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Preliminary Report on the 

Geologic Events Associated 

With the May 31, 1 970, 

Peru Earthquake 
CO WATER RESOURCES UBRARV 

US GtoLOGICAL SURVEY 

WHO, COL3RAOO DISTRICT 

25046, MS 415 OfC 

' : · ~;-:c; CO 80225-0046 
Preliminary Report on the 

Geologic Events Associated 

With the May 31, 1970, 

Peru Earthquake 

By George E. Ericksen and George Plafker, 

U.S. Geological Survey, and 

Jaime Fernandez Concha, Lima, Peru 

G E 0 l 0 G I C A l S U R V E Y C I R C U l A R 639 

CO WATER RESOURCES UBRARY 

U.S. GEOlOGICAl SURVEY 

WRD, COlORADO DISTRICT 

BOX 25046, MS 415 DFC 

DENVER, CO 80225-0046 

Washington 19 70 
United States Department of the Interior 

WALTER J. HICKEL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1970 

Second printing 1970 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page Page 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Earthquake effects--Continued 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 1 Ground effects--Continued 

Seismicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Surficial cracking, compaction, and 

Geologic and physiographic setting . . . . . . . . . . . . . . 5 differential subsidence . . . . . . . . . . . . . . . . . . 16 

Sand boils and water ejection . . . . . . . . . . . . . 17 

Soil slips and fragmentation . . . . . . . . . . . . . . . 17 

Tectonic deformation . . . . . . . . . . . . . . . . . . . . 18 

Cordillera IDanca . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Cordillera Negra . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Earthquake effects . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Santa Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Geologic hazards and reconstruction ............. 18 

Nevados Huascar{n debris avalanche . . . . . . . . . 7 

Yungay-Ranrahirca area . . . . . . . . . . . . . . . . 7 

Huaraz ................................. 18 

Santa Valley south of Huaraz . . . . . . . . . . . . . . . . 20 

Effects downstream from Yungay ......... 12 Yungay and vicinity . . . . . . . . . . . . . . . . . . . . . . . 20 

Landslides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Caraz ................................... 20 

Landslide-dammed lakes . . . . . . . . . . . . . . . . . . 13 Chimbote ............................... 21 

Faults and fissures ....................... 14 Casma .................................. 23 

Geologic foundation conditions Huarmey ................................ 23 

and related earthquake damage ............ 15 Trujillo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

Coastal region ............................ 15 Conclusions and recommendations . . . . • . . . . . . . . . 24 

Ground effects ........................... 16 References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

Figure 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

ILLUSTRATIONS 

Page 

Map of central western Perii showing area affected by the earthquake of 

May 31, 1970 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Oblique aerial view of Nevados Huascaran and the HuascarS:n debris avalanche 

that destroyed Yungay and Ranrahirca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Destruction of adobe buildings in Huaraz as a result of the May 31, 1970, 

earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

A lobe of the Huascar{n debris avalanche that now covers the former city 

of Yungay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Large impact crater in field of south side of Quebrada Llanganuco that was made 

by a huge block of rock or cluster of rocks hurled several hundred meters 

by the Huascariin debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Block of granodiorite (estimated to weigh 700 tons) that was transported by 

the May 31, 1970, Huascaran debris avalanche, located near the site of the 

former town of Ranrahirca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Rotational slide in unconsolidated fluvioglacial sediments, east side of Rio 

Santa at Recuay ................................................................ 14 

Slumping and fissuring of paved road near the bay shore in western Chimbote due 

to liquefaction and lateral spreading of water-saturated beach sediments ..................... 16 

Chimbote·Huallanca railroad damaged as the result of differential compaction 

and lateral spreading of water-saturated unconsolidated sediments ......................... 17 

Concrete-block house, Corporaci6n Peruana del Santa housing development, 

Chimbote, which was torn apart by liquefaction and lateral flow of 

beach sediments that constitute the foundation material ................................. 18 

Sand boil in water-saturated unconsolidated sediments of the Casma Valley; during 

earthquake jets of muddy and sandy water squirted from the two pits ...................... 19 

Destruction of houses in a residential area of southeastern Chimbote where the 

ground settled due to compaction of water-saturated sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

III 

Preliminary Report on the Geologic Events Associated 

with the May 31 j 1970, Peru Earthquake 

By George E. Ericksen and George Plafker, U.S. Geological Survey, 

and Jaime Fernandez Concha, Lima, Peru 

INTRODUCTION 

The Peru earthquake of May 31, 1970, was 

by far the most destructive historic earthquake 

in the Western Hemisphere and ranks high 

among the world's catastrophic natural disasters. 

Within an area of about 65,000 sq km (square 

kilometers) in west-central Peru (fig. 1) the 

earthquake took an estimated 70,000 lives, 

caused 50,000 injuries, and destroyed or 

rendered uninhabitable roughly 186,000 

buildings, representing 80 percent of all 

structures in the area (statistics from Comit~ 

Nacional de Emergencia del Peru, July 2, 19':'0). 

Within the affected area many landslides and 

one cataclysmic debris avalanche disrupted 

communications and temporarily blocked 

ground access to many of the afflicted 

communities in the Andes Mountains. As a 

consequence, rescue and relief operations were 

seriously hampered, and the full extent of the 

disaster did not become known until weeks after 

the earthquake. 

Much information relating to the 

earthquake, aftershocks, geologic effects, and 

structural damage is still being processed and 

will eventually appear in comprehensive reports. 

The purpose of this report is to make 

immediately available the results of our 

reconnaissance observations on the geologic 

effects of the earthquake, as well as some 

conclusions and recommendations regarding 

geologic hazards that should be considered in 

reconstruction. This preliminary report is based 

largely on studies of the effects of the 

May 31, 1970, earthquake made by Fernandez 

Concha from June 1 to 17 and by Ericksen and 

1 

Plafker between June 13 and July 4. During this 

investigation one or more of us visited most of 

the damaged cities and larger towns in the area, 

observed and measured ground effects, and 

made helicopter and fixed-wing photographic 

flights over the Santa Valley. 

Acknowledgments 

We wish to express our deep appreciation to 

the many persons in the earthquake-devastated 

area who cheerfully and informatively answered 

our questions about their observations of 

phenomena related to the seismic movement and 

resulting destruction. Their poise and 

cheerfulness in the aftermath of this cataclysmic 

tragedy have been truly magnificent. 

Thanks are given to our. Peruvian and 

American colleagues who supported our 

investigations and furnished invaluable 

information. We especially wish to express our 

gratitude to Ing. Alberto Giesecke, Director, 

Instituto Geoffsico del Pertl, and Enrique 

Gajardo, Director, Centro Regional de 

Sismologia para America del Sur, for advice, 

information, and logistical support. 

We wish to thank the Corporaci6n Peruana 

del Santa for furnishing us with a copy of the 

report of Ing. Jaime Femlindez Concha, of 

which we have made liberal use in the 

preparation of this report. We also wish to thank 

Ing. Gilberta Tisnado of the Instituto Geoflsico 

del Peru who assisted us with the field 

investigations, logistic arrangements, and in 

obtaining data from various Government and 

private sources. The Servicio Aerofotogr,fico 
10" 

12" 

Santiago de Chuco ................. 

0 /~, 

, ' ( ' j' Co~ngo '\ 

~ \ 

MAIN SHOCK. 

• 

~ 

• 

• 

• 3

• 3 

• 

•• 9 2 

200 

• 

• 

• 

EXPLANATION 

.3 

N 

Location of aftershocks 

More than one shock 

indicated by number 

Approximate limit of 

moderate to heavy damage 

0 50 MILES 

~--------------~ 

Huaylas o 

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NEVA DOS 

A HU.ASCA,AN % 

Y~Yungay 

~ uaz 1 

\ 0 Huari 

~ s'an Marcos ~01 ) I , 

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Chiquia~ 

0 80 KILOMETERS 

ne contours in meters 

oeogota 

Figure 1.-Map of central western PerU' showing area affected by the earthquake of May 31, 1970. Epicenters 

indicated for main shock and larger aftershocks are from U.S. Coast and Geodetic Survey (Mark Spaeth, or111 

commun., July 9, 1970). 

2 
Nacional del Peru kindly furnished the oblique 

aerial photograph of the Huascaran debris 

avalanche (fig. 2). lng. Daniel Huaco also 

accompanied us during investigations in the 

coastal region and provided us with data 

obtained during his studies in the vicinity of 

Chimbote. 

Figure 2.-0blique aerial view of Nevados Huascar'n and the Huascar3:n debris avalanche that destroyed Yungay and 

Ranrahirca. 

3 
Although Ericksen and Plafker were sent to 

Peru as direct representatives of the U.S. 

