Reservoir related seismicity changes around the Gotvand Dam (south west of Iran)

SUMMARY Over the past few decades, the global number of dams has increased substantially. Water impounded behind these dams, resulting in elevated crustal pore pressure and altered stress distribution around reservoirs, could potentially trigger or suppress the failure of nearby faults, leading to transient changes in seismicity. In this study, we analyse 14 yr (2006–2019) of spatiotemporal seismicity recorded by a dense local network in the Gotvand area (SW Iran), covering about 5.5 yr before and 8.5 yr after impoundment. The initial catalogue, comprising over 48 000 relocated earthquakes, was reduced to 6464 background events by adopting a 3-D Epidemic-Type Aftershocks Sequence declustering model with a cut-off magnitude of 1.3. We analyse the spatiotemporal background seismicity pattern in the Gotvand area in comparison with calculated reservoir-related spatiotemporal stress changes relative to the initial stress state before water impoundment, approximating the Gotvand reservoir by 726-point sources covering the reservoir surface. We find that following the initiation of impoundment, the local background seismic activity slightly increased during the impounding, pointing to induced/triggered seismicity. However, most importantly, the impoundment of Gotvand lake has altered the spatial seismicity patterns, leading to a notable reduction in seismic activity in the central area of the reservoir, which is in agreement with the calculated negative Coulomb stress changes in the same area. Using the Coulomb-Rate-and-State seismicity model, we find that the spatiotemporal seismicity response due to the calculated stress changes is consistent with the observations.