Empirical Ground-Motion Relations in Northeastern México from intraplate moderate earthquakes

Abstract

ABSTRACT We present attenuation relations for peak ground velocity (PGV) and acceleration (PGA) for northeastern Mexico, where seismic activity has been underestimated over the years. We tested four models derived from a parametrical form previously proposed for intraplate earthquakes in Central Mexico but using regional data recorded by the Mexican National Seismological Service (Servicio Sismológico Nacional de Mexico, SSN). For comparison and as a selection criteria, we used the decaying trend of the models, the ratio between the observed and the predicted data (logarithmic) as a function of the distance and its standard deviation, and we determined the following attenuation relations: Where MW is the moment magnitude, R the hypocentral distance, and H the focal depth (both in km). and correspond to the intra-event and inter-event standard deviations. For PGV are 0.18 and 0.004, while for PGA and are 0.25 and 0.30, respectively. We found that by using the original distance definition R, (dependent on the hypocentral distance and on the magnitude altogether), the results are basically the same as when using the hypocentral distance directly. We also tried to incorporate an independent variable to characterize the local soil conditions of the recording stations. Despite the fact of finding a good residual behavior with that incorporation, the empirical functions of acceleration (PGA) stopped decaying around 250 km, implying a physical impasse and making the models inadequate for greater distances, situation that did not occur with velocities (PGV). The obtained constants found for the local site conditions on both PGA and PGV were similar between stations, showing no strong impact on the site effects on the predictive equations. The proposed models fulfill the purposes of having small residual fitting and simple attenuation relations that depend only on magnitude, hypocentral distance and focal depth, making them practical and easy to apply for engineering or seismic risk assessment purposes with the available data from local or regional networks.