200‐m‐deep earthquake swarm in Tricastin (lower Rhône Valley, France) accounts for noisy seismicity over past centuries

Abstract

In the lower Rhône Valley (France), the Tricastin area was struck in 2002–2003 by an earthquake swarm with a maximum ML‐magnitude of 1.7. These shocks would have gone unnoticed if they had not occurred beneath habitations and close to the surface, some events being only 200‐m deep. A several months’ monitoring of the seismic activity by a 16‐station mobile network showed that earthquakes clustered along a N–S‐trending, at least 5‐km long, shallow rupture zone, with no corresponding fault mapped in the surface. Half of the seismic events occurred in a massive, c. 250‐m‐thick, Lower Cretaceous limestone slab that outcrops near by. Since the late eighteenth century, several much more severe earthquake swarms have struck Tricastin. The 1772–1773 and 1933–1936 swarms were prolific and protracted, with reports of numerous detonations and even damage. Obviously, the abnormal noises that caused panic in the past centuries can be explained by the shallowness of the phenomena, a 200‐m focal depth being perhaps a record value for tectonic earthquakes.

Surface rupture and shallow fault reactivation during the 2019 Mw 4.9 Le Teil earthquake, France

“…1a). Twenty kilometers southward from Le Teil village, on the eastern bank of the Rhône River, a very shallow seismic swarm (the so-called Tricastin swarm 17 ) occurred over several months in 2002-2003 with no corresponding fault at the surface (Fig. 1a).…”

The French seismic CATalogue (FCAT-17)

“…They correspond to areas covered by regional catalogues and where observatory earthquake locations have been chosen as the preferred solution in the SiHex catalogue; • European Cenozoic Rift System (ECRIS) zone delimits boundaries of the extensive structures (upper and lower Rhine grabens, together with the Bresse and Limagne grabens). Extension of this zone is deduced from Dezes et al (2004); • Provence zone, composed for the Provence Panel of a mesozoic shelf basin and characterized by shallow seismicity (Baroux et al 2001;Cushing et al 2008); • Tricastin zone, characterized by very shallow depth seismicity occurring through swarms (Thouvenot et al 2009); • Hainaut zone, also characterized by swarm seismic activity with shallow hypocenters (Camelbeeck 1985); • Atlantic zone, that corresponds to the oceanic front (including southernmost part of United Kingdom and Chanel domain). The strategy adopted to estimate an a priori depth in each of these regions consists in performing a statistical analysis of regional depths computed through the WLSQ inversion for events falling in ''complete'' and ''simplified'' ET strategies.…”

The fracture permeability of Hard Rock Aquifers is due neither to tectonics, nor to unloading, but to weathering processes

“…Moreover, at shallow depths like those of the fissured layer of HRA (several tens to one hundred metres), it appears difficult to invoke the hypothesis of the creation of new fractures outside the main active faults that are identified by ruptures at the surface. As a matter of fact on the one hand such shallow foci have never been observed at the subsurface (Thouvenot et al. , 2009) and on the other hand reservoir‐induced or natural piezometric changes‐induced seismicity, all being surface‐induced phenomena, have never been the subject of any shallow observation.…”

“…The energy dissipated during a seismic event depends on the surface area of the fault plane on which the displacement occurs (Wells and Coppersmith, 1994). Most of the hypocenters being located in depths ranging from several kilometres to several hundred kilometres (Thouvenot et al. , 2009), only the shallow earthquakes or those generating a move over a very large fault plane are likely to propagate to the surface.…”

“…, 2009), only the shallow earthquakes or those generating a move over a very large fault plane are likely to propagate to the surface. Seismologists consider that ‘shallow’ earthquakes are those occurring in the first 40 km (Thouvenot et al. , 2009) and a literature thorough search performed by these authors does not provide any evidence for foci shallower than 1–2 km.…”