Velocity-strengthening Friction Research Articles

Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals

HypothesisThis study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties. ExperimentsThis work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3–10 % RH) by AFM. FindingsThree logarithmic regimes for the velocity-dependence of friction are identified. BelowTc ∼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W1) to velocity-strengthening friction (S1). AboveTc ∼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S0) precedes velocity-weakening friction (W1). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above Tc –likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S0).These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.

Deep Postseismic Creep Following Large Earthquakes Revealed by Repeating Aftershocks in the Southeastern Tibetan Plateau

Abstract Large earthquake occurrence and the subsequent postseismic period are the most dramatic part of a seismic cycle that usually lasts months to years. However, the fault dynamics that account for the postseismic events are yet to be fully understood. Here, we use the repeating aftershock sequences (RASs) to investigate postseismic slips following the Mw 6.6 Lushan, Mw 6.5 Jiuzhaigou, Mw 6.1 Jinggu, and Mw 6.2 Ludian earthquakes in the southeastern Tibetan Plateau and find 135 RASs following the mainshocks. The RAS seismicity suggests that seismogenic faults began to creep in depth within a few hours after the Lushan, Jiuzhaigou, and Jinggu mainshocks. The deep creeps mainly follow a velocity-strengthening friction mode and decay with an Omori law p-value of ∼1. The results suggest that the combination of fault healing and geometry together controls deep fault behaviors. We develop two conceptual models to explain our observations. Our results provide new insights into spatiotemporal fault evolution after large earthquakes.

Short-Term Deep Postseismic Slips Following Large Earthquakes in Southern Taiwan

Abstract Deep postseismic slips that reflect the rheology and the deformation of the seismogenic fault after large earthquakes are usually investigated less than shallow slips because of the difficulty of obtaining direct observations. In this study, we used the seismicity of repeating aftershock sequences (RASs) to improve our understanding of the deep postseismic slips after three large earthquakes in southern Taiwan. To more precisely characterize the evolution of the RASs, we applied a template-matching technique to continuous waveform data for 90 days before and after the mainshocks. We identified 28 RASs that mainly occurred in regions near the mainshock hypocenters with relatively high VP/VS ratios. The deep fault slips estimated from the RASs show that the postseismic slip rates decreased logarithmically with increasing duration time, suggesting that the faults were creeping according to the velocity-strengthening friction law. We inferred that the high hydraulic pressure environment contributed to the fault creep, but the dynamic stress perturbation may have been the main factor affecting the fault creep. The results of this study improve our understanding of the behavior of deep faults and may aid in future seismic hazard assessments in Taiwan.

Structural controls on coseismic rupture revealed by the 2020Mw 6.0 Jiashi earthquake (Kepingtag belt, SW Tian Shan, China)

SUMMARYThe Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The 2020 January 19 Mw 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centred on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ∼7 km depth and dips ∼7° northward, consistent with the inferred position of the décollement. Our calibrated relocation of the main shock hypocentre is consistent with eastward, unilateral rupture of this fault. The narrow slip pattern (length ∼37 km but width only ∼9 km) implies that there is a strong structural or lithological control on the rupture extent, with updip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between main shock slip constrained by InSAR and teleseismic waveform modelling (∼7 km) and well-relocated aftershocks (∼10–20 km) may suggest that faults within sediments above the décollement exhibit velocity-strengthening friction.

Surface reverse afterslip of the 2008 Wenchuan Earthquake at Yingxiu, Sichuan, China: A case of velocity-strengthening fault friction at shallow depths

