Frictional Strength Of Faults Research Articles

Time‐Dependent Weakening of Granite at Hydrothermal Conditions

AbstractThe evolution of a fault's frictional strength during the interseismic period is a critical component of the earthquake cycle, yet there have been relatively few studies that examine the time‐dependent evolution of strength at conditions representative of seismogenic depths. Using a simulated fault in Westerly granite, we examined how frictional strength evolves under hydrothermal conditions up to 250°C during slide‐hold‐slide experiments. At temperatures ≤100°C, frictional strength generally increases with hold duration but, at 200 and 250°C, an initial increase in strength transitions to rapid time‐dependent weakening for holds longer than 14 hr. Forward modeling of long hold periods at 250°C using the rate and state friction constitutive equations requires a second, strongly negative, state variable with a long evolution distance. This implies that significant hydrothermal alteration is occurring at 250°C, consistent with microstructural observations of dissolution and secondary mineral precipitation.

The spectrum of fault slip in elastoplastic fault zones

Natural faults are typically surrounded by damage zones that exhibit inelastic material response. This study investigates the role of fault zone strength in modulating the spectrum of fault slip across different spatio-temporal scales. We carry out long-term simulations of seismic and aseismic slip for an elastoplastic spring slider model with rate-and-state friction as well as a continuum model of a 2D anti-plane rate-and-state fault embedded in an elastoplastic bulk. Results of the elastoplastic spring slider model show the emergence of a new stability boundary, depending on the bulk yield strength relative to fault frictional strength, that limits the rupture size regardless of the fault length. Continuum simulations generate a spectrum of slip analogous to the spring slider model including localized or migrating events of slow and fast slip. A fault may remain locked for yield strength sufficiently low and close to fault reference strength even if it is intrinsically rate weakening and larger than the nucleation length scale predicted by the elastic analysis. These findings shed new light on the nature of fault frictional stability and suggest the critical role of the fault zone rheological properties in modulating the spectrum of fault slip.

Numerical investigation of the effect of fluid pressurization rate on laboratory-scale injection-induced fault slip

The effect of normal stress variations on fault frictional strength has been extensively characterized in laboratory experiments and modelling studies based on a rate-and-state-dependent fault friction formalism. However, the role of pore pressure changes during injection-induced fault reactivation and associated frictional phenomena is still not well understood. We apply rate-and-state friction (RSF) theory in finite element models to investigate the effect of fluid pressurization rate on fault (re)activation and on the resulting frictional slip characteristics at the laboratory scale. We consider a stepwise injection scenario where each fluid injection cycle consists of a fluid pressurization phase followed by a constant fluid pressure phase. We first calibrate our model formulation to recently published laboratory results of injection-driven shear slip experiments. In a second stage, we perform a parametric study by varying fluid pressurization rates to cover a higher dimensional parameter space. We demonstrate that, for high permeability laboratory samples, the energy release rate associated with fault reactivation can be effectively controlled by a stepwise fluid injection scheme, i.e. by the applied fluid pressurization rate and the duration of the constant pressure phase between each successive fluid pressurization phase. We observe a gradual transition from fault creep to slow stick–slip as the fluid pressurization rate increases. Furthermore, computed peak velocities for an extended range of fluid pressurization rate scenarios (0.5 MPa/min to 10 MPa/min) indicate a non-linear (power-law) relationship between the imposed fluid pressurization rate and the peak slip velocities, and consequently with the energy release rate, for scenarios with a fluid pressurization rate higher than a critical value of 4 MPa/min. We also observe that higher pressurization rates cause a delay in the stress release by the fault. We therefore argue that by adopting a stepwise fluid injection scheme with lower fluid pressurization rates may provide the operator with a better control over potential induced seismicity. The implications for field-scale applications that we can derive from our study are limited by the high matrix and fault permeability of the selected sample and the direct hydraulic connection between the injection well and the fault, which may not necessarily represent the conditions typical for fracture dominated deep geothermal reservoirs. Nevertheless, our results can serve as a basis for further laboratory experiments and field-scale modelling studies focused on better understanding the impact of stepwise injection protocols on fluid injection-induced seismicity.

