Significance of directivity effects during the 2011 Lorca earthquake in Spain

Abstract

The May 11th 2011, Lorca earthquake in Southeastern Spain was a moderate magnitude event (Mw 5.1) yet it caused nine fatalities, more than 300 injuries and more than 462 million euros in economic loses. Peak ground accelerations as well as response spectral ordinates far exceed expected values from various ground motion prediction models. In particular, spectral ordinates computed from recorded ground motions significantly exceed those in current Spanish probabilistic seismic hazard models, as well as those in the Spanish and European building codes. The objective of this paper is to assess directivity effects on ground motions recorded during the 2011 Lorca earthquake, and to evaluate the significance of these effects in earthquake resistant design on moderate seismic regions. In the first part of this paper, we study the likelihood of the presence of a directivity pulse, by conducting a comparison of different parameters of recorded ground motions to analytical pulses. In the second part, we relate the recorded ground motion and its inelastic displacement spectra to some recent statistical models that try to capture the displacement demand features of earthquakes presenting directivity-pulse characteristics. It is shown that simple analytical pulses are capable of reproducing very well pulse-type near-fault ground motions recorded during the event. It is concluded that directivity effects played a major role in the large impact caused by this relatively small event. Furthermore, directivity effects which are typically ignored, both in probabilistic seismic hazard analysis and in most building codes, may lead to important underestimations of ground motions.