Slip on the geometrically complex Rhyolite Ridge Fault System and associated local stresses in the Desert Peak Geothermal Field in Nevada, were modeled with the boundary element method (BEM) implemented in Poly3D. The impact of uncertainty in the fault geometry at depth, the tectonic stresses driving slip, and the potential ranges of frictional strength resisting slip on the likely predictions of fracture slip and formation in the surrounding volume due to these local stresses were systematically explored and quantified. The effect of parameter uncertainty was evaluated by determining the frequency distribution of model predicted values. Alternatively, Bayesian statistics were used to determine the best fitting values for parameters within a probability distribution derived from the difference of the model prediction from the observed data. This approach honors the relative contribution of uncertainties from all existing data that constrains the fault parameters. Lastly, conceptual models for different fault geometries and their evolution were heuristically explored and the predictions of local stress states were compared to available measurements of the local stresses, fault and fracture patterns at the surface and in boreholes, and the spatial extent of the geothermal field. The complex fault geometry leads to a high degree of variability in the locations experiencing stress states that promote fracture, but such locations generally correlate with the main injection and production wells at Desert Peak. In addition, the strongest and most common stress concentrations occur within relays between unconnected fault segments, and at bends and intersections in faults that connect overlapping fault segments associated with relays. The modeling approach in this study tests the conceptual model of the fault geometry at Desert Peak while honoring mechanical constants and available constraints on driving stresses and provides a framework that aids in geothermal exploration by predicting the spatial variations in stresses likely to cause and reactivate fractures necessary to sustain hydrothermal fluid flow. This approach also quantifies the relative sensitivity of such predictions to fault geometry, remote stress, and friction, and determines the best fitting model with its associated probability. / Geology

Volcanic subsidence, caused by partial emptying of magma in the subsurface reservoir has long been observed by spaceborne radar interferometry. Monitoring long-term crustal deformation at the most notable type of volcanic subsidence, caldera, gives us insights of the spatial and hazard-related information of subsurface reservoir. Several subsiding calderas, such as volcanoes on the Galapagos islands have shown a complex ground deformation pattern, which is often composed of a broad deflation signal affecting the entire edifice and a localized subsidence signal focused within the caldera floor. Although numerical or analytical models with multiple reservoirs are proposed as the interpretation, geologically and geophysically evidenced ring structures in the subsurface are often ignored. Therefore, it is still debatable how deep mechanisms relate to the observed deformation patterns near the surface. We aim to understand what kind of activities can lead to the complex deformation. Using two complementary approaches, we study the three-dimensional geometry and kinematics of deflation processes evolving from initial subsidence to later collapse of calderas. Firstly, the analog experiments analyzed by structure-from-motion photogrammetry (SfM) and particle image velocimetry (PIV) helps us to relate the surface deformation to the in-depth structures. Secondly, the numerical modeling using boundary element method (BEM) simulates the characteristic deformation patterns caused by a sill-like source and a ring-fault. Our results show that the volcano-wide broad deflation is primarily caused by the emptying of the deep magma reservoir, whereas the localized deformation on the caldera floor is related to ring-faulting at a shallower depth. The architecture of the ring-fault to a large extent determines the deformation localization on the surface. Since series evidence for ring-faulting at several volcanoes are provided, we highlight that it is vital to include ring-fault activity in numerical or analytical deformation source formulation. Ignoring the process of ring-faulting in models by using multiple point sources for various magma reservoirs will result in erroneous, thus meaningless estimates of depth and volume change of the magmatic reservoir(s).