Postseismic Stress Relaxation Research Articles

Stress Heterogeneity as a Driver of Aseismic Slip During the 2011 Prague, Oklahoma Aftershock Sequence

AbstractThe interaction of aseismic and seismic slip before and after an earthquake is fundamental for both earthquake nucleation and postseismic stress relaxation. However, it can be difficult to determine where and when aseismic slip occurs within the seismogenic zone because geodetic techniques are limited to detecting moderate to large slip amplitudes or long duration small slip amplitudes. Here, we use repeating earthquakes (earthquakes that re‐rupture the same fault patch) as a proxy for aseismic slip during the 2011 Prague, Oklahoma earthquake sequence. We find that aseismic slip in the Prague earthquake sequence occurs both within the granitic basement and the overlying sedimentary rocks. The repeating earthquakes show that patches of aseismic slip are mostly located at fault intersections. These fault intersections hosted possible mainshock slip, abundant aftershocks, and afterslip. We estimate that ∼40% of the aftershocks are driven by afterslip. We interpret that aseismic slip occurs at fault intersections where stress heterogeneity creates patches of lower stress that are stable within a nonsteady state, rate‐state framework.

Effects of Earthquake Recurrence on Localization of Interseismic Deformation Around Locked Strike‐Slip Faults

AbstractLocalized geodetic deformation of arctan shape around locked strike‐slip faults is widely reported, but there are also important exceptions showing distributed deformation. Understanding the controlling mechanism is important to hazard assessment and geodynamic analysis. Here we use simple finite element viscoelastic earthquake cycle models to investigate the basic mechanics of this process. Our models feature a vertical strike‐slip fault in an elastic layer overlying a viscoelastic substrate of Maxwell or Burgers rheology, with or without a low‐viscosity shear zone representing deeper extension of the fault. We demonstrate that the primary control on the localization of interseismic deformation is the recurrence interval of past earthquakes. Given viscosity, shorter recurrence leads to greater localization, regardless of the rheological model used. The presence of a low‐viscosity deep fault does not change this conclusion, although it tends to lessen localization by promoting faster postseismic stress relaxation. Distributed deformation, although less reported, is a natural consequence of very long recurrence and in theory should be as common as localized deformation. We think that the apparent propensity of the latter is likely associated with the much greater quantity and better quality of geodetic observations from higher‐rate and shorter‐recurrence faults. Our results also show the important role of nearby earthquakes along the same fault. For faults of relatively short recurrence, frequent ruptures of nearby segments, modeled using a migrating rupture sequence, further enhance localization. For faults of very long recurrence, faster near‐fault deformation induced by a recent earthquake may give a false impression of localized interseismic deformation.

Patterns of the Seismic Cycle in the Kuril Island Arc from GPS Observations

The 2006–2007 Simushir earthquakes started a new episode in the rupture history of the Kuril-Kamchatka subduction zone. This earthquake doublet terminated the seismic silence in the Central Kurils, which had lasted for almost a century. The analysis of seismologic, geologic and GPS data over the Kuril Islands region allows us to reveal the patterns of deformation of the lithosphere in the Kuril subduction zone, in particular, the peculiarities of accumulation and relaxation of elastic stresses during the seismic cycle. We analyzed > 9 years of continuous GPS observations on the Kuril Islands to determine the surface displacements caused by the geodynamic processes in the subduction zone. We used available GPS data to model the coseismic stress release and postseismic stress relaxation related to the 2006–2007 Simushir earthquakes. We showed that the duration of postseismic relaxation after the 2006 earthquake exceeds 10 years, preventing transition of the central Kurils to the interseismic stage and thus delaying the beginning of the new seismic cycle. The value of Maxwell viscosity of the asthenosphere found in this study (3 × 1017 Pa s) in the Kuril Island arc is substantially lower than the values determined for several other subduction zones. This contrast in viscosity agrees with Kogan et al. (J Geophys Res 118:3691–3706, 2013), who used shorter GPS time series than in our study. The results of our analysis show that multidirectional motions along the Kurils Islands are explained by the fact that segments of the island arc are at different stages of the seismic cycle.

