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Seismic cycle and seismic hazard of a network of active faults: the case of the Corinth-Patras rift (Greece)

Aurélien Boiselet has defended his thesis on the 26th March 2014 in ENS, Paris.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/DEI/SARG/BERSSIN

Authors > BOISELET Aurélien

Publication Date > 26/03/2014

Summary

The Corinth rift (Greece) is one of the regions with the highest strain rates (16 mm/y extension rate) in the Euro-Mediterranean area and as such it has long been identified as a site of major importance for earthquake studies in Europe (20 years of research by the Corinth Rift Laboratory and 4 years of in-depth studies by the ANR-SISCOR project). This enhanced knowledge, acquired in particular, in the western part of the Gulf of Corinth (CRL region), an area about 50 by 40 km2, between the city of Patras to the west and the city of Aigion to the east, provides an excellent opportunity to compare fault-based (FB) and classical seismotectonic (ST) approaches currently used in seismic hazard assessment studies. An homogeneous earthquake catalogue was thus constructed for the purpose of this study along with a comprehensive database of all relevant geological, geodetical and geophysical information available in the literature and recently collected within the ANR-SISCOR project. The homogenized Mw earthquake catalogue is composed of data from the National Observatory of Athens and from the university of Thessaloniki as well as data acquired through historical and instrumental work performed within the ANR-SISCOR group for the CRL region. A frequencymagnitude analysis confirms that seismicity rates are governed by Gutenberg-Richter (GR) statistic for 1.2<Mw<5 earthquakes, however beyond M≥5, observed seismicity rates tend to be higher than expected from the GR-scaling law. This result suggests that the CRL region is actually behaving similarly to some individual faults that show a “characteristic” frequency-magnitude behaviour. Indeed, based on the comprehensive database, the highly fractured nature of the fault network is confirmed, with individual fault segments that do no appear to exceed 15 km in length at the surface and thus preferentially producing at most only 6 <M<6.5 earthquakes. However, occasionally, multiple fault segment ruptures, capable of generating M~7.0 earthquakes, had also to be considered in order to reconcile estimated fault slip rates (e.g. Psathopyrgos fault with the highest value of ~ 5 mm/y) with paleoseismologically and seismically derived magnitude rates (available only for some faults). A logic tree methodology was implemented to explore different geometries and rupture scenarios reflecting the range of opinions within the ANR-SISCOR group. On the basis of this consensual logic tree, probability of occurrences of M>=6 earthquakes were computed for the region of study. Timedependent models (Brownian Passage time and Weibull probability distributions) were also explored. The probability (normalized by area) of a M>=6.0 earthquake is found to be greater in the CRL region compared to the eastern part of the Corinth rift. Probability estimates corresponding to the 16th and 84th percentile are also provided, as a means of representing the range of uncertainties in the results. Probability estimates based on the ST-approach are then compared to those based on the FBapproach approach. In general ST tends to overestimate probabilities of occurrence compared to FB estimates along slow slipping faults and underestimate them along faster moving faults (e.g. Psathopyrgos, Aigion). The FB approach in this region is still affected by a high degree of uncertainty, because of the poor constraints on the 3D geometries of the faults and the high uncertainties in their slip rates and maximum magnitude. Thus, for example, the Psathopyrgos fault is the fault that shows the highest probability (12%) of hosting a M>=6.0 event in the next 30 years, however, the associated uncertainty is also the greatest (5%-27%). Finally, the cumulative seismic moment rate of the faults is comparable to the seismic moment rate estimated from the earthquake catalogue. Interestingly, the overall seismic moment rate of the CRL region shows a deficit of 50+/-20 % when compared with an estimate of the equivalent geodetic moment rate. This suggests an important component of aseismic deformation.


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