1946 Ancash earthquake

The 1946 Ancash earthquake in the Andes Mountains of central Peru occurred on November 10 at 17:43 UTC. The earthquake had a surface-wave magnitude of 7.0, and achieved a maximum Mercalli intensity scale rating of XI (Extreme). About 1,400 people are thought to have died from the event.

Tectonic setting
The west coast of Peru lies a convergent boundary where the Nazca Plate subducts beneath the South American Plate. Despite being located in a zone of compression, shallow continental extension occur beneath the Andes mountains, within the overriding South American Plate. Some of these normal faults are dated to the Quaternary and are considered young. The 1946 event is the only recorded normal faulting earthquake within the Andes mountains.

Earthquake
The earthquake was triggered by slippage along a shallow-angle normal fault at a depth of ~17 km, as suggested by the focal mechanism. A surface-wave magnitude of 7.0 was assigned by the ISC while the calculated moment magnitude was 7.0 ± 0.1 or 6.8. The fault was a shallow crustal structure on the South American Plate, as surface ruptures were observed with a maximum offset of 3.5 m. Surface rupture associated with the earthquake was the first on a large-scale ever observed in South America. It had a length of 28 km. This event also reactivated several older thrust faults. These faults are part of the Marańón fold and thrust belt, which formed in the Tertiary. The associated surface rupture of this event had a length of 21 km on the Quiches Fault, an active normal fault named after the town which suffered extensively during the earthquake. The surface rupture was considered a pioneering instance of a major one being well-documented in South America. Due to the highly inaccessible nature of the affected area and road closures due to landslides and fractures, surveyors who documented the surface rupture said it may be more extensive than observed. Shallow intraplate earthquakes within the South American Plate are rare due to the long recurrence intervals on faults. The Quiches Fault have an average recurrence interval of 13,000 years.

Impact
Shaking was felt over a 600,000 km2 area, being most severe in the departments of Ancash and La Libertad. At least 45 landslides were triggered by the strong ground motions which was the reason for the high death toll and destruction. One of these slides completely buried the village of Acobamba in Suytucocha Valley. At least 217 people died in Acobamba and the town was buried under 20 m of debris. Three of the largest landslides had volumes of 36,550,546 m3, 13,503,994 m3 and 10,582,563 m3, respectively.

The earthquake was felt over a 450,000 km2 area. Shaking was reportedly so strong that residents in the village of Mayas were unable to stand. A resident described first hearing a blast followed by shocks in the ground that caused him to fall twice, injuring his knee on the second occasion. The maximum Modified Mercalli intensity, X–XI, was felt in a thin strip of area from Cerro Pelagatos to the villages of Mayas and Quiches. Intensity IX was assigned a northwest–southeast elliptical-shaped area measuring 1,600 km2. Large landslides were reported within the isoseismal VIII–IX. Many fatalities were the result of poor construction practices. Villages experienced amplified shaking on soft alluvial deposits. In the village of Quiches, 677 people were killed. Hundreds of deaths were recorded in Acobamba, Sihuas, Conchucos and Pampas.