GEOLOGIC AND SEISMOLOGIC ASPECTS OF THE 

MANAGUA, NICARAGUA, EARTHQUAKES OF 

DECEMBER 23, 1972 
Air view of central Managua looking south. Fault D passes obliquely across photograph and through 

the Central Bank which is heavily damaged. The adjacent Bank of the Americas is essentially undamaged. Many of the smaller structures that remain standing are badly damaged and will be razed. 

Extensive open areas in foreground are where structures have collapsed due to the earthquake and 

(or) fire. Much of the debris was already cleared away in the right foreground. 
Geologic and Seismologic Aspects of the 

Managua, Nicaragua, Earthquakes of 

December 23, 1972 

By R. D. Brown, Jr., P. L. vVard, and George Plafker 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 838 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1973 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600128 

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

Washington, D.C. 20402 

Stock Number 2401-00340 
Abstract ----------------------------------------

Introduction -------------------------------------

Purpose and scope ---------------------------

Acknowledgments ----------------------------

Geologic aspects of the earthquakes ----------------

Earthquake faults ----------------------------

Surface expression ---------------------------

Fractures with dextral displacement ----------

Lack of evidence for fault creep ---------------

Relationship of faults to structural damage ___ _ 

Similarities of 1972 faults to the 1931 earthquake 

fault ------------------------------------- Geologic evidence for previous faulting ---------

Landslides and surficial effects ----------------

Seisrnologic aspects of the earthquakes -------------

~ethods ------------------------------------

CONTENTS 

Page 

1 

1 

2 

3 

4 

4 

4 

13 

13 

14 

16 

17 

17 

18 

19 

Seisrnologic aspects-Continued 

Distribution of aftershocks -------------------

Nodal plane solutions ------------------------

Comparison to similar earthquakes ------------

Setting of the earthquakes ------------------------

Regional tectonic relations -------------------- Physiography _______________________________ _ 

Near-surface rock units ---------------------- Ground-water relations ____________________ ---

Volcanic risk at ~anagua ------------------------

Seismic risk at ~anagua -------------------------

Historic seismicity ---------------------------

A comparison -------------------------------

Conclusions -------------------------------------

Recommendations --------------------------------

References cited __ - _ -- ________ - _ -- ___ - ------------

ILL USTR.A. TIONS 

FRONTISPIECE. Air view of central ~anagua. 

Page 

20 

21 

26 

26 

2() 

27 

28 

29 

29 

30 

30 

30 

32 

33 

33 

Page 

PLATE 1. ~ap showing faults and fractures related to the Managua earthquakes of December 23, 1972 _______ In pocket 

FIGURE 1. 

2-15. 

16. 

17-19. 

20. 

21. 

22. 

23. 

Geologic map of Managua area showing earthquake faults ---------------------------------------

Photographs: 

2. Fractures and broken waterline ------------------------------------------------------- 3. Fault-displaced sidewalk _____________________________________________________________ _ 

4. Fault offset in railroad line ___________________________________________________________ _ 

5. Offset pavement tiles -------------------------------------------------------------- ___ _ 

6. Fault-displaced curb ------------------------------------------------------------------

7. Open fracture in fault zone -----------------------------------------------------------

8. En echelon fractures in fault zone -----------------------------------------------------~ 

9. Compressional effects in curb ----------------------------------------------------------- 10. Dextral displacement of curb --------- ________________________________________ -- ____ -- _ 

11. Collapsed Customs House office building -----------------------------------------------

12. Damaged horne in fault zone ----------------------------------------------------------

13. Partly damaged horne in fault zone -----------------------------------------------------

14. Collapsed tarquezal structures in fault zone --------------------------------------------- 15. View of fault scarp at Lake Tiscapa __________________________________________________ _ 

Schematic diagrams illustrating effect of faulting at the Tiscapa pit crater -----------------------

Photographs: 

17. Head of rotational landslide at Tiscapa crater rim ----------------------------------------

18. Large landslide near krn 11 on Leon-Managua Highway --------------------------------- 19. Minor debris slide along creek bank ____________________________________________________ _ 

~ap showing aftershock epicenters ------------------------------------------------------------

Sections showing aftershock hypocenters ____ ----------------- ________ --------------------------

Composite nodal plane solutions ----------------- ______ ----------------------------------------

Acceleration versus distance to fault relationships----------------------------------------------

v 

5 

10 

10 

10 

11 

11 

12 

13 

13 

13 

14 

15 

15 

16 

18 

19 

20 

21 

22 

23 

24 

25 

31 
VI 

TABLE 

CONTENTS 

TABLES 

1. Characteri:stics of fractures along the Managua faults ------------------------------------------

2. Crustal structure models used in the studY-----------------------------------------------------

3. Fault dimensions and slip for five earthquakes with magnitudes of 5.5 to 6.5 -----------------------

4. Comparison of fault density at Managua and vicinity with other urban areas in seismically active 

Page 

6 

20 

26 

zones ------------------------------------------------------------------------------------ 32 
GEOLOGIC AND SEISMOLOGIC ASPECTS OF THE 

MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

By R. D. BRowN, JR., P. L. WARD, and GEORGE PLAFKER 

ABSTRACT 

The Managua, Nicaragua, earthquake of December 23, 1972 

(Richter magnitude of 5.6, surface-wave magnitude of 6.2), 

and its aftershocks strongly affected an area of about 27 

square kilometers centered on Managua. Within this area, 

over 11,000 people were killed and 20,000 were injured. 

About 75 percent of the city's housing units were destroyed 

or rendered uninhabitable leaving between 200,000 and 

250,000 people homeless, and property damage exceeded half 

a billion dollars. As a consequence, the economy and government of the city, and to a large extent the entire country, 

were severely disrupted. 

Surface geology shows that there are at least four subparallel strike-slip faults spaced 270 to 1,150 meters apart 

in the Managua area that slipped in a predominantly sinistral (left-lateral) sense during the earthquake. Aftershock 

studies show that at least one of these northeast-trending 

faults extends from the surface to a depth of 8 to 10 km 

(kilometers) over a maximum length of about 15 km. The 

faults are mappable on land for 1.6 km, 5.1 km, 5.9 km, and 

2. 7 km; aftershock data indicate that faulting extends at 

least 6 km northeast of the city beneath Lake Managua. 

Horizontal displacements vary, with the maximum aggregate sinistral slip ranging from 2.0 to 38.0 centimeters. There 

is also a local small down-to-the-southeast vertical component of slip on three of the four faults. The nature and distribution of the surface faulting are consistent with a tectonic 

origin for the earthquake. 

The extensive destruction and loss of life in the Managua 

area were caused by a combination of the following factors: 

( 1) occurrence of the earthquake on faults directly beneath 

the city, (2) poor behavior of structures, chiefly tarquezal 

(wood frame and adobe) and masonry, during strong seismic 

shaking, and ( 3) direct displacement of structures, streets, 

and utilities by faulting. The historic record of seismicity 

and geologic evidence of active Holocene faulting and volcanism together show that Managua is an unusually high 

risk area in terms of geologic hazards and that these hazards 

should be a primary consideration in evaluating reconstruction of Managua. 

INTRODUCTION 

Managua, Nicaragua's political capital, its business and industrial center, and by far its largest 

city, was struck by three moderate-sized earthquakes within less than an hour in the early morning of December 23, 1972. The earthquakes 

and related surface faulting severely damaged the 

central part of the city, interrupted essential services, and, by their effect on Managua, severely disrupted the entire Nicaraguan economy. The first and 

largest earthquake was felt at 12 :30 a.m., local time. 

It was assigned a Richter magnitude, Mb, of 5.6 

(surface-wave magnitude, M87 of 6.2) by seismologists of the U.S. National Oceanic and Atmospheric 

Administration (National Oceanic and Atmospheric 

Administration, 1973). The two largest aftershocks 

were felt at about 1:18 a.m. and 1 :20 a.m. Both 

were smaller (Mb, 5.0 and 5.2) than the main shock, 

but were large enough to cause substantial additional damage. According to eyewitness accounts, 

many buildings that were structurally weakened but 

still standing after the main earthquake suffered 

additional damage or collapsed during these aftershocks. 

The earthquake sequence killed over 11,000 people 

and injured another 20,000, caused more than half a 

billion dollars property damage, and destroyed or 

rendered uninhabitable 75 percent of the city's housing units leaving between 200,000 and 250,000 people 

homeless out of a total Managua population of 

around 500,000. Interviews with residents of Managua indicate that many left their homes and moved 

into the streets as the shaking from the first earthquake subsided. Many of these people were still in 

open areas when the aftershocks were felt and thereby escap2d possible injury or death in the further 

collapse of buildings. Aftershock activity continued 

for weeks after the initial earthquake, with the frequency and magnitude of aftershocks progressively 

diminishing with time. All of the significant damage 

resulted either from the first three shocks or from 

fires that followed shortly thereafter. The earthquakes were of moderate size but caused extensive 

damage because (1) they occurred at shallow depth 

under the city, (2) at least four surface faults broke 

in and near Managua, and ( 3) most buildings had 

little resistance to seismic shaking. 

1 
2 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

An accurate evaluation of the geologic hazards 

and the possibility of future earthquakes like those 

of December 23 is critical to future development 

plans. Such evaluations have obvious applications in 

formal planning and in plan implementation by governmental bodies. Less obvious perhaps is the degree 

to which such evaluations are used by financial institutions, insurance companies, and by business and 

industry. In recent years, in various parts of the 

world, geologic knowledge concerning recognized active faults and other clearly identifiable geologic 

hazards has been increasingly applied by private 

industry to decisions on site selection, mortgage loan 

evaluation, and the setting of insurance rates. These 

nongovernmental decisions can profoundly affect the 

pattern of growth and development simply by directing or influencing the flow of investment capital 

away from high-risk sites and towards those where 

the level of risk is deemed more acceptable. 

Much current planning, both at governmental and 

private levels, reflects the viewpoint that earthquake 

safety in modern cities involves designing for the 

interaction of two complex systems : the manmade 

system that is the city itself, and the natural system 

consisting of the geologic processes that cause or 

accompany a major earthquake. Successful planning 

for earthquake safety involves far more than the 

prevention of structural failure in buildings. It 

should include, as well, ensuring the integrity of 

communication lines, water service, sanitation facilities, and emergency services such as police, fire, and 

hospital facilities. Such planning should also recognize that massive economic loss will recur in accordance with the recurrence rates of catastrophic geologic processes. Such losses are largely independent 

of structural design and construction practices, 

which are directed primarily to the safety of human 

lives, at least insofar as earthquake-resistant characteristics are concerned. Comprehensive urban 

planning for earthquake safety depends first of all 

on a clear understanding of the processes that accompany earthquakes and how these processes may 

affect the works of man. 

PURPOSE AND SCOPE 

This report on the earthquakes of December 23, 

1972, is intended to (1) record and interpret preliminary geologic and seismologic data and (2) 

evaluate these data as an aid for those who must 

make difficult decisions regarding future development and reconstruction in the Managua area. 

In order to assist the reader in finding the type of 

information he is interested in, we have separated 

the sections with data relevant to the 1972 earthquakes and their setting from those sections concerned primarily with the overall geologic hazards 

at Managua. Data pertaining to the earthquakes and 

their setting are in the following three sections: 

"Geologic Aspects of the Earthquakes," "Seismologic Aspects of the Earthquakes," and "Setting of 

the Earthquakes." Readers who are concerned primarily with risk at Managua as related to geologic 

hazards may wish to skip the data sections and turn 

directly to the sections entitled "Volcanic Risk" and 

"Seismic Risk." 

Many of the painful lessons learned in Managua 

may save hundreds of lives and millions of dollars if 

they are used to guide policy and planning at Managua and in other earthquake-prone regions. 

Among the topics that are critical to decisions on 

land use and redevelopment plans for Managua are 

several that are essentially geologic in nature. Those 

that are addressed here include: 

Identification of the various geologic processes that 

accompanied the earthquakes of December 23. 

An assessment of the relative importance of these 

processes. 

An estimate of the future hazard from similar or 

greater earthquakes within the Managua area. 

Geologic conditions that may suggest constraints or 

limitations on certain types of functions, land use, 

or structural or design types within the Managua 

area. 

The scope of this report is restricted to earthquake-related effects in the Managua area (that is, 

within a few miles of the city center), to geologic 

conditions within that area, and to the relations between observed damage patterns and geologic conditions. This range of topics is dictated by the brief 

nature of our field investigation and by its focus on 

these explicit problems. 

Nongeologic factors that also contributed to the 

extensive damage in the central part of Managua 

include design and construction practices, vulnerability of parts of the water system to fault rupture, 

age and stage of repair of structures, and effects of 

the emergency on disaster relief response. These 

nongeologic factors are being studied by other investigators and will not be discussed here. 

Many planning decisions may require answers to 

other geologic questions that are not addressed in 

this report. For example, a logical and reasonable 

question is: Are there sites within a few kilometers 

of Managua that are significantly safer from geologic hazards than is the site of the present city? 
INTRODUCTION 3 

Although the answer to this question may be yes, 

this is not very helpful unless such sites are identified and delineated. To do so, however, requires an 

evaluation of both earthquake and volcanic hazards 

and a careful appraisal of engineering geologic conditions. The present level of knowledge of the 

geology near Managua is inadequate to answer many 

important questions like this one, but a relatively 

modest geologic investigation could provide much of 

the essential data. The level of effort required probably amounts to 1 or 2 man-years and would cost less 

than a hundred thousand dollars. In view of the 

massive commitment of millions of dollars for reconstruction and redevelopment, this investment in 

evaluating alternative courses of action from a geologic perspective seems an obvious and essential step 

in the planning process. 