Broadscale postseismic deformation and lower crustal relaxation in the central Bayankala Block (central Tibetan Plateau) observed using InSAR data

We have generated a more than 500 km long postseismic deformation rate map and cumulative displacement time series in the central Bayankala Block of the Tibetan Plateau using ENVISAT/ASAR data from 2003 to 2010 by the π-RATE stacking algorithm. This rate map spans a period of ∼7.2 years and reveals that postseismic motion of 2001 Kokoxili earthquake exhibits a striking signal, dominating crustal deformation of the central Bayankala Block with a cross-fault magnitude ∼9–11 mm/yr in line of sight (LOS) (∼93.1°E). The southern and northern parts of the postseismic deformation field exhibit different patterns and variable magnitudes, reflecting asymmetry of the displacement distribution. Postseismic motion affects eastward extrusion of the central Bayankala Block, which reaches ∼15 km north of the Ganzi-Yushu fault. To further investigate viscoelastic relaxation of the lower crust in this area after approximately two years, E-M, E-M-M and E-S models are constructed. The result shows that the best fit viscosity for the lower crust is about 1 × 1019 Pa·s. Comparison between cumulative displacements resolved by these three models shows that viscoelastic relaxation of the lower crust makes the most significant contribution to postseismic stress relaxation after 2001 event.

Recurrence interval of the 2008 Mw 7.9 Wenchuan earthquake inferred from geodynamic modelling stress buildup and release

The destructive 2008 Mw 7.9 Wenchuan earthquake ruptured the Longmen Shan (LMS) fault zone and devastated cities in Sichuan province, China. Estimates of the recurrence interval of the large earthquake is important to understand the feature of seismic activity and to analyze the seismic hazard in the fault area. In this research we introduce the method of geodynamic modelling to estimate the recurrence interval of the Mw7.9 earthquake based on the basic physics of earthquakes-stress buildup and release on fault. The inter-seismic stress accumulation prior to the 2008 Wenchuan earthquake is extracted from our previous study, which developed 3D finite element visco-elastic lithospheric model of the LMS fault zone. The co-seismic stress release and the post-seismic stress relaxation of the earthquake are simulated through dislocation source models. The recurrence interval, which is the duration needed to accumulate the magnitude of the stress drop of the Mw7.9 earthquake, is estimated to be about 4200–6500 years. Sensitivities of the estimated recurrence interval relying on model dependent parameters, such as the viscosity and slip models, are discussed. This research provides a preferred method to estimate recurrence interval of large earthquake in fault zone.

Open Fissure Folds record coseismic loading and postseismic stress relaxation

Open Fissure Folds hosted by high pressure/low temperature metamorphic rocks of south Evia (Greece) are introduced, their structural and microstructural record is analysed, and a mechanical model is proposed. Open Fissure Folds are preserved as at least two parallel folded quartz–feldspar veins separated by narrow buckled rock columns. The veins originated as tensile cracks that propagated in the middle crust driven by high differential stress. Features diagnostic for Open Fissure Folds indicate that the rock columns represented the layers of high viscosity, and not the veins as consistently reported in many previous studies on folded veins. This record is taken to indicate that buckling of the rock columns initiated after arrest of the fractures and terminated prior to complete vein sealing. Accordingly, mechanical decoupling by open fissures allowed for buckling of the rock columns in response to episodic creep of the host rocks according to stress relaxation, as expected for postseismic deformation in the earthquake cycle. I propose that the parental fractures propagated in response to quasi-instantaneous coseismic loading of the middle crust. Buckling was attributed to transient postseismic creep and stress relaxation. Complete sealing of the veins occurred when stresses were largely relaxed. Each Open Fissure Fold records the stress and strain history of a single earthquake.