Department of the Interior, they worked as part 

of a special UNESCO team that was formed to 

make a preliminary study of the earthquake. We 

wish to express our thanks to our structural 

engineer colleagues on this team, Prof. Glen Berg 

of the University of Michigan and Dr. Raul 

Husid of the University of Chile, and to their 

associate Mr. John Meehan of the State of 

California Department of Schools for 

information that helped us to evaluate the 

relationship of structural failures to foundation 

conditions. 

SEISMICITY 

According to the U.S. Coast and Geodetic 

Survey (P. D. E. card, June 1, 1970), the initial 

shock, Richter magnitude of 7. 7, occurred on 

May 31, 1970, at 3:23 p.m. Peruvian time 

(3:23 e.s.t.). The epicenter was at lat 9.2° S., 

long 78.8° W., or at sea 25 km (kilometers) 

west of the port city of Chimbote in the 

northwestern part of the Department of Ancash 

(fig. 1). Data from local seismic records in Peru 

(Enrique Gajardo W., Rept. to the Smithsonian 

Institution Center for Short-Lived Phenomena, 

June 23, 1970) suggest that the epicenter may 

actually be somewhat farther seaward than 

shown in figure 1, but that it would lie on the 

continental shelf. The hypocenter, or focus, was 

determined by the Coast and Geodetic Survey to 

be at a depth of 56 km. 

According to eyewitness accounts, the 

earthquake began without prior warning as a 

gentle swaying motion that lasted for a few 

seconds and was followed by hard shaking 

variously estimated as lasting from 30 to 90 

seconds (the best estimates seem to be about 45 

seconds). There were no strong-motion 

accelerographs in the affected area at the time of 

the main shock, but several portable instruments 

that were installed later recorded some of the 

aftershocks. Most eyewitnesses described the 

earthquake tremors as having a pronounced 

side-to-side motion that made it d:fficult or 

impossible to walk or run, but not strong 

enough to throw people to the ground. In most 

places visited, the adobe structures reportedly 

began to collapse after only about 15 seconds of 

hard shaking. 

4 

The area of structural damage extends along 

the coast from north of Trujillo to slightly south 

of Las Zorras, and it extends inland nearly to 

the Rio Marallon (unpub. data, American 

Andean Relief Expedition to Ancash) roughly 

12 0 km from the coast (fig. 1 ). Our 

investigations suggest a maximum intensity of 

shaking in the coastal area between Casma and 

Chimbote (VII-VIII on the Modified Mercalli 

scale). Intensity of shaking was slightly less in 

the Santa Valley, attaining a probable maximum 

of VII on the MM scale in the region from 

Huaraz northward to Huallanca. 

The epicenters of 37 aftershocks recorded at 

teleseismic distances on the Worldwide 

Seismograph network through June 8 are shown 

in figure 1. Most of these have magnitudes 

between 4 and 6% and hypocentral depths on 

the order of 45-66 km. Many strong 

aftershocks continued to be felt in the disaster 

area throughout the time we were working 

there. 

The spatial distribution of the main shock 

and all but two of the aftershocks plotted in 

figure 1 suggests that the faulting associated 

with the May 31, 1970, earthquake occurred at 

depths between 45 and 66 km on a fault surface 

about 140 km long parallel to the coast and 

6 5 km wide. Fault movement at depth is 

indicated by the absence of movement on 

known surface faults within the region, or of 

tectonic warping along the coast, as well as the 

lack of a tsunami (maremoto or seismic sea 

wave) that would suggest tectonic deformation 

of the sea floor. The significance of the two 

aftershocks having epicenters east of the 

Cordillera Blanca (fig. 1) is not apparent from 

the available data. 

Neither the nature of displacement nor the 

dip of the fault are known, but it is reasonable 

to expect that focal mechanism solutions of 

earthquakes in this sequence will eventually 

provide information on the displacement at the 

earthquake source. The May 31, 1970, 

earthquake occurred in a well-defined belt of 

seismicity that follows the Peru-Chile Arc for 

7,000 km from Venezuela to southern Chile. 

The high seismic activity, youthful mountain 

chains, volcanoes, and oceanic trenches that 

characterize this belt are believed by many 

geologists and geophysicists to result from 

progressive underthrusting of the Pacific Ocean 
basin beneath the South American continent. 

The nature and distribution of the 

May 31, 1970, earthquake and its aftershocks 

suggest that this earthquake also was probably 

genetically related to this fundamental zone of 

underthrusting along the western margin of 

South America. Although numerous large 

earthquakes have been recorded throughout the 

Peruvian sector of the arc during this century, it 

is noteworthy that the area affected by the 

May 31, 1970, event had been relatively free of 

destructive shallow-focus earthquakes 

(Gutenberg and Richter, 1954; Silgado, 1968). 

This is in accord with the testimony of Santa 

Valley residents who stated that a major 

destructive earthquake had not occurred in that 

area previously for a period of at least three 

generations. 

GEOLOGIC AND PHYSIOGRAPHIC SETTING 

dissected, fluvioglacial and mudflow material, 

which locally attains thicknesses of several 

hundred meters. A considerable part of the 

mudflow and debris-flow material originated 

from bursting of glacial lakes and from ice-rock 

avalanches in the Cordillera Blanca. Although 

this material is coherent enough to stand in 

near-vertical roadcuts and high streambanks, it is 

relatively unstable and slides readily when 

saturated or subjected to seismic shaking. Most 

of the cities and towns in the valley have been 

built on this type of material or on uncemented 

alluvial and fluvioglacial deposits. 

Cordillera Blanca 

The imposing Cordillera Blanca takes its 

name from a nearly continuous capping of 

glaciers and snowfields. The highest mountain in 

the range is Nevados Huascarc1:n. It culminates in 

a glacier-covered north peak 6,663 meters 

(21,860 ft) high and in an even higher south 

The principal populated areas affected by peak that attains an altitude of 6, 764 meters 

the earthquake of May 31, 1970, include the (22,190 ft) (fig. 2). Many other ice-covered 

coastal region from Las Zorras in the south t,o peaks in the range are 5,500 meters or more in 

Trujillo on the north, and the valley of the Rio altitude. Deep U-shaped glacial valleys of the 

Santa south of Huallanca (fig. 1). This part of Cordillera Blanca drain westward into the Rfo 

the Santa Valley commonly is referred to as the Santa and eastward into the headwaters of the 

Callejon de Huaylas (Huaylas Corridor) but in Rio Maranon (fig. 1). Several of these valleys 

the strictest sense, the Callejon de Hua:ylas is now contain morainal lakes and formerly 

only that part of the Santa Valley between (perhaps during the last 10,000 years) probably 

Huaraz and Huallanca. The lower part of the all contained one or more lakes. Some of the 

Callejon " de Huaylas is a deep, steep-walled older lakes have gradually filled with sediment 

canyon, about 15 km long, at the bottom of and now appear as flat valley floors or swampy 

which is a narrow vertical-walled slot, as much as areas. Some lakes gradually cut through their 

several hundred meters deep, called the Canon morainal dams and thus were drained. Other 

del Pato. lakes were drained by sudden bursting of their 

< The upper Rw Santa flows between two morainal dams, probably owing mostly to 

major mountain ranges of the Andes, the overtopping of the dams by waves generated by 

Cordillera Blanca on the east and the Cordillera ice or rockfalls. Sudden drainage of these lakes 

Negra on the west. The Cordillera Blanca has a can result in catastrophic mudflows such as the 

core of granodiorite; Mesozoic marine mudflow of December 1941 that destroyed 

sedimentary rocks lie on the flanks and at places one-fourth of Huaraz. 

cap the granodiorite in the center of the range. The upper slopes in the Cordillera Blanca are 

In contrast, the Cordillera Negra consists largely oversteepened, commonly ranging from 45° to 

of Tertiary volcanic flow and volcanic clastic nearly 9 0° ; they are thus locally in an 

rocks, chiefly andesitic, but also rhyolitic in extremely unstable condition. In some areas, as 

composition. Granodiorite and Mesozoic observed by Ericksen in 1948-49, small icefalls 

sedimentary rocks are widely exposed in the take place several times a day. At other localities 

northern part of the Cordillera Negra. The valley major ice avalanches are reported every few 

of the Rfo Santa is partly filled with deeply months or few years. Evidently the north peak 

5 
of Huascaran is presently one of the most 

unstable ice-covered areas in the Cordillera 

Blanca. The northwest side of this peak, the 

source of the avalanche that destroyed Yungay 

and Ranrahirca at the time of the May 31, 1970, 

earthquake, also was the source of a smaller 

avalanche that destroyed most of Ranrahirca in 

1962. The distribution of debris-flow deposits 

throughout the area suggests that repeated ice 

and rock avalanches have originated on the 

flanks of Huascifan during Holocene time. 

Cordillera Negra 

The Cordillera Negra, in contrast to the 

Cordillera Blanca, has a more gentle and 

subdued topography, and its relatively broad, 

gently undulating crest is generally about 14,000 

feet in altitude. It lacks glaciers and snowfields. 

