ALBERT

Description: Geologic observations indicate that faults are fractally rough surfaces, with deviations from planarity at all length scales. Fault roughness introduces complexity in the rupture process and resulting ground motion. We present a 2D kinematic rupture generator that emulates the strong dependence of earthquake source parameters on local fault geometry observed in dynamic models of ruptures on nonplanar faults. This pseudodynamic model is based on a statistical analysis of ensembles of 2D plane strain rupture simulations on fractally rough faults with rate-weakening friction and off-fault viscoplasticity. We observe strong anticorrelation of roughness-induced fluctuations in final slip, rupture velocity, and peak slip velocity with the local fault slope for right-lateral strike-slip ruptures. Spatial variability in these source parameters excites high-frequency seismic waves that are consistent with observed strong-motion records. Although accurate modeling of this high-frequency motion is critical to seismic-hazard analysis, dynamic rupture simulations are currently too computationally inefficient to be of practical use in such applications. We find that the seismic waves excited by the pseudodynamic model have similar intensity and spectral content to the corresponding dynamic model. Although the method has been developed in 2D, we envision that a similar approach could be taken for the 3D problem, provided that computational resources are available to generate an ensemble set of 3D dynamic rupture simulations. The resulting methodology is expected to find future application in efficient earthquake simulations that accurately quantify high-frequency ground motion.

Description: INTRODUCTION Large earthquakes strike infrequently and close-in recordings are uncommon. This situation makes it difficult to predict the ground motion very close to earthquake-generating faults, if the prediction is to be based on readily available observations. A solution might be to cover the Earth with seismic instruments so that one could rely on the data from previous events to predict future shaking. However, even in the case of complete seismic data coverage for hundreds of years, there would still be one type of earthquake that would be difficult to predict: those very rare earthquakes that produce very large ground motion. These extreme-ground-motion events are so unlikely that most engineers would not even consider designing facilities to withstand the possibility of their occurrence. An exception would be a structure that needs to remain functional for an unusually long period of time. One example of a planned long-life structure has been the high-level nuclear waste repository at Yucca Mountain, Nevada. This structure has been envisioned as one that would perform reliably over tens of thousands of years (CRWMS M&O, 1998). The problem of predicting the maximum possible ground motion in the Yucca Mountain region has been studied using two approaches: a geological approach that examines evidence from the past, and a seismological approach that predicts possibilities for the future via computer simulations. Both strategies are described in detail in Hanks et al. (forthcoming). The seismological approach involved computer simulations that invoked a "physical limits" perspective. Calculations were performed to numerically simulate the largest possible earthquake-generated ground motions that could occur, while remaining faithful to the current state of knowledge about rock physics and wave propagation. These "physical limit" simulations were specifically applied to scenario earthquakes on the faults on and near Yucca Mountain (Andrews et al. 2007). In...

Description: There is strong evidence that the 11 March 2011 Tohoku earthquake rupture reached the seafloor. This is surprising because the shallow portion of the plate interface in subduction zones is thought to be frictionally stable, leading to the widely held view that coseismic rupture would stop several tens of kilometers downdip of the seafloor. Various explanations have been proposed to reconcile this seeming inconsistency, including dynamic weakening (e.g., thermal pressurization) and extreme stress release around shallow subducted seamounts. We offer a simpler explanation supported by 2D dynamic rupture simulations of the Tohoku earthquake. Our models account for depth-dependent material properties and the complex geometry of the fault, seafloor, and material interfaces, based on seismic surveys of the Japan Trench. The fault obeys rate-and-state friction with standard logarithmic dependence of shear strength on slip velocity in steady state. In our preferred model, the uppermost section of the fault is velocity strengthening. Rupture nucleates on a deeper, velocity-weakening section. Waves released by deep slip reflect off the seafloor, transmitting large stress changes to the upper section of the fault driving the rupture through the velocity-strengthening region to the trench. We validate the model against seafloor deformation and 1-Hz Global Positioning System (GPS) data. The seafloor displacements constrain the seismogenic depth and overall amount of slip, particularly near the trench. Our simulations reproduce many features in the GPS data, thereby providing insight into the rupture process and seismic wave field. Sensitivity to parameters is explored through an extensive suite of simulations. Neither static seafloor deformation nor onshore 1-Hz GPS data can uniquely determine near-trench frictional properties due to trade-offs with average stress drop. While conducted specifically for the Japan Trench region, our simulations suggest that rupture to the trench in megathrust events is quite possible, even if velocity-strengthening properties extend tens of kilometers landward from the trench. Online Material: Mp4 movies of particle velocities.