FMHex20: A database of earthquake focal mechanisms in metropolitan France and conterminous Western Europe

We present a compilation of over 1700 focal mechanisms for nearly 1300 earthquakes in metropolitan France and conterminous Western Europe. It is based on both published and unpublished sources (articles, reports, observatory websites) for which the focal mechanism solutions have been verified for internal consistency, corrected in cases of minor errors and rejected in cases of major inconsistencies between the parameters. The database, labeled FMHex20, is a first version and should be regularly updated in the future as part of an ongoing effort within the Seismicity Transverse Action of the French RESIF research infrastructure. We also present first-order seismotectonic analyses for the whole metropolitan France and for a couple of example regions (Western France and Northern Alps-Jura-Vosges) to illustrate how the FMHex20 database can serve as a basis for geodynamic or seismic hazard zonation studies. Combined with complementary datasets, it can improve our understanding of the kinematics of potentially active faults, including in very-low-strain-rate regions as is the case for most of metropolitan France.

Time-Dependent Stresses From Fluid Extraction and Diffusion With Applications to Induced Seismicity

Over recent decades, it has become clear that the extraction of fluids from underground reservoirs can be linked to seismicity and aseismic deformation around producing fields. Using a simple model with uniform fluid extraction from a reservoir, Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17(10), pp. 942–946) illustrated how poroelastic stresses resulting from fluid withdrawal may be consistent with earthquake focal mechanisms surrounding some producing fields. Since these stress fields depend on the spatial gradient of the change in pore fluid content within the reservoir, both quantitative and qualitative predictions of the stress changes surrounding a reservoir may be considerably affected by assumptions in the geometry and hydraulic properties of the producing zone. Here, we expand upon the work of Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17, pp. 942–946 and 1985, “Stress and Subsidence Resulting From Subsurface Fluid Withdrawal in the Epicentral Region of the 1983 Coalinga Earthquake,” J. Geophys. Res. Solid Earth, 90, pp. 6801–6816) to provide a quantitative analysis of the surrounding stresses resulting from fluid extraction and diffusion in a horizontal reservoir. In particular, when considering the diffusion of fluids, the spatial pattern and magnitude of imposed stresses is controlled by the ratio between the volumetric rate of fluid extraction and the reservoir diffusivity. Moreover, the effective reservoir length expands over time along with the diffusion front, predicting a time-dependent rotation of the induced principal stresses from relative tension to compression along the ends of the producing zone. This reversal in perturbed principal stress directions may manifest as a rotation in earthquake focal mechanisms or varied sensitivity to poroelastic triggering, depending upon the criticality of the pre-existing stress state and fault orientations, which may explain inferred rotations in principal stress directions associated with some induced seismicity.