Stability of faults with heterogeneous friction properties and effective normal stress

Abstract

Abundant geological, seismological and experimental evidence of the heterogeneous structure of natural faults motivates the theoretical and computational study of the mechanical behavior of heterogeneous frictional interfaces. Fault zones are composed of a mixture of materials with contrasting strength, which may affect the spatial variability of seismic coupling, the location of high-frequency radiation and the diversity of slip behavior observed in natural faults. To develop a quantitative understanding of the effect of strength heterogeneity on the mechanical behavior of faults, here we investigate a fault model with spatially variable frictional properties and pore pressure. Conceptually, this model may correspond to two rough surfaces in contact along discrete asperities, the space in between being filled by compressed gouge. The asperities have different permeability than the gouge matrix and may be hydraulically sealed, resulting in different pore pressure. We consider faults governed by rate-and-state friction, with mixtures of velocity-weakening and velocity-strengthening materials and contrasts of effective normal stress. We systematically study the diversity of slip behaviors generated by this model through multi-cycle simulations and linear stability analysis. The fault can be either stable without spontaneous slip transients, or unstable with spontaneous rupture. When the fault is unstable, slip can rupture either part or the entire fault. In some cases the fault alternates between these behaviors throughout multiple cycles. We determine how the fault behavior is controlled by the proportion of velocity-weakening and velocity-strengthening materials, their relative strength and other frictional properties. We also develop, through heuristic approximations, closed-form equations to predict the stability of slip on heterogeneous faults. Our study shows that a fault model with heterogeneous materials and pore pressure contrasts is a viable framework to reproduce the full spectrum of fault behaviors observed in natural faults: from fast earthquakes, to slow transients, to stable sliding. In particular, this model constitutes a building block for models of episodic tremor and slow slip events.Keywords: fault heterogeneity; heterogeneous pore-pressure; slip instability; earthquakes; rate-and-state friction; linear stability analysis Highlights:1. A comprehensive study of slip behaviors in fault models with mixed velocity-strengthening / velocity-weakening materials and heterogeneous effective normal stress 2. Fault stability investigated with numerical rate-and-state simulations and analytical linear stability analysis 3. Rich range of slip behaviors including steady-slip, slow aseismic slip transients, ruptures localized in part of the fault (P-instability), ruptures breaking the whole fault (T-instability) and combinations of both 4. Closed form equations that accurately describe the stability conditions for P-instabilities and T-instabilities 5. A useful framework to understand the full spectru...

Variations in precursory slip behavior resulting from frictional heterogeneity

“…Consistent with previous studies (e.g., Skarbek et al 2012;Dublanchet et al 2013;Luo and Ampuero 2017;Yabe and Ide 2017), we observed several different types of slip behavior in our parameter studies. The first is the "total seismic" regime, in which a mainshock event ruptures the entire seismogenic zone on the fault (parameter sets A-C in Fig.…”

“…Our results show that simple frictional heterogeneity on the fault can explain the variations in activity levels of precursory slip before large earthquakes, in addition to transitions in slip behavior (Skarbek et al 2012;Dublanchet et al 2013;Luo and Ampuero 2017;Yabe and Ide 2017) and seismicity between mainshocks. Because frictional parameters and effective normal stresses should vary both along strike and along dip, complex seismicity in subduction zones could be explained by variations in frictional parameters.…”

“…The transition from the total seismic regime to the slow slip regime corresponds to the slip behavior transitions documented by Skarbek et al (2012). This transition is controlled by the spatially averaged values of frictional parameters on an infinite fault subjected to constant external stress (Yabe and Ide 2017), though the conditions of the transition vary in the finite fault system or with increasing external stress (Skarbek et al 2012;Dublanchet et al 2013;Luo and Ampuero 2017;Yabe and Ide 2017). In this study, we conduct parameter studies only for the a value.…”

“…However, other parameters, such as cell size and the ratio of VWZ size to VSZ size, also affect the changes in the conditions of the transition. Detailed parameter studies of these changes were conducted by Luo and Ampuero (2017). Hereafter, we focus on precursory slip behavior in the total seismic regime.…”

“…In contrast, Skarbek et al (2012) documented transitions from seismic to slow slip on a finite frictionally heterogeneous fault. Luo and Ampuero (2017) performed a thorough stability analysis of an infinite frictionally heterogeneous fault. Yabe and Ide (2018;hereafter YI18) reproduced aftershocks within the mainshock rupture area (e.g., Beroza and Zoback 1993;Woessner et al 2006) by considering the partial rupture of a frictionally heterogeneous fault.…”