Physics-Based 3-D Simulation for Earthquake Generation Cycles at Plate Interfaces in Subduction Zones

The generation of interplate earthquakes can be regarded as a process of tectonic stress accumulation and release, driven by relative plate motion. We completed a physics-based simulation system for earthquake generation cycles at plate interfaces in the Japan region, where the Pacific plate is descending beneath the North American and Philippine Sea plates, and the Philippine Sea plate is descending beneath the North American and Eurasian plates. The system is composed of a quasi-static tectonic loading model and a dynamic rupture propagation model, developed on a realistic 3-D plate interface model. The driving force of the system is relative plate motion. In the quasi-static tectonic loading model, mechanical interaction at plate interfaces is rationally represented by the increase of tangential displacement discontinuity (fault slip) across them on the basis of dislocation theory for an elastic surface layer overlying Maxwell-type viscoelastic half-space. In the dynamic rupture propagation model, stress changes due to fault slip motion on non-planar plate interfaces are evaluated with the boundary integral equation method. The progress of seismic (dynamic) or aseismic (quasi-static) fault slip on plate interfaces is governed by a slip- and time-dependent fault constitutive law. As an example, we numerically simulated earthquake generation cycles at the source region of the 1968 Tokachi-oki earthquake on the North American-Pacific plate interface. From the numerical simulation, we can see that postseismic stress relaxation in the asthenosphere accelerates stress accumulation in the source region. When the stress state of the source region is close to a critical level, dynamic rupture is rapidly accelerated and develops over the whole source region. When the stress state is much lower than the critical level, the rupture is not accelerated. This means that the stress state realized by interseismic tectonic loading essentially controls the subsequent dynamic rupture process.

Cyclic ductile and brittle deformation related to coseismic thrust fault propagation: Structural record at the base of a basement nappe (Preveli, Crete)

AbstractThe structural record at the base of a basement nappe (Preveli nappe, Crete, Greece) thrust upon sedimentary rocks is investigated, aimed on understanding mechanisms which result in decoupling of the thrust sheet from its original substratum. We identify several superimposed deformation stages, each with characteristic structural style and indications of episodic deformation at initially high differential stress. The final stage involves formation of a matrix supported breccia transected by pseudotachylytes, comprising the lowermost 30 m of the nappe. Brecciation and pseudotachylyte formation occurred in a single event, and structures were not modified afterward. Complete induration of breccia and composition of phengite crystallized during devitrification of pseudotachylytes place the sequence of events into the middle crust. We propose a model relating episodic deformation and cyclic stress history to propagation of a thrust fault in a limited number of seismic events. Terminal brecciation and frictional fusion record passage of the fault front beneath the site of observation and decoupling of the thrust sheet. Absence of discernible further deformation is consistent with negligible basal friction during transport as a nappe. Brecciation and pseudotachylyte formation mark the switch from a history of repeated coseismic loading and postseismic stress relaxation in the plastosphere, driven by seismic events on the approaching thrust fault, to passive transport with deformation localized in a weak thrust plane. For a sequence of superimposed ductile to brittle structures, our model provides an alternative to progressive cooling and exhumation concomitant with deformation over millions of years.

Amphibolite-facies pseudotachylytes in Premosello metagabbro and felsic mylonites (Ivrea Zone, Italy)

Pseudotachylytes (solidified friction-induced melts produced during seismic slip) that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle–ductile (elasto-frictional/viscous-plastic) transition in the crust. Thin, sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite-facies Premosello metagabbro and inside country felsic mylonites of the Ivrea Zone (Italian Southern Alps). Petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies (500–650°C) metamorphic conditions. The occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites. The association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients, related to downward propagation of seismic ruptures from the upper brittle crust, followed by post-seismic stress relaxation by ductile flow.

Lower crustal relaxation beneath the Tibetan Plateau and Qaidam Basin following the 2001 Kokoxili earthquake

