Viscous Shear Zones Research Articles

Spatial migration of temporal earthquake clusters driven by the transfer of differential stress between neighbouring fault/shear-zone structures

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry.

Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting

AbstractThe Malawi Rift is localized within Precambrian amphibolite‐granulite facies metamorphic belts, bounded by up to 150 km long border faults, and generates earthquakes throughout ∼40 km thick crust. Rift‐related faults are inferred to exploit pre‐existing weaknesses that allow rifting of otherwise dry and strong crust. It is unclear what these weaknesses are, and how localization into weak zones can be reconciled with strength required for lower crustal seismicity. We present results of mineral equilibria modeling, which indicate that Proterozoic metamorphism generated dry crust dominated by a quartz‐feldspar assemblage that is metastable at current conditions. For rift propagation to be possible at current cool thermal gradients and in mechanically strong, dry quartzofeldspathic rocks, mid‐ to lower‐crustal strain must be localized into relatively weak, inherited shear zones that deform primarily by aseismic, viscous creep. These shear zones are embedded within high‐strength crust, and interaction between creeping shear zones and enveloped or surrounding rocks may locally increase stress and trigger frictional, seismic slip at mid‐ to lower‐crustal depths. Over time, this interaction may produce a fracture network that allows infiltration of fluids. We therefore suggest that during rifting of previously deformed and metamorphosed crust, major faults are most likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zones. In this case, fluids may infiltrate and locally weaken metastable lower crust, including allowing time‐dependent fluid‐driven seismicity and local partial melting, but length‐scales of this weakening is limited by the scale of the permeability network.

СТРУКТУРЫ ЦЕНТРАЛЬНОГО ТИПА В ЮЖНОМ СИХОТЭ-АЛИНЕ В ВЕРОЯТНОСТНЫХ ГРАВИТАЦИОННЫХ МОДЕЛЯХ: ГЛУБИННОЕ СТРОЕНИЕ, РЕОЛОГИЯ, ИЕРАРХИЯ, ФРАКТАЛЬНОСТЬ И МЕТАЛЛОГЕНИЯ

As a result of the analysis of 3D-distributions of density contrast in the crust of the South Sikhote-Alin, connections are established between ore districts, ore clusters, and ore fields, on the one hand, and hierarchically subordinate structures of the central type of three taxonomic levels, on the other. Based on the coincidence of the areas of spatial distribution of ore mineralization with the projections of deep density heterogeneities, the probable depth of occurrence of fluid-magmatic sources of ore mineralization and vertical extent of ore-magmatic systems of different specialization were determined: tin (depth of 2‒6 km), tin-polymetallic (4‒12 km), polymetallic (6‒20 km), and gold-tungsten (14‒24 km). The spatial distribution of central-type structures and the associated areas of ore mineralization are coincident with viscous shear zones, within which subsidence of the crustal upper layer is observed (Samarkinsky terrane, Kavalerovsky district and Soboliny ore cluster), as well as displacement of the contours of density heterogeneities in different depth sections (Dalnegorsky region). Deep boundaries of the ancient continental crust and Mesozoic island-arc wedges subducted under accretionary-fold complexes were determined. The joint of these structures is characterized by distinct features of a transform fault.

Ring fault creep drives volcano-tectonic seismicity during caldera collapse of Kīlauea in 2018

Basaltic caldera collapses are episodic, producing very-long-period (VLP) earthquakes up to Mw 5.4, with prolific inter-collapse (between collapses) volcano-tectonic (VT) seismicity. During the 2018 caldera collapse of Kīlauea Volcano, VT seismicity ceased following each collapse, and then accelerated to a quasi-steady rate prior to the next collapse, marking a temporal pattern distinct from typical foreshock/aftershock sequences. There is currently no consensus on the mechanism(s) that generates the VT seismicity. Here we demonstrate that inter-collapse ring fault creep, induced by chamber depressurization, was the main driver of VT seismicity at Kīlauea in 2018. This is evidenced by: 1) the correlation between cumulative number of VT events and GNSS-derived ring fault creep; 2) agreement between repeating earthquake and GNSS derived creep rates; and 3) consistency between the time dependence of mechanically modeled, creep-driven seismicity and observations. We further show that, ring fault creep can be explained by velocity strengthening friction alone or in conjunction with viscous shear zone rheology. The simultaneous occurrence of creep and seismicity highlights the spatially heterogeneous velocity weakening/strengthening friction on the ring fault. If the VT seismicity-creep correlation can be replicated at other basaltic volcanoes, it would demonstrate that VT seismicity can be used as a proxy for ring fault creep in the absence of GNSS measurements on subsiding caldera block(s).

