This paper reviews rock friction and the frictional properties of earthquake faults. The basis for rate- and state-dependent friction laws is reviewed. The friction state variable is discussed, including its interpretation as a measure of average asperity contact time and porosity within granular fault gouge. Data are summarized showing that friction evolves even during truly stationary contact, and the connection between modern friction laws and the concept of “static” friction is discussed. Measurements of frictional healing, as evidenced by increasing static friction during quasistationary contact, are reviewed, as are their implications for fault healing. Shear localization in fault gouge is discussed, and the relationship between microstructures and friction is reviewed. These data indicate differences in the behavior of bare rock surfaces as compared to shear within granular fault gouge that can be attributed to dilation within fault gouge. Physical models for the characteristic friction distance are discussed and related to the problem of scaling this parameter to seismic faults. Earthquake afterslip, its relation to laboratory friction data, and the inverse correlation between afterslip and shallow coseismic slip are discussed in the context of a model for afterslip. Recent observations of the absence of afterslip are predicted by the model.

/pdf/laboratory-derived-friction-laws-and-their-application-to-44s35v6won.pdf

Laboratory-derived friction laws and their application to seismic faulting

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion No seismicity was associated with this event The sudden displacements are best explained by ∼2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 67 This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses

https://science.sciencemag.org/content/sci/292/5521/1525.full.pdf

A silent slip event on the deeper Cascadia subduction interface.

Slow slip, a mechanism by which faults can relieve stress, was thought to be distinct from earthquakes. However, a global review of slow-slip phenomena suggests that instead there is a continuum between the two types of event.

/pdf/an-integrated-perspective-of-the-continuum-between-563cgxkcox.pdf

An integrated perspective of the continuum between earthquakes and slow-slip phenomena

With the availability of the Global Positioning System and other technical advances, a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized. They include deep episodic tremor, low-frequency earthquakes, slow slip events and 'silent' earthquakes. Based on data chiefly from western Japan, Ide et al. report that these 'slow' seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of 'regular' earthquakes. A diverse array of unusual earthquake phenomena occurring at relatively long periods, 'slow' events, follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes, indicating that they comprise a new earthquake category. Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic tremor1, low-frequency earthquakes2, very-low-frequency earthquakes3, slow slip events4 and silent earthquakes5,6,7,8,9. Each of these has been demonstrated to arise from shear slip, just as do regular earthquakes, but with longer characteristic durations and radiating much less seismic energy. Here we show that these slow events follow a simple, unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes. We find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant, with a spectral high-frequency decay of f-1. This scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category. The observed scale dependence of rupture velocity for these events can be explained by either a constant low-stress drop model or a diffusional constant-slip model. This new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation.

A scaling law for slow earthquakes

[1] It has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements. However, only recently has a fuller spectrum of strain release processes, including normal, slow, and silent earthquakes (or slow slip events) and continuous and episodic slip, been observed and generated by numerical simulations of the earthquake cycle. Despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive. The concurrence of seismic tremor with slow slip episodes in Cascadia and southwestern Japan provides insight into the process of slow slip. A perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface. Alternatively, evidence is accumulating to support the notion that tremor results from shear failure during slow slip. Global observations of the location, spatial extent, magnitude, duration, slip rate, and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation. Most slow slip events occur just downdip of the seismogenic zone, consistent with rate- and state-dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation. At a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation. Slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault, bringing it closer to or farther from earthquake failure, respectively. This paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide, with a focus on circum-Pacific subduction zones. Trends in global observations of slow slip events suggest that (1) slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it, (2) different frictional properties likely control fast versus slow slip, (3) the depth range of slow slip may be related to the thermal properties of the plate interface, and (4) the equivalent seismic moment of slow slip events is proportional to their duration (Moατ), different from the Moατ3 scaling observed for earthquakes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006RG000208

Slow slip events and seismic tremor at circum-Pacific subduction zones

EARTHQUAKES typically release stored strain energy on timescales of the order of seconds, limited by the velocity of sound in rock. Over the past 20 years, observations1–13 and laboratory experiments14 have indicated that rupture can also occur more slowly, with durations up to hours. Such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones. The detection of events with larger timescales requires near-field deformation measurements. In December 1992, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and we show here that the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes. The largest earthquakes associated with these slow events were small (local magnitude 3.7) and contributed negligible strain release. The importance of slow earthquakes in the seismogenic process remains an open question, but these observations extend the observed timescale for slow events by two orders of magnitude.

A slow earthquake sequence on the San Andreas fault

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Pinon Flat. Small-scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short-baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small-scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross-coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

Tidal calibration of borehole strain meters : Removing the effects of small-scale inhomogeneity

Over the past 6 years, repeated strain steps recorded on a tensor strainmeter installed close to the San Andreas fault trace at San Juan Bautista in California correlated closely with episodic creep events registered on a nearby creepmeter. The strain events were remarkably similar in character, were of about 1 hour duration, and were followed within hours to days by creep events of 2–6 days duration. The episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data. We postulate that the additional stress transferred to this region by the Loma Prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region, thus accounting for the increased frequency of the creep/strain events following the Loma Prieta earthquake.

Measurements of the strain field associated with episodic creep events on the San Andreas Fault at San Juan Bautista, California

Two continuous borehole plane strain monitoring sites have been operational in California since late 1983, using borehole tensor strain monitors implanted at a depth of 150 m. Shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals. At Pinon Flat Observatory, data indicate a constant shear strain accumulation of 0.6 microstrain per annum with the axis of maximum compression oriented 50° ± 5° west of north. This result differs significantly from regional geodetic estimates, the amplitude being dominated by continued viscoelastic response of the hole. Variations of measured strain rate exceeding 100 nstrain/yr would be significant at this instrument site. At San Juan Bautista, measurements of the angle of maximum compression (10° ± 3° west of north) agree closely with previous geodetic and hydrofracture estimates. After 2 years of operation the residual shear strain rate is 1 μstrain/yr. Preliminary analysis of strain steps observed at San Juan Bautista during the Morgan Hill earthquake of April 24, 1984, show good agreement with calculations from seismically determined source parameters for this event.

https://espace.library.uq.edu.au/view/UQ:718455/UQ718455_OA.pdf

Ross L. Gwyther

Papers

A slow earthquake sequence on the San Andreas fault

Tidal calibration of borehole strain meters : Removing the effects of small-scale inhomogeneity

Measurements of the strain field associated with episodic creep events on the San Andreas Fault at San Juan Bautista, California

Borehole tensor strain measurements in California