Modelling array beams for the 1979, 1984 and 2021 Granada earthquakes (southern Spain)

Abstract

Despite the occurrence of large, past earthquakes in the Central Betic Range (Southern Spain), seismicity recorded with digital seismographs is limited to small magnitude events. Here we are interested in the three strongest events (M_W 4.5 to 5.0), of which source models are unclear for different reasons: The June 20th 1979 and June 24th 1984 earthquakes are still characterized by a lack of regional recordings, while the August 12th 2021 earthquake occurred during a teleseismic M8 event. We use beamforming at distant seismic arrays and waveform modelling of depth phases to estimate source parameters for seven earthquakes altogether. The technique is successful at reproducing P-waveforms and at estimating the depth of four recent (1997–2021) earthquakes with M_W > 4. In addition, it is also used along with an inverse scheme that yields source mechanisms similar to regional moment tensor solutions. Inversion suggests normal faulting at depths of 7 km and 9 km for the 1984 and 2021 events, which is consistent with our understanding of regional seismotectonics. Beamforming has been able to extract the 2021 waveforms from the M8 coda wavefield, and could be a suitable approach also for other cases of earthquake coincidence. The most noteworthy result is a strike-slip mechanism at 60 km depth for the 1979 earthquake, which is a singular subcrustal event in this area and might be related to tearing at the edge of the Gibraltar slab.