Tectonic Shear Stress Research Articles

Precursory stress changes before large earthquakes; on a new physical law for earthquakes

In this article, we first review the progress in our understanding of tectonic stresses in the Earth's crust achieved in last three to four decades. We briefly review the literature on roughly four groups of studies; (1) World's Stress Map project (WSM), (2) fault-slip data and paleostress analyses, (3) Coulomb stress transfer, and (4) theories of the generalized continua with emphasis on the Cosserat continuum. Based on these studies, we develop a new numerical algorithm to calculate tectonic strain rate, tectonic stress, and normalized tectonic shear stress fields from the earthquake focal mechanisms. We show that this field changes over time, and is therefore far from being constant. Further on, we focus our efforts on the analysis of the relationship between the magnitudes of the earthquakes and the normalized tectonic shear stress. We discover a new and previously unknown physical law, which we call the maximum magnitude law (MM-law). We discuss further the origin and physical background of this law and its importance in earthquake studies.

Breather-type oscillations of the global tectonic shear stress fields

In this paper, two observations of the breather-type oscillating tectonic shear stress field in the Earth's crust are described. The normalized tectonic shear stress from the regional earthquake focal mechanisms is calculated by using the modified moment tensor summation method. The results show that the observed time-development of the normalized tectonic shear stress can be modelled as a breather, which is a spatially localized, time-periodic, oscillatory solution of the sine-Gordon equation. It represents the stress and strain fields, which periodically oscillate in time and decay in space as the distance from the center increases. Two case studies are described. The first was observed during the years 2017–2019 in South America, while the second was observed during the years 1977–1979 south of Japan. The first example is an almost ideal breather. The second example contains some natural noise, but can still be modelled as a breather.

Dual-Driven Fault Failure in the Lower Seismogenic Zone

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

The evolution and formation mechanisms of closed pores in coal

Closed pores, which have been confirmed to exist in coal, have a dual significance in illustrating the tremendous gas outburst content in coal mining and ascertaining the exact methane content in coal, although it is unknown if this methane can be exploited and utilized. In this paper, the principal objective is to explore the evolution and formation mechanisms of closed pores, regarding which there is a lack of knowledge and understanding. We investigated the varieties of closed pore contents in coalification, combining the small angle X-ray scattering (SAXS) and liquid nitrogen adsorption methods (LNA), and simultaneously analyzed the surface topography of closed pores by scanning electron microscopy (SEM). The closed pore volume represents a low-high-low evolution trend during coalification owing to the appearance of gas pores in the pyrogenic hydrocarbon generation stage. However, the specific surface area increases uniformly due to the increase in the number of micro-pores. The tectonic deformation effect impels the increase of closed pores, which reflected in the larger pore volume and specific surface area in strong deformation coal than that in weak deformation coal. The genesis of closed pores can be summarized as a result of heterogeneous matrix shrinkage, the tectonic compressive stress and the tectonic shear stress. Finally, this work provides a novel method to investigate the mechanism of geological CO2 sequestration.

Dynamics of the Salton block: Absolute fault strength and crust-mantle coupling in Southern California

Analysis of force and torque balance on the Salton block, Southern California, yields estimates of time-averaged shear stress acting on the bounding faults and the base of the crust. Averaged over a depth of 30 km, the San Andreas and San Jacinto faults sustain time-averaged shear stress of ;21-35 and 24-43 MPa, respectively. This implies that tectonic shear stress at seismogenic depths is greater than a typical 1-10 MPa earthquake stress drop and, with a corresponding effective friction coefficient of 0.10-0.21, lower than that predicted by laboratory experiments. Basal stress of 3-14 MPa also is required to drive the Salton block into the Transverse Ranges. Thus, the forces driving mountain building, basin formation, and the generation of earthquakes south of the Transverse Ranges in Southern California stem from stresses transmitted laterally across weak faults and from below.

