ALBERT

Description: Estimating small-scale V P / V S variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local V P / V S can be extracted, because the V P / V S is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007) , in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the V P / V S ratio is estimated by fitting the points on this plane. We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of V P / V S only in the case of a medium with homogeneous V P / V S , whereas, when a V P / V S gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with V P / V S decreasing with depth, we found that V P / V S is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90°. Moreover, we calculated analytically the dependence of this bias on the takeoff angles. Our simulations also showed that the difference between the calculated and the expected V P / V S is reduced for simple horizontally layered velocity structures (〈0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone.

Description: In this note, we derive an attenuation function for computing magnitude values equivalent to M w using strong-motion data. We analyze 106 earthquakes of the 1 April 2014 M w  8.1 Pisagua sequence, which occurred along the 1877 seismic gap in northern Chile. We considered both foreshocks and aftershocks with moment magnitude available from moment tensor inversion in the GEOFON bulletin and recorded by the Integrated Plate boundary Observatory Chile strong-motion network. The maximum peak displacement measured over the double integrated traces is used to construct the magnitude scale, following a nonparametric approach. A bootstrap analysis is performed to assess the uncertainty of the model parameters, and cross-validation tests are performed to proof the suitability of the derived model in predicting the M w in the analyzed area, with an uncertainty of 0.2 magnitude units. The derived scale is applied to an early aftershock, which occurred about 155 s after the mainshock, initially missed in bulletins published by rapid global earthquake monitoring agencies (e.g., National Earthquake Information Center and GEOFON), because its phase arrivals at regional/teleseismic distances mix with those of the mainshock and its later arrivals. The estimated magnitude equivalent to M w is 6.6±0.3, which rank this event as the second largest aftershock of the sequence, after the M w  7.6 earthquake that occurred on 3 April 2014.