Garagash, Dmitry
Permanent URI for this collectionhttps://hdl.handle.net/10222/44138
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Item Open Access Nucleation and arrest of dynamic slip on a pressurized fault(2012-10) Garagash, Dmitry I.; Germanovich, Leonid N.Elevated pore pressure can lead to reactivation of slip on pre-existing fractures and faults when the static Coulomb failure is reached locally. As the pressurized region spreads diffusively, slip can accumulate quasi-statically (paced by the pore fluid diffusion) or dynamically. In this work, we consider a prestressed fault with a locally peaked, diffusively spreading pore pressure field to study (1) conditions leading to the escalation of slip and nucleation of dynamic rupture and (2) rupture run-out distance before it is arrested. Nucleation appears in this model when the fault friction decreases from its peak value with slip, while arrest of dynamic propagation is imminent on aseismic faults (i.e., such that prestress tau(b) is less than the residual fault strength tau(r) at ambient conditions). When fluid overpressure is a small-to-moderate fraction of the ambient value of normal effective stress (and prestress is large enough for fault slip to be activated by overpressure), dynamic rupture always nucleates, and the nucleation length increases with decreasing prestress practically independently of the overpressure value. Transition from the ultimately unstable (tau(b) > tau(r)) to the ultimately stable (tau(b) < tau(r)) fault loading is marked by a strong increase of the nucleation length (proportional to 1/(tau(b) - tau(r))(2)) as tau(b) approaches tau(r) from above. For aseismic faults (tau(b) < tau(r)), no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress. The largest run-out distances of dynamic slip on aseismic faults correspond to overpressure/prestress just sufficient for slip activation. In such cases, the dynamically accumulated slip can lead to enhanced, dynamic fault weakening, resulting in a sustained dynamic rupture and generating a large earthquake. This is consistent with field observations when the largest injection-induced seismicity occurred after fluid injection ended.Item Open Access Seismic and aseismic slip pulses driven by thermal pressurization of pore fluid(2012-04) Garagash, D. I.There are several lines of evidence that suggest that thermal pressurization (TP) of pore fluid within a low-permeability fault core may play the key role in the development of earthquake slip. To elucidate effects of TP on spontaneous fault slip, we consider solutions for a steadily propagating slip pulse on a fault with a constant sliding friction, the level of which may reflect other thermally-activated processes at the rupture front (such as the flash heating on asperities). Upon arrival of the pulse front, essentially undrained-adiabatic TP takes place during the initial slip acceleration from the locked state with a corresponding reduction of the fault strength. With passage of time, the diminishing rate of heating (due to the reduced fault strength) and increasing rate of hydrothermal diffusion from the shear zone offset TP and result in partial recovery of the strength, slip deceleration and eventual locking and healing of the slip. We show that the rupture speed nu(r) decreases with thickness h of the principal shear zone. For lab-constrained values of fault-gouge parameters, the TP-pulse solution predicts seismic (nu(r) similar to km/s) slip on a millimeter-to-cm thin principal shear zone; and aseismic slip with nu(r) similar to 10 km/day and slip rates 1-2 orders above the plate rate on a relatively thick (h similar to 1 m) shear zone. These and other predictions of the TP-pulse model are consistent with the independent sets of observational constraints for large crustal and subduction interplate earthquakes, and slow slip transients (North Cascadia), respectively. Locking of the slip soon after the diffusive transport of the heat and pore fluid becomes efficient significantly limits the maximum co-seismic temperature rise to values well below previous theoretical estimates. As a result, the onset of macroscopic melting and some of thermal decomposition reactions, recently suggested to explain strong co-seismic fault weakening, are precluded over much of the seismogenic zone.