Abstract. We conducted laboratory experiments to study frictional healing and the effects of normal stress vibrations on healing. The experiments were carried out using a servo-controlled double-direct shear apparatus on 10 cm x 10 cm blocks separated by a 3 rnm-thick gouge layer of fine-grained (grain size of 75-212 pro) quartz powder. We performed slide-hold-slide tests in which sliding surfaces were driven at a constant velocity, halted for a given interval, then restarted at the prior driving velocity. Healing varied systematically with cumulative displacement, and by conducting several sets of identical slide-hold-slides we calibrated and removed these effects. Forward modeling of the healing and relaxation curves using the rateand state-dependent friction laws shows that a displacement-dependent increase in the parameter b can account for our observations. To study the effects of vibration, we varied the mean normal stress of 25 MPa during holds by double amplitudes ranging from I to 13 MPa at a frequency of I Hz. Vibrations increased rates and magnitudes of frictional relaxation and healing, most likely due to increased gouge compaction. These effects increased with increasing amplitude of vibration. We performed normal stress step tests and used •he results to model the vibrational slide-hold-slide tests. Rate-and state-dependent constitutive laws cannot adequately describe the behavior we observed experimentally because they neglect gouge compaction. Mechanisms such as normal force oscillations may explain faster fault healing rates than would be predicted by standard laboratory measurements a• constant stress.

Abstract. Data on synthetic fault gouge previously collected by Richardson and Marone [1999] were compared with the predictions of a unified theory for rate-and state-dependent friction compiled by Sleep [1997]. The theory treats the gouge as a continuum one-dimensional fluid sheared between parallel plates. It is predicted that the strain rate localized into a shear band of width called Wss during steady state sliding from the nominal width of the gouge zone Wnom. The critical displacement during velocity stepping tests is predicted to be Wss Eint, where Ein t is the critical strain, an intrinsic material property. It is predicted that the strain rate for renewed sliding after holds delocalizes to a width Wnew which is greater than W•,s and for long holds approaches the full gouge zone width Wnom. The displacement for recovery of the shear traction to its steady state value is predicted to be WnewEint, which for long holds is much greater than the critical displacement obtained by velocity stepping. Only the macroscopic effects of this process could be studied using the laboratory data in hand. Compaction during the hold and the difference between peak shear traction upon restart and the steady state shear traction during sliding (healing) were measured. To simulate more complex normal traction variations on real faults, the normal traction was varied sinusoidally about its previous value during some holds. The theory reasonably predicts the observed relationship between healing and compaction and healing versus hold time. It predicts the slip needed for recovery of shear traction following holds but poorly predicts the shear traction versus time during recovery. We attribute this failure to the fact that the laboratory gouge is a heterogeneous three-dimensional substance. Qualitatively, the delocalized width Wnew varies with position within the gouge plane, and slip is required for localized shear to organize in three dimensions. As strain rate was not observed as a function or time and position within the gouge, other explanations for the observed long recovery times following holds remain viable, including consolidation strengthening.

We have studied induced seismicity associated with five deep gold mines located in the Far West Rand district, Republic of South Africa, focusing on the digital data recorded from January 1994 until February 2000 by in-mine arrays of three-component geophones. The observed seismicity, which exceeds 1,000 events per day, can be divided into two kinds of events, designated as Types A and B. Type A events are tightly clustered in time and space and generally occur within 100 m of an active mining face or development tunnel; their spectra are comparatively enriched in high frequencies, and they have an upper moment-magnitude cutoff at M max Ͻ 1. We associate these events with the "fracture-dominated" rupture of competent rock induced by dynamic stresses during blasting and quasi-static stress perturbations from the excavation and closure of individual stopes. In contrast, Type B events are temporally and spatially distributed throughout the active mining region; they represent "friction-dominated" slip in existing shear zones such as faults or dikes and have source-scaling properties that agree well with extrapolations from tectonic earthquakes. In the Far West Rand region, Type B events can have large magnitudes (M Ͼ 3), but they show a distinct lower magnitude cutoff at M min ϳ 0. We interpret this cutoff in terms of a critical-patch size for nucleation of shear failure, and we show that the data are consistent with a rate-and state-dependent friction model in which the critical slip distance D c ϳ 10 ‫4מ‬ m. Both the spectral predictions of this model and accelerograms of Type B events agree that f max ϳ 200 Hz.

We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down‐dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude ofMw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.

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