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)

摘要

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 degrees 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.