Do upper-plate material properties or fault frictional properties play more important roles in tsunami earthquake characteristics?

Tsunami earthquakes are a type of shallow subduction zone events that rupture slowly (<1.5 km/s) with exceptionally long duration and depleted high frequency radiation, resulting in a large discrepancy of Mw and Ms magnitudes and abnormally large tsunami along coastal areas. Heterogeneous fault frictional properties at shallow depth have been thought to dominate tsunami earthquake generation. Some recent studies propose heterogeneous upper-plate material properties determine rupture behavior of megathrust earthquakes, including characteristics of tsunami earthquakes. In this study, we use a recently developed 3D dynamic earthquake simulator to explore tsunami earthquake generation and systematically examine roles of upper-plate material properties and fault frictional properties in tsunami earthquake characteristics in a physics-based framework. For heterogeneous fault friction, we consider isolated asperities with strongly velocity-weakening properties embedded in a conditionally stable zone with weakly velocity-weakening properties. For heterogeneous upper-plate properties, we consider a generic depth profile of seismic velocity and rigidity constrained from seismic surveys. We design a set of models to explore their effects on tsunami earthquake generation and characteristics. We find that the conditionally stable zone can significantly slow down rupture speeds of earthquakes that nucleate on asperities to be <1.5 km/s over a large depth range (1–20 km), while heterogeneous upper-plate properties can only reduce rupture speeds to be ∼1.5–2.0 km/s over a narrow depth range (1-3 km). Nevertheless, heterogeneous upper-plate properties promote cascading rupture over multiple isolated asperities on the shallow subduction plane, contributing to large tsunami earthquake generation. We also find that fault frictional properties play much more important roles than upper-plate material properties in long normalized duration, high-frequency depletion and low moment-scaled radiated energy in tsunami earthquakes. In addition, the effective normal stress on the subduction plane, which affects fault frictional strength, also influences the characteristics of tsunami earthquakes, including duration, stress drop and moment-scaled radiated energy.

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

The seismic history of the Mosha fault, the largest and most active fault of Eastern Tehran metropolis, and its relation to the Damavand active volcano, the highest mountain in the Middle East, is investigated. We deduce that the central Mosha, near the Damavand, has a higher seismicity than either its western or eastern segments. On 7 May 2020, an Mw 5.1 earthquake occurred on the central Mosha, about 40 km east of Tehran and 10 km southwest of the Damavand crest, and it was felt intensely in Tehran. Its rupture is imaged and located in a region that presented a relative seismic quiescence compared to its eastern and western parts, during the last 14 years, suggesting its partial locking and heterogeneous distribution of fault frictional strength on this segment of Mosha. Its significant directivity to the west is confirmed by the mainshock rupture model, its PGA distribution, and distribution of early aftershocks. The rupture model suggests a relatively small stress drop of 2.6 bar, which is consistent with the comparatively high rupture dimension of 9 km for a Mw 5.1 earthquake, and indicates the easy rupture expansion on the central Mosha near the Damavand Volcano. The central Mosha experienced earthquakes in 1930, 1955, and 1983, as well as high microseismic activity and the 2020 seismic sequence, all of which strongly point to a possible influence of the Damavand Volcano on the seismicity of the central Mosha. This is corroborated by the observation of hydrothermal zones on the Mosha fault and the extension of a sill-like Damavand young magma chamber until central Mosha in tomography studies. We propose that the existing heat may increase the pore pressure on the fault, which lowers the effective normal stress, facilitates the nucleation-expansion of the rupture, and unclamps the fault. Damavand could act as a fuse and nucleate earthquakes, and if the rupture extends toward the west, it could have a significant directivity effect on low-frequency seismic waves that reach Tehran without attenuation and affect tall structures. In addition, high site amplification for frequencies up to 16 Hz due to the deep sedimentary basin, mainly in the mid-city of Tehran, will be remarkable for short buildings.

