Total Fault Slip Research Articles

Modeling earthquake cycles and wear on rough faults with the mortar finite element method

SUMMARYThis paper presents a mortar-based finite element formulation for modelling earthquake cycles and wear on rough faults governed by rate and state friction. The method allows large sliding on the fault and accounts for all stages in the earthquake cycle, using a variable time step size with a transition between quasi-static and fully dynamic time discretizations. Wear laws with linear and power law forms are discretized and implemented into the mortar method, as well as a minimum level of normal traction constraint to treat fault opening. We examine the effect of wear laws on the slip behaviour and near-fault stresses during simulations of earthquake cycles on rough faults. The simulations demonstrate that the implementation of wear allows more realistic modelling of the earthquake cycle without the development of unrealistically large stresses and with less reduction of earthquake magnitude with total fault slip. Moreover, the method enables to study the effects of roughness and fault slip on the gouge zone thickness.

Regional‐Scale Low‐Angle Normal Fault Friction and Cohesion Constrained From Mohr‐Coulomb Models of Active and Abandoned Range‐Front Faults in Papua New Guinea

AbstractLow‐angle normal faults (LANFs) are poorly understood, as their slip at <30° dips appears inconsistent with Byerlee friction and Andersonian stresses. Rider blocks are fault slices formed when the uppermost part of a LANF is abandoned in favor of a new, steeper fault. Mohr‐Coulomb modeling of rider blocks can constrain fault frictional and cohesive strength provided known fault geometries. Analytical modeling shows that LANF viability depends on fault geometry and on the frictional and cohesive strength of the fault relative to that of the surrounding crust. Modeling of the Gwoira rider block, located atop the Mai'iu LANF, southeastern Papua New Guinea, indicates that this fault segment is frictionally weak (0.04 ≤ µf ≤ 0.17) in the shallow crust, with neither non‐Andersonian stresses nor supra‐hydrostatic fluid pressures required. Such weakness favors slip localization onto a single fault, enabling rapid slip rates at shallow dips. We show that slip at a second site, which lacks a rider block, does not require a weak fault because of a large fault/crust cohesion contrast and a rapid steepening of dip on the convex‐upward fault from only 21° at the surface to >30° down‐dip. From our results, we characterize four possible styles of faulting on convex‐up normal faults: (a) total fault slip, (b) footwall damage, (c) fault partial abandonment, and (d) total fault abandonment. Our results reveal that LANF faulting style depends on the strength of the fault compared to the surrounding crust, fault geometry, and the distribution of differential stress required for slip as a function of depth.

Filling the Gap in Cascadia: The Emergence of Low‐Amplitude Long‐Term Slow Slip

AbstractLong‐term slow slip events have been observed at several subduction zones around the globe, where they play an integral part in strain release along megathrust faults. Nevertheless, evidence for long‐term slow slip has remained elusive in the Cascadia subduction zone. Here we conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low‐amplitude long‐term slow slip event on the Cascadia subduction zone, with the possibility of others that are less resolved. Starting in mid‐2012, a 1.5‐year transient is observed in southern Cascadia, with a group of coastal GNSS stations moving ∼2 mm to the west. The data are modeled as a Mw 6.4 slow slip event occurring at 15–35 km depth on the plate interface, just updip of previously recognized short‐term slow slip and tremor. The event shares many characteristics with similar long‐term transient events on the Nankai subduction zone. However, the total fault slip amplitude is an order‐of‐magnitude smaller in Cascadia when compared to large events elsewhere, making long‐term slow slip detection challenging in Cascadia. While there are other westward long‐duration transients in the refined data set, the surface displacements are below the level of the noise or are limited spatially to a few neighboring stations, making interpretation unclear.