ACKNOWLEDGMENTS 

The authors of this report spent less than a month 

on the ground in the Managua area after the earthquake. To accomplish much of value in so brief an 

investigation requires the support, cooperation, and 

assistance of many people. It is not possible to acknowledge all here, but everywhere we received the 

most courteous cooperation from both Nicaraguan 

officials and private citizens during a time of great 

hardship for them. We are especially indebted to 

Capt. and Ing. Orlando Rodriguez M., Director, Servicio Geologico N acional, and to Ing. Humberto 

Porta C., Director General, Instituto Geografico N acional. These two men, and the organizations which 

they head, provided information, data~ logistics support, liaison, and coordination with other activities 

in connection with the disaster. Ing. Rodriguez deserves special thanks for providing our transportation and lodging and for arranging official permission for our investigations within restricted areas. 

We are also indebted to Mr. Lloyd Cluff, who preceded us to Managua by about 10 days. Mr. Cluff 

reported his observations of active fault breaks in 

Managua to us on January 2, and those observations 

helped guide our investigations and make them more 

effective. Investigations by Cluff and Mr. Gary Carver defined and accurately located the two central 

and largest fault breaks and established the amount 

and direction of movement on these. Our studies 

confirm their findings, add substantiating new data 

on these two faults, and provide similar data on two 

parallel but smaller faults. 

Evidently, others also identified these faults prior 

to our visit. A.n undated report by Juan Kuan S. and 

Carlos Valle G. "Informe Tecnico sobre el Origin del 

Terremoto de Managua," contains a map showing 

the approximate location of the four fault breaks 

and some of the points at which evidence of fault 

movement was observed. The Kuan-Valle report, 

although brief and relatively undocumented by observations, presents an accurate picture of the fault 

pattern. Unfortunately, Brown and Plafker did not 

learn of this report until after their fieldwork was 

completed. 

Aerial photography is critically important to postearthquake studies, especially where there is extensive surface faulting or other surface geologic effects 

that require precise location and careful measurements. Superb vertical stereophotographic coverage 

in both color and color infrared, at optimum altitude 

and sun angles, of the entire affected area was provided by the NASA Manned-Spacecraft Center in 

Houston, Texas, and was used in this study. Many 

NASA people at Houston worked long and irregular 

hours to provide timely post-earthquake aerial photography. We especially acknowledge and appreciate 

the efforts of the Mission Manager, Mr. Charles 

Harlan; the Earth Resources Program Office, Mr. 

Olav Smistad; and Mr. A. J. Roy, aircraft commander of NASA C-130 no. 929. 

Tent facilities during our field study were provided on the grounds of the residence of the U.S. 

Ambassador, Mr. Turner Shelton. We acknowledge, 

with thanks, the efforts by Ambassador Shelton and 

others on the U.S. Embassy staff to accommodate 

our field investigations at a time of extreme difficulty 

for the Embassy. 

The aftershock study was carried out with the 

skilled and dedicated assistance of Ing. Arturo 

Aburto Q. of the Servicio Geologico N acional, who 

worked 12 to 14 hours a day, 7 days a week, maintaining equipment and making arrangements with 

the various landowners and watchmen at the instrument locations. Dave Harlow, Dan Marquez, Jim 

Gibbs and AI Vaugh of the U.S. Geological Survey 

also participated in parts of the fieldwork, and Robert Page, Rob Wesson, Bill Ellsworth, Jim Ellis, 

John Lahr, and Bill Gawthrop worked hard to prepare the instruments for the field during the Christmas holidays. Mr. Leroy Anstead, Inter American 

Geodetic Survey Representative in Nicaragua, was 

most helpful in providing office space for data analysis and a place for the various earth scientists working in Managua to congregate and exchange ideas. 

He also assisted with many of the logistical arrangements. 
4 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

GEOLOGIC ASPECTS OF THE EARTHQUAKES 

By far the most important geologic effect of the 

earthquakes of December 23, 1972, was the tectonic 

movement that occurred on at least four subparallel 

surface faults in the Managua area (fig. 1). Warping along the fault zones and displacements on fractures along the fault caused direct damage to many 

buildings, streets, and utilities. Relatively minor 

secondary geologic effects of the earthquake include 

displacement on surface fractures not obviously related to the faulting and a variety of downslope mass 

movements. There was no evidence that compaction 

or liquefaction of the unconsolidated deposits played 

a significant role in the damage distribution. A reconnaissance study of the shoreline along the south 

shore of Lake Managua indicates that there was no 

significant earthquake-related regional tilting or 

relative land-level changes in that area. 

EARTHQUAKE FAULTS 

Four faults were identified in the Managua area 

along which displacement occurred during the earthquake of December 23d or its aftershocks (pl. 1, fig. 

1). The faults are manifested in the unconsolidated 

alluvial and pyroclastic surficial materials as continuous lines of open fractures or zones of en echelon 

fractures that consistently show a sinistral (leftlateral) sense of motion and locally show subordinate extensional and vertical components of displacement. In a few localities, particularly where 

there is appreciable topographic relief, it was not 

always possible to differentiate surface fractures related to faulting from fractures that may have been 

formed through surficial processes such as downslope slumping or lurching. To the extent possible, 

however, fractures mapped in the field and shown 

on plate 1 are those believed to be primarily of tectonic origin. 

Numbers on plate 1 show data points where surface faulting was observed. Details of the observation at each data point are given in table 1. Within 

the limitations of the map scale, we have tried to 

plot and describe as accurately as possible the distribution of surface fractures observed in the field. 

High-resolution 1:6,000- and 1: 10,000-scale vertical 

color photographs of the Managua area taken by 

NASA on December 27th and 28th enabled precise 

location of data points in the field. These photographs were also used to update the 1: 10,000-scale 

topographic base map of the Managua area (used 

for pl. 1) in the immediate vicinity of the mapped 

faults so that th2 data po~nts could be plotted accurately relative to streets, highways, major buildings, 

and other features. 

It is entirely possible that faults other than the 

four described herein moved during the earthquake 

sequence but were not identified during our brief 

geologic reconnaissance of the earthquake-affected 

area. The combined surface geologic and seismologic 

data described in the following sections clearly indicate, however, that the faults we have mapped include the most important ones along which displacement occurred during the earthquakes. 

SURF ACE EXPRESSION 

The four surface faults along which displacement 

occurred during the December 23d earthquake or 

during its aftershocks are subparallel and _trend 

northeastward across the Managua area. On plate 1 

these faults are labeled A through D from east to 

west. Faults A and B are about 850 m (meters) 

apart, faults B and C are 270 to 500 meters apart, 

and faults C and D are roughly 850 to 1,150 meters 

apart. The faults can be traced on land for the following distances: A-1.6 km (kilometers), B-5.1 

km, C-5.9 km, and D-2.7 km. All the faults die 

out on land to the southwest. Towards the northeast, fault A dies out on land but the other three 

faults extend to the shore of Lake Managua. The 

distribution of aftershocks, described in the following section, is in good agreement with the mapped 

southwestern limits of faulting and further suggests 

that one or both of faults B and C probably extend 

at least 6 km northeastward beneath Lake Managua 

approximately as indicated in figure 1. 

Zones of surface fractures along the faults vary 

considerably in width and number of constituent 

fractures, depending upon both the amount of displacement and the nature of the ground surface. In 

open fields displacement tends to be concentrated in 

a single fracture or in a well-defined band of en 

echelon fractures a few meters to 20 m wide. The 

fractures along fault C are effectively masked in 

cultivated and planted fields between the Circumferential Highway and the Nejapa Country Club. In 

built-up areas where rigid structures such as streets, 

curbs, sidewalks and buildings locally tend to bridge 

the shear zone, displacement may be distributed over 

broad areas 60 m or more wide in which there are 

as many as 20 fractures. Buildings commonly hide 

fractures that pass beneath them, unless displacement is large enough to visibly affect the structure. 

The fractures in urban areas commonly ruptured 

underground utility lines, so that in many places the 

fault trace was marked by flowing water or utility 
GEOLOGIC ASPECTS OF THE EARTHQUAKES 

EXPLANATION Q) 

= Q) 

~] ~ KNt.:·~-%~ ALLUVIUM, LAKE DEPOSITS, AND SOIL 

.i) 0 

5:: :t MAFIC AND INTERMEDIATE PYROCLASTIC ROCKS 

C INTERBEDDED FLOWS AND PYROCLASTIC ROCKS 

Q) ~ '""~~~ ~ 'g 8 3:::: MAFIC PYROCLASTIC ROCKS 

g = g ///// 

E: -~ 

5:: 

---Contact "T'MTTT Volcanic collapse --- Fault or lineament 

---1931 earthquake fault 

2 

2 

1972 earthquake fault - Dotted 

~ where inferred from aftershock 

distribution 

0 2 

0 

LAKE 

MANAGUA 

(elev 39m) 

a 

4 

. 

, 

. 

2 

. . 

, 

. 

. 

, 

Huete Pt 

. . 

, 

. 

,' 

, 

. 

. 

6 8 KILOMETERS 

4 MILES 

FIGURE 1.-Generalized geologic map of the Managua area showing faults related to the 1972 and 1931 earthquakes. 

Geology modified from 1 :250,000-scale Managua Sheet (ND 16-15) of the Instituto Geognifico Nacional, Nicaragua. 

5 
6 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

TABLE 1.-Cha,racteristics of fractures along the Managua faults 

[Measured aggregate displac,ement: Tr., trace. N.m., cracks observed but displacement not measured; (?), measured displacement may not be true value. Sense of displacement: S, sinistral; D, dextral; V, vertical (down-to-southeast); E, wall separation (extension). Observations by R. D. Brown, Jr., and George Plafker, January 6-11, 1973] 

1 ----

2 

3 

4 

5 

7 

9 

10 

11 

12 

13 ----

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 ---- 27 

28 

29 ----

30 

31 

32 

33 

34 ---- 35 

36 ----

N. 25 E. 

N.10E. 

N. 32 E. 

76 

30 

9 

76 

15 

5 

5-9 

5 

12 

23 

52 

12-15 

23 

2 

2 

46 

12 

4 

2 

1 

1 

1 

3 

3 

2 

1 

3 

>8 

10 

1 

6 

2 

1 

2 

2 Many 

N. 35 E. 

---------------- { 

---------------- { 

---------------- { 

N. 15 E. 

N. 10-40 E. 

N. 

N. 

N. 

N. 15 W.-N. 4 E. 

----------------- { 

N. 

N. 

N.-S. 

N. 10 E. 

N. 42 E. N. 10 E. 

N. 45 E. 

N. 15 E. 

--"N~-5~io·w-~-­

N. 5 W.-N. 32 E. 

Tr. 

Tr. 

N.m. 

>1.2 

1.2 

>1.2 

1.6 

N.m. 

10.2 

N.m. 

<1.2 

>1.9 

>1.2 

Tr. 

Tr. 

2.5 

Fault A 

Fault B 

<1.2 

N.m. 

3.8-10.2 

N.m. 

>5.1 

>LO 

5.1 

2.5 

22.9 

10.2 

N.m. 

10.2 

7.6 

19.0 

10.2-15.2 

5.1-7.6 

(?)25 

10.2 

10.2 

N.m. 

N.m. 

N.m. 

>3.8 

N.m. 

>2.5 

s 

s 

v 

s 

v 

s 

E 

s 

s 

s 

s 

s 

s 

s 

D 

D 

D 

s 

s 

E 

s 

E 

s 

s 

s 

s 

s 

E 

s 

s 

s 

s 

s 

s 

} ----------

) ----------

) ----------

2.0 

25.9 

) ----------

Ground surface 

Open field --------

___ do ------------ Dirt road and lawn. 

Asphalt pavement _ 

Interlocking tile pavement. 

Asphalt pavement and o}:.en field. 

Remarks 

Open fractures with no horizontal displacement. Do. En echelon, poorly exposed 

fractures. 

Broken underground pipes. 

Offset concrete block fence. 

Southeast-facing slope break. 

Severely damaged new con- crete home astride fracture zone. Asphalt pavement - Damage to concrete homes astride fracture zone. Tile floor 

... do ------------- Concrete curbs and dirt road. 

••• do -------------

Concrete fence foundation. 

Dirt road and open field. 

Dirt road --------- Railroad embank- ment, asphalt 

pavement, con- crete sidewalks and curb. Dirt road and bare ground on low hill (Chico Pelon) . Asphalt pavement and concrete curbs. 

___ do ------------- ___ do -------------

Cutbank 12 m high and concrete re- taining wall. 

Asphalt pavement and concrete curbs. 

••• do -------------

Asphalt and tile pavement. 

Asphalt pavement and concrete curbs. 

___ do ------------- Open field Asphalt pavement and concrete curbs. 

Asphalt pavement, concrete curbs, and sidewalks. 

Asphalt pavement and concrete curbs. 

Asphalt pavement -

Dirt road --------- Open field Asphalt pavement -

Open field -------- Interlocking tile pavement. 

Fracture exposed in ditch; 

dip approx 85 ° SE. Fracture zone appears to die out near here. 

Fracture zone concealed to north beneath garbage 

dump. Broken waterline and con- crete fence foundation. Broken waterline. 

Entire area intensely frac- tured. Includes lurch cracking. 

Measurement on curb. 

Curved fracture in asphalt 

pavement west of sta. 36 

convex towards northeast. 