Valleys on the flanks are V-shaped, and are 

deeply incised in their lower parts, indicating 

that they were cut by streams rather than by 

glaciers. Small cirques and lateral moraines near 

the crest of the range attest to former local 

glaciation. Volcanic rocks on steep slopes of the 

Cordillera Negra are locally deeply weathered or 

strongly fractured and consequently are subject 

to sliding during the rainy seasons or in response 

to seismic movement. 

EARTHQUAKE EFFECTS 

Most of the effects of the May 31, 1970, 

earthquake herein described are of the kind that 

are characteristic of great earthquakes; they have 

been thoroughly documented in many reports, 

including reports describing earthquakes that 

have affected the Andean region. Because this 

report is of preliminary nature, we have not 

attempted to compare destructive effects of the 

May 31, 1970, Perti earthquake with other great 

earthquakes that have taken place along the west 

coast of South America. However, the 

Huascar~ avalanche of May 31, 1970, appears 

to be a geologic event that, in size and 

destructiveness, exceeds any avalanche known to 

have accompanied any other earthquake. 

6 

Santa Valley 

It is estimated that throughout the length of 

the Santa Valley above Huallanca, a distance of 

about 130 km, 90 percent of the buildings have 

been seriously damaged or destroyed. The 

widespread damage is due largely to (1) adobe 

construction (fig. 3), which offers little 

resistance to earthquake shock, (2) to thousands 

or tens of thousands of small landslides and 

rockfalls, and (3) to the debris avalanche that 

destroyed the city of Yungay ahd the nearby 

town of Ranrahirca (fig. 2). Two- and 

three-story adobe buildings with heavy tile roofs 

were the most severely damaged. In addition, 

some brick buildings and a few reinforced 

concrete buildings were damaged or destroyed. 

Brick or concrete-block buildings without a 

reinforced concrete framework offered only a 

little more resistance to earthquake shock than 

did adobe buildings, although rarely did they 

collapse totally. The few reinforced concrete 

buildings that were severely damaged were of 

poor construction; they either had very poor 

quality concrete that crumbled during seismic 

movement, or they lacked proper pillar support 

and shear walls. 

An accurate estimate of deaths in the Santa 

Valley is not yet available, and it is doubtful 

that the true number will ever be known. By far 

the greatest number of casualties were at 

Yungay (320 known dead; 15,000 missing and 

presumed dead) and in Huaraz (800 dead; 

16,000 missing and presumed dead). (Statistics 

of the Comit~ Nacional de Emergencia of Peril, 

July 2, 1970.) In addition to the known dead 

and missing inhabitants of Yungay, an unknown 

additional number of Sunday visitors and 

marketers of farm products, perhaps totaling 

several thousand persons, were also killed by the 

avalanche that destroyed Yungay. 

Yungay had a pre-earthquake population of 

about 19,000 and Huaraz, the largest city in the 

Santa Valley and the capital of the Department 

of Ancash, had about 65,000 residents. Most 

deaths in Huaraz were in the older section of the 

city where collapse of 2- and 3-story buildings 

killed both occupants and people who fled into 

the narrow streets for safety. One area of some 
J 

( 

\ 

.. .. 

Figure 3.-Destruction of adobe buildings in Huaraz as a result of the May 31, 1970, earthquake. 

100 square blocks now is a mass of rubble and 

broken adobe walls where streets are filled with 

rubble to depths of as much as 10 feet. Figure 3 

is a typical street scene in this area of Huaraz. At 

the time of our visit (end of June 1970), local 

authorities estimated that several thousand 

people were still buried beneath the rubble of 

collapsed buildings. In striking contrast to this 

adobe disaster, most of the few buildings in 

Huaraz that were built of brick with reinforced 

concrete frames, including one of four stories 

and one of five stories, were only slightly to 

moderately damaged. On the other hand, a 

four-story school building of reinforced concrete 

and brick with inadequate shear walls did 

collapse. 

Roughly 3,100 people out of a population 

of 5,000 are dead or missing at Mancos where 

roughly 70 percent of the adobe buildings were 

destroyed. Ranrahirca was also destroyed by a 

tongue of the same avalanche that buried 

Yungay, and an estimated 1,800 of the 1,850 

residents were killed. Other villages and towns in 

7 

the Santa Valley reported deaths ranging from 

less than 10 to several hundred. Damage to 

communities on the east side of the Cordillera 

Blanca was relatively slight and casualties few. 

Although many of the adobe houses in this area 

collapsed, surprisingly few occupants were 

injured. Innumerable small landslides and 

rockfalls triggered by the earthquake caused 

several hundred injuries and an unknown 

number of fatalities through the region. 

Nevados Huascar~n debris avalanche 

Yungay-Ranrahirca area 

By far the most destructive and geologically 

fascinating aspect of the earthquake was the 

cataclysmic avalanche of ice and rock that fell 

from the glacier-covered north peak of Nevados 

Huascar~n (fig. 2). The main part of the debris 

avalanche appears to have involved a slab of ice 

and rock about 800 meters wide that broke 
away from the sheer west face of the peak, at an 

estimated altitude between 5,500 and 6,500 

meters. Our observations of the configuration of 

the slide source area, and on the composition, 

volume, and flow characteristics of the 

avalanche material indicate that the original slide 

mass probably involved a volume of ice and rock 

that was considerably greater than 25 million 

cum (cubic meters). The amount of rock was 

probably much greater than the amount of ice. 

By means of comparison of post-earthquake 

vertical and oblique aerial photographs with 

photographs taken prior to the earthquake, we 

hope to be able to make a more accurate 

estimate of the volumes of ice and rock that fell. 

As the avalanche moved down the side of 

Huascaran and down the Llanganuco Valley it 

took on the character of a highly fluid mudflow, 

owing to melting of part of the ice (as a result of 

friction and other energy release in the rapidly 

moving mass) and to water and water-saturated 

sediments picked up in Llanganuco Valley. 

Within a few minutes after the first tremors of 

the earthquake were felt, an enormous mass of 

rock, ice, and mud had sped 16 km from 

Huascaran to the Rio Santa. The avalanche 

disrupted the Rio Santa temporarily, so that it 

flowed upstream for a distance of about 2 km; it 

also lapped up onto the lower slopes of 

the Cordillera Negra, and flowed downstream. 

An area of more than 15 sq km was 

devastated. Yungay was obliterated by a 

relatively small tongue of mud ;md rock several 

meters thick that swept over a ridge 100-200 

meters high that stood between Yungay and the 

main stream of the avalanche, which followed 

the Rio Llanguanuco Valley to the south 

(fig. 4). Only the tops of a few palm trees in the 

central plaza and part of the main cathedral now 

protrude above the mud, and mark the site of 

this formerly prosperous and picturesque city. 

Figure 4.-A lobe of the Huascar{n debris avalanche that now covers the former city of Yungay; the avalanche 

flowed over a 100- to 200-meter-high ridge (arrow) between Yungay and Llanganuco Valley. 

8 
The main part of the debris avalanche, which 

was channeled along the Rio Llanganuco Valley, 

buried most of Ranrahirca as well as parts or all 

of several ~mailer villages along th~fertile valleys 

of the Rio Llanganuco and Rio Santa. An 

estimated 1, 800 people were killed at 

Ranrahirca and an unknown, but probably large, 

number of additional casualities were sustained 

elsewhere in the extensive area covered by the 

debris lobe. Only 8 years earlier, on 

January 10, 1962, the Ranrahirca area was 

devastated by an avalanche that originated from 

the same part of the north peak of Nevados 

Huascaran as the recent one. According to a 

published account by Benjamin Morales (1966), 

the 1962 avalanche contained an estimated 2.5-3 

million cu m of ice and rock; it wiped out nine 

small towns, including Ranrahirca, and killed 

4,000 people. Unlike the May 31, 1970, 

avalanche, however, the 1962 event was not 

triggered by a felt earthquake, and the debris did 

not have sufficient velocity or volume to flow 

over the ridge between Yungay and the 

Llanganuco Valley. 

According to eyewitnesses, the 1970 debris 

avalanche on Huascaran was triggered during the 

earthquake. It moved downslope at high velocity 

with a deafening noise and was everywhere 

accompanied or preceded by a strong turbul;mt 

airblast. Accounts of survivors suggest that the 

debris avalanche traveled the 14.5-km distance 

from its source to the vicinity of the cemetery at 

Yungay in 2-4 minutes-an average velocity of 

between 217 and 435 km per hr (kilometers 

per hour). A velocity on the order of 400 

km per hr is indicated near the middle part of 

the course by the trajectories of thousands of 

boulders, many weighing more than 3 tons, that 

in some places were hurled more than 700 

meters across the Llanganuco Valley (fig. 5). 

