C. del Fresno

Study of the Damaging Earthquakes of 1911, 1999, and 2002 in the Murcia, Southeastern Spain, Region: Seismotectonic and Seismic-Risk Implications

A detailed study of four earthquakes that occurred in the Murcia region (southeastern Spain) in 1911, 1999, and 2002 has been carried out. New intensity maps have been plotted for the March and April 1911 shocks. These show maximum values of VII-VIII (EMS). We have found values of VI and V, respectively, for the 1999 and 2002 earthquakes. Surface wave magnitudes range from 5.2 for the Bullas 2002 event to 5.7 for the March 1911 event. Focal mechanisms for the Mula 1999 and Bullas 2002 events indicate reverse and strike slip motions, with scalar seismic moments of 5.9 � 10 16 N m and 8.6 � 10 15 N m, respectively, and focal dimensions of 4.0 and 1.5 km. Both earthquakes were recorded at epicentral distances greater than 21 km, showing low values of peak ground acceleration (PGA )( 0.020 g). These records involve a ground motion less than the ones expected in the region, according to different hazard studies. The seismic hazard map that we obtained for the Murcia region (for a return period of 475 years) shows higher values than those given in the Spanish Building Code NCSE-02 (2002) (e.g., 0.24 g for PGA in the city of Murcia, compared with 0.16 g in the code).

The 2010 Granada, Spain, Deep Earthquake

The deep earthquake ( h =650 km) that occurred on 11 April 2010 south of Granada, Spain, has been studied using the inversion of body waves at teleseismic and regional distances. We have obtained a solution of dip-slip motion on either a vertical plane or a nearly horizontal plane with the pressure axis dipping 45° to the east. The horizontal plane is chosen as the rupture plane, with rupture propagating from east to west on the basis of directivity effects at teleseismic distances and differences of the waveforms at regional distances. The comparison of these results with the focal mechanisms of four other deep earthquakes that occurred in the same area shows similar rupture processes. The origin of this deep seismic activity remains an open question. Tomographic studies have shown the existence of an anomalous body in this region that extends from 200- to 700-km depth. The olivine-spinel phase transitions or shear melting along horizontal planes inside of this body may be an explanation for the occurrence of these earthquakes.