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: We use Global Positioning System (GPS) velocities and kinematic Finite Element models (FE-models) to infer the state of locking between the converging Nazca and South America plates in South-Central Chile (36[degree sign]S - 46[degree sign]S) and to evaluate its spatial and temporal variability. GPS velocities provide information on earthquake-cycle deformation over the last decade in areas affected by the megathrust events of 1960 (Mw= 9.5) and 2010 (Mw= 8.8). Our data confirm that a change in surface velocity patterns of these two seismotectonic segments can be related to their different stages in the seismic cycle: Accordingly, the northern (2010) segment was in a final stage of interseismic loading whereas the southern (1960) segment is still in a postseismic stage and undergoes a prolonged viscoelastic mantle relaxation. After correcting the signals for mantle relaxation, the residual GPS velocity pattern suggests that the plate interface accumulates slip deficit in a spatially and presumably temporally variable way towards the next great event. Though some similarity exist between locking and 1960 coseismic slip, extrapolating the current, decadal scale slip deficit accumulation towards the ~ 300-yr recurrence times of giant events here does neither yield the slip distribution nor the moment magnitude of the 1960 earthquake. This suggests that either the locking pattern is evolving in time (to reconcile a slip deficit distribution similar to the 1960 earthquake) or that some asperities are not persistent over multiple events. The accumulated moment deficit since 1960 suggests that highly locked patches in the 1960 segment are already capable of producing a M ~ 8 event if triggered to fail by stress transfer from the 2010 event.