A structural model for the seismicity of the Arudy (1980) epicentral area (Western Pyrenees, France)

Abstract

S U M M A R YThe Western Pyrenees presents a diffuse and moderate (M ≤ 5.7) instrumental seismicity. It nevertheless historically suffered from strong earthquakes (I = IX MSK). The seismic sources of these events are not yet clearly identified. We focus on the Arudy (1980) epicentral area (M = 5.1) and propose here the reactivation of early Cretaceous normal faults of the Iberian margin as a potential source. The late Cretaceous inversion of this basin, first in a left-lateral strike-slip mode and then in a more frontal convergence, resulted in a pop-up geometry. This flower structure attests of the presence of a deep crustal discontinuity.The present-day geodynamic arrangement suggests that this accident is reactivated in a right lateral mode. This reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts, rooted on this discontinuity. These thrusts accommodate the shortening component of the strain. The distribution of the instrumental seismicity fits well the structural model of the Arudy basin. Whatever the compressive regional context, the structural behaviour of the system explains too the extensive stress tensor determined for the Arudy crisis if we interpret it in terms of strain ellipsoid. Indeed numerical modelling has shown that this concomitant activity of strike-slip and thrust faulting results in an extensive component that can rise 50 per cent of the finite strain.We identify too a 25-30 km long potential seismic source for the Arudy area. The size of the structure and its potential reactivation in a strike-slip mode suggest that a maximum earthquake magnitude of ∼6.5 could be expected. The extrapolation of this model at the scale of the Western Pyrenees allows to propose other potential sources for major regional historical earthquakes.

The 2006 November,ML= 5.0 earthquake near Lourdes (France): new evidence for NS extension across the Pyrenees

“…The M L = 5.1, 1980 Arudy earthquake with a normal faulting CMT focal mechanism was also followed by numerous normal faulting aftershocks (Gagnepain‐Beyneix et al 1982). Recent geomorphological and geological studies in the Arette‐Arudy region, 30–50 km west of the Argelès event location, also evidenced normal faults dipping to the north at crustal scale in the North Pyrenean Zone, with clear quaternary activity (Dubos‐Sallée et al 2007). Northward dipping normal faults have also been evidenced in the eastern part of the range (Briais et al 1990), but their quaternary activity is still debated (Philip et al 1992).…”

Active Tectonics of the Pyrenees: A review

“…1 and 2). The instrumental seismicity of the NWPZ clusters between the cities of Bagnères-deBigorre and Arette, in an 80 km long and 20 km wide area (Gagnepain-Beyneix et al, 1982;Souriau et al, 2001;Dubos, 2003;Rigo et al, 2005;Ruiz et al, 2006a;Dubos-Sallée et al, 2007). This E-W Pyrenean band of seismicity seems to be limited to the east by the Adour Fault (or Bigorre Fault, structure 7 on Table 1 and Fig.…”

“…Also for this area and using seismological, structural and geomorphic data, Dubos-Sallée et al (2007) proposed that the late Cretaceous inversion of the former Iberian Margin in a strike-slip mode resulted in a pop-up flower-like geometry limited to the south by the Mail Arrouy Thrust (structure 8 in Table 1, Fig. 3).…”

Microseismicity of the Béarn range: Reactivation of inversion and collision structures at the northern edge of the Iberian plate

“…The depth distribution of the microseismicity underlines a north dipping zone beneath the Béarn massif, down to at least 20 km (Figure ) [ Souriau and Pauchet , ]. The core of the cluster is located beneath the Aspe valley (Oloron; Figure , section B), and its shape underlines a steep (~50–60°) north dipping zone, which tends to flatten shallowing upward toward the south (Figure ) [ Souriau and Pauchet , ; Dubos‐Sallée , ]. This sigmoid shape is fairly consistent with the distorted trace of the ECLT at depth, as shown by Lacan [] and Lacan and Ortuño [].…”