The vertical component of surface afterslip of the 2008 Mw7.9 Wenchuan Earthquake, China, has been observed since 2011 using repeated levelings at the Yingxiu site, which spans the southwestern segment of the 2008 rupture. We present the geological, co-seismic rupture, and aftershock features around the Yingxiu site and employ the rate- and state-dependent friction theory to analyze the friction mechanism and depth of the afterslip. The logarithmically increasing afterslip series with a cumulative amount of ∼30 mm obeys a velocity-strengthening friction law, from which the afterslip depth is estimated to be 2.6–6 km. This depth is further constrained to ∼5 km that probably represents the thickness of a shallow velocity-strengthening layer of the fault, from information of sedimentary strata and aftershock distribution. The coseismic reverse vertical slip was <3 m around the Yingxiu site during the 2008 mainshock, whereas that on the adjacent rupture section to the northeast was ≥8 m above ∼15 km depth, including the largest slip of 12.5 m at ∼13 km depth (Zhang et al., 2012). These features suggest that an apparent deficit of coseismic slip existed on the rupture section at the Yingxiu site, compared with coseismic slip on the adjacent section. Furthermore, the surface-observed afterslip is uncorrelated with the aftershock slip on the fault in the underlying velocity-weakening layer. Therefore, the surface afterslip at the Yingxiu site is due to post-2008 relaxation that compensates for the coseismic slip deficit in the shallow layer of the fault and is probably governed by velocity-strengthening friction.

Velocity-weakening friction induced by laboratory-controlled lithification

Regarding the occurrence of seismicity on major plate-boundary fault zones, one leading hypothesis is that the processes of lithification is responsible transforming loose, unconsolidated sediment that does not host earthquake nucleation into the frictionally unstable rocks that inhabit the seismogenic zone. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis. However, systematically quantifying frictional behavior as a function of lithification remains a challenge. Here, we simulate the lithification process in the laboratory by consolidating mixtures of halite and shale powders with halite-saturated brine, which we then desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We quantify lithification by: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. We observe that powdered samples of each halite-shale proportion exhibit predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Analysis of the individual rate-dependent friction parameters shows that the occurrence of velocity weakening is due to relatively low values of a for lithified samples. Larger velocity weakening is associated with cohesion of >∼1 MPa, and porosity reduction of >∼50 vol%. Microstructural images reveal that the shear surfaces for powders tend to exhibit small cracks not seen on the lithified sample shear surfaces. Our results suggest that lithification via cementation and porosity loss can facilitate slip instability, supporting the lithification hypothesis for seismogenic slip.

Characteristics of earthquake ruptures and dynamic off-fault deformation on propagating faults

Abstract. Natural fault networks are geometrically complex systems that evolve through time. The evolution of faults and their off-fault damage patterns are influenced by both dynamic earthquake ruptures and aseismic deformation in the interseismic period. To better understand each of their contributions to faulting we simulate both earthquake rupture dynamics and long-term deformation in a visco-elasto-plastic crust subjected to rate- and state-dependent friction. The continuum mechanics-based numerical model presented here includes three new features. First, a 2.5-D approximation is created to incorporate the effects of a viscoelastic lower crustal substrate below a finite depth. Second, we introduce a dynamically adaptive (slip-velocity-dependent) measure of fault width to ensure grid size convergence of fault angles for evolving faults. Third, fault localization is facilitated by plastic strain weakening of bulk rate and state friction parameters as inspired by laboratory experiments. This allows us to simulate sequences of episodic fault growth due to earthquakes and aseismic creep for the first time. Localized fault growth is simulated for four bulk rheologies ranging from persistent velocity weakening to velocity strengthening. Interestingly, in each of these bulk rheologies, faults predominantly localize and grow due to aseismic deformation. Yet, cyclic fault growth at more realistic growth rates is obtained for a bulk rheology that transitions from velocity-strengthening friction to velocity-weakening friction. Fault growth occurs under Riedel and conjugate angles and transitions towards wing cracks. Off-fault deformation, both distributed and localized, is typically formed during dynamic earthquake ruptures. Simulated off-fault deformation structures range from fan-shaped distributed deformation to localized splay faults. We observe that the fault-normal width of the outer damage zone saturates with increasing fault length due to the finite depth of the seismogenic zone. We also observe that dynamically and statically evolving stress fields from neighboring fault strands affect primary and secondary fault growth and thus that normal stress variations affect earthquake sequences. Finally, we find that the amount of off-fault deformation distinctly depends on the degree of optimality of a fault with respect to the prevailing but dynamically changing stress field. Typically, we simulate off-fault deformation on faults parallel to the loading direction. This produces a 6.5-fold higher off-fault energy dissipation than on an optimally oriented fault, which in turn has a 1.5-fold larger stress drop. The misalignment of the fault with respect to the static stress field thus facilitates off-fault deformation. These results imply that fault geometries bend, individual fault strands interact, and optimal orientations and off-fault deformation vary through space and time. With our work we establish the basis for simulations and analyses of complex evolving fault networks subject to both long-term and short-term dynamics.

