Physics-Based Scenario of Earthquake Cycles on the Ventura Thrust System, California: The Effect of Variable Friction and Fault Geometry

Abstract

The Ventura Thrust system in California is capable of producing large magnitude earthquakes. Geological studies suggest that the fault geometry is complex, composed of multiple segments at different dips: thrust ramps dipping 30°-50°linked with bed-parallel décollements dipping \ 10°. These latter types of gently dipping faults form due to preexisting weaknesses in the crust, and therefore have different frictional parameters from thrust ramps; the faults also experience different stresses because of how stresses are resolved onto the fault planes. Here, we use a twodimensional fault model to assess how geometry and frictional properties of the ramp/décollement system should affect the seismic cycle. We test velocity-strengthening, velocity-weakening, and conditionally stable décollements, and in addition explore how the dip angle of the décollement changes the earthquake behavior. A velocity-strengthening décollement cannot replicate the throughgoing earthquake ruptures that have been inferred for the Ventura fault system. We therefore suggest that this and other décollements may be better represented using a velocity-weakening or conditionally stable response. Our results show that minor variations in fault geometry produce slip amounts and recurrence intervals that differ only by 10-20%, but do not fundamentally alter the types of earthquakes and interseismic slip. We conclude that geological constraints on fault geometry are typically sufficient to produce modeled earthquake sequences that are statistically consistent with paleoseismic records. However, both frictional parameters along the fault and effective normal stress influence earthquake rupture patterns significantly. More research is needed to adequately constrain these quantities in order for earthquake rupture models to work as effective predictors of fault behavior.

The Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS)

“…Although our initial benchmarks have a simple setup, comparison of results for tens of models have yielded some unexpected and important insights, affirming the importance of starting simple in a community code verification exercise. The results and lessons from our initial benchmarks prepare us for future benchmark problems that incrementally incorporate additional, potentially dominating physical factors, including fully dynamic ruptures, coupling with fluids, multiple fault segments, nonplanar fault geometries, and inelastic bulk constitutive behavior (e.g., Segall and Rice, 1995;Noda and Lapusta, 2010;Segall and Rice, 2006;Segall et al, 2010;Erickson et al, 2017;Lambert and Barbot, 2016;Qiu et al, 2016;Barbot, 2018;Ong et al, 2019). For future verification exercises, we plan to address important issues in SEAS simulations, such as 3D effects, heterogeneous fault frictional properties, and full dynamics, which should advance the state-of-the-art computational capabilities in our field.…”

“…Methods based on the finite difference method (FDM) or a hybrid finite element/spectral BIEM have been used to simulate quasi-dynamic ruptures on faults with more complex bulk rheologies (Erickson and Dunham, 2014;Erickson et al, 2017;Allison and Dunham, 2018;Mckay et al, 2019;Abdelmeguid et al, 2019). Other SEAS modeling approaches include boundary element methods (BEM) for simulating slow slip and tremor (e.g., Tse and Rice, 1986;Rice and Tse, 1986;Ong et al, 2019;Goswami and Barbot, 2018;Luo and Ampuero, 2011;Nakata et al, 2012;Liu, 2013;Wei et al, 2013), coupling faulting with fluid/heat transport and inelastic dilatancy (Segall and Bradley, 2012a), effects of surface topography (Ohtani and Hirahara, 2015), frictional heterogeneities (Kato, 2016) and viscoelastic response (Kato, 2002;Lambert and Barbot, 2016;Barbot, 2018). A spectral element method (SEM) has also been developed for simulating fully dynamic earthquakes in a heterogeneous bulk (Kaneko et al, 2010).…”

Structural control and system-level behavior of the seismic cycle at the Nankai Trough

“…The constitutive law is motivated by a wealth of laboratory experiments (Dieterich 1978;Marone et al 1990;Marone and Kilgore 1993;Nakatani 2001;Yamashita et al 2014;Lyu et al 2019, and references therein) and is supported by various micro-physical models for the evolution of strength in fault gouge or bare contacts (Chester 1989; Sleep and Blanpied 1992;Sleep 1995Sleep , 2006; Barbot 2019a). The framework of rate-and-state friction has been useful to model a wide spectrum of rupture styles, including slow and fast ruptures (Rice and Tse 1986;Lapusta and Rice 2003;Hori et al 2004;Liu and Rice 2005;Rubin 2008;Chen and Lapusta 2009;Kaneko et al 2010;Barbot et al 2012;Wei et al 2015;Veedu and Barbot 2016;Lambert and Barbot 2016;Lui and Lapusta 2016;Salman et al 2017;Yu et al 2018;Ong et al 2019) and the dynamics of quasi-static deformation in dominantly aseismic periods Avouac 2004, 1992;Barbot et al 2004;Bruhat et al 2011;Rousset et al 2012;Rollins et al 2015).…”

“…(7) and (8) can then be evaluated numerically with algebraic relationships. The approach allows us to include non-planar fault geometry (Ong et al 2019) and to accommodate nonlinear rheological laws. The dynamics of fault slip coupled with viscoelastic flow is then obtained based on the Runge-Kutta method with adaptive time steps (Press et al 1992).…”

Physics of Megathrust Earthquakes: Introduction

“…The three following papers in this issue focus on the physics of megathrust earthquake ruptures by investigating the role of frictional properties on rupture style (Senatorski 2019), the role of geometrical segment boundaries on the down-dip segmentation of the megathrust (Ong et al 2019), and the coupling between rupture propagation and tsunami generation (Lotto et al 2018).…”