Historic Earthquakes

Ancash, Peru
1946 November 10 17:42 UTC
Magnitude 7.3

Fourteen hundred people killed. Nearly all buildings were destroyed or heavily damaged in the Sihuas-Quiches-Conchucos area of Ancash Department. Many landslides occurred: one buried the village of Acobamba and another dammed the Pelagatos River. The quake was felt from Guayaquil, Ecuador to Lima, Peru. Several segments of surface faulting were seen in a zone about 18 km (11 mi) long from Quiches to Hacienda Mayas. The faulting was purely dip-slip (vertical) with as much as 3.5 m (11 ft) offset.

The first well-observed instance of major faulting accompanying an earthquake in South America was provided by the Ancash earthquake in central Peru on November 10, 1946. This shock disturbed an enormous area, and extreme violence was reported in the meizoseismal region near the fault. Structures were demolished, and the death toll reached about 1400, which is remarkable for so thinly populated a region. This devastation was in large part due to great landslides, one of which covered the village of Acobamba to a depth of 20 meters.

The region is one of principally Cretaceous rocks dissected into deep canyons by the Marañon river and its tributaries. The principal fault scarp appeared on the relatively level summit of a ridge which rises over 1000 meters above the almost completely demolished town of Quiches. This scarp, 5 kilometers long, has an average trend of N 42° W, paralleling the ridge, but it is downthrown southwestward, away from the canyon, and the fracture surface dips 58° southwest. This surface in places shows clean slickensides in limestone, with striation indicating that the entire displacement was dip-slip, with no sign of strike-slip. The manner in which this trace cuts across the topographic irregularities of the elevated surface is so characteristic that anyone with a little experience will recognize it as faulting. The throw reached a maximum of 3.5 meters. Northward the trace disappears on approaching a deep canyon, but 10 kilometers beyond it resumes with similar characteristics for a length of 3 kilometers. West of the main fault is another fracture, paralleling it with opposite dip (30° northeast) and striking N 40° W; this is downthrown a maximum of 1 meter northeast, so that there is a graben about 2 kilometers wide.

The country is not easy to travel over under any conditions, and the investigators were further hampered by the effects of slides and fissuring which closed some of the roads. It is therefore quite possible that the actual surface faulting was more extensive, and more complex, than that described.

Abridged from Richter, Charles F., 1958, Elementary seismology: San Francisco, W.H. Freeman and Company, 768 p.

1946 10 November. The earthquake which occurred on this day in the provinces of Pallasca and Pomabamba in the department of Ancash was notable for having been associated in its production with a visible case of tectonic dislocation, and furthermore for being one of the most destructive that occurred in this high Andean region, where some 1396 people died, despite the sparse population in these districts. The seismic movement had a felt area which probably exceeded 450,000 km2. The epicentral region, situated between 8°10' and 8°26'S and between 77°27' and 77°52' W, comprises the eastern part of the continental divide and the valley of the Marañon River, with elevations varying from 4,800 to 1,600 m. The relief corresponds to an old erosional plateau dissected by rivers rejuvenated by Pliocene and post-Quaternary uplifts of the Andes; glacial valleys are encountered between 3,800 and 4,000 m a.s.l. The structural zones, according to Wilson and Keyes (1965), are characterized: a) by a folded area associated with numerous reverse faults; b) by areas where Mesozoic sediments are imbricated by large reverse faults and overthrusts; and c) by tectonic trenches which more or less follow the course of the Marañon River, transversely affected by transcurrent faults.

Description. This earthquake occurred on Sunday the 19th, at 12 hrs, at the time when the majority of the people were at lunch. At the Mayas ranch, near the epicentral area, a strong detonation was heard, followed by the earthquake. Señor Octavio Canoza, owner of the ranch, reported these moments in the following manner: "On the day of November 10 I was resting in the study of the ranch house, when at about 12:40 I felt a terrible explosion in the direction of the Shuitocoha dairy, situated on the road which leads to the district of Conchucos. Frightened, I ran to see whether something serious had occurred in that dairy, and instantly I felt the first shock, which was followed by many others. Finding it impossible to keep on my feet on level ground, I fell down twice, the second time resulting in a lesion in the left knee ... "

The maximum intensity, estimated as X-XI MM, was probably confined to a long narrow area from the snow-capped Peak Pelagatos to the villages of Mayas and Quiches, where the destructive effects were great and where topographic transformations and slides occurred. The line of the IX isoseist, elliptical in shape and oriented NW-SE, took in approximately 1600 square kilometers. Secondary effects: a) After the terraqueous commotion, there was evident in the high part of the village of Quiches a fault scarp six kilometers long which showed a sinuous surface with average trend of N42°W and dip of the fault plane of 58°SW. The displacement was clearly vertical, judging by the scarps of the surface of the fault surface and which in some places reached a vertical throw of up to 3.50 meters. The fault was followed until it was lost on reaching the deep Llama ravine. After approximately ten kilometers in a straight line from this last place, the fault reappeared for an extent of three kilometers, in the slopes of Angaschaj hill on the road from the Quilca ranch to Mayas, with a N55°W trend and maximum displacement of one meter, again losing itself on descending into the Angajlle ravine. At Quiches, Cerro Paltas, and in the Sillapata hills (Tayabambita) there were fractures dipping to the NW, in contrast to the main fault. The one on the Sillapata hills was two kilometers long. Average strike N40°W and vertical throw one meter. As this fracture is encountered at a distance of 14 kilometers from the trend of the main one, it seems that the zone between the two was downdropped.

b) Large landslides were produced in the ravines of Palagatos, Shuitococha, Llama, San Miguel, where damming occurred.

In the Pelagatos ravine it is estimated that about 25 million cubic meters of decomposed granitic material slumped into the valley. In the Huaychayaco landslide, in the high part of Quiches, approximately 5 million cubic meters of limy-marly material broke loose from the slopes of the hill. A landslide of large proportions buried the hamlet of Acobamba, where 217 people died. Other notable slides occurred in Portachuelo, near the snow-capped Pelagatos Peak and on the Marañon River.

c) Numerous fissures in the ground were observed near Quinches, Mayas, Huallabamba, Conchucos, and Citabamba.

d) The effects on structures in the small populated centers of the region were great; almost total destruction of recent mud-wall structures and primitive and old buildings of the same material, in Quiches, Sihuas, Mayas, and Conchucos.

Instrumental study: the magnitude is estimated at 7.25. The author attempted a refinement of the epicentral determinations made in the United States and Europe, which led to the following results:
Geographic latitude: 8°20'
Geocentric latitude: 8°17'
Geographic longitude: 77°50'
Origin time in GMT: 17:42:54

The focal depth was estimated as about 30 to 40 km, based on the pp-p readings of the network of stations of the Seismological Laboratory of California and the observatories of Harvard and Ottawa.

Abridged from Historia de los sismos mas notables ocurridos en el Perú (1513-1970): Geofísica Panamericana, v. 2, no. 1, January 1973, Enrique Silgado F.