Historic Earthquakes

1970 May 31 20:23:27 UTC
Magnitude 7.9

Peru On Sunday, May 31, at 27.3 seconds after 4:23 pm EDT, a magnitude 7.9 earthquake centered at 9.2° South, 78.8° West, about 22 miles west of Chimbite, a major Peruvian fishing port.

Damage reached from Chiclayo in the north to Lima in the south, but coastal towns near the epicenter and towns in the Santa River Valley to the east suffered most.

Casma, a coastal city south of the earthquake epicenter with about 13,000 population sustained severe damage to 90 percent of its structures, but had comparatively few deaths. Chimbote, north of Casma, reported almost 3,000 deaths, and sustained much damage to 70 to 80 percent of its structures. According to preliminary reports, most of the fatalities resulting from the earthquake were caused by building collapses. Townsites were too often located on alluvial fill, and dwellings were mostly of adobe.

Although the earthquake itself caused much death and damage, severe losses were also caused by a huayco which swept down the steep slopes of the Cordillera Blanca from Nevados Huascarán into the Callejón del Huaylas, a steep valley paralleling the coast. Yungay and thousands of its residents were buried under tens of feet of mud, earth, water, boulders, and debris. Huarás, capital of the Ancash Department, was struck by a huayco 29 years ago and 6,000 lives were lost. On May 31, the earthquake and its side-effects damaged more than 70 percent of the buildings in the valley city, and took an estimated 20,000 lives.

What caused the earthquake?

According to present theory and a rapidly developing body of geophysical and geological evidence, the continental block of South America is drifting westward, overriding and forcing down the thinner Pacific Ocean crustal slab. As this slab bends downward, strains accumulate until the breaking point is reached.

The ocean slab at Peru dips under the continent at an angle of about 60 degrees, and earthquakes occur at increasingly greater depths toward the east, reaching a maximum of 380 miles near the Peru-Brazil border. The crustal region off Chimbote reached it breaking point on the afternoon of May 31, when a strain-relieving rupture occurred at a depth of 25 miles. The aftershock pattern indicates that, from the initial break, the rupture then proceeded southward more than 100 miles.

How severe was the May 31 earthquake?

The losses in Peru have not yet been completely described. Like a grim legend, which grows grimmer with each telling, the death toll leaps upward week by week. Five days after the earthquake, Peruvian officials feared a death toll as high as 30,000. Several days later that figure was tentatively raised to 50,000. An Associated Press dispatch on July 14 said Peru's Minister of Health now estimated the number of dead and missing persons to be 70,000. It may be impossible ever to assess fully the toll of this seismic disaster.

Property damage has been tentatively put at $530 million, but here again, the assessment is difficult. The destruction of towns like Yungay was virtually complete - how does one put a value on a community? In a country of 13 million persons, nearly a million are homeless. What is the cost of such a massive dislocation?

Huyco: An Incan world used to describe the avalanches which are a chronic threat to residents of steep Andean valleys.

Abridged from Earthquake Information Bulletin, September - October 1970, Volume 2, Number 5.