Subshear Rupture Research Articles

Dynamic modeling of the 2004 Mw 6.0 Parkfield, California, earthquake

We present two spontaneous rupture models of the 2004 Mw 6.0 Parkfield earthquake constrained by near‐source ground motions. We start with a stress drop distribution calculated from a kinematic slip distribution. Using a linear slip‐weakening friction law, we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records. The material contrast across the San Andreas Fault is incorporated using different one‐dimensional velocity structures on each side of the fault. An approximately constant S parameter of 0.3 and a uniform slip‐weakening distance of 0.15 m are used in the dynamic models. In our preferred dynamic model, consistent with the ground motion and GPS, the slip is bounded by seismicity streaks at 5 and 10 km depths, confirming a locked zone at depth. The stress drop is approximately 10 MPa in the hypocentral region and about 2 MPa elsewhere. The material contrast across the fault causes significant normal stress variations (∼1 MPa), leading to a larger strength drop to the southeast than to the northwest. The main rupture front propagates at nearly a constant subshear rupture velocity ∼3 km/s in both directions. The total radiated energy determined from the preferred dynamic model is 1.1 × 1013 J, seismic moment is 1.0 × 1018 Nm, and fracture energy is 3.0 × 1013 J. The limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone.

Aftershock asymmetry on a bimaterial interface

To better understand the asymmetric distribution of microearthquake aftershocks along the central San Andreas fault, we study dynamic models of slip‐weakening ruptures on an interface separating differing elastic half‐spaces. Subshear ruptures grow as slightly asymmetric bilateral cracks, with larger propagation velocities, slip velocities, and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed (the southeast front, in the context of the San Andreas). When the SE front encounters a stress barrier, the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse. This slip pulse smooths the stress field and reduces the static stress change beyond the SE front. Furthermore, because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon, the SE rupture front can be left far below the failure threshold, while the NW front remains quite close to failure. Both mechanisms could contribute to the observed aftershock asymmetry. Formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip‐weakening law. To achieve this while minimizing off‐fault damage requires either substantial velocity contrasts or small reductions in friction. The simulations also show a pronounced asymmetry in the timescales over which barriers to the SE and NW experience increasing stresses, a result that has implications for the asymmetric distribution of subevents in compound earthquakes.

Characteristics of Near-Fault Ground Motions by Dynamic Thrust Faulting: Two-Dimensional Lattice Particle Approaches

Two-dimensional lattice particle models have been used to simulate dynamic thrust faulting and its effects on near-fault ground motions. The lattice particle modeling approach has been demonstrated as an efficient way to model the dynamic rupture phenomena observed from a foam rubber experiment on dipping faults. We constructed a 42° dipping fault model to simulate near-fault ground motions under different site conditions in which different combinations of fault geometry (blind fault underlying a sedimentary layer and outcropping thrust models) and near-surface sedimentary layers were investigated. In particular, dynamic behaviors between a blind fault with an overlying sedimentary layer and an outcropping fault without a sedimentary layer were compared. In this simulation, a dynamic slip pulse accompanied by fault separation was initiated at the deepest part of the fault and propagated updip along the fault under a subshear rupture velocity. A strong asymmetrical ground-motion pattern on the hanging wall and footwall, caused by near-source rupture effects, was observed. Rupture directivity played an important role in determining the size and distribution of peak ground velocities and accelerations on the hanging wall and footwall. In the case of an outcropping thrust without a sedimentary layer, the hanging wall underwent a stronger ground motion, caused by the near-surface breakout phase, as the rupture reached the free surface. In the case of a blind thrust overlying a sedimentary layer, the peak ground particle velocity and acceleration could be much larger on the footwall. This is a result of the amplification effect of trapped long-period seismic energy in the sedimentary layer. The seismic energy emitted from the rupture-stopping phase was incident to the sedimentary layer and radiated under the rupture directivity effect. The radiated long-period seismic energy was trapped in the sedimentary layer, propagating away from the fault trace toward the footwall side. The numerical results show that, for an initial slip of 5 to 5.5 m, the horizontal peak ground velocity and acceleration could reach about 1.5 m/sec and 2 g , respectively, on the footwall for a blind thrust with a sedimentary layer. With the same initial slip level, the peak ground velocity and acceleration were about 1. 1 m/sec and 1.5 g , respectively, on the hanging wall for an outcropping thrust without a sedimentary layer. These results can partially explain the field observations of precarious rock distributions and overturned transformers in the vicinity of the White Wolf Fault during the 1952 M s 7.6 Kern County earthquake. Furthermore, the current simulation can be used for near-fault strong-motion prediction for large thrust faults in the Los Angeles Basin or similar tectonic settings around the world.

