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5. - 8. März 2012
DDC:
550 - Earth sciences
Abstract:
On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. With a magnitude of 8.8, it is the sixth-strongest earthquake since the beginning of the instrumental record; rapid response teams from Chile, the US, Germany and the UK installed a dense network for monitoring aftershocks along the whole rupture zone. We analysed a subset of this network (in total 139 stations) and detected over 100000 aftershocks following the main earthquake in the period from March to September 2010 alone, using automatic detection algorithms. Picks are refined by an auto-picking algorithm (MPX) and events are relocated in a minimum-1D model. About 20000 events are designated as very well located with at least 16 high quality automatic picks and a residual rms no larger than 0.2 s. Besides crustal seismicity, the aftershock sequence is dominated by intense plate interface seismicity near and immediately downdip of the most intense coseismic rupture. We also observe a second separate band of deeper aftershocks below the downdip end of the seismogenic zone at a depth of 40-50 km and a distance to the trench of 130-180 km, with a gap of 20-30 km to the main plate interface seismicity. In this presentation we concentrate on the analysis of this deep seismic band. The seismicity in this band is not truly continuous along the rupture zone but it is present along the whole rupture zone and forms clusters elongated along strike. Focal mechanisms derived from first motion polarities show that these events tend to be thrust type events, well aligned with the plate interface. A second deep separate group of plate interface aftershocks is not known from other subduction zone aftershock sequences. To get a better idea about the distribution of these 6000 deep aftershocks (30 to 50 km), a waveform- and catalogue-based clustering of aftershocks was carried out, followed by double-difference relocation. We also identified over 700 groups of events with highly similar waveforms. Most of the clusters are doublets and triplets, but the largest cluster contains 12 events. In more than 50 clusters events occurred semi-periodically 6-8 times with intervals of around 2 weeks, suggesting these are repeating events. For about 3000 aftershocks in the deep band precise relative locations could be determined based on catalogue and waveform-based double difference times, with formal uncertainties down to 100 m.
