Patterns of Reservoir‐Triggered Seismicity in a Low‐Seismicity Region of FrancePatterns of Reservoir‐Triggered Seismicity in a Low‐Seismicity Region of France

We analyzed the impact of the 26 largest impounded reservoirs on reservoir-triggered seismicity (RTS) patterns in the low-seismicity region of continental France. We treat reservoir-triggered earthquakes as tectonic earthquakes and apply similar concepts in our analysis. Generally, the spatial extent of an aftershock zone is controlled by the mainshock rupture length. In a similar manner, we use reservoir length as an equivalent length to the rupture length to assess the spatial extent of reservoir-triggering earthquakes and one to three reservoir lengths as a proxy for the nearfield distance where the stress change induced by reservoir impoundment may trigger seismicity. Accordingly, we define the 1L(r) distance as the near-field reservoir effect on seismicity and the 10L(r) distance as the far field, null effect of reservoir stress change on background seismicity. We find that (1) about a quarter of the reservoirs trigger M-max = 2.5-4.7 within the 1L(r) distance in a 15 yr space time window, and (2) as tested against a randomized series, superposed epoch analysis demonstrates a robust increase in the average seismicity rate within 2 yrs for the 1-3L(r) distance from reservoirs. The reservoirs that trigger in the (1L(r)) near-field distance are significantly larger than the nontriggering ones. While considering the distance of triggering of earthquakes from the reservoir, it is more appropriate to consider the normalized distance (the distance normalized by the reservoir length) to identify earthquake triggering reservoirs at a 1L(r) distance. While considering reservoir dimensions, the reservoir length appears to be a more important parameter than the reservoir depth, as the length is proportional to the area of significant stress change. Our results suggest that the RTS mimics the aftershock sequence of a slow reservoir-impoundment loading, with a corresponding M*reservoir = M(L-r) mainshock magnitude. Further, when considering mainshock-aftershock interactions, our analysis and observations support that the M-max for RTS for a given reservoir remains, on average, smaller than the reservoir magnitude equivalent.