Benchun Duan

Heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike‐slip faults

[1] We combine a viscoelastic model for the interseismic process and an elastodynamic model for the coseismic process to explore the dynamics (over multiple earthquake cycles) of two parallel strike-slip faults embedded in a two-dimensional full space. The step over fault geometry results in a buildup of heterogeneous fault stress near the step over. This heterogeneous stress accumulates at the early stage of the evolution of the fault system, and finally stabilizes after a number of earthquake cycles. The heterogeneity in fault stress varies with the geometrical parameters (e.g., width and along-strike overlap/ gap) of the step over, as well as the rupture history of the fault system. This heterogeneous fault stress from previous earthquakes has significant effects on earthquake rupture initiation, propagation, and termination. The locations with a low normal stress level near a step over are favorable points for earthquake initiation. Rupture can jump a 4 km wide compressional step over and a 8 km or wider dilational step over if the fault system has historically experienced many earthquakes. A young step over with less induced heterogeneity allows rupture to jump only smaller step over widths. These results may have important implications for seismic hazard analysis in areas where segmented strikeslip faults predominate, particularly for estimating maximum earthquake potential. Citation: Duan, B., and D. D. Oglesby (2006), Heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike-slip faults, J. Geophys. Res., 111, B05309, doi:10.1029/2005JB004138.

Nonuniform prestress from prior earthquakes and the effect on dynamics of branched fault systems

[1] To examine the effects of branched fault geometry on the dynamics of fault systems in the long term, we perform multicycle simulations on generic faulting models. An explicit finite element algorithm is used to simulate spontaneous dynamic rupture of earthquakes. The fault stress during the interseismic period is evaluated by an analytical viscoelastic model. We find that the fault prestress field becomes highly nonuniform near the branch point and on the two branch segments over multiple earthquake cycles, owing to the branched fault geometry and stress interaction between the two segments. The principal prestress on faults rotates over multiple earthquake cycles and departs from the regional stress field significantly near the branch point. After a number of earthquake cycles, the branched fault systems evolve to a steady state in which several patterns of the fault prestress and earthquake rupture repeat. The nonuniform prestress developed from previous earthquakes has large effects on the rupture and slip patterns. Several different rupture scenarios can occur on a given branched fault system. In addition, backward branching can occur in the nonuniform prestress field, either driven by slip on the ‘‘stem’’ of the fault system or through a triggering mechanism. These modeling results may have important implications for understanding fault-branching behavior observed in the 1992 Landers, the 1999 Hector Mine, and the 2002 Denali fault earthquakes and for seismic hazard analysis in the areas where branched fault systems exist.

The Effects of Double Fault Bends on Rupture Propagation: A Geometrical Parameter Study

We use the 2D finite element method to determine how geometrical parameters determine whether rupture will propagate across a linked stepover in a strike-slip fault. The end segments of the fault system are aligned in the direction of maximum shear, and the length and angle of the linking segment are allowed to vary. We observe that ruptures propagate through extensional stepovers with steeper angles and longer linking segments than otherwise equivalent compressional step- overs. These different rupture behaviors form distinct regions in angle-stepover-length parameter space; the boundary between these regions takes the shape of an asymptotic curve in both the extensional and compressional cases. Models in which the size of the entire fault system was made larger or smaller revealed that the location of the bound- aries between regions of different rupture behavior do not scale linearly with the system size; it was easier to rupture steeper and relatively longer stepovers in fault systems that were larger overall. A separate set of models in which the stress field is rotated so that the parallel end segments were optimally aligned for rupture significantly altered the rupture behavior curves; in this stress field, it was easier to rupture compressional stepovers with steeper angles and longer linking segments than it was to rupture equivalent extensional stepovers. In both the case in which the end segments are aligned with the direction of maximum shear and the case in which the end segments are optimally oriented for rupture, the angles at which rupture could no longer propagate through the entire fault corresponded with peaks in the faults S value.

The Dynamics of Thrust and Normal Faults over Multiple Earthquake Cycles: Effects of Dipping Fault Geometry

We perform dynamic simulations of thrust and normal faults over multiple earthquake cycles. Our goal is to explore effects of asymmetric fault geometry on the long-term seismicity and dynamics of dipping faults. A dynamic finite-element method is used to model the interseismic and coseismic processes, with a dynamic relaxation technique for the former. The faults are loaded by stable sliding along the downward continuation of the faults. The asymmetric fault geometry of dipping faults with respect to the free surface cause changes in normal stress during the interseismic and coseismic periods. These changes are of opposite sign in the two periods, resulting in a stabilization of the normal and shear stresses over many earthquake cycles. Both faults develop relatively stable event patterns, in which a large event that ruptures the entire fault is preceded by a number of small events with various rupture lengths. A strong asymmetry in fault and ground motion exists between thrust and normal faults, and between the hanging wall and the footwall of both faults on faults dipping less than 70°. In both normal and thrust faults, the horizontal component of ground motion dominates on the footwall, while the vertical component dominates on the hanging wall. The above results may have implications in seismic hazard analysis and building design in regions where dip-slip faults predominate.

