Analysis of seismic activity in the Valllorcine-Chamonix area : a case of induced activity ?

4-5 months starting between February and April 2019
Laboratory : ISTerre, Grenoble
Supervisor : Agnes Helmstetter, agnes.helmstetter
Duration : 4-5 months, starting in March or April 2019
Background : M2R or equivalent
Salary : classical internship salary (554€/month)
Key-words : earthquakes, swarms, induced seismicity, template-matching filter

The objective of this study is to analyze seismic activity in the Vallorcine and Chamonix area (northern French Alps). This area is particularly interesting has it produced some of the largest historical earthquakes in the French Alps (Chamonix 1905, magnitude M between 5.5 and 6, Vallorcine 2005, M=4.5). Moreover, seismic activity often occurs as "swarms" of earthquakes. Swarms are bursts of seismicity that can last for days to years, with a rate of activity much larger than the long-­term rate. Unlike regular aftershock sequences, the rate of seismicity increases progressively and decays slowly with time, and the largest earthquake does not often occur at the beginning of the sequence. In space, the size of the cluster is usually much larger than the rupture length of the largest earthquake. This suggests that swarms are not only triggered by tectonic stress or earthquake interactions, but that another loading process is active. In many cases, the size of the swarm extends with time. This diffusion of activity is also observed for seismicity induced by fluid injections (geothermal fields, reservoir dams, oil fields, etc.), suggesting that fluid flow is also responsible for the triggering of natural swarms. In addition to fluid diffusion, aseismic slow slip events have also been proposed as a triggering process, both for natural and induced swarms. Swarms are also particularly interesting because they have been observed several times prior to large earthquakes (2009 L’Aquila earthquake in Italy, 2011 Tohoku-oki earthquake in Japan). However, we cannot predict yet how a swarm sequence will evolve and if it may lead to a large event.

In the Vallorcine-Chamonix area, we suspect that earthquakes may be triggered by the nearby dam reservoir. But temporal changes in snow and ice load may also be responsible for earthquake triggering. The goal of this work will be to test these assumptions by analyzing the spatial and temporal distribution of earthquakes in this area. The candidate will first analyze the existing seismic catalog of the regional seismic network Sismalp, which is complete above M=1.5 in this area. Template­‐matching methods will then be applied to improve the detection threshold down to M=0 and therefore to increase considerably the number of detected earthquakes. The student will have to run and adapt a code for template-matching detection and classification of earthquakes.

Expected background for the candidate ; The candidate should have a solid background in geophysics and a very good level in signal processing. Being familiar with Matlab will be an advantage to begin the internship in good conditions.

Follow up in PhD : Yes, funding available. This M2R may be followed by a PhD on the same subject funded by ITN project URBASIS, starting in september 2019. Eligible candidates for this PhD can be of any nationality, but must not have resided in France for more than 12 months in the 3 years preceding his/her recruitment.
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