Frictional Characteristics of Oceanic Transform Faults: Progressive Deformation and Alteration Controls Seismic Style

AbstractOceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well‐explained. We test the effects of fault‐rock evolution on oceanic transform fault frictional strength and stability using direct‐shear experiments (at room temperature, 10 MPa normal stress, and fluid‐saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong (μ = 0.52–0.85) and velocity‐weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix‐rich chlorite‐bearing fault breccias and gouges are frictionally weak (μ = 0.25–0.48) and velocity‐strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically.

On the Effect of Grain Fragmentation on Frictional Instabilities in Faults With Granular Gouge

AbstractThe evolution of frictional strength during stick‐slip dynamics of a fault system is key to understanding earthquake nucleation and rupture patterns. In mature faults, granular gouge is produced by wear, comminution or fragmentation during tectonic movements. In this work, we introduce a fragmentation model in the simulation of a sheared granular fault to explore the influence of grain breakage on the stick‐slip dynamics. With fragmentation of highly stressed particles, the fault frictional strength increases accompanied by many small slip events triggered by particle breakage. The small fragments produced by particle breakage are not only stronger and more difficult to break, but they also change the distribution of contact forces, leading to the strengthening of the fault system. Based on statistical analyses on the size distribution of slip events with different particle strengths, we find that with lower particle strength, slip events become more correlated with particle fragmentation events and that the number of large slip events decreases. In addition, our analyses on the relationship between slip and particle fragmentation events reveal three types of correlations: In the first and second types, particle fragmentation events trigger micro or major slips, respectively. In the third category, large‐scale particle fragmentations take place at the end of large slip events owing to stress localization during post‐slip particle rearrangements. Our results in this work highlight the role of micromechanics of particle fragmentation in the failure of fault damage zones and help in understanding the relation between particle breakage and frictional failures.

Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

Abstract. Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.

Preparation zones for large crustal earthquakes consequent on fault-valve action

A combination of geological evidence (in the form of hydrothermal vein systems in exhumed fault systems) and geophysical information around active faults supports the localized invasion of near-lithostatically overpressured aqueous fluids into lower portions of the crustal seismogenic zone which commonly extends to depths between 10 and 20 km. This is especially the case for compressional–transpressional tectonic regimes which, beside leading to crustal thickening and dewatering through prograde metamorphism, are also better at containing overpressure and are ‘load-strengthening’ (mean stress rising with increasing shear stress), the most extreme examples being associated with areas undergoing active compressional inversion where existing faults are poorly oriented for reactivation. In these circumstances, ‘fault-valve’ action from ascending overpressured fluids is likely to be widespread with fault failure dual-driven by a combination of rising fluid pressure in the lower seismogenic zone lowering fault frictional strength, as well as rising shear stress. Localized fluid overpressuring nucleates ruptures at particular sites, but ruptures on large existing faults may extend well beyond the regions of intense overpressure. Postfailure, enhanced fracture along fault rupture zones promotes fluid discharge through the aftershock period, increasing fault frictional strength before hydrothermal sealing occurs and overpressures begin to reaccumulate. The association of rupture nucleation sites with local concentrations of fluid overpressure is consistent with selective invasion of overpressured fluid into the roots of major fault zones and with observed non-uniform spacing of major hydrothermal vein systems along exhumed brittle–ductile shear zones. A range of seismological observations in compressional–transpressional settings are compatible with this hypothesis. There is a tendency for large crustal earthquakes to be associated with extensive (L ~ 100–200 km) low-velocity zones in the lower seismogenic crust, with more local Vp/Vs anomalies (L ~ 10–30 km) associated with rupture nucleation sites. In some instances, these low-velocity zones also exhibit high electrical conductivity. Systematic, rigorous evaluation is needed to test how widespread these associations are in different tectonic settings, and to see whether they exhibit time-dependent behaviour before and after major earthquake ruptures.