Constraints on deformation kinematics across the Yumu Shan, NE Tibetan Plateau, based on fluvial terraces

Abstract In compressive ranges, the kinematics of thrusts and related folds are the key to understanding mountain building processes; however, relatively little attention has been paid to folding on the hanging-wall of a thrust fault which has broken the surface. On the hanging-wall of the Yumu Shan thrust fault, which lies on the north side of the Yumu Shan at the NE edge of the Tibetan Plateau, we found that significant folding deformation had occurred simultaneously with faulting at the surface. To determine the kinematics of the folding and faulting, we surveyed deformed fluvial terrace surfaces across the fold and fault. The geometry of the surface of terrace T4 of the Liyuan River exhibits a faulting displacement of 55 ± 11 m at the fault trace, while in the hinterland, there is an active anticline with a wave length of ~4000 m. Limb rotation on the forelimb is recorded by the terrace geometry. Based on terrace geometry and the deformation of bedrock strata, we infer the geometry of the fault and fold, and conclude that the forelimb was formed by simple-shear deformation above the fault ramp, and that the backlimb was formed by the down-lowering of the fault dip angle. Using the geometry of the fault and fold and previous age control for the terraces, an average rock uplift of 1.2 ± 0.1 mm/a is estimated across the range, which produces a total fault slip rate of 1.8 ± 0.4 mm/a below the Yumu Shan. This new uplift rate indicates an onset age of 2.5 ± 0.5 Ma for the uplift of the Yumu Shan, suggesting that this is a young mountain range. This result suggests that the outward accretion of the NE Tibetan Plateau occurred at ~3 Ma, and it provides an insight into the kinematics of deformation within relatively recently deforming structures, suggesting that folding is a significant process in accommodating the crustal shortening where the thrusting has broken the surface.

Controls on Mid‐ocean Ridge Normal Fault Seismicity Across Spreading Rates From Rate‐and‐State Friction Models

AbstractRecent seismic and geodetic observations have led to a growing realization that a significant amount of fault slip at plate boundaries occurs aseismically and that the amount of aseismic slip varies across tectonic settings. Seismic moment release rates measured along the fast‐spreading East Pacific Rise suggest that the majority of fault slip occurs aseismically. By contrast, at the slow‐spreading Mid‐Atlantic Ridge seismic slip may represent up to 60% of total fault displacement. In this study, we use rate‐and‐state friction models to quantify the seismic coupling coefficient, defined as the fraction of total fault slip that occurs seismically, on mid‐ocean ridge normal faults and investigate controls on fault behavior that might produce variations in coupling observed at oceanic spreading centers. We find that the seismic coupling coefficient scales with the ratio of the downdip width of the seismogenic area (W) to the critical earthquake nucleation size (h*). At mid‐ocean ridges, W is expected to increase with decreasing spreading rate. Thus, the relationship between seismic coupling and W/h* predicted from our models explains the first‐order variations in seismic coupling coefficient as a function of spreading rate.

Path and amount of dextral fault slip in the Eastern California shear zone across the central Mojave Desert

New total fault slip estimates for the central Mojave Desert portion of the Eastern California shear zone support through-going dextral shear of ∼18 km that is transferred northward to the Garlock fault. The Eastern California shear zone accommodates ∼25% of the plate-boundary shear, with the transversely oriented Garlock fault forming its northern boundary with the Walker Lane belt. Total fault slip estimates for the major faults in the central Mojave Desert were determined using detailed geologic mapping supplemented with aeromagnetic data. Offset data for the Blackwater fault show a consistent 1.8 ± 0.1 km of dextral slip along 55 km of strike length, and they indicate that slip initiated at or after 3.8 Ma. The faults adjacent to the Blackwater fault have dextral slip of: 4.8 ± 0.3 km (Harper Lake fault); 2.9 ± 0.5 km (Mount General fault); 1.0 ± 0.7 km (Lockhart fault); and 3.2 ± 0.3 and 0.53 ± 0.05 km (Paradise fault). These slip data augment published data for the central Mojave Desert and allow evaluation of the strain path in the central Mojave Desert, showing consistent along-strike, through-going dextral slip. Contractional and extensional step-overs in the central Mojave Desert accommodate a change from focused slip in the southern central Mojave Desert to a wider and more dispersed fault system northward with a shift in the locus of dextral shear westward. We interpret these changes in the fault system to be a response to the Garlock fault, which inhibits through-going faulting but allows through-going dextral shear via clockwise deflection of its trace.

