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Seismic Stereometry Reveals Preparatory Behavior and Source Kinematics of Intermediate‐Size Earthquakes

Abstract : Although moderate-size earthquakes are poorly studied by lack of near-fault observations, they can provide key information about larger damaging earthquakes. Here we propose a new approach, inspired by double-difference relocation, that uses high-coherency waveforms recorded at neighboring sensors, to study the preparation phase and dynamics of moderate-size earthquakes. We validate this technique by analyzing the 2016, M w 5.2 Borrego Springs earthquake in Southern California and find consistent rupture velocities of 2 km/s highlighting two main rupture asperities. The analysis of the 2019, Ml5.2 Le Teil earthquake in France reveals slow nucleation at depth that migrates to the surface and propagates northward with a velocity of ∼2.8 km/s, highlighting two main rupture events also imaged by InSAR. By providing unprecedented resolution in our observation of the rupture dynamics, this approach will be useful in better understanding the preparation phase and rupture of both tectonic and induced earthquakes. Plain Language Summary Small and moderate-size earthquakes are much more numerous yet studied in much less detail than large and damaging earthquakes. This is because to study small earthquakes precisely, one needs seismometers very close to the rupture, which is rarely the case. It is still unclear if small and large earthquakes start the same way, thus studying more systematically small earthquakes could help to answer this question. Here we show that we can use two close-by seismometers, as we would do with 3-D glasses, to observe with a stereoscopic effect, the evolution of intermediate-size earthquakes when they rupture along a fault. We can retrieve the direction of rupture, the speed at which the fault breaks, and the length of the rupture, important parameters that help to characterize the earthquakes. Using this new method, we can sometimes also observe the behavior of the fault before the main rupture which will be useful to better understand how earthquakes start.
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Submitted on : Monday, September 7, 2020 - 4:12:25 PM
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A. Mordret, Florent Brenguier, M. Causse, P. Boue, C. Voisin, et al.. Seismic Stereometry Reveals Preparatory Behavior and Source Kinematics of Intermediate‐Size Earthquakes. Geophysical Research Letters, American Geophysical Union, 2020, 47 (17), pp.e2020GL088563. ⟨10.1029/2020GL088563⟩. ⟨hal-02932273⟩