# 1947 Satipo earthquake

The 1947 Satipo earthquake was the largest earthquake in the sub-Andean region of Peru. It occurred on November 1 at 09:58:57 local time with an epicenter in the Department of Junín. The earthquake had an estimated moment magnitude (M$_w$ ) of 7.7 and focal depth of 20 km (12 mi). Damage was severe in the towns of Satipo and La Merced, and at least 233 people died.

## Tectonic setting

Earthquakes in Peru is mainly concentrated at the subduction zone where the Nazca Plate subducts beneath the South American Plate at the Peru–Chile Trench off its west coast. Another zone of seismicity is recorded within the South American Plate in the sub-Andean region. Most of these shallow earthquakes are the result of normal-faulting such as the 1946 Ancash earthquake which ruptured a shallow-dipping fault oriented parallel to the range. Most normal-faulting occurs in the high Andes. Ongoing extension occurs perpendicular to the range in the country. Some earthquakes display reverse-faulting such as in a 1969 event. The occurrence of west-dipping thrust faults in the sub-Andean suggest the Amazonian Craton is actively underthrusted beneath the range. Reverse-faulting in the sub-Andes occur as a result of subduction-induced compression which could account for its deformation style. While the crust thickens and uplifts, gravitational forces in the high Andes trigger normal-faulting. The horizontal stresses on the range sides are stabilised by gravity acting on the high Andes and crustal buoyancy.

## Earthquake

The mainshock was a shallow intraplate earthquake that occurred within the South American Plate. It was the largest shallow earthquake recorded in the sub-Andean region. The United States Geological Survey measured the earthquake at 7.5 on the surface-wave magnitude scale and 7.7 by the International Seismological Centre on the moment magnitude scale. A maximum Modified Mercalli intensity of IX (Violent) was assigned to an area immediately southeast of the epicenter.
A seismic station in Huancayo determined that the aftershocks had epicenters distributed east of the mainshock. Using observations from Weston, Pasadena and Saint Maur, the mainshock's focal mechanism indicated purely vertical faulting on a north-oriented fault plane that dipped 30°. The inferred dip angle was shallower than that of faults observed in the region. Within the epicenter region, two other earthquakes larger than magnitude 6.0 were recorded. In 1976, a M$_s$  6.6 strike-slip earthquake occurred south of the 1947 earthquake epicenter at 5 km (3.1 mi) depth. A M$_w$  6.5–6.8 shock in 1982 occurred north of the 1947 earthquake with a left-lateral–reverse mechanism.

## Impact

The earthquake was felt across a 1,300,000 km$^2$ (500,000 sq mi) area encompassing nearly all of Peru. Large portions of Brazil from Tabatinga and headwaters of the Acre River also felt the earthquake. A maximum Modified Mercalli intensity of VIII–IX (Severe–Violent) was assigned to a 4,000 km (2,500 mi) area. The peak ground acceleration was calculated at 309 mm (12.2 in) per second-squared in Satipo, based on evaluating a collapsed brick pilar. Hundreds of kilometers away in Iquitos and Lima, the shaking corresponding to III–IV (Weak–Light). At least 233 deaths were recorded; deaths were only recorded in Satipo, Andamarca, Acobamba, La Merced, Víctor, Comas, Perené and other towns, and the total figure was not finalised.
The earthquake destroyed or seriously damaged 63 percent of all adobe constructed homes in La Merced. The town hotel was undamaged. Another 36 percent of homes had moderate damage, while the remaining 1 percent were undamaged. Most reinforced concrete homes withstood the earthquake shaking, although some were reportedly cracked. At least half of all limestone structures were destroyed. In Satipo, the earthquake collapsed an entire school complex. A partially constructed church with thick brick walls and reinforced iron beams was razed. Many new buildings of similar construction were also demolished. Many adobe buildings in Jauja and Cerro de Pasco were severely damaged. The settlements of Andamarca, Acobamba and Comas were also devastated.
Landsliding was extensive in the elevated forested region and deposited in ravines between San Ramón and Satipo that formed landslide dams. A failed landslide dam caused inundation in Puerto Ocopa. Many small-scale landslides destroyed forested areas near the Satipo River. Parts of a highway were buried under landslide debris. In the aftermath some injured victims were airlifted to Lima. Pedro Venturo Zapata visited the affected area and led in the recovery efforts.