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The importance of pore fluid pressure (Pf) for fault strength, stress state and slip behavior holds promise for explaining spatio-temporal subduction zone megathrust behavior, but the coseismic state of Pf and its distribution with depth are poorly constrained. Here, we analyze fault stress states and 3D rupture dynamics of six scenarios based on the 2004 Mw 9.1 Sumatra-Andaman earthquake. We vary Pf from hydrostatic to lithostatic under two different gradients that result in depth-dependent versus constant effective normal stress on the seismogenic part of the megathrust. As Pf magnitude increases, fault strength, moment magnitude, cumulative slip, peak slip rate, dynamic stress drop and rupture velocity decrease. When Pf follows the lithostatic gradient, depth-constant effective normal stress results, as theoretically proposed. We find that such a near-lithostatic pore fluid pressure gradient shifts peak slip and peak slip rate up-dip. We study the dynamically modeled apparent co-seismic principal stress rotations and absolute post-seismic stress state. In all earthquake dynamic rupture scenarios, the mean apparent stress rotations are larger in the accretionary wedge than below the megathrust. Scenarios with higher Pf exhibit lower mean apparent principal stress rotations in the accretionary wedge. By comparison against observations of the 2004 Sumatra-Andaman earthquake, two preferred scenarios emerge. These support the presence of very high coseismic pore fluid pressure at 97 % of the lithostatic pressure, producing average shear and effective normal traction magnitudes of 4-5 MPa and 22 MPa, respectively. The mean dynamic stress drop for both scenario earthquakes is 3 MPa and the mean rupture velocity is 2400-2600 m/s, comparable to observations of the 2004 Sumatra earthquake. The heterogeneous post-seismic stress states in these scenarios are consistent with the variety of aftershock focal mechanisms observed after the 2004 earthquake. These two preferred scenarios differ in pore fluid pressure gradient and thus in slip on the shallow megathrust. Under conditions of very high pore fluid pressure that lead to weak megathrusts in terms of the low static shear strength and low dynamic friction during rupture, near-trench strength and constitutive behavior are crucial for megathrust hazard, as peak slip and peak slip rate occur at shallower depths. This condition also is consistent with observations that the stress drops of small earthquakes in subduction zones are only weakly depth-dependent.

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred - the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3-D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high-stress asperities along the down-dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference.

Winners of the 2016 Outstanding Student Paper Awards announced

Large dynamic stresses near earthquake rupture fronts may induce an inelastic response of the surrounding materials, leading to increased energy absorption that may affect dynamic rupture. We systematically investigate the effects of off-fault plastic energy dissipation in 2-D in-plane dynamic rupture simulations under velocity-and-state-dependent friction with severe weakening at high slip velocity. We find that plasticity does not alter the nature of the transitions between different rupture styles (decaying versus growing, pulse-like versus crack-like, and subshear versus supershear ruptures) but increases their required background stress and nucleation size. We systematically quantify the effect of amplitude and orientation of background shear stresses on the asymptotic properties of self-similar pulse-like ruptures: peak slip rate, rupture speed, healing front speed, slip gradient, and the relative contribution of plastic strain to seismic moment. Peak slip velocity and rupture speed remain bounded. From fracture mechanics arguments, we derive a nonlinear relation between their limiting values, appropriate also for crack-like and supershear ruptures. At low background stress, plasticity turns self-similar pulses into steady state pulses, for which plastic strain contributes significantly to the seismic moment. We find that the closeness to failure of the background stress state is an adequate predictor of rupture speed for relatively slow events. Our proposed relations between state of stress and earthquake source properties in the presence of off-fault plasticity may contribute to the improved interpretation of earthquake observations and to pseudodynamic source modeling for ground motion prediction.

Although kinematic earthquake source inversions show dominantly pulse-like subshear rupture behavior, seismological observations, laboratory experiments and theoretical models indicate that earthquakes can operate with different rupture styles: either as pulses or cracks, that propagate at subshear or supershear speeds. The determination of rupture style and speed has important implications for ground motions and may inform about the state of stress and strength of active fault zones. We conduct 2D in-plane dynamic rupture simulations with a spectral element method to investigate the diversity of rupture styles on faults governed by velocity-and-state-dependent friction with dramatic velocity-weakening at high slip rate. Our rupture models are governed by uniform initial stresses, and are artificially initiated. We identify the conditions that lead to different rupture styles by investigating the transitions between decaying, steady state and growing pulses, cracks, sub-shear and super-shear ruptures as a function of background stress, nucleation size and characteristic velocity at the onset of severe weakening. Our models show that small changes of background stress or nucleation size may lead to dramatic changes of rupture style. We characterize the asymptotic properties of steady state and self-similar pulses as a function of background stress. We show that an earthquake may not be restricted to a single rupture style, but that complex rupture patterns may emerge that consist of multiple rupture fronts, possibly involving different styles and back-propagating fronts. We also demonstrate the possibility of a super-shear transition for pulse-like ruptures. Finally, we draw connections between our findings and recent seismological observations.