area of heavy landsliding, 

many adobe houses sustained no damage. Landsliding implies intensity X, but undamaged adobe 

houses suggest much lower intensities (fig. 41). 

Also, numerous houses near landslides along the 

highways toward the Pacific were not damaged.

Rails bent greatly are not related directly to 

ground shaking, but this effect is related to ground 

movement due to faulting, as seen in Gualan (fig. 

42A) and near El Jicaro (fig. 425), or to ground 

compaction, as observed in Puerto Barrios (fig. 

42C). Another factor that yields higher intensities

is surface faulting, examples of which are observed 

in Gualan (see cover photograph) and along the 

Motagua fault, near Las Ovejas (fig. 42D).

The Agua Caliente Bridge was destroyed, and the 

Benque Viejo Bridge was at the verge of collapse 

owing to large ground displacement in those areas. 

The displacements sustained by the Agua Caliente 

Bridge were larger than those planned in the original design for the structure. The damage to these 

structures gives an indication of the severity of the 

ground deformation but does not indicate the level 

or the time duration of the seismic disturbance. The 

Benque Viejo Bridge is similar in construction to 

some of the highway overpasses in the San Fernando Valley of California, which collapsed as a result of the 1971 San Fernando earthquake.

The isoseismal map shown in figure 43 represents a preliminary Modified Mercalli intensity dis-
54 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT 

TABLE 9.—Casualties and damage, by municipalities

j*This consecutive number identifies the total number of municipalities 

I in a department (as listed in table 8). J

Municipality Population Deaths Injuries % Damage

1.*

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

1.

2.

3.

4.

5.

Ch ima 1 1 enan go

San Jose Poaquie

San Martin Jil'otepeque

Zaragoza

Patzicia

Sta. Cruz Balanya

Tecpan

Patzun

Parramos

El Tejar

San Andres Itzapa

Yepocapa

Comalapa

Sta. Apolonia

Guatemala

San Pedro Sacatequepez

San Juan Sacatepequez

Chuarrando

Sn. Raymundo

San Pedro Ayampuc

Mixco

Amatitlan

Palencia

Villa Canales

Sn. Miguel Petapa

Sta. Catarina Pinula

Chinautla

Progreso Cabecera

El Jicaro

San Agustin Acasagustlan

Morazan

Sanarate

20,194

9,795

33,066

7,317

10,585

2,903

24,181

18,900

3,237

3,039

8,447

10,457

18,163

4,182

700,504

10,714

43,116

6,985

9,225

10,481

129,878

26,412

18,982

31,774

8,078

12,934

32,763

11,048

6,197

17,344

7,080

15,253

600

1,000

2,920

300

811

100

3,023

309

200

50

150

87

3,050

900

1,195

720

720

42

118

54

346

16

68

2

2

9

50

1,300

372

126

134

69

3,000

2,657

5,000

1,000

2,248

500

7,000

390

900

900

728

289

5,000

844

5,550

1,667

2,400

1,789

1,543

316

2,400

80

157

100

140

70

15

3,500

2,538

917

570

137

25%

90%

100%

100%

90%

80%

100%

85%

90%

85%

90%

90%

95%

85%

45%

100%

100%

60%

60%

90%

80%

20%

85%

20%

70%

75%

80%

95%

100%

50%

100%

70%

tribution of the main event in Guatemala. The isoseismal for an intensity rating VII follows the general trend of the mapped Motagua fault. The isoseismal VIII, and higher, in the Departments of 

Sacatepequez, Chimaltenango, Guatemala, and the 

southern part of Quiche, follows the general trend 

of maximum adobe-damaged areas.

The high intensities attenuate faster in the eastern part of the country near Los Amates. However, 

as one progresses west, from El Jicaro to near Sanarate, the intensities increase in a narrow area, 

and then, outside Sanarate, there is a sudden intensity decrease for the next 35 km and again a 

rather large increase to Modified Mercalli intensities of VIII and IX in the Mixco area. Guatemala 

City, as it appears on this map, has been assigned 

an average intensity of VII and, in the northern part 

of the city, an intensity rating of VIII. A detailed

mapping of the intensity distribution in Guatemala 

City associated with the February 4 earthquake is 

now being done and will be presented in a subsequent separate report.

A study of intensity distributions unexpectedly 

showed that a number of small villages near the 

causative fault sustained no damage. The intensity 

ratings attenuate rather rapidly in a north-south 

direction in the eastern part of the country.

The epicenter was located west of the town of Los 

Amates, approximately 12 km away. The highest intensities were in Gualan and 145 km due west in 

Mixco. In Guatemala City, the intensity was IX in 

the center of the city. To the northwest of the city, 

the intensity was VIII along the strike of some faults 

mapped after the earthquake.

The intensity VII isoseismal has an east-west 

trend, from Los Amates, parallel to the Motagua
INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD OBSERVATIONS 

TABLE 9.—Casualties and damage, by municipalities—Continued

55

Municipality Population Deaths Injuries % Damage

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

1.

2.

3.

4.

1.

2.

1.

2.

3.

4.

5.

6.

7.

1.

2.

1.

2.

3.

1.

2.

3.

A.

5.

1.

2.

3.

Sacatepequez

Sumpanjo

Magdalena Milpas Altas

Jocotenango

San Lucas Sacatepequez

San Antonio Aguas Calientes

Pastores

Sta. Domingo Xenaxoj

Sn. Miguel Duenas

Santiago Sacatepequez

San Maria de Jesus

San Bartolome Milpas Altas

Quiche

Joyabaj

Chinique

Chichicastenango

Jutiapa

Asuncion Mita

Zacapa

Gualan

Rio Hondo

Cabanas

Huite

Usumatlan

Teculutan

Baja Verapaz

Rabinal

Izabal

Los Amates

Morales

Totonicapan

St. Maria Chiquimula

Momostenango

San Cristobal Totonicapan

San Fco. el Alto

Chiquimula

Esquipulos

Sn. Jacinto

26,9A5

10,232

2,921

3,426

A,3AA

3,866

A, 592

2,759

A, 215

7,9A3

7,144

1,513

35,147

32,13A

A, 353

A5.733

54,680

29,071

3A,703

23,375

9,637

5,817

3,941

3,771

5,933

21,913

20,393

38,903

45,537

52,677

52,688

15,161

43,398

16,623

19,329

38,872

19,304

3,851

277

315

135

118

157

113

127

57

7

218

2

27

56

600

35

140

13

198

187

95

89

67

26

31

119

33

30

14

29

3

3

21

10

20

20

1,251

1,300

584

582

1,170

544

567

560

524

1,247

218

246

175

5,497

18

30

475

550

281

240

152

150

150

377

341

167

158

54

10

10

11

3

55

110

110

158

25%

100%

50%

50%

40%

50%

30%

70%

30%

40%

20%

40%

95%

50%

99%

80%

95%

75%

50%

60%

75%

90%

50%

2%

50%

50%

1%

fault for a distance of 150 km to near San Antonio 

La: Paz in the Department of El Progreso. From 

San Antonio to Zaculeu in the Department of 

Quiche, an east-west distance of another 85 km, the 

intensity VII isoseismal broadens considerably to 

72 km in width. The intensity VIII and IX isoseismals follow a trend parallel to the trend of surface 

faulting. The dashed line is questionable for the intensity VIII isoseismal continuation to the west between Sanarate and to the east of Guatemala City. 

The number of landslides in this area was very 

high, on the average one landslide per kilometre.

From Guatemala City toward El Progreso, there 

were 32 landslide areas as far as Kilometre 29 near 

the town of El Chato and a total of 54 landslide 

zones in the first 48 km on this main highway toward the Atlantic Ocean. A landslide zone consists of 

one to three large landslides obstructing the highway.

On this road there were two bridges that suffered 

considerable damage. The Agua Caliente Bridge collapsed and impeded traffic, and the Benque Viejo 

Bridge was on the verge of collapse (Husid and 

others, this report). Severe landslides occurred also
56 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT 

TABLE 9.—Casualties and damage, by municipalities—Continued

Municipality Population Deaths Injuries Damage

1. Solola 25,819 110 300

1. Jalapa 45,425 27 254 50%

2. San Pedro Pinula 23,846 9 97 25%

3. Mataquescuinla 16,145 55 122 20%

1. Sta. Rosa 14,127 40 291

1. Alta Verapaz 43,505 15 700 60%

2. Sta. Cruz Verapaz 3,508 3 253 70%

1. Quezaltenango 65,526 14 228

1. Aguacataman 18,492

2. San Sebastian Huehuetenango 7,824

3. San Pedro Necto 11,371

4. San Miguel Acatan 15,011

5. Concepcion 8,102

6. Newton 12,613

7. St. Ana Huista 4,755

8. La Libertad 14,756

9. Colotenango 9,458

10. San Caspar Ixchil 3,058

1. Villa Nueva (Guate) 5 12 8%

2. Acatenango (Chimaltenango) 22

	Total 22,525 74,027

along the main highway to the Pacific Ocean, between Guatemala City and Antigua. Landsliding interrupted road traffic along these two main throughways and also disrupted railroads near Las Ovejas, 

Gualan, El Progreso, Rio Hondo, and Puerto Barrios.

