Seismic Source Parameters of Normal-Faulting Inslab Earthquakes in Central Mexico

Abstract

We studied 62 normal-faulting inslab earthquakes in the Mexican subduction zone with magnitudes in the range of 3.6 ≤ M w ≤ 7.3 and hypocentral depths of 30 ≤ Z ≤ 108 km. We used different methods to estimate source parameters to observe differences in stress drop, corner frequencies, source dimensions, source duration, energy-to-moment ratio, radiated efficiency, and radiated seismic energy. The behavior of these parameters is derived. We found that normal-faulting inslab events have higher radiated seismic energy, energy-to-moment ratio, and stress drop than interplate earthquakes as expected. This may be explained by the mechanism dependence of radiated seismic energy and apparent stress reported in previous source parameter studies. The energy-to-moment ratio data showed large scatter and no trend with seismic moment. The stress drop showed no trend with seismic moment, but an increment with depth. The radiated seismic efficiencies showed similar values to those obtained from interplate events, but higher than near-trench events. We found that the source duration is independent of the depth. We also derived source scaling relationships for the mentioned parameters. The low level of uncertainties for the seismic source parameters and scaling relationships showed that the obtained parameters are robust. Therefore, reliable source parameter estimation can be carried out using the obtained scaling relationships. We also studied regional stress field of normal-faulting inslab events. Heterogeneity exists in the regional stress field, as indicated by individual stress tensor inversions conducted for two different depth intervals (Z 40 km, respectively). While the maximum stress axis (σ 1) appears to be consistent and stable, the orientations of the intermediate and minimum stresses (σ 2 and σ 3) vary over the depth intervals. The stress inversion results showed that the tensional axes are parallel to the dip direction of the subducted plate. At depths greater than 40 km, the maximum horizontal stresses are oblique to the dip direction following the general trend of the slab geometry.