Detecting repeating aftershocks in the Three Gorges Reservoir region, Central China

SUMMARYSlippage on a deep fault, which cannot be directly measured, can be inferred from repeating earthquakes. Here, we use a matched filter technique (MFT) to detect missing earthquakes near the Xiannvshan fault in the Three Gorges Reservoir region of Central China. We detected 13 repeating aftershock sequences (RASs) containing 107 events after a Ms 4.4 local earthquake that occurred on 26 March 2014. The RASs occurred in the vicinity of the Ms 4.4 main shock hypocentre and were concentrated within a depth range of 4–7 km. The short-term slip rates estimated from these RASs varied from ∼0.001 to 0.31 mm d–1. The slip rates of the RASs followed an approximately logarithmic decay with RAS duration, suggesting that the deep Xiannvshan fault behaviour tended to follow the logarithmic velocity-strengthening friction law. Relatively high-stress RAS events seemed to influence the occurrence of other RAS events whereas we found no evidence that high-stress nearby events, including a Ms 4.6 local earthquake that occurred on 29 March 2014, triggered the RAS events. We suggest that the fault stress change caused by the Ms 4.4 main shock played a role in triggering the RAS events. Our results demonstrate that the MFT can effectively help identify repeating aftershocks, which could help decipher fault zone rheology.

Implications of basement rock alteration in the Nankai Trough, Japan for subduction megathrust slip behavior

The Nankai Trough is an exceptionally well-studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip. Outcrop studies of exhumed analogues for the modern subduction thrust, as well as seismic reflection images of the active subduction zone, suggest that the basaltic basement participates in plate-boundary shearing at depths of a few km and greater. We obtained altered basaltic upper basement from the modern Nankai Trough seaward of the trench, recovered by drilling on IODP Expedition 333. We performed laboratory friction experiments on bare surfaces and gouge powders of this material, in addition to and in combination with other materials for comparison. The altered basalt exhibits predominantly velocity-strengthening frictional behavior, indicating a tendency for stable slip. For bare surface experiments, few instances of velocity-weakening friction occur and are restricted to slip rates of <10−6 m/s. Thin sections and XRD analyses of the starting material indicate that the velocity-strengthening behavior is likely associated with the presence of clay minerals, mostly Mg-smectite, which coat the rims of larger, stronger grains. Based our friction data, we suggest that in addition to creep the altered Nankai basalts may also allow the possibility of slow slip events for two reasons: (1) velocity-weakening friction may be expected at low slip rates, but velocity-strengthening at higher rates will damp any potential slip instabilities; and (2) based on a critical stiffness criterion for accelerating (stable) slip, SSEs are capable of nucleating in the altered Nankai basalt despite velocity-strengthening friction. In light of these observations, and given the depth at which altered basement appears to be incorporated along the plate interface, we raise the possibility that some shallow slow slip events in the Nankai accretionary prism could originate from shearing in altered basalt.

A physics-based approach of deep interseismic creep for viscoelastic strike-slip earthquake cycle models

