Abstract
In 2017–2019, a seismic swarm was triggered in the Maurienne valley (French Alps), with more than 5000 events detected by the regional SISmalp network. The population, who asked SISmalp to provide information on the processes and the associated risk, felt many earthquakes. In a post-L’Aquila trial context, we conducted a reflection on the scientific and social operational management of the crisis. The geological and tectonic analysis, the deployment of a temporary seismic network, an automatic double-difference relocation procedure (HypoDD) after clustering earthquakes, as well as the interactions with the population and the risk managers, have been carried out jointly. The length and unpredictability of the sequence complicated crisis management and the relations between local authorities and civil protection. The involvement of SISmalp, beyond its main scientific and observation prerogatives, has contributed to moderate the fears of the population by providing scientific explanations.
Highlights
Earthquakes occur as main-shock/aftershocks sequences, and in some cases, as seismic swarms
Close to Grenoble, the dextral Belledonne border fault (BBF) seems to be linked with old structure: the south part of the Belledonne Middle Fault [BMF, Thouvenot et al, 2003, Le Roux et al, 2010], but more north in Belledonne massif and close to the Maurienne swarm, the situation between actual/historical seismicity and tectonic features are still unclear
At the external crystalline massif (ECM) scale a relay (N60), between major faults, brings local adjustments on secondary faults that partly resume ancient faults, which is a segment of the Fond de France Fault system (FFF) system
Summary
Earthquakes occur as main-shock/aftershocks sequences, and in some cases, as seismic swarms. Earthquakes clustered in time and space, without a clear onset and main event Apart from those triggered by human activity, it is usually thought that swarms can be triggered by fluid pressure or by aseismic sliding [e.g., Vidale and Shearer, 2006, Roland and McGuire, 2009, Chen et al, 2012, Duverger et al, 2015]. Distinguished through the migration velocities of events within the swarm, from a few m/day rate for fluid-triggered events to several m/hour rate for aseismic events [Shapiro et al, 1997, Lohman and McGuire, 2007, Shelly et al, 2013] This distinction requires precise data and location of the events. Compared to main-shock/aftershocks sequences, long swarm sequences of low to moderate seismic events (some of which are felt by the public) raise questions about crisis management over such long periods of time, especially in moderate seismic prone regions. The operational sequence provides the analysis of interactions between scientists and risk managers, pointing out some avenues for improvements elaborated with local authorities
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