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LABORATORIE DE GÉOLOGIE HISTORIQUE UNIVERSITÉ DE PARIS SUD, ORSAY, France
CENTRE DE RECHERCHE D'ASTRONOMIE D'ASTROPHYSIQUE ET DE GÉOPHYSIQUE, BOUZAREAH, Algeria
LABORATOIRE DE GÉOLOGIE STRUCTURALE UNIVERSITÉ DE MONTPELLIER, MONTPELLIER 34060, France
LABORATOIRE DE GÉOCHIMIE C.R.P.G.-E.N.S.G., 504501 VANDOEUVRE CÉDEX, France
LABORATOIRE DE SISMOLOGIE I.P.G. DE PARIS, PARIS, France
Abstract
During the EI Asnam earthquake of 10 October 1980 (Ms = 7.3), a clear active thrust fault with left-lateral offset was observed. Three trenches have been excavated across this fault in order to determine slip rate and recurrence intervals between large earthquakes, and thus reconstruct its past activity.
Exposure I was excavated in the flood area created in 1980 by a pressure ridge across the Cheliff and Fodda Rivers. Six flood deposits (silty-sandy and muddy horizons) alternating with paleosoils appear in this exposure; they are affected by normal faults associated with the main thrust fault. Assuming that every flood deposit results from a tectonic event of magnitude greater than 7, we can correlate previous flood deposits with these events. Exposures II and III display thrust faults displacing different paleosoils. We propose a sequence of reconstructions based on the thickness of the various deposits and the dip-slip of each tectonic event.
The Late Holocene slip rate is 0.65 mm/yr for the dip-slip and 0.46 mm/yr for each of the horizontal and the vertical movements. Radiocarbon dates of coseismic movements indicate a rather irregular seismic activity during the past 7000 yr. Two sequences of large earthquakes around 4000 yr B.P. and around the modern age are separated with a period of quiescence. The average Late Holocene recurrence interval of large earthquakes is 1061 yr; however, during the active faulting episodes, the recurrence time varies from approximately 300 to 500 yr.
Footnotes
* Present address: Laboratoire de Tectonique, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cédex 05, France.
Present address: Laboratoire de Sismologie, Institut de Physique du Globe de Strasbourg, Strabourg 67084, France.
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