The preliminary intensity distribution in Guatemala (fig. 43) suggests that the shaking intensity 

was greater in the western part of the country. This 

isoseismal pattern suggests a fault propagation 

rupture from east to west. The isoseismals broaden 

to the west, a phenomenon similar to a Doppler 

effect, which creates a constructive interference pattern to the west. Several small villages were located 

near and at intermediate distances from the causative fault; for example, adobe construction in Jones, 

about 8 km from the fault, sustained no damage. 

Also, in several communities south of the Motagua 

fault, such as San Pedro Pinula nearly 25 km from 

the fault, adobe construction sustained no damage. 

Numerous small villages in which adobe buildings 

sustained no damage were observed 8 to 30 km from 

the causative fault. Other towns, such as Entre Rios 

approximately 37 km due east of Morales, near the

extension of the Motagua fault, had an intensity 

•rating of only V.

The pattern of isoseismals displayed in figure 43 

may be the effect of a moving source in the near 

field. This effect is shown schematically in figure 44 

(Benioff, 1955) to be the progression of a discrete 

number of points. The initiation of the fault motion is near Los Amates, at point 0, and terminates 

at point 8, toward Guatemala City. The largest 

circle represents a wavelet at point 0, which in the 

time domain is shown at the lower part of this diagram and is identified with a 0, and the succesive 

circles represent the wavelet position as it propagates from points 1, 2, 3..., and so on. There is a 

time delay between these points, as is seen in the 

two lower diagrams. The lower left diagram represents the signal from each point as seen at a station 

west of the fault, and the lower right diagram represents the signal as seen at a station east of the 

fault. The composite signal for each direction of 

propagation is shown as the resultant in the lower 

part of figure 44. The energy can be measured as 

the square of the velocity amplitude; hence, the resultant wavelet traveling to the west, shown at the

*U.S. GOVERNMENT PRINTING OFFICE: 19770-240-961/84
INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD OBSERVATIONS 57

I5°N

14°

7

f Chajul

San Sebastia'n V. 

/S| • ^» ® X^_T

LX'^'~\ JT^

7^ ^

(~^ >—! 7i Santa Cruz \ 

\ | j ^Del Quiche' x

•y\ ^Chim'altenango vs_ ^--^/ S p?nuieadr° ""*' I, % •

City

A .^fc '/

Escuintla; . ~ ^^ Jutiapa % "3^

- S • > ^

.. r5 " ^

PACIFIC OCEAN

50 KM 

1

North

92° 91° 90°W 89°

FIGURE 40.—Intensity sampling distribution. Each dot represents the location where one or more questionnaires was completed during a survey taken in Guatemala. Largest circle indicates epicenter location of main event. (Base map modified 

from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

lower left in figure 44, has a larger concentration of 

energy than the slightly dispersed wavelet traveling 

in the opposite direction.

The possibility of a fault rupture traveling from 

east to west (suggested in fig. 43) could be verified 

with teleseismic data at western azimuths. Also, the 

suggestion of a double rupture or a multiple earthquake from the isoseismal distribution is plausible. 

If this second alternative is adopted, the first earthquake will be constrained to the observed surface 

faulting from Los Amates to Kilometre 15 southwest of El Progreso. The second event could be associated with the secondary faulting observed in the 

Mixco area.

Other factors that may enter into the intensity 

distribution pattern shown in figure 43 are seismicwave amplification effects, topographic seismicwave amplification, influence of the surficial soil 

conditions, and depth of the water table.

The high-level isoseismals VI, VII, and VIII represent the shape of the radiation pattern in the 

near field, assuming the rupture started near the

epicentral region and propagated west. A similar 

suggestion made by Hanks (1975) correlates the 

intensity VI and VII isoseismals from the San Fernando earthquake with the radiation pattern for 8-s 

Rayleigh waves and also with the azimuthal variations of the amplitude ground displacements.

The isoseismal map was plotted on a 1:500,000 

geologic map of Guatemala (edition by Bonis, 

Bohnenberger, and Dengo, 1970), and no simple 

correlation was found between the gross surficial 

geology and the intensity distribution. There is a 

correlation between the fault that slipped during 

the February 4 earthquake and the intensity distribution. West of Guatemala City is the Mixco 

fault, which has a north-south trend. In this region, the intensity rating attains a maximum of IX.

The earthquake was felt by nearly everyone over 

an area of at least 93,125 km2 , suggesting that an 

intensity V or higher extended over that area. The 

areas felt for intensities VI and higher are given in 

table 10.
58 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 41.—One of the many landslides obstructing the main highway from Guatemala City toward El Pirogreso.

TABLE 10.—Felt area for Modified Mercalli intensities VI and

higher

Modified Mercalli intensity

VI and higher _ _ _ _ _ _ _

VII and higher _ _ _ _ _ _ _ _

VIII and higher _ _ _ _ _

IX (small area) _ _ _ _ _

Felt area (km 2 )

32,697

6,437

2,495

125

DISTRIBUTION OF ADOBE DAMAGE

The map shown in figure 45 represents the overall distribution of adobe damage in the Republic of 

Guatemala. This map was prepared with information from a variety of sources. First, we studied 

reports from the Comite Nacional de Emergencia, 

which gathered a large amount of information 

through the local committees. These reports were 

supplemented by detailed lists of damage and by our 

assessment of the sustained damage in the field. 

Newspaper clippings from the national press were

also consulted. Other sources of information were 

gathered by the Universidad de San Carlos and by a 

team of the Camara de Construccion in Guatemala 

City.

The adobe damage distribution map shows the 

amount of damage to a given village. On this map, 

a scale of 4 means that 91 to 100 percent of all the 

adobe houses in a village collapsed (fig. 46) ; a 3 

implies 76 to 90 percent; a 2 means 51 to 75 percent; 1 means 26 to 50 percent; 0 in the scale implies negligible to 25 percent damage to adobe 

houses. The maximum adobe damage was found in

FIGURE 42.—A, Rails bent in Gualan, Department of Zacapa. 

B, Rails repaired between El Jicaro and Las Ovejas, Department of El Progreso; also shown in B is the surface 

faulting with an east-west trend. This photograph was 

taken from a helicopter in a eastward direction. C, Rail* 

bent on the Puerto Barrios wharf. D, Aerial view of 

fault trace near Las Ovejas. See figure 40 for town locations.
'A'
60 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

\5°N\-

Intensity Distribution

FIGURE 43.—Modified Mercalli intensity distribution in Guatemala from the main event. Circle indicates epicenter location 

of the February 4 earthquake; dashed line indicates approximate isoseismal. (Base map modified from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

the Departments of Chimaltenango, Guatemala, and 

lower Quiche. These regions of the country are the 

highlands of Guatemala. The affected region of 51- 

to 100-percent damage covers a surface area of approximately 8,725 km2 . There are two well-defined 

areas of maximum damage; one follows a trend 

parallel to the Motagua fault located in the Departments of Izabal, Zacapa, and El Progreso. A gap is 

found between the 4 and 2 contours for nearly 40 

km. Within the next 35 km there is another concentration of high damage (fig. 45). The damage 

pattern diminished rapidly to the west into the 

state of Totonicapan.

TABLE 11.—Damage to adobe-type structures

Scale l

2 and higher _ _

3 and higher _ _.

4

Percent 

damage

51-75

76-90

91-100

Damage 

area 

(km 2 )

8,725

2,825

1 790

1 Scale values taken from figure 45.

A number of cracks, fractures, and short fault 

scarps were found due west of Guatemala City in

the Mixco area. Damage decreases more rapidly to 

the east than to the west of the zone of strongest 

shaking. The areas damaged for different percentiles of collapsed adobe houses are given in table 11.

Adobe structures are apt to collapse more easily 

than bajareque (Husid and others, this report) and 

wooden-type construction. The latter construction is 

found throughout the Motagua River Valley in the 

lowland near the coastal area of Guatemala, In 

Puerto Barrios and in Santo Tomas, many of the 

houses are built of wood. The wharf of Puerto Barrios collapsed because of structural failure and possible ground compaction (Ray Wilson, oral commun., 1976) (fig. 47). There was no damage to 

well-built structures in the port of Santo Tomas, 5 

km south of Puerto Barrios. In the port of Santo 

Tomas, a number of ground cracks were observed, 

and ground-level changes of 10 to 15 cm in the 

maritime port were due to ground compaction.

