Ground motion assessment summary

October 18, 2015 by  
Filed under Ground Motions

Ericson Encina & Ana Gabriela Haro

Chile is one of the most earthquake-prone countries in the world. On an interview, the deputy head of the National Seismological Centre (CSN), Dr. Mario Pardo, stated  “… statistically, each 8 to 10 years some Chilean location is struck by a destructive earthquake”, and then he added “And for the same location, a coastal earthquake will occur every 80 to 150 years”. Additional information about Chilean Seismicity and Earthquakes (in Spanish)  is available in the CSN website. To illustrate the point, the chart presented below renders the historical seismicity of Chile since 1868. It is worth noting that the last big earthquake close to Illapel was a 8.2 Mw magnitude in April 1943.

Historical Sesimicity of Chile - Source: CNS

Historical seismicity of Chile – Source: CSN

Several seismic motions have been felt since the main 8.4 Mw magnitude earthquake on 16 September at 7:45 p.m. local Chilean time. As per 2 October, CSN had processed around 1100 aftershocks with magnitudes over 3.0, being the largest aftershock the one occurred 22 minutes after the main shock that same day, at 8:07 p.m. scoring magnitude 7.6 Mw. An interesting animation of the aftershock sequence between 16 to 26 september was presented by IRIS. The two following figures show the seismicity evolution of the Coquimbo’s Region from July to September 2015, and the depth profile of the aftershocks as by 30 September 2015.

Evolution of Coquimbo Region’s seismicity – Source: CSN

Aftershocks’ depth profile – Source: CSN

According to Dr. Sergio Barrientos, Head of CSN, the main motion had a duration of about 70 seconds and the fault zone ruptured was estimated on 200-250km long with a displacement of 6-7 meters (University of Chile news). The earthquake features were consistent with a thrust-fault mechanism caused by the subduction of the Nazca plate beneath the South American plate. The images below present the shake map (Intensity) chart and the displacement estimation chart released by CSN by the end of September.

Shake Map – Source: CSN

 

Displacement estimation – Source: CSN

Acceleration records were recorded by several stations over the Chilean territory. Illapel’s main shock acceleration records could be found in CSN and Strong Motion Center databases. According to a preliminary report developed by Rubén Boroschek the maximum horizontal ground acceleration was 0.83 g – it was registered at Monte Patria station, 100km north from Illapel. At Monte Patria station was also produced the highest horizontal ground velocity and displacement, reaching 385 mm/sec and 149 mm, respectively.  In general, the maximum ground accelerations registered by accelerometers other than those in Monte Patria station were below 0.30 g, on average maximum ground accelerations were between 0.05 g and 0.10 g with a tendency to be greater northward than southward of Illapel. The acceleration and displacement response spectra illustrated in the figures below were adapted from Boroschek’s preliminary report.

Acceleration spectrum - Source: R. Boroschek

Acceleration spectrum – Source: R. Boroschek

Displacement spectrum - Source: R. Boroschek

Displacement spectrum – Source: R. Boroschek

Several quick/preliminary studies has been published since the earthquake. A list of the relevant webpages found so far is given below:

Where would it be the next? Scientists cannot predict seismic events but they have identified 3 regions that haven’t had seismic activity for a long time. As depicted in the figure below, in Peru the region from Ilo to Cuya hasn’t had strong seismic activity since 1868, in Chile from Iquique to Tocopilla since 1877, and in Chile from Taltal to Huasco since 1922. Although these regions have higher probabilities to suffer an earthquake it doesn’t mean any of them will be the place for the next strong earthquake.

Seismic gaps in northern Chile – Source: CSN

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