Revisiting the shallow M-w 5.1 Lorca earthquake (southeastern Spain) using C-band InSAR and elastic dislocation modelling
Résumé
Space geodetic techniques such as interferometric synthetic aperture radar (InSAR) and global positioning systems (GPS) have demonstrated to be useful in mapping the displacement fields of large earthquakes (moment magnitude (M-w) approximate to 6 or higher). However, the displacement fields of smaller earthquakes (M-w < 5.5), such as those that typically result from the collision of the European and African plates, are less often analysed by space geodetic techniques, and their characterization, in terms of slip along the fault plane at depth and focal depth location, often challenges current seismological techniques. This letter presents the results of InSAR analysis of the 11 May 2011, M-w 5.1, Lorca earthquake. The Lorca earthquake occurred close to an area undergoing rapid subsidence due to sediment compaction related to water pumping. Therefore, it is challenging to separate the InSAR signals due to the earthquake from those due to human activity. We used four sets of SAR data acquired from the European C-band Advanced SAR (ASAR) sensor on board the Environmental Satellite (ENVISAT) to map the surface-displacement field in the Lorca region. Then, we use a simple elastic dislocation model to characterize the fault plane geometry and the fault slip at depth. We find that the InSAR signals can be explained by approximate to 21 cm reverse slip with a approximate to 6 cm left-lateral component on a 3 kmx3 km segment centred at 4.2 km depth dipping 45 degrees NW and striking N65 degrees E, consistent with the rupture of a segment of the Alhama de Murcia fault and consistent with recent published analyses. Interestingly, the InSAR signal can also be explained by approximate to 21 cm reverse slip with a approximate to 6 cm left-lateral component on a 3 kmx3 km segment centred at approximate to 4.2 km depth dipping 50 degrees SE and striking N230 degrees E, consistent with preliminary focal plane solutions indicating a rupture on a previously unmapped blind structure. We conclude that the second model cannot be rejected on the base of the InSAR results, the complex surface-displacement pattern (containing both seismic and non-seismic displacement), the different preliminary moment tensor solutions and the published locations of aftershocks at depth.