Characteristics and possible origins of the seismicity in northwestern France ´ E. Beucler, M. Bonnin, C. Hourcade, B. Van Vliet-Lano¨ e, C. Perrin, L. Provost, A. Mocquet, J. Battaglia, L. Geoffroy, P. Steer, B. Le Gall, J.M. Douchain, D. Fligiel, P. Gernigon, B. Delouis, J. Perrot, S. Mazzotti, G. Mazet-Roux, S. Lambotte, M. Grunberg, J. Vergne, C. Cl´ ement, ´ E. Calais, J. Deverch` ere, L. Longuevergne, A. Duperret, C. Roques, T. Kaci and C. Authemayou eric.beucler@univ-nantes.fr November 16, 2021

A polyphased geological setting Beucler et al. NW France seismicity November 16, 2021 2

A polyphased geological setting Beucler et al. NW France seismicity November 16, 2021 3

Historical data 6°W 6°W 4°W 4°W 2°W 2°W 0° 0° 2°E 2°E 4°E 4°E 46°N 46°N 47°N 47°N 48°N 48°N 49°N 49°N 50°N 50°N 577 1579 1799 1853 1930 1490 1711 1926 1889 1959 Epicentral intensity II III IV V VI VII VIII 462-1962 565 events in SISFRANCE database (Jo- mard et al., 2021) Beucler et al. NW France seismicity November 16, 2021 4

Instrumental data 6°W 6°W 4°W 4°W 2°W 2°W 0° 0° 2°E 2°E 4°E 4°E 46°N 46°N 47°N 47°N 48°N 48°N 49°N 49°N 50°N 50°N −10 −5 0 5 10 dVS /V0 (%) MW 1 2 3 4 5 z=02 km, V0 =3.09 km/s 1962-2020 1962-2020 dataset 1962-2009: SI-Hex (Cara et al., 2015) 6631 NW events vs 38028 (MW ≥ 2.5, 597/2591 23%) 2010-2020: unified catalogue BCSF-R´ eNaSS+CEA 2689 NW events vs 38048 (MW ≥ 2.5, 847/3326 25%) Total: 9320 events in the northwestern part of France (including NW of Auvergne) VS model from (Gaudot et al., 2021) Beucler et al. NW France seismicity November 16, 2021 5

Permanent seismic coverage evolution Beucler et al. NW France seismicity November 16, 2021 6

Magnitudes Respective distributions of moment magnitudes for the western part (green) and for the rest of metropolitan France (orange) during two different time ranges. The green histograms are computed for a region comprised between 2° E and 6° W and 45.4° N and 50° N, using bins of 0.4 Beucler et al. NW France seismicity November 16, 2021 7

Focal mechanisms fmhex database (Mazzotti et al., 2021) 6°W 6°W 4°W 4°W 2°W 2°W 0° 0° 2°E 2°E 4°E 4°E 46°N 46°N 47°N 47°N 48°N 48°N 49°N 49°N 50°N 50°N 1984-02-25 1972-09-07 2016-04-28 2005-04-18 2015-07-28 2003-02-14 2018-02-12 2013-11-21 2005-06-22 2002-09-30 1987-01-10 2003-08-23 2016-12-11 1975-08-30 2017-01-07 2014-07-11 1996-11-26 1996-06-01 1983-07-07 1985-09-30 2019-06-21 2013-09-25 1977-04-27 2016-05-02 OL HE VA LA VANNES NANTES RENNES ANGERS BREST LIMOGES POITIERS CAEN LE-MANS CHINON ORLÉANS SAINTES CLERMONT-FERRAND Proterozoic and Paleozoic metasedimentary and crystalline terrains Beucler et al. NW France seismicity November 16, 2021 8

Toward a comprehensive catalog? 2-year of continuous seismic signal Template matching + discrimination (see C. Hourcade poster for ML implementation and details) 6°W 6°W 4°W 4°W 2°W 2°W 0° 0° 47°N 47°N 48°N 48°N 49°N 49°N 50°N 50°N 100 km 2018-08-01/2020-07-31 FR RD GB detection station BCSF+CEA (192 +60 ) This study (+131 ) 6°W 6°W 4°W 4°W 2°W 2°W 0° 0° 47°N 47°N 48°N 48°N 49°N 49°N 50°N 50°N 100 km 2018-08-01/2020-07-31 BCSF+LDG (192 ) This study (131 ) 0 5 10 15 20 25 30 35 Depth (km) Beucler et al. NW France seismicity November 16, 2021 9

Discussion Possible geodynamic factors controlling the seismicity 1 Plate-scale stress field Overall NW-SE orientation of the maximum horizontal stress and a general extensive to transtensive tectonic style with a NE-SW deviatoric tension 2 Local and temporal stress modulations Spatial variations of gravitational potential energy / isostatic adjustment to erosion and sedimentation / mechanical response to hydrological or meteorological transients / post-glacial adjustmets / tides 3 Tectonic and fault inheritance Are earthquake locations compatible with known (active) faults? → back to the question about a comprehensive catalogue Frequency: 0.4 → 1 event/day Beucler et al. NW France seismicity November 16, 2021 10

Cara, M., Y. Cansi, A. Schlupp, P. Arroucau, N. B´ ethoux, ´ E. Beucler, S. Bruno, M. Calvet, S. Chevrot, A. Deboissy, B. Delouis, M. Denieul, A. Deschamps, C. Doubre, J. Fr´ echet, S. Godey, O. Golle, M. Grunberg, J. Guilbert, M. Haugmard, L. Jenatton, S. Lambotte, D. Leobal, C. Maron, V. Mendel, S. Merrer, M. Macquet, A. Mignan, A. Mocquet, M. Nicolas, J. Perrot, B. Potin, O. Sanchez, J.-P. Santoire, O. S` ebe, M. Sylvander, F. Thouvenot, J. Van Der Woerd & K. Van Der Woerd (2015). “SI-Hex: a new catalogue of instrumental seismicity for metropolitan France”. Bull. Soc. G´ eol. France 186(1), 3–19, doi:10.2113/gssgfbull.186.1.3. Gaudot, I., ´ E. Beucler, A. Mocquet, M. Drilleau, M. Haugmard, M. Bonnin, G. Aertgeerts & D. Leparoux (2021). “3-D crustal VS model of western France and the surrounding regions using Monte Carlo inversion of seismic noise cross-correlation dispersion diagrams”. Geophys. J. Int. 224(3), 2173–2188, doi:10.1093/gji/ggaa552. Jomard, H., O. Scotti, S. Auclair, P. Dominique, K. Manchuel & D. Sicilia (2021). “The SISFRANCE database of historical seismicity. State of the art and perspectives”. C. R. Geoscience, subm. Mazzotti, S., C. Aubagnac, L. Bollinger, K. Oscanoa, B. Delouis, D. Paco, C. Doubre, M. Godano, H. Jomard, C. Larroque, A. Laurendeau, F. Masson, M. Sylvander & A. Trilla (2021). “FMHex20: An earthquake focal mechanism database for seismotectonic analyses in metropolitan France and bordering regions”. BSGF - Earth Sci. Bull. 192, 10, doi:10.1051/bsgf/2020049. Beucler et al. NW France seismicity November 16, 2021 10