Near collapsed part of Customs House. 

East side (uphill) overrides west side of fracture, 

Compression with spalling at joints of curbs that trend north-south. 

Broken waterline. Horizontal displacement measured on curved road. Broken waterline. 

Possible lurch cracking. 
37 ----

38 ----

39 ----

40 

41 

42 

43 

44 

45 ---- 46 

47 

48 

49 

50 

51 

52 

53 

54 

55 

56 

57 

58 

59 ---- 60 ----

61 ----

62 

63 

64 

65 

66 

67 

68 

69 ----

70 

71 

N. 42 E. 

N. 25E. 

N.12 E. 

N. 32 E. 

N. 24E. 

N. 22 E. N. 8 E. 

N. 34 E. 

N. 32 E. 

N.4fiE. 

GEOLOGIC ASPECTS OF THE EARTHQUAKES 7 

TAELE l.-Chamcte1·istics of fractures along the Managua faults-Continued 

76 

18 

50 

46 

50 

43 

46 

-90 6 

12 

23 

9-12 

12 

9-12 

8 

15 

37 

12 

61 

55 

}4 

24 

Many 

1-3 

1-3 

1 

1 

1 

1 

>1 

1 

2 

1-3 

1 >4 

9 

>2 

2 

12 

7 

N. 25 E. 

N. 25 E. 

----------------- { N. 17 E. 

N.-N. 30 E. 

N. 10 E. 

N. 10 E. 

N. 10 E. 

N. 5 W.-N. 10 E. 

N. 20 E. 

N. 15 W. N. 22 E. N. 

N. 45 E. 

----------------- { 

N. 

N. 

N. 

N. 

N. 

Fault B-Continued 

N.m. 

N.m. 

5.1 

5.1 

5.1 15.2-17.8 

2.5-3.8 

15.2- (?) 48.::1 

(?)22.3 5.1 

N.m. 

>2.5 

N.m. 

N.m. 

(?) 35.6 

>15.2 

N.m. 

N.m. 

N.m. 

N.m. 

N.m. 

N.m. 12.7-16.5 

N.m. 

N.m. 

N.m. 

33.0 

28.6 

17.0 

30.5 

N.m. 

Tr. 

Fault C 

Tr. 

17.8 15.2-17.8 

27.9 

4.3 N.m. 

N.m. 

N.m. 

s 

E 

s 

s 

v 

s 

s 

v 

s 

s 

s 

s 

s 

s 

D 

s 

s 

s 

s 

s 

D 

s 

s 

s 

s 

s 

E 

s 

s 

D 

s 

s 

E 

s 

v 

s 

s 

s 

16.8-19.6 

) ----------

(?)41.1 

38.1 

33.0 

35.2 

31.3 

Ground surface 

Interlocking tile pavement and 

open field. Asphalt pavement 

and tile sidewalk. 

Asphalt pavement _ 

___ do -------------

Asphalt pavement, concrete curbs, and open field. Asphalt pavement, concrete curbs, and tile sidewalk. 

Asphalt pavement and concrete curbs. Dirt road, concrete- lined ditch, open field. Asphalt pavement _ 

Dirt road ---------

Interlocking tile pavement and open field. 

Open field --------

___ do ------------- ___ do -------------

••• do ------------- ___ do ------------- ___ do------------- ___ do------------- Asphalt pavement _ Asphalt pavement 

and concrete curbs. 

Lake shore -------

Open field and cliff. Open field Interlocking tile pavement. 

Asphalt and inter- locking tile pave- ment, and con- crete curbs. 

Concrete slab sidewalk. Asphalt pavement, concrete curbs, railroa:i embank- ment. Asphalt pavement and concrete curbs. 

___ do -------------

___ do ------------- - __ do ____________ _ 

___ do ______ -------

___ do -------------

___ do ___ ----------

___ do -------------

Remarks 

Probable en echelon offset to west between stas. 36 and 37. At Baptist Hospital. 

Broken waterline. 

Do. 

Pressure ridges trend east- west in field. Two broken waterlines in street. Horizontal displacement measured on curved road and sidewalk. 

Broken waterline. 

Possible lurch cracking. En echelon offset between stas. 46 and 47. 

Offset fence. Posts may not have been perfectly aligned 

prior to faulting. Do. Several centimeters dextral offset of fence. Fracture dies out within 100 m to south . 

In baseball field. Do. 

Prominent zone dextral frac- tures across Pan American Highway. Cannot be traced to northeast. 

Broken sewer outfall along fault trace. Slump in cliff at old lake shoreline. 

Powerplant parking lot. 

Measured on north curb of highway. 

Broken waterline. Measure- ment on curb south of railroad line. 

Broken waterline. 

Few poorly exposed cracks with slight horizontal displacement. Do. Broken waterline. 

Poorly exposed cracks in street and market floor. Down-to-southeast slope break on east side of fracture zone. 
8 

72 

73 

74 

75 

76 

77 

78 

79 

so 

81 

82 

83 

84 

85 

86 

87 

88 

89 

90 

91 

92 

93 

94 

95 

96 

97 

98 

99 

100 

101 

102 

103 

104 

1~!) 

106 

107 

108 

109 

110 

111 

112 

113 

;::; 

i 

= :3 

_g 

U1 

114 ----

Cl) 

0 = 

N 

v 

... 

.a 

<:.> 

o:l 

... 

.... 

.... 

0 

"' = <II 

... 

E-< 

N. 28 E. 

N. 40E. 

N.20 E. 

N. 32 E. 

N. 45 E. 

N.-N. 5 E. 

N. N.10W. 

N.28E. 

N. 45E. 

N. 20 E. 

MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

TABLE !.-Characteristics of fractures along the Managua faults-Continued 

<+-<S 0_. 

~~ ·-

~N 0 

~~ 

~t P.o:l 

<.;: 

88 

14 

30 

49 

5 

55 

15 

15-18 

3 

35 

.... 

0 

~~ ..0~ 

.,_ s = 

=m 

X <II 

z~ p.1) 

P.o:l 

<.e:: 

>20 

3 

3 

1 

>4 

1 

>3 

>7 

>4 

5 

>6 

2 

1 

2-3 

3 

1-2 

>6 

1 

1 

1-2 

1-2 

1-3 

1 

3 

g) 

... 

.a 

<:.> 

o:l 

... .... 

.... 

0 

"' = <II 

... 

E-< 

----------------

---------------- N. 

----------------

---------------- N. 40 E. 

----------------

----------------

---------------- N. 16 E. N. 

N. 1().-15 E. 

N. 20 E. 

----------------

----------------

N. 

N. 

----------------

----------------

N. 10 E.-N. 20 w. 

-----------------

N. 5-10 E. 

N. 10-40 w. 

N. 20 w. 

N. 15 w. N. 15 w. 

----------------

N. 

----------------

N. 8-20 E. 

N. 30 E. 

----------------

----------------

N. 30 E. 

N. 6 W. N. 40 E. 

N. 25 E. 

N. 

N. 

I 

~~ ~ 

Ql::l Cl) = ~ .... "'<:.> s bll~ <II 

<:.> 

~ .... o:l t>o=-- ~ ~~ E :.a 

"'='""' .... ~s~ 0 

Ul<IIO 

o:l<:JN ~ 

= Cl) o:l <II 

~D..S 00. 

Fault C-Continued 

>5.1 s 

N.m. s 27.9-30.5 s 

>10.2 s >6.3 s >15.2 s N.m. s N.m. s 27.9 s >5.1 s N.m. 

>5.1 s 1.3-2.5 v N.m. s 

>5.1 s N.m. s 

N.m. s N.m. s N.m. s 7.6 s 2.5 v 7.5-10.2 s 

(?) 33.0 s 

N.m. s 5.1 v N.m. s 

(?) 15.2 s 5.1-10.2 v N.m. s N.m. s 

N.m. s 

N.m. s Tr. s 

N.m. s 

N.m. s 2.5 s N.m. 

Fault D 

31.9-34.!1 

29.3 

Ground surface 

Asphalt pavement and concrete curbs. ___ do-------------

___ do -------------

___ do ------------- ___ do ------------- ___ do------------- ___ do ___ ----------

___ do------------- ___ do-------------

Open field -------- Asphalt pavement and dirt road. 

Open field --------

Dirt road and earthen floor of house. 

___ do ------------- Earthen floor of house. 

Dirt road --------- ___ do -------------

Open field --------

___ do -------------

___ do -------------

Interlocking tile pavement in highway. 

Open field --------

___ do -------------

___ do -------------

___ do ------------- Golf course fair- way. 

___ do-------------

___ do ------------- ___ do -------------

__________ Open field ----------

___ do ------------- Asph'llt pavement - Golf course fair- way. Golf course fairway and asphalt pave- ment. 

Remarks 

Down-to-southeast slope break on east side of fracture zone. 

Broken waterline. 

Broken waterline. 

Fracture zone obscured by brush in field to south. 

Broken waterline. Fracture intersects wood- frame shack. 

Pressure ridges that trend east-west between fractures. En echelon cracks along base of southeast-facing slope. 

Approx 15.2 em down-to-east displacement, probably partly due to fill compaction. 

At base of southeast-facing 

slope. 

Fracture zone concealed. 

Fracture zone dies out to south. 

Fracture zone dies out to south. En echelon fractures in arcu- ate zone concave to east. 

-------------------------------------------------- Tr. 

Tr. 

N.m. 

1.9 

(?) 5.1 

>1.3 

1.3 

N.m. 

s 

s 

s 

s 

2.2 

(?)5.9 

Dirt road 

Open field -------- Asphalt pavement and concrete curbs. ___ do -------------

___ do -------------

- __ do -------------

Asphalt pavement, concrete curbs, and sidewalk. Asphalt pavement, concrete curbs, and interlocking tile pavement. 

Vacant lot and asphalt pave- ment. 

Open fracture with no measurable horizontal displacement. Do. 

Measurement at painted 

yellow line on road. Broken waterline. Measurement on south curb. No offset of north curb. Measurement on Banco de Americas sidewalk. 

Broken waterline. 
GEOLOGIC ASPECTS OF THE EARTHQUAKES 9 

TABLE 1.-C haTacteristics of fTactures along the Managua faults-Continued 

. ., ;6E .., 0 = = s"i:""' .3E ., "' .... ~ s ;jQ.I~ ., .... s 0 "" s s ... ... <1> ... ~<" <1> ... " '>< ~: t 0~ <1>" E ., ... 

" ;:; ....... (lj~ !=: 

~~ .:;,0 " ~<::~ -a Ground surface Remarks " s ~ " s "'0 i ... ~os "' ... "" .... ~ 0 ;a "'"' .... .... ~ N .,- +>" ~;a.s 

~ ~ .... "'"~ .... 0 ~ !" 0 

~s~ ... - = 0 =~a:l 

:3 "' 25 0" ""' :;! ";4j;::::_ 

" .. -;:; "~ " <n<1>0 

~-a e s " <1> "'" <1> "<> N 

00 " ... ~~ CoOl ... <1>" 

E-< <.t: E-< ~"ii..S r./1 "' ~-;n~~ 

Fault D-Continued 

115 ---- 1- 2 N. N.m. 

116 ---- 1-3 N. 10 E. N .m. 

117 ---- 3 N. 10 E. N .m. 118 ---- N. 16 E . . 1 N. 10- 12 E. N.m. 

!19 ---- 44 8-10 N . 8 E . >1.3 

120 ---- 6 3 ----------------- N.m. 121 ---- 44 3 ----------------- N.m. 122 ---- 3 N. 35 E. N.m. 

123 ---- N. 36 E . >3 N . 11}--18 E. >1.0 

excavations in the streets (fig. 2). Localities at 

which waterline breaks were observed are indicated 

by an "X" on plate 1. Throughout much of the central part of Managua, where earthquake damage 

was greatest, the fracture zones were concealed by 

rubble. This is especially true along fault D between 

stations 109 and 116. 

The traces of faults A, C, and D are straight to 

slightly sinuous, with uniform average strikes for 

most of their lengths of N. 38° to 40° E. Towards 

the southwestern end of fault C in the vicinity of 

the Nejapa Country Club, where the trace is marked 

by several short, linear fracture zones over a broad 

area, there is a gradual change in strike to northsouth or even N. 10° W. In contrast, fault B is a 

more complex feature, consisting of three major segments showing en echelon offsets and considerable 

variability in strike. The overall trend of the zone 

is approximately N. 40° E., but individual segments 

have average strikes of N. 23 ° E. in the area north 

of the Pan American Highway (Highway 1), N. 

43 ° E. from the highway to the vicinity of Lake 

Tiscapa, and N. 32° E. in the area south of the lake. 

The three en echelon segments of fault B are connected by broad complex zones of fractures near the 

Pan American Highway and the Managua-Masaya 

Highway (Highway 4). At the Pan American Highway there is an offset of approximately 200 m between segments of the fault, with a large number of 

intervening fract ures, some of which have small 

components of dextral ::;lip in the vicinity of stations 

---------- Asphalt pavement_ Broken waterline. 

s ---------- Asphalt pavement 

and concrete curbs. 

s ---------- __ _ do ------------ s ---------- Asphalt pavement Severe damage to Texaco 

and open field. station astl·ide fracture zone. 

s ---------- Asphalt pavement Severe damage to concrete and concrete homes astride fracture 

curbs. zone. 

s ---------- ___ do ------------ s ---------- ___ do ------------ s ---------- ___ do ------------ Severe damage to concrete homes astride fracture zone. 

s ---------- Open field and Fracture zone appears to earthen floor of die out to south. warehouse. 

