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.