ALBERT

Description: On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35 km depth and 50–120 km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~ 50 km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50 km depth and further inland at 150–160 km trench perpendicular distance, with mostly shallow dipping (~ 28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120 km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~ 34.5°S). These crustal events occur in a ~ 30 km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw = 6.9) on 11 March 2010.

Description: On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35 km depth and 50–120 km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50 km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50 km depth and further inland at 150–160 km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120 km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30 km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw=6.9) on 11 March 2010.

Description: On 27 February 2010 theMw 8.8 Maule earthquake in Central Chile ruptured a well known seismic gap, which last broke in 1835. Shortly after the mainshock, Chilean agencies (UC Santiago, UC Concepción) and the international seismological community (USA (IRIS), France (IPGP), UK (University of Liverpool), Germany (GFZ)) installed a total of 142 portable seismic stations along the whole rupture zone in order to capture the aftershock activity. Most stations were in the field until September 2010, with a subset remaining until January 2011; the UK stations will remain in the field beyond this time. The data from the initial deployment are open and are being distributed through the IRIS and GEOFON data centres. We will present preliminary aftershock distributions based on automatic detection algorithms. In total, for the period between March and September 2010 we detected _60,000 locatable earthquakes, of which we form a subset of _7,000 events with high quality locations. The depth of events in the high quality subset is generally well constrained such that the plate interface is clearly defined, and can be separated from overriding plate seismicity. First order features that can be identified are: 1.) A pronounced cluster of seismicity is apparent at 25-35 km depth and 50-120 km perpendicular distance from the trench (with some NS variation). 2.) A secondary band of seismicity can be identified at 40-50 km depth and _150-160 km perpendicular trench distance and between 34_ and 37_S. Although the secondary band lies along the continuation of the primary one, it is clearly separated from it by a gap with sparse seismicity. 3.) Intense crustal seismicity is found in the region of Pichilemu. This region hosted the strongest aftershock (Mw=6.9), a normal faulting event with NW strike. The aftershocks extend from the plate interface to the surface and are aligned on a NNW-SSE oriented band in map view. 4.) An isolated shallow cluster of crustal seismicity occurs beneath the volcanic arc (36.42_S, 71.1_W) near Laguna del Dial. Ongoing research is concerned with calculating first motion focal mechanisms for the larger events and improving locations by relative location methods.

Description: 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.

Description: On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic landstations was installed along the whole rupture zone in order to capture the aftershock activity. We present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. Processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, Caltech and GFZ, we determined 19,908~hypocentres with magnitudes Mw between 1 and 6.2. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25-35 km depth and 50-120 km distance to the trench. Along strike, the aftershocks occur largely within the zone of co-seismic slip but extend ~50 km further north. Along dip, the events are either within the zone of co-seismic slip, or downdip from it, depending on the slip model used. 3.) A third band of seismicity is observed further downdip at 40-50 km depth and further inland at 150-160 km trench perpendicular distance, with mostly shallow dipping thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the co-seismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 to 120 km depth are present north of 36°S. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the mainshock 6.) Pronounced crustal aftershock activity is found in the region of Pichilemu (~34.5°S). The time-series of postseismic deformation analyzed here show rapid transient deformation immediately following the Maule earthquake. We examine the relation between the spatial-temporal properties of the aftershock distribution and postseismic displacements from GPS. First results show a linear relationship between cumulative displacement and cumulative number of aftershocks at large times (〉25d, when the local aftershock catalog is available). This relationship may be use to infer rheological properties. Similar relations have been observed in other large subduction zone earthquakes.

Description: 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.