The patterns of triggered seismicity and subsidence effects from conventional geothermal operations are often irregular in their timing and location, and therefore difficult to predict, particularly in the absence of detailed knowledge of the local subsurface stress conditions, rock properties and permeability structure. Reinjection is usually identified as the principal cause of triggered seismicity, and ground subsidence effects are usually attributed to pressure decline from production. The real situation is more complex, however, and the two processes of subsidence and seismicity can be closely entwined. Both processes are products of subsurface stress changes acting on clays, rocks or fault surfaces, usually exhibiting anomalous geo-mechanical properties. Adaptive mitigation for adverse effects from either process calls for a coordinated approach, using injection management to control induced stress and strain changes, while considering the potential adverse effects of both deformation processes. This paper reviews several geothermal cases, and shows that interlinked mechanisms, involving temperature and chemical changes, as well as transient pressures, are implicated. Interlinked mechanisms such as ‘slow-deformation’ events or earthquakes, and ‘seismicity-induced’ subsidence, are probably more common in geothermal settings than we previously thought. Examples from New Zealand of triggered seismicity favor a mechanism associated with the indirect effects of increased fluid flow. The flow is driven by pressure gradients through a fracture network, but seismic failure is triggered only on pre-existing, favorably-oriented fracture-networks, and can occur throughout the fracture network affected by moving fluid. The triggering mechanism can be local temperature, pressure or chemical transients, or local stress perturbations, unlocking asperities on stressed fractures. Some subsidence and seismicity mechanisms require consideration of the transition between brittle and ductile behavior across a range of temperatures, pressures and rock types. Settlement can also originate from shaking of seismic origin and non-linear stress-strain relationships such as yielding. To simulate such interactions and deformation processes, what is required is a better conceptual understanding of deformation processes, and truly inter-coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) modeling. Some of the more fundamental rock properties used in traditional reservoir simulation, such as permeability, porosity and stress state, which are usually treated as constant parameters in history matching and subsequent scenario predictions, are, in reality, time-variables, and this needs to be incorporated into the inter-coupled modeling.

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