Fault Slip Velocity Research Articles

Reply to “Comment on ‘Earthquake cycles and physical modeling of the process leading up to a large earthquake’”

In response to Cocco and Bizzarri’s (2002) argument that “there is no need to assume that friction must become independent of slip rate at high speeds to resemble slip weakening”, I remarked in my previous paper (Ohnaka, 2004) that “this argument seems logically inconsistent, because the effect of high velocity cutoff has been incorporated into the equation used in their simulation.” The most recent comment by Bizzarri and Cocco (2006) represents their counterargument to this remark. The basis of their reasoning is that even if the +1 term was included in the argument of the logarithm in equations (1c) and (1d) described in the paper (Ohnaka, 2004), the traction dependence on slip rate would remain in the governing equation, because the state variable depends on the fault slip velocity (Bizzarri and Cocco, 2006). I feel the need to point out, however, that their counterargument still appears to be logically inconsistent. In their simulations of the dynamic rupture regime within the framework of the rateand state-dependent formulation, they used the equation from which the direct effect of high slip rates on the shear traction had been cut off, which they admit in their comment (Bizzarri and Cocco, 2006). In addition, they also used the equation from which the effect of slip rate on state evolution had been removed; it would seem they are unaware of this fact. The state variable has the dimensions of time and is interpreted to be the age of the load supporting contacts across the fault surface (see Dieterich and Kilgore, 1996). According to Dieterich and Kilgore (1996), the evolution of state at constant slip speed under constant effective normal stress is given by

Two‐dimensional numerical modeling for near source strong ground motions of the Chelungpu fault during the 1999 Chi‐Chi Taiwan earthquake

Two‐dimensional numerical modeling based on the finite element method is employed to simulate near fault tip strong motion records of the Chi‐Chi Earthquake on September 21, 1999, in order to examine the kinematics of earthquake faulting and near field wave propagation properties. The modified ‘split‐node’ technique of Melosh and Raefsky is employed to simulate the equivalent body forces system for a double couple without moment. Different source parameters are examined to synthesize ground motions to simulate the observed near source strong motion records. Based on waveform modeling, a low angle thrust fault 30 km in length with a dip angle of 30°?is suggested for the ruptures near its northern end. The rupture is interpreted as bilateral from the center of the fault. The determined source rupture velocity and risetime are 2.0 km/sec and 5 sec, respectively, while the dislocation is about 6 meters. The estimated peak fault slip velocity is about 3.5 km/sec, showing a largest value than previously reported. Results of this study show that there was lower rupture velocity and longer rise‐time of the fault slip on the northern part of the Chi‐Chi Earthquake fault. Numerical modeling of this study indicates that the directivity of source rupture does not show significant low frequency ground motion near the earthquake fault tip. However, the effects of surface breaking of the fault movement have contributed large ground deformations near the fault tip, consequently, inducing large damage.

On a Mechanism of Clay Smear Emplacement in Synsedimentary Normal Faults

Faults that contain clay smears that may act as effective barriers against fluid flow. It is therefore of interest to understand the mechanism by which clay smears are emplaced in fault zones. Such a mechanism was inferred by the authors from observations made in the openpit browncoal mines at Frechen near Cologne, Germany, where synsedimentary (growth) faults were exposed by openpit mining in a paralic sequence of gravel, slightly consolidated sands, shales, and lignite of the Late Tertiary Rhine delta. The fault zones at Frechen contain persistent clay intercalations, a few millimeters to as much as 1 m in thickness, derived from extremely plastic shale beds in the upthrown and downthrown blocks, and smeared-out-usually without substantial admixing of sandy material from the fault walls - over distances as great as 70 m in dip direction. In the Frechen mine, these 'clay smears' are found to seal faults against transverse groundwater flow. The effectiveness of this important fault sealing mechanism, which is likely to operate only in soft sediments and at sufficiently slow fault slip velocities, can be quantified in an approximative way in terms of a dimensionless ratio of the clay extrusion rate over fault slip velocity. While it appearsmore » from this study that clay smears start to form at shallow depth, field observations reported elsewhere suggest that they may grow and preserve their sealing properties during subsequent burial.« less

A physical understanding of the earthquake source mechanism. II. The fault-slip velocity and acceleration.

The fault-slip motion of an earthquake is described on the basis of the recognition that frictional characteristics of the fault surfaces are important factors to be considered insofar as an earthquake source is a shear-type dislocation accompanying fracture or stick-slip. Expressions for the fault-slip velocity and acceleration are derived in terms of the stress drop (or the stress defined by the difference between the critical stress necessary to initiate slippage or fracture and the kinetic frictional stress in solid friction), the rigidity, the shear wave velocity, a parameter p standing for the magnitude of viscous friction on the fault plane and another parameter c, whose magnitude may depend upon the surface conditions such as roughness, of a natural fault and upon the bounded nature of the fault. It is shown that whether the stress after the earthquake dislocation is larger than the average kinetic frictional stress or not depends upon p and c.