Numerical Simulations Reproduce Field Observations Showing Transient Weakening During Shear Zone Formation by Diffusional Hydrogen Influx and H2O Inflow

AbstractExposures on Holsnøy island (Bergen Arcs, Norway) indicate fluid infiltration through fractures into a dry, metastable granulite, which triggered a kinetically delayed eclogitization, a transient weakening during fluid‐rock interaction, and formation of shear zones that widened during shearing. It remains unclear whether the effects of grain boundary‐assisted aqueous fluid inflow on the duration of granulite hydration were influenced by a diffusional hydrogen influx accompanying the fluid inflow. To better estimate the fluid infiltration efficiencies and the parameter interdependencies, a 1D numerical model of a viscous shear zone is utilized and validated using measured mineral phase abundance distributions and H2O‐contents in nominally anhydrous minerals of the original granulite assemblage to constrain the hydration by aqueous fluid inflow and diffusional hydrogen influx, respectively. Both hydrations are described with a diffusion equation and affect the effective viscosity. Shear zone kinematics are constrained by the observed shear strain and thickness. The model fits the phase abundance and H2O‐content profiles if the effective hydrogen diffusivity is approximately one order of magnitude higher than the diffusivity for aqueous fluid inflow. The observed shear zone thickness is reproduced if the viscosity ratio between dry granulite and deforming, reequilibrating eclogite is ∼104 and that between dry granulite and hydrated granulite is ∼102. The results suggest shear velocities <10−2 cm/a, hydrogen diffusivities of ∼10−13±1 m2/s, and a shearing duration of <10 years. This study successfully links and validates field data to a shear zone model and highlights the importance of hydrogen diffusion for shear zone widening and eclogitization.

Surface faulting earthquake clustering controlled by fault and shear-zone interactions

Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.

Embrittlement Within Viscous Shear Zones Across the Base of the Subduction Thrust Seismogenic Zone

AbstractGeophysical observations indicate that patches of localized fracturing occur within otherwise viscous regions of subduction plate boundaries. These observations place uncertainty on the possible down‐dip extent of the seismogenic zone, and as a result the maximum magnitude of subduction thrust earthquakes. However, the processes controlling where and how localized fracturing occurs within otherwise viscous shear zones are unclear. We examined three exposures of exhumed plate boundary on Kyushu, Japan, which contain subducted sediments and hydrated oceanic crust deformed at ∼300 to ∼500°C. These exposures preserve subduction‐related viscous deformation, which in two of the studied exposures has a mutually overprinting relationship with quartz veins, indicating localized cyclical embrittlement. Where observed, fractures are commonly near lithological contacts that form viscosity contrasts. Mineral equilibrium calculations for a metabasalt composition indicate that exposures showing cyclical embrittlement deformed at pressure‐temperature conditions near dehydration reactions that consume prehnite and chlorite. In contrast, dominantly viscous deformation occurred at intervening pressure‐temperature conditions. We infer that at conditions close to metamorphic dehydration reactions, only small stress perturbations are required for transient embrittlement, driven by localized dehydration reactions reducing effective stress, and/or locally increased shear stresses along rheological contrasts. Our results show that the protolith composition of the subducting oceanic lithosphere controls the locations and magnitudes of dehydration reactions, and the viscosity of metamorphosed oceanic crust. Therefore, compositional variations might drive substantial variations in slip style.

“Ghost Transient” Corrections to the Southern California GPS Velocity Field from San Andreas Fault Seismic Cycle Models

Abstract Seismic cycle models of the northern and southern San Andreas fault (SAF) were developed to estimate corrections for viscoelastic transients embedded in the present-day Global Positioning System (GPS) velocity field. These models incorporate a viscous shear zone embedded in either a layered or a 3D viscoelastic structure. In the southern SAF model, the 3D effective viscosity structure is based on the Southern California Earthquake Center (SCEC) community rheology model (CRM) and the SCEC community thermal model. Viscoelastic materials are represented with Burgers or Maxwell viscoelastic rheologies, and ranges of rheological parameters and earthquake chronologies are explored. For the northern and southern SAF models, surface velocity perturbations or “ghost transients” (GTs) of up to 3.5–6 mm/yr are obtained for a wide range of conditions. In all cases, the GT velocities are consistent with left-lateral rather than right-lateral motion across the SAF, and subtracting them from the present-day GPS velocity field increases the apparent SAF offset rate. For the southern SAF model, GT velocities are fairly insensitive to the timing of large SAF earthquakes prior to 1857, but they are sensitive to rheology (i.e., temperatures and strain rates assumed for computing CRM effective viscosities). For the northern SAF model, GT velocities are sensitive to both rheology (mantle, lower crust and shear zone effective viscosities, and Burgers Body parameters Δb and RB) and mean earthquake recurrence interval. For the reference northern and southern SAF models provided to U.S. Geological Survey National Seismic Hazard Mapping Project deformation modelers, the maximum GT velocities are 3.73 and 4.84 mm/yr, respectively.

Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone

AbstractFluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ∼315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates build-up of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.

Influence of Shear Heating and Thermomechanical Coupling on Earthquake Sequences and the Brittle‐Ductile Transition

AbstractLocalized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.

An Explanation of Episodic Tremor and Slow Slip Constrained by Crack‐Seal Veins and Viscous Shear in Subduction Mélange

AbstractEpisodic tremor and slow slip (ETS) occurs in the transition zone between the locked seismogenic zone and the deeper, stably sliding zone. Actual mechanisms of ETS are enigmatic, caused by lack of geological observations and limited spatial resolution of geophysical information from the ETS source. We report that quartz‐filled, crack‐seal shear and extension veins in subduction mélange record repeated low‐angle thrust‐sense frictional sliding and tensile fracturing at near‐lithostatic fluid pressures. Crack‐seal veins were coeval with viscous shear zones that accommodated deformation by pressure solution creep. The minimum time interval between thrusting events, determined from a kinetic model of quartz precipitation in shear veins, was less than a few years. This short recurrence time of low‐angle brittle thrusting at near‐lithostatic fluid overpressures within viscous shear zones may be explained by frequent release of accumulated strain by ETS.

Influence of microscale weak zones on bulk strength

AbstractShear zones have different rheological properties than the surrounding rocks, indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the host rock rheology alone. We demonstrate the value of this concept at the microscale. We first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure. Rather, three‐dimensional weak zones, analogous to viscous shear zones, can interconnect or bridge weak phases. These zones typically form at high stress sites, comprise multiple minerals, and deform by mechanisms independent of those in the surrounding minerals. The presence of weak zones strongly affects the bulk strength of the rock, disproportionate to the mode of the weak zones. For example, the development of 1% mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude. Calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength. Instead, accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones. For this reason, we advocate use of the scale‐independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases. More generally, the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock.

Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?

Recent friction experiments carried out under upper crustal P–T conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults. These mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled experiments have been performed on carbonate gouges at seismic slip rates (1 m s−1), to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust. At relatively low displacements (<1 cm) and temperatures (≤100 °C), brittle fracturing and cataclasis produce shear localisation and grain size reduction in a thin slip zone (150 μm). With increasing displacement (up to 15 cm) and temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms start to accommodate intragranular strain in the slip zone, and play a key role in producing nanoscale subgrains (≤100 nm). With further displacement and temperature rise, the onset of weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low dislocation densities, possibly due to high temperature (>900 °C) grain boundary sliding (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those predicted by theoretical studies, and those observed during experiments on metals and fine-grained carbonates, where superplastic behaviour has been inferred. To a first approximation, the measured drop in strength is in agreement with our flow stress calculations, suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature.

Scaling of viscous shear zones with depth-dependent viscosity and power-law stress–strain-rate dependence

Scaling of viscous shear zones with depth-dependent viscosity and power-law stress–strain-rate dependence

Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

AbstractWe have developed a suite of earthquake cycle models for strike‐slip faults to investigate how finite ruptures and lithosphere‐scale viscous shear zones affect interseismic deformation. In particular, we assess whether localized and stationary interseismic deformation and large‐scale, rapidly decaying postseismic transients may be explained with models incorporating either or both of these features. Models incorporating viscous shear zones give more stationary interseismic deformation than layered half‐space, Maxwell viscoelastic models producing similar early postseismic deformation in the near field. This tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time. Models with finite (200 km long) ruptures produce time‐dependent deformation similar to that from models with infinitely long ruptures, though with smaller‐magnitude and somewhat more localized surface velocity fluctuations. Models incorporating a stiff lithosphere, a low‐viscosity mantle asthenosphere, and a crust‐ to lithosphere‐scale viscous shear zone can replicate postseismic and interseismic deformation typical of large, strike‐slip earthquakes. Such models require depth‐dependent, power law, or transient rheologies for the viscous shear zone material in the crust, as long as the effective viscosity per unit shear zone width is approximately 1015 Pa s/m at crustal depths during the postseismic interval. Below the Moho, the shear zone effective viscosity per unit width must be higher throughout the seismic cycle, at least over a short depth interval. For example, if shear zone viscosity per unit width is 5×1016 Pa s/m in the mantle lithosphere, a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5×1018 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.