Structural geometry of the source region for the 2013 Mw 6.6 Lushan earthquake: Implication for earthquake hazard assessment along the Longmen Shan

The 2013 Mw 6.6 Lushan earthquake occurred in the Longmen Shan fold-and-thrust belt, Sichuan Province, China, near the five-year anniversary of the devastating 2008 Mw 7.8 Wenchuan earthquake. To define the fault that generated the 2013 earthquake and its relationship with the Beichuan fault, which ruptured in the Wenchuan earthquake, we construct several cross sections and a 3D structural model. The sections and models reveal that the main-shock of the Lushan earthquake occurred on a portion of the Range Front blind thrust (RFBT) and that the structural geometry of this fault varies along strike. The Lushan main-shock occurred at a location along the strike of the fault where the geologic shortening and total fault slip are greatest. A lateral ramp of the RFBT appears to coincide with the northern limit of aftershocks from the Lushan earthquake, leading to a 75 km seismic gap between the Wenchuan earthquake and the 2013 earthquake sequence. Although both the Wenchuan and Lushan earthquakes occurred within the Longmen Shan fold-and-thrust belt, different faults generated the two events. Based on this structural characterization and analysis of the aftershocks of the Wenchuan and Lushan earthquakes, we suggest that the Lushan earthquake may have been triggered by the 2008 rupture but is best considered as an independent event rather than an aftershock of the Wenchuan earthquake. The RFBT that generated the Lushan earthquake is linked to a detachment that extends into the Sichuan basin along the Triassic evaporite layer. The coulomb stress change simulation suggests that other faults linked to this detachment may have been loaded by the 2008 and 2013 earthquake, posing the risk of future earthquakes along the Longmen Shan and in the densely populated Sichuan basin.

The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries

Geophysical observations from the 2011 moment magnitude (M(w)) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the M(w) 8.8 2010 Maule, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation--a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.

On the relations between fracture energy and physical observables in dynamic earthquake models

We explore the relationships between the fracture energy density (EG) and the key parameters characterizing earthquake sources, such as the rupture velocity (vr), the total fault slip (utot), and the dynamic stress drop (Δτd). We perform several numerical experiments of three‐dimensional, spontaneous, fully dynamic ruptures developing on planar faults of finite width, obeying different governing laws and accounting for both homogeneous and heterogeneous friction. Our results indicate that EG behaves differently, depending on the adopted governing law and mainly on the rupture mode (pulselike or cracklike, sub‐ or supershear regime). Subshear, homogeneous ruptures show a general agreement with the theoretical prediction of EG ∝ , but for ruptures that accelerate up to supershear speeds it is difficult to infer a clear dependence of fracture energy density on rupture speed, especially in heterogeneous configurations. We see that slip pulses noticeably agree with the theoretical prediction of EG ∝ utot2, contrarily to cracklike solutions, both sub‐ and supershear and both homogeneous and heterogeneous, which is in agreement with seismological inferences, showing a scaling exponent roughly equal to 1. We also found that the proportionality between EG and Δτd2, expected from theoretical predictions, is somehow verified only in the case of subshear, homogeneous ruptures with RD law. Our spontaneous rupture models confirm that the total fracture energy (the integral of EG over the whole fault surface) has a power law dependence on the seismic moment, with an exponent nearly equal to 1.13, in general agreement with kinematic inferences of previous studies. Overall, our results support the idea that EG should not be regarded as an intrinsic material property.

