By David Jacobson and Volkan Sevilgen, Temblor
** Read about the seismic swarm that preceded this earthquake **
Updated: 9:00 am PDT 25 Apr 2017
At just past 5:30 p.m. local time, a M=6.9 earthquake shook central Chile, including the capital city of Santiago (EMS reported the earthquake was a M=6.7). This earthquake’s epicenter was offshore of the port city of Valparaiso, and occurred at a depth of 25 km according to both the USGS and EMS. So far, 89 aftershocks have followed the M=6.9, with the largest being a M=5.4. Based on data from the USGS, severe shaking was recorded close to the epicenter. Despite this, the USGS PAGER system estimates that both economic losses and fatalities will remain low. So far, over 700 people have reported feeling the quake on the USGS website, though the region is home to millions of people.
This earthquake occurred within an earthquake swarm that started on Saturday, which was highlighted by a M=6.0 Saturday evening. Up to this point, 54 M=3.0+ earthquakes have occurred offshore of Valparaiso in the last 2 days. To read more about the swarm, click here. Based on the initial depth reported by both the USGS and EMS, this quake likely occurred above the megathrust (subduction zone) on a secondary shallowly dipping splay.
Chile is one of the most seismically active countries on earth, with the large majority of quakes occurring on or near the subduction zone. Based on the Global Earthquake Activity Rate (GEAR) model, which forecasts earthquake magnitudes globally using strain rates and seismicity since 1977, this earthquake should not be considered surprising, as the region is susceptible to M=7.5+ quakes. Nonetheless, this is a major quake and caused buildings to sway in the capital. We will post a more in-depth post on this quake later.