Near Puerto Barrios, mud spouts occurred during 

the main earthquake. The damage in this region to 

wood-frame and adobe structures was minimal.
INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD OBSERVATIONS 61

toward ,,<;,' 

Guatemala City il.V^

active fault segment

direction of fault progression c

Time Time

resultant in direction 

of fault propagation

resultant in reverse direction

FIGURE 44.—Schematic of slip progression and its effect on 

wave amplitudes and shapes due to a moving source 

(modified from Benioff, 1955). Fault propagation from 

east, near Los Amates, to west, toward Guatemala City.

The pattern of adobe damage (fig. 45) is similar 

to the intensity distribution shown in figure 43. The 

similarity suggests that more energy was released 

to the west than to the east. It also suggests that 

seismic energy was released along and near the 

known fault zone of the Motagua fault. The maximum concentration of damage, mostly in the Department of Chimaltenango, is suggestive of a constructive interference of seismic waves due to a 

moving dislocation.

INTENSITIES IN GUATEMALA CITY

The distribution of earthquake intensity was investigated by canvassing Guatemala City in a manner similar to the way in which the Republic of 

Guatemala was canvassed. In the process of canvassing Guatemala City, data were obtained from a 

representative number of questionnaires solicited 

from each of the 16 zones into which the city is 

divided. A total of 1,050 questionnaires was completed and collected.

A map of intensity distribution for Guatemala 

City associated with the earthquake is now being 

prepared. The following cursory comments are the 

preliminary result of our fieldwork.

The maximum Modified Mercalli intensity in the 

city was IX, as shown by the partial collapse of reinforced-concrete structures, and there were pockets 

of high intensities in different zones of the city. A 

cursory examination of the questionnaires shows 

that some of the localized high intensities may be 

related to possible ground-amplification effects of 

seismic waves. Similar findings were reported elsewhere on a seismic-zonation study in Lima, Peru 

(Espinosa and others, 1976).

A number of chimneys cracked and collapsed in 

different suburbs of the city (fig. 48). The variations of the intensity ratings in the city proper 

varied from VI to IX. A number of wells showed an 

increase in temperature of more than 2° C, and 

their water level changed drastically, sometimes 

more than 60 cm, due to the February 4 earthquake. 

In the northern part of the city, damage to adobetype construction was intense. Also, throughout the 

city, there were a number of damaged reinforcedconcrete structures (Husid and others, this report). 

A number of landslides occurred to the northwest 

in the Colonia 1° de Julio, Zone 19, on the Barranco 

de las .Guacamayas. The barrancos have an almost 

vertical drop of nearly 90 m. Figure 49 shows the 

head of a landslide in the area, at approximately 

long 90°33.5' W., lat 14°40.6' N.

In Zone 7,13 Calle B. No. 31-14, a one-story brick 

house sustained a large amount of damage from a 

ground crack, possibly a small fault with a northsouth trend, which crossed through the house. This 

ground crack could be traced very easily through 

the suburb, across damaged streets and houses. In 

other zones, such as Zone 19, 8 Avenue No. 6-96 

there was considerable damage because of ground 

failure. In Zone 11, 8 Calle No. 20-62, Colonia Mirador, there was moderate damage to a one-story 

brick house. These examples show the damage that 

can be caused by faulting in the immediate vicinity 

or below some of these houses in Guatemala City.

INTENSITIES IN NEIGHBORING COUNTRIES

The Modified Mercalli isoseismal V covered an 

area from Ilepango, Izales, and San Salvador in El 

Salvador to Santa Barbara, San Pedro Sula, and 

Puerto Cortes in Honduras. In Tegucigalpa, an intensity rating of IV was assigned. To the north,
62 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

I6°00'

15° 00'

92°00' 91°00' 90°00' 89° 00' 88° 00'

FIGURE 45.—Contour map showing damage to adobe-type structures in Guatemala owing to the February 4 earthquake. See 

figure 1 for Department names. (Base map modified from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

Belice had a IV and Mexico City a IV rating. Poctun in the Guatemalan northern territory had an 

intensity of V. The total felt area for an intensity 

V or higher was 93,125 km2 .

The earthquake of 1773, called the Santa Marta 

earthquake, caused extensive damage in the Guatemalan highlands down to El Salvador. The February 4 earthquake and the Santa Marta earthquake affected similar areas in Guatemala and El 

Salvador, but there was no damage to El Salvador 

from the February 4 earthquake.

GROUND MOTION AT INTERMEDIATE DISTANCES

Strong ground displacements were recorded on a 

seismoscope in Guatemala City (Knudson, this report), but, unfortunately, no accelerographs were 

operational in the area at the time of the earthquake.

Strong ground motions were experienced in the 

Mixco area, in particular at the Licorera Mixco, 

where a large oil-fired boiler was displaced horizontally a maximum of 41 cm.

In La Colonia 1° de Julio, a northwestern suburb 

of Guatemala City, a large water tank sustained 

damage to its back bracings at the upper level 

(nearly 12 m from the ground). The shear bracings 

also were bent. The water tank sustained damage at

a height of 4 m from the bottom of the tank and 16 

m from the ground, where the bolts tying the steel 

walls of the water tank were sheared and water 

leaked out.

In Guatemala City, in the chemistry building of 

the ICAITI (see Glossary) complex, a large machine 

weighing 1,298 kg was displaced 7 cm horizontally. 

To the southeast of the city in Zone 10, in the Herrera's grounds, four medium-sized marble statutes 

were thrown 40 cm from their stands (fig. 50). 

There were some reports that hanging lamps dented 

the ceilings of homes and that outside hanging lamps 

left a fan of dents on the walls.

In the maritime port of Santo Tomas, a large upright unloading cargo crane weighing 60 tons was 

displaced from its rails 5 cm in a northeasterly 

direction. This port is northeast of Los Amates.

SOURCE PARAMETERS FROM FIELD OBSERVATIONS

Many major earthquakes have occurred in close 

association with major faults; as a result, a number 

of empirical relations that correlate magnitude, 

length of surface faulting, fault displacement, and 

magnitude have been derived. King and Knopoff 

(1968) developed the following relation:

log M-4.99. (1)
FIGURE 46.—Photographs showing the sustained damage in the towns of: A, Joyabaj; B, Comalapa; C, Tecpan; and D, San

Martin Jilotepeque.

Using this equation, one determines the expected cm (Plafker and others, this report). This yields a

magnitude for the Guatemalan earthquake by sub- magnitude of 7.4.If King and Knopoff's expression

stituting the field observations of L = 300 km and (modified from Tocher) is used,

the average horizontal fault displacement ^ = 100 log Lu2 = 2.75 M-8.93, (2)
64 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 47.—Puerto Barrios wharf, Department of Izabal, destroyed by February 4 earthquake. Arrows show large warehouse partially submerged. See figure 40 for location of town.

one obtains an Ms value of 7.4. These values are in 

good agreement with the value Ms = 7.5 determined 

for the Guatemalan earthquake from teleseismic observations (Person and others, this report).

Having established that Ms = 7.5 is a reasonable 

estimate for this earthquake, we proceed to determine the seismic energy, Es, from the GutenbergRichter energy-magnitude relationship given by 

Richter (1958) :

log E8 = 11.8 +1.5 MB . (3)

This equation yields E$ = l.lxW23 ergs for the 

Guatemalan earthquake of Ms = 7.5.

The stress drop (Keilis-Borok, 1959; Aki, 1966; 

Brune, 1970, eq. 30) is denned by

7

fW ITT 

T 16

(4)

where r is the radius of a circular dislocation, assumed in this case to be 150 km, the rigidity representative of volume around the faulted area is

FIGURE 48.—Chimney collapse from a one-story house in 

Guatemala City, Zone 11.
INTENSITY DISTRIBUTION AND SOURCE PARAMETERS FROM FIELD OBSERVATIONS 65

FIGURE 49.—Head of large landslide (shown by arrow) in the 

Barranco de las Guacamayas, Guatemala City.

assumed to be ^=3x10" dyne cm- 2, and ^=100 cm; 

then,

Aa=3 bars.