Crustal deformation in northwestern Arabia from GPS measurements in Syria: Slow slip rate along the northern Dead Sea Fault

SUMMARY New Global Positioning System (GPS) measurements in NW Syria provide the first direct observations of near-field deformation associated with the northern Dead Sea fault system (DSFS) and demonstrate that the kinematics of the northern section of this transform plate boundary between the Arabian and Sinai plates deviate significantly from plate model predictions. Velocity estimates based on GPS survey campaigns in 2000, 2007 and 2008, demonstrate left-lateral shear along the northern DSFS with 1σ uncertainties less than 0.7 mm yr−1. These velocities are consistent with an elastic dislocation model with a slip rate of 1.8–3.3 mm yr−1 and a locking depth of 5–16 km. This geodetically determined slip rate is about half of that reported farther south along the central section (Lebanese restraining bend) and the southern section (Jordan Valley and Wadi Araba) of the transform and consequently requires some deformation to occur away from the transform along other geological structures. The factor of two difference in slip rates along the transform is also consistent with differing estimates of total fault slip that have occurred since the mid Miocene: 20–25 km along the northern DSFS (in NW Syria) versus about 45 km along the southern DSFS segment. Some of the strain deficit may be accommodated by north–south shortening within the southwestern segment of the Palmyride fold belt of central Syria. Additionally, a distinct change in velocity occurs within the Sinai plate itself. These new GPS measurements, when viewed alongside the palaeoseismic record and the modest level of present-day seismicity, suggest that the reported estimates of recurrence time of large earthquakes (M > 7) along the northern section of the DSFS may be underestimated owing to temporal clustering of such large historical earthquakes. Hence, a revised estimate of the earthquake hazard may be needed for NW Syria.

Present-day strain distribution across the Minab-Zendan-Palami fault system from dense GPS transects

The Strait of Hormuz area is a transition zone between the continental collision of the Zagros (west) and the subduction of an oceanic part of the Arabian Plate beneath the Makran wedge (east). Geology and recent GPS measurements indicate that about 15 mm yr−1 of relative motion in N10°E direction is accommodated by two major fault systems: (1) the NNW-trending Minab-Zendan-Palami (MZP) fault system that connects the Main Zagros Thrust (MZT) to the inner Makran thrust system and the Frontal subduction thrust and (2) the N-trending Sabzevaran-Kahnuj-Jiroft (SKJ) fault system that bounds the Jazmurian depression to the west. We use dense GPS measurements along four transects across these fault systems in order to determine the strains spatial distribution. The northern GPS transect confirms the total fault slip rates for both fault systems estimated by the tectonic analyses (about 10 and 7.3 mm yr−1 in N10° direction across the MZP and SKJ fault systems, respectively). For both fault systems, the elastic deformation spreads over shear zones that are several tens of kilometres wide. However, transects located close to latitude 27°N reveal a much narrower shear zone (∼10 km) for the MZP fault system. Moreover, we confirm that most of the present-day strain is transferred towards the frontal subduction thrust rather than towards the inner Makran thrusts. In order to complement this new GPS velocity field with spatially dense measurements, we processed a set of ERS radar images by the radar interferometry (InSAR) technique. We used both a ‘stacking’ and a ‘persistant-scatterers’ approach to differentiate the ground deformation signal which spatial gradient is expected to be very low, from the atmospheric signal. Results from these interferograms appear to be relatively in agreement with the GPS-determined strain distribution. Nevertheless, they confirm the absence of any superficial creep behaviour since no sharp discontinuity on interferometric phase can be noted on any interferogram. Finally, we use a purely kinematic ‘block model’ inversion process to calculate slip rates and locking depths for each fault system from our GPS measurements. These models suggest that the relative quiescence over the last 200 yr has certainly produced a slip deficit as high as 2 m. So, we may wonder if the MZP fault system is not late in the interseismic phase of its earthquake cycle.

