A geodetic coseismic fault-slip model for the May, 11th 2011 Lorca earthquake using radar interferometry and GPS

Abstract

The Alhama de Murcia Fault (AMF) is a compound multi-segmented oblique left-lateral fault system. The AMF is one the longest faults in the Eastern Betics Shear zone (Southeastern Spain). In the last decades its seismogenic potential has been carefully evaluated based on paleoseismological data. On May 11th, 2011 a moderate (Mw 5.1) earthquake shook the region, causing nine casualties and severe damage in Lorca city (Murcia region). The early reported location of the aftershock sequence did not draw any particular trend; furthermore in-situ geology surveys did not identify any surface coseismic slip-related ground deformation. In order to provide better seismic hazard assessments, we need to locate and, if possible, characterize the fault-slip distribution that generated this earthquake. In this work, we detected small but significant ground deformation close to the epicentral area of the Lorca earthquake by using geodetic (satellite radar interferometry and GPS) data. Geodetic data was processed by using a stack of differential radar interferograms (corrected for a known long-term subsidence contribution), daily GPS estimated coordinates and high-rate 1-Hz GPS data. We jointly inverted the detected static coseismic displacements for the fault plane geometry parameters by using a rectangular dislocation model embedded in a homogeneous elastic half-space. The best-fitting fault plane closely follows the geologically derived AMF geometry (NE-SW strike trend and dipping ~70º to NW). Later, the obtained model geometry was extended and divided into patches to allow for a detailed analysis of the fault slip distribution pattern. Slip distribution indicates that slip occurred in a main patch –4-5 km long– with reverse and leftlateral motion (with peak fault slip magnitude of ~20 cm). However, the modelling results also indicate that fault slip occurred close to the surface along the centre and southwest of the city of Lorca. The shallower character of the slip and the effect of a finite dynamic rupture fault arrest probably caused the relatively intense ground acceleration recorded in the city of Lorca (~0.4g) and increased the building damage. This study also represents the first modern geodetically observed ground deformation signature due to seismic activity in the Iberian Peninsula.