This deadly rain of rocks killed and injured 

many people and was extremely destructive to 

buildings, livestock, and vegetation. The 

unusually high velocity and large volume of the 

debris avalanche allowed it to override large 

topographic irregularities including the ridge 

between the Llanganuco Valley and Yungay. Its ,( 

momentum at the R10 Santa, almost 16 km 

from the source, carried it across the river and as 

much as 50 meters vertically and several 

hundred meters horizontally up the opposite 

bank, where it destroyed part of a small village. 

The avalanche's velocity was due primarily 

to the combination of steep slopes (as much as 

70° ) in the source area and to the great vertical 

relief j 3, 7 00 meters) along i t s 16-km path to 

the Rio Santa. In the upper half of its course the 

vertical drop is almost 3,000 meters at an 

average slope of 23° ; in the lower half the drop 

is only 700 meters and the slope averages 5°. 

Frictional resistance to sliding of the mass may 

have been significantly reduced by admixture of 

snow and ice and locally, perhaps, by entrained 

air beneath the debris. Air-cushioned flow near 

the source is suggested by the fact that the 

debris avalanche apparently moved across ridges 

of unconsolidated morainal material without 

disrupting them. 

Most of the damage caused by the avalanche 

was in the populated and cultivated lower part 

of its course below about 2,600 meters altitude. 

Here, the debris spread across an area of low 

relief in three lobes in an area of about 9 sq k~. 

Maximum width across this area, near the Rio 

Santa, is 4.3 km. These lobes buried Yungay, 

most of Ranrahirca, and several smaller 

communities. They also destroyed 2.3 km of the 

main highway in the Santa Valley and about an 

equal amount of the primary electric 

transmission line from the hydroelectric plant at 

Huallanca. 

The thickness of the debris deposit, as 

measured at a number of places along its margins 

and in incised stream channels, ranges from a 

few centimeters to 10 meters and is 

conservatively estimated to average about 3 

meters. The thickness is about 5 meters at the 

site of the plaza in Yungay, as estimated by 

comparing the heights of four palm trees that 

extend above the.debris surface with the overall 

height of palm trees from the same place that 

were uprooted by the debris flow. Assuming an 

average depth of 3 meters, the volume of debris 

in the three lobes alone is on the order of 27 

million cum. An additional 5-10 million cum 

of debris (mainly boulders) was deposited in the 

upper part of the avalanche over an area of 

about 10 sq km, and an even larger volume of 

debris has been flushed down the Santa River. 

Thus, the total volume of solid material moved 

by the debris avalanche could easily be on the 

order of 50 million cum. 

The material deposited by the debris 

avalanche is mainly mud and boulders. Nearthe 

9 
Figure 5.-Large impact crater (2.5 meters deep, 7 meters wide, and 11 meters long) in field at the south side of 

Quebrada IJanganuco, which was made by a huge block of rock. or cluster of rocks hurled several hundred meters 

by the Huarasca'n debris avalanche. All rocks in or near crater are projectiles or fragments of projectiles from the 

avalanche. The two large rocks in the crater are about 2% meters in longest dimension. 

terminus, where the water content of the mass 

was increased by melting of incorporated ice and 

snow and by the overrunning of streams, it 

locally spread out as a viscous debris flow 

consisting of mud with embedded boulders. The 

mud characteristically is a medium-gray silt 

containing subordinate very fine sand. It 

probably contains a high percentage of glacial 

rock flour derived from morainal deposits along 

the mountain front. It is soft and sticky when 

wet, and very hard, with deep dessication cracks, 

when dry. One survivor said that the mud near 

the terminus was too soft to walk on until about 

8 days after the avalanche. Except for a few 

poorly drained soft spots, the surface was 

generally as hard as adobe at the time of our 

visit 4 weeks after the earthquake. Residents of 

the area also claim that the surfaces of the debris 

lobes were covered with blocks of ice after the 

avalanche; numerous conical collapse pits in the 

mud, as much as several meters across, clearly 

mark the sites of ice blocks that have melted out 

of the mud. Some of the local soft areas that we 

noted on the debris lobes were said by 

road-construction crews to be over buried blocks 

of partly melted ice. 

Surface features of the debris lobes, such as 

steep margins, grooved surfaces, mud spatters on 

boulders, concentrically grooved mud-covered 

boulders (indicating slow rolling), and 

depressions or "wakes" behind obstacles in the 

path of flow, all suggest that the mud had 

10 
appreciable viscosity near the termini of the 

lobes. Conversely, splattering of mud on the 

upstream side of obstacles and far from the 

margins of the flow are indicative of local high 

surface fluidity. Areas of smooth-surfaced, 

boulder-free mud within the debris flow 

apparently represent the melting of ice and 

"dewatering" df fluid mud from between the 

coarser material (fig. 6). 

Quantities of boulders and angular blocks of 

rock vary considerably from place to place, 

generally constituting an estimated 10-50 

percent of the total volume of the debris lobes. 

Cobble- and peoble-size rock fragments seem to 

be relatively sparse in the material covering 

Y u ngay and Ranrahirca. The overwhelming 

majority of the rock debris consists of massive 

granodiorite of the type that makes up Nevados 

Huascar~n. The largest block observed is a 

14,000-ton giant that was transported by the 

avalanche and deposited on the Ranrahirca lobe 

near the site of Ranrahirca. The largest boulder 

measured on the Yungay lobe was about 7,000 

tons. Most of the rock fragments are angular and 

fresh as though recently derived from the 

outcrop; such an angular block, estimated to 

weigh about 700 tons, is shown in figure 6. A 

number of them, however, were previously 

transported by earlier debris flows and .(or) 

glaciers, as indicated by adhering soil, 

weathering of one or more surfaces, or multiple 

glacial faceting and striations. A few of the 

fragments consist of weakly cemented 

Quaternary gravels and deeply weathered 

granodiorite. The presence of large blocks of 

such semicoherent material in the debris flow 

Figure 6.-~lock of granodiorite (estimated to weigh 700 tons) that was transported by the May 31, 1970, 

Huascaran debris avalanche, located near the site of the former town of Ranrahirca. Layer of mud showing 

polygonal cracks resulted from seepage from the avalanche after it had come to rest. Top of block is covered with 

mud and rock fragments deposited by the avalanche after the block had come to rest. 

11 
and the general absence of percussion marks 

suggest that the boulders did not abrade one 

another significantly within the moving 

avalanche. Many of the larger boulders now rest 

directly on the original ground surface beneath 

the debris lobes or are embedded in a layer of 

mud that is thin relative to boulder height. 

However, such boulders are covered with mud 

and commonly have smaller rocks on their upper 

surfaces (see fig. 6), implying that the mud was 

thick enough to cover them at some stage during 

the debris avalanche. 

The debris avalanche probably moved as a 

high-speed wave pulse or slug that picked up and 

moved even the largest boulders at its front 

while dropping others out behind. Such a 

mechanism would be consistent with eyewitness 

accounts of a "wall of debris as high as a 10 

story building," with the pronounced air blast 

that accompanied the avalanche, with the great 

disparity between the height of the mud 

coatings on boulders and other objects in the 

path of the debris avalanche, and with the 

relatively thin residual layer of mud over much 

of the area traversed by the debris flow. 

Furthermore, a large destructive slug of muddy 

water is known to have passed on down the Ri'o 

•Santa to the sea. A thorough ground and 

photogeologic investigation of the debris flow 

deposits and extensive interviews of 

eyewitnesses to the event undoubtedly would 

shed light on the intriguing problem of the 

transport mechanism of this debris mass. 

Effects downstream from Yungay 

The flow of muddy water and rock debris , continued down the Rio Santa, causing 

extensive damage to structures, transportation 

routes, and communications networks 

downstream from Yungay. According to 

eyewitness accounts, the flow reached 

Huallanca, 52 km downstream from Yungay, at 

about 5:00p.m. This rate of flow indicates an 

average velocity of roughly 35 km per hr. This 

approximate velocity is consistent with 

information that the flow reached the Mirador 

railroad station, 100 km downstream from 

12 

Yungay, at 6:00p.m. and Tablones, a distance 

of 130 km from Yungay, at approximately 

7:00p.m. 

Eleven kilometers downstream from Yungay 

the debris flow overflowed the Ri~ Santa's 

channel and inundated the Caraz airport, much 

of the highway, and extensive tracts of 

agricultural lands. Caraz itself was not damaged 

by the flow. However, the Rio Paron, which 

flows through Caraz into the Rio Santa, did have 

a minor debris flow that locally overflowed its 

banks and damaged small areas of the town. 

From aerial inspection, it appears as if this 

debris flow originated in an extensive area of 

surficial earthflows and rockslides just a few 

kilometers east of Caraz. 

Between Caraz and the Canon del Pato, the 

highway was locally covered by mud where the 

debris flow overflowed the Rio Santa channel. 

The highway bridge across the Rio Santa at 

Choquechaca was destroyed. 