Slip rate and mode of the Feldbiss normal fault (Roer Valley Graben) after removal of groundwater effects

The Feldbiss fault is the main active fault bounding to the southwest the Roer Valley Graben of NW Europe. In order to investigate the reasons of the discrepancy between its short- and long-term slip rate estimates, we have repeatedly surveyed a 2.3 km-long line by levelling at Sittard (The Netherlands) from April 2001 to December 2004. Simultaneously, three superposed aquifers were monitored on both sides of the fault. The across-fault differential groundwater variations are shown to be responsible for a seasonal 0.87 mm/m vertical motion of the fault, mainly through the effect of pore pressure changes in confined aquifers. After removal of this effect, I get a residual trend of −0.60±0.11 mm/yr for the relative motion of the hangingwall. This trend was disturbed in the summer of 2002 by transient millimetre-level up and down motions temporally associated with the small Eschweiler earthquake that occurred on July 22, 2002 on the Feldbiss fault at a distance of 35 km from Sittard. I explain this as a creep event triggered by the passage of trapped seismic waves. The subsisting discrepancy between the long-term slip rate of 0.06 mm/yr and short-term rates of ∼ 1.5 mm/yr (from 1962 to 2001) and 0.6 mm/yr (from 2001 to 2004) is tentatively interpreted as resulting from the current occurrence of a decade-long episode of aseismic slip involving a large part of the fault plane. Occasionally associated with minor seismicity, this aseismic slip event could betray a thickening of the transitional region located below the upper crustal zone of stable sliding. It causes a total fault slip of 5–10 cm in a few tens of years, releasing smoothly all, or a great part of, the strain accumulated during the previous 1–2 ky of fault quiescence.

Influence of depth, focal mechanism, and tectonic setting on the shape and duration of earthquake source time functions

Source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by Tanioka and Ruff [1997] and coworkers were compared in a systematic way. They were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration. Time function durations picked by Tanioka and Ruff [1997] are proportional to the cube root of seismic moment if moments from the Harvard centroid moment tensor catalog are used. The average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones, with a distinct change occurring at ∼40 km depth. The complexity of the time functions, as quantified by the number of subevents, appears to decrease below ∼40 km depth. Furthermore, among events shallower than 40 km, the average duration of scaled time functions is shorter, and their average shape has a more abrupt termination (1) for events with strike‐slip focal mechanisms compared to thrust events and (2) for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate (subduction) boundary. In each of these cases, events in more technically and seismically active settings have a longer duration and a more gradual termination. This can be interpreted in terms of lower stress drops and/or slower rupture velocities at active plate boundaries, suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates. Furthermore, differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones.Supporting data table is available via Web browser or via Anonymous FTP from ftp://kosmos.agu.org, directory “append” (Username = “anonymous”, Password = “guest”); subdirectories in the ftp site are arranged by paper number. Information on seraching and submitting electronic supplements is found at http://www.agu.org/pubs/esupp_about. html.

Tectonic geomorphology of the Ash Hill fault, Panamint Valley, California

ABSTRACTPanamint Valley, in eastern California, is an extensional basin currently bounded by active, dextral‐normal oblique‐slip faults. There is considerable debate over the tectonic and topographic evolution of the valley. The least‐studied structure, the Ash Hill fault, runs for some 50 km along the valley’s western edge, and active strands of the fault continue south into the neighbouring Slate Range. Vertical displacement on the fault is valley‐side up, creating topography that conflicts with the gross morphology of the valley itself. We use this topography, along with kinematic and geological markers, to constrain the Quaternary slip rate and orientation of the Ash Hill fault. The fault offsets all but the active channel deposits in the valley, and slickenlines indicate a strike‐slip to dip‐slip ratio of 3.5:1. An offset volcanic unit dated at 4 Ma provides a minimum slip rate of 0.3±0.1 mm yr−1, and a long‐term strike‐slip to dip‐slip ratio of 5.2:1. Slip on the fault has warped a palaeolake shoreline within the valley. Simple elastic dislocation modelling of the vertical deformation of the shoreline suggests total fault slip of ≈60 m, valley‐side up. The shoreline probably dates to 120–150 ka, implying a late Quaternary slip rate of 0.4–0.5 mm yr−1. We suggest two possible mechanisms for the apparently anomalous slip behaviour of the Ash Hill fault. The fault may be a listric structure related to the proposed low‐angle fault underlying Panamint Valley. Alternatively, the Ash Hill fault is a high‐angle fault, implying that the valley is currently bounded by high‐angle dextral‐slip faults. Lack of detailed subsurface information precludes any knowledge of the true relationships between the presently active faults.

