Keith B. Richards-Dinger

Generic Earthquake Simulator

Many of the papers in this topical issue concern earthquake simulators and their results. The goals and history of the project leading to this work are described in the preface to this topical issue. Earthquake simulators are computer programs that use physics of stress transfer and frictional resistance to describe earthquake sequences. Some are capable of generating long earthquake histories on many faults. They necessarily adopt a variety of simplifications to make computation feasible. The amount of detail computed within individual earthquakes depends on the simulator. None of those capable of generating long histories includes elastodynamics, but some make approximations of it. Nevertheless, seismic waves are not computed in any of the many‐fault simulators focused on here. The faults are typically approximated by many rectangular elements, although the future use of triangles would allow more accurate representation of curved fault surfaces. This paper briefly describes the features that are common to all of the earthquake simulators discussed in this topical issue of SRL . Following it are four papers (Pollitz, 2012; Richards‐Dinger and Dieterich, 2012; Sachs et al. , 2012; Ward, 2012) authored by each of the groups, which present features of their simulator that go beyond this generic description. Results from using these simulators are not presented in those papers but are contained within a subsequent paper (Tullis et al. , 2012) that compares the results of using these five simulators on an all‐California fault model, allcal2. A detailed description of this fault model can be found at http://scec.usc.edu/research/eqsims/, and the formats used for input and output by our group are described by Barall (2012). ### Fault Geometry and Slip Rates UCERF …

A Comparison among Observations and Earthquake Simulator Results for the allcal2 California Fault Model

Online Material: Supplemental figures of space‐time and frequency‐magnitude relations, scaling plots, mean and covariance plots of interevent times, probability distribution functions of recurrence intervals, and earthquake density plots. In order to understand earthquake hazards we would ideally have a statistical description of earthquakes for tens of thousands of years. Unfortunately the ∼100‐year instrumental, several 100‐year historical, and few 1000‐year paleoseismological records are woefully inadequate to provide a statistically significant record. Physics‐based earthquake simulators can generate arbitrarily long histories of earthquakes; thus they can provide a statistically meaningful history of simulated earthquakes. The question is, how realistic are these simulated histories? This purpose of this paper is to begin to answer that question. We compare the results between different simulators and with information that is known from the limited instrumental, historic, and paleoseismological data. As expected, the results from all the simulators show that the observational record is too short to properly represent the system behavior; therefore, although tests of the simulators against the limited observations are necessary, they are not a sufficient test of the simulators’ realism. The simulators appear to pass this necessary test. In addition, the physics‐based simulators show similar behavior even though there are large differences in the methodology. This suggests that they represent realistic behavior. Different assumptions concerning the constitutive properties of the faults do result in enhanced capabilities of some simulators. However, it appears that the similar behavior of the different simulators may result from the fault‐system geometry, slip rates, and assumed strength drops, along with the shared physics of stress transfer. This paper describes the results of running four earthquake simulators that are described elsewhere in …

Deterministic model of earthquake clustering shows reduced stress drops for nearby aftershocks

While a number of viable physical mechanisms have been offered to explain the temporal clustering of aftershocks, the spatial clustering of aftershocks, in particular the concentrated productivity of aftershocks very near the mainshock rupture area, has been difficult to reproduce with physical models. Here we present a new deterministic physical model capable of reproducing both the spatial and temporal clustering. We apply this new model to a longstanding puzzling question raised by ground motion observations, which suggest that nearby aftershocks show reduced ground motions relative to similar magnitude mainshocks. In the model, the physical basis for these observations is reduced stress drops for nearby aftershocks compared to similar magnitude mainshocks. These reduced stress drops are due to nearby aftershocks rerupturing incompletely healed parts of the fault which ruptured in the mainshock.