The dependence of response spectrum on the tectonic ambient shear stress field

It has been analyzed the influence of the tectonic ambient shear stress value on response spectrum based on the previous theory. Based on the prediction equation BJF94 presented by the famous American researchers, CLB20, a new prediction formula is proposed by us, where it is introduced the influence of tectonic ambient shear stress value on response spectrum. BJF94 is the prediction equation, which mainly depends on strong ground motion data from western USA, while the prediction equation SEA99 is based on the strong ground motion data from extensional region all over the world. Comparing these two prediction equations in detail, it is found that after BJF94’s prediction value lg(Y) minus 0.16 logarithmic units, the value is very close to SEA99’s one. This case demonstrates that lg(Y) in extensional region is smaller; the differences of prediction equation are mainly owe to the differences of tectonic ambient shear stress value. If the factor of tectonic ambient shear stress value is included into the prediction equation, and the magnitude is used seismic moment magnitude to express, which is universal used around the world, and the distance is used the distance of fault project, which commonly used by many people, then regional differences of prediction equation will become much less, even vanish, and it can be constructed the universal prediction equation proper to all over the world. The error in the earthquake-resistant design in China will be small if we directly use the results of response spectrum of USA (e.g. BJF94 or SEA99).

Heterogeneous distribution of the dynamic source parameters of the 1999 Chi‐Chi, Taiwan, earthquake

The spatial and temporal distribution of the stress on the fault plane of the 1999 Chi‐Chi, Taiwan, earthquake is calculated from kinematic inversion results using a three‐dimensional finite difference method for solving the elastodynamic equations. We analyze the relations between stress and slip for all grid positions on the fault, and use these relations to infer the friction law for the rupture. The dynamic source parameters were also determined. Our results show that for most of the points on the fault, the relation between stress and slip was consistent with the slip‐weakening law during the rupture process, especially for those points with large slip. However, consistency with the velocity‐weakening law is not clear from the observed relation between stress and slip velocity. The distributions of the dynamic parameters on the fault are very heterogeneous. The peak value of the static stress drop is ∼35 MPa. In general, high stress drop occurred in the areas with large slip. The estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock. The slip‐weakening distance Dc and the fracture energy Gc are proportional to the final slip. The aftershock activity correlates with the spatial distribution of dynamic source parameters. Usually, the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop, and there were few aftershocks in regions which had large values of critical slip‐weakening distance Dc.

A model for fluid-injection-induced seismicity at the KTB, Germany

SUMMARY The 9.1 km deep KTB (Kontinentale Tiefbohrung, Germany) drilling hole is one of the best investigated deep-drilling sites in the world. Among other parameters, in situ measurements revealed continuous profiles of principal stresses, pore fluid pressure and fracture geometry in the vicinity of the borehole. The present study combines these parameters with hydraulic and seismicity data obtained during fluid-injection experiments conducted at the KTB to derive a conceptual model for fluid-injection-induced seismicity at the KTB. This model rests on the well constrained assumptions that (1) the crust is highly fractured with a permeable fracture network between 9 km depth and the Earth's surface and (2) the crust is in near-failure equilibrium, whereby a large number of fracture planes are under near-critical condition. During the injection experiment, the elevated pore fluid pressure remained well below the least principal stress and thus was too small to cause hydraulic opening of existing fractures. Consequently, the geometry of the fracture network was assumed to have not changed during fluid injection with induced seismicity occurring solely as a result of lowering of the effective normal stress, consistent with observed source mechanisms. The key parameter in the present model is the fracture permeability, which exhibits large spatial and directional variations. These variations are proposed to primarily control fluid migration paths and associated propagation of elevated fluid pressure during fluid injection. In contrast with common models based on isotropic fluid diffusion or spatially averaged permeability, highly permeable branches of the fracture network strongly affect the propagation of fluid pressure and prohibit the concept of a smooth ‘pressure front’. We find evidence that major fluid flow exists at comparatively low fluid pressure (below the critical pressure required to cause seismic failure) without being detected seismically. This might also explain the difference between 1011 J of hydraulic energy inserted into the system during fluid injection and ∼108 J of seismic energy: a major part of the hydraulic energy might be converted to potential energy of the ground water level caused by upward migrating fluid. From the fluid level response to changes of injection rate observed in a second borehole we estimate fluid signal velocities to be as large as 300 m d−1. Importantly, the suggested model also accounts for the occurrence of repeating earthquakes (multiplets), a large number of which were observed during the injection experiment. The present model also suggests that coseismic changes of the stress field caused by tectonic shear stress release are very local and of small magnitude. This is consistent with the observation that none of the larger induced events is followed by aftershock series that would be expected if coseismic processes had noticeably perturbed the local stress field.

