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Zusammenfassung:
Geothermal reservoir operations can lead to seismic activity, for instance, due to increased pore pressure, but some observations are difficult to explain by pressure changes alone. Such observations include unexpected, induced events after injection well shut-in when pore pressures are thought to decrease. Here, we analyze induced seismicity up to ML 1.5 and surface deformation in Blue Mountain, Nevada using seismic, geodetic, and hydraulic data between 2016 and 2020. This period is characterized by long-term surface subsidence of up to 1 cm/yr above the reservoir. The long-term deformation is associated with modest seismic activity but also short-lived seismicity spikes during rapid maintenance shutdowns in 2017 and 2018. The seismicity is concentrated within the geothermal reservoir at 0.5 - 2.5 km depth, similar to injection operations.
We present a numerical model of abrupt seismicity rate changes during the two shutdowns that explains the observation through short-term poroelastic effects leading to increased Coulomb stressing rates. In contrast, the long-term surface subsidence can be modeled statically by superimposing localized fault slip, and thermal contraction of the reservoir. Our findings demonstrate that poroelastic coupling drives the abrupt increase in seismic activity following well shutdown, while aseismic fault slip and thermal contraction dictate the long-term static deformation.