The hydroelectric plant at the mouth of the 

Canon del Pato near Huallanca, having an 

installed capacity of 100,000 kw (kilowatts) and 

an ultimate capacity of 150,000 kw, was not 

damaged by the earthquake or debris flow. 

Safety gates at the tunnel intakes were shut in 

time to prevent entrance of debris into the 

turbines. However, the diversion dam at the 

tunnel intake was destroyed and the access road 

through the precipitous Canon del Pato was 

largely buried or swept away by landslides. Also, 

the railroad bridge that provided access to the 

powerplant at the lower end of the Canon del 

Pato was destroyed by the debris flow. 

Low-lying parts of the town of Huallanca 

and the camp of the hydroelectric company 

(Corporacion Peruana del Santa) immediately 

below the Canon del Pato were damaged by the 

debris flow, and five persons in one of the , homes were swept away. The Rio Santa has a 

channel that averages about 100 meters wide at 

Huallanca. According to residents in Huallanca, 

the river dried up for about an hour after the 

earthquake and prior to the arrival of the debris 

flow. Upon arrival of the debris-laden waters, 

the river rose 20 meters above its prequake level, 

and reportedly ran full for 2-4 hours before 

beginning to subside. Residents describe the 
front of the flow as a dark-colored, noisy, 

turbulent wave, containing abundant blocks of 

ice, that was competent enough to carry off 

"room-size" boulders from the town site. After 

the main part of the debris flow passed, dozens 

of corpses, and vehicles and other manmade 

materials were deposited near Huallanca. 

In addition to the damage from the flooding, 

most adobe structures in Huallanca and the 

camp at the hydroelectric plant were extensively 

damaged by the earthquake, and a few were 

struck by boulders shaken loose from the steep 

valley walls. Two or three people were 

reportedly killed in houses that collapsed. 

The part of the Santa Valley between 

Huallanca and the coast could not be visited 

during our reconnaissance investigations, but 

was examined from the air. Large segments of 

the railroad line between Huallanca and 

Chimbote are covered with debris deposits or 

were washed away by the debris flow, and 

railroad stations were destroyed by the shaking 

or by inundation by the debris flow. 

Landslides 

Landslides of all types ranging from several 

tens of feet to a few thousand feet in diameter 

or maximum length are found throughout the 

steep slopes both of the Cordillera Negra and 

Cordillera Blanca. Such landslides were triggered 

by the combination of lateral and vertical 

accelerations imparted to the slide masses during 

the seismic shaking. Most of the rockfalls and 

soilfalls are on steep, relatively dry slopes. Some 

larger rotational slides and surficial slumps, 

particularly those near the bottom of the Santa 

Valley and intermediate slopes of the Cordillera 

Blanca, are in water-saturated ground where 

dynamic ground-water loading in the pores and 

joints of the affected materials probably 

increased the susceptibility to sliding. A record 

rainfall during the past wet season, which 

included unusually heavy rains as late as May 17, 

probably contributed significantly to the 

amount of landsliding observed. 

In general, the larger landslides were 

concentrated along the lower courses of the R1o 

Santa and its tributaries, which commonly are 

deeply incised into unconsolidated Quaternary 

fill. In this area, many landslides were clearly 

related to roadcuts and to the terraced and 

irrigated cultivated fields on steep hillsides. 

These landslides can be classified according to 

the type of failure, and each of the many classes 

can be correlated closely with the nature of 

material involved and the slope inclination. 

Most of the landslides observed were loose 

rockfalls and soilfalls or slides along the steep 

valley walls and roadcuts. Thousands of 

landslides of this type occur throughout the 

region. They are particularly numerous along the 

steep valley walls in the Canon del Pato area and 

in an area of particularly unstable volcanic rock 

and steep slopes in the Cordillera Negra 

downstream from Huallanca. Several small slides 

downstream from the Carton del Pato were 

observed during our overflights and visits to the 

area on June 26 and 28, and visibility was often 

seriously impaired by the dense dust clouds 

arising from these slides. The great abundance of 

soilfalls and rockfalls, as well as loose individual 

rocks that tumbled down the hillsides during the 

earthquake, caused many injuries and a 

substantial, but unknown, number of deaths 

throughout the Santa Valley. Rockfalls 

reportedly also accounted for most, or all, of the 

casualties sustained on the east flank of the 

Cordillera Blanca. Rockfalls and soilfalls have 

blocked road and railroad routes at many places 

and they constitute a continuing hazard, as 

many are likely to be reactivated during the next 

rainy season. 

A large rotational slide was observed along 

the east bank of the Ri~ Santa at Recuay 

(fig. 7). This slide involves a block of 

fluvioglacial sediments about 1 km long and 

100-200 meters high above river level, and has a 

headwall scarp estimated to be as much as 15 

meters high. At its northern end, the toe of the 

slide extends across the Rio Santa, which is 

about 50 meters wide at this point. The 

rotational movement thrust the river bottom 

upward about 5 meters, thereby blocking flow 

of the Rio Santa. This landslide-dammed lake is 

described below. 

Landslide-dammed lakes 

As described in the preceding section, a 

small lake was formed on the Rio Santa behind 

13 
I 

Figure 7.-Rotational slide in unconsolidated fluvioglacial sediments, east side of Rio Santa at Recuay; a small lake 

formed behind toe of slide. Headwall of slide (arrows) is as much as lQ-15 meters high. 

the rotational landslide at the town of Recuay. 

At the time of our visit on June 25, this 

landslide-dammed lake was about 300 meters 

long and 150 meters wide. The lake had been 

somewhat larger previously but in , order to 

remove the hazard of flooding, Cia. Minera 

Alianza, a mining company headquartered at the 

nearby town of Ticapampa, lowered the level 

about 211:! meters by two trenches 6 meters long 

that were cut through the dam. 

A rockslide dammed the stream draining 

upper Llanganuco Lake, causing the lake level to 

rise 7-8 meters between the time of the 

earthquake on May 31 and the end of June. The 

lake more than doubled its length as it flooded 

the nearly flat alluvial plain at its head. This 

slide was first observed by Fernandez Concha 2 

days after the earthquake. Still later the 

Corporacion Peruana del Santa started to cut a 

trench through the slide to limit the rise in lake 

level to about 8 meters instead of an estimated 

15 meters that was expected if such a trench 

were not cut. This trench was completed in early 

July. 

Faults and fissures 

Many faults exist in the Santa Valley, but 

apparently none moved during the 

May 31, 1970, earthquake. The most prominent 

recent faults of the area are along the foot of the 

Cordillera Blanca. They are marked by an en 

echelon system of fault scarps, as much as 20 

feet high, that cut Holocene glacial moraines. 

Although it is evident that these faults have 

moved recently (perhaps at some time during 

the past few hundred years), there is no evidence 

of movement during the May 31 earthquake. We 

were able to examine a strand of this fault 

system on the ground in Quebrada Querococha, 

to the east of Ticapampa, and to observe other 

scarps near Huaraz during a helicopter flight. 

Fissuring related to incipient landsliding or 

differential compaction of unconsolidated 

14 

------- - - -
sediments is extensive in the Santa Valley. 

Fissures are most clearly visible in the main road 

through the valley. Most of these are parallel or 

subparallel to the road and to the Ri~ Santa, and 

most were evidently opened by incipient sliding 

and slumping of unconsolidated sediment 

toward the Rio Santa. Such open fissures are as 

much as a foot wide and several feet deep. Some 

are due to differential compaction of the 

unconsolidated sediments. Fissures related to 

lateral extension and incipient sliding were also 

observed at many places in relatively flat fields 

near the bottom of the Santa Valley and on 

moderately inclined to steep hillslopes on the 

sides of the valley. 

Geologic foundation conditions and related 

earthquake damage 

Nearly all the cities, towns, and villages in 

the valley of the Ri'o Santa are built on 

unconsolidated to semiconsolidated fluvioglacial 

material and debris-flow deposits in which the 

ground-water table is generally less than 5 

meters below the surface. During the earthquake 

this foundation material locally was subject to 

differential compaction and to downslope 

slumping. In some places, damage to structures 

is clearly related to foundation failure, whereas 

in others the pattern of damage is somewhat 

variable and seemingly unrelated to geologic 

foundation conditions. For example, some 

villages were almost totally destroyed, whereas 

near by villages of apparently similar 

construction and geologic foundation show only 

moderate damage. Similarly, parts of Huaraz 

were leveled, whereas other areas in the city 

were only moderately damaged or are 

undamaged. We have no explanation for the 

cause of these striking variations. Subtle 

variations in composition and (or) water content 

of the foundation material could be an 

important factor, but slight differences in 

construction and the pattern of seismic wave 

propagation may be equally important. Detailed 

studies of damage distribution by teams of 

geologists, structural engineers, and seismologists 

are needed before firm conclusions may be 

drawn regarding the cause for this erratic 

damage distribution. 

