Source parameters of induced earthquakes using regional earth models: Application to the 2013 sequence off the northeast coast of Spain

Abstract

We have obtained source parameters (focal depth, moment magnitude and moment tensors) for the 8 largest earthquakes of the 2013 seismic sequence occurred near an underground gas storage off the NE coast of Spain. To determine the regional centroid moment tensors we have used two methodologies: one based on the inversion of complete waveforms and another based on the inversion of spectral amplitudes of Love and Rayleigh waves. Both methodologies give compatible results in terms of focal mechanisms and focal depths, although the method based on waveform inversion provides more consistent results, with almost identical mechanisms and focal depths for all the events in the sequence. The mechanisms of the largest events are predominantly strike-slip (75%) with a smaller normal component (25%). The orientation of the P axes is approximately N-S, consistent with the known current state of stress in the region. The optimum focal depth obtained for all the analyzed events is 8 km, significantly deeper than the depth of injection (approximately 2 km below sea level). To elucidate the cause of this discrepancy we have investigated the influence of the earth model in the results of the source inversion. Therefore we have obtained new shear wave velocity models for the region (both 1D and 3D) based on surface wave dispersion measurements from ambient noise and from the earthquakes of the sequence. Using the 1D model we have determined new centroid moment tensor solutions. These new estimates do not differ significantly from those obtained initially using an earth model representative of the western US, indicating that the depth difference is not caused by the earth model.