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Abstract

The Chile earthquake of 27 February 2010 nucleated just north of the city of Concépcion, and ruptured to the north and to the south along 350-400 km, with most slip (up to 15 m) accumulating in the northern patch. 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 and grid-search based phase association algorithms (binder and CMM). 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. 2500 earthquakes of these belong to a large cluster downdip of the main slip patch and the most intense shallow aftershock seismicity in the overriding crust and along the plate interface. Although the reasons for the sharp limits for the deep seismic band are not known (below the Arauco peninsula at the southern end of the rupture, the plate interface does not yet intersect the mantle at these depths, based on a tomographic study), the location on the plate interface downdip of the coseismic rupture suggests it is driven by afterslip.