Dual-Driven Fault Failure in the Lower Seismogenic Zone

ABSTRACTFrictional instability leading to fault rupture may be driven by increasing differential stress or by increases in pore-fluid pressure within the rock mass. Geological evidence (from hydrothermal vein systems in exhumed faults) together with geophysical information around active faults support the localized invasion of near lithostatically overpressured hydrothermal fluids, derived from prograde metamorphism at greater depths, into lower portions of the crustal seismogenic zone at depths of about 10–15 km (250°C&amp;lt;T&amp;lt;350°C). This is especially true of compressional–transpressional tectonic regimes that lead to crustal thickening and dewatering and are better at containing overpressure. Extreme examples are associated with areas undergoing active compressional inversion where existing faults, originally formed as normal faults during crustal extension, undergo reverse-slip reactivation during subsequent shortening though poorly oriented for reactivation. Extreme fault-valve action is likely widespread in such settings with failure driven by a combination of rising fluid pressure in the lower seismogenic zone lowering fault frictional strength, as well as by rising tectonic shear stress—dual-driven fault failure. Localized overpressure affects rupture nucleation sites, but dynamic rupturing may extend well beyond the regions of intense overpressuring. Postfailure, enhanced fracture permeability along fault rupture zones promotes fault-valve discharge throughout the aftershock period, increasing fault frictional strength before hydrothermal sealing occurs and overpressures begin to reaccumulate. The association of rupture nucleation sites with concentrated fluid overpressure is consistent with selective invasion of overpressured fluid into the roots of major fault zones and with nonuniform spacing of major vein systems along exhumed brittle–ductile shear zones.

Grain Friction Controls Characteristics of Seismic Cycle in Faults With Granular Gouge

AbstractMature faults at their core contain granular gouge, created due to communition of host rocks, which its frictional behavior controls earthquake nucleation and rupture patterns. In this work, we consider a fault system with granular gouge to study the effect of grain friction on the characteristics of seismic cycles. Our results show that particle friction controls the evolution of fault frictional strength as well as accumulation and release of elastic strain energy. Our discrete element simulations show that the stick‐slip frictional strength and dilation of the fault, as well as their variations, nonlinearly increase with the particle friction, but at high particle friction saturate. By statistical analyses on a large number of slip events, we find that the average recurrence time and its variations decrease with particle friction. A fault with higher grain friction shows more small slip events and also contains a limited number of extreme events. High particle friction introduces a more complex nucleation phase with higher stored energy and many recurrent small failures. We analyze the pseudo acoustic emission, which is based on monitoring the velocity signal of particles, and find higher temporal and more spatially distributed pseudo acoustic emissions for fault with higher grain friction. Our findings in this study show that, in faults with granular gouge, where the fault zone walls are totally engaged to the gouge layer, the friction at grain‐scale controls the characteristics of stick‐slip cycles including timing and amount of energy release.

How fast can low-angle normal faults slip? Insights from cosmogenic exposure dating of the active Mai’iu fault, Papua New Guinea

Abstract Is there an upper limit to normal fault slip rates? The Mai’iu fault, located within the rapidly extending Woodlark Rift, Papua New Guinea, is one of few active continental low-angle normal faults (LANFs) globally. There is ongoing debate regarding how commonly normal faults slip at shallow (&amp;lt;30°) dips, and at what rates. We present a global compilation of reported slip rates on active and inactive extensional detachments that suggests that such faults may slip at &amp;gt;10–20 mm/yr—faster than any reported high-angle normal fault. Cosmogenic nuclide exposure dating (10Be in quartz) of the lowermost Mai’iu fault scarp supports this finding, indicating slip at 11.7 ± 3.5 mm/yr over the past ∼5.5 k.y. Our results highlight the long-term viability of LANFs, and show that the Mai’iu fault represents one of Earth’s fastest active continental normal faults. Rapid and large-displacement slip is likely enabled by extremely low fault frictional strength.

