Active Faulting in the French Western Pyrenees: Paleoseismic and Macroseismic Evidence

Details Active Faulting in the French Western Pyrenees: Paleoseismic and Macroseismic Evidence Active Faulting in the French Western Pyrenees: Paleoseismic and Macroseismic Evidence

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.t53b0492a&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #T53B-0492

Mathematics

Logic

7230 Seismicity And Seismotectonics, 8107 Continental Neotectonics, 1208 Crustal Movements: Intraplate (8110)

Scientific paper

We have identified a 50-km-long active fault scarp between Lourdes and Arette in the French Pyrenees. This region was affected by large and moderate earthquakes in 1660 (Io = VIII-IX), in 1750 (Io = VIII, Lourdes) and in 1967 (M=5.7, Io = VIII, Arette). Historical and instrumental catalogues that extend back to the 14th century suggest an East-West elongated active zone parallel to the Lourdes fault scarp. Most earthquakes in this area are shallow but available focal mechanism solutions do not indicate a coherent pattern. We have compared the felt areas of the main earthquakes in order to estimate their macroseismic magnitude. In addition we re-calculate the instrumental magnitude of the 1967 earthquake (the largest instrumental earthquake recorded in the Pyrenees) and determine a new focal mechanism solution according to WWSSN network data. Paleoseismic studies conducted in this area indicate a fault scarp characterised by an East-West Trending 50-m-high geomorphologic structure, composed of 3 continuous and linear segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces along strike. Evidence for active faulting is indicated by stream channels which are deviated and abandoned likely due to the successive uplift of the northern block of the fault. In Capbis and Arcizac, geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the successive fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip, and thrust ruptures with deformed alluvial units in buried channels. Geomorphologic and paleoseismic investigations, and magnitude-length scaling laws (Wells and Coppersmith, 1994) reveal that the Lourdes fault has the potential of producing an earthquake with Mw 6.1 to 6.9. The fault would thus correspond to a major seismic source in the western Pyrenees with possible magnitude larger than the 1660 main historical event. \hspace \hline Reference : Wells, D.L., and Coppersmith, K.J., 1994, Bull. Seism. Soc. Am., 84/4, 974-1002.

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