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Abstract Many natural faults are believed to consist of velocity weakening (VW) patches surrounded by velocity strengthening (VS) sections. Numerical studies routinely employ this framework to study earthquake sequences including repeating earthquakes. In this laboratory study, we made a VW asperity, of lengthL, from a bare Poly(methyl methacrylate) PMMA frictional interface and coated the surrounding interface with Teflon to make VS fault sections. Behavior of this isolated asperity was studied as a function ofL(ranging from 100 to 400 mm) and the critical nucleation length, , which is inversely proportional to the applied normal stress (2–16 MPa). Consistent with recent numerical simulations, we observed aseismic slip for < 2, periodic slip for 2 < < 6, and non‐periodic slip for 10 < . Furthermore, we compared the experiments whereLwas contained by VS material to standard stick‐slip events whereLwas bounded by free surfaces (i.e.,L = the total sample length). The free surface case produced ∼10 times larger slip during stick‐slip events compared to the contained fault ruptures, even with identical . This disparity highlights how standard, complete‐rupture stick‐slip events differ from contained events expected in nature, due to both the free surface conditions and the heterogeneous normal stress along the fault near the free ends, as confirmed by Digital Image Correlation analysis. This study not only introduces the Teflon coating experimental technique for containing laboratory earthquake ruptures, but also highlights the utility of as a predictive parameter for earthquake behavior.
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The effects of characteristic slip distance on earthquake nucleation styles in fully dynamic seismic cycles
Earthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate-and-state friction, it was found that the ratios of a to b parameters control whether earthquakes nucleate as an expanding crack or a fixed length patch. However, as an essential parameter in earthquake physics, critical slip distance DRS controls the weakening efficiency of fault strength and can influence the nucleation styles. Here we investigate the effects of DRS on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. The inferred values of DRS from small-scale laboratory faults are 1-100 μm, several orders smaller than those obtained from geophysical observations on large natural faults. Considering the scale-dependence of widely observed DRS, the ratio of DRS to velocity weakening asperity size W is applied to substitute the absolute value of DRS in this study. We find when DRS/W is relatively large (~10-5), a/b=0.5 can separate two nucleation styles as found previously. For a relatively small DRS/W (~10-6), however, a/b larger than 0.7 is necessary to produce the typical expanding crack-like nucleation style. When DRS/W<4x10-7 and a/b<0.8, the fixed length nucleation style dominates. For some cases with a/b>0.75, the initial yielding phase accelerates to a considerable slip velocity just before the subsequent expanding fracture phase, which may explain the generation of foreshock activities. Specially, the first yielding phase is possible to trigger dynamic events without a secondary fracture phase. Furthermore, when the nucleation site is not in the middle of the asperity, large enough a/b (e.g., 0.8) could induce a complex nucleation style as well as abundant interseismic aseismic transients. We also recognize a special twin nucleation style that incorporates a failed acceleration phase. Our results reveal the critical role of DRS on earthquake nucleation styles and suggest that the fixed length nucleation style may be more common for the range of DRS/W (~10-4-~10-7) observed on natural and laboratory faults.
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- Award ID(s):
- 1943742
- PAR ID:
- 10492003
- Publisher / Repository:
- 2023 AGU Annual Meeting
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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