Parallel Simulations of Dynamic Earthquake Rupture along Geometrically Complex Faults on CMP Systems

Chip multiprocessors (CMP) are widely used for high performance computing and are being configured in a hierarchical manner to compose a CMP compute node in a CMP system. Such a CMP system provides a natural programming paradigm for hybrid MPI/OpenMP applications. In this paper, we use OpenMP to parallelize a sequential earthquake simulation code for modeling spontaneous earthquake rupture along geometrically complex faults on two CMP systems, IBM POWER5+ system and SUN Opteron server. The experimental results indicate that the OpenMP implementation has the accurate output results and the good scalability on the two CMP systems. We apply the optimization techniques such as large page and processor binding to the OpenMP implementation to achieve up to 7.05% performance improvement on the CMP systems without any code modification. Further, we illustrate an element-based partitioning scheme for explicit finite element methods. Based on the partitioning scheme and what we learn from the OpenMP implementation, we discuss how efficiently to use hybrid MPI/OpenMP to parallelize the sequential earthquake rupture simulation code in order to not only achieve multiple levels of parallelism of the code but also to reduce the communication overhead of MPI within a CMP node by taking advantage of the shared address space and on-chip high inter-core bandwidth and low inter-core latency. Our initial experimental results indicate that the hybrid MPI/OpenMP implementation obtains the accurate output results and has good scalability on CMP systems.

Inelastic response of compliant fault zones to nearby earthquakes

[1] Using dynamic rupture models, we examine the response of compliant fault zones that surround pre-existing faults to nearby earthquakes. We find that some portions of a compliant fault zone can experience inelastic deformation due to dynamic stress perturbations, while the remaining portions deform elastically. Inelastic response of the fault zone results in sympathetic fault-parallel motion (i.e., consistent with long-term geologic slip) across the fault, while elastic response causes retrograde motion (i.e., opposite to long-term geologic slip) in the static displacement field. Inelastic deformation signals along fault zones detected geodetically may be used to constrain the stress state in the crust because inelastic response occurs only when the prestress level is close to material strength, which can be measured in the laboratory.

Parallel Earthquake Simulations on Large-Scale Multicore Supercomputers

Earthquakes are one of the most destructive natural hazards on our planet Earth. Hugh earthquakes striking offshore may cause devastating tsunamis, as evidenced by the 11 March 2011 Japan (moment magnitude Mw9.0) and the 26 December 2004 Sumatra (Mw9.1) earthquakes. Earthquake prediction (in terms of the precise time, place, and magnitude of a coming earthquake) is arguably unfeasible in the foreseeable future. To mitigate seismic hazards from future earthquakes in earthquake-prone areas, such as California and Japan, scientists have been using numerical simulations to study earthquake rupture propagation along faults and seismic wave propagation in the surrounding media on ever-advancing modern computers over past several decades. In particular, ground motion simulations for past and future (possible) significant earthquakes have been performed to understand factors that affect ground shaking in populated areas, and to provide ground shaking characteristics and synthetic seismograms for emergency preparation and design of earthquake-resistant structures. These simulation results can guide the development of more rational seismic provisions for leading to safer, more efficient, and economical50pt]Please provide V. Taylor author e-mail ID. structures in earthquake-prone regions.

A Suite of Exercises for Verifying Dynamic Earthquake Rupture Codes

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.

An OpenMP Approach to Modeling Dynamic Earthquake Rupture Along Geometrically Complex Faults on CMP Systems

Chip multiprocessors (CMP) are widely used for high performance computing and are being configured in a hierarchical manner to compose a CMP compute node in a parallel system. OpenMP parallel programming within such a CMP node can take advantage of the globally shared address space and on-chip high inter-core bandwidth and low inter-core latency. In this paper, we use OpenMP to parallelize a sequential earthquake simulation code for modeling spontaneous dynamic earthquake rupture along geometrically complex faults on two CMP systems, IBM POWER5+ system and SUN Opteron server. The experimental results indicate that the OpenMP implementation has the accurate output results and the good scalability on the two CMP systems. Further, we apply the optimization techniques such as large page and processor binding to the OpenMP implementation to achieve up to 7.05% performance improvement on the CMP systems without any code modification.

Dynamics of Parallel Strike‐Slip Faults with Pore Fluid Pressure Change and Off‐Fault Damage

We use a 2D finite‐element program to investigate how effects of time‐dependent pore pressure and off‐fault damage in the form of plastic yielding could affect earthquake rupture on parallel strike‐slip faults with a stepover. From single‐fault tests, we find that the positive Coulomb stress (PCS) region at the end of the first fault controls the rupture initiation time and location on the second fault. Plastic deformation could significantly reduce the effective normal stress and adjust the shear stress in a specific direction, resulting in a narrowband with PCS in a dilatational stepover favoring the initiation of rupture. For a compressive stepover, the effect of plastic deformation is less obvious and the crescent‐shaped PCS region triggers rupture initiation on the second fault. The undrained pore pressure increases the effective normal stress in a dilatational stepover, which significantly reduces the jumping ability of rupture. When both undrained pore pressure and significant off‐fault damage are present, the effect of undrained pore pressure dominates in the dilatational stepover, whereas plastic deformation in the compressive stepover slightly reduces the maximum jumpable width.