The high damaging mw 5.1 lorca 2011 earthquake: Possible role of local seismic amplification

On May 11, 2011, a seismic series struck the city of Lorca (SE Spain) causing widespread damage and 9 fatalities. The series was characterized by a Mw 4.5 foreshock occurred at 15:05 (GMT), a main event occurred at 16:47, Mw 5.1, and many other aftershocks of smaller magnitude. All focuses were shallow and located about 3 km N of this city. The acceleration records of both Mw 4.5 and 5.1 events at the station located at Lorca (LOR) show that main direction of the motion was almost perpendicular to the strike of the north segment of the Lorca fault (N50oE). To understand the local seismic response of the city area in general, and of the LOR site in particular, ambient noise and Vs30 was measured through the city and the whole set of accelerograms recorded at LOR was analyzed (period 1993–2013; epicentral distances from 2 to 150 km). The HVSRs do not show a significant amplification while from the accelerograms recorded at LOR, a possible “fault effect” related to the near fault shaking, as well as to a “fault-zone” effect, i.e. due to the dynamic properties of the fault zone rocks with respect to the adjacent rock masses was recognized. These effects are only visible in the case of earthquake records whose epicenters are closer than 15 km (i.e. produced by the fault zone). Such effect combined with large accelerations that occurred may explain the damages produced by the main event.