To verify the above stress-drop determination, 

the seismic moment M0 and the energy E (Brune, 

1968), defined by

and

are combined to obtain a

E

(5)

(6)

(7)

Assuming ^ and u as given above and A, the dislocation area, to be 300 km in length and 20 km in 

width (assumed, Brune, 1968, table 2), M0 =L8 x 1027 

dyne-cm. Using E, determined from equation (3)

FIGURE 50.—A marble statue thrown 40 cm from its pedestal; 

it is 120 cm high, weighs approximately 200 kg, and is 

located in Zone 10 in Guatemala City.

to be l.lxlO23 ergs, and substituting these values in 

equation (7), one obtains a =18 bars.

From the above computations, it seems that the 

February 4 earthquake was a low-stress-drop earthquake. Independently, Dewey and Julian (this report) have found Aa=6.6 bars, using information 

determined from the spectral density of G-waves.

Using a Modified Mercalli intensity rating of 

VII for Guatemala City with an epicentral distance 

of 157 km, one determines the particle horizontal 

velocity (Espinosa, unpub. data, 1975) from

log x = 1.27-0.79 log A + 0.16 /mm (8) 

to be 4.5 cm/s, where A is the epicentral distance 

in kilometres and 7mm is the Modified Mercalli intensity rating. If, instead of the epicentral distance, 

one uses the distance from the causative fault to 

Guatemala City (25 km), then one obtains a maximum particle velocity of 19.3 cm/s. The above quantities give an indication of the level of ground motion of the main event experienced in Guatemala 

City.
66 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

An earthquake similar to the Guatemala earthquake was the Varto-tistukran earthquake of 

August 19, 1966, on the Anatolian fault system in 

Turkey (Ambraseys and Zatopek, 1968; Wallace, 

1968), and data obtained from it are very similar 

to the observations made by the authors after the 

Guatemalan earthquake. These two earthquakes are 

strike-slip faults, the former right-handed and the 

latter left-handed. In terms of fault displacement,

magnitude, and length of faulting, the February 4 

earthquake is similar to the November 26, 1943, 

Turkish earthquake, which had a magnitude of 7.6, 

a length of rupture of 280 km, and a relative horizontal displacement of 110 cm. The February 4 

earthquake had a magnitude of 7.5, a length of rupture of 240 km, and an average relative horizontal 

displacement of 100 cm.
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

DAMAGE AND ENGINEERING IMPLICATIONS

By RAUL HUSID, ALVARO F. ESPINOSA, and ANTONIO QUESADA 5

INTRODUCTION

In this report, we discuss the damage done by the 

February 4 earthquake and the engineering implications in greater detail. We report on the damage 

to selected structures in the capital city and on a 

few structures in the rest of the affected area but 

do not attempt to include all the important failures.

EARTHQUAKE-RESISTANT DESIGN PRACTICE IN 

GUATEMALA

When the February 4 earthquake occurred, no 

earthquake-resistant-design code had been enacted 

into law in Guatemala, and therefore it was not 

mandatory to design structures to withstand seismic forces. Each engineer or architect selected a 

foreign code and designed accordingly (J. Asturias, 

oral commun., 1976). The same professional was 

usually in charge of supervising the construction 

process. Review of design and construction by 

specialized structural engineers, independent of the 

original designer, was not required, as it is in Chile, 

Mexico, and the United States.

According to two local structural engineers, J. 

Arias and R. Zepeda (oral commun., 1976), many 

professionals used elements of a version (not necessarily the latest) of the Structural Engineers Association of California code. Thus, the .structures in 

Guatemala City were not designed according to 

common standards. In the short time available for 

the study, it was difficult to assess, from the condition of the buildings, whether the various standards employed are suitable for the local soil conditions, quality of construction materials, dynamic 

characteristics of the structures, and other important factors. It is noteworthy to mention that 

the material characteristics, such as the strength of 

steel reinforcing bars, are frequently assumed by 

the engineer without any supporting technical evidence.

5 Organization of American States, Washington, D.C.

TYPES OF STRUCTURES

Guatemala City has many modern buildings; most 

are reinforced concrete, but a few are high-rise steel 

structures. The predominant type of modern construction appears to be the reinforced-concrete 

frame structure having flat beams in one or two 

directions and masonry (reinforced or unreinforced) 

filler walls. It is common to find filler walls made of 

poorly reinforced hollow brick or hollow tile.

One of the most common forms of construction is 

adobe, which is used for the majority of houses, 

churches, and small structures throughout the 

country. Roofs are generally tile on wood-pole rafters.

Reinforced mud or bajareque construction is also 

used extensively in Guatemala. It consists of a wood 

frame covered with lath, the wall space being filled 

with mud and plastered. Bajareque is similar to 

quincha, which is frequently used for building 

houses in the coastal region of Peru. Quincha construction sustained extensive damage in the 1970 

Peruvian earthquake (Husid and Gajardo, 1970; 

Berg and Husid, 1971, 1973).

Wooden construction was common in Puerto Barrios and in the port of Santo Tomas. Corrugatedsteel and reinforced-concrete grain silos were used 

in the area affected by the earthquake. Water tanks 

were predominantly elevated and built of reinforced 

concrete or steel.

DAMAGE SURVEY

Although the capital city was not damaged as 

severely as towns along the Motagua River Valley 

and some towns in the highlands west of Guatemala City, there was extensive damage in several 

zones, and some reinforced-concrete and steel structures completely collapsed.

The types of construction found outside Guatemala City are adobe, bajareque, and wood. Adobe 

construction in many towns sustained the same

67
68 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 51.—Damage to a wooden structure in Puerto Barrios, 

caused in part by ground compaction. Note the large offset 

of 11 cm shown in the photograph.

heavy damage that has been observed after many 

previous earthquakes in other countries (Husid and 

Gajardo, 1970; Eisenberg and others, 1972; Husid 

and Espinosa, 1975; Espinosa and others, 1975). 

Wooden construction withstood damage well even 

when extensive damage was caused by ground compaction (Ray Wilson, oral commun., 1976) beneath 

the building (fig. 51).

Many school buildings were severely damaged by 

this earthquake, and, if the earthquake had occurred 

during class time, the death toll would have been 

larger. The second story of a three-story reinforcedconcrete frame structure with masonry walls at the 

Colegio San Javier collapsed (fig. 52). In the same 

school complex, a second building, next to the one 

that partially collapsed, was extensively damaged. 

There was no available information about the 

lateral loads used in the design of the school structures.

The Institute Guatemalteco Americano, a fivestory reinforced-concrete frame structure with 

poorly reinforced hollow brick walls sustained extensive damage. This structure has rather large 

cantilevered slabs in its perimeter supporting very 

heavy concentrated loads (reinforced-concrete ornaFIGURE 52.—Collapse of the second story of a building at the 

Colegio San Javier (Zone 12), Guatemala City.

ments and hollow brick walls) at their free end. 

Most of the walls were on the verge of collapse, 

and the slabs showed severe cracks in the cantilevered area. A slab on the penthouse partially collapsed, and reinforced-concrete columns and beams 

showed severe damage at the same level. It is important to note that this school building, which 

sometimes houses more than 2,000 students, has 

only one stairway. If the earthquake had occurred 

when 2,000 students were attending classes, many 

could have been injured as a result of panic and lack 

of adequate exits.

A three-story framed reinforced-concrete structure (fig. 53) was partially collapsed (Zone 12) 

when columns on its second floor failed.

Severe damage to several hospitals in Guatemala 

City created a serious problem because of the large 

number of injured people therein. Included were the 

Hospital Neuro-Psiquiatrico (Zone 7), Sanatorio 

San Vicente (Zone 7), Hospital Roosevelt (Zone 

11), and the Nursery School of Casa del Nine No. 

1 (Zone 1).

The Cathedral of Guatemala City, which was partially destroyed during the 1917-18 earthquakes
DAMAGE AND ENGINEERING IMPLICATIONS 69

FIGURE 53.—Partial collapse of a three-story reinforced-concrete structure due to failure of columns in its second 

floor (Zone 12), Guatemala City.

(Penney, 1918; Seismological Society of America 

Bulletin, 1911-1975, vol. 8, no. 1, p. 38-39), was extensively damaged, and both front towers were on 

the verge of collapse.

A new church, Iglesia del Divino Redentor (fig.

54), sustained extensive damage, including collapsed 

roof and walls (Zone 11). The roof was supported 

from the bottom chord of steel trusses that failed 

and caused the collapse of the walls. The overlapping 

of vertical steel bars was done at the same level, 

and the separation of ties contributed to the weakness of the structure. Other churches were also 

seriously affected in Guatemala City, and some were 

completely or partially collapsed.

The Hotel Terminal (Zone 4), a reinforced-concrete frame, flat-slab, six-story building, collapsed 

when several columns at the third floor failed (fig.

