Search

The effect of plasticity on dynamic frictional sliding along an interface between two identical elastic–viscoplastic solids is analyzed. The configuration considered is the same as that in Coker et al. (J Mech Phys Solids 53:884–992, 2005) except that here plane strain analyses are carried out and bulk material plasticity is accounted for. The specimens have an initial compressive stress and are subject to shear loading imposed by edge impact near the interface. The material on each side of the interface is modeled as an isotropically hardening elastic–viscoplastic solid. The interface is characterized as having an elastic response together with a rate- and state-dependent frictional law for its inelastic response. Depending on bulk material properties, interface properties and loading conditions, frictional slip along the interface can propagate in a crack-like mode, a pulse-like mode or a train-of-pulses mode. Results are presented for the effect of material plasticity on the mode and speed of frictional slip propagation as well as for the partitioning of energy components between stored elastic energy, kinetic energy, plastic dissipation in the bulk and frictional dissipation along the interface. Some parameter studies are carried out to explore the effects of varying the interface elastic stiffness and the impact velocity. [ABSTRACT FROM AUTHOR]

We examine the characteristics of seismic radiation produced by localized fault-opening and shear motions in a homogeneous solid, along with the effects of having dissimilar solids across the fault on the seismic radiation. The study employs calculations of synthetic seismograms generated at various receiver locations by shear and tensile dislocation sources. The results indicate that, in contrast to the standard case of shear dislocation, the body wave amplitudes of the fault-parallel component generated at near-fault seismograms by tensile dislocation are considerably larger than those of the fault-normal component. The P and S arrivals from a shear dislocation at receivers on the opposite sides of a fault in a homogeneous solid have the same polarities on the fault-normal component and opposite polarities on the fault-parallel and vertical components. However, for a tensile dislocation source the situation is exactly reversed. The existence of a velocity contrast across the fault produces additional phases that mask somewhat the above signals. However, the generated amplitudes and other waveform characteristics may be used to distinguish between the physical fault plane and the auxiliary plane. The recording and analysis of the discussed signals for regular earthquakes that are dominated by shear motion will require high-resolution receivers located very close to the fault. [ABSTRACT FROM AUTHOR]

We perform 2-D finite-difference calculations of mode II rupture along a bimaterial interface governed by slip-weakening friction, with the goal of clarifying rupture properties and the conditions leading to the development of unilateral wrinkle-like pulses. The simulations begin with an imposed bilateral rupture in a limited source region. Rupture properties outside the imposed source are examined for ranges of values of the degree of material contrast γ across the fault, the difference between static and dynamic coefficients of friction, and the difference between static friction and initial shear stress. The results show that mode II rupture evolves with propagation distance along a bimaterial interface, for broad ranges of realistic conditions, to a unilateral wrinkle-like pulse in the direction of slip on the compliant side of the fault. These conditions span in our calculations the ranges and γ > 2–5 per cent. When the difference between the static friction and initial shear stress is smaller, the evolution to unilateral wrinkle-like pulses occurs for smaller values of γ. The amount of slip increases with propagation distance, due to the incorporation of slip-weakening friction, in contrast to earlier results based on Coulomb and Prakash–Clifton friction laws with slip-independent coefficient. In all cases leading to wrinkle-like pulses, the rupture velocity in the preferred (+) propagation direction is , where is the generalized Rayleigh wave speed. Simulations with imposed rupture speed in the source region close to the slower P wave speed can excite, in addition to the primary wrinkle-like pulse in the preferred direction with , a weak pulse in the opposite (−) direction with . In some cases leading to bilateral crack-like propagation (e.g. ), the rupture velocities in the opposite directions are (the faster P wave speed) and , with the initial supershear crack front in the + direction followed by a pulse with . [ABSTRACT FROM AUTHOR]

We study volumetric deformation structures in stepover regions using numerical simulations and field observations, with a focus on small-scale features near the ends of rupture segments that have opposite-polarity from the larger-scale structures that characterize the overall stepover region. The reversed-polarity small-scale structures are interpreted to be generated by arrest phases that start at the barriers and propagate some distance back into the rupture segment. Dynamic rupture propagating as a symmetric bilateral crack produces similar (anti-symmetric) structures at both rupture ends. In contrast, rupture in the form of a predominantly unidirectional pulse produces pronounced reversedpolarity structures only at the fault end in the dominant propagation direction. Several observational examples at different scales from strike-slip faults of the San Andreas system in southern California illustrate the existence of reversed-polarity secondaty deformation structures. In the examples shown, relatively-small pressure-ridges are seen only on one side of relatively-large extensional stepovers. This suggests frequent predominantly unidirectional ruptures in at least some of those cases, although multisignal observations are needed to distinguish between different possible mechanisms. The results contribute to the ability of inferring from field observations on persistent behavior of earthquake ruptures associated with individual fault sections. [ABSTRACT FROM AUTHOR]

We investigate the seismic velocity contrast across the San Andreas fault (SAF) in the Parkfield area using fault zone head waves (FZHW) that propagate along the bimaterial fault interface and direct P waves. We systematically analyse large data sets of near-fault waveforms recorded by several seismic networks over the period 1984–2005. Clear FZHW are observed at many stations on the NE side of the fault in the creeping section of the SAF north of Middle Mountain (MM). This indicates the presence of a sharp bimaterial interface and that the NE side of the fault has lower seismic velocities in that region. The obtained P-wave velocity contrast is about 5–10 per cent north of MM, and it systematically decreases to 0–2 per cent near Gold Hill (GH). The along-strike variations of the velocity contrast are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF near GH, associated with the GH fault, and existing 3-D seismic tomography results. The obtained imaging results offer an explanation for the mixed rupture directions of the M6-type Parkfield earthquakes. The strong velocity contrast around MM is expected to produce a preferred propagation direction to the SE for earthquakes that nucleate near MM (e.g. the 1934 and 1966 Parkfield earthquakes). In contrast, the near-zero velocity contrast and multiple fault branches near GH imply that earthquakes that nucleate near GH (e.g. the 2004 Parkfield earthquake) are not expected to have a preferred propagation direction to the SE, and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities. The observed systematic reduction of the velocity contrast along the SAF from NW of MM to SE of GH provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the Parkfield section and propagate to the SE, and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the NW. [ABSTRACT FROM AUTHOR]