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Abstract Earthquake nucleation is a fundamental problem in earthquake science and has practical implications for forecasting seismic hazards. Laboratory experiments performed on large, meter‐scale fault systems offer unique insights into the nucleation process because the migration and expansion of the nucleation zone can be precisely detected, measured, and characterized using arrays of local strain and slip measurements. We report on a series of laboratory experiments conducted on a 1‐m direct shear machine. We sheared layers of quartz gouge between roughened acrylic forcing blocks over a range of normal stresses between 3 and 12 MPa, generating a spectrum of slip modes, ranging from aseismic creep to fast‐dynamic rupture. Co‐seismic slip, peak slip velocity, and high‐frequency acoustic energy content of laboratory earthquakes increases systematically with both cumulative fault slip and normal stress. Slower and smaller laboratory earthquake sequences have larger nucleation zones, creep more during their inter‐seismic period, and are deficient in high‐frequency energy compared to larger and faster rupture sequences. We find that the critical nucleation length scale,H*, scales inversely with cumulative fault slip and normal stress. A reduction inH*and an increase in event size can be explained by a decrease in the critical slip distance,Dc, or an increase in the frictional rate parameterb–aand is likely driven by shear localization. Together, our results indicate that homogeneous, mature fault zones that have undergone more cumulative fault slip are expected to have smallerH*and can more easily host dynamic instabilities, relative to immature faults.
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The State of Pore Fluid Pressure and 3‐D Megathrust Earthquake Dynamics
Abstract We study the effects of pore fluid pressure (Pf) on the pre‐earthquake, near‐fault stress state, and 3‐D earthquake rupture dynamics through six scenarios utilizing a structural model based on the 2004Mw9.1 Sumatra‐Andaman earthquake. As pre‐earthquakePfmagnitude increases, effective normal stress and fault shear strength decrease. As a result, magnitude, slip, peak slip rate, stress drop, and rupture velocity of the scenario earthquakes decrease. Comparison of results with observations of the 2004 earthquake support that pre‐earthquakePfaverages near 97% of lithostatic pressure, leading to pre‐earthquake average shear and effective normal tractions of 4–5 and 22 MPa. The megathrust in these scenarios is weak, in terms of low mean shear traction at static failure and low dynamic friction coefficient during rupture. Apparent co‐seismic principal stress rotations and absolute post‐seismic stresses in these scenarios are consistent with the variety of observed aftershock focal mechanisms. In all scenarios, the mean apparent stress rotations are larger above than below the megathrust. Scenarios with largerPfmagnitudes exhibit lower mean apparent principal stress rotations. We further evaluate pre‐earthquakePfdepth distribution. IfPffollows a sublithostatic gradient, pre‐earthquake effective normal stress increases with depth. IfPffollows the lithostatic gradient exactly, then this normal stress is constant, shifting peak slip and peak slip rate updip. This renders constraints on near‐trench strength and constitutive behavior crucial for mitigating hazard. These scenarios provide opportunity for future calibration with site‐specific measurements to constrain dynamically plausible megathrust strength andPfgradients.
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- Award ID(s):
- 2121568
- PAR ID:
- 10369466
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 4
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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