55). Simple ties were used, and they were widely 

spaced. This column failure appears to be similar 

to failures seen in the upper story of the Student 

Union Building of the Universidad Agraria Nacional in Lima, Peru, after the October 3, 1974, 

earthquake (Husid and others, 1976).

Numerous gas-station structures, designed in the 

form of inverted pendulums, collapsed when the 

welding failed (fig. 56).

The International Airport building sustained 

minor damage, and, in one area, ground compaction 

occurred beneath the western part of the structure. 

An inspection of the airport building was made at 

the request of the airport commander. The two 

side concourses (fingers) off the main structure were 

designed as two-story inverted-pendulum-type structures. These fingers had one stair exit, a severe hazard in case of fire or earthquake.

Various elevated steel water tanks collapsed in 

Guatemala City and vicinity. Figure 57 shows one 

type of tank that suffered complete failure. Figure 

58 shows the engineering plan of another type of 

elevated steel water tank (50,000-gallon capacity) 

that was used extensively throughout the city and 

failed in several locations. Three water tanks of 

this design that were full of water at the time of 

the earthquake collapsed (fig. 59). Two other tanks 

of this design that were empty sustained extensive 

damage; the anchor bolts had sheared, and most of 

the diagonal bracing failed.

A few corrugated-steel grain silos collapsed (fig. 

60), and some that remained standing sustained extensive damage where they were connected to the 

foundation.

An inspection was made of the XAYA-PIXCAYA 

project, which, when it is completed, will provide 

water for Guatemala City. The sedimentation tank 

and the Plant Lo De Coy are located near Mixco. 

The main event, or possibly its aftershocks, created 

a new system of fractures that crosses beneath the 

sedimentation tank and other structures (fig. 61). 

These fractures may be related to renewed movements on an old fault. The existence of steep slopes 

in this area creates a potential for landslides, especially if cracks should develop in the sedimentation tank and water should leak into the ground.

Surface breakage on north- to northeast-striking 

secondary faulting (Zone 19) in Colonia San Francisco locally generated extensive damage to local 

residences (fig. 62).

In San Jose Rosario, a subdivision of Guatemala 

City, surface breakage with both vertical and rightlateral displacement occurred on a secondary fault 

parallel with the San Francisco fault. This surface 

rupture was in an area where plans have been made 

for construction of high-cost new homes (fig. 63).

Three central spans of the Agua Caliente Bridge, 

about 36 km northeast of Guatemala City, collapsed, 

but the piers remained undamaged. The spans were 

pinned at one end and had rocking rollers at the 

other. The failure appears to have been generated 

by the local failure of the supports caused by large
70 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 54.—Collapse of a new church, Iglesia del Divino Redentor, Guatemala City (Zone 11).

displacements of the deck (fig. 64). An old steeltruss railroad bridge near the Agua Caliente Bridge 

sustained no damage.

The Benque Viejo multiple-span steel-truss bridge 

across the Platanos River was near collapse because 

of the large relative displacements between the 

trusses and their supports. The rollers have snapped 

flat between the upper and lower bearing plates 

(fig. 65).

GENERAL OBSERVATIONS

During the damage survey, it was found that 

similar problems recurred many times. Some of 

them are briefly described below.

The use of nonstructural masonry walls in reinforced-concrete framed structures was frequently

neglected in the design, as has been observed in 

previous earthquakes, for example, the 1970 and 

1974 Peruvian earthquakes (Husid and Gajardo, 

1970; Berg and Husid, 1973; Husid and others, 

1976). When lateral displacements occur in structures, resisting elements are loaded in proportion 

to their stiffnesses. Hence, short columns will be 

loaded with far greater shear forces than long 

columns. A column having a free height H has a 

stiffness approximately eight times greater than 

that of a column of equal cross section and height 

2H. Hence, the short column carries a lateral load 

approximately eight times larger than that carried 

by the larger column. One of the many examples 

of this kind of failure observed in Guatemala is 

shown in figure 66.
DAMAGE AND ENGINEERING IMPLICATIONS 71

FIGURE 55.—Collapse of the Hotel Terminal, caused by the failure of reinforced-concrete columns in its third story. This

building is located in Guatemala City (Zone 4).

The lack of reinforced-concrete columns framing 

masonry walls has been shown to be responsible for 

heavy losses during earthquakes. Experimental 

studies concerning the behavior of masonry walls 

subjected to lateral loads have shown that, when 

such walls are designed with integral framing or 

edge members, their behavior is almost ductile, even 

after cracking. When the masonry walls are not 

so framed, their lateral failure is extremely brittle 

and sudden, even for smaller lateral loads (Jorquera, 1964).

A common practice in Guatemala is to build nonstructural unreinforced masonry walls in tall structures. The behavior of these walls was very poor, 

as in previous earthquakes. An example is shown 

in figure 67, where brick falling from the 10th and 

llth stories of a building destroyed the slab roof

and part of the supporting structure of the first 

floor. A similar problem was reported by Husid, 

Espinosa, and de las Casas (1976) at the Industrial 

Bank of Peru in downtown Lima, after the October 

3, 1974, earthquake.

Several one-story reinforced-concrete framed 

structures collapsed completely, and the quality of 

concrete did not seem to be a factor that could 

justify the failure. It was observed that distances 

between ties in columns were rather large and exceeded the American Concrete Institute specifications, especially close to the slab and ground levels 

(fig. 68).

Heavy parapets located in the upper part of front 

facades, which are severe hazards for the population during earthquakes, collapsed in many areas 

throughout the capital city.
72 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 56.—Collapse of inverted-pendulum gas-station structure, Guatemala City (Zone 7).

Although chimneys are not common on homes in 

Guatemala City because of the mild weather, several 

of those that did exist collapsed.

Adobe construction is not earthquake resistant; 

some examples of the unsatisfactory performance 

of adobe is shown in figures 69A, B, C, and D. Considering the fact that it is not economically feasible 

to eliminate adobe construction in Guatemala, it 

would be desirable to make an inventory of the different types of adobe used countrywide and prepare recommendations for simple modifications that 

might improve the strength of adobe houses subjected to earthquakes.

A common design for multistory structures in 

Guatemala City utilizes principal frames in the 

transverse direction only. The horizontal loads in 

the longitudinal direction are resisted with a 

"pseudorigid" frame in which the normal beams 

are replaced by wide strips of slab (flat beams) 

that join the tops of the columns. As an example, 

figure 70 shows a five-story reinforced-concrete 

structure that sustained serious damage when flat 

beams were almost destroyed; the building had to 

be evacuated. Buildings of this design are usually 

very flexible and have long fundamental periods. It 

is probable that the behavior of the multistory buildings would have been less satisfactory if they had 

been subjected to a stronger ground shaking or to 

shaking of similar amplitude but longer periods.

A close look at the effects of the February 4 

earthquake in Guatemala City shows the following 

aspects:

1. Strengths of reinforced-concrete lateral-loadresisting elements were often unrelated to 

their stiffnesses.

2. Masonry filler walls in multistory buildings 

lacked minimum reinforcement.

3. Numerous heavy parapets collapsed and created 

a serious hazard in Guatemala City.

4. Reinforced-concrete column ties were frequently 

too widely spaced and sometimes not adequately hooked.

5. Brick walls often lacked reinforced-concrete 

corner columns, and long walls also lacked 

intermediate reinforced-concrete columns.

6. Surface breakage on secondary faulting occurred in developed areas.

7. Adobe construction sustained heavy damage. 

Adobe houses did not have any edge members.

8. Heavy roofs of adobe and unreinforced masonry 

houses frequently collapsed.

9. Elevated water tanks often failed. From the 

number of collapses, types of connections, 

and sizes of resisting elements, it is suspected 

that the lateral-force seismic coefficient used 

for the design was too low.

10. Corrugated-steel grain silos frequently failed, 

and several collapses were observed.
DAMAGE AND ENGINEERING IMPLICATIONS 73

FIGURE 57.—Collapse of an elevated steel water tank in the Institute Tecnico Vocacional in Guatemala City (Zone 13) .

Foundation Plant 

Tank Cross Section

0,6m 0.6m

SCALE 1 = 75

1,12m

6,10m

1.22m

1.12m

= 0.65cm

6.10m -t= 0.65cm

t = 0.65 cm

8.15m

1.22m

15.0m

0.4m

-6,10m- Plate t=l.3cm27.3cm

Elevation View

n at 16cm— -^

Footing

t — -• -

0w

L_

t^-6(^2.2cm

,

1 1 <£2.2cm Both Directions

FIGURE 58.—Engineering plans of an elevated water tank shown in figure 59.

(0, diameter.)
74 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 59.—Collapse of an elevated steel water tank in Villanueva, Colonia de los Planes, 22 km south of Guatemala City.