Near‐source ground motion from steady state dynamic rupture pulses

Ground motion from two‐dimensional steady state dynamic ruptures is examined for both subshear and supershear rupture velocities. Synthetic seismograms demonstrate that coherent high‐frequency information about the source process rapidly attenuates with distance from the fault for subshear ruptures. Such records provide almost no resolution of the spatial extent of the stress breakdown zone. At supershear speeds, S waves radiate away from the fault, preserving the full source spectrum and carrying an exact history of the slip velocity on both the fault‐parallel and fault‐normal components of motion, whose amplitudes are given by a function of rupture speed that vanishes at times the S‐wave speed. The energy liberated from the strain field by the passage of a supershear rupture is partitioned into fracture energy dissipated within the fault zone and far‐field S‐wave radiation. The partition depends on both the rupture velocity and the size of the breakdown zone.

Near-Source Ground Motions from Simulations of Sustained Intersonic and Supersonic Fault Ruptures

We examine the long-period near-source ground motions from simulations of M 7.4 events on a strike-slip fault using kinematic ruptures with rupture speeds that range from subshear speeds through intersonic speeds to supersonic speeds. The strong along-strike shear-wave directivity present in scenarios with subshear rupture speeds disappears in the scenarios with ruptures propagating faster than the shear-wave speed. Furthermore, the maximum horizontal displacements and velocities rotate from generally fault-perpendicular orientations at subshear rupture speeds to generally fault-parallel orientations at supersonic rupture speeds. For rupture speeds just above the shear-wave speed, the orientations are spatially heterogeneous as a result of the random nature of our assumed slip model. At locations within a few kilometers of the rupture, the time histories of the polarization of the horizontal motion provide a better diagnostic with which to gauge the rupture speed than the orientation of the peak motion. Subshear ruptures are associated with significant fault-perpendicular motion before fault-parallel motion close to the fault; supershear ruptures are associated with fault-perpendicular motion after significant fault-parallel motion. Consistent with previous studies, we do not find evidence for prolonged supershear rupture in the long-period (>2 sec) ground motions from the 1979 Imperial Valley earthquake. However, we are unable to resolve the issue of whether a limited portion of the rupture (approximately 10 km in length) propagated faster than the shear-wave speed. Additionally, a recording from the 2002 Denali fault earthquake does appear to be qualitatively consistent with locally supershear rupture. Stronger evidence for supershear rupture in earthquakes may require very dense station coverage in order to capture these potentially distinguishing traits.

Rupture characterization of a low magnitude earthquake of central Apennines (Italy)

This paper describes a detailed study of the rupture mechanism of a M L = 3 aftershock of the 1984 Abruzzo (Central Italy) earthquake ( M s = 5.8). To study the rupture extension, shape and directivity, the time durations of P and S far-field pulses were analyzed in different directions corresponding to six recording stations. The path corrected far-field displacement P and S pulses at six seismic stations were determined using the empirical Green function method. The fault plane solution has been estimated by joint inversion of P polarities and S polarizations measured on three component records. Finally, we used a method based on the plot of ‘isochrons’ associated with the observed pulse widths and projected on each nodal plane. The comparison of isochron plots for the two nodal planes suggests a rupture fault plane striking along the Apenninic direction. A weak source directivity up and west-ward on the fault plane was observed. A maximum ruptured surface of 0.4 km 2 was estimated assuming a sub-shear rupture process. The estimate of the seismic moment from spectral measurements gives the static stress drop ranging from 1 to 66 bar for the rupture velocity ranging between 0.2 β and β, where β is the shear wave velocity at the source.

Three-dimensional simulation of spontaneous rupture: The effect of nonuniform prestress

abstract We use a finite difference method to study crack propagation in a three-dimensional continuum, for conditions of both uniform and nonuniform prestress. The rupture criterion employed satisfies two fundamental physical requirements: it ensures finite stresses in the continuum and finite energy dissipation in crack extension. The finite stress numerical simulations exhibit abrupt jumps in rupture velocity when sharp changes in prestress are encountered on the crack plane, behavior analogous to that predicted theoretically for two-dimensional, singular cracks. For uniform prestress conditions, the slip velocity function is approximately a low-pass filtered version of that of a singular, constant rupture velocity crack. For nonuniform prestress, spatial variations of peak slip velocity are strongly coupled to spatial variations of rupture velocity. For uniform prestress and low cohesion, rupture velocity is predicted to exceed the S-wave velocity in directions for which mode II (inplane) crack motion dominates. A subshear rupture velocity is predicted for directions of predominantly mode III (antiplane) crack motion. Introduction of stress heterogeneities is sufficient, in each of the three cases studied, to reduce average rupture velocity to less than the S velocity, but local supershear rupture velocities can still occur in regions of high prestress. Rupture models with significant segments of supershear propagation velocities may be consistent with seismic data for some large earthquakes, even where average rupture velocity can be reliably determined to be subshear.