Chile Earthquake

Update 3 | Summary
Posting Date: March 11, 2010, 12:00:00 PM

Three strong aftershocks of magnitude 6.0 and above struck Chile today in quick succession, including the strongest quake to hit the country since February 27th, when the M8.8 earthquake occurred. Among the numerous aftershocks that day was one that registered M6.9. This morning, at 11:39 am local time (9:39 am EST), another M6.9 earthquake struck at an estimated depth of 21.7 miles about 90 miles southwest of Chile’s capital, Santiago, and 85 miles south of Valparaiso. In Valparaiso the quake caused buildings to shake, and some people fled from the Inauguration ceremony of the country’s recently elected president that was in progress. Just 16 minutes later another shock, of M6.7, struck within a few miles of the first, and 11 minutes later yet another struck, having a magnitude of 6.0. Shortly after the Inauguration ceremony the Parliament building was evacuated. All together, nine aftershocks having a magnitude larger than 5.0 took place today in less than five hours. All occurred within 30 kilometers (less than 20 miles) of each other along the northern segment of the rupture zone of the February 27th quake. The original shock on February 27th occurred at the interface of the subducting Nazca plate and the overriding South American plate. Preliminary analysis of the location of today’s events and their seismic-wave radiation patterns indicates that the largest shocks happened as the result of normal faulting. However, it remains unclear as to which plate the shocks may have taken place in. The focal depths of today’s earthquakes have not as yet been reliably defined, so whether the two large events in particular (M6.9 and M6.7) occurred at a shallow depth within the overriding plate or at a deeper depth within the subducting plate cannot be confidently pinpointed. (The US Geological Survey originally said the first aftershock had a magnitude of 7.2, which it later revised.) Whichever turns out to be the case, both possibilities can generate larger ground motion than if they occurred on the plate interface as the February 27th earthquake did. Today’s M6.9 earthquake rocked buildings and shook windows in Santiago. In Constitución along Chile’s coast, buildings emptied and streets became crowded with people seeking higher ground. Chile’s National Emergency Office quickly issued a tsunami alert for central areas of the country, including Valparaiso, but the Hawaii-based Pacific Tsunami Warning Center said there was no threat of a destructive widespread tsunami. The likelihood and type of damage that could result from today’s shocks remain uncertain. On the one hand, the weakest buildings already have been severely damaged if not destroyed; on the other hand, partially damaged buildings are likely to be more severely impacted. However, there have not been immediate reports of significant damage or injuries. The AIR earthquake modeling team has run scenarios for today’s aftershocks and does not expect them to add significantly to the losses already caused by the main shock. The AIR team continues to monitor events in Chile and will continue to report as new developments warrant. AIR’s post-disaster survey team will be arriving in Chile tomorrow, and AIR’s Dr. Guillermo Franco, Senior Research Engineer, will be joining the EERI (Earthquake Engineering Research Institute) fact-finding team.

Update 3 | Downloads
Posting Date: March 11, 2010, 12:00:00 PM