FIGURE 60.—Collapse of a corrugated-steel grain silo in Villalobos, 5 km southeast of Guatemala City.
DAMAGE AND ENGINEERING IMPLICATIONS

Operations 

Building

75

1

FIGURE 61.—Plan of sedimentation tank and Plant Lo de Coy 

of the XAYA-PIXCAYA project and cracks (heavy lines) 

running under and across the sedimentation tank.

FIGURE 62.—Severe damage to reinforced-masonry construction * 

caused by secondary faulting in Colonia San Francisco 

(Zone 19). The fault strikes north to northeast.

200m
76 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 63.—Rupture across the San Jose Rosario subdivision.

Fault with 13 cm vertical and 5 cm right-lateral displace- FlGURE 64.—Collapse of three central spans of the Agua 

ments (Zone 19) Caliente Bridge, Kilometre 36 on the road to the Atlantic Ocean. This bridge was constructed in 1959.

„ __ _ TT . . „ .. , „ FIGURE 66.—Failure of short column in three-story framed FIGURE 65.—Benque Viejo Bridge, on the verge of collapse • f •, . . . ^ , i r,., /r7

,. ^.. .J , , remforced-concrete structure in Guatemala City (Zone aiter failure of supports, shown by arrow. „,
DAMAGE AND ENGINEERING IMPLICATIONS 77

FIGURE 67.—Partial collapse of protruding first-story structure of the Cruz Azul 11-story .reinforced-concrete building caused 

by falling (shown by arrows) of masonry walls from the two topmost stories, in Guatemala City (Zone 1).

FIGURE 68.—Collapse of one-story reinforced-concrete framed structure in the Licorera Mixco, Mixco.
78 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 69.—A, Massive destruction of some adobe houses in Antigua, Guatemala, near the center of town. B, Large-scale destruction of adobe houses in Guatemala City, Calle 2 and Avenida 9A (Zone 2). C, Collapse of adobe houses in Guatemala City, Calle 22A and 34 Avenida (Zone 5). D, Collapse of adobe houses in Guatemala City, Calle 22 and Avenida 

32 (Zone 5).
DAMAGE AND ENGINEERING IMPLICATIONS 79

FIGURE 70.—Severe damage to flat beams and slab in the five-story reinforced-concrete Edificio ELGIN, in Guatemala City

(Zone 14).
THE GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

STRUCTURAL ENGINEERING OBSERVATIONS IN GUATEMALA CITY

By KARL V. STEINBRUGGE

INTRODUCTION

This paper reports on earthquake damage to 

buildings in Guatemala City and places special emphasis on earthquake-resistant structures and 

other building types having engineering relevance 

to construction in the United States.

The author inspected in detail the interiors and 

exteriors of about 25 major structures and, in a few 

instances, the construction drawings. His engineering colleagues, who formed a team under the auspices of the Earthquake Engineering Research Institute, collaborated on data collection, and they 

have had some input to this report. The views expressed in this report, however, are those of the 

author. This report is preliminary, and some details 

may change with further evaluation upon completion of the data collected.

LIFE LOSS AND CONSTRUCTION

The population of Guatemala City is about 

700,000, about 1,200 of whom may have died in the 

earthquake, according to one source (table 9). The 

fatality rate of 1 person in about 600 for Guatemala 

City contrasts sharply with the very high rates 

for some of the villages and small cities located much 

closer to the fault rupture. The deaths in Guatemala City resulted essentially from the collapse of 

adobe construction; this type of construction seems 

to be more prevalent in the northern and northwestern sections of the city. These zones were closer 

to the fault and to the source of seismic energy. In 

contrast to the damage patterns found in some locations near the fault (Espinosa and others, this report), widespread flattening of city blocks of adobe 

construction was not found in Guatemala City, 

which lies about 25 km from the fault break (Plafker and others, this report).

In contrast to the heavy mass construction such 

as adobe, light-mass all-metal structures, including

one-story warehouses and aircraft hangars, performed excellently. Any comparative wind versus 

earthquake analysis would show that, if these lightmass structures can withstand a moderately strong 

windstorm, they can survive a major earthquake.

DESIGN AND CONSTRUCTION PRACTICES

As in many Latin American countries, Guatemala has no effective building code or enforcement 

procedures comparable to those found in the United 

States. Each design professional is allowed to establish his own design criteria and supervise his own 

construction without independent scrutiny, on the 

basis of his status as a registered professional. This 

practice does not preclude good construction, but, 

in effect, it places no limitations on the extent of 

poor construction. On the other hand, the local structural engineers and architects are, in general, excellent design professionals and comparable to their 

U.S. counterparts. Many Guatemalan professionals 

have been competently trained, as was seen from 

a brief review of several sets of construction drawings and from discussions with them. This is not to 

say, however, that poor design does not exist.

The quality of construction appears to be generally good for the newer buildings. High-strength 

concrete seems to be common. The quality of the 

reinforcing steel, however, may be open to question in some instance, since this steel does not appear to be locally tested and the strength characteristics (and other qualities) of imported steel often 

are not known.

There apparently are only two structural-steel 

multistory buildings in Guatemala City, the Finance 

Ministry Building (fig. 71) and the National Theater (which is only partially steel frame). In these 

two known instances, special quality-control efforts 

were made during fabrication and erection, and no 

significant damage occurred to them.

80
STRUCTURAL ENGINEERING OBSERVATIONS IN GUATEMALA CITY 81

FIGURE 71.—The Finance Ministry Building (in Guatemala 

City) has 19 stories above ground plus 3 below. The 

undamaged steel-frame building is in the final stages of 

construction. (Photograph by W. H. Smith, American 

Iron and Steel Institute.)

Reinforced-concrete floors and roofs in major 

multistory buildings normally were of "waffle" construction. Forms for the waffles were customarily 

wooden rather than metal or plastic. Lateral-force 

resistance was commonly provided by frame action 

that utilized the waffle slabs and concrete columns, 

but some designs utilized reinforced-concrete shear 

walls.

Exterior walls were normally of reinforced-unitmasonry construction and were considered to be 

nonstriictural for design purposes. These unitmasonry panel walls were strengthened by the inclusion of reinforced-concrete-bond beams and 

equivalent vertical members. This bracing system 

did what it was designed to do. There was no major 

fallout of these panels where the building remained 

intact. The exterior wall panels that fell from the 

Seguros Building were not reinforced.

Since the large majority of multistory buildings 

have been built in recent years, design and construction practices are modern. Guatemalan practices generally follow U.S. practices, and therefore 

many Guatemalan buildings will perform like those 

in the United States. For the usual building design, 

the Uniform Building Code appears to be the norm 

for seismic design.

SPECIFIC DAMAGE OBSERVATIONS

Tall buildings that were in close contact pounded 

together. Structural damage was minimal in most 

cases, and architectural damage was usually not 

excessive. However, whenever a building consisted 

of two independent structural units, such as a stair 

and/or elevator tower that was structurally independent from the rest of the building, then the electrical, plumbing, and mechanical equipment, and so 

on was broken at the floor lines, and the functional 

capacity of the building was reduced. One example 

of pounding damage was observed between the stair 

tower and the main structure of the 13-story Seguros Building where, at the roof, a 7-cm separation 

was observed.

Folded plates and shells performed well, and no 

instances of significant internal damage are known. 

(The author, however, understands that there were 

one or more instances of such damage, but they remain unconfirmed.) The International Airport 

shown in figure 72 is one example of a folded-plate 

structure where minor spalling occurred at the tops 

and bases of columns, footing rocking was observed, 

some glass broke (see open windows in fig. 72), 

structural separations showed significant differential movements, and the stairs ajcting as diagonal 

braces were damaged, but the folded-plate roof sustained no damage.

A second example of folded plates and shells is 

the Universidad del Valle de Guatemala y Colegio 

Americano de Guatemala. The one-story hyperbolic 

paraboloid roofs performed well, but certain reinforced-concrete columns supporting these roofs were 

badly damaged. This damage can be attributed to 

the "structural performance of non-structural" 

brick panel walls that framed against these columns. 

These well-built brick walls did not fail, but they 

stiffened the columns to the point that they resisted a disproportionate share of the lateral forces.

Clearly, a better understanding of the importance 

of "non-structural" walls is vital to Guatemalan 

engineers as well as to many U.S. architects and 

engineers. Brick-infill walls between exterior columns as well as brick interior "partitions" used for 

tenants' improvements within a building were often 

well constructed and of rather substantial strength. 

