D. Roten

Scalable Earthquake Simulation on Petascale Supercomputers

Petascale simulations are needed to understand the rupture and wave dynamics of the largest earthquakes at shaking frequencies required to engineer safe structures (> 1 Hz). Toward this goal, we have developed a highly scalable, parallel application (AWP-ODC) that has achieved “M8”: a full dynamical simulation of a magnitude-8 earthquake on the southern San Andreas fault up to 2 Hz. M8 was calculated using a uniform mesh of 436 billion 40-m3 cubes to represent the three-dimensional crustal structure of Southern California, in a 800 km by 400 km area, home to over 20 million people. This production run producing 360 sec of wave propagation sustained 220 Tflop/s for 24 hours on NCCS Jaguar using 223,074 cores. As the largest-ever earthquake simulation, M8 opens new territory for earthquake science and engineering—the physics-based modeling of the largest seismic hazards with the goal of reducing their potential for loss of life and property.

Quantification of Fault-Zone Plasticity Effects with Spontaneous Rupture Simulations

Previous studies have shown that plastic yielding in crustal rocks in the fault zone may impose a physical limit to extreme ground motions. We explore the effects of fault-zone non-linearity on peak ground velocities (PGVs) by simulating a suite of surface-rupturing strike-slip earthquakes in a medium governed by Drucker–Prager plasticity using the AWP-ODC finite-difference code. Our simulations cover magnitudes ranging from 6.5 to 8.0, three different rock strength models, and average stress drops of 3.5 and 7.0 MPa, with a maximum frequency of 1 Hz and a minimum shear-wave velocity of 500 m/s. Friction angles and cohesions in our rock models are based on strength criteria which are frequently used for fractured rock masses in civil and mining engineering. For an average stress drop of 3.5 MPa, plastic yielding reduces near-fault PGVs by 15–30% in pre-fractured, low strength rock, but less than 1% in massive, high-quality rock. These reductions are almost insensitive to magnitude. If the stress drop is doubled, plasticity reduces near-fault PGVs by 38–45% and 5–15% in rocks of low and high strength, respectively. Because non-linearity reduces slip rates and static slip near the surface, plasticity acts in addition to, and may partially be emulated by, a shallow velocity-strengthening layer. The effects of plasticity are exacerbated if a fault damage zone with reduced shear-wave velocities and reduced rock strength is present. In the linear case, fault-zone trapped waves result in higher near-surface peak slip rates and ground velocities compared to simulations without a low-velocity zone. These amplifications are balanced out by fault-zone plasticity if rocks in the damage zone exhibit low-to-moderate strength throughout the depth extent of the low-velocity zone (

A Suite of Exercises for Verifying Dynamic Earthquake Rupture Codes

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The ShakeOut earthquake scenario: Verification of three simulation sets

∼5 km). We also perform dynamic non-linear simulations of a high stress drop (8 MPa) M 7.8 earthquake rupturing the southern San Andreas fault along 250 km from Indio to Lake Hughes. Non-linearity in the fault damage zone and in near-surface deposits would reduce peak ground velocities in the Los Angeles basin by 15–50%, depending on the strength of crustal rocks and shallow sediments. These results show that non-linear effects may be relevant even at long periods, in particular in earthquakes with high stress drop and in the presence of a low-velocity fault damage zone.

3D Simulations of M 7 Earthquakes on the Wasatch Fault, Utah, Part I: Long-Period (0–1 Hz) Ground Motion

The omission of nonlinear effects in large-scale 3D ground motion estimation, which are particularly challenging due to memory and scalability issues, can result in costly misguidance for structural design in earthquake-prone regions. We have implemented nonlinearity using a Drucker-Prager yield condition in AWP-ODC and further optimized the CUDA kernels to more efficiently utilize the GPUs memory bandwidth. The application has resulted in a significant increase in the model region and accuracy for state-of-the-art earthquake simulations in a realistic earth structure, which are now able to resolve the wavefield at frequencies relevant for the most vulnerable buildings (> 1 Hz) while maintaining the scalability and efficiency of the method. We successfully run the code on 4,200 Kepler K20X GPUs on NCSA Blue Waters and OLCF Titan to simulate a M 7.7 earthquake on the southern San Andreas fault with a spatial resolution of 25 m for frequencies up to 4 Hz.

A comparison of observed and simulated site response in the Rhône valley

We describe a set of benchmark exercises that are designed to test if computer codes that simulate dynamic earthquake rupture are working as intended. These types of computer codes are often used to understand how earthquakes operate, and they produce simulation results that include earthquake size, amounts of fault slip, and the patterns of ground shaking and crustal deformation. The benchmark exercises examine a range of features that scientists incorporate in their dynamic earthquake rupture simulations. These include implementations of simple or complex fault geometry, off‐fault rock response to an earthquake, stress conditions, and a variety of formulations for fault friction. Many of the benchmarks were designed to investigate scientific problems at the forefronts of earthquake physics and strong ground motions research. The exercises are freely available on our website for use by the scientific community.