SCEC Award Number 19074 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Simulations of Fluid-Driven Aseismic Slip and Fault Valving
Investigator(s)
Name Organization
Eric Dunham Stanford University
Other Participants Yuyun Yang (PhD student)
SCEC Priorities 3f, 3d, 1c SCEC Groups FARM, SDOT, CXM
Report Due Date 04/30/2020 Date Report Submitted 04/29/2020
Project Abstract
 Fluids permeate Earth’s crust and the pressure exerted by pore fluids alters fault strength and seismicity. The departure of fluid pressure from hydrostatic conditions drives flow through the crust, especially along permeable fault damage zones. Permeability is altered by cracking and dilatancy during fault slip, as well as healing and sealing processes acting over interseismic time scales. Thus steady fluid input at the base of faults, from mantle dehydration and/or metamorphic reactions, will lead to cyclic fluctuations in fluid flux, pore pressure, and fault strength—a concept known as fault valving. We have introduced a fault valving dynamics into our 2D antiplane shear earthquake sequence modeling code, SCycle (available under an open-source license at bitbucket.org/kallison/scycle). Using a simple parameterization of permeability evolution processes, we model fault valving and quantify overpressure build-up and release. A key finding is the occurrence of fluid-driven aseismic slip near the base of the seismogenic zone, wherein high pressure fluids ascending from depth weaken the fault, initiating aseismic slip that increases permeability and allows fluids to ascend and pressurize and slip the next section of the fault. The model predicts shallowing of the locking depth in the final decades of the earthquake cycle, with a rate that can vary from ~30 m/a to ~3 km/a, depending on the healing/sealing time scale. Fluid-driven slip is also predicted to occur in the seismogenic zone, but with microseismicity accompanying aseismic slip—behavior suggestive of earthquake swarms.
Intellectual Merit Fluid migration and pore pressure dynamics are typically neglected in earthquake sequence simulations, but fault weakening from pressurization is arguably just as important as frictional weakening. The model developed here provides a fully coupled framework for friction slip dynamics and fluid effects, with potential applications ranging from naturally occurring seismicity and earthquake swarms in tectonic and/or volcanic areas to induced seismicity due to anthropogenic fluid injection/extraction. In particular, the modeling makes testable predictions of fluid-driven aseismic slip (which might be observed in geodetic and/or seismic data as time variations in locking depth and/or limiting depth of microseismicity) and pore pressure.
Broader Impacts This project trained two PhD students and one postdoctoral fellow. The modeling software has been released to the community using an open-source license and can be obtained from bitbucket.org/kallison/scycle.
Exemplary Figure Figure 2. Slip histories from sequence simulations in the reference model (bottom row, with pore pressure and effective normal stress held fixed to steady state solution in Figure 1d) and a model with permeability and pore pressure evolution (top row). In the reference model, periodic earthquakes rupture the entire seismogenic zone, and the locking depth changes by only ~1 km over the earthquake cycle. In contrast, permeability and pressure evolution, when coupled to frictional dynamics, give rise to a complex sequence of aseismic slip, microseismicity, and large ruptures. Black arrow shows aseismic creep at depth (i.e., the locking depth) infiltrating the locked seismogenic zone at a rate of 0.5 km/yr. Two similar creep events, coexisting at different depth, are seen from 10-20 yr. White arrow tracks an earthquake swarm that ascends upward through the mid-seismogenic zone. Figure 3 explains how these behaviors are associated with ascending high pressure fluids. From Zhu et al., 2020.
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