Analysis of the 2002 May earthquake sequence in the central Pyrenees, consequences for the evaluation of the seismic risk at Lourdes, France

Abstract

S U M M A R YThree earthquakes of magnitudes 4.6, 4.3 and 3.7 occurred in 2002 May at two locations 20 km from the pilgrimage city of Lourdes in the French Pyrenees. They were well recorded by the permanent Pyrenean seismic networks, by a temporary local network, as well as by accelerometric stations. In order to understand their tectonic contexts, and to come to a better evaluation of the seismic risk at Lourdes, a detailed analysis of these events is performed.The first two events are located south of Lourdes in an area where only a few earthquakes have occurred up to now. Their focal solutions derived from first-motion polarities indicate reverse faulting, with a N110 • E strike consistent with the geological structures. 10 aftershocks were recorded and relocated with respect to the main events, benefiting from the waveform similarity of the various events. This analysis reveals that the two main events concern probably the same fault, the second rupture being in the prolongation of the first one, whereas the other small aftershocks are located on fault segments in the vicinity of the hypocentre of the second event.The third large event, located to the SE of Lourdes, involves a normal mechanism with a N120 • E plane parallel to the main geological structures. It occurred in a region of intense activity, including in particular an event of maximum macroseismic intensity IX in 1660.The first two events are at the boundary of a large quiet zone. In order to understand the related structural context, a new crustal tomographic model has been computed. It reveals that this quiet zone coincides with a block of high P-velocity. In contrast, the seismicity appears to be stronger at the northern and eastern boundaries of this block.The accelerometric data of the three main events recorded at Lourdes have been used to estimate the maximum peak ground accelerations in this city if a large event occurred, similar to those which damaged the city in the seventeenth and eighteenth centuries. Horizontal accelerations of 0.25 ± 0.07 g are predicted in the frequency domain 1-5 Hz at the location of the Sanctuary for a magnitude 6 event occurring 10 km away from the city. Taking into account the error bars, these values could in some cases exceed those specified by the building codes in this region.

Active Tectonics of the Pyrenees: A review

“…This E-W Pyrenean band of seismicity seems to be limited to the east by the Adour Fault (or Bigorre Fault, structure 7 on Table 1 and Fig. 3), which does not show any geomorphological expression (Souriau et al, 2001;Dubos, 2003;Dubos et al, 2004).…”

The Sounds of Small Earthquakes: Quantitative Results from a Study of Regional Macroseismic Bulletins

“…Note that the LDG magnitudes are overestimated by roughly one unit with respect to the Richter M L scale (i.e., M LDG ϳ M L ‫ם‬ 1). Finally, to provide additional upperbound information in the high-magnitude/ long-range domain, we also incorporated in the database the results of a macroseismic survey conducted by the BCSF after a M LDG 4.8 event in the Central Pyrenees (Dubos et al, 2004;BCSF internal report, 2002).…”

Absolute earthquake locations using 3-D versus 1-D velocity models below a local seismic network: example from the Pyrenees

“…(ii) the Cauterets sequence following two moderate earthquakes (M L = 4.6 and 4.3) between 2002 May 16 and2002 May 20 is distributed in two small very clustered swarms that were relocated by a double-difference relative location process (Dubos et al 2004).…”

“…The relative earthquake location based on HYPO-DD using cross-correlation waveforms from Dubos et al (2004) are used as reference for the first sequence and a HYPO71 process for the second reference. In both case, the azimuthal gap ranges between 95…”