Martin Galis

The Earthquake‐Source Inversion Validation (SIV) Project

Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake-source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward-modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source-model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake-source imaging problem.

Simulation of the Planar Free Surface with Near-Surface Lateral Discontinuities in the Finite-Difference Modeling of Seismic Motion

Kristek et al. (2002) developed a technique for simulating the planar free surface in the 3D fourth-order staggered-grid finite-difference (FD) modeling of seismic motion. The technique is based on (1) explicit application of zero values of the stress-tensor components at the free surface and (2) adjusted FD approximations (AFDAs) to vertical derivatives at and near the free surface. The technique was shown to be more accurate and efficient than the standard stress-imaging technique in 1D models. In this study, we tested accuracy of the AFDA technique in media with lateral material discontinuities reaching the free surface. We compared the FD synthetics with synthetics calculated by the standard finite-element (FE) method because the FE method naturally and sufficiently accurately satisfies the boundary conditions at the free surface and the traction interface continuity conditions at internal material discontinuities. The comparison showed a very good level of accuracy of the AFDA technique. We also demonstrated the very good sensitivity of our FD modeling to different positions of the same physical model in the spatial FD grid.

Induced seismicity provides insight into why earthquake ruptures stop

Our theoretical model of rupture arrest indicates that most of the injection-induced earthquakes have been self-arrested. Injection-induced earthquakes pose a serious seismic hazard but also offer an opportunity to gain insight into earthquake physics. Currently used models relating the maximum magnitude of injection-induced earthquakes to injection parameters do not incorporate rupture physics. We develop theoretical estimates, validated by simulations, of the size of ruptures induced by localized pore-pressure perturbations and propagating on prestressed faults. Our model accounts for ruptures growing beyond the perturbed area and distinguishes self-arrested from runaway ruptures. We develop a theoretical scaling relation between the largest magnitude of self-arrested earthquakes and the injected volume and find it consistent with observed maximum magnitudes of injection-induced earthquakes over a broad range of injected volumes, suggesting that, although runaway ruptures are possible, most injection-induced events so far have been self-arrested ruptures.

Fault roughness and strength heterogeneity control earthquake size and stress drop

An earthquake’s stress drop is related to the frictional breakdown during sliding and constitutes a fundamental quantity of the rupture process. High-speed laboratory friction experiments that emulate the rupture process imply stress drop values that greatly exceed those commonly reported for natural earthquakes. We hypothesize that this stress drop discrepancy is due to fault-surface roughness and strength heterogeneity: an earthquake’s moment release and its recurrence probability depend not only on stress drop and rupture dimension but also on the geometric roughness of the ruptured fault and the location of failing strength asperities along it. Using large-scale numerical simulations for earthquake ruptures under varying roughness and strength conditions, we verify our hypothesis, showing that smoother faults may generate larger earthquakes than rougher faults under identical tectonic loading conditions. We further discuss the potential impact of fault roughness on earthquake recurrence probability. This finding provides important information, also for seismic hazard analysis. 1. Background and Motivation Earthquakes can be regarded as frictional phenomena that release tectonically or otherwise accumulated stresses in the form of slip along generally preexisting fault surfaces [e.g., Scholz, 2002; Aki and Richards, 2009]. The coseismically released static stress drop Δτ—defined as the average change in shear stress on a rupture surface before and after a slip event—is a fundamental quantity of the rupture process, bearing information on an earthquake’s frictional breakdown during sliding, its seismic energy release, the frequency content of radiated seismic waves, and earthquake recurrence probability [e.g., Reid, 1910; Brune, 1970; Scholz, 2002; Aki and Richards, 2009]. Static stress drop is relevant for hazard assessment and the general understanding of earthquake physics. Estimates of Δτ based on seismological observations employ a simplified representation of the earthquake source that correlates fault slip, moment release, or frequency content of radiated seismic waves to stress drop [e.g., Brune, 1970; Kanamori and Anderson, 1975; Hanks, 1977; Aki and Richards, 2009; Allmann and Shearer, 2009]. The corresponding values of Δτ are centered at ~3–4MPa and do not change systematically with earthquake size, which is taken as evidence for self-similar earthquake scaling [e.g., Kanamori and Anderson, 1975; Hanks, 1977; Allmann and Shearer, 2009]. On the other hand, laboratory friction experiments indicate an almost complete breakdown in frictional resistance during sliding when coseismic slip velocities are reached [e.g., Han et al., 2007; Di Toro et al., 2011]. The observed large change in friction (typically Δμ ≥ 0.5) in such experiments, combined with effective normal stresses at seismogenic depths (σeff), yields coseismic stress drops Δτ =Δμσeff that exceed those derived from seismological observations by multiples of 10. Consequently, laboratoryand field-based estimates of coseismic stress drop Δτ are incompatible, questioning the validity of current Δτ estimates and the conclusions that are based on them. We conjecture that the strong discrepancy in Δτ estimates is due to the nonplanarity of natural rupture surfaces [e.g., Power et al., 1988; Sagy et al., 2007; Candela et al., 2012; Brodsky et al., 2016], the spatial heterogeneity of rock strength on the fault (here strength refers to a fault’s potential to sustain some amount of shear stress before slippage occurs [e.g., Ripperger and Mai, 2004; Konca et al., 2008; Mai and Thingbaijam, 2014]), and their combined effect on an earthquake’s slip distribution and moment release. We employ large-scale numerical simulations to investigate how the surface roughness of a fault and its strength heterogeneity affect average slip D and seismic moment M0 that are associated to stress drop Δτ. After describing the numerical model that was used in this study, we present our results and conclusion. The online supporting information contains additional data on model formulation and adopted physical parameters. ZIELKE ET AL. FAULT ROUGHNESS AND EARTHQUAKE STRESS DROP 777 PUBLICATIONS Geophysical Research Letters

Accounting for Fault Roughness in Pseudo-Dynamic Ground-Motion Simulations

Geological faults comprise large-scale segmentation and small-scale roughness. These multi-scale geometrical complexities determine the dynamics of the earthquake rupture process, and therefore affect the radiated seismic wavefield. In this study, we examine how different parameterizations of fault roughness lead to variability in the rupture evolution and the resulting near-fault ground motions. Rupture incoherence naturally induced by fault roughness generates high-frequency radiation that follows an ω−2 decay in displacement amplitude spectra. Because dynamic rupture simulations are computationally expensive, we test several kinematic source approximations designed to emulate the observed dynamic behavior. When simplifying the rough-fault geometry, we find that perturbations in local moment tensor orientation are important, while perturbations in local source location are not. Thus, a planar fault can be assumed if the local strike, dip, and rake are maintained. We observe that dynamic rake angle variations are anti-correlated with the local dip angles. Testing two parameterizations of dynamically consistent Yoffe-type source-time function, we show that the seismic wavefield of the approximated kinematic ruptures well reproduces the radiated seismic waves of the complete dynamic source process. This finding opens a new avenue for an improved pseudo-dynamic source characterization that captures the effects of fault roughness on earthquake rupture evolution. By including also the correlations between kinematic source parameters, we outline a new pseudo-dynamic rupture modeling approach for broadband ground-motion simulation.