18, 19, and 20. The exact location of the fault trace 

in the Pan American Highway area is further complicated by pervasive lurch- and slump-fracturing 

related to settling and spreading in loose pyroclastic 

deposits that underlie a small hill (Chico Pelon, sta. 

17, pl. 1) situated approximately in the en echelon 

offset between the linear fault segments. The en 

echelon offset in the vicinity of the Managua-Masaya 

Highway is marked by a diffuse zone of cracks over 

100 m wide. At this locality some of the fracturing 

may be due to downslope lurching of highway fill 

and the loose pyroclastic material that makes up the 

slopes of the Tiscapa crater. 

Displacement on fractures within fault zones associated with the December 23d earthquakes is predominantly horizontal and in a sinistral sense: that 

is, to an observer looking across the surface cracks, 

the opposite side has moved toward the left. The 

sense of lateral displacement can be ascertained 

from the fact that en echelon fractures have more 

northerly trends than the fault zones, as determined 

by offsets of streets, curbs, railroad tracks, walls, 

and fences, or by matching irregularities in the 

walls of open fractures (figs. 3 to 8). The amount of 

displacement across individual fractures and the 

aggregate displacement across the zones were measured where suitable linear reference features were 

available. Inasmuch as the streets, curbs, or fences 

on which horizontal slip was measured are oblique 

to the trend of the fracture zones, the measurements 

give a vector component of the slip, rather than the 
10 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

FIGURE 2.-0ne of many flowing waterline breaks along earthquake fractures. (Located on fault B, sta. 40, pl. 1.) 

total slip. The calculated true sinistral slip is shown 

in table 1 and on plate 1 at those localities where we 

FIGURE 3.-Displaced sidewalk blocks at fault C along north 

side of Pan American Highway ( sta. 2, pl. 1). Aggregate 

displacement across zone 12 m wide here is 28.6 em sinistral and 17.0 em extensional. 

FIGURE 4.-Sinistral offset in railroad lines where it is 

crossed by fault C (near sta. 63, pl. 1). Rails had already 

been straightened somewhat before this photograph was 

taken. 
GEOL OGIC ASPE CTS OF THE EARTHQUAKES 11 

,, 

/ 

j 

• ' 

( 

,) 

,; 

,) 

,) 

# 

.) 

, 

)' 

_, 

t 

j 

I 

,1 

5 

i 

} 

j 

FIGURE 5.-Fault displacement in gas station pavement along 

fault B. Sinistral offset of line is 12.7 em; absence of 

scraping along edges of offset interlocking concrete tiles 

indicates that strike-slip motion occurred after tiles pulled 

apart about 6.4 em. 

obtained the largest reliable measurements of horizontal slip across each of the fracture zones. Although the aggregate displacements vary from fault 

to fault and along the trace of individual faults, this 

variation is generally systematic. Maximum aggregate displacements of 22.9 em (centimeters) (sta. 

22) and 33.0 em (sta. 61) measured on faults Band 

C, respectively, in the vicinity of the Pan American 

Highway give calculated total sinistral displacements at these localities of 25.9 em and 38.1 em. 

For both of these faults the displacement is reasonably constant southwestward to the vicinity of Lake 

Tiscapa and appears to diminish progressively to 

the southwest of the lake. Aggregate sinistral displacement on faults A and D is small: It is 2.0 em 

or less on fault A and at least 2.2 em, but possibly as 

much as 5.9 em, on fault D. 

Minor vertical displacements in which the southeast block is relatively downthrown (fig. 8) are 

evident across the fracture zones or on individual 

fractures within the zones at a number of localities 

along faults A, B, and C. Most of the observed vertical displacements are along fault A and the southwestern two-thirds of fault C where there is clear 

evidence for prior vertical movements in the form 

FIGURE 6.-Sinistral offset of street and curb on fault C near 

U.S. Embassy (view towards south, sta. 80, pl. 1). Measured aggregate displacement is 27.9 em across a zone more 

than 49 m wide. Note asphalt-patched fractures in street 

and severe "shear" cracking in five-story office building. 

of southeast-facing topographic slope breaks. In 

many places, vertical offset on the faults related to 

the December 23d earthquakes is difficult to ascertain 

because of preexisting topographic slopes that parallel the faults. Maximum aggregate displacement 

across the zones as determined from measurements 

of the vertical component across constituent fractures is 1.6 em for fault A, 5.1 em for fault B, and 

possibly as much as 10.2 em for fault C. More accurate values for the vertical component of displacement related to faulting should become av;ilable 

when the Institute Geografico N acional finishes resurveying level lines in and near Managua. 

Several features of the two main faults, B and 

C, suggest the possibility that they merge into a 

single master fault at some unknown depth beneath 

the thick fill of unconsolidated deposits that underlies Managua. The two faults are within a few hundred meters of one another at the surface and 

judging from their trends, they could intersect i~ the vicinity of the Nejapa Country Club (pl. 1). 

Both underwent roughly equal amounts of strikeslip displacement of 30 to 40 em. On a gross scale, 

they may be considered as a single rupture with 
12 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

FIGURE 7.- 0pen fracture along fault C in vacant lot north of U.S. Embassy (sta. 8r1, pl. 1). Both the fault tren.d and opening direction on the fracture parallel the ruler. Scale is 15 em long. 

sinistral displacement of about 64 em, the sum of 

the maximum observed offsets. 

Fault C has a continuous linear surface trace 

along which there is clear geologic evidence for 

repeated recent movements involving large vertical 

displacements, as will be discussed in a following 

section. In contrast, fault B has a discontinuous, 

irregular surface trace, and there is no geologic evidence along it for preexisting movements. Thus, the 

irregular, en echelon segments of fault B may be 

interpreted as splays resulting from upward spreading of the rupture zone along fault C within the 

near-surface unconsolidated deposits. Arguing 

against this interpretation is the fact that none 

of the en echelon segments that make up fault B 

merge into fault C or come closer to it than 270 m. 

Unfortunately, the resolution of aftershock locations 

is inadequate to permit a unique solution to this 

question. 

In addition to the sinistral displacement across 

the fracture zones, there typically is a subordinate 

gaping or extensional east-west component, a local 

compressional component in a general north-south 

direction, and a minor vertical component in which 

the southeast block is relatively downthrown. The 

maximum measured extensional components across 

the fracture zones, 10.2 em on fault B (stas. 22 and 

29) and 17.8 em on fault C (sta. 68), are between 

one-third and two-thirds of the measured sinistral 

displacement (figs. 3, 5, and 7). A large extensional 

component (10.2 em) was also measured at one locality on fault A (sta. 6), but at this locality, the 

gaping is probably due in part to slump on a prominent southeast-facing slope. The north-south compressional component in the fracture zones is manifested by east-west-trending buckles and overthrusts 

that connect en echelon fractures (fig. 8) or by 

local compressive buckles and overthrusts of northsouth streets and pavements (fig. 9). The amount of 

north-south compressive shortening in the fault 

zones cannot be ascertained from the available data 
GEOLOGIC ASPECTS OF THE EARTHQUAKES 13 

FIGURE 8.-En echelon fractures along fault B (between stas. 

42 and 43, pl. 1). There is a down-to-southeast component 

of vertical displacement on fracture in foreground and a 

prominent compressional bulge at east-west trending fracture connecting en echelon fractures near the belt and 

hammer in middle ground. 

but appears to be smaller than either the sinistral 

or extensional components. 

FICIJRE D.- Compressional rupture and lateral buckle of 

north-south trending- curb along- fault zone B (near sta. 

25, pl. 1). 

FRACTURES WITH DEXTRAL DISPLACEMENT 

Fractures with predominantly dextral (rightlateral) displacements were observed at several localities near the four earthquake faults. They are 

located along the Pan American Highway in the gap 

between faults B and C (stas. 18, 19, 20, 56) and in 

an open pasture less than 100 m west of fault B (sta. 

50). The largest amount of dextral slip, 16.5 em, was 

measured at station 56 across a zone 9 m wide that 

causes a pronounced offset in the pavement and 

curbs of the Pan American Highway (fig. 10). Dextral slip on fractures at stations 18, 19, and 20 

amounted to >5.1 em, 1.0 em, and about 5.1 em, 

respectively; the slip at station 50 may be as much 

as several centimeters but could not be accurately 

measured. Unlike the fractures on the zones along 

the trend of the earthquake -f'aults, the dextral fractures appear to be local effects that do not extend 

along strike for more than a few hundred meters; 

for instance, those at the highway were not seen on 

parallel streets either to the north or south. They 

appear to be local movements related to the sinistral 

movement on the faults. 

FIGURE 10.-Dextral offset of between 12.7 and 16.5 em in 

curb of Pan American Highway between faults B and C 

(sta. 56, pl. 1). 

LACK OF EVIDENCE FOR FAULT CREEP 

We could not find evidence for creep deformation 

along any of the surface faults. Absence of prequake creep is suggested by the fact that all observed surface fractures in paved streets and in 

curbs appeared to be new and there was no patchwork to suggest movement along them prior to the 
14 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

earthquake. By the time we made our study of the 

fau lting (January 6-11), many of the larger fractures in paved streets had been patched with asphalt 

(fig. 6) . Now here did we see evidence of additional 

cracking through the asphalt patches. Although this 

is not conclusive evidence against postquake creep 

because the patches may not be deforming together 

with the pavement, it is strongly suggestive that the 

major part of the displacement occurred at the time 

of the earthquake or prior to the date when these 

earthquake fractures were patched. 

RELATIONSHIP OF FAULTS TO STRUCTURAL 

DAMAGE 

Displacement along surface faults was directly 

responsible for severe localized damage to the streets 

and underground utilities of Managua and to many 

buildings along the fault traces. Virtually all underground utilities along the faults were disrupted, 

most critical of which were the waterlines. Fires 

raged out of control in downtown Managua for 

weeks after the earthquake. Early control of the 

fires was prevented by the loss of most fire-fighting 

equipment due to station house collapse during the 

earthquake. However, even had the equipment survived intact, the loss of water pressure throughout 

the city as a result of waterline breaks along faults 

(fig. 2) would have hindered effective control of 

major fires. In addition to the waterlines, t here was 

extensive, but less critical, damage along the faults 

to sanitary and storm sewers, as well as to street 

pavements and curbs. 

Many structures located on or close to the faults 

appear to exhibit more damage than structures of 

comparable design and construction away from the 

faults. This may be because, in addition to being 

subjected to the seismic shaking, which affected the 

entire Managua area, the foundations and structural 

frames of such buildings were also distorted or 

physically ruptured by the faulting. 

The most obvious localization of damage by faulting is along faults B and C, which had the largest 

displacement. Heavily damaged reinforced concrete 

buildings on or close to these faults include the Customs House office building, Baptist Hospital, U.S. 

Embassy, and Nejapa Country Club (pl. 1). The 

Customs House office building, a three-story concrete structure astride a segment of fault B that had 

25.9 em sinistral displacement, was the most dramatic failure of the larger structures (fig. 11). Even 

many of the better constructed residential dwellings 

along these faults were severely damaged, whereas 

nearby buildings of identical construction that were 

subjected only to shaking sustained little or no loss 

FIGURE 11.-Collapsed three-story reinforced concrete Customs House office building. This structure is astride fault B at a 

locality where aggregate sinistral slip is 25.9 em (near sta. 22, pl. 1). 
GEOLOGIC ASPECTS OF THE EARTHQUAKES 15 

FIGURE 12.-Severely damaged small home on trace of fault B (near sta. 43, pl. 1). Note fractures in street, curb, and driveway. Homes of similar construction in this subdivision that were no-t on earthquake fau lts generally h ad negligible damage. 

(figs. 12 and 13). In some areas where structures 

were mainly unreinforced concrete block or older 

tarquezal (wood frame and adobe) construction, the 

fault trace appeared to be marked by a distinct 

swath of near-total destruction (fig. 14). 

Localization of damage was even noted along 

faults A and D, which underwent only a few centimeters slip. Modern two- and four-story concrete 

buildings of the Pureza de Maria School (Colegio 

Pureza de Maria) located close to fault A and a 

four-story building of the American School ( Colegio 

Americano) that is astride the fault exhibit severe 

structural damage. Similarly, along the southern 

part of the trace of fault D in the Barrio de Bolonia, 

a number of reasonably well-constructed newer 

homes sustained severe damage due to foundation 

displacement. Fault D passes through the commercial center of Managua and intersects the 13-story 

Central Bani( building (Banco Central), which had 

extensive nonstructural damage possibly caused in 

FIGURE 13.- Masonry and wood home damaged by foundation 

displacement along fault C ( sta. 78, pl. 1). Fractures intersect the near half of the house which is in a state of 

incipient collapse; the part of the house which is off the 

fault zone is relatively undamaged. Some fractures in the 

street have a vertical slip component. 
16 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

FIGURE 14.-Swath of destroyed buildings along fault C (sta. 64, pl. 1). Fault trace is through center of photograph. 

Open fractures that trend north-south in street pavement are en echelon to the fault. Structure on the right is typical of tarquezal (wood and adobe) construction that was extensively damaged in the Managua area. 

part by foundation displacement. (See frontispiece.) 

In contrast, the adjacent 16-story Bank of the 

Americas (Banco de Americas) building, which is 

off the fault zone, sustained less severe earthquake 

damage. 