The role of effective normal stress, frictional properties, and convergence rates in characteristics of simulated slow slip events

Slow slip events (SSEs) observed in subduction zones around the world exhibit a wide range of recurrence intervals, durations, and propagation speeds. To illuminate possible mechanisms responsible for the observed variability and guide future modeling efforts, we employ the earthquake simulator RSQSim to explore the effects of variations in the convergence rates, frictional parameters, and effective normal stress on SSE characteristics. Both recurrence intervals and durations of SSEs are essentially directly proportional to the effective normal stress, while along-strike propagation speeds are inversely proportional. Effective normal stresses in the range of 3–9 MPa produce fault slip consistent with observed SSEs. Changes in rate-state frictional instability (b − a) have a moderate inverse effect on propagation speeds but much weaker effects on recurrence intervals and durations. Convergence rates are inversely proportional to recurrence intervals, propagation speeds show a weak positive relationship, and event durations appear insensitive.

Delayed Seismicity Rate Changes Controlled by Static Stress Transfer

On 15 June 2010, a M_w5.7 earthquake occurred near Ocotillo, California, in the Yuha Desert. This event was the largest aftershock of the 4 April 2010 M_w7.2 El Mayor-Cucapah (EMC) earthquake in this region. The EMC mainshock and subsequent Ocotillo aftershock provide an opportunity to test the Coulomb failure hypothesis (CFS). We explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative CFS change imparted by the Ocotillo event. Based on simple CFS calculations we divide the Yuha Desert into three subregions, one triggering zone and two stress shadow zones. We find the nominal triggering zone displays immediate triggering, one stress shadowed region experiences immediate quiescence, and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown. We quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of CFS change with the rate-state earthquake rate formulation of Dieterich (1994), assuming that each subregion contains a mixture of nucleation sources that experienced a CFS change of differing signs. Our modeling reproduces the observations, including the observed delay in the stress shadow effect in the third region following the Ocotillo aftershock. The delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations. We find that rate-state models of time-dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity.

A physics-based earthquake simulator replicates seismic hazard statistics across California

An earthquake simulator needing fewer inputs and assumptions replicates seismic hazard estimates. Seismic hazard models are important for society, feeding into building codes and hazard mitigation efforts. These models, however, rest on many uncertain assumptions and are difficult to test observationally because of the long recurrence times of large earthquakes. Physics-based earthquake simulators offer a potentially helpful tool, but they face a vast range of fundamental scientific uncertainties. We compare a physics-based earthquake simulator against the latest seismic hazard model for California. Using only uniform parameters in the simulator, we find strikingly good agreement of the long-term shaking hazard compared with the California model. This ability to replicate statistically based seismic hazard estimates by a physics-based model cross-validates standard methods and provides a new alternative approach needing fewer inputs and assumptions for estimating hazard.

Sensitivity of Induced Seismic Sequences to Rate‐and‐State Frictional Processes

It is well established that subsurface injection of fluids increases pore-fluid pressures that may lead to shear failure along a pre-existing fault surface. Concern amongst oil and gas, geothermal, and carbon storage operators has risen dramatically over the past decade due to the increase in the number and magnitude of induced earthquakes. Efforts to mitigate the risk associated with injection induced earthquakes include modeling of the interaction between fluids and earthquake faults. Here, we investigate this relationship with simulations that couple a geomechanical reservoir model and RSQSim, a physics-based earthquake simulator. RSQSim employs rate-and state-dependent friction (RSF) which enables the investigation of the time-dependent nature of earthquake sequences. We explore the effect of two RSF parameters and normal stress on the spatiotemporal characteristics of injection induced seismicity. We perform >200 simulations to systematically investigate the effect of these model components on the evolution of induced seismic sequences and compare spatiotemporal characteristics of our synthetic catalogs to observations of induced earthquakes. We find that the RSF parameters control the ability of seismicity to migrate away from the injection well, the total number and maximum magnitude of induced events. Additionally, the RSF parameters control the occurrence/absence of premonitory events. Lastly, we find that earthquake stress drops can be modulated by the normal stress and/or the RSF parameters. Insight gained from this study can aid in further development of models that address best practice protocols for injection operations, site-specific models of injection induced earthquakes, and probabilistic hazard and risk assessments of induced earthquakes.