SUMMARYMost geodetic inversions of surface deformation rates consider the depth distribution of interseismic fault slip-rate to be time invariant. However, some numerical simulations show downdip penetration of dynamic rupture into regions with velocity-strengthening friction, with subsequent updip propagation of the locked-to-creeping transition. Recently, Bruhat and Segall developed a new method to characterize interseismic slip rates, that allows slip to penetrate up dip into the locked region. This simple model considered deep interseismic slip as a crack loaded at its downdip end, and provided analytical expressions for stress drop within the crack, slip and slip rate along the fault. This study extends this approach to strike-slip fault environments, and includes coupling of creep to viscoelastic flow in the lower crust and upper mantle. I use this model to investigate interseismic deformation rates along the Carrizo Plain section of the San Andreas fault. This study reviews possible models, elastic and viscoelastic, for fitting horizontal surface rates. Using this updated approach, I develop a physics-based solution for deep interseismic creep which accounts for possible slow vertical propagation, and investigate how it improves the fit of the horizontal deformation rates in the Carrizo Plain region. I found solutions for fitting the surface deformation rates that allow for reasonable estimates for earthquake rupture depth and coseismic displacement and improves the overall fit to the data. Best-fitting solutions present half-space relaxation time around 70 yr, and very low propagation speeds, less than a metre per year, suggesting a lack of creep propagation.

Slip-Size Distribution and Self-Organized Criticality in Block-Spring Models with Quenched Randomness

We study slowly pulling block-spring models in random media. Second-order phase transitions exist in a model pulled by a constant force in the case of velocity-strengthening friction. If external forces are slowly increased, nearly critical states are self-organized. Slips of various sizes occur, and the probability distributions of slip size roughly obey power laws. The exponent is close to that in the quenched Edwards--Wilkinson model. Furthermore, the slip-size distributions are investigated in cases of Coulomb friction, velocity-weakening friction.

The effect of compliant prisms on subduction zone earthquakes and tsunamis

Earthquakes generate tsunamis by coseismically deforming the seafloor, and that deformation is largely controlled by the shallow rupture process. Therefore, in order to better understand how earthquakes generate tsunamis, one must consider the material structure and frictional properties of the shallowest part of the subduction zone, where ruptures often encounter compliant sedimentary prisms. Compliant prisms have been associated with enhanced shallow slip, seafloor deformation, and tsunami heights, particularly in the context of tsunami earthquakes. To rigorously quantify the role compliant prisms play in generating tsunamis, we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the Earth and the acoustic response of the ocean. Gravity is included in our simulations in the context of a linearized Eulerian description of the ocean, which allows us to model tsunami generation and propagation, including dispersion and related nonhydrostatic effects. Our simulations span a three-dimensional parameter space of prism size, prism compliance, and sub-prism friction – specifically, the rate-and-state parameter b−a that determines velocity-weakening or velocity-strengthening behavior. We find that compliant prisms generally slow rupture velocity and, for larger prisms, generate tsunamis more efficiently than subduction zones without prisms. In most but not all cases, larger, more compliant prisms cause greater amounts of shallow slip and larger tsunamis. Furthermore, shallow friction is also quite important in determining overall slip; increasing sub-prism b−a enhances slip everywhere along the fault. Counterintuitively, we find that in simulations with large prisms and velocity-strengthening friction at the base of the prism, increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height.

Velocity‐weakening behavior of Westerly granite at temperature up to 600°C

AbstractThe deep limit to seismicity in continental crust is believed to be controlled by a transition from velocity‐weakening to velocity‐strengthening friction based on experimental measurements of the rate dependence of friction at different temperatures. Available experimental data on granite suggest a transition to stable creep at about 350°C (∼15 km depth). Here we present results from unconfined experiments on Westerly granite at both dry and hydrated conditions that show increasingly unstable slip (velocity‐weakening behavior) at temperature up to 600°C. A comparison of previously published experimental results with those presented in this study suggests that the rate and state friction parameters strongly depend on normal stress and pore pressure at high (&gt;400°C) temperature, which may help explain regional variations in the depth distribution of earthquakes in continental crust. Temperature dependence of the rate and state friction parameters may also contribute to strong dynamic weakening observed in high‐speed friction experiments on crystalline rocks such as granite and gabbro.

