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Enhancing Nuclear Safety


Research

Theses in progress

Modelling seismicity along faults in a probabilistic seismic hazard calculation


Host laboratory: Seismic Risk Assessment Section (BERSSIN)

Beginning of the thesis: October 2016

Student name: Thomas CHARTIER


Subject description (in French)
Taking faults into account in probabilistic seismic hazard assessment (PSHA) often requires to simplify geological data and uncertainties. This study proposes a novel methodology to model faults that considers multiple fault rupture scenarios as an aleatory rather than an epistemic uncertainty while preserving the information attached to each fault. The methodology, conceived so as to be flexible and applicable to regions where data on faults, geodesy and seismicity may be sparse, is based on modelling seismicity rates using a geological slip rate budget approach. The advantage of this approach is that each fault consumes its own cumulated slip rate, alone or in combination with neighboring faults and depending on the fault-to-fault rupture scenario (FtF) considered but always in coherence with a regional target for the modeled magnitude frequency distribution (MFD). Depending on the rules set for the target, part of the geological fault slip rate may not contribute to the seismic hazard. This part of the geological slip rate (hereafter referred to as non-main shock slip -NMS) is then considered as after slip, distributed deformation, aftershock slip or aseismic slip. Modeled seismicity rates along each fault are then compared to data (earthquake catalogues, geodetic moment rate estimate, and paleoseismological records) in order to weight the different branches of the PSHA logic tree.


We test this methodology on the western Corinth Rift fault system, Greece, where a large number of active faults has been identified. It is shown that the methodology is very sensitive to the nature of FtF scenarios considered. In spite of the imposed Gutenberg-Richter shape for the regional MFD the resulting MFD of each individual fault varies, depending on the position of the fault in the system. Furthermore, the proportion of NMS can vary dramatically (from 30 to 80%) depending on the FtF ruptures allowed. Estimated geodetic moment rates, corrected for the modeled NMS, remain however still three times larger than either seismological or geological moment rates. Coherence between modeled geological M>= 6.0 earthquake rates and those deduced from paleoseismological and seismological data available for the Aigion fault, on the other hand, are encouraging and point to a mean of proposing relative weights between the different FtF rupture scenarios for a site-specific PSHA at Aigion.


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