On the effects of thermally weakened ductile shear zones on postseismic deformation

We present three‐dimensional (3‐D) numerical models of postseismic deformation following repeated earthquakes on a vertical strike‐slip fault. Our models use linear Maxwell, Burgers, and temperature‐dependent power law rheology for the lower crust and upper mantle. We consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate‐strengthening friction on a deep aseismic fault root. We find that the thermally activated shear zones have little effect on postseismic relaxation. In particular, the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. Stronger rheologies can give rise to an opposite polarity of vertical displacements, but the amplitude of the predicted transient deformation is generally negligible. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. We also investigate the discriminating power of models incorporating Burgers and power law rheology. These rheologies were proposed to explain postseismic transients following large (M7) earthquakes in the Mojave desert, Eastern California. Numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high‐quality geodetic observations for observation periods less than a decade. Longer observations, however, may potentially allow discrimination between the competing models, as illustrated by the model comparisons with available GPS and interferometric synthetic aperture radar data.

Simple shear is not so simple! Kinematics and shear senses in Newtonian viscous simple shear zones

AbstractThis work develops an analytical model of shear senses within an inclined ductile simple shear zone with parallel rigid boundaries and incompressible Newtonian viscous rheology. Taking account of gravity that tends to drive the material downdip and a possible pressure gradient that drives it upward along the shear zone, it is shown that (i) contradictory shear senses develop within two sub-zones even as a result of a single simple shear deformation; (ii) the highest velocity and least shear strain develop along the contact between the two sub-zones of reverse shear; (iii) for a uniform shear sense of the boundaries, a zone of reverse shear may develop within the top of the shear zone if the pressure gradient dominates the gravity component; otherwise it forms near the bottom boundary; (iv-a) a ‘pivot’ defined by the intersection between the velocity profile and the initial marker position distinguishes two sub-zones of opposite movement directions (not shear sense); (iv-b) a pivot inside any non-horizontal shear zone indicates a part of the zone that extrudes while the other subducts simultaneously; (v) the same shear sense develops: (v-a) when under a uniform shear of the boundaries, the shear zone remains horizontal and the pressure gradient vanishes; or alternatively (v-b) if the shear zone is inclined but the gravity component counterbalances the pressure gradient. Zones with shear sense reversal need to be reinterpreted since a pro-sheared sub-zone can retro-shear if the flow parameters change their magnitudes even though the same shear sense along the boundaries is maintained.

Continental margin deformation along the Andean subduction zone: Thermo-mechanical models

The Chilean Andes extend north–south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally at ∼100 km depth. This situation might explain the flat Wadati–Benioff zone of Central Chile.

Multiscale Brittle-Ductile Coupling and Genesis of Slow Earthquakes

We present the first attempt to explain slow earthquakes as cascading thermal-mechanical instabilities. To attain this goal we investigate brittle-ductile coupled thermal-mechanical simulation on vastly different time scales. The largest scale model consists of a cross section of a randomly perturbed elasto-visco- plastic continental lithosphere on the order of 100 9 100 km scale with no other initial structures. The smallest scale model investigates a km-scale subsection of the large model and has a local resolution of 40 9 40 m. The model is subject to a constant extension velocity applied on either side. We assume a free top surface and with a zero tangential stress along the other boundaries. Extension is driven by velocity boundary conditions of 1 cm/a applied on either side of the model. This is the simplest boundary condition, and makes it an ideal starting point for understanding the behavior of a natural system with multiscale brittle-ductile coupling. Localization feedback is observed as faulting in the brittle upper crust and ductile shearing in an elasto-viscoplastic lower crust. In this process brittle faulting may rupture at seismogenic rates, e.g., at 10 2 -10 3 ms -1 , whereas viscous shear zones propagate at much slower rates, up to 3 9 10 -9 ms -1 . This sharp contrast in the strain rates leads to complex short-time-scale interactions at the brittle-ductile transition. We exploit the multiscale capabilities from our new simulations for understanding the underlying thermo-mechanics, spanning vastly different, time- and length-scales.

A periodic shear-heating mechanism for intermediate-depth earthquakes in the mantle

Intermediate-depth earthquakes, at depths of 50-300 km in subduction zones, occur below the brittle-ductile transition, where high pressures render frictional failure unlikely. Their location approximately coincides with 600 to 800 degrees C isotherms in thermal models, suggesting a thermally activated mechanism for their origin. Some earthquakes may occur by frictional failure owing to high pore pressure that might result from metamorphic dehydration. Because some intermediate-depth earthquakes occur approximately 30 to 50 km below the palaeo-sea floor, however, the hydrous minerals required for the dehydration mechanism may not be present. Here we present an alternative mechanism to explain such earthquakes, involving the onset of highly localized viscous creep in pre-existing, fine-grained shear zones. Our numerical model uses olivine flow laws for a fine-grained, viscous shear zone in a coarse-grained, elastic half space, with initial temperatures from 600-800 degrees C and background strain rates of 10(-12) to 10(-15) s(-1). When shear heating becomes important, strain rate and temperature increase rapidly to over 1 s(-1) and 1,400 degrees C. The stress then drops dramatically, followed by low strain rates and cooling. Continued far-field deformation produces a quasi-periodic series of such instabilities.