High-resolution catalog of the the Maurienne Swarm (French Alps) based on template matching and double-different relocation

Abstract

<p><span>Since 2017, the Maurienne Valley (French Alps) has been affected by an episode of seismic unrest. In this study we focused on the seismic swarm that occurred in 2017 and 2018, which was characterized by 8 events with ML > 3 and a maximum magnitude of 3.7. The goal was to extend the existing SISmalp catalog, and also to provide accurate locations and magnitude estimations.</span></p><p><span>The employed data was recorded by a local seismic network composed of 6 broadband stations. The use of template matching allowed us to detect more than 70000 events, increasing the detection rate by more than ten times compared to the original catalog. We obtained high resolution locations applying a double difference relocation method, providing as input differential times calculated by cross-correlating templates with their respective detections. Finally, we estimated magnitudes using template-family-based linear regression analysis, in order to include even the weakest events. The seismic locations will be discussed in the tectonic and geological setting of the Maurienne Valley.</span></p>

Unprecedented seismic swarm in the Maurienne valley (2017–2019) observed by the SISmalp Alpine seismic network: operational monitoring and management

“…Before and after the installation of the temporary network, mc are 1.635 and 0.871, respectively. However, we do not observe a clear variation of the b-values in time, also confirmed by Minetto et al [2020Minetto et al [ , 2021 using a high resolution catalog obtained by template matching. The maximal magnitude recorded during the swarm activity is 3.8 on Friday 27, October 2017.…”

“…In addition, geophysical and seismological studies are currently underway to confirm the spatio-temporal distribution of the events within the swarm in order to clarify their triggering. Several mechanisms are suspected (e.g., aseismic sliding or fluids, or both) that will be constrained by magneto-telluric imaging and template-matching application [e.g., Minetto et al, 2020] in further studies.…”

“…Errors are less than 200 m, much more dispersed along the vertical axis. The errors computed with HypoDD can be underestimated for clusters with more than 100 events [Waldhauser and Ellsworth, 2000] but other estimation by bootstrap method [Minetto et al, 2021] are with the same order of magnitude.…”

“…This allows, during the swarm, to find each day several "undetected automatically" events. This method is not as powerful as more proven methods such as, for example, the template-matching methods, currently applied to all continuous streams of temporary and permanent stations [Minetto et al, 2020[Minetto et al, , 2021]. However, this method is automatically integrated into the operational seismicity monitoring system conducted by SISmalp.…”

“…Discussion on the accuracy of the absolute and relative location of earthquakes must be improved through, for example, bootstrap analysis in order to valid the splitting into several parallel sub-faults toward the northern parts of the swarm. Moreover, further analyses will be carried out, both on the exploration of continuous data by Template Matching [Minetto et al, 2020], and the analysis of the processes at the origin of the swarm triggering through the calculation of spatiotemporal migration rates.…”