Not only did these "non-structural" walls change 

the dynamic characteristics of the building, but 

often they also led to column failure or other significant damage. Another example of this kind of 

damage was observed at the two-story administration building adjacent to the airport control tower.
82 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

* I ? 7 I7TI17 7

FIGURE 72.—International Airport at Guatemala City. Note that some windows are broken.

FIGURE 73.—Hotel Terminal in Guatemala City. Columns in 

third story collapsed.

Restated, a brick "non-structural" panel wall is an 

effective shear wall in a concrete frame as long as 

the "non-structural" wall remains intact.

Few instances of precast-concrete construction 

were inspected. One hospital structure, being a 

one-story "shear wall" building, had no damage to 

its roof, which consisted of a long span of precast 

concrete planks ("Spancrete"). There was no interconnection between the planks. A precast-concrete 

double-tee roof at the Universidad del Valle collapsed, but this building was in the course of construction, and its final bracing system was incomplete.

In general, building performance ranged from 

good to excellent, but several spectacular failures occurred. For example, in the Hotel Terminal (fig. 

73), columns failed in the third story (fig. 74). 

Another example is the dormitory-classroom unit 

of the Catholic Boys School (Colegio San Javier), 

which collapsed owing to failure of its second-story 

columns (fig. 75). Figure 76 shows a collapsed 

column at the second floor of this unit. The time 

of night at which this earthquake occurred prevented major life loss in this building, since the 

first and second floors are used as classrooms.

A large number of hospitals were evacuated in 

Guatemala City owing to structural damage and to 

functional impairments.

Functional problems caused in multistory buildings were commonly in the form of elevator outages, 

and most elevators were still nonoperational 2 weeks 

after the event. Standby power remained in service
STRUCTURAL ENGINEERING OBSERVATIONS IN GUATEMALA CITY 83

FIGURE 74.—Typical collapsed column in Hotel Terminal.

FIGURE 75.—Collapsed second story in one unit of the Catholic Boys School in Guatemala City. Roofs were originally at the

same level.
84 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

FIGURE 76.—Detail of column failure in second story at the Catholic Boys School. Story height now measured in inches.

at almost all the locations inspected by the author, 

but, undoubtedly, instances of failure did occur. 

Batteries required for standby power generally did 

not shift or, if they shifted, did not fall or break 

their connecting cables.

The nearest American counterparts to Guatemalan adobe buildings are the brick structures in 

the older sections of almost every U.S. city, including San Francisco and Los Angeles. These nonearthquake-resistant brick buildings, with their 

sandlime mortar that "holds brick apart," are a 

major potential source of loss of life.
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86 GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT

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CHRONOLOGICAL HISTORICAL RECORD OF 

DAMAGING EARTHQUAKES IN GUATEMALA, 1526-1976

[Reference sources listed at end of table. Dashes mean no data, h is the depth of focus 

for the earthquake. Asterisks mark earthquakes plotted in fig. 3. Station codes are given 

in the Glossary]

NONINSTRUMENTAL DATA

Date Time (local time) Place Reference

1526 July 19 or 20 _____ _____ 3

Damaging earthquake that caused removal of Spanish settlement to 

Ciudad Vieja.

1541 September 10* _____ Ciudad Vieja 1,3

Two destructive earthquakes killed approximately 150 Spaniards and 

at least 600 Indians and Negroes. Two days of rain prior to the 

earthquakes added damage from avalanches and landslides. The 

primary Spanish settlement then moved to Antigua.

1565 February _____ _____ 3

Series of violent earthquakes caused extensive damage in and near 

Antigua.

1575 ____ _______ ______ 1,3

Several large shocks caused damage in San Salvador and also in 

Antigua, Guatemala.

1577 November 30 _____ _____ 3 

Earthquake swarm. Largest shock caused much damage in Antigua.

1586 December 23* _____ _____ 1,3,6

Long earthquake sequence beginning January 16, 1585, and ending 

with largest shock on above date. Accompanied by eruption of Fuego 

Volcano. Antigua was destroyed, causing many deaths.

1607 April* ______ _____ 3,6 

Many buildings collapsed, killing a number of people in Antigua.

1651 February 18 13:00 _____ 1,3 

Extensive damage in Antigua.

1681 July 22 _____ _____ 3 

A swarm of earthquakes caused extensive damage in Antigua.

1684 August _____ _____ 3 

Earthquake swarm caused notable damage in Antigua area.

1689 February 12 _____ _____ 1,3,6

Earthquake swarm caused extensive damage and loss of life in 

Antigua area. Stronger than shock of 1651.

1702 August 4* _____ _____ 3,6 

A strong earthquake caused extensive damage in Antigua.

1717 September 29 19:00 and 9:00 _____ 1,3,6 

and 30*

Antigua damaged on September 29, destroyed September 30; loss 

of life was extensive. Large aftershock on October 3. Earthquakes 

accompanied by violent eruptions of Fuego.

1751 March 4 8:00* _____ 1,3,6 

Antigua damaged. Cathedral dome destroyed.

87
88 CHRONOLOGICAL HISTORICAL RECORD—Continued

NONINSTRUMENTAL DATA—Continued

Date Time (local time) Place Reference

1765 April April 20* £f\j' ________ ________ o

Fifty killed and many injured; many towns destroyed in the Department of Chiquimula. Earthquake may have originated on the 

Motagua fault.

1765 October _____ _____ 1

The earthquake of "San Rafael" severly damaged many towns in 

Guatemala.

1773 July 29 15:45* _____ 1,3,6

This major event was part of an earthquake swarm beginning in 

May and continuing until December. Very strong shocks occurred 

on June 11 at 5:00 and 17:00 hours (local time). Large aftershocks occurred on September 7 and December 14. Antigua was 

completely destroyed, and many deaths resulted. The capital was 

then moved to Guatemala City. These earthquakes were felt even 

more strongly in Chimaltenango and Quezaltenango, nearer to the 

Motagua fault, and thus the Motagua fault may have been the 

source of these earthquakes.

1830 April 1 _____ _____ 3

Swarm, similar to that of 1773, destroyed many buildings in Antigua. 

Major aftershock on April 23.

1852 May 16 _____ _____ 1 

Damage in the vicinity of Quezaltenango.

1853 February 9* _____ _____ 3

Major earthquake caused great alarm in Quezaltenango. Also strongly felt in Antigua and Amatitlan.

1855 January 1-26 _____ _____ 3

Swarm with main events on the 18th and 26th. Damage at Cantel 

and Zunil.

1859 Decembers 20:15 _____ 1,3

Major earthquake near El Salvador-Guatemala border. Houses were 

shattered in Escuintla and Amatitlan. Tsunami at Acajutla, El 

Salvador.

1860 December 19* _____ _____ 6

Extensive damage to churches and homes in Escuintla. Aftershocks 

continued until December 31.

1861 August 27 _____ _____ 3 

Damage to homes and churches in Conquaco and Jalpatagua.

1862 December 19* _____ _____ 1,3

Antigua, Amatitlan, Escuintla. Tecpan, and neighboring towns were 

severely damaged. Damage to many churches and ancient constructions. Slight damage to old churches in Guatemala City; 

astronomical observatory reported tilt of 3'28''.

1863 December 12' _____ _____ 6

The earthquake, centered near Guatemala City, caused changes in 

flow of springs in the northern part of the city, and earth fractures opened in the areas of Jocotenango and El Bosque, causing 

much panic throughout the city.

1870 June 12 15:00 _____ 1,3,6

Extensive damage in the regions of Chiquimulilla, Cuilapa, and Ixhuatan. A later quake at 18:23 (local time) caused serious damage 

in Cuilapa. Aftershocks continued until the 23d.

1874 Septembers 21:00* _____ 1,3,6 

(or possibly 18)

Antigua, Chimaltenango, and Patzicia were destroyed and 200 people 

killed.
CHRONOLOGICAL HISTORICAL RECORD—Continued 89

NONINSTRUMENTAL DATA—Continued

Date Time (local time) Place Reference

1881 August 13 12:30 _____ 3 

Earthquake swarm felt in San Marcos; possible damage in Chinigue.

1885 November 22 --——__ _____ 3 

Strong shocks; damage at Amatitlan.

1885 December 18 17:36 _____ 3

Amatitlan destroyed (intensity—IX, Rossi-Forel). Cracking of 

ground; new hot springs on shores of Lake Amatitlan. Many foreshocks and aftershocks into January 1886. Volcano Pacaya increased level of activity.

1902 April 19 20:20 lat 14.0° N., long 91.0° W. 4,5,6

Magnitude 8.3 (PAS)

Destroyed the city of Quezaltenango. Extensive loss of life. Activity 

continued until September 23, when an earthquake was strongly 

felt and an eruption of volcano Santa Maria began.

