Yuko Kase

Slip-Length Scaling Law for Strike-Slip Multiple Segment Earthquakes Based on Dynamic Rupture Simulations

Most large earthquakes occur on several faults, and accordingly it is important to the prediction of earthquake size to know whether several adjacent faults rupture simultaneously and how slips are distributed on the simultaneously ruptured faults. In this article, I investigate slip-length scaling law, simulating spontaneous rupture processes on multisegmented strike-slip faults in a 3D half-space. Because of fault interaction, the amount of slip caused by simultaneous ruptures on two or more segments is larger than that of a rupture on a single segment. The fault interaction decreases with the distance between segments, thus the amount of slip attains a constant value when more than two segments rupture. The decrease of fault interaction also causes the small rate of slip increase for simultaneous ruptures on long segments. The numerical results physically support the empirical scaling law, which is characterized by a strong increase of slip with length and a tendency for slip to saturate for very long faults.

Rupture characteristics of the 2005 Tarapaca, northern Chile, intermediate-depth earthquake: Evidence for heterogeneous fluid distribution across the subducting oceanic plate?

[1] We examined the rupture of the 2005 Tarapaca, northern Chile, earthquake at about 110 km depth with respect to both kinematic and dynamic characteristics by using regional and teleseismic waveforms. The earthquake has a downdip tensional focal mechanism. The subhorizontal rupture is characterized by two patches of large slip and high stress drop which are aligned nearly in the east-west direction, being perpendicular to the direction of the Chile Trench. Rupture initiated in the eastern patch and then propagated to the western patch. Between the two patches, there exists a region of nonpositive stress drop and high strength excess, which can cause subshear rupture to propagate from the eastern to the western patches but radiates little seismic waves. Seismic radiation energy from this earthquake tends to be low, which is consistent with the nonpositive stress drop and high strength excess between the two patches. While the physical mechanism of intermediate-depth earthquakes is still controversial, current leading hypotheses are associated with dehydration within subducting plates. The rupture characteristics of the Tarapaca earthquake can be related to heterogeneous fluid distribution due to the dehydration. The spatial separation and dominant stress of the two large-slip patches agree with the characteristics of the previously reported double seismic zone beneath Chile. The two patches may be the manifestation of the double seismic zone where dehydration reactions can release fluid. Using a numerical simulation of 3-D dynamic rupture, we have shown that weakening due to fluid can account for the rupture characteristics of the Tarapaca earthquake.

Spontaneous Dynamic Rupture Propagation beyond Fault Discontinuities: Effect of Thermal Pressurization

Abstract A range of several fault segments often sequentially ruptures during an earthquake. We investigated the effects of thermal pressurization (TP) on dynamic rupture propagation beyond fault discontinuities by simulating spontaneous rupture propagation on two vertical strike‐slip fault segments. We revealed that a rupture can jump wider stepovers owing to TP, and that TP on a primary (nucleating) fault enables a rupture to jump at deep portions. In previous numerical studies on dry fault systems, it was found that a rupture sometimes fails to propagate to an unconnected fault, which is observed in the case of real earthquakes, and a rupture that successfully propagates is usually triggered near the surface of the Earth, unlike rupture evolution images obtained by seismic waveform modeling. TP can explain the inconsistencies between the previous numerical simulations and the observations, without depending on the heterogeneity of the initial stress and/or friction. Under depth‐dependent stress, we showed that TP enables a rupture to jump much wider stepovers at deep portions. If TP is in effect on faults, hydraulic diffusivity along with fault geometry can strongly control the characteristics of rupture propagation at fault discontinuities.

Suppression of slip and rupture velocity increased by thermal pressurization: Effect of dilatancy

We investigated the effect of dilatancy on dynamic rupture propagation on a fault where thermal pressurization (TP) is in effect, taking into account permeability varying with porosity; the study is based on three-dimensional (3-D) numerical simulations of spontaneous ruptures obeying a slip-weakening friction law and Coulomb failure criterion. The effects of dilatancy on dynamic ruptures interacting with TP have been often investigated in one- or two-dimensional numerical simulations. The sole 3-D numerical simulation gave attention only to the behavior at a single point on a fault. Moreover, with the sole exception based on a single-degree-freedom spring-slider model, the previous simulations including dilatancy and TP have not considered changes in hydraulic diffusivity. However, the hydraulic diffusivity, which strongly affects TP, can vary as a power of porosity. In this study, we apply a power law relationship between permeability and porosity. We consider both reversible and irreversible changes in porosity, assuming that the irreversible change is proportional to the slip rate and dilatancy coefficient ɛ. Our numerical simulations suggest that the effects of dilatancy can suppress slip and rupture velocity increased by TP. The results reveal that the amount of slip on the fault decreases with increasing ɛ or exponent of the power law, and the rupture velocity is predominantly suppressed by ɛ. This was observed regardless of whether the applied stresses were high or low. The deficit of the final slip in relation to ɛ can be smaller as the fault size is larger.

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.

Effect of water phase transition on dynamic ruptures with thermal pressurization: Numerical simulations with changes in physical properties of water

Phase transitions of pore water have never been considered in dynamic rupture simulations with thermal pressurization (TP), although they may control TP. From numerical simulations of dynamic rupture propagation including TP, in the absence of any water phase transition process, we predict that frictional heating and TP are likely to change liquid pore water into supercritical water for a strike-slip fault under depth-dependent stress. This phase transition causes changes of a few orders of magnitude in viscosity, compressibility, and thermal expansion among physical properties of water, thus affecting the diffusion of pore pressure. Accordingly, we perform numerical simulations of dynamic ruptures with TP, considering physical properties that vary with the pressure and temperature of pore water on a fault. To observe the effects of the phase transition, we assume uniform initial stress and no fault-normal variations in fluid density and viscosity. The results suggest that the varying physical properties decrease the total slip in cases with high stress at depth and small shear zone thickness. When fault-normal variations in fluid density and viscosity are included in the diffusion equation, they activate TP much earlier than the phase transition. As a consequence, the total slip becomes greater than that in the case with constant physical properties, eradicating the phase transition effect. Varying physical properties do not affect the rupture velocity, irrespective of the fault-normal variations. Thus, the phase transition of pore water has little effect on dynamic ruptures. Fault-normal variations in fluid density and viscosity may play a more significant role.