Time Dependence of Shear Wavespeeds in Northern Chile Related to the 2014 Mw8.3 Pisagua Earthquake

The 2014 Mw8.3 Pisagua earthquake in northern Chile ruptured an area of about 160 x 70 km on the boundary between the Nazca and South American plates. Taking advantage of the availability of continuously recorded broadband data from the Integrated Plate Boundary Observatory Chile (IPOC) network in northern Chile from three years before to two years after this event, we determined empirical Greens functions (EGFs) from ambient noise to investigate the time dependence of shear wavespeeds throughout the crust in the upper (South American) plate. Daily EGFs are phase-weight stacked over the entire five-year period using an S-transform technique based on that described by Baig et al. (2009). These pilot stacks are then interferometrically compared to similarly stacked subsets over shorter time windows varying from one month to five days. Instead of analyzing the slope of the phase spectrum within moving windows as is usually done, we determine differences in phase velocity using the vertical component of fundamental mode Rayleigh waves by comparing the phase of the spectra of the EGFs within the time window expected for the wave train. Preliminary results show that such an approach can detect variations in phase velocity on the order of 0.1%. These variations can then be used in an inversion scheme to map the corresponding location and change in the shear wavespeeds within the crust of northern Chile relative to a background model.