Dynamic instabilities of frictional sliding at a bimaterial interface

Understanding the dynamic stability of bodies in frictional contact steadily sliding one over the other is of basic interest in various disciplines such as physics, solid mechanics, materials science and geophysics. Here we report on a two-dimensional linear stability analysis of a deformable solid of a finite height H, steadily sliding on top of a rigid solid within a generic rate-and-state friction type constitutive framework, fully accounting for elastodynamic effects. We derive the linear stability spectrum, quantifying the interplay between stabilization related to the frictional constitutive law and destabilization related both to the elastodynamic bi-material coupling between normal stress variations and interfacial slip, and to finite size effects. The stabilizing effects related to the frictional constitutive law include velocity-strengthening friction (i.e. an increase in frictional resistance with increasing slip velocity, both instantaneous and under steady-state conditions) and a regularized response to normal stress variations. We first consider the small wave-number k limit and demonstrate that homogeneous sliding in this case is universally unstable, independent of the details of the friction law. This universal instability is mediated by propagating waveguide-like modes, whose fastest growing mode is characterized by a wave-number satisfying kH∼O(1) and by a growth rate that scales with H−1. We then consider the limit kH→∞ and derive the stability phase diagram in this case. We show that the dominant instability mode travels at nearly the dilatational wave-speed in the opposite direction to the sliding direction. In a certain parameter range this instability is manifested through unstable modes at all wave-numbers, yet the frictional response is shown to be mathematically well-posed. Instability modes which travel at nearly the shear wave-speed in the sliding direction also exist in some range of physical parameters. Previous results obtained in the quasi-static regime appear relevant only within a narrow region of the parameter space. Finally, we show that a finite-time regularized response to normal stress variations, within the framework of generalized rate-and-state friction models, tends to promote stability. The relevance of our results to the rupture of bi-material interfaces is briefly discussed.

Lithification facilitates frictional instability in argillaceous subduction zone sediments

Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths>~5–10km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225°C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

Velocity-strengthening friction significantly affects interfacial dynamics, strength and dissipation.

Frictional interfaces abound in natural and man-made systems, yet their dynamics are not well-understood. Recent extensive experimental data have revealed that velocity-strengthening friction, where the steady-state frictional resistance increases with sliding velocity over some range, is a generic feature of such interfaces. This physical behavior has very recently been linked to slow stick-slip motion. Here we elucidate the importance of velocity-strengthening friction by theoretically studying three variants of a realistic friction model, all featuring identical logarithmic velocity-weakening friction at small sliding velocities, but differ in their higher velocity behaviors. By quantifying energy partition (e.g. radiation and dissipation), the selection of interfacial rupture fronts and rupture arrest, we show that the presence or absence of strengthening significantly affects the global interfacial resistance and the energy release during frictional instabilities. Furthermore, we show that different forms of strengthening may result in events of similar magnitude, yet with dramatically different dissipation and radiation rates. This happens because the events are mediated by rupture fronts with vastly different propagation velocities, where stronger velocity-strengthening friction promotes slower rupture. These theoretical results may have significant implications on our understanding of frictional dynamics.

Instabilities at frictional interfaces: Creep patches, nucleation, and rupture fronts

The strength and stability of frictional interfaces, ranging from tribological systems to earthquake faults, are intimately related to the underlying spatially extended dynamics. Here we provide a comprehensive theoretical account, both analytic and numeric, of spatiotemporal interfacial dynamics in a realistic rate-and-state friction model, featuring both velocity-weakening and velocity-strengthening behaviors. Slowly extending, loading-rate-dependent creep patches undergo a linear instability at a critical nucleation size, which is nearly independent of interfacial history, initial stress conditions, and velocity-strengthening friction. Nonlinear propagating rupture fronts-the outcome of instability-depend sensitively on the stress state and velocity-strengthening friction. Rupture fronts span a wide range of propagation velocities and are related to steady-state-front solutions.