SUMMARY In 2001 November a magnitude 7.8 earthquake ruptured a 400 km long portion of the Kunlun fault, northeastern Tibet. In this study, we analyse over five years of post-seismic geodetic data and interpret the observed surface deformation in terms of stress relaxation in the thick Tibetan lower crust. We model GPS time-series (first year) and InSAR line of sight measurements (years two to five) and infer that the most likely mechanism of post-seismic stress relaxation is time-dependent distributed creep of viscoelastic material in the lower crust. Since a single relaxation time is not sufficient to model the observed deformation, viscous flow is modelled by a lower crustal Burgers rheology, which has two material relaxation times. The optimum model has a transient viscosity 9 × 1017 Pa s, steady-state viscosity 1 × 1019 Pa s and a ratio of long term to Maxwell shear modulus of 2:3. This model gives a good fit to GPS stations south of the Kunlun Fault, while displacements at stations north of the fault are over-predicted. We attribute this asymmetry in the GPS residual to lateral heterogeneity in rheological structure across the southern margin of the Qaidam Basin, with thinner crust/higher viscosities beneath the basin than beneath the Tibetan Plateau. Deep afterslip localized in a shear zone beneath the fault rupture gives a reasonable match to the observed InSAR data, but the slip model does not fit the earlier GPS data well. We conclude that while some localized afterslip likely occurred during the early post-seismic phase, the bulk of the observed deformation signal is due to viscous flow in the lower crust. To investigate regional variability in rheological structure, we also analyse post-seismic displacements following the 1997 Manyi earthquake that occurred 250 km west of the Kokoxili rupture. We find that viscoelastic properties are the same as for the Kokoxili area except for the transient viscosity, which is 5 × 1017 Pa s. The viscosities estimated for the Manyi and Kokoxili areas are consistent with constraints obtained from other earthquakes in the northwest and south central parts of the Tibetan Plateau.

Complex states of stress during the normal faulting seismic cycle: Role of midcrustal postseismic creep

Numerical models are used to analyze coseismic and postseismic change in the state of stress near the lower tip of a normal fault directly below the brittle‐ductile transition. The models are compared with information obtained from the geological record of exhumed metamorphic rocks regarding the magnitude and the geometry of coseismic stress increase close to the lower fault termination and the mechanisms involved during postseismic stress relaxation. The numerical results predict a drop in differential and fault‐parallel shear stress in the upper crust and a stress increase in the lower crust due to the tapering off in fault slip. The coseismic deformation field in the crust causes significant deflection of the principal stresses from the horizontal and the vertical. During the postseismic period, the recovery of the crustal stress field remains incomplete, even for long recurrence intervals. Instead, the stress field evolves in self‐similar cycles, controlled by repeated earthquake activity. The numerical models show that postseismic viscous creep localized beneath the fault contributes to the recovery of all stress tensor components in the upper crust close to the fault. This result implies that a detailed understanding of the processes and conditions active during postseismic stress relaxation in the middle and lower crust is essential in order to estimate reloading rates in the upper crust surrounding the fault.

Post-seismic stress relaxation with a linear transient rheology

We performed an analysis of post-seismic stress relaxation, taking into account generalized linear rheologies. We compared the stress field (and its derived functions) obtained with a classical Maxwell rheology with that obtained with a transient Burgers body. From a set of synthetic case studies, we have revealed quantitative and qualitative differences both in relaxation times and in local stress values when a transient rheology is introduced. As a practical application, we modeled the time evolution of the Coulomb failure function following the 2009 L'Aquila earthquake, and we show that a transient rheology can lead to non-monotonic time dependence.

Postseismic Relaxation Along the San Andreas Fault at Parkfield from Continuous Seismological Observations

Seismic velocity changes and nonvolcanic tremor activity in the Parkfield area in California reveal that large earthquakes induce long-term perturbations of crustal properties in the San Andreas fault zone. The 2003 San Simeon and 2004 Parkfield earthquakes both reduced seismic velocities that were measured from correlations of the ambient seismic noise and induced an increased nonvolcanic tremor activity along the San Andreas fault. After the Parkfield earthquake, velocity reduction and nonvolcanic tremor activity remained elevated for more than 3 years and decayed over time, similarly to afterslip derived from GPS (Global Positioning System) measurements. These observations suggest that the seismic velocity changes are related to co-seismic damage in the shallow layers and to deep co-seismic stress change and postseismic stress relaxation within the San Andreas fault zone.

Coupled stress and pore fluid pressure changes in the middle crust: Vein record of coseismic loading and postseismic stress relaxation

