Local Vp/Vs ratio in the vicinity of the Tocopilla (Chile) earthquake (Mw 7.7, 14/11/2007) inferred by differential P- and S- travel times

We have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction margin corresponding to the rupture area of the Tocopilla earthquake (Mw 7.7, 14/11/2007). This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km Northern Chile Southern Peru seismic gap, which at the time had not seen an earthquake of this magnitude since the M~9 event of 1877. Tocopilla event activated two main co-seismic slip patches: one around the epicenter and another north-east of the Mejillones Peninsula. We have applied the Lin and Shearer (2007) approach to the aftershock sequence of the Tocopilla event. In this approach, the relative time shift between the S phases of a pair of nearby events at one station are plotted as function of the time shifts between the P phases of the same pair. The process is repeated for a cluster of events. If the events are close enough to assume a locally uniform Vp/Vs ratio and the P-reciprocal wavefront can be approximated as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The technique is extended to a set of stations demeaning the time shifts from each pair of events. The time shifts are inferred maximizing the cross-correlation function between the event pairs. The technique has been applied to clusters of events sharing similar waveforms and spatially clustered hypocentres. We have adopted a robust linear regression and have assigned a statistical error to the best fit. Clusters have been identified along the whole profile of the subducting slab, although most clusters falls within a sub-vertical branch of the subduction interface hosting a major aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones Peninsula, and shows Vp/Vs mostly in the range 1.7-1.8. Clusters of the plate interface shallower than about 30 km show Vp/Vs around 1.9, while at intermediate depths (30-40 km) Vp/Vs is ~1.8. We speculate on the existence of hydrated crust producing the highest Vp/Vs (~1.9 or larger) observed at shallow depths.