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.