P. M. Mai

The seismic cycle at subduction thrusts: 1. Insights from laboratory models

[1] Subduction megathrust earthquakes occur at the interface between the subducting and overriding plates. These hazardous phenomena are only partially understood because of the absence of direct observations, the restriction of the instrumental seismic record to the past century, and the limited resolution/completeness of historical to geological archives. To overcome these restrictions, modeling has become a key-tool to study megathrust earthquakes. We present a novel model to investigate the seismic cycle at subduction thrusts using complementary analog (paper 1) and numerical (paper 2) approaches. Here we introduce a simple scaled gelatin-on-sandpaper setup including realistic tectonic loading, spontaneous rupture nucleation, and viscoelastic response of the lithosphere. Particle image velocimetry allows to derive model deformation and earthquake source parameters. Analog earthquakes are characterized by “quasi-periodic” recurrence. Consistent with elastic theory, the interseismic stage shows rearward motion, subsidence in the outer wedge and uplift of the “coastal area” as a response of locked plate interface at shallow depth. The coseismic stage exhibits order of magnitude higher velocities and reversal of the interseismic deformation pattern in the seaward direction, subsidence of the coastal area, and uplift in the outer wedge. Like natural earthquakes, analog earthquakes generally nucleate in the deeper portion of the rupture area and preferentially propagate upward in a crack-like fashion. Scaled rupture width-slip proportionality and seismic moment-duration scaling verifies dynamic similarities with earthquakes. Experimental repeatability is statistically verified. Comparing analog results with natural observations, we conclude that this technique is suitable for investigating the parameter space influencing the subduction interplate seismic cycle.

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

Modeling the seismic cycle in subduction zones: The role and spatiotemporal occurrence of off‐megathrust earthquakes

Shallow off-megathrust subduction events are important in terms of hazard assessment and coseismic energy budget. Their role and spatiotemporal occurrence, however, remain poorly understood. We simulate their spontaneous activation and propagation using a newly developed 2-D, physically consistent, continuum, viscoelastoplastic seismo-thermo-mechanical modeling approach. The characteristics of simulated normal events within the outer rise and splay and normal antithetic events within the wedge resemble seismic and seismological observations in terms of location, geometry, and timing. Their occurrence agrees reasonably well with both long-term analytical predictions based on dynamic Coulomb wedge theory and short-term quasi-static stress changes resulting from the typically triggering megathrust event. The impact of off-megathrust faulting on the megathrust cycle is distinct, as more both shallower and slower megathrust events arise due to occasional off-megathrust triggering and increased updip locking. This also enhances tsunami hazards, which are amplified due to the steeply dipping fault planes of especially outer rise events.