SIMILARITIES OF 1972 FAULTS TO THE 1931 

EARTHQUAKE FAULT 

All the earthquake faults related to the 1972 

event are roughly parallel to a fault that was 

mapped in the northwestern part of the city of 

Managua after a destructive earthquake on March 

31, 1931 (pl. 1, fault E). In a study made on the 

day after the 1931 earthquake, U.S. Army Corps of 

Engineers personnel identified a fault zone trending 

N. 36° E. that extended 2 km through the present 

General Somoza Stadium to the shore of Lake Managua (Durham, 1931; Sultan, 1931). They found 

numerous cracks, none of which was more than 5 

em wide or had more than 10 em vertical displacement, generally with the southeast side relatively 

downdropped. No horizontal displacement was observed on fractures formed at the surface along the 

mapped fault, but the observation that individual 

fractures had more northerly trends than the strike 

of the zone is strongly suggestive of en echelon ruptures with the proper sense of rotation for sinistral 

faulting. 

The zone of cracks was less than 150 m wide, and 

there was extreme damage along the fault trace, 

especially to the penitentiary and market building, 

which were directly over the faultline. The water 

main leading from the reservoir to the city was 

pulled apart where it crossed the fault. As a consequence, the Engineer troops were badly handicapped 

by lack of water in fighting the fires that broke out 

after the earthquake-a situation exactly comparable to that which occurred in 1972. 

The December 1972 earthquakes do not appear to 

have caused renewed movement on this fault in the 

segment we examined to the northeast of the Gen-
GEOLOGIC ASPECTS OF THE EARTHQUAKES 17 

eral Somoza Stadium; we did not work along that 

part of the trace to the southwest of the stadium. 

GEOLOGIC EVIDENCE FOR PREVIOUS FAULTING 

Of the five faults shown on plate 1 that are related 

to the 1972 and 1931 earthquakes, only faults A and 

C have clear indications of previous Holocene displacement. Both faults were mapped on the 1:50,000 

Managua sheet (Kuang and Williams, 1971) of the 

geologic map of Nicaragua as normal faults with 

the southeast side relatively downthrown, presumably on the basis of the prominent topographic 

scarps that are locally developed along t hem. Faults 

A, B, and C all show local earthquake-related vertical displacements in which the southeast block was 

relatively downthrown in the same sense as the topographic slopes along faults A and C. All of fault A 

(named the "Escuela fault") was delineated on the 

geologic map, and it is shown intersecting a northsouth trending lineament to the south of Managua 

(fig. 1). The part of fault C (named the "Tiscapa 

fault") extending from a few hundred meters northeast of Lake Tiscapa through the lake and southwestward past the Nejapa Country Club was also 

delineated on the geologic map. A number of other 

faults that cut Quaternary deposits are shown on the 

geologic map, most notable of which is a zone of 

north-south-trending faults associated with the Nejapa line of volcanic centers to the west of Managua 

(fig. 1). The existence of northeast-trending faults 

with large vertical components of displacement is 

also suggested by the prominent linear reentrant in 

the northeast shore of Lake Managua from Punta 

Huete northeastward, a feature with roughly the 

same strike as the 1931 and 1972 earthquake faults 

at Managua (fig. 1). 

The topography at the Tiscapa pit crater provides 

some information on t he history of previous displacement along fault C. The trace of the fault on 

the northeast side of the crater is marked by a degraded southeast-facing scarp more than 15 m high 

at the crater rim and by lakeshore offsets of about 

50 m in a sinistral sense on the northeast and 30 m 

in a dextral sense on the southwest (fig. 15). Both 

the rim scarps and opposing lakeshore ofrsets appear 

to result from relative downdropping of the southcast part of the crater, which is essentially an inYcrtecl <:one whose walls slope inward 50° to 60°. 

Asymmetry in the amount of horizontal offset of the 

lakeshores <..:otilcl result from a relatively small sinistral fau lt displacement either concurrent with, or 

after, the ,·e rti<..:al movements. The postulated faulted 

origin for the displm:ement <:rater rim and lakeshore is illustrated diagrammatically in figure 16. 

The amount of inferred vertical and lateral displacement on the fault is subject to large uncertainties 

regarding the original crater shape and the extent 

to which that shape was modified by landslides along 

the crater walls. The most direct geometric reconstruction indicates that on the order of 30 m vertical and 10 m sinistral displacement provides the 

best fit for the lakeshore. Although the calculated 

amount of vertical slip is nearly double that which 

is indicated by scarp heights at the crater rim or 

further sotith along the fault trace, it is clear that 

any reasonable model requires a vertical component 

that is larger than the horizontal component. This is 

at variance with the predominantly strike-slip sense 

of displacement observed after the 1972 earthquakes, 

and may indicate a late Holocene change in style of 

tectonic deformation. The Tiscapa data, and other 

evidence for young faulting cited previously, indicate an extremely complex and active Holocene tectonic history in the Managua area involving recurrent horizontal and vertical movements over a broad 

zone of faulting. It is noteworthy that about 30 fault 

movements, equivalent in displacement to that which 

occurred during the 1972 event would be required 

to produce just the 10 m of sinistral displacement 

that has offset the shores of Lake Tiscapa- an indication that there must have been many repeated displacements on the fault since the Tiscapa crater was 

formed. 

LANDSLIDES AND SURFICIAL EFFECTS 

S2condary geologic effects related to seismic shaking during the earthquake were relatively minor. 

Small slope failures affected steeper slopes in the 

Managua area, most notably along parts of the inner 

walls and rim of the Tiscapa crater, where the upper 

part of a slide showed rotational tilting (fig. 17), 

and on Highway 2 southwest of Managua at about 

km 11, where one major slide and several incipient 

slides in cuts and embankments temporarily blocked 

part of the highway (fig. 18) . A number of rockfalls 

and debris slides were triggered along the steep 

slopes of the Asososca pit crater west of Managua 

(fig. 1), and small areas of artificial fill failed in the 

road along the south rim of Asososca crater. Minor 

slumping, debris falls, and ravelling were widespread along steep natural and artificial slopes in 

loose pyroclastic deposits and alluvium throughout 

the area (fig. 19). 

Although most of Managua is underlain by thick 

deposits of unconsolidated materials, there was no 

obvious damage related to difrerential compaction, 
18 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

FIGURE 15.- Northeast margin of the Tiscapa pit crater and lake showing approximate trace of fault C a long which t here is 

a degraded scarp at the crater rim and 50 m apparent sinistral offset of the lake shore (arrows). The opposite lake 

shore along the fault trace, which cannot be seen in t he photograph, is offset about 40 m in a dextral sense. (Photo 

taken from a point near sta. 45, pl. 1, looking northward.) 

liquefaction, and lateral spreading of foundations. 

Lack of such effects is probably due to the high permeability of the predominantly pyroclastic deposits 

that underlie the city, a low water table, an unusually dry rainy season preceding the earthquake 

(Santos, 1972), and the short duration of seismic 

shaking. The only clear indications of surficial 

slumping and lateral spreading were found along 

the banks of a sewer outfall along the shore of Lake 

Managua, where the water table was within 45 em 

of the surface. 

SEISMOLOGIC ASPECTS OF THE EARTHQUAKES 

The hypocenter of the main Managua earthquake 

was located by the National Oceanic and Atmospheric Administration (National Oceanic and Atmospheric Administration, 1973) at 12.4°N., 86.1 o W., at 

an assumed depth of 5 km. This location could be in 

error by at least 50 km because of the lack of local 

seismic stations and the difficulties of accurately 

locating earthquakes from data recorded around the 

world. Furthermore, the hypocenter, or point in the 

earth where a fault begins to rupture as located with 

the first seismic waves to arrive at the various recording stations, is not particularly relevant to the 

discussion of damage in Managua since the earthquake did not occur at a point but was caused by the 

sudden release of energy along a fault plane with an 

area of more than 100 km2 (square kilometers). 

Following a large earthquake, there are many 

smaller earthquakes or aftershocks in the same region. In a number of well-documented cases, the zone 

of aftershocks has been observed to outline the fault 

that moved during the main event. Therefore, nine 
SEISMOLOGIC ASPECTS OF THE EARTHQUAKES 19 

A B c 

.· 

FIGURE 16.-Schematic diagrams illustrating possible fault-controlled topographic modifications at the Tiscapa pit crater 

(oblique views above) and Lake Tiscapa (plan views below). For clarity, the vertical and horizontal displacements are 

shown sequentially, although it is likely that they were at least in part simultaneous. A, Inferred initial shape; B, Formation of rim scarp and symmetrically offset lake shore due to vertical fault slip; C, asymmetrical offset of lake shore 

due to sinistral slip. Diagrams are not to scale. 

portable seismographs were operated in the Managua area from January 4 to February 7, 1973, to 

locate as many aftershocks as possible and thereby 

to determine the source characteristics of the main 

earthquake. The locations and nodal plane solutions 

of 94 aftershocks with magnitudes of about 0 to 4 

that occurred between January 4 and January 17 

are discussed in the following sections. 

METHODS 

The nine portable seismographs were each selfcontained stations with a sensor, amplifier, smokedpaper recorder and clock. They were operated at 

amplifications of about 250,000 to 1,000,000 times 

(at a frequency of 20 cycles per second), depending 

on the level of the ground noise at the various sites 

caused by human activities and wind. A master clock 

with a drift rate of less than 0.05 seconds per day 

was carried daily to each instrument to synchronize 

all clocks. The overall relative precision of timing 

between stations was better than 0.1 second. 

The records were analyzed using a binocular microscope with adjustable magnification of up to 

about 30 times. The timing accuracy was thus better 

than 0.1 second. The earthquakes were located using 

the standard method of minimizing the root-meansquare (RMS) of the travel-time residuals. After a 

few mistakes in reading and card punching were 

corrected, all earthquake locations had RMS values 

of less than 0.1 second. 

Because data on crustal velocity are lacking, the 

nature of the geologic structure of the region under 

Managua to a depth of 10 km must be assumed in 

order to calculate the earthquake locations. In order 

to cover the range of reasonable possibilities, three 

different crustal velocity models (table 2) were 

assumed for the calculations. Model A is our best 

estimate, though it is based on scanty data of the 

probable structure under Managua. Model B is a 

crustal structure determined for the summit area 

of Kilauea Volcano in Hawaii (Ward and Gregersen, 

1973) that most likely has higher velocities in the 
20 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

FIGURE 17.- Head of rotational slump along rim of Tiscapa 

pit crater. Note back-tilted benches and paved headwall 

cracks in rim road. (Location near sta. 45, pl. 1.) 

TABLE 2.- Crustal str11cture models used in this study 

Velocity (km/sec) Thickness (km) 

Depth to the top of the layer (km) 

A. Model used for the final data analysis 

2.5 - - ------ - ---- 3.5 

5.0 

6.0 

6.8 

8.0 

1.0 

2.0 

3.0 

9.0 

10.0 

0.0 

1.0 

3.0 

6.0 

15.0 

25.0 

B. High-velocity Hawaiian-type crust 

1-.=8- _-_ _-_-_-_-__ - _-_-_-_-_ 0.2 - -,-- 0--,- .0--

3.1 ---------- - - - 1.5 0.2 

5.1 ----- - - ------ 3.7 1.7 

6.7 - ------------ 3.8 5.4 

7.4 ------------- 4.0 9.2 

8.3 - ------------ 13.2 

1.8 - ------------ 3.0 

4.0 

5.0 

6.0 

6.8 

8.0 

C. Low-velocity crust 

0.2 

1.5 

1.3 

7.0 

5.0 

10.0 

0.0 

0.2 

1.7 

3.0 

10.0 

15.0 

25.0 ------------

upper crust than the Managua area. Model C is a 

crustal structure made up of much lower velocity 

material to depths of 10 km. This model assumes the 

volcanic ash and pyroclastics under Managua extend 

to a depth of 10 to 15 km and is considered to have 

about the lowest average velocities possible in this 

area. All earthquakes were located with each of the 

three models. The latitudes and longitudes rarely 

differed by more than 0.5 km, and the depths for the 

events located using Model B were generally 1 to 

1.5 km shallower than those using the other models. 

Thus, the choice of an appropriate crustal velocity 

structure does not critically affect the results given 

here. 

Times of arrival of earthquake waves at a minimum of four stations are sufficient to locate an 

earthquake, but additional readings provide a redundancy that permits more accurate locations. It 

was found in analyzing the data that earthquake 

locations determined with less than six arrival times 

scattered more than those determined with six or 

more. Although locations were determined for over 

165 events during this period, the epicenters of only 

94 events located near the network and with six or 

more stations are discussed here. 

DISTRIBUTION OF AFTERSHOCKS 

The locations of the aftershocks are shown in figure 20. The polygons represent the error in location, 

assuming a possible error in reading the arrival 

times at each station of 0.1 second. This standard 

error, which is statistically the 68-percent confidence 

limit, is calculated as an ellipsoid. Each polygon 

plotted is the shadow of an ellipsoid on the plane of 

the map projection where, to save computer time, 

the shadow is plotted as an 18-sided polygon rather 

than as a smooth ellipse. Thus we are 68 percent 

certain that the epicenter for each earthquake lies 

within the polygon plotted on the map. The largest 

symbols represent the least accurate locations. These 

error limits do not include the possible errors in location caused by incomplete understanding of the 

crustal structure. As discussed above, those errors 

are small and would cause a systematic shift in the 

locations. 

