EQNEWS / November 1996
Cover Photograph

Ancash, Peru, M7.4 Earthquake, November 10, 1946

At about 12:40 p.m. on November 10, 1946, a powerful earthquake registering 7.4 rocked north-central Peru, near the small mountain villages of Ancash and Quiches. The quake is considered by many as one of the most out- standing earthquakes of historic South America, not only for is destructive power, but because of its extraordinary display of surface fault rupture. The earthquake and its destruction are well documented by Enrique Salgado F. in Bulletin of the Seismological Society, Vol. 41, No. 2, pp 83-100, 1951.

(Click on image for larger size)

The quake was felt over 680 km away in Guayaquil, Ecuador, and 410 km away in Lima, producing a felt area covering 450,000 sq. km. Maximum shaking intensities of X occurred with a 1,600 sq. km region of isoseismal IX. An estimated 1,400 people were killed and several structures were completely wrecked or sustained total collapse. Numerous landslides were reported, including one which buried the village of Acobambra to a depth of 20 meters. Another 200 persons were killed in the steep, narrow Llama valley which was ravaged by several small landslides burying native farmers and herders and wiping out many homes. Nearly every building in Quiches and Conchucos were totally destroyed and 50 percent of the structures in the town of Silhaus sustained partial to total collapse. Interviews conducted by E. Salgado a few days after the quake reported fantastic stories of a load thunder clap followed by intense shaking. Phenomenal accounts of people, cattle and buildings being thrown into the air were common. A strong aftershock sequence persisted for a couple years and included on strong jolt (VII) on February 14, 1948, which killed an additional seven people.

The earthquake produced a sinuous surface fault rupture which extended along a N42oW trend for about 5 km, with maximum displacements of 3.5 m along a 58oSW dipping plane (See photograph; notice the man at the foot of the fault just right of center for scale). Field exposures showed nearly verticle striations indicating movement occurred without a dip-slip component. Interestingly, the northward trace disappear for 10 km then reappeared for an additional length of 3 km. West of the main trace were additional, sub parallel ruptures, but faced to the northeast - back to the main trace; maximum offsets on these ruptures were on the order of 1 m. Calculations by Salgado suggest the focal depth was on the order of 30-40 km deep and field evidence suggested normal fault motion. However, some disagreement has been noted in the literature whether the focal mechanism was acctually reverse (thrust) not normal, and the "main" trace of the fault may have been secondary ruptures sympathic to a principal fault. Never-the-less, the fault exposures at Ancash, Peru, remain some of the most remarkable geologic features of recent time.

Image: Steinbrugge Collection, EERC, University of California, Berkeley.

Rev. Nov. 13, 1996