Seismological analysis of flood dynamics and hydrologically-triggered earthquake swarms associated with storm Alex Małgorzata Chmiel, M. Godano, M. Piantini, M. Bakker, F. Gimbert, J.-P. Ampuero, P. Brigode, A. Sladen, D. Rivet, D. Ambrois, M. Chapuis Rencontres scientifiques et techniques RESIF, 17/11/2021

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Storm Alex: a very destructive “Mediterranean episode” On October 2, 2020, the southern Alps were struck by a heavy precipita8on event caused by Storm Alex 3

Storm Alex: a very destructive “Mediterranean episode” Saint-Martin Vésubie before after 4 www.ign.fr Limited hydrological observa3ons But! Both turbulent flow and sediment transport during floods generate ground mo2on (Schmandt et al., 2013; Gimbert et al., 2014; Bur2n et al., 2016; Cook et al., 2018). On October 2, 2020, the southern Alps were struck by a heavy precipitation event caused by Storm Alex

Seismic data 5 after Fr. (Hz) Vel [m s -1 ] Acc. [m s -2 ] Vel [m s -1 ] 1570 m 630 m 5970 m 1570 m 630 m 5970 m 5 Storm Alex

6 SPIF BELV TURF 1) The seismic power averaged in 1-20 Hz stations show a rapid increase from 10:00 UTC (SPIF & BELV) 2) 3 seismic power local maxima (SPIF & BELV) 3) The first two seismic power maxima have pronounced high-amplitude peaks 4) The timing of the main seismic power maximum at TURF station and the third seismic power maximum of BELV station is well correlated with the second maximum of the simulated runoff (Q, Kinematic Local Excess Model, Borga et al., 2007) Seismological observations of the flash flood (mm) 6 SPIF BELV TURF

varying relationship between low frequency and high frequency seismic power 7 SPIF BELV TURF The relative contributions of low- (2-10 Hz) and high- (10-45 Hz) frequency seismic power Seismological observations of the flash flood (mm) PSD 7 SPIF BELV TURF

The Vésubie river dynamics during the flash flood 8 1) The flash flood on the Vésubie river started at about 10:00 UTC 2) The backazimuth of ∼110°: bending of the Vésubie river channel, a ∼2.5 km long downstream reach of the Vésubie river, or the confluence of the Venanson stream with the Vésubie river 3) Similar peaks in seismic power generated by flood waves were observed during glacial lake outburst floods in the Himalayas (Cook et al., 2018). These peaks may share similar characteristics to sediment pulses (Piantini et al., 2021). 4) From maximum 1 to 3 there is a shift from short lived peaks to a much more spread distribution of power through time. v1=~5.8 m/s v2=~4.8 m/s (mm) 8 SPIF BELV TURF

The Vésubie river dynamics during the flash flood Małgorzata Chmiel 9 v1=~5.8 m/s v2=~4.8 m/s (mm) SPIF BELV TURF 1) The flash flood on the Vésubie river started at about 10:00 UTC 2) The backazimuth of ∼110°: bending of the Vésubie river channel, a ∼2.5 km long downstream reach of the Vésubie river, or the confluence of the Venanson stream with the Vésubie river 3) Similar peaks in seismic power generated by flood waves were observed during glacial lake outburst floods in the Himalayas (Cook et al., 2018). These peaks may share similar characteristics to sediment pulses (Piantini et al., 2021). 4) From maximum 1 to 3 there is a shift from short lived peaks to a much more spread distribution of power through time. Summary 1 Seismological observations constrain time- and space-dependent processes during the flood and rainfall-runoff relationship at the catchment scale

