The seismicity of southern Spain is characterized by low-to-moderate earthquake activity (magnitudes <5.5) related to the convergence of the African and Eurasian plates. The detailed structure of this complex boundary is not well understood and the study of faulting parameters and depths of the small-to-moderate-size events can contribute much to our understanding of the regional stresses. In this work, we present time domain, least-square moment-tensor and dislocation grid-search inversion results obtained by modeling 20 regional seismograms of the main shock in the sequence of 45 events that occured near Mula, Spain, between 2 and 18 of February, 1999. The mb 4.7 main shock occured on February 2, 1999 at 13:45:17 UTC, and was located at 38.11 °N, - 1.49°E. Our formal, best least-squares moment-tensor solution includes a substantial 72% non-double-couple component. This exotic component of the source is difficult to interpret in terms of simple faulting processes of small earthquakes, and very poorly constrained by the observations. We quantified the significance of the non-double-couple component using a dislocation grid-search to identify the optimal double-couple solution. The analysis suggests that the non-double-couple component of the moment-tensor inversion result is likely a consequence of source-receiver geometry, and is unnecessary to match the main features in the regional waveforms. Since depth resolution from the regional waveforms was minimal, we used teleseismic P-wave observations to refine our source depth estimate. Our preferred faulting solution is a pure double-couple mechanism that strikes 50°N, dips 70°, has a rake of -30°, and a depth of 7.5-12.5 km. Independent work suggests that the shallower depth is more likely. The seismic moment is 1.29 × 1023 dyn cm, which corresponds to an Mw of 4.7. Alignment of the aftershocks in a northeast-to-southwest direction suggests that the event was predominantly left lateral movement along a near-vertical strike-slip fault.
All Science Journal Classification (ASJC) codes
- Earth-Surface Processes