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Address : CNRS-U. Pierre & Marie Curie, France
Celebration date : 9 Tuesday September 2008
Author name : Sebrier, Michel

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  • Article title : Review of active faults in slowly deforming regions: The western Europe case
    Article type : Tectonic & Seismotectonic
    Location : International Geological Congress,oslo 2008

    Fulltext :

    SStudies on seismicity catalogues of Western Europe showed their incompleteness. Thus, seismic risk assessment in such regions of low to moderate seismicity must take into account geological data in order not to underestimate seismic hazard. However, due to erosion and sedimentation processes, vegetation cover or/and human interactions, geomorphic signals, which permit to delineate active faults, decline as fault slip rates decrease. Two approaches have been considered to address this problem. (1) Historical data on large historical earthquakes is a reliable starting point to investigate active fault sources in Western Europe. (2) Quantitative geomorphologic analyses are a robust tool to identify active deformation. Nevertheless, the low strain rates of western European regions imply that the time-window of landscape evolution must cover most of the Quaternary. Then, final identifications of slowly slipping active faults also involve geophysical techniques to the near-surface fault geometry. Such methodologies have been successfully used during the last decades either as national efforts or as European projects. These have increased the knowledge on such slow-slipping active faults.
    Active faults that have been identified and analysed belongs either to extensional or compressional provinces. Very few strike-slip fault have been analyzed so far. Most of the potentially seismogenic faults of Western Europe are characterized slip-rates on the order of 10-1 to 10-2 mm/yr. However, these slow active faults have produced destructive earthquakes with Mw magnitudes estimated between 6 and 7. Even if such destructive earthquake may repeat in the future with an annual probability of occurrence relatively low (10-3 to 10-4 for individual sources), they have strong incidence on seismic hazard assessment of hazardous plants. Slip rates for individual faults around the Western Mediterranean appear slightly higher, especially for normal faulting in the Apennines and strike-slip faulting in southern Spain that are closer to the Africa/Eurasia plate boundary. In extensional tectonic provinces (eastern Iberia Peninsula, Lower Rhine Embayment, Upper Rhine Graben, Apennines, Inner Alps...), slow-slipping normal faults appear to provide the same kind of seismic parameters than faster active faults (Mw, recurrence interval, elapsed time, slip-rate...). In compressional tectonic provinces (Southern Spain, Po Plain, Provence, and southernmost Upper Rhine Graben bordering Jura), slow-slipping reverse or thrust faults are frequently hidden or blind so that paleoseismic observations are limited to address their seismogenic potential. However, combining multiscalar DEM-aided landscape analyses with subsurface data, completed by near-surface geophysics permit to estimate both Mw and slip rates for the potentially seismogenic sources. In conclusion, obtained results in Western Europe demonstrate that seismic strain release, even very low, localises repeatedly on same sources.