Coastal region 

Severe earthquake damage occurred in the 

coastal region of Peru from Trujillo 

(lat 8° 10' S.) on the north to the village of Las 

Zorras (lat 10° 17' S.) on the south, a 

longitudinal distance of about 260 km. 

Throughout this area there has been major 

damage or total destruction of adobe buildings, 

which constitute a large proportion of the 

residences. New brick houses that lack 

reinforced concrete pillars, such as are found in 

some communities and particularly in one entire 

subdivision of Chimbote, have been seriously 

damaged or destroyed. Well-constructed houses 

and buildings, consistin_g of reinforced concrete 

pillars and connecting stringers with brick wall 

panels, generally show little or no damage, 

although they may be entirely surrounded by 

demolished adobe structures. The few 

woodframe buildings in the area were virtually 

unaffected by the earthquake. 

Damage of large reinforced concrete or 

concrete and brick structures varied from 

negligible to total collapse depending upon the 

design and quality of construction. In Chimbote, 

some of the tallest buildings of this type (to five 

stories) were undamaged. Others that have 

extremely heavy concrete roofs and inadequate 

resistance to lateral forces were subject to shear 

failure of pillars and bearing walls. In a few 

structures, damage is attributable to 

poor-quality concrete in the bearing members. 

Because the earthquake occurred on a 

Sunday afternoon, when school was not in 

session and businesses were closed, the number 

of deaths in the coastal area was low, probably 

about 1,000. Of these casualities, about 700 

were in Chimbote and resulted from collapse of 

structures on their occupants or on people who 

fled outdoors into the narrow streets. The total 

number is small considering the regional 

population of close to 200,000, and the extent 

of destruction. By rare good fortune, Casma, 

which formerly had a population of several 

thousand, and which was almost totally 

destroyed, recorded only 70 deaths. 

15 
Ground effects 

Surficial cracking, compaction, and 

differential subsidence 

Fissuring, . slumping, and differential ground 

subsidence are extensive in swampy areas and 

areas of near-surface ground-water table in the 

valleys and along low-lying coastal segments. 

Although such areas are of rather limited extent 

in this desert region, virtually the entire 

population and most engineering works along 

the coast are concentrated in them. In these 

areas, fissures as much as 50 em (centimeters) 

wide, settlement of roadbeds relative to bridges, 

and cracking and slumping of roadway 

embankments caused damage and temporary 

disruption of transportation routes (figs. 8 and 

9). Fissures also formed in foundation materials 

of cities and towns owing to differential 

settlement along the junction of filled areas and 

natural deposits and to lateral spreading of 

water-saturated sand. At the Port of Casma and 

in near-shore areas of Chimbote, lateral 

spreading caused by liquefaction of deltaic and 

beach deposits produced many extension cracks 

in the ground as much as 20 em wide that 

literally tore apart all the structures they 

intersected (fig. 10). 

In some areas of near-surface ground-water 

table, seismic compaction of the surficial 

deposits lowered the land surface sufficiently to 

cause flooding. A large flooded area formed in 

this way extends over nearly a square kilometer 

of a residential area in eastern Chimbote. Houses 

here were almost totally destroyed, and water 

Figure 8.-Slumping and fissuring of paved road near the bay shore in western Chimbote due to liquefaction and 

lateral spreading of water-saturated beach sediments. 

16 
Figure 9.--<:himbote-Huallanca railroad damaged as the result of differential compaction and lateral spreading of 

water-saturated unconsolidated sediments. 

covers house floors to depths of as much as 

30 em. The entire area has been abandoned. 

Surficial compaction and consequent flooding 

also has affected a large segment of the Port of 

Casma. 

Sand boils and water ejection 

In a few areas of near-surface ground-water 

table, sand boils are numerous and large. These 

resulted from squirting of muddy and sandy 

water from fissures that opened during 

earthquake shaking in response to compaction 

and cracking of the saturated surficial deposits. 

At some places, eyewitnesses report that water 

was ejected to heights about a meter above the 

ground surface. The sand boils are commonly 

marked by a central hole a few centimeters to a 

meter in diameter, surrounded by a mound of 

ejected sand and silt as much as 15 meters in 

diameter (fig. 11). 

17 

Soil slips and fragmentation 

Throughout the region from Huarmey to 

Chimbote, seismic movement caused small 

landslips (no large landslides were observed) on 

steep slopes and rockfalls on hillsides and steep 

roadcuts. The intensity of shaking was sufficient 

to shift rock fragments and boulders, ranging 

from a few centimeters to about 40 em in 

maximum length, from their original position on 

flat surfaces, or to cause them to slide a few 

centimeters down very gentle slopes. In these 

areas, the soil surface commonly is covered with 

a weak evaporite-cemented crust, a few 

millimeters thick, which crumbles easily in the 

hand. The seismic shaking caused these crusts to 

be widely fragmented on gentle to steep slopes 

and to form a network of fine cracks on many 

level surfaces. 
: 

Figure 10 -concre~e-block house, Corporaci;n Peruana del Santa housing development, Chimbote, which was torn 

apart by liquefaction and lateral flow of beach sediments that constitute the foundation material. 

Tectonic deformation 

Active tectonic faulting was not observed, 

and, to judge from the location of the epicenter 

and major aftershocks offshore from the coast ' would not be expected on shore. There was no 

significant vertical tectonic warping along the 

coast or observable seismic sea wave (tsunami or 

maremoto) of the type that have accompanied 

some previous great coastal earthquakes in Peru. 

This suggests an absence of shallow dip-slip 

faulting on the sea floor. 

18 

GEOLOGIC HAZARDS AND 

RECONSTRUCTION 

Huaraz 

Huaraz, the major city of the Santa Valley, 

is estimated to be 90 percent destroyed. 

Foundation conditions are as suitable for 

reconstruction at the present site of the city as 

they are at any other locality in the Santa 

Valley. However, some of the rubble of former 

buildings will have to be removed from the 
Figure 11.-Sand boil in water-saturated unconsolidated sediments of the Casma Valley; during earthquake jets of 

muddy and sandy water squirted from the two pits. The mound of ejected sand is more than 1 meter in diameter. 

townsite and the remaining material carefully 

leveled and compacted before reconstruction. If 

reconstruction is to include hillslopes near 

Huaraz, as has been suggested in some 

preliminary plans for reconstruction, a careful 

engineering geologic study should be made of 

the potential for fissuring and sliding in these 

areas. 

Two small lakes exist in the Cordillera 

Blanca at the head of Quebrada Cojup which 

drains through Huaraz. One of these is a 

remnant of the morainal lake that burst in 1941 

and partly destroyed Huaraz. The remnant of 

this lake is small and is reported to be in no 

danger of future rupture. The other lake is in a 

rock basin carved by a glacier, and also is 

reported to be in no danger of rupture 

(Benjamin Morales, oral commun., Corporacion 

Peruana del Santa, June 1970). 

The glaciers and snowfields in the Cordillera 

Blanca east of Huaraz are less extensive and in 

an area of less relief than those at the Nevados 

Huascaran and northward in the Cordillera 

Blanca. Thus, the potential for development of 

ice-originated avalanches or debris flows that 

could damage Huaraz is considered to be slight 

to nonexistent. 

19 

In summary, there are no geologic 

conditions precluding reconstruction of Huaraz 

at its present site. The main preoccupations in 

reconstruction should be the widening of streets 

and limiting of height of unreinforced adobe 

buildings to one story. 
Santa Valley south of Huaraz 

Recuay, the major town in the upper Santa 

Valley, was moderately damaged by seismic 

shaking. Several other smaller towns and villages 

in this area show variable damage, ranging from 

about 2 Q percent to almost 100 percent 

destruction. Geologic foundations of these 

settlements consist of alluvium or fluvioglacial 

material that is suitable for reconstruction. In 

general, hillslopes in this part of the valley are 

more gentle than they are to the north in the 

Callejon de Huaylas, and consequently are more 

stable. 

The only potential geologic hazard in the 

southern part of the Santa Valley is the Laguna 

Querococha. This lake, in Quebrada Querococha 

east of Ticapampa, is impounded behind 

coalescing alluvial fans from the north and south 

sides of the Quebrada. The resulting earth dam is 

broad, and because of its mode of formation, is 

more stable than the more typical morainal dam 

in the Cordillera Blanca. However, during the 

May 31, 1970, earthquake, fissures were opened 

in alluvial and morainal material at the 

northwest side of the lake. These fissures 

extended into the alluvial fan that forms the 

northern part of the dam (Emilio Caballero, oral 

commun., General Superintendent, c{a. Minera 

Alianza, June 1970). Ing. Caballero also 

reported that the surface flow at the lake outlet 

had diminished since the earthquake, which 

indicates an increase in ground-water flow into 

the alluvial dam. Such infiltration is potentially 

dangerous for it might lead to destruction of the 

alluvial dam due to piping and (or) sliding of 

newly unstable water-saturated material. As a 

means of evaluating the potential hazard of the 

Laguna Querococha, the lake discharge should 

be kept under close observation and the fracture 

system studied to determine whether incipient 

sliding continues. The slide area should be 

surveyed and a permanent triangulation net 

established for periodic resurvey. The most 

critical period for observation of the slide will 

probably be during the next rainy season. 