Seventy-nine aftershocks (84 percent of the 

events) lie in a narrow zone striking about N. 30° 

to 35° E. The apparent widening of this zone to the 

northeast can clearly be attributed to the increased 

errors in locating earthquakes that occurred farther 

and farther outside the network of stations. The 

zone is so narrow that 72 of these 79 events could be 

assumed to occur on one vertical plane. The data do 

not preclude the possibility that there is more than 

one fault within the aftershock zone, which is approximately half a kilometer wide. The other seven 

events that occur near but not on this zone either 

represent normal statistical scatter in the locations 
SEISMOLOGIC ASPECTS OF THE EARTHQUAKES 21 

FIGURE 18.-Part of large landslide 55 m wide in highway embankment and cut near km 11 on Highway 2 (Leon-Managua 

Highway). This was the largest landslide seen in the earthquake-affected area. 

or show that a small amount of deformation was 

occurring away from the central fault during the 

period of this study. The fault zone outlined by 

aftershocks extends southwest to northeast for 15 

km or at most 19 km, depending on where one assumes the main seismically active zone ends. 

The depths of these events clearly range from 

about 2 km to about 8 or 10 km, or at most 16 km. 

All earthquakes are shown in figure 21A where they 

are projected onto a vertical plane striking N. 58° W 

through the area and perpendicular to the main 

epicentral trend. The locations of the earthquakes 

in this northeast-trending zone are shown in figure 

21B where they are projected onto a vertical plane 

passing along the zone. Note that most locations 

define a narrow vertical zone of seismic activity. 

Thus, aftershock locations considered with the observed surface faulting clearly imply that the fault 

that broke during the main earthquake on December 23, 1972, is 10 to 15 km long, extends to a depth 

of 8 or 10 km, and strikes approximately N. 30° to 

35° E. 

Twelve earthquakes were located in a group about 

6 km northwest of the main fault just south of the 

Chiltepe volcano, and three were located about 8 km 

to the southeast of the main fault. Both groups lie 

near minor faults observed or inferred from the 

geology (Kuang and Williams, 1971). This type of 

minor aftershock activity off the main fault has been 

observed in other areas (for instance, Hamilton, 

1972). Chinnery (1963) calculated the stress 

changes around a strike-slip fault or dislocation 

surface. He showed that while the greatest increase 

in stress after an offset is at the ends of the dislocation, there is a significant increase in shear stress 

to the side of the dislocation and centered at a distance of about one dislocation length from its center. 

Thus these 15 aftershocks may be related to stress 

changes resulting from the main earthquake. 

NODAL PLANE SOLUTIONS 

The first seismic wave to arrive at a station moves 

the ground either up or down. By noting the direction of this first motion and projecting it back along 
22 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

li'IGURE 19.-Minor slide of unconsolidated deposits in steep bank of dry creek (near sta. 46, pl. 1). Failures such as these 

were common in steep creek banks and artificial cuts throughout the Managua area. 

the ray to its point of origin on an imaginary sphere 

around the focus of the earthquake, we can infer two 

possible directions of fault motion during the earthquake. A reasonable choice between these two a lternatives can usually be made from geologic evidence. 

First-motion plots for the well-recorded earthquakes 

are shown in figure 22, where earthquakes with 

similar first-motion patterns are grouped in each 

plot. 
SEISMOLOGIC ASPECTS OF THE EARTHQUAKES 23 

0 

I I It I I 

Laguna 

Jiloa 

Laguna d~ .... : :·· ... · ... 

Asososca :. \..___.): 

··.·.·.: ·.· 

5 

.KILOMETERS 

Lake 

Managua 

..4---------1 I )---------1 * 

PAN AMERICAN HJGHWA Y j 

Tiscapa 

* 

FIGURE 20.-Locations of 94 aftershocks for the period from January 4 to January 17, 1973. The polygons represent the error 

in location assuming a possible error in reading the arrival times at each station of 0.1 second. Station locations are 

designated by stars. Polygons for earthquakes with nodal plane solutions other than type A (fig. 22) are crosshatched 

as follows: type B, east-west trending lines; type C, southwest-trending lines; D, southeast-trending lines. 

There are four different first-motion patterns. The 

locations of events with these patterns are shown 

with different symbols in figures 20 and 21. The 

northeast-striking nodal plane is assumed in each 

case to be the fault plane since the ground fractures 

all trend north to northeast. 
24 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

A N. 58° w. 

4 

en 

a: 

w 

1-

w 

~ 

0 

..J 

~ 8 

z 

:c 

1-

0.. 

w 

0 

12 

16 

8 s. 32° w. 

0 

DISTANCE, IN KILOMETERS 

S. 58° E. 

DISTANCE, IN KILOMETERS 

0 ~--~----~----~--~----~----~--~----~--~r----. 

4 

en 

a: 

w 

1-

w 

~ 

0 

..J 

' ~ 8 

z 

:c 

1-

0.. 

w 

0 

12 

16 

FIGURE 21.-Hypocenters of the aftershocks projected onto vertica1l planes. The strike of plane A is N. 58° W., and for plane 

B, the strike is N. 32° E. The polygons and crosshatching are the same symbols used in figure 20. 
SEISMOLOGIC ASPECTS OF THE EARTHQUAKES 25 

A 

w 

c 

w 

~ • r 

• • 

. i 

• 

.. 

• 

N 

s 

N 

E w 

0 0 

s 

N 

E 

s 

E 

• 100°/80°N 0 • 

• • 

• • 

• • 

•• • 

s 

FIGURE 22.-Composite nodal plane solution for 59 type A events, 5 type B events, 19 type C events, and 9 type D events. 

The plots are an equal area stereographic projection of the lower half of an imaginary sphere around the focus of the 

earthquake. Waves traveling up directly through the upper half of the sphere are projected through the center of the 

sphere onto the lower half. Compressions or upward motions of the ground are represented by solid circles; dilatations 

or downward motions of the ground are represented by open circles. Arrows designate the direction of motion on the 

most likely nodal plane chosen because of the trend of the zone of aftershocks and the trend of the surface faulting. 

A. Fifty-nine earthquakes show sinistral slip along 

a vertical plane roughly parallel to the main 

zone of seismic activity. These events are located predominantly on the northeast half of 

the main seismic zone and in the cluster of 

events to the northwest. Some are located to 

the southwest. 

B. Five earthquakes near the northeast end of the 

seismic zone have sinistral slip but with a 

major component of normal faulting down to 

the southeast. 

C. Nineteen events in the cluster of activity near 

the southwest end of the fault show sinistral 

slip along a nearly vertical fault striking N. 

55° E. 
26 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

D. Nine events, five along the southwest part of the 

main fault and four in the northwest cluster 

of earthquakes, have apparent dextral slip on 

the northeast-trending plane. 

The few inconsistent points in each plot were reexamined and are correctly read. They show that 

while there is great consistency between earthquakes, the nodal planes for individual aftershocks 

may change by ±5° to 10° in strike. The nodal plane 

solutions show sinistral slip with a slight rotation 

of stresses at the northeast (solution B) and southwest (solution C) ends of the fault. In addition, 

minor local reversal of the fault motion is suggested 

by solution D. 

COMPARISON TO SIMILAR EARTHQUAKES 

The fault slip during the Managua earthquakes 

was greater than that associated with earthquakes 

of comparable size and mechanism in California. 

The seismic moment (Mo) has been shown by Aki 

( 1966) to be proportional to the product of the fault 

area A and the average displacement u: 

Mo=~Au, 

where fL is the shear modulus in the source region. 

The moment can be calculated directly from the 

spectral density of the seismic waves. It has been 

related empirically (Wyss and Brune, 1968), however, to the body wave magnitude in the magnitude 

range of interest here by the equation 

log Mo~l.7ML+15.1. 

Thus the larger the earthquake, the larger the product of the fault area times average displacement. 

Fault dimensions and slips for the main Managua 

earthquake and four well-studied strike-slip earthquakes in California are summarized in table 3. The 

slip along the fault is twice as large for the Managua 

earthquake as it is for the Parkfield, Truckee, or 

Borrego Mountain earthquakes, which were of similar magnitude. The slip was about similar for the 

Coyote Mountain earthquake, but this event apparently was confined to a fault at a depth of 10 to 13 

km which did not break the surface. Thus the Managua earthquake was accompanied by twice as much 

slip on the fault and therefore by more severe 

ground fracturing than similar earthquakes in 

California. 

SETTING OF THE EARTHQUAKES 

REGIONAL TECTONIC RELATIONS 

Managua lies within the trend of volcanic ·and 

earthquake activity that girdles the Pacific Ocean 

basin and that popularly is referred to as the "Pacific Ring of Fire." According to modern geologic 

theory, the earthquakes and volcanic activity around 

the Pacific result from relative movement between 

large plates of the earth's crust. Certain boundaries 

between such mobile plates are defined by long, 

linear trenches in the seafloor, well-defined zones of 

earthquake activity that are shallow near the trench 

and deepen toward adjoining continental areas, and 

linear chains of volcanoes that parallel both the 

trench and the trend of the zone of earthquakes. All 

of these characteristic features occur in Central 

America and have been active there for several milTABLE 3.-Fault dimensions and slip for five earthquakes with magnitudes of 5.5 to 6.5 

~ ~ i Q) 0 -;; ..= 0 C) rn ..= "' .-!! "' rJl Cil 

]-s ~ "' OJ-. e OJ ltl. .-!! ~ .e .;;§ ~ ;:; rn~ 

=-- [iJ Pt ........... Reference .......... .... s s ....... 0 ., ...,·a 0 0 s "C "' ..c::~ .;;~ 0~ Q) O"" "C C' s:::t.) .. .. s::: 

~ OJ, t~ ..c::'-" ., .... Q) rn 

.s s Q) t>l)Q) ... OJ 

~ p.8 $ 

r2 .-!! 

~ ~ 0~ §'3 S:::s;:: 

~§ ·- 0 .b Q ~"C ..:l~ OJo H>l Q,; ~ Ei:iS 00 

Managua ------------ 12-23-72 5.6 6.2 13 >6 10-15 From 2 to 8 >67 70 13 National Oceanic Atmos- or 10. pheric Administration 

Parkfield 6-27-66 5.5-5.8 6.2 ( 197 3) , and this report. ------------ 37 40 From 0 to 10 18 10 0.7 Aki (1966), Eaton and others or 14. ( 1970), Brown and others 

Truckee 9-12-66 6.0-6.5 (1967). ------------- 0.8 16 10 From 0 to 9 None >30 20 Kachadoorian and others (minor) or 12. (1967), Greensfelder 

( 1968) , Ryal and others (1968), Tsai and Aki 

Borrego Mountain 4- 9-68 6.4 (1970). 31 45-56 From 0 to 10 38 30 9 Allen and Nordquist (1972), or 12. Hamilton ( 1972), Clark 

( 1972), Wyss and Hanks 

Coyote Mountain ---- 4-28-69 5.8 0.5 None -10 From 10 to 13 None 60 (1972). 80 Thatcher and Hamilton (1973). 

1 Determined from the surface-wave magnitude. 
SETTING OF THE EARTHQUAKES 27 

lion years (Dengo, 1968; McBirney and Williams, 

1965). Clearly, the historic volcanism and earthquakes are natural and continuing processes that 

man must understand and plan for if he wishes to 

live and prosper here. 

Major geologic features in Central America are 

the Middle America Trench, a pronounced linear feature 4 to 5 km deep along the Pacific Coast from 

central Mexico to Costa Rica (shown on index map, 

fig. 1), and the chain of young andesitic stratovolcanoes extending from western Guatemala to Panama. Most earthquake activity in Central America is 

in a belt about 200 km wide that parallels the trench. 

Where the focal depths of these earthquakes can be 

well determined, they exhibit a systematic distribution-shallow near the trench and deeper with increasing distance towards the northeast (Molnar 

and Sykes, 1969). The zone of earthquake activity 

thus dips about 45° NE. and extends from very near 

the surface at the Middle America Trench to more 

than 170 km deep at points farthest from the trench. 

In Nicaragua, earthquake activity related to this 

dipping zone extends as far inland as Lake Managua 

and Lake Nicaragua. The line of volcanoes that extends through most of Nicaragua approximately follows the northeasternmost limit of earthquake activity. Earthquakes along this zone since 1963, when 

the data are most complete, have ranged up to magnitude 6 (National Oceanic and Atmospheric Administration, 1973), but Gutenberg and Richter 

(1954) report some events as large as magnitude 

7.7 in the period since 1913. Because Managua lies 

100 to 200 km above this zone, even large earthquakes are unlikely to cause severe damage, although 

shallow earthquakes in this zone could cause damage 

in the Pacific coastal areas of Nicaragua. 

The northeast-dipping zone of earthquake activity 

marks the boundary between two crustal plates. The 

Caribbean plate on the northeast includes most of 

Central America and extends northeast into the 

Caribbean. The Cocos plate on the southwest extends 

into the Pacific Ocean from the Middle America 

Trench. Geologic and geophysical evidence suggests 

that the Pacific, or Cocos plate, is moving relatively 

towards the northeast and is slowly being driven 

beneath the Caribbean plate along the plate boundary. 

