Earthquakes involve frictional sliding along fractures extending through the Earth’s solid crust, called "faults." Geologic faults can be saturated by fluids like water which affect the frictional resistance to sliding, hence the size and timing of earthquakes. Fluid pressure within the fault zone, an important parameter, is controlled by the production and migration of that fluid. It is critical to quantify the effects of fluids on fault sliding to better assess earthquake hazards. Here, the researchers use and develop state-of-the-art computer simulations to model earthquakes on faults that are thought to have active fluid flow along them, like the San Andreas fault. The project focuses on fluid migration and changes in pressure and frictional resistance. Its outcomes have relevance to naturally occurring earthquakes, as well as to induced seismicity; these latter earthquakes are triggered by human activities that involve injection or extraction of fluids from the Earth. The project also provides support to a female graduate student and training to undergraduate students. The developed modeling software will be released with an open-source license for use by the scientific community.
Earthquake sequence simulations capture the slow tectonic loading, aseismic slip, earthquake nucleation, and rupture propagation. They are becoming major tools to study earthquake processes and interpret observations. Fault strength is the product of a rate-and-state friction coefficient and the effective normal stress (total normal stress minus pore pressure). In this project, pore pressure is determined using a fault-zone fluid-transport model, rather that set a priori as is typically done. Fluids ascend through the crust in a fault zone whose permeability varies in time; it decreases interseismically from healing and sealing processes and increases from cracking and dilatancy during fault slip. This behavior is called "fault valving." Preliminary simulations reveal fault valving behavior with earthquake recurrence influenced by cyclic overpressure build-up and release. The fluids model is here merged into a thermomechanical earthquake sequence framework. It accounts for the transition from localized frictional sliding to distributed viscous flow at depth using a temperature-dependent power law. Temperature obeys the heat equation with frictional and viscous shear heating that create a thermal anomaly along the fault and its viscous root. The model is generalized to a poroviscoelastic framework that accounts for creep closure of pores, thermal expansion of fluids (and pores), and fluid pressurization. The system behavior is quantified in terms of dimensionless ratios of relevant time scales, such as earthquake recurrence interval, healing/sealing time scale, and pore pressure diffusion time across the seismogenic depth. Outcomes of the simulations predict crustal deformations, seismic and aseismic slip patterns, heat flow, stress profiles, and other features which can be compared to geophysical data and geological observations.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