Y ungay and vicinity 

Further data are required before the danger 

of future debris avalanches at Yungay can be 

properly evaluated. Because there is a 

"protective" ridge between the site of Yungay 

and the main path of debris avalanches from 

Huascar~n down the Llanganuco River valley, 

only enormous, high-speed avalanches such as 

the one of May 1970 are likely to pose a threat 

to the Yungay area. Conceivably, such an event 

may not occur again for thousands of years. A 

detailed geologic study is required to determine 

whether catastr-ophic debris avalanches have 

overrun the protective ridge in. the past and, if 

possible, the approximate frequency with which 

such events have recurred. In an overflight of 

Nevados Huascaran, we did not observe any 

major fractures that would indicate a possibility 

for other avalanches in the near future. 

Nevertheless, the near-vertical west face of the 

north peak of Huascaran is a potentially 

unstable feature, and other rockfalls and icefalls 

may be expected to take place here in the 

future. 

Ranrahirca was in a more hazardous position 

than Yungay because it was in the direct path of 

any slide or fall from the north peak of 

Huascaran. It was largely destroyed in December 

1962 by a debris flow which originated on the 

north peak of Huascarln, at the same site as the 

debris avalanche of May 1970. It is reasonable to 

assume that renewed accumulation of snow and 

ice in this area will result in other avalanches in 

the future, perhaps within 10-20 years. 

Other villages near Ranrahirca also are in the 

paths of potential avalanches from Nevados 

Huascar~n, and consideration should be given to 

moving inhabitants to safer areas. Potential areas 

for reconstruction of small towns are on terraces , of the Rio Santa just south of Ranrahirca and 

north of Yungay. However, before 

reconstruction in these areas, an evaluation of 

potential avalanche paths should be made to 

determine the safety of the sites. 

Caraz 

Caraz, estimated to be 70-80 percent 

destroyed, is on an alluvial fan at the mouth of 

Quebrada Partin. The geological foundation is 

similar to that of Huaraz and is suitable for 

reconstruction. However, Caraz potentially is in 

one of the most hazardous locations of any of 

the larger towns or cities in the Santa Valley. 

Laguna Paron near the head of Quebrada Paron, 

20 
the largest lake in the Cordillera Blanca, is partly 

dammed by a glacier and partly by a moraine. 

The lake drainage is entirely by subsurface flow 

through the morainal dam (Benjamin Morales, 

oral commun., June 1970). Furthermore, this 

lake and the Quebrada are between Nevados 

Huascaran on the south and Nevados Huandoy 

on the north. Extensive glaciers and snowfields 

overhang the valley on both mountains, and ice 

avalanches or rockfalls into the lake could cause 

it to rupture and to form a debris flow even 

larger than that of May 1970. 

Relocation of Caraz a short distance up or 

down the Santa Valley would place it in an even 

more hazardous position than its present one. 

To the south at lower altitude, the town would 

be subject to destruction by any debris flow that 

originated upstream in the Santa Valley. For 

example, the airport at the south edge of Caraz 

was partly covered by the May 1970 debris flow. 

The area immediately north of Caraz, however, 

would be in the direct path of a debris flow 

originating from Laguna Paron. 

A possible solution to the problem of Caraz 

is the partial draining of Laguna Paron. The 

Corporacion Peruana del Santa currently has 

plans to effect this drainage by driving a 1.2-km 

tunnel in rock, starting downstream from the 

lake and intersecting the lake at a depth of 40 

meters below the present surface. Drainage to 

this level would largely eliminate the danger of 

rupture of the lake and consequently the danger 

of destruction to Caraz by a debris flow. A 

second alternative would be to relocate the city ,/ 

on the west bank of the R10 Santa. 

Chimbote 

Chimbote had 70-80 percent serious 

damage or destruction to buildings. Various 

parts of Chimbote have been built on beach and 

deltaic deposits, mudflow deposits, and slope 

debris. In some places, the ground-water table 

was at or near the surface before the earthquake, 

and at others it was at depths of several meters 

or more. 

At several places, differential compaction 

and (or) incipient slumping and sliding of 

foundation material were major causes of 

damage in Chimbote. Among structures and 

21 

areas .that were affected by foundation 

movement are the following: 

1. At the steel plant in Chimbote, 

differential compaction of dry-slope 

debris and mudflow material on 

which the plant was constructed 

resulted in severe damage to some 

reinforced concrete structures, by 

causing breaking and partial settling 

of heavy concrete floors and by 

differential settling of units of a 

newly built rolling mill. The 

differential compaction in this area 

ranged from less than an inch to 

about a foot. 

2. The Corporacion Peruana del Santa 

housing development in Chimbote 

was built in part on an inactive beach 

or berm that is backed by a lagoonal 

swamp, near the bay shore in the 

westernmost part of Chimbote. The 

back slope of the beach surface and 

row masonry houses on it were 

severely cracked during the 

earthquake; some houses were torn 

apart as shown in figure 10. The 

nature of the cracking suggests that 

it was caused by liquefaction of the 

sand and lateral spreading toward the 

adjacent swamp. 

3. Mudflats, roadways, and artificial seawalls 

near the dock of the Chimbote steel 

plant subsided as much as a meter 

because of compaction and seaward 

spreading. The massive concrete 

dock itself did not settle because it 

was built on piers driven down 

through the soft, unconsolidated 

surface deposits. However, 

reinforced concrete supporting 

pillars on the shoreward side of the 

dock were broken and apparently 

tilted gently seaward. 

4. A residential area in eastern Chimbote 

built on low-lying lagoonal and 

swamp deposits subsided during the 

earthquake. In this area the 

ground-water table was near the 

surface before the earthquake, but 

seismic shaking resulted in 

compaction and settling of the 
unconsolidated deposits, and as a 

consequence a low-lying area of 

about a square kilometer is now 

partly flooded with water a few 

inches to about 30 em deep (fig. 12). 

5. In the business section of Chimbote 

bordering the bay and near the Hotel 

ChimG is an elongate zone 

subparallel to the bay shore, several 

blocks long, where many open 

fissures cut across paved streets and 

through buildings; some 2- to 4-story 

buildings in this area have settled 

differentially as much as a foot, 

relative to nearby streets and 

buildings. This is evidently an area of 

liquefaction and incipient seaward 

spreading as well as of differential 

compaction of foundation material. 

6. A few kilometers northeast of Chimbote, 

the narrow-gage Chimbote-Huallanca 

railroad passes through farmland in 

which the ground-water table is near 

the surface. Differential compaction 

during the earthquake caused 

irregular settling of the railroad bed 

in such a manner that the once 

straight rails are now undulating; 

highs and lows show differences in 

elevation of as much as 1-2 feet in 

distances of 100 feet (fig. 9). 

Furthermore, slumping and lateral 

spreading of water-saturated ground 

resulted in several feet of horizontal 

displacement of the rails in some 

areas. 

Chimbote can be rebuilt at the same site, but 

certain residential areas should probably be 

Figure 12.-Destruction of houses in a residential area of southeastern Chimbote where the ground settled due to 

compaction of water-saturated sediments; flooded area was dry before the earthquake. 

22 
abandoned. One of these areas is that in the 

eastern part of the city, now flooded because of 

soil compaction during the earthquake. This area 

would be subject to additional compaction and 

flooding during future earthquakes. Inasmuch as 

the central part of Chimbote is virtually at sea 

level, the city is in potential danger of 

destruction by tsunamis originating at almost 

any place in the Pacific region. Although it 

would be unreasonable to consider relocation of 

the city because of this danger (Callao is in an 

equally or even more dangereus. position for 

tsunami destruction), it is not unreasonable to 

consider reconstructing the destroyed part of 

the city on higher ground. 

An evaluation of the danger of a destructive 

tsunami to Chimbote may show that the 

configuration of the Chimbote bay may greatly 

reduce the danger of widespread inundation. On 

the other hand, the tsunami associated with the 

1960 Chilean earthquake produced the highest 

waves yet recorded in the bay, and there is no 

guarantee that even higher waves might not be 

generated by closer earthquakes or by 

earthquakes in the western Pacific. 

Casma 

Casma was almost completely destroyed, 

and after the earthquake appeared as though it 

had been bombed; few building walls of heights 

of more than a meter or two were still standing. 

