Mapping Subduction Zone Fault Slip with Teleseismic and Geodetic Data

<p>Contours of ground deformation (10 cm interval) from some large subduction zone earthquakes in the central Andes as measured by the ERS-1, ERS-2 and Envisat satellites of the European Space Agency. Deformation is in the line-of-sight of the radar beam, which is about 23 degrees from vertical and projected onto the map as a black arrow. The largest magnitude of ground deformation during the earthquakes is off-shore, but these dense on-land images of ground displacement provide new information about the details of the fault motion during the earthquake.</p>

<p>In an effort to understand the behavior of the megathrust fault in the Peru- Chile subduction zone, we have used seismic and geodetic observations to determine the location of fault slip during different parts of the earthquake cycle (co-seismic, post-seismic, and inter-seismic). During the co-seismic time period, we can invert teleseismic waveforms from the GSN and geodetic data (InSAR and GPS) both jointly and separately to constrain the rupture process. We have combined the seismic and geodetic data to study 8 earth- quakes (6.7 < Mw < 8.4) in southern Peru and northern Chile that occurred between 1993 and 2007 (Fig. 1), providing one of the most detailed space-time rupture histories available in a subduction zone and yielding new discoveries about the generation of large earthquakes in this area [Pritchard et al., 2006; Pritchard et al., 2007; Pritchard & Fielding, 2008; Loveless et al., 2010]. In northern Chile, because of the spatially and temporally dense geodetic data and constraints on earthquake locations and slip from the GSN, we can clearly separate co-seismic and post-seismic deformation. We document a complex mosaic of phenomena including large earthquakes, post-seismic after-slip with a spatial distribution that appears to be tied to variations in coastal mor- phology, and the first observations with sufficient resolution in both time and space to infer triggering of a large (Mw > 7) earthquake by a so-called ``silent earthquake'' [Pritchard & Simons, 2006]. Using the teleseismic and geodetic data together, we have shown that in order to make reliable two-dimensional models of earthquake slip for subduction zone earthquakes larger than magnitude 7.5 (necessary for hazard and other types of studies), we cannot use standard techniques. We must either combine teleseismic data with the geodetic data [Pritchard et al., 2007; Pritchard & Fielding, 2008] or use new anal- ysis methods recently developed by others [Lay et al., 2010]. We used our detailed models of the 6 largest earthquakes to study potential relationships between the rupture areas and other observables in an effort to better forecast the location of large earthquakes. We find no obvious or simple relationship between various other proposed physical processes and the earthquake slip in these earthquakes. However, analysis of the forearc gravity field and its gradients shows correlation with many of the observed slip patterns, as suggested by previous studies [Loveless et al., 2010].</p>


<p>J. P. Loveless, M. E. Pritchard, and N. Kukowski, Testing mechanisms of subduction zone segmentation and seismogenesis with slip distributions from recent Andean earthquakes, Tectonophysics, 2009, in press.</p>

<p>M. E. Pritchard, and E. J. Fielding, A study of the 2006 and 2007 earthquake sequence of Pisco, Peru, with InSAR and teleseismic data, Geophys. Res. Lett., 35, 10.1029/2008GL033374, 2008.</p>

<p>M. E. Pritchard, E. Norabuena, C. Ji, R. Boroschek, D. Comte, M. Simons, T. Dixon, and P. A. Rosen, Teleseismic, geodetic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes, J. Geophys. Res., 112, 10.1029/2006JB004294, 2007.</p>

<p>M. E. Pritchard, and M. Simons, An aseismic fault slip pulse in northern Chile and along-strike variations in seismogenic behavior, J. Geophys. Res., 111, 10.1029/2006JB004258, 2006.</p>

<p>M. E. Pritchard, C. Ji, and M. Simons, Distribution of slip from 11 Mw > 6 earthquakes in the northern Chile subduction zone, J. Geophys. Res., 111, 10.1029/2005JB004013, 2006.</p>


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