1912 June 12 12:43 42.0 lat 17.0° N., long 89.0° W. 4

Magnitude 6.8 (PAS)

1913 March 9 9:49* _____ 6

Strong earthquake felt in the central region of Santa Rosa and also in 

the Departments of Cuilapa and Santa Rosa of Lima. Felt over 

large parts of the country. Many deaths and much destruction.

1915 September? 01:20 48.0 UTC lat 14.0° N., long 89.0° W. 2,4,5

Magnitude 7.9 (PAS)

Heavy damage in Jutiapa. Felt strongly over large areas of Guatemala and El Salvador.

1917 December 25 22:20* _____ 2,6,8

A series of earthquakes that began on November 17, 1917, and continued into January 1918. December 25 main event had 

magnitude of 6 plus and maximum Modified Mercalli intensity of 

VIII-IX. In Guatemala City, cracks opened in the streets, and 

about 40 percent of the houses were destroyed or seriously damaged. The Colon Theatre collapsed while filled with people; school 

buildings, churches, asylums, hospitals, sugar mills, the post office, 

the railway station, and the British and American legation buildings 

were thrown down, and many occupants were killed or injured. 

Later destructive earthquakes occurred on December 29 (14h), 

January 3, 1918 (22:37), and January 24 (19:30). The most 

destructive of this series was the January 3, 1918, earthquake.

INSTRUMENTAL DATA

Date

1919 Apr. 17

1921 Feb. 4

1929 Jan. 17 1

1931 Sept. 26

1931 Sept. 26

1932 May 22

1934 May 19

1939 Sept. 28

1939 Dec. 5

1940 July 27

1942 Apr. 11

1942 Aug. 6 2

1942 Aug. 8

1943 Aug. 31

1943 Sept. 23

1944 Oct. 2

1945 Aug. 10 3

1945 Oct. 27

1946 Jan. 5

1946 June 26

Time 

UTC

20 53 03.0

08 22 44.0

19 00

19 50 30.0

20 03 07

22 40 02.0

10 47 37.0

14 58 27.0

08 30 07.0

13 32 30.0

01 25 12.0

23 36 59.0

22 36 34.0

16 10 40.0

15 00 44.0

17 22 00.0

11 20 03

11 24 41.0

01 15 11.0

07 53 40

Location

N. Lat W.

14.5

15.0

15.0

15.5

14.2

14.8

15.5

14.5

14.2

14.7

14.0

14.2

14.2

15.0

14.5

15.4

15.0

15.0

14.7

Long

91.7

91.0

92.0

91.5

90.0

91.2

91.5

91.5

91.5

91.5

91.0

91.5

91.5

91.5

89.8

88.8

91.2

91.0

90.8

h 

(km)

120

60

——

80

120

110

90

140

___

80

110

160

200

210

90

Magnitude 

(Richter)

7.0

7.5

6.0

6.25

6.0

6.25

6.25

6.75

6.75

6.5

8.3

6.5

6.75

6.75

6.5

6.75

6.0

6.5

Station 

code

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

PAS

Reference

4

4,5

2

4,5

4,5

4,5

4,5

4,5

4,5

4,5

4,5

4,5,6

4,5

4,5

4,5

4,5

2

4,5

4,5

4,5
90 CHRONOLOGICAL HISTORICAL RECORD—Continued

INSTRUMENTAL DATA—Continued

Date

1948 July 16 4

1949 July 8

1950 Feb. 17

1950 Oct. 23 s

1950 Oct. 23

1950 Oct. 23

1950 Oct. 24

1950 Oct. 24

1950 Oct. 28

1950 Nov. 5

1951 Jan. middle

1951 July 25

1953 Aug. 24

1953 Nov. 17 7

1954 Oct. 21

1955 Aug. 28

1955 Sept. 3

1957 July 8

1959 Feb. 20

1959 Mar. 9

1960 Apr. 13 8

1960 Aug. 20

1961 June 17

1961 Sept. 1

1965 Aug. 5 9

1966 Aug. 18 10

1969 Apr. 21

1971 Oct. 12 n

1972 Jan. 22 TM

1973 June 7 13

1974 Dec. 31 M

Time 

UTC

07 12 28

07 19 39

12 40 47.0

03 47 16.0

16 13 20.0

17 47 51.0

23 38 43.0

00 52 03.0

05 50 15.0

22 15 47.0

16 35 20.0

of month *

18 42 19.0

13 21 02.0

13 29 52.0

06 51 44.0

20 13 30.0

12 36 21.0

15 30 33.0

18 16 20.0

22 03 03.0

12 37 43.0

00 19 35.2

15 07 33.7

18 50 35.4

19 05 07.7

10 33 17.7

02 19 07.1

09 44 59.3

13 08 50.3

18 32 42.9

20 21 09.0

Location

N. Lat W.

14.6

14.3

14.0

13.9

14.5

14.5

14.3

15.0

14.5

14.3

14.5

14.5

14.1

13.8

13.9

14.0

13.8

14.5

15.9

15.1

15.5

14.5

14.2

13.6

14.8

14.6

14.1

15.8

14.0

14.3

14.1

Long

91.2

91.2

91.5

90.8

91.5

92.0

91.7

92.0

92.0

91.7

92.0

90.5

91.4

91.8

90.6

91.0

90.8

91.0

90.6

91.0

92.0

91.5

92.0

92.5

91.0

91.7

91.0

91.2

91.0

92.0

91.8

h 

(km)

_

100

__

___

—

64

——

64

96

——

60

64

150

48

171

30

115

85

37

59

85

821

36

102

78

39

Magnitude 

(Richter)

6.25

6.75

6.0

6.25-6.50

7.1

6.5

6.1

6.2

6.0

6.2

6.25

6.25

6.5

7.25-7.50

6.5

6.75

6.50-6.75

6.0

6.5

6.3

6.0

6.0

6.0

6.5

4.0mb

6.0

5.9-6.2

6.0-6.25

5.9 nib

6.0

6.25

5.5 mb

6.0

5.7 Ms

5.7 mb

6.0

5.5 mb

6.2

5.5 m,b

6.0

6.1 Ms

5.7 m.b

Station 

code

PAS

PAS

PAS

PAS

PAS

PAS

PAS

BRK

TAG

PAS

PAS

PAS

PAS

PAS

PAS

PAS

BRK

PAS

TAG

PAS

PAS

PAS

PAS

COS

PAS

BRK

PAL

COS

PAS

BRK

COS

BRK

ERL

ERL

PAS

ERL

PAS

ERL

BRK

COS

COS

Reference

2

4,5

4,5

2,4,5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5,7

5

5

5

5,7

2,5

5

2,5

2,5

2,5

2,5

1 Considerable damage in Puerto Barrios.

2 Felt strongly throughout the central region of Guatemala; alarmed a large part of the population and 

caused considerable damage.

3 Numerous earth cracks near Quirigua and significant (moderate) damage in Quirigua. Felt in Departments of Chiquimula, Zacapa, and part of Izabal. 

* People were tossed from their beds in Guatemala City.

5 Near the coast of Guatemala. Damage in San Marcos area.

6 A series of strong earthquakes in the region of Ixhuatan in the Department of Santa Rosa that caused 

considerable damage in the area. 

T Near the coast of Guatemala.

8 Guatemala—Mexico border; one killed, 14 injured, and damage to the San Marcos area; also damage 

at Heuhuetenango.

9 One killed and four injured at a hydroelectric project when several workers were buried under dirt.

10 Felt at San Salvador.

11 Mexico-Guatemala region.

12 Felt (intensity V) at San Salvador, El Salvador.

13 Felt in Guatemala City area. 

References:

(1) Brigham (1887).

(2) Seismological Notes, Seismological Society of America Bulletin, v. 5-15, v. 8-18, v. 38-48.

(3) Montessus de Ballore (1888).

(4) Gutenberg and Richter (1954).

(5) NO A A Hypocenter Data File (1976, written commun.).

(6) Vassaux (1969).

(7) U.S. Earthquakes (1960, 1965).

(8) Claudio Urrutia E. (1976, written commun.).

COVER PHOTOGRAPH:

Surface fractures along the Motagua fault crossing 

a soccer field at Gualan, Department of Zacapa. The 

fractures, formed during the February 4, 1976, earthquake, show left-lateral strike-slip displacement; the 

ground on the right side of the photograph moved 

west (away from the viewer), and the ground on the 

left side of the photograph moved east (toward the 

viewer). Superimposed on this picture is the trace of 

the horizontal long-period low-gain seismogram, 

east-west component, recorded at Albuquerque Seismological Center, Albuquerque, New Mexico, U.S.A. 

Epicentral distance is 2,778 km.