Earthquake Nucleation on Faults With Heterogeneous Frictional Properties, Normal Stress

AbstractWe examine the development of an instability of fault slip rate. We consider a slip rate and state dependence of fault frictional strength, in which frictional properties and normal stress are functions of position. We pose the problem for a slip rate distribution that diverges quasi‐statically within finite time in a self‐similar fashion. Scenarios of property variations are considered and the corresponding self‐similar solutions found. We focus on variations of coefficients, a and b, respectively, controlling the magnitude of a direct effect on strength due to instantaneous changes in slip rate and of strength evolution due to changes in a state variable. These results readily extend to variations in fault‐normal stress, σ, or the characteristic slip distance for state evolution, Dc. We find that heterogeneous properties lead to a finite number of self‐similar solutions, located about critical points of the distributions: maxima, minima, and between them. We examine the stability of these solutions and find that only a subset is asymptotically stable, occurring at just one of the critical point types. Such stability implies that during instability development, slip rate and state evolution can be attracted to develop in the manner of the self‐similar solution, which is also confirmed by solutions to initial value problems for slip rate and state. A quasi‐static slip rate divergence is ultimately limited by inertia, leading to the nucleation of an outward expanding dynamic rupture: asymptotic stability of self‐similar solutions then implies preferential sites for earthquake nucleation, which are determined by distribution of frictional properties.

Temporal variation of frictional strength in an earthquake swarm in NE Japan caused by fluid migration

AbstractTemporal variations of the fault frictional strength was investigated based on the diversity of focal mechanisms in the source area of the Yamagata‐Fukushima border earthquake swarm, a significant earthquake swarm that occurred in central Tohoku, NE Japan, which started just after the 2011 M9.0 Tohoku‐Oki earthquake. The focal mechanisms of events in this swarm activity were determined using P wave polarity data as well as short‐period (1.5–2.5 Hz) waveform data from the direct P wave. The stress field in the source area of this swarm was estimated by applying stress tensor inversions to these focal mechanism data. Based on the estimated stress field, and under the assumption of uniform stress, we calculated relative frictional strengths for individual focal mechanisms. The calculated relative frictional strengths vary over a wide range, but their average value exhibits a characteristic temporal variation, which is at first small, but steadily increases with time for 100 to 150 days, and then becomes approximately constant. We confirmed this characteristic temporal variation of the average relative frictional strength by assuming the stress to be nonuniform. Similar temporal variations of the average relative frictional strength are obtained for even these cases, confirming the variation. The most likely cause for the observed temporal variation of the average relative frictional strength is the temporal variation of the pore fluid pressure in the source area of the swarm, facilitated by the Tohoku‐Oki earthquake and the subsequent fluid diffusion.

Fast-moving dislocations trigger flash weakening in carbonate-bearing faults during earthquakes.

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.

Earthquake Rupturing in Fluid-Overpressured Crust: How Common?

Whether or not ruptures nucleate in fluid-overpressured crust (λ v = P f/σ v > 0.4) is important because pore-fluids overpressured above hydrostatic lower fault frictional strength and may also vary through the earthquake cycle, acting as an independent variable affecting fault failure. Containment of fluid overpressure is precarious because pressure-dependent activation of faults and fractures allows drainage from overpressured portions of the crust. Discharge of fluids through activated fault-fracture permeability (fault-valve action) decreases overpressure so that subsequent failure depends on the cycling of both overpressure and frictional strength as well as tectonic stress. Geometric and mechanical considerations suggest that fluid overpressures are more likely to develop and be sustained in compressional/transpressional regimes as opposed to extensional/transtensional tectonic settings. On the basis of geophysical observations and force-balance analyses, subduction interface shear zones appear to be strongly but variably overpressured to near-lithostatic levels (λ v > 0.9) over the full depth range of seismogenic megathrusts. Strong overpressuring at seismogenic depths is also documented in active fold-thrust belts and in areas of ongoing compressional inversion (e.g., northern Honshu) where inherited normal faults are reactivated as steep reverse faults, requiring near-lithostatic overpressures (λ v → 1.0) at depths of rupture initiation. Evidence for overpressuring around strike-slip faults is less clear but tends to be strongest in areas of transpression. In areas of extensional tectonics coincident with particularly high fluid discharge, there is some evidence of overpressuring concentrated towards the base of the seismogenic zone. In general, because of the limited resolution of geophysical techniques, it is easier to make the case for rupture propagation through overpressured crust than to make a definitive case for the direct involvement of overpressured fluids in rupture nucleation, though in some instances the circumstantial evidence is compelling. An unresolved related issue is the heterogeneity of overpressuring. Do the active fault zones themselves serve as fluid conduits that are locally overpressured with respect to the surrounding crust?