The Managua earthquakes of December 23, 1972, 

were at much shallower depths than the inferred 

crustal boundary between the Cocos and Caribbean 

plates, ancf the observed surface faulting, described 

in this report, exhibits a much different geometry 

than that of the plate boundary. For these and other 

reasons, it is unlikely that the December 23 earthquakes are a simple and direct result of relative 

plate movement between these two major crustal 

blocks. More likely they are caused by relatively 

shallow adjustment to accumulating crustal strain 

within the southwesternmost part of the Caribbean 

plate. This interpretation is favored both by the historic record of shallow-focus earthquakes in the 

Managua area and by the surface trend of the volcanic chain which passes through the Pacific coastal 

part of Nicaragua. The line of recent volcanoes in 

Nicaragua exhibits a marked bend or offset to the 

south in the segment between the volcano Momotombo on the northwest shore of Lake Managua and 

Masaya Caldera to the southeast of Managua. Detailed crustal structure and geology are not known 

well enough in the Managua area to specify the relations between the plate boundary, the line of volcanic activity offset to the south in a dextral sense, 

and shallow-focus earthquakes like those of December 23 with sinistral offset of the ground. A close 

relationship between all three, although still unproven, is an attractive hypothesis for testing and 

studying. 

PHYSIOGRAPHY 

The nature of the land surface in and around 

Managua provides important clues both to the geologic history of the area and also to the kinds of 

damage that may be expected in future earthquakes. 

Many of the surface effects of the December 23 

earthquakes are likewise related directly to easily 

observed topographic features. 

Much of the city of Managua and most of the surrounding areas affected by the earthquakes are on a 

surface that dips a few degrees towards the north. 

A few north-flowing washes drain this surface and 

feed into Lake Managua, but all are small and none 

are incised more than a few tens of meters into the 

surface. More deeply incised ravines are common 

further south, however, in the upland area lying 

west of Masaya Caldera. Except near the Chiltepe 

Peninsula, similar low relief is also found along the 

shoreline of Lake Managua, and at most places near 

Managua the lake appears to be very shallow for 

considerable distances offshore. 

This gently north-dipping surface is interrupted 

in several places by low hills, most of which are 

clearly of relatively recent volcanic origin. Examples 

include Tiscapa near the south edge of the city, the 

hill enclosing Lake Asososca on the west, and the 

ridgeline on which the Nejapa pits southwest of 

Managua are located. Few of the hilly areas rise 

more than about 100 meters above the general sur-
28 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

face, and fe·w exhibit steep slopes. Steep slopes are 

found, however, in the crater walls at Tiscapa, Asososca, and in most of the other interior depressions 

of volcanic origin. 

Several lines of evidence suggest that the gentle, 

relatively undissected surface at Managua and extending generally to the southeast is very young. 

This surface appears to be graded to Masaya Caldera, and locally perhaps to other nearby volcanic 

centers. Its essentially planar form has not yet been 

modified greatly .by erosion, sedimentation, or other 

geologic processes, and the rock materials that underlie it exhibit generally the same inclination as 

does the surface. Most of these near-surface rocks 

are lapilli or ash derived from nearby volcanic 

sources such as Masaya. 

If, as appears likely, the surface in and near Managua is a relatively young constructional feature, 

the task of evaluating earthquake risk becomes more 

difficult. Geologists commonly recognize and evaluate active faults, those which are capable of generating destructive earthquakes, by their surface topographic expression. Recurrent movement on faults 

produces well-defined scarps, trenches, alined stream 

courses, and other linear topographic features that 

not only mark the fault trend, but provide clear evidence of repeated activity along the same lines. 

These identifying characteristics, however, can be 

destroyed by other geologic processes, and if such 

other processes are operative, the record of faulting 

is apt to be blurred or completely obliterated. However, a very young surface provides a useful means 

of dating fault-formed features that clearly cut or 

offset it. Hence, if the young surface near Managua 

does locally show evidence of fault displacement, 

such displacement must be very young indeed. 

The general low relief and absence of steep slopes 

in and near Managua also had an important bearing 

on the kinds of damage that resulted from the earthquake. Landslides and other kinds of slope failure 

are often among the most important causes of property damage in large and moderate earthquakes. Although many slope failures of different kinds could 

be observed after the earthquake, most of these were 

small; there were far fewer than are usually seen in 

areas with even moderate slopes. Other factors probably also contributed to the low incidence of slope 

failures, but the low relief and the relatively small 

area covered by steep slopes were major ones. 

NEAR-SURFACE ROCK UNITS 

The severity and distribution of damage resulting 

from destructive earthquakes depend to a large extent upon the nature of the near-surface geology. 

Different kinds of rock units respond to shaking in 

quite different ways, and in many well-observed 

earthquake areas, a very close correlation has been 

noted between the geology and the intensity of damage. Although the relation between damage from 

shaking and geology is far from simple, damage is 

commonly greatest over thick accumulations of 

poorly compacted water-saturated deposits and is 

least over relatively dense well-consolidated rocks. 

Our knowledge of the geologic units that underlie 

Managua comes from published geologic maps of the 

area, from published scientific papers, from our own 

observations of scattered exposures of bedrock units, 

and from a few unpublished records from water 

wells. The data are inadequate for a detailed analysis of the geology, and they allow us to "see" only 

about 200 m beneath the surface. Nevertheless, the 

different lines of evidence are consistent, and they 

indicate that the city is underlain by a relatively 

homogeneous sequence of rocks, predominantly volcanic but with many interbeds of water-worked volcanic debris. 

Exposures in and near Managua show that most 

of the volcanic debris is composed of lapilli-sized 

( 4 to 32 mm) angular basaltic scoria. The scoria, 

or cinder deposits, contains almost no fine-grained 

ash except as thin beds a few centimeters thick. 

Both the scoria and the thin beds of ash are pyroclastic debris and appear to be derived either from 

Masaya or from the line of volcanic vents immediately to the west of Managua. Locally, these beds 

contain interbeds of more compact fine-grained rocks 

that are the products of volcanic mudflows. Unlike 

the scoria, the mudflow deposits are firm and relatively well lithified. They are thick and firm enough 

to be quarried for building stone west and southwest 

of Managua, and Williams (1952) has described 

quarried localities at which the imprints of human 

feet can be seen on exposed bedding surfaces. 

The sequence of interbedded scoria, ash, and mudflow deposits appears to underlie nearly all of Managua, or at least those parts of the city that exhibited the greatest damage (fig. 1). The relative 

proportions of each rock type vary somewhat in 

different exposures and in the logs of wells, and the 

sequence is characterized by lensing and by channeling where water-worked deposits are evident. 

Despite these variations, lapilli-sized scoria appears 

to be the dominant lithology at least to the depths 

known from drilling, about 200 m. 

Some confidence in extrapolating units for considerable distances from outcrops, wells, or artificial 
VOLCANIC RISK AT MANAGUA 29 

exposures is gained from the structural attitude of 

the rocks. In spite of the several faults described in 

this report, the rocks are little deformed and generally dip about 4 ° N. They are more steeply inclined, however, within a few hundred feet of the 

faults. 

The lack of interstitial fine-grained matrix in the 

scoria, the rough exterior and vesicularity of individual granules, and the angularity of the granules 

together contribute to form a rock unit that is extremely porous and permeable and that has a low 

bulk density. Largely because of the angular, rough 

surface of the lapilli-sized fragments, this rock is 

fairly stable under static loads, and where it is undisturbed it will stand in near-vertical slopes. It is 

clearly much less stable under dynamic load conditions, such as the shaking that accompanies earthquakes. This was well shown by the numerous small 

debris-falls (fig. 19) that accompanied the earthquakes of December 23. 

Somewhat different geologic relations are evident 

west of Managua along the line of volcanic centers 

that extends south from Lake Jiloa through Lake 

Asososca. There, relatively dense lava flows and vent 

debris are associated with pyroclastic deposits (fig. 

1). Damage in this area was much less intense than 

in the central city, and although a major part of the 

difference in intensity is due to distance from the 

epicenter of the main shocks, some of the difference 

may be related to the differences in geologic conditions between the two areas. 

Despite the general uniformity of ground response 

in the damaged area, it is likely that shaking was 

more intense than it would have been in an area 

underlain by well-consolidated, relatively dense bedrock. This conclusion, however, is based more on 

knowledge of other earthquakes and research results 

than on direct observation of ground effects at 

Managua. 

GROUND-WATER RELATIONS 

A major factor controlling damage in many earthquakes is ground water. Ground water in permeable 

zones can result in liquefaction and loss of strength 

in foundation materials. A near-surface water table, 

even if unconfined, can lead to slope failures, lateral 

spreading on low slopes, and to other kinds of 

failure. 

Ground-water levels in the Managua area appear 

to be well below the surface except in the northernmost part of the city, where they are at or near the 

level of Lake Managua. An unpublished map of the 

ground-water surface prepared by Hazen and Sawyer, Engineers, New York-lYianagua (1964), shows 

that the surface of the ground water is from 10 to 

30 m beneath the ground surface in most of the area 

that was damaged, and that the piezometric surface 

slopes northward somewhat more gently than the 

land surface. The high porosity and permeability of 

the rock units that contain the ground water, and 

the lenticular nature of most of the impermeable 

interbeds, are considered by us as evidence that the 

ground-water system is relatively open and that confined aquifers are relatively unimportant in the part 

of the geologic section penetrated by wells. 

VOLCANIC RISK AT MANAGUA 

In addition to geologic hazards related to earthquakes, the Managua area has had a long and active 

history of volcanism, and the future risk from destructive volcanic eruptions should be considered in 

reconstruction planning. A thorough discussion of 

the volcanic risk is far beyond the scope of this fieldwork and report but nevertheless, we feel that the 

seriousness of this risk warrants a brief outline and 

evaluation of the available data. 

There are three types of recent volcanoes in Nicaragua. According to McBirney ( 1955), 

the first, and by far the most common group is the Strombolian type, characterized by a ste·ep sided structure of ash, 

cinders, and vesicular lava. This group includes the volcanoes 

El Viejo, Telica, Cerro Negro, Asososca de Leon, Santa Clara, 

Momotombo, Chiltepe, Concepcion, Madera, and a host of 

minor cinder cones. The activity of these volcanoes, which 

is often intermittent over many years, is normally solfataric, 

the volume of solid ejecta being subordinate to that of 

steam and other gaseous elements. 

The second group is of the Krakatoan type usually characterized by a low, shie·ld-like structure composed of successive layers of massive lava flows and a large, steep-walled 

collapse crater. Thes·e volcanoes have been notable for sudden, paroxysmal eruptions, usually culminating long periods 

of dormancy, during which enormous quantities of gas and 

pumice are ejected in the short period of a few days. At 

the final stage of such eruptions a cylindrical portion of the 

dome has usually collapsed into the vacated magma chamber 

forming a large, vertical-walled caldera. In this class we 

find Cosequina, Apoyeque, * * * and Apoyo. 

The third type is the Masaya type, of which Masaya is the only example in Nicaragua. Masaya is 

quite similar to those Hawaiian volcanoes that consist of a caldera formed by repeated collapses of 

vents within the summits of a flattish basaltic shield 

volcano as magma migrates upward from great 

depth. It has been the most consistently active volcano in Central America in historic times (McBirney, 1956). There is "no trace of the characteristic 

pumice beds, which are so voluminous about the 

other [more explosive] calderas * * *" (McBirney, 
30 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

1956), and McBirney concludes that while gas emission may damage crops, as happened in the period 

around 1927 and 1954, "* * * little is to be feared 

from lava eruptions because of the large volume that 

must be filled before any of the existing craters 

overflow. Even an eruption of lava from the flanks 

of the Nindiri-Masaya group or from any other 

vents on the caldera floor would not be likely to 

endanger any center of population." No lava flow 

has covered the Managua area in historic time. One 

flow, however, believed to have erupted in 1670 (MeBirney, 1956), did run 9 km northward, within 31j2 

km of the present site of the international airport. 

Masaya could pose a threat to substantial development in the region between Managua and the City 

of Masaya. 

Two of the three most explosive and potentially 

devastating volcanoes in Nicaragua, Apoyeque and 

Apoyo, are within 35 km of the center of Managua. 

The vent occupied by Lake Jiloa on the flank of 

Apoyeque also "appears to be the source of thick 

pumice beds typical of an explosive eruption" (MeBirney, 1955). Furthermore, Lakes Tiscapa, Asososca, and Nejapa are collapse craters from recent 

volcanic activity (McBirney, 1955). These calderas 

and craters appear to be dormant. McBirney (1955), 

however, reports that the temperatures in Lake 

Nejapa are abnormally high and that the chemical 

content of the water implies that the lake is fed by 

hot springs. On June 8, 1852, the first indication of 

a new eruption of Masaya was "when Lake Masaya, 

together with Lakes Tiscapa, Asososca, Apoyo, and 

others began to 'boil.' Most likely this 'boiling' was 

actually an emission of gases from the lake bottom" 

(McBirney, 1956). This observation shows, however, 

that these features are merely dormant and not 

dead. 

Thus there is significant volcanic risk in the Managua area not only from lava flows but from the 

possibility of a truly devastating eruption. What 

makes evaluation of volcanic risk particularly difficult is the question of time scale. There has been no 

historic Krakatoan-type eruption in the Managua 

area, but there may have been one large eruption 

since human habitation of the area (Williams, 

1952). Another eruption may be thousands of years 

away. Devastating eruptions typically occur, however, only hours to weeks after the first visible signs 

of a reawakening of activity at previously dormant 

vents. The volcanic risk needs to be carefully evaluated and taken into account in the reconstruction of 

Managua. 

SEISMIC RISK AT MANAGUA 

HISTORIC SEISMICITY 

Damaging earthquakes have occurred frequently 

in Nicaragua. Montessus De Ball ore ( 1888) lists 

earthquakes in 1528, 1663, 1844, 1849, 1858, 1862, 

1881 and 1885, but from his descriptions, it is difficult to tell where these events occurred. The earthquakes of 1844, 1858, and 1881, however, caused 

damage in the region of Managua. Earthquakes in 

1898, 1913, 1918, 1928, and 1931 also caused damage 

in Managua (list compiled by Ken Jorgensen, Panama Canal Company, written commun., 1966). 

