Objectives

The purpose of the project is to develop a methodology to identify active faults in presently low seismicity areas of Europe and to demonstrate and measure their activity in terms of historical or prehistorical large earthquakes.

As compared with the seismic activity along plate boundaries, the occurrence of large earthquakes, rupturing the whole brittle crust and producing surface ruptures, is relatively rare in intraplate regions. However, because of the higher vulnerability caused by the development of urban areas, widespread damage is to be expected for populated regions. In the densely populated areas of northwest Europe, the seismic activity is apparently low and strongly damaging earthquakes are virtually absent in the human memory. Nevertheless, evidence exist for the occurrence of large earthquakes (Camelbeeck and Meghraoui, 1996, 1998) and this part of Europe would be at high risk if for example an event similar to the Basel earthquake of 1356 would occur. The Pyrenees and the Alps, in a tectonic environment linked to the complex boundaries between the African and the European plates, are also regions where the present-day seismic activity is relatively low but strong earthquakes have occurred during historical times and indication of paleoearthquakes exist (Combes et al., 1993), indicating a real potential for the occurrence of large seismic events.

The time period covered by the scientific observations (historical and instrumental seismic data) in the areas where the recent seismicity is weak is less than the probable duration of the seismic cycle (time interval between two large earthquakes) which is of the order of some thousand to hundred thousand years along the few studied active faults on the continental interior (Crone et al., 1997). Thus, fundamental problems of estimating the size of large earthquakes and the return period of major seismic events along active faults must be studied using geological investigations of Holocene and Late Pleistocene deposits.

The proposed methodology (paleoseismology) is based on the fact that repeated coseismic displacements along an active fault that may expose young deposits, allow past large earthquakes to be geologically recorded with a typical morphological expression. On the other hand, severe shaking due to earthquakes can also induce surface ground cracking, sand injections, shallow local landslips and other liquefaction features. Furthermore, in natural caves, strong ground motions can cause speleothem fractures and dislocations (Postpischl et al., 1991).

The methodology will be applied in four zones from three areas with diverse seismotectonic framework: The Lower and Upper Rhine graben system characterized by extensional continental deformation, the Trentino-Alto Adige (Eastern Alps) and the Eastern Pyrenees which are regions under compressive tectonic stress field. The applicability of the different aspects of the methodology will of course be different from one region to the other and will depend on the geological, archeological and historical context.

Final objectives of the project are:

1. TO IDENTIFY PALEOEARTHQUAKES ALONG SELECTED ACTIVE FAULTS IN THE STUDIED REGIONS.
2. TO ESTIMATE THE MAGNITUDE AND RECURRENCE TIME OF THESE EVENTS.
3. TO ESTABLISH A FORMULATION OF THE RESULTS WHICH IS DIRECTLY USABLE FOR SEISMIC HAZARD ANALYSIS.

References:

Camelbeeck, T., and Meghraoui, M. (1996), Large earthquakes in northern Europe more likely than once though, EOS, Trans. Am. Geophys. Union 77 (42), 405,409.

Camelbeeck T. and Meghraoui M. (1998), Geological and geophysical evidence for large palaeo-earthquakes with surface faulting in the Roer Graben (northwest Europe). Geophys. J. Int., 132, 347-362.

Combes Ph., Carbon D., Cushing M., Granier Th. et Vaskou Ph. (1993), Mise en évidence d'un paléoséisme pléistocène supérieur dans la vallée du Rhône: implications sur les connaissances de la sismicité en France. Comptes Rendus de l'Académie des Sciences, t.317, série II, n° 5, Septembre 1993, p.689-696.

Crone A., Machette M. and Bowman J. (1997), The episodic nature of earthquake activity in stable continental regions revealed by paleoseismicity studies of Australian and North American Quaternary faults, Australian Journal of Earth Sciences, 44, 203-214..

Postpichl, D., Agostini, S., Forti, P., and Quinif, Y. (1991), Paleoseismicity from karst sediments: "Grotta del Cervo" cave, Central Italy, Tectonophysics, 193, 33-44.