The structural, microstructural, and fluid inclusion record of late‐stage quartz veins in metamorphic rocks of southern Evia (Greece) is analyzed and a mechanical model is proposed. The veins are monogenetic. They developed in a single stage from tensile cracks at temperatures similar to 300°C. The orientation of the veins is uniform on the kilometer scale and indicates an extensional tectonic regime, with σ1 = σv (σv denoting the overburden stress). The length to aperture ratio of the veins is incompatible with an elastic distortion and requires ductile deformation of the host rock after fracture arrest, with crack parallel shortening by less than 2%. The fluid inclusion densities as a function of position reveal an increase in pore fluid pressure Pf during progressive sealing. Fracture mechanics suggests that Pf in an open fissure should be similar to the least principal stress, Pf ≈ σ3. For the given crustal stress regime, σ1 = σv can be taken as constant, as vein formation takes place within a sufficiently short time span, for which the overburden does not change significantly. If so, the observed increase in Pf during progressive opening and sealing of the veins reflects a decrease in differential stress, as expected for postseismic deformation in the earthquake cycle. We propose that the parental fractures formed in consequence to coseismic loading of the middle crust near the tip of an active fault in the upper crust. Each vein, which developed from such a fracture during postseismic stress relaxation, records the stress and Pf history of a single seismic cycle and the strain accumulated during postseismic creep.

Vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust

Discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 °C in an extensional tectonic regime. The vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude. Non-elastic ductile deformation of the host rock must have controlled widening of the arrested cracks. Fracture parallel shortening is low. Due to the low accumulated strain, the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock. Information is exclusively obtained from the microfabrics of the vein sealing quartz. There, the density of geometrically necessary dislocations, derived from orientation maps created by electron backscatter diffractometry (EBSD), decreases markedly from the vein wall to the centre. The gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing. Early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein. The gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing. In view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins, the monogenetic veins are interpreted to record a single mid-crustal stress transient, related to the earthquake cycle. Fracturing is proposed to be a consequence of coseismic loading, while progressive opening and sealing record the deformation during postseismic stress relaxation by creep.

Simulating coseismic deformation of quartz in the middle crust and fabric evolution during postseismic stress relaxation — An experimental study

Non-steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake-driven episodic deformation in the middle crust. Three types of experiments were carried out using a modified Griggs-type solid medium deformation apparatus. All three start with high stress deformation at a temperature of 400 °C and a constant strain rate of 10 − 4 s − 1 (type A), some are followed by annealing in the stability field of α-quartz for 14–15 h at zero nominal differential stress and temperatures of 800–1000 °C (type A + B), or by annealing for 15 h at 900 °C and at a residual stress (type A + C). The quartz samples reveal a very high strength > 2 GPa at a few percent of permanent strain. The microstructures after short-term high stress deformation (type A) record localized brittle and plastic deformation. Statisc annealing (type A + B) results in recrystallisation restricted to the highly damaged zones. The new grains aligned in strings and without crystallographic preferred orientation, indicate nucleation and growth. Annealing at non-hydrostatic conditions (type A + C) results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation. In this case, however, the recrystallised zone is several grain diameters wide, the grains are elongate, and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation. Quartz microstructures identical to those produced in type A + B experiments are observed in cores recovered from Long Valley Exploratory Well in the Quaternary Long Valley Caldera, California, with considerable seismic activity. The experiments demonstrate the behaviour of quartz at coseismic loading (type A) and subsequent static annealing (type A + B) or creep at decaying stress (type A + C) in the middle crust. The experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks.

New insights into the mechanism of postseismic stress relaxation exemplified by the 23 June 2001 Mw = 8.4 earthquake in southern Peru

The 2001 Mw = 8.4 southern Peru subduction earthquake marked the beginning of a transient postseismic surface motion in direction of the coseismic displacement at the permanent GPS station Arequipa. In general this motion is assigned to afterslip. Our hypothesis is that the observed transient signal can be explained by stress relaxation processes in the overriding plate. We use a 2D finite element model incorporating non‐linear viscoelastic Maxwell rheology. Our model results indicate that coseismically induced stresses are relieved by viscoelastic stress relaxation in the lower crust. The trenchward directed creep motion is transferred to the upper crust due to elastic coupling leading there to an instantaneous relief of elastic stresses. In contrast to the existing conceptual models for stress relaxation, which incorporate shear stresses, we conclude that tensional elastic stresses throughout the crust and upper mantle are the main driving forces for the transient GPS signal.