Hydrologically triggered earthquake swarm? Year 2014 2015 2016 2017 2018 2019 2020 44 12' 44 06' 44 00' 43 54' 5 km MVIF N Days since 00:00 October 2, 2020 0 10 60 50 40 30 20 Elevation [m] B Tinée Var Monthly earthqua rate 30 0 10 20 A 7 06' 7 12' Depth [km] C 12 10 -2 0 2 8 4 6 0 500 3000 2500 2000 1500 1000 3500 0.5 1.0 2.5 1.5 3.5 3.0 2.0 ML 80 0 20 40 60 Daily earthquake rate ML October November December 01 15 15 01 01 07 07 07 21 21 D Storm Alex -1.0 0.0 1.0 2.0 10 Year 2014 2015 2016 2017 2018 2019 2020 44 12' 44 06' 44 00' N 00 October 2, 2020 60 50 40 Elevation [m] B Tinée Var Monthly earthquake rate ● Storm Alex 30 0 10 20 A C 0 500 3000 2500 2000 1500 1000 3500 0.5 1.0 2.5 1.5 3.5 3.0 2.0 ML 44 00' 43 54' 5 km Days since 00:00 October 2, 2020 0 10 60 50 40 30 20 Tinée Var 7 06' 7 12' Depth [km] 12 10 -2 0 2 8 4 6 0.5 1.0 2.5 1.5 2.0 80 0 20 40 60 Daily earthquake rate ML October November December 01 15 15 01 01 07 07 07 21 21 D Storm Alex -1.0 0.0 1.0 2.0 Before template matching (22 earthquakes Seiscomp3) Template matching on MVIF (+ 91 earthquakes ML>=-0.5) 3 main clusters <

Hydrologically triggered earthquake swarm? Year 2014 2015 2016 2017 2018 2019 2020 44 12' 44 06' 44 00' 43 54' 5 km MVIF N Days since 00:00 October 2, 2020 0 10 60 50 40 30 20 Elevation [m] B Tinée Var Monthly earthqua rate 30 0 10 20 A 7 06' 7 12' Depth [km] C 12 10 -2 0 2 8 4 6 0 500 3000 2500 2000 1500 1000 3500 0.5 1.0 2.5 1.5 3.5 3.0 2.0 ML 80 0 20 40 60 Daily earthquake rate ML October November December 01 15 15 01 01 07 07 07 21 21 D Storm Alex -1.0 0.0 1.0 2.0 11 Year 2014 2015 2016 2017 2018 2019 2020 44 12' 44 06' 44 00' N 00 October 2, 2020 60 50 40 Elevation [m] B Tinée Var Monthly earthquake rate ● Storm Alex 30 0 10 20 A C 0 500 3000 2500 2000 1500 1000 3500 0.5 1.0 2.5 1.5 3.5 3.0 2.0 ML 44 00' 43 54' 5 km Days since 00:00 October 2, 2020 0 10 60 50 40 30 20 Tinée Var 7 06' 7 12' Depth [km] 12 10 -2 0 2 8 4 6 0.5 1.0 2.5 1.5 2.0 80 0 20 40 60 Daily earthquake rate ML October November December 01 15 15 01 01 07 07 07 21 21 D Storm Alex -1.0 0.0 1.0 2.0 Before template matching (22 earthquakes Seiscomp3) Template matching on MVIF (+ 91 earthquakes ML>=-0.5) 3 main clusters Possible origin. 1) Pore pressure increase at depth caused by fluid migration from the surface through fractures (Saar and Manga, 2003; Kraft et al., 2006) 2) An elastic stress perturbation induced by hydrological loadings (Rigo et al., 2008) 3) Pore pressure increase in deep fluid-saturated poroelastic rock in response to overlying hydrological loading (Miller, 2008; D’Agostino et al., 2018). <

• Further analysis based on the presented unique dataset to study surface and deep earth processes associated with storm Alex. • Seismometers can constrain interac8on between the different Earth’s systems, ime- and space-dependent processes during the flood and rainfall-runoff relaionship at the catchment scale • 114 earthquakes between local magnitude ML 0.5 and 2.5 were triggered by the hydrological loading and/or by the resuling in-situ pore pressure increase in the Tinée valley. 12 Summary

Małgorzata Chmiel 13 Seismological observa