We study the mechanical response of two‐dimensional vertical strike‐slip fault to coseismic damage evolution and interseismic healing of fault damage zones by simulating fully dynamic earthquake cycles. Our models show that fault zone structure evolution during the seismic cycle can have pronounced effects on mechanical behavior of locked and creeping fault segments. Immature fault damage zone models exhibit small and moderate subsurface earthquakes with irregular recurrence intervals and abundance of slow‐slip events during the interseismic period. In contrast, mature fault damage zone models host pulse‐like earthquake ruptures that can propagate to the surface and extend throughout the seismogenic zone, resulting in large stress drop, characteristic rupture extents, and regular recurrence intervals. Our results suggest that interseismic healing and coseismic damage accumulation in fault zones can explain the observed differences of earthquake behaviors between mature and immature fault zones and indicate a link between regional seismic hazard and fault structural maturity.
- Award ID(s):
- 1735630
- NSF-PAR ID:
- 10065220
- Date Published:
- Journal Name:
- Nature Geoscience
- ISSN:
- 1752-0894
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)The fault damage zone is a well-known structure of localized deformation around faults. Its material properties evolve over earthquake cycles due to coseismic damage accumulation and interseismic healing. We will present fully dynamic earthquake cycle simulations to show how the styles of earthquake nucleation and rupture propagation change as fault zone material properties vary temporally. First, we will focus on the influence of fault zone structural maturity quantified by near-fault seismic wave velocities in simulations. The simulations show that immature fault zones promote small and moderate subsurface earthquakes with irregular recurrence intervals, whereas mature fault zones host pulse-like earthquake rupture that can propagate to the surface, extend throughout the seismogenic zone, and occur at regular intervals. The interseismic healing in immature fault zones plays a key role in allowing the development of aseismic slip episodes including slow-slip events and creep, which can propagate into the seismogenic zone, and thus limit the sizes of subsequent earthquakes by releasing fault stress. In the second part, we will discuss how the precursory changes of seismic wave velocities of fault damage zones may affect earthquake nucleation process. Both laboratory experiments and seismic observations show that the abrupt earthquake failure can be preceded by accelerated fault deformation and the accompanying velocity reduction of near-fault rocks. We will use earthquake cycle simulations to systematically test the effects of timing and amplitudes of such precursory velocity changes. Our simulations will provide new insights into the interplay between fault zone structure and earthquake nucleation process, which can be used to guide future real-time monitoring of major fault zones.more » « less
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Abstract Fluid injection stimulates seismicity far from active tectonic regions. However, the details of how fluids modify on‐fault stresses and initiate seismic events remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw‐cut granite fault and compared events induced at different levels of background shear stress. Water was injected at 10 mL/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa‐level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We found that the initiation of slow slip was broadly consistent with a Coulomb failure stress, but that initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on‐fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid‐induced rupture will “runaway” or whether a fluid‐induced rupture will remain localized to the fluid pressurized region.
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Abstract Foreshocks are not uncommon prior to large earthquakes, but their physical mechanism remains controversial. Two interpretations have been advanced: (1) foreshocks are driven by aseismic nucleation and (2) foreshocks are cascades, with each event triggered by earlier ones. Here, we study seismic cycles on faults with fractal roughness at wavelengths exceeding the nucleation length. We perform 2‐D quasi‐dynamic, elastic simulations of frictionally uniform rate‐state faults. Roughness leads to a range of slip behavior between system‐size ruptures, including widespread creep, localized slow slip, and microseismicity. These processes are explained by spatial variations in normal stress (
σ ) caused by roughness: regions with lowσ tend to creep, while highσ regions remain locked until they break seismically. Foreshocks and mainshocks both initiate from the rupture of locked asperities, but mainshocks preferentially start on stronger asperities. The preseismic phase is characterized by feedback between creep and foreshocks: episodic seismic bursts break groups of nearby asperities, causing creep to accelerate, which in turns loads other asperities leading to further foreshocks. A simple analytical treatment of this mutual stress transfer, confirmed by simulations, predicts slip velocities and seismicity rates increase as 1/t , wheret is the time to the mainshock. The model reproduces the observed migration of foreshocks toward the mainshock hypocenter, foreshock locations consistent with static stress changes, and the 1/t acceleration in stacked catalogs. Instead of interpreting foreshocks as either driven by coseismic stress changes or by creep, we propose that earthquake nucleation on rough faults is driven by the feedback between the two. -
null (Ed.)ABSTRACT Numerical models of rupture dynamics provide great insights into the physics of fault failure. However, resolving stress interactions among multiple faults remains challenging numerically. Here, we derive the elastostatic Green’s functions for stress and displacement caused by arbitrary slip distributions along multiple parallel faults. The equations are derived in the Fourier domain, providing an efficient means to calculate stress interactions with the fast Fourier transform. We demonstrate the relevance of the method for a wide range of applications, by simulating the rupture dynamics of single and multiple parallel faults controlled by a rate- and state-dependent frictional contact, using the spectral boundary integral method and the radiation-damping approximation. Within the antiplane strain approximation, we show seismic cycle simulations with a power-law distribution of rupture sizes and, in a different parameter regime, sequences of seismogenic slow-slip events. Using the in-plane strain approximation, we simulate the rupture dynamics of a restraining stepover. Finally, we describe cycles of large earthquakes along several parallel strike-slip faults in three dimensions. The approach is useful to explore the dynamics of interacting or isolated faults with many degrees of freedom.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41561-018-0160-2
