Analysis of the Spatiotemporal Evolution of the Maurienne Swarm (French Alps) Based on Earthquake Clustering

Abstract

A seismic swarm is generally defined as a seismic sequence with no clear dominant earthquake (mainshock) (Mogi, 1963). Seismic swarms also do not have a clear onset, and the number of events do not decrease with time according to the Omori law (Utsu, 1957) as observed in classical mainshock-aftershock sequences. This suggests that stress triggering (Stein, 1999) alone cannot explain the genesis of seismic swarms. Seismic swarms are observed in many geological environments, such as volcanic and hydrothermal regions (Hill, 1977), oceanic ridges (Sykes, 1970), pure tectonic regions (e.g., Špičák, 2000), and also during hydraulic stimulations (Healy et al., 1968).Fluids are commonly cited as one of the driving sources of seismic swarms. Pore fluid pressure would indeed lower the effective stress, thus promoting rupture (Hubbert & Rubey, 1959). This is supported by experiments during fluid injections (e.g., Ohtake, 1974;Raleigh et al., 1976), and by the observation of changes in groundwater level and surface discharge during seismic activity (Kisslinger, 1975). In instances of natural swarms where no subsurface effects are evident, a migration of seismicity compatible with fluid diffusion is the main indicator of the presence of fluids (e.g., Chen et al., 2012;Shelly et al., 2013).Aseismic slip is another proposed triggering mechanism of swarms, which is commonly inferred by comparing geodetic and seismological observations (Lohman & McGuire, 2007;Ozawa et al., 2007). Swarms caused by aseismic slip have a higher migration velocity (km/h) (Lohman & McGuire, 2007;Roland & McGuire, 2009) compared to seismic swarms caused by fluids (m/day) (Shapiro et al., 1997). The periodic occurrence of identical events called repeaters is also thought to be evidence of aseismic slip (e.g., Nadeau et al., 1995), but it has been shown that repeaters are also possible during fluid diffusion (Deichmann et al., 2014). Fluid diffusion and aseismic slip may also coexist (e.g., Bourouis & Bernard, 2007;De Barros et al., 2020;Hatch et al., 2020), and earthquake-earthquake interactions may also play a role in the migration (e.g., Fischer & Hainzl, 2021). Even though several triggering mechanisms have been proposed, it remains a challenge to identify the driving processes of a swarm and to explain the reason why a swarm has a specific migration velocity, propagation direction, and duration.The lack of understanding of why seismic swarms occur does not only derive from the complexity of the processes, but also from specific difficulties inherent to these seismic sequences. Studying the spatiotemporal evolution of swarms requires high-resolution locations and high detection rates, usually obtained with template matching

Cluster Analysis of Seismicity in the Eastern Gulf of Corinth Based on a Waveform Template Matching Catalog

“…These events of magnitudes between −0.5 and 2.0 were triggered by hydrological loading or increase in the resulting in situ underground pore-pressure at the Maritime Alps, southern France, after the area was hit by Storm Alex in October 2020. Minetto et al [ 39 ] applied a single-station matched filter to detect 79,503 events using 1330 templates in the area of Maurienne Valley in the western French Alps, during a swarm that occurred between August 2017 and March 2019. This enabled the authors to study the spatiotemporal migration of the sequence, the geometry of the activated faults, and the variation of the b -value of the Gutenberg–Richter relation, as well as gain a better understanding of the underlying physical mechanisms.…”