Origin of Rollover (1)

Rollover is the folding of hanging-wall fault blocks by bending or collapse in response to slip on nonplanar--commonly listric--normal faults. The shapes of rollover folds are controlled by a number of variables, including (1) the shape of the fault, (2) the total fault slip after a bed is deposited, (3) the direction of relative particle motion in hanging-wall collapse, (4) the history of sedimentation rate relative to fault slip rate, and (5) compaction. The importance and role of each of these variables is illustrated by a two-dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments. In this modeling, an active axial surface, oriented parallel to the direction of relative p rticle motion in hanging-wall collapse, emanates from each fault bend and is the instantaneous locus of folding. The quantitative correctness of this theory of rollover is tested by modeling natural structures from the Gulf of Mexico for which both fault shape and fold shape are known from high-quality seismic and well sections. The direction of hanging-wall collapse commonly is in the antithetic or synthetic normal-fault or Coulomb-shear orientations, although sliding along weak bedding planes also is an important collapse mechanism in some regions. Collapse is in the antithetic-shear direction for concave fault bends and in the synthetic-shear direction for convex bends. These collapse directions can be observed directly in some high-quality seismic images as axial surfaces emanating from fault bends. The shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate, as well as the total slip after a bed is deposited. The crests of classic Gulf Coast rollovers are growth axial surfaces, along which are abrupt changes in sedimentation rate within the growth st atigraphic interval. These changes are produced by deformation of the sediment-water interface along active axial surfaces. Compaction can substantially modify the relationship between fault shape and rollover shape; however, under certain common conditions, the history of compaction can be neglected if the folding is modeled in the compacted state.

Anelastic response of the Earth to a dip slip earthquake

The deformation of the earth's surface immediately following an earthquake is well modeled by a dislocation in an elastic half space. Some time after the event, however, the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle (the asthenosphere). This relaxation causes further slow displacements observable at the earth's surface. This paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30° dip thrust fault. A variety of rheological models are studied. These models encompass variations in asthenosphere thickness, mesosphere viscosity, fault depth, and, most important, power law flow. Both Newtonian (n = 1) and non‐Newtonian (n = 3) Maxwell rheologies are considered for the asthenosphere and mesosphere. The results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault (a comparable downwarp develops on the upthrown side). These peripheral warps grow to a maximum height of about 5% of the total fault slip some time after the earthquake event and then collapse at much greater times. The vertical motions for a 30° dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone. Horizontal displacements propagate asymmetrically away from the rupture zone, the largest displacements occurring in the overthrust plate. The principal result of the comparison of n = 1 and n = 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar: the major difference is the time dependence of the development of the pattern. In spite of the complex geometrical structure of the subduction zone the displacements ui(1)(t) for Newtonian rheology are nearly the same as ui(3)(t′) for the non‐Newtonian rheology if t′ ∼ t3. Thus although the qualitative pattern of the displacements is not diagnostic of power law flow, the great difference in time dependence allows a clear discrimination of the two flow laws.

Implications of an elastic analysis of in situ stress measurements near the San Andreas Fault

Twenty‐nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.

Containment of stick‐slip failures on a simulated fault by pore fluid injection

Unstable stick‐slip events have been confined to a portion of a finely ground, 40‐ by 200‐cm simulated fault in a granite sample by selectively injecting water onto the ends of the stressed fault. Source parameters of eight confined events generated at normal stresses between 0.7 and 4.1 MPa have been measured. Dynamic stress drops increased from 0.06 to 0.38 MPa with increasing normal stress such that the ratio of stress drop to normal stress is about 0.09. Fractional shear stress drops of these events are less than 0.2. Total fault slip measured at the center of the slipped portion ranged from 8.5 to 38.9 µm. Maximum relative slip velocities at the same location ranged from 2.5 to 34.7 cm/s. Slip velocity increased with increasing dynamic stress drop.