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lunes, 30 de enero de 2012

Felt earthquakes in Peru during 2011

Rapid subduction of the Nazca plate beneath South America (of around ~6cm/yr, Kendrick, et al., (2003)),  causes strong seismic coupling along the entire subduction interface plate, giving raise to the continuous occurrence of interplate and intraplate earthquakes of different seismic moment release and located at diverse depths. On a yearly basis the seismicity rate in Peru vary from hundreds up to thousands of  earthquakes, but from the the total, only a certain number of events are felt by the population. In Peru, the "Instituto Geofísico del Peru" (IGP) is the institution in charge of the monitoring and surveillance of the seismic activity in the national territory, and whenever a seismic event is felt by the population, the information of the source parameters as well as the intensity reports are given as fast as possible and provided to the population in order be informed.

During 2011 the seismological service of IGP reported the occurrence of 138 felt earthquakes in the peruvian territory. From this dataset the largest magnitude event ML=7.0 occurred the August 24th at 17:46 (GMT) in the Peruvian Amazon, more precisely at 44 km SE from the Pucallpa city with a depth=149Km (intermediate depth) and called by IGP "The Contamana Earthquake". The second largest magnitude event ML=6.7occurred the October 28th at 18:54 (GMT) causing severe shaking on the western central coast, the hypocenter was located 63km south of Ica city with a depth=35 km, reported intensities reached values up to VI in the modified Mercalli scale, despite its moderate magnitude, this earthquakes seems to have filled the small seismic gap between the Nasca 1996 (M7.6) and the Pisco 2007 (M8.0) earthquakes, this hypothesis was also suggested in the IGP report for this earthquake. The magnitudes for the other events are ranging between 3.9 up to 6.6 ML, and all them have caused from small to moderate intensities in the cities near to the epicenter. The Figure shows the location of all the earthquakes occurred during 2011, red circles correspond to superficial/shallow earthquakes (depths between 0-60km) and green circles to intermediate events (depths between 61-300km). The size of each circle describes the magnitude assigned to each event according to the legend.  
The map shows that the superficial events are mostly located between the coast-line and the Peru-Chile trench, although in southern and northern Peru the seismicity distribution is quite scarce compared to central Peru, this suggests that the interaction between these two plates in central Peru during this year was important,  moreover in less proportion, some superficial events are also located inland over the andean cordillera and eastward in the sub-andean region, suggesting the interaction of the active quaternary faults  (note that colors define the topography). Intermediate events are mainly distributed inland showing a sparse distribution in northern Peru but most concurrent in Central and Southern Peru, where the geometry of the plate changes from sub-horizontal to normal. Note that in southern Peru the shallow seismicity is not important. In this region a large earthquake Mw=8.4 occurred in Jun. 2001 releasing part of the stress accumulated since the greatest 1868 southern Peru Earthquake. If someone wants to compare with the earthquakes reported for 2010 follow this link.

posted by JCVillegas and SYauri

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