A multidisciplinary study of a slow-slipping fault for seismic hazard assessment: the example of the Middle Durance Fault (SE France)

Abstract

SUMMARY Assessing seismic hazard in continental interiors is difficult because these regions are characterized by low strain rates and may be struck by infrequent destructive earthquakes. In this paper, we provide an example showing that interpretations of seismic cross sections combined with other kinds of studies such as analysis of microseismicity allow the whole seismogenic source area to be imaged in this type of region. The Middle Durance Fault (MDF) is an 80‐km‐long fault system located southeastern France that has a moderate but regular seismicity and some palaeoseismic evidence for larger events. It behaves as an oblique ramp with a left‐lateral‐reverse fault slip and has a low strain rate. MDF is one of the rare slow active fault system monitored by a dedicated dense velocimetric short period network. This study showed a fault system segmented in map and cross section views which consists of staircase basement faults topped by listric faults ramping off Triassic evaporitic beds. Seismic sections allowed the construction of a 3‐D structural model used for accurate location of microseismicity. Southern part of MDF is mainly active in the sedimentary cover. In its northern part and in Alpine foreland, seismicity deeper than 8 km was also recorded meaning active faults within the crust cannot be excluded. Seismogenic potential of MDF was roughly assessed. Resulting source sizes and estimated slip rates imply that the magnitude upper limit ranges from 6.0 to 6.5 with a return period of a few thousand years. The present study shows that the coupling between 3‐D fault geometry imaging and accurate location of microseismicity provides a robust approach to analyse active fault sources and consequently a more refined seismic hazard assessment.

Accurate hypocentre locations in the Middle-Durance Fault Zone, South-Eastern France

“…Most of the earthquakes are confined to the upper 10 km of the crust; however, as noted by Cushing et al [14], a few seismic events are located deeper than the sedimentary cover. These deep earthquakes could be linked to potentially seismogenic blind thrusts within the pre-Triassic basement.…”

“…[14]: a) the P-wave velocity of each layer is the area weighted average of the velocity sampled at that depth interval by the data; b) the topmost layer and the station corrections reflect the basic features of near-surface structure; c) equally high precision locations should be found for all well-locatable earthquakes occurring anywhere within the seismic network.…”

A morphological study of Durance River terraces from Tallard to Avignon, South-East France

“…The reach between Mirabeau and Sisteron has the complete range of terraces revealing increased elevations upstream. This reach attracted earlier terrace studies [43,49,50] and it has been affected by recent tectonics [16,17,24,38,[51][52][53][67][68][69][70][71][72].…”

“…Even though the basement rocks are much closer to the surface east of the MDF [48], implying an initial down throw to the west, it has been suggested that there was reverse uplift on the west side to trap the Miocene Valensole conglomerates [53]. However, to the south between Manosque and Mirabeau, conglomerates are found west of MDF [25].…”

“…1b), which has been extensively studied [13,24,46,47,[51][52][53]. On-going tectonic activity has occurred (faulting of the middle terrace at la Brillane and last glacial striation of low terrace conglomerates [53]). The slip fault is mainly near the right-bank valley wall and is segmented.…”