Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004geoji.156..527d&link_type=abstract
Geophysical Journal International, Volume 156, Issue 3, pp. 527-540.
Astronomy and Astrophysics
Astronomy
6
Earthquake, Lourdes, Pyrenees, Relocation, Seismic Risk, Tomography
Scientific paper
Three 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.
Benahmed Sébastien
Chevrot Sébastien
Dubos Noalwenn
Fels Jean-François
Ponsolles Christian
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