Spatio-temporal Slip, and Stress Level on the Faults within the Western Foothills of Taiwan: Implications for Fault Frictional Properties

We use preseismic, coseismic, and postseismic GPS data of the 1999 Chi-Chi earthquake to infer spatio-temporal variation of fault slip and frictional behavior on the Chelungpu fault. The geodetic data shows that coseismic slip during the Chi-Chi earthquake occurred within a patch that was locked in the period preceding the earthquake, and that afterslip occurred dominantly downdip from the ruptured area. To first-order, the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate-and-state friction. Comparison with the distribution of temperature on the fault derived from thermo- kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200 at depth. This inference is consistent with the temperature induced transition from velocity-weakening to velocity-strengthening friction that is observed in laboratory experiments on quartzo-feldspathic rocks. The time evolution of afterslip is consistent with afterslip being governed by velocity-strengthening frictional sliding. The dependency of friction, l, on the sliding velocity, V, is estimated to be ol=o ln V ¼ 8 � 10 � 3 : We report an azimuthal difference of about 10-20 between preseismic and postseismic GPS velocities, which we interpret to reflect the very low shear stress on the creeping portion of the decollement beneath the Central Range, of the order of 1-3 MPa, implying a very low friction of about 0.01. This study highlights the importance of temperature and pore pressure in determining fault frictional sliding.

Conditions for Consecutive Rupture of Adjacent Asperities

Numerical simulation of earthquake cycles is carried out using a three-dimensional infinite uniform elastic model with a planar fault in which frictional stress obeys a rate- and state-dependent law. Two circular asperities with velocity-weakening frictional properties are embedded in a model fault plane, and velocity-strengthening friction is assumed in remaining regions. Different values of the characteristic slip distance &lt;u&gt;L&lt;/u&gt; are given to the two asperities so that seismic rupture nucleated at the asperity with smaller &lt;u&gt;L&lt;/u&gt; may be arrested at the asperity with larger &lt;u&gt;L&lt;/u&gt;. Seismic rupture is arrested in the large &lt;u&gt;L&lt;/u&gt; asperity for high contrast in &lt;u&gt;L&lt;/u&gt; when shear stress at the large &lt;u&gt;L&lt;/u&gt; asperity is low. The condition for rupture arrest is examined using a phase plane plot of frictional stress and slip velocity to show that varying effective stiffness of an asperity is important for consecutive rupture. Effective stiffness of an asperity decreases during an interseismic period due to inward propagation of aseismic sliding.

Modelling temporal variation of surface creep on the Chihshang fault in eastern Taiwan with velocity-strengthening friction

SUMMARY The active Chihshang fault at the boundary between the Eurasian and the Philippine Sea plates along the Longitudinal Valley in eastern Taiwan is creeping near the surface but has also produced large earthquakes at mid-crustal depth such as the 2003, M w 6.5, Chengkung earthquake. The creep rate measured at the surface shows strong seasonal fluctuations before the Chengkung earthquake, correlated with groundwater pressure variations measured at nearby wells. The Chengkung earthquake did not rupture the fault near the surface but induced a sudden increase of creep rate that decayed with time during the postseismic period. These observations suggest that the near surface fault obeys a rate-strengthening friction law. We conduct numerical simulations based upon the observed variations of creep rate with regard to a velocity-strengthening friction law and 1-D groundwater diffusion model to investigate the fault rheology behind this phenomenon. The model, which assumes a creeping fault segment extending from the surface to a depth of 5 km, yields a good fit to the creep data when the friction parameters are assumed to vary with depth or to have changed at the time of the Chengkung earthquake. Our best model suggests that the creeping zone is characterized by a rate parameter a = ∂μ/∂log (V )o f 1.3× 10 −2 and a friction coefficient of 0.84, and a long-term slip rate of 25.9 mm yr −1 at depths less than 87 m. By contrast, the lower segment of the creeping zone exhibits a much lower friction coefficient of 0.19, a smaller a of 6.6 × 10 −3 and a higher long-term slip rate of 38.1 mm yr −1 at depths between 0.087 and 5.0 km. This change in rate parameters implies that the lithologies or physical properties of the fault rocks may vary with depth. Alternatively, a model assuming a dramatic increase in rate parameter from 5.6 × 10 −4 to 6.2 × 10 −3 by the strong ground shaking of the Chengkung earthquake can also yield a fair agreement with the observed data. The lack of a notable deceleration in fault creep rate during the dry period from 2002 to mid-2003 suggests that the real recharging system for the fault zone may be more complicated than what we assumed in this study.