The town had only a few reinforced concrete 

buildings, most of which show little or no 

damage. A large one-story school building of 

reinforced concrete and cement block, 

apparently of good construction, collapsed. Had 

the earthquake been during school hours, 

hundreds of school children would surely have 

been killed or injured. The extensive destruction 

of adobe buildings in Casma is partly due to the 

extremely poor quality of the adobe bricks and 

mortar used in their construction. The adobe 

and mortar were made of a local silty sand, low 

in clay. Further, the bricks lack the usual straw 

binder which is essential to good adobe, as it 

increases bearing strength and resistance to 

shear. 

The foundation material at Casma is suitable 

for reconstruction at the present site of the 

town. The site is not in danger of flooding, 

debris flows, or landslides. 

The fishing port north of Casma shows 

evidence of lateral extension and settling. The 

town is on a former tidal and deltaic mudflat 

and has a near-surface ground-water table. Some 

streets that were dry before the earthquake now 

are covered with 2 feet of water, evidently due 

to differential compaction of water-saturated 

soil during the earthquake. By measuring open 

cracks in walls of a fishmeal plant and concrete 

sewage pipes, as well as slipping of wooden 

timbers in a nearby pier, we were able to 

establish that the seaward side of the area moved 

seaward approximately 1.4 meters in a distance 

of 300 meters. 

Huarmey 

Huarmey appears to have been 70-80 

percent destroyed; as in other towns in the 

affected area, the major damage was to adobe 

buildings. The town has been built on alluvial 

sediments that offer no particular danger to 

reconstruction at the present site. 

Trujillo 

Trujillo was less damaged than the 

above-mentioned coastal cities. Serious damage 

or destruction was probably on the order of 

10-20 percent and almost entirely limited to 

adobe buildings. Towers of the Cathedral on the 

Plaza de Armas were seriously damaged and the 

center part of a dome fell, although the dome 

itself remained in place. A huge and apparently 

unstably mounted statue in the Plaza de Armas 

remained intact during the earthquake, which 

indicates that the shaking was relatively mild 

there. 

Like· Chimbote, part of Trujillo is very near 

sea level; it consequently is subject to damage or 

destruction by a tsunami. 

The port of Trujillo, Salaverry, shows little 

damage, chiefly because most buildings are of 

wood construction. Small churches and a few 

adobe buildings were damaged. An area at the 

port slumped, causing cracking of a cement 

platform near the major port warehouse. 

23 
CONCLUSIONS AND RECOMMENDATIONS 

The extensive destruction effected by the 

May 31, 1970, PerU: earthquake to cities, towns, 

and villages was due largely to poor construction 

of buildings, chiefly adobe, which had little 

shear resistance to lateral forces imposed by 

earthquake shock. The degree of destruction, 

however, was affected in some areas by 

differential compaction of geologic foundation 

material, generally unconsolidated sediments 

and rock debris of varied origin. In some areas 

the damage was enhanced by incipient slumping 

or sliding of foundation material down hillslopes 

or by lateral extension of liquefied sediments 

toward free faces with attendant surficial 

extension fissuring. 

The major catastrophic geologic result of the 

earthquake was the debris avalanche that 

originated on the north peak of Nevados 

Huascaran. This avalanche buried the towns of 

Yungay and Ranrahirca, killing at least 17,000 

inhabitants, and damaged or destroyed several 

other small settlements and farms in its path. As 

it moved down the Rio Santa, it swept over 

farms and small settlements, caused extensive 

damage to road and railway routes, destroyed 

the diversion dam and access bridge to the 

important Huallanca hydroelectric plant near 

the north end of the Canon del Pato, and 

damaged power tranmission and communication 

lines. 

No evidence of displacement was found on 

any of the exposed faults within the region or of 

tectonic warping along the coast. 

The following recommendations are made 

regarding reconstruction of cities and towns 

damaged or destroyed during the May 31, 1970, 

earthquake. 

1. The geological environment favors 

reconstruction of cities, towns, and 

villages at their present sites, except 

for the following: 

(a) A low-lying swampy and flooded 

zone in eastern Chimbote should 

be abandoned as a residential 

area. Because of the ever-present 

danger of tsunamis associated 

with earthquakes of this region, 

preference should be given to 

relocating new residential areas 

on higher ground north and 

24 

northeast of Chimbote. In future 

construction of multistory 

buildings in Chimbote, attention 

should be given to foundation 

construction to avoid problems 

due to differential settling and 

liquefaction of water-saturated 

sediments during seismic shock. 

Furthermore, the hazards of 

seaward sliding of sand deposits 

along the shore need to be 

considered in any future 

construction along the 

waterfront. 

(b) Yungay probably should not be 

rebuilt. However, more study is 

required to properly assess the 

potential danger of another 

debris flow at Yungay (see 

page 8). 

(c) Ranrahirca definitely should not be 

rebuilt at its previous site 

because of the danger of other 

debris avalanches originating 

from the north peak of Nevados 

Huascar an. 0 ther small 

settlements along the lower part 

of the Rio Llanganuco that were 

destroyed by the Ranrahirca lobe 

also should not be rebuilt. 

Potential safe sites for small 

towns can be found on the 

alluvial terr.aces just south of 

Ranrahirca and north of Y ungay. 

(d) Caraz, at the mouth of Quebrada 

Paron, is in great potential 

danger of being destroyed should 

Laguna Paron burst. Two 

possibilities for reconstruction 

exist: (1) relocate the town in a 

nearby area at the west side of 

the Santa Valley or (2) drain 

Laguna Paron, as has been 

proposed by the Corporacion 

Peruana del Santa (Benjamin 

Morales, oral commun., 1970), in 

which case the town can be 

rebuilt at its present site. 

2. An engineering geologic appraisal should 

be made of cities and towns to 

evaluate systematically the 

foundation conditions and to serve 
as a gUlde for reconstructiOn. Such 

appraisals should be made by 

expenenced personnel as soon as 

possible. 

3. The systematic hydrologic and geologic 

study of lakes m the Cordillera 

Blanca should be contmued on a 

permanent basis, at about the 

present level of endeavor, to 

anticipate and correct poten tml 

floodmg hazards. 

4. An- appraisal of distnbution and stability 

of glaciers and snowfields m the 

Cordillera Blanca should be made. 

5. Geologic research Is needed mto the 

distnbution and ages of prehistoric 

debris flow deposits m the Santa 

Valley to (1) dehneate areas 

subjected to destructive debns flows 

m the past, and (2) to determme 

their recurrence mterval as closely as 

possible. 

6. A coordmated program of geological and 

geophis~I-cal research should be 

undertaken to study active tectonic 

features m the region m order to gam 

an msight mto the nature of 

deformation now m progress, 

regional variatiOns m Lhe stram 

accumulatiOn pattern, and possible 

recurrence mtervals of earthquakes. 

As a mimmum th1s study should 

mvolve (a) mappmg known Holocene 

faults and estabhshmg monuments, 

(b) regional stram measurements by 

precise tnangulat10n and levehng, 

(c) mvestigat10n of the coastal and 

nver terrace sequences, and 

(d) detailed seismologlC studies of 

the spacial distnbutwn, focal 

mechamsm, and frequency of 

earthquakes. 

These mvesbgatwns, as well as research mto 

other phases of the May 31, 1970, earthquake, 

such as a detailed study of the 

Yungay-Ranrahrrca debns avalanche, study of 

seismic charactenstics of the earthquake and 

aftershocks, engmeermg constructiOn study, and 

other research InvestigatiOns should be 

coordmated and ultimately be the subject of a 

symposium leadmg to a publication that would 

place upon permanent record the charactenstlc 

features of the earthquake. Such a symposmm 

would lead to better understandmg of 

earthquakes, not only m Peru but m the world as 

as whole, and would be of aid m better 

preparmg for catastrophes of this kmd. 

REFERENCES CITED 

Gutenberg, Beno, and Richter, C. F., 1954, Se1sm1c1ty of 

the earth and assoc1ated phenomena (2d ed ) 

Prmceton, New Jersey, Pnnceton Umv. Press, 322 p. 

Morales, BenJamm, 1966, The Huascaran avalanche m 

the Santa Valley, Peru: BelgiUm lnternat 

Symposmm on Scientific aspects of snow and 1ce 

avalanches: L'assoc mt. d'hydrologie scJentifique 

pub.69,p 304-315. , 

SJ!gado 1F., Enrique, 1968, H1stona de los s1smos mas 

notables ocumdos en el Peru (1515-1960) Inst 

Panamencano de Geograf1a e Hlstona, Com1t~ 

Panamencano de Cienc1as Geof{sJcas, Bol. 

B1bhografico de Geofls1ca y Oceanograf1n 

Amencanas, v 4, p 191-241 

25 

*U S GOVI:RNMEN'T PRINTING OFFICE 1970 0 - 408-518 




ERICKSEN AND OTHERS-GEOLOGIC EVENTS ASSOCIATED WITH THE MAY 31, 1970, PERU EARTHQUAKE-GEOLOG

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