Adhesion energy between mica surfaces: Implications for the frictional coefficient under dry and wet conditions

AbstractThe frictional strength of faults is a critical factor that contributes to continuous fault slip and earthquake occurrence. Frictional strength can be reduced by the presence of sheet‐structured clay minerals. In this study, two important factors influencing the frictional coefficient of minerals were quantitatively analyzed by a newly developed computational method based on a combination of first‐principles study and thermodynamics. One factor that helps reduce the frictional coefficient is the low adhesion energy between the layers under dry conditions. Potassium ions on mica surfaces are easily exchanged with sodium ions when brought into contact with highly concentrated sodium‐halide solutions. We found that the surface ion exchange with sodium ions reduces the adhesion energy, indicating that the frictional coefficient can be reduced under dry conditions. Another factor is the lubrication caused by adsorbed water films on mineral surfaces under wet conditions. Potassium and sodium ions on mica surfaces have a strong affinity for water molecules. In order to remove the adsorbed water molecules confined between mica surfaces, a differential compressive stress of the order of tens of gigapascals was necessary at room temperature. These water molecules inhibit direct contact between mineral surfaces and reduce the frictional coefficient. Our results imply that the frictional coefficient can be modified through contact with fluids depending on their salt composition. The low adhesion energy between fault‐forming minerals and the presence of an adsorbed water film is a possible reason for the low frictional coefficient observed at continuous fault slip zones.

Processes of faulting in jointed rocks of Canyonlands National Park, Utah

Detailed studies of 11 segmented normal faults from five grabens in the Needles District of Canyonlands National Park, Utah, demonstrate key processes in the growth, linkage, and evolution of grabens. Field observations and stereophotogrammetry reveal a ubiquitous asymmetry in cross-sectional geometry, based on distinct map patterns of graben-bounding faults, rollover anticlines with attendant joint dilation, footwall uplift with joint closure, and spoon-shaped graben floors. Master and antithetic faults across Devils Lane and several other grabens are defined quantitatively by the displacement distributions along the faults. Two-dimensional shape parameters that characterize the displacement profiles indicate that inelastic processes such as changes in fault frictional strength influence strain accumulation along the faults. The degree of graben asymmetry increases systematically with distance from the Colorado River; greater symmetry is associated with locally greater age and/or strain of grabens nearer the river. Scatter in plots of maximum displacement vs. fault length was attributed previously to linkage of fault segments alone but is here shown to correlate additionally with distance from the Colorado River and, therefore, to spatial strain gradients within the graben array. Extensional strain across the fault array, accumulating at rates of perhaps 1.5 to 2 cm/yr or 10−14 to 10−13 s−1, is accommodated at depth by salt flow and formation of reactive salt diapirs beneath the grabens and was probably initiated when the Colorado River had cut sufficiently deep into the section for active salt diapirism to commence.

Free surface effects on the propagation of dynamic rupture

Dynamic rupture of reverse and normal fault intersecting the surface are investigated. In the case of a normal fault nucleating at depth and propagating upwards, coupling of rupture‐radiated stress and free boundary conditions at the surface may induce a shallow secondary nucleation anticipating up to a few seconds the arrival of the main rupture front. Indeed, the free surface induces normal stress fluctuations modifying the fault frictional strength. No significant effect on rupture velocity is observed in the case of reverse faulting. These incidences are explained by a stress analysis and illustrated by some numerical simulations in the case of dynamic normal faulting in a homogeneous half‐space. The described effects could explain some observations of high frequency radiation close to the surface in documented shallow earthquakes like in Kalamata, 1986 [Bouin, 1994].