1947 Satipo earthquake

The 1947 Satipo earthquake occurred on November 1 at 09:58:57 local time with an epicenter in the Peruvian Amazon jungle in the Department of Junín. The earthquake had an estimated moment magnitude of 7.7 and a shallow focal depth of 20 km.

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 earthquake 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 happens perpendicular to the range in the country. Some earthquake show reverse-faulting mechanism such as the 1969 Huaytapallana earthquake. The presence of west-dipping thrust faults in the sub-Andean suggest the Amazonian Craton is actively being underthrusted beneath the range. In 1983, a model was proposed to explain the deformation style; subduction-induced compression promotes reverse-faulting in the sub-Andes. As crustal thickening and uplift occurrs, gravitational forces in the high Andes causes normal-faulting. The horizontal stresses on the range sides are stablilised by gravity acting on the high Andes and crustal buoyancy.

Earthquake
The mainshock was a shallow intraplate earthquake occurring within the South American Plate. It is the largest ever recorded shallow earthquake within the sub-Andean region. The magnitude was assigned 7.5 by the United States Geological Survey on its surface-wave magnitude scale and 7.7 by the International Seismological Centre on the moment magnitude scale. In an area immediately southeast of the epicenter, the maximim Modified Mercalli intensity was assigned IX (Violent). An analysis of aftershocks recorded by a seismic station in Huancayo determined that their epicenters were situated east of the mainshock and where present-day seismicity takes place. Using observations from Weston, Pasadena and Saint Maur, the focal mechanism was inferred to be purely vertical faulting on a north-oriented fault plane that dips 30°. The inferred dip angle is shallower than the faults observed in the region. In 1976, a 6.6 earthquake occurred south of the 1947 earthquake epicenter at 5 km depth; its focal mechanism was determined to be strike-slip. A 6.5–6.8 shock occurred north of the 1947 earthquake with a left-lateral–reverse mechanism.

Impact
The earthquake produced 45 to 60 seconds of violent ground shaking in the Department of Junín, and was felt within a 1,300,000 km2 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 (Severe) to IX (Violent) was assigned to an area estimated to be 4,000 km in size. The peak ground acceleration was calculated at 309 mm/s² in Satipo, based on evaluating the damage to a brick pilar. In the city of Lima, 240 km from the epicenter, the earthquake caused light to weak shaking corresponding to IV (Light) or III (Weak).

Landsliding was extensive in the elevated forested region and deposited in ravines between San Ramón and Satipo causing landslide dams. A failed landslide dam caused inundation in Puerto Ocopa. Many of small-scale landslides and destroyed vegetation near the Satipo River. Parts of a highway were buried under landslide debris. Approximately 233 to 2,233 people were killed during the earthquake. The total number of deaths is unknown and only deaths were recorded in Satipo, Andamarca, Acobamba, La Merced, Víctor, Comas, Perené and other towns.

The earthquake destroyed or seriously damaged 63 percent of all adobe constructed homes in La Merced; the town's hotel was however, 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 buildings were destroyed. In Satipo, the earthquake collapsed an entire school complex. A partially constructed church with thick brick walls reinforced with iron beams collapsed. Many new buildings of similar construction were also demolished. There was also destruction in Andamarca, Acobamba and Comas. Many adobe buildings in Jauja and Cerro de Pasco were also severely damaged.