Great Quake, Chile

An earthquake of magnitude 8, classified as a “great quake,” struck off the coast of Chile this afternoon, generating a damaging tsunami.

Thursday, 17th September 2015

A magnitude 8.3 earthquake struck 46 km west of Illapel, Chile, 229 km north-north-west of Santiago, Chile, at 10:55 this morning, New Zealand Standard Time. The undersea quake was 25 km deep.

The earthquake occurred on a thrust fault on the interface between the Nazca and South America tectonic plates according to the U.S. Geological Survey. The USGS reports that an area of seabed 230 km x 100 km slipped during the quake.

The area has a history of very large earthquakes, and readers may remember the magnitude 8.8 Maule earthquake which ruptured a 400 km long section of the plate boundary to the south of today’s event in 2010. A magnitude 8.2 earthquake struck off the cost of Chile in April 2014.

The Pacific Tsunami Warning Centre has issued a number of bulletins for this event which it reports as magnitude 8.3 at a depth of 10 km. A tsunami was generated by the movement of the seafloor during the earthquake, causing evacuations and damage to coastal areas in Chile. Sea level changed by 4.75 metres at Coquimbo, Chile at 25 minutes after midday today (NZST), the largest of several waves to come ashore. Maximum sea-level changes of 1.62 metres were recorded at Valparaiso, Chile and 1.3 metres at Talcahuano, Chile. These locations are on the coast to the west and close to the epicentre of the earthquake.

The tsunami wave has been propagated away from land as well, and local authorities are assessing the risk it imposes as the disturbance travels across the Pacific Ocean. It is currently thought that sea level changes of between 0.3 and 1 metre may be experienced along eastern coasts of New Zealand at various times after midnight tonight.

Experience shows that the first wave of a tsunami disturbance may not be the largest, and this is confirmed by the records made at Coquimbo in Chile where the largest sea-level change was only 3.11 metres at 11:39 this morning. The sea level disturbance continues for some hours after the first wave comes ashore as the disturbance bounces off local topography and additional “waves” come ashore.

The latest report from the Pacific Tsunami Warning Centre (bulletin 11 released at 7:26 this evening) records a sea-level change of 0.83 metres at Easter Island at 5:05 p.m. and 0.56 metres at Santa Cruz Galapagos at 6:43 p.m. this evening New Zealand time.

New Zealand’s Ministry of Civil Defence has a warning for eastern coasts and the Chatham Islands issued at 3:50 p.m. this afternoon on its website. This is an active link, and should update if MCDEM update their posting.

The U.S. Geological Survey has reported 21 significant aftershocks since this morning’s earthquake. Of these one was magnitude 7.0, six were magnitude 6 and 14 were magnitude 5.

Leave a Reply