Morpho-structural evolution of the Cordón Caulle geothermal region, Southern Volcanic Zone, Chile: Insights from gravity and 40Ar / 39Ar dating

The Cordón Caulle geothermal region (40.5°S) is a 13 km × 6 km, NW-SE elongate, volcano-tectonic depression hosting numerous monogenetic volcanic centers, fumaroles, and hot-springs, most of which occur spatially associated with the edges of the depression. Volcanism started at approximately 0.3 Ma, with dominantly mafic effusions, and expressed ultimately in post-glacial and historic dacite–rhyolite eruptions (1921–1922 and 1960). Three NE-trending gravimetric profiles were carried out across the depression of Cordón Caulle, yielding a residual negative anomaly with the greatest amplitude (− 20 mGal) at the center of the depression. A 2D-gravity model given by a 490 m thick shallow body with a density contrast = − 650 kg/m 3, interpreted as a shallow package of dacite–rhyolite lavas and tephra, was found to match the residual anomaly. A dense rock equivalent volume of felsic material on the order of 28 km 3 (25% of the total output of Cordón Caulle) is derived from this thickness. Previous workers explain the origin of felsic rocks at Cordón Caulle in terms of the orientation of the borders of Cordón Caulle relative to the maximum stress direction σ 1 (i.e. NE-trending) prevailing during the Quaternary, according to which NW-trending domains were compressional, favoring stagnation, storage and differentiation of magmas. The dominantly monogenetic style of volcanism, the plateau-like morphology and the negative Bouguer anomaly, on the contrary, are interpreted as an indication of extensional tectonics. Intra-arc extension is proposed to be related to coseismic or postseismic stress relaxation accompanying major thrust events, such as the great 1960 Chilean earthquake, which is consistent, at Cordón Caulle, with the direction of propagation of dikes of the 1960 eruption (i.e. coseismic extension) and present-day subsidence inferred from InSAR data (i.e. postseismic extension). Prior to the Quaternary (8.2–1.6 Ma), nearly east-west compression prevailed at the intra-arc zone. A 5.44 ± 0.04 Ma old granodiorite intrusion bracketed by two sub-parallel branches of the regional, margin-parallel Liquiñe–Ofqui Fault Zone (LOFZ) in a prominent pop-up structure, 15 km to the east of Cordón Caulle, accounts for reverse reactivation of the LOFZ and rapid fault-related exhumation of basement rocks (> 1 mm/year). We argue that under compression, reverse reactivation of the LOFZ was facilitated by the combined effect of cyclic increase in fluid pressure induced by magmatic–hydrothermal input, and removal of lithostatic load induced by glacial erosion (> 1.5 mm/year in the vicinity of Cordón Caulle).

Deformation mechanism maps for feldspar rocks

Deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates. The maps display constant temperature contours in stress-grain size space for strain rates ranging from 10 −16 to 10 −12 s −1. Two fields of dominance of grain boundary diffusion-controlled creep and dislocation creep are separated by a strongly grain size-sensitive transition zone. For wet rocks, diffusion-controlled creep dominates below a grain size of about 0.1–1 mm, depending on temperature, stress, strain rate and feldspar composition. Plagioclase aggregates containing up to 0.3 wt.% water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks. The strength of water-bearing feldspar rocks is moderately dependent on composition and water fugacity. For a grain size range of about 10–50 μm commonly observed in natural ultramylonites, the deformation maps predict that diffusion-controlled creep is dominant at greenschist to granulite facies conditions. Low viscosity estimates of 10 18–10 19 Pa·s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures >900 °C or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high strain shear zones. For similar thermodynamic conditions and grain size, lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks.

A Physical Basis for Time Clustering of Large Earthquakes

We develop a theory that links stress interaction between earthquakes and the occurrence of temporal clustering. Coseismic static stress change in the vicinity (50 km) of large earthquakes suggests that perturbations of 0.1 to 1 bars may affect the occurrence of other earthquakes. At larger distances, interactions also seem to exist: four M 8 earthquakes have occurred in Mongolia on distant faults (400 km) during the last century. Also, paleoseismic observations documenting much longer time periods display a time clustering of major events. We demonstrate with simple mechanical concepts that postseismic stress relaxation magnifies the coseismic stress change and has a major effect on fault interaction during the seismic cycle. In the simple case where two distant faults are coupled, the probabilistic occurrence of triggered earthquakes may increase dramatically due to long range postseismic coupling.