At 3:23 p.m. local time, on May 31, 1970, an earthquake of magnitude 7 ¾; occurred approximately 15 miles off the coast of Peru, west of Chimbote. It is estimated that the earthquake took the lives of 70,000 people, caused 50,000 injuries, destroyed roughly 200,000 homes and buildings, and left approximately 800,000 people homeless.
Within the affected region, roughly 40,000 square miles, many villages were almost totally destroyed and several were swept away. The single, most devastating event was the large debris avalanche that originated from the north peak of Huascaran, falling 12,000 ft and traveling 7 miles at an average speed of roughly 200 miles per hour to destroy the villages of Yungay and Ranrahirca. Field evidence strongly suggests that the area near Yungay and Ranrahirca will continue to be overridden by future debris avalanches.
Damage resulting from vibrational effects was severe. The most intense vibrational damage occurred along the coast at Chimbote and Casma where most houses and buildings of adobe construction were destroyed or severely damage. Severe vibrational damage also occurred in cities and villages located in the Santa Valley at epicentral distances ranging from 60 to 100 miles inland.
The single, most important factor contributing to the massive damage from ground motion was the poor adobe construction. The influence of the underlying geological and soil conditions was an important secondary factor.