All accounts of the earthquake of March 31, 1931, 

indicate that it was remarkably similar to the 1972 

earthquake in most respects. The event was of magnitude 5.3 to 5.9 (Gutenberg and Richter, 1954) and 

caused ground fracturing along a northeast-trending 

fault in the western part of Managua. The downtown area was heavily burned. About 1,000 people 

(Sultan, 1931) were killed out of a population of 

about 40,000 (Durham, 1931). Most homes were 

destroyed, and utilities were seriously damaged. 

A small earthquake (magnitude 4.6) occurred in 

Managua on January 4, 1968. It caused the heaviest 

damage in the Colonia Centroamerica, but no loss of 

life occurred (Brown, 1968). 

These few data on historic seismicity show how 

common earthquakes are in the Managua area. From 

these data and the regional tectonic relations discussed above, it seems certain that damaging earthquakes will occur again in the Managua area. 

The data are inadequate for determining a statistical recurrence rate of earthquakes, but it seems 

reasonable to expect an earthquake in Managua 

similar to that of December 23, 1972, within the 

next 50 years. 

A COMPARISON 

No method has been developed to quantify the 

earthquake risk in one area as compared to another. 

Too many factors, many of them as yet poorly understood, must be taken into account. Considerable 

research is being done and needs to be done in the 

future to find methods for defining comparative risk. 

Some qualitative comparisons, however, can be made 

on the basis of existing data. 

All three of the earthquakes that shook Managua 

between 12:30 and 1:30 a.m. on December 23 were 

of moderate magnitude. The greatest of these, at 

magnitude 5.6, was smaller than the San Fernando, 

Calif., earthquake (6.6) of February 9, 1971, and 

I much smaller than such great earthquakes as the 

Alaskan earthquake of March 27, 1964 (8.4), the 
SEISMIC RISK AT MANAGUA 31 

San Francisco earthquake of April 18, 1906 (8.3), 

the Niigata, Japan, earthquake of June 16, 1964 

(7.5), or the Peruvian earthquake of May 31, 1970 

(7.7). Because the magnitude scale is exponential, 

each integer step-for example, from 6.0 to 7.0-

1.0 

0 

0.52g 

0.08km 

represents an increase in released energy of about X 

30 times. Accordingly, a magnitude 8 earthquake 

releases nearly 1,000 times the energy of a magnitude 6 earthquake. The area of the fault that slipped ~ in the Managua earthquake is on the order of 100 

km2

, whereas faults that slip during events of mag- ~ 

nitude 6.5 and 7.5 typically have areas on the order i= 

of 500 km2 (Hamilton, 1972) and 2,000 km2 (Aki, ~ 

1966), respectively. In view of the complex regional ~ 

tectonics in the Managua area, we would guess that ~ 

it is unlikely that there are faults with areas much ~ 

larger than 500 km2 • On this basis, there appears 

little likelihood that earthquakes much greater than 

magnitude 6.5 will occur in the immediate vicinity of 

Managua. Of course, an earthquake with magnitude 

larger than 8.0 might easily occur on the large faults 

associated with the Middle America Trench and the 

zone of underthrusting of the Cocos plate, but the 

energy source from such earthquakes would be 100 

EARTHQUAKE 

MAGNITUDE 

• 5.0-5.9 0 6.0-6.9 

• 7.0-7.9 

• 

•• 

CID • 

to 200 km distant from Managua. 

Maximum expected magnitude is, however, not 

the only consideration. Damage caused directly by 

an earthquake is primarily related to the amount 

that the ground accelerates during the event, the 

duration of the shaking, the number of fractures 

going through buildings and other structures, and 

the amount of displacement on these. fractures. Acceleration is attenuated logarithmically with distance. The data in figure 23 show that the peak 

acceleration of 0.31g (F. Matthiesen, oral commun., 1973) recorded at the ESSO refinery during 

the main Managua earthquake is about the same as 

might be expected somewhere between 30 and 50 km 

from an earthquake of magnitude 7.7. The duration 

of shaking also is attenuated with distance in a 

roughly similar way (Page and others, 1972). Thus 

the intensity and duration of ground shaking in Managua were large compared with that observed in 

many cities shaken by larger earthquakes because the 

Managua earthquake occurred almost directly below 

the central part of the city. The acceleration would 

probably have been 10 times less if the earthquake 

had occurred only 20 to 40 km distant. For instance, 

there was no noteworthy damage at Masaya, Tipitapa, or other nearby cities. 

Statistically, seismologists find that in a region 

where there is one earthquake of magnitude 8 in a 

given period of time, there are approximately 10 

DISTANCE (KM) 

FIGURE 23.-Peak horizontal acceleration versus distance to 

the slipped fault as a function of magnitude (after Page 

and others, 1972). The X is the peak acceleration of 0.3lg 

observed at the ESSO refinery for the Managua earthquake. 

earthquakes greater than magnitude 7, 100 greater 

than magnitude 6, 1,000 greater than magnitude 5, 

and so forth. Although it is dangerous to extrapolate 

this relation from region to region, a city that is so 

close to a fault and is built in such a way that it can 

be destroyed by a magnitude 6.0 earthquake might 

be destroyed much more often than a city that could 

sustain an earthquake of magnitude 7.5 with little 

damage. 

Proximity to faults and ground displacement beneath structures can significantly increase damage. 

No place in the central two-thirds of Managua is 

more than one-half kilometer from one of the four 

faults that moved during this earthquake sequence or 

the fault that moved during the 1931 earthquake. 

Within the approximately 15-km2 city limits of Managua there are 11 km of faults active within the last 

42 years-a fault density of roughly 0.73 km/km~. 

We are not aware of a similar density of faults in 

any other city. Even in the entire 50 km~ area included on the 1: 10,000-scale topographic map of Ma-
32 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

TABLE 4.-Comparison of fault density at Managua and vicinity with other urban areas in seismically active zones 

[Faults included are only those with known Holocene or historic displacement] 

Approximate Fault 

Community population length 

(km) 

Managua, Nicaragua ------------ 400,000 11.0 

Berkeley, Calif ----------------- 116,716 11.1 

Oakland, Calif ------------------ 362,100 55.9 

Managua and vicinity, Nicaragua _ 500,000 18.0 

Fukui, Japan ------------------ 744,230 1.5 

Hayward, Calif ----------------- 100,000 23.4 

San Bruno, Calif --------------- 36,254 3.2 

San Leandro, Calif -------------- 70,300 7.9 

Woodside, Calif ---------------- 4,875 6.4 

Fremont, Calif ----------------- 123,273 34.9 

Greater Los Angeles area, Calif __ 6,755,000 46.2 

nagua and vicinity (part of which is shown as plate 

1), there are at least 18 km of active faults with a 

density of 0.36 km/km2 • In table 4, fault density at 

Managua is compared with the density of faults 

along which there has been late Quaternary movement in other seismically active urbanized areas 

elsewhere. Clearly, the hazard from active faults is 

as great, if not greater, at Managua than at any 

other large city for which data are available. 

The pattern of active faults in Managua differs 

from those in most other urban areas crossed by 

faults, and it differs in such a way as to increase 

the hazard. In most urban areas crossed by active 

faults, the fault breaks are simple-either a single 

continuous break or a narrow band of subparallel or 

en echelon breaks a few tens of meters to several 

hundred meters wide-so that the hazard from surface displacements can be well defined. The four 

faults recognized and described in this report, and 

a fifth which moved in 1931, together constitute a 

wide band of active faults which trends northeastward across the central part of the city. Together 

these fi,ve 'active fault traces pose a major threat to 

much of the urbanized area and to yet undeveloped 

la:nd lying on their trend and immediately south of 

the city. New displacements may occur on any or 

all of these faults during future earthquakes, for at 

least two of them show clear evidence of repeated 

movement in the past. This pattern of faulting, 

which defines a band 3 km wide, suggests also that 

future surface displacements may not be confined 

only to those faults which are now known. New 

branch faults and subsidiary faults may occur within 

the zone or outside of it. 

CONCLUSIONS 

The extensive destruction and loss of life in the 

Managua earthquakes of December 23, 1972, were 

Area 

(km2 ) 

15.0 

25.8 

138.0 

50.0 

6.2 

96.7 

14.4 

38.7 

36.1 

246.4 

590.8 

Length per km2 (km) 

0.73 

.43 

.41 

.36 

.24 

.24 

.22 

.21 

.18 

.14 

.08 

Data source 

This report. 

Radbruch (1967). 

Radbruch (1967). 

This report. 

Collins and Foster (1949). 

Radbruch ( 1967). 

Brown (1970). 

Radbruch (1967). 

Schlocker and others ( 1965). 

Radbruch ( 1967). 

Wentworth and others (1970). 

caused almost entirely by the following: 

1. Occurrence of the earthquakes directly beneath 

the city. 

2. Poor construction of the buildings, chiefly of tarquezal and masonry, which had very little 

shear resistance to lateral forces imposed by 

the strong seismic shaking. (These effects are 

being studied and reported in detail by other 

investigators.) 

3. Direct displacement on four subparallel surface 

faults through the Managua area. 

From the standpoint of risk from earthquakes, 

and possibly also volcanism, Managua is situated in 

an exceptionally hazardous location. 

On the basis of available geologic and seismologic 

data the following conclusions appear warranted: 

1. Earthquakes comparable in magnitude to those 

of 1931 and 1972 can reasonably be expected 

within the next 50 years. 

2. Some of these earthquakes will be accompanied 

by surface faulting like that in 1931 and 1972. 

3. Maximum hazard from surface faulting is along 

the trace of known active faults, five of which 

have been recognized. 

4. New surface faulting is possible, and even likely, 

within a broad zone that includes all of the 

present area of Managua. 

5. Other conditions of foundation materials, design, 

and construction being equal, maximum damage from shaking will be controlled largely by 

the proximity of structures to the surface ruptures and, in the case of a dipping fault, to the 

fault plane at depth. 

6. In terms of the damage they cause, secondary 

geologic effects such as slope failure, liquefaction, and compaction will be far less significant 

than shaking and fault displacement. 

7. The nature and distribution of the surface faulting are consistent with a tectonic origin for 
REFERENCES CITED 33 

the 1931 and 1972 earthquakes. 

8. Catastrophic eruptions from nearby volcanic 

centers pose a hazard that may be as great as 

that from earthquakes, but one that is as yet 

largely unevaluated. 

RECOMMENDATIONS 

A reconstruction and redevelopment plan for Managua that is sound and economically feasible should 

be based on informed evaluations by experts from a 

number of disciplines. Key roles in the long-range 

decisions that will govern future development should 

be played by earth scientists, engineers, city planners, economists, and political scientists. The required action can take several routes simultaneously, 

among the most critical of which are: 

1. Evaluation of the present and potential sites for 

development so that the seismologic-volcanologic hazards can be minimized. 

2. Development of adequate emergency facilities 

and response systems to reduce the impact of 

natural or other disasters. 

3. Adoption and strict enforcement of building 

codes and zoning ordinances that would ensure 

the integrity of vital utilities and emergency 

services such as communications, water, police, 

fire, and hospital facilities. 

Comprehensive planning for the future of Managua depends first of all on an understanding of the 

geologic hazards and how these hazards may affect 

the works of man. The problems of emergency response systems as well as construction and zoning 

practices are beyond the scope of this report and 

require the expertise of others. However, some of 

the specific recommendations that can be made regarding the geologic and seismologic problems are: 

1. A full evaluation of the hazard from earthquakes 

is required as a basis for local zoning and 

structural design criteria. This would involve 

detailed geologic and seismologic studies primarily directed towards delineating active 

faults and predicting the level of shaking and 

acceleration that can be expected in future 

earthquakes. Other potential geologic hazards 

such as the possibility of landslide damage to 

existing and planned critical facilities, such as 

the Lake Asososca water intake and pumping 

facility, should also be considered. 

2. The hazard from catastrophic volcanic eruptions 

should be evaluated. This would entail detailed 

geologic studies to deduce the eruptive histories of volcanoes in the Managua area and 

geophysical monitoring to determine their 

present state of activity. 

3. To the extent possible, essential underground 

service facilities, such as sewer and waterlines, 

electric power and telephone lines, should be 

routed so that they cross known active fault 

zones in the fewest possible places. Where 

crossings are unavoidable, design provisions 

should be made for fault displacements of at 

least the amounts reported here. 

4. Emergency and critical facilities, such as hospitals, fire stations, police stations, powerplants, 

schools, and important government buildings, 

should be sited well away from known active 

faults and, to the extent possible, outside of 

the zone in which surface faulting is prevalent. 

5. Disaster relief planning for future destructive 

earthquakes should be undertaken and periodically reviewed; the 1931 and 1972 earthquakes provide patterns that should be incorporated into such plans. Especially important 

are the fault trends, amount and nature of 

displacement, the rupture of waterlines at fault 

crossings, and the effects of sucl: ruptures on 

postearthqnake fire hazar~ 

6. Regional earth science studies shonld be undertaken on a long-range basis to evaluate safe 

sites in Nicaragua for future growth and development. Such studies should include both 

geological field investigations and monitoring 

of seismic and volcanic processes. 

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34 MANAGUA, NICARAGUA, EARTHQUAKES OF DECEMBER 23, 1972 

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*u.s. GOVERNMENT PRINTING OFFICE: 1973-515-657/53