Fluid‐Induced Swarms and Coseismic Stress Transfer: A Dual Process Highlighted in the Aftershock Sequence of the 7 April 2014 Earthquake (Ml 4.8, Ubaye, France). Issue 4 (11th April 2019) - De Barros, Louis
Abstract: The upper part of the Ubaye Valley (French Alps) is characterized by alternating mainshock‐aftershock sequences and swarms. Particularly, during the 2012–2015 crisis, four mainshocks with Ml > 3.5 occurred. We here focus on the aftershocks of the largest one (Ml = 4.8, 7 April 2014), in order to better understand the involved processes behind this peculiar seismic behavior. We use template matching detection, waveform classification, and double‐difference relocations to analyze this seismicity, on average and at the scale of the clusters that compose it. Most event sources are aligned along a plane consistent with the mainshock fault (N165, 65°W), but occurred on conjugate structures. A few clusters of seismicity are also observed far from the mainshock source. Our analysis shows that distinct, spatially separated processes are at play. While coseismic stress transfer explains the seismicity close to the mainshock source, fluid‐pressure diffusion and distant stress triggering are required to generate events farther away. The overall distributions in time and magnitude followed a slow Omori's decay and a Gutenberg‐Richter relationship, respectively. However, these classical responses arise from the superposition of very different behaviors, associated with different processes at depth. Plain Language Summary: Earthquakes have regularly shaken the upper part of the Ubaye valley, near the town of Barcelonnette in the Southwestern French Alps, for nearly 20 years. The earthquake behavior is peculiar, as swarms of numerous low magnitude events alternate with larger earthquakes, such as the Ml 4.8 one occurring on 7 April 2014. To understand this dual behavior, we performed an in‐depth analysis of the aftershock of this event. The seismicity mainly aligns on a plane consistent with the mainshock fault (N165, 65°W). However, most of the events occurred on branching structures, belonging to the damaged zone of this fault, or on some other small faults far away. While the average behavior of this aftershock sequence is close to a standard one, we show that two different processes occurred at depth. Events occurring close to the mainshock are triggered by coseismic stress transfer, while fluid‐pressure diffusion are likely required to explain the seismicity further away. Such dual process should be considered for seismic hazard assessment. Key Points: Improved detection, classification, and location of the aftershock sequence following the Ml 4.8 earthquake in the French Alps Event sources align consistently along the mainshock fault, but on structures of its damaged zone or to small faults further away Spatial separation of two different processes: coseismic stress transfer close to the mainshock and fluid‐pressure diffusion further away