April 24, 2017 Earthquake: Valparaiso, Chile

The April 24, 2017 M 6.9 earthquake west of Valparaiso, Chile, occurred as the result of thrust faulting on or near the subduction zone interface between the Nazca and Pacific plates. At the latitude of this event, the Nazca plate is moving towards the east-northeast at a velocity of 74 mm/yr with respect to South America, and begins its subduction beneath the continent at the Peru-Chile Trench, 80 km to the west of the April 24 earthquake. The size, location, depth and mechanism of this event are all consistent with its occurrence on the megathrust interface in this region.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Events of the size of the April 24, 2017 earthquake are typically about 40x20 km in size (length x width).

Chile has a long history of massive earthquakes, including the 2010 M 8.8 Maule earthquake in central Chile, which ruptured a ~400 km long section of the plate boundary immediately south of this 2017 event. The April 24, 2017 earthquake lies close to where the Juan Fernandez Ridge enters the subduction zone, in a section of the plate boundary that ruptured in a M 8.0 earthquake in March 1985, and previously in the 1906 M 8.2 Valparaiso earthquake (reported by some to be as large as M 8.6). To the north of the Juan Fernandez Ridge, the subduction zone last ruptured in the M 8.3 Illapel earthquake in September 2015. This subduction zone also hosted the largest earthquake on record, the 1960 M 9.5 earthquake in southern Chile. Over the century prior to the April 24, 2017 earthquake, the region within 400 km of this event has hosted 19 M 7+ earthquakes. Prior to the April 24 M 6.9 event, the region immediately surrounding this earthquake hosted 9 other events of M 4.5 or larger, including a M 5.9 earthquake on April 23, 2017. The 6.9 shock was also preceded by 4 seconds by a M~4 foreshock. Within an hour of the M 6.9 earthquake, 4 aftershocks of M 4.7-5.4 had been located.

One car damaged due to a rockfall at Colina

Rengo, Libertador General Bernardo OʿHiggins, Chile - CDI: 7; Quillota, Valparaiso, Chile - CDI: 6; Vina del Mar, Valparaiso, Chile - CDI: 6; Valparaíso, Valparaiso, Chile - CDI: 6; Calera, Valparaiso, Chile - CDI: 6

(reference #13456)

The U.S. Geological Survey said the quake was centered in the Pacific about 22 miles (35 kilometers) west of the port city of Valparaiso and hit around 6:40 p.m. Buildings swayed in Santiago, the capital 70 miles (115 kilometers) miles to the east. The USGS revised the quake's magnitude down from an initial reading of 7.1. Chile's emergency services office said no damage to infrastructure was reported from the quake, which was felt more than 600 miles away in Cordoba province and other parts of neighboring Argentina. (reference #11784)

This sequence, which began on 22 April 2017, was punctuated by a M6.9 rupture on 24 April, and ended in early May 2017, was confined to a small area in a region of low slip between the two major asperities of the 1985 M8.0 Valparaíso earthquake. The southern extent of 2017 seismicity coincides with the 10 day foreshock sequence of the 1985 event, and the entire 2017 sequence defines an area that also hosted a similar sequence of earthquakes in 1973, punctuated by a M6.5 event. Foreshocks and aftershocks of the 2017 M6.9 event surround the main slip patch of this large earthquake, as has been observed in a number of recent megathrust earthquake sequences. (reference #11980)