Historic Earthquakes

Peru
1974 October 03 14:21:29 UTC
Magnitude 8.1

Severe damage to adobe and quincha buildings in Chorrillos district. Hundreds of families were left homeless in the aftermath of the earthquake.


The walls of this reinforced concrete water tank located on the second floor of a building in Miraflores district collapsed in the earthquake.


An example of the extensive damage in La Molina district. This classroom and a laboratory building at the Agrarian University partially collapsed in the earthquake.


Lima residents wander through the desolation of their homes wrecked by the earthquake.


Heavily damaged building in the Lima area after the shock.

The shock centered approximately 80 kilometers southwest of Lima rocked the southern coastal area of Peru inflicting heavy damage in the Lima area. The National Earthquake Information Service is Golden, Colorado, located the epicenter at latitude 12.3 South and longitude 77.8 West. The focal depth was 13 kilometers. The tremor, which killed 78 and injured several thousand, was Peru's worst earthquake disaster in terms of lives lost since May 31, 1970, when a magnitude 7.9 shock killed an estimated 50,000.

Peru is located on the western edge of the South American crustal plate, one of several large lithospheric plates that comprise the Earth's crust and slowly move with respect to one another. The boundary between the South American plate and the Nazca plate to the west is one of the most seismically active areas of the world. The Nazca plate is being overridden and driven beneath the westward-moving South American plate. This collision between two large segments of the lithosphere is the source of most of Peru's earthquakes. Offshore, where the two plates meet, the shocks occur at shallow depth. To the east, as the Nazca plate is pushed downward, the earthquakes occur at progressively greater depth - to as much as 600 kilometers near the Peru-Brazil border. The October 3 earthquake occurred offshore at shallow depth. Shallow earthquakes are potentially more destructive than deep shocks of the same magnitude because they generate stronger surface waves.

Soon after the October 3 earthquake the Organization of American States requested the U.S. Geological Survey (USGS) to conduct a field evaluation. A team of five geophysicists was sent to Lima by the Survey. Their investigation included intensity and damage surveys, strong-motion studies and an aftershock program. The damage and intensity survey involved collecting a total 400 questionaires from various parts of the Lima area and assigning a Modified Mercalli intensity to each location canvassed. From this information, an intensity distribution map of the city was drawn. The intensity and damage survey was performed by A.F. Espinosa of the USGS, R. Husid of the Colorado School of Mines, and J. de Las Casas of the National University of Peru.

Lima, a city of nearly 4 million, is characterized by a great diversity of architectural styles that range from attractive, well-designed buildings to poorly constructed adobe homes. Most of the homes in the south part of the city are built of adobe or quincha, a mixture of mud and sugarcane stalks somewhat more earthquake resistant than adobe.

Peru had no seismic design code until after the 1970 earthquake; most structures built before then are not earthquake resistant. For this reason damage to buildings was extensive. South of the city in the suburb of Chorillos many people were left homeless. East of the city in La Molina a number of reinforced concrete structures at the new Agrarian University were partially collapsed. Liquification, the transformation of a granular material from a solid state to a liquified state as a result of increased pore water pressure caused by ground shaking, occurred in several parts of the Lima area, principally in the port of Callao to the northwest. Part of this district is built on landfill and water-saturated sediment. These factors, combined with poor design and construction, contributed to the collapse of a four-story reinforced concrete building in Callao.

The intensity of the October 3 earthquake in the Lima area generally ranged between V and VIII; however, in several isolated places the reported intensity was as low as IV, and at one place, as high as IX.

The results of the study conducted by the Geological Survey in the Lima area will be used by civil and structural engineers to develop criteria for designing earthquake resistant buildings in the area. In regions of high seismic activity such as Peru the loss of life and property from earthquakes can be greatly reduced by such measures.

Abridged from Earthquake Information Bulletin, January-February, Volume 7, Number 2.

This grain elevator complex in the port of Callao partially collapsed in the earthquake.