After the 2010 Mw 8.8 Maule earthquake : Tectonics in central Chile derived by an automated analysis of aftershocks from an amphibious seismic network
The Chilean subduction zone is among the seismically most active plate boundaries in the world and coastal ranges suffer from a magnitude 8 or larger megathrust earthquake about every ten years. The Constitución-Concepción or Maule segment in central Chile between about 35.5°S and 37°S was considered to be a mature seismic gap, rupturing last in 1835 and being seismically quiet without any magnitude 4.5 or larger earthquakes reported in global catalogues. It is located to the north of the nucleation area of the 1960 magnitude 9.5 Valdivia earthquake and to the south of the 1928 magnitude 8 earthquake near Talca. On 27 February 2010 this segment ruptured in a Mw 8.8 earthquake, nucleating near 36°S and affecting a 500 km-long segment of the margin between 34°S and 38.°S. Most of the aftershocks occurred offshore. Therefore, a network of 30 ocean-bottom seismometers (OBS) was deployed in the northern part of the rupture area for a three month period, recording local offshore aftershocks between 20 September 2010 and 25 December 2010. In addition, data of a network consisting of 33 land stations of the GeoForschungsZentrum Potsdam were included into the network, providing an ideal coverage of both the rupture plane and areas affected by post-seismic slip as deduced from geodetic data. Two years prior to the Maule event the Collaborative Research Center SFB 574 "Volatiles and Fluids in Subduction Zones" operated an amphibious seismic network in the same area. Both data sets gave a great opportunity to compare seismicity and stress distributions before and after a megathrust event and to study the evolution of a subduction zone within the seismic cycle of a megathrust event. In this study the aftershocks of the Mw 8.8 Maule earthquake are analysed in order to gain information about the rupture zone, stress distributions, and faulting in the forearc after a megathrust event. As most of the temporary and permanent seismic networks are located on land, automatic picking routines have been developed with land station data and there are few studies with automatically determined phase arrivals from OBSs in the literature. The analysis of aftershocks in this study is performed in an automated approach to show that an automated determination of phase arrivals and polarisation, focal mechanisms and magnitudes can be accomplished with OBS data as well. Aftershock seismicity analysis in the northern part of the survey area reveals a well resolved seismically active splay fault in the accretionary prism of the Chilean forearc. Splay faults, large thrust faults emerging from the plate boundary to the sea floor in subduction zones, are considered to enhance tsunami generation by transferring slip from the very shallow dip of the megathrust onto steeper faults, thus increasing vertical displacement of the sea floor. These structures are predominantly found offshore, and therefore, hard to detect in seismicity studies as most seismometer stations are located onshore. Application of critical taper theory analysis suggests that in the northernmost part of the rupture zone, co-seismic slip likely propagated along the splay fault and not the subduction thrust fault while in the southern part it propagated along the subduction thrust fault and not the splay fault. The most profound features of a comparison of aftershocks to data collected in 2008 before the Maule event are: (1) a sharp reduction in intraslab seismic activity after the Maule earthquake, (2) an increase in seismic activity at the slab interface above 50 km depth, where large parts of the rupture zone were mainly aseismic prior to the Maule earthquake. Further, the aftershock seismicity shows a broader depth distribution above 50 km depth, shifting the updip limit of the seismogenic zone about 30 km closer to the trench, and (3) an active seismic cluster in the 2008 data while in 2010 there is a seismic gap in about 40 to 50 km depth along the plate boundary probably related to a relic mantle body.
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