Paleo-earthquakes investigations in the Upper Rhine Graben in the framework of the PALEOSIS project.

Marc CUSHING, Francis LEMEILLE, Fabrice COTTON, Bertrand GRELLET, Jean-Christophe AUDRU and François RENARDY Rapport Final. HAN2000 – potential for large earthquakes in low seismic activity regions of Europe, 39-43.

A global investigation in order to find paleo-earthquakes evidences in the Upper Rhine Graben (URG) has been performed in the framework of the CCE PALEOSIS project. Several methodological themes have been selected in the French part of the URG to target possible recent morphological evidences of faulting. In the southern URG (Basel to Mulhouse area), morphological, geophysical and field analyses were performed. In the northern URG, a complete multidisciplinary study of the Selestat to Lauterbourg area was perfornied (DEM, aerial photos, field, geophysical prospecting and reinterpretation of oil companies seismic profiles). Results of this first approach and the presence of relatively well known quaternary levels lead us to concentrate on the Hangenbieten-Achenheim and Mundoslheim area (Lemeille et al., 1999). This study shows evidences of quaternary faulting in the URG: (1) in Hangenbieten, normal faults with 16 m offset in Upper Crmerian formations (400 to 450 ky) and (2) in Achenheim, involving formations (150 to 300 ky) with an offset of 6 m. Complementary high resolution seismic profiles have been realised in order to assess the deep rooting of these surface rupturing. Moreover, the microtectonic analysis suggests a seismic origin for the offsets. For a 6.0-6.5 magnitude earthquake (metric displacement) and an inferred vertical deformation rate (0.04 mm/y), recurrence period could be about 25 000 years.