Seismic hazard on the French Riviera: observations, interpretations and simulations

Abstract

S U M M A R YWe present here a detailed analysis of a seismic data set recorded by a dense seismological network installed over 6 months in the southeast of France. This experiment was set-up at the boundary between the Ligurian basin and the southern subalpine thrust belt (the Nice arc), which is a complex tectonic region that undergoes low to moderate seismicity. We recorded more than 500 microearthquakes, among which 348 occurred exactly in the centre of the network during a very active seismic sequence that lasted mainly over 2 months.We performed an absolute location of all of the events and calculated the magnitudes. Then we applied a cross-correlation technique to gather similar events and to relocate relatively few of them. This method revealed a very clear alignment of 19 events in a direction N120 • oblique to the N20 • general trend of seismicity. Focal mechanisms were determined for the four largest events and composite solutions for 32 smaller ones. Both the alignments of the earthquakes and the focal solutions revealed that two oblique segments of the fault were activated during the crisis.The main segment (8 km long) that was oriented NNE with a left-lateral strike slip movement is called the Blausasc fault. Taking into account the tectonic evolution and the relationships between surface structures and the distribution of earthquakes, and through a paleaoreconstruction of the tectonic evolution, we propose that the Blausasc fault is the hidden root of the Peille-Laghet fault, which has a mapped length of at least 15 km. The smaller segment (0.6 km long) that was activated during the crisis could be interpreted as an antithetic Riedel fracture.The active Blausasc fault is located in a densely populated zone, at only 10 km from the crowded cities of Monaco and Nice. It is thus particularly interesting to analyse it for hazard assessment. In the last section, we present a simulation that is aimed at predicting what the ground motion in the city of Nice would be like if an earthquake of magnitude 5.7 occurs on this fault. For this, we used the recordings of the largest event of the seismic sequence (Ml = 3.2) and an empirical Green's function summation scheme to simulate the ground motion at two stations situated in urban environments. The values obtained show that especially on soft soil sites, the effects of such an earthquake would be considerable in the city of Nice.

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps–Ligurian basin junction during the Cenozoic

“…The development of the subalpine Castellane and Nice subalpine chains was mainly completed during the Miocene (w20 to 5 Ma, Laurent et al, 2000). However, deformation was still active, at least in the Nice subalpine chain, during the Pliocene and the Quaternary Clauzon and Rubino, 1996;Courboulex et al, 2007;Dubar and Perez, 1989;Larroque et al, 2001;Laurent et al, 2000;Schroetter, 1998). The second Alpine phase started at about 26e23 Ma in the Argentera massif area located w100 km north of Figure 3.…”

Cone penetration testing to assess slope stability in the 1979 Nice landslide area (Ligurian Margin, SE France)

“…Some of the tectonic deformation in the Western Alps resulted in presently still seismically active features such as the Peille-Laghet fault in the Maritime Alps (Courboulex et al 2007) or the Marcel fault (Fei et al 2014). The southwestward trace of the Peille-Laghet fault turns into the "Blausasc fault" (sensu Courboulex et al 2007) and affects the area south of the city of Nice. The nearly E-Wtrending Marcel fault is located further downslope at the Nice margin, but may equally affect slope stability in the Var canyon and adjacent slopes (Fei et al 2014).…”

“…Long-term pore pressure records at the 1979 NAIL area reveal a significant correlation between precipitation events and Δu, associated with the beginning of the snowmelt and heavily rainfall events over a period of 3 months (Stegmann et al 2011). In addition, a seismological network was installed covering the region between Nice and the Italian border (Courboulex et al 2007). More than 500 micro-earthquakes recorded between 2000 and 2001 reveal a NNE alignment on the socalled Blausasc fault, which is probably the "hidden root of the Peille-Laghet fault".…”

“…Simulations to assess the peak ground accelerations (PGAs) in the city of Nice were performed assuming a magnitude 5.7 earthquake if the entire 8 km portion of the Blausasc fault fails (Courboulex et al 2007). The average PGAs were quantified between 0.3 to 1.5 m/s 2 and potentially strong enough to trigger small to large landslides ( Fig.…”

Detailed quantification of delta subsidence, compaction and interaction with man-made structures: the case of the NCA airport, France

“…Larroque et al, 2011). The deformation of the margin, associated to moderate seismicity (Courboulex et al, 2007), is mostly seen through a set of thrust faults at the foot of the continental slope which likely hosted the 1887 earthquake rupture of which the magnitude is estimated around M w = 6.8-6.9 (Larroque et al, 2012). However, in this part of the margin, the main faults identified by morpho-geological studies are oriented parallel to the margin and are thus not compatible with a motion restricted to the sole riverbed.…”