A comparison of tectonic ambient shear stress value in China with that in western USA

A method is proposed to estimate average tectonic ambient shear stress value for a region. Thus the average stress values of 19 regions in western USA, and 43 regions (each region is 10°×10°) in Chinese mainland and its surroundings have been obtained. The data of 15 993 earthquakes are from the Internet Centroid Moment Tensor solution made by Harvard University from 1997 to 1999. The results demonstrate that there are highest average stress values in the regions of south California of USA and its off coast sea, reach to 12.0 MPa and 13.7 MPa respectively, then gradually decrease toward north, south, and east. The lowest value is 8.7 MPa and 63% of highest value. The average stress values in northern Xinjiang and in the Chayu region of Tibet are 17.2 and 12.9 MPa respectively. They are highest values in China and higher than USA’s. The average stress value in North China, Yunnan, Sichuan, Taiwan is similar to south California of USA. The average stress value in South-North Seismic zone is about 13 MPa, a little higher than south California. The distribution of average stress value for two important regions provides basic data for geology. These results are useful to research earthquake activity background and attenuation relation of strong ground motion parameters (e.g. peak acceleration and response spectra).

Tectonic significance of the cold seepage zones in the eastern Nankai accretionary wedge – an outcome of the 15 years’ KAIKO projects

The tectonic significance of the cold fluid venting in the eastern Nankai accretionary wedge has been elucidated based upon results of precise mapping and submersible observation executed since 1984 under the French–Japanese cooperative projects (KAIKO I, II, KAIKO-Nankai and KAIKO-Tokai) in conjunction with studies of the Japan Trench, Sagami Bay and other regions. The very sharp boundary of benthic communities associated with temperature anomalies indicates the occurrence of focused upflow of nutrient-rich fluids through cracks in the carbonate crust existing on a near-surface horizon of permeable sediment. The linear alignment of venting zones is a surface expression of the shallow detachment faults. Methane-rich fluids expelled from the conduit along a fault migrate in the overlying permeable sediment and react with circulating sea water containing sulfate to produce hydrogen sulfide and carbonate precipitate. En echelon or zigzag patterns of the cracks are developed in the carbonate crust under the influence of tectonic shear stress. It has been concluded that bivalve communities grow on a thin veneer of sediment above the cracks of carbonate through which fluid containing hydrogen sulfide diluted with sea water flows out. On the other hand, bacterial mats seem to flourish either on thin sediment overlying the fault conduit or over a thick cover of sediment without carbonate crust. In the former case, bivalves cannot grow because venting fluids are too dense with methane and hydrogen sulfide to maintain the oxygen-rich condition of the bottom water. In the western Nankai Trough fine-grained less permeable sediment cover seems to prevent circulation of fluids, whereas methane appears to be widespread in the deeper sedimentary layer there. Major distinctions in modes of seepage among sites are presumed to be caused by: (1) the tectonic regime causing channels of upward fluid venting; (2) rates and composition of venting fluids affected by mixing rates of sea water; and (3) the permeability and thickness of the near-bottom sediment.

Postglacial induced surface motion and gravity in Laurentia for uniform mantle with power-law rheology and ambient tectonic stress

Abstract Previous investigations of postglacial sea levels around Laurentia in a non-linear earth without ambient stress found that non-linear uniform mantle models cannot explain the relative sea level data around the ice margin. In this study, a 3D finite element model of postglacial readjustment is used to calculate the Earth’s response due to realistic ice and eustatic water loads on stratified mantle with non-linear rheology and ambient tectonic shear stress. By varying the creep parameter A* and the ambient stress level in a uniform mantle, it is found that the combination A* around 3×10−35 Pa−3 s−1 and ambient stress level around 10 MPa is able to explain the sea level data both outside and inside the Laurentian ice margin – although for certain sites inside, a slightly thicker ice is required to improve the fit to the data. Other geophysical and geodetic observables – namely uplift rate, horizontal velocity, free air gravity and the rate of change of gravity – have also been calculated for this model.

Some characteristics of the stress field of the 1995 Hyogo‐ken Nanbu (Kobe) earthquake

We investigate the characteristics of the stress field associated with the 1995 Hyogo‐ken Nanbu (Kobe) earthquake. We use for the study a tomographic model of the fault slip inferred from the numerous near‐field recordings, and we calculate the space and time evolution of shear stress on the fault during the earthquake. We show that the spatial distribution of stress drop is very heterogeneous. The apparent strength of the fault at the onset of the earthquake is low (of the order of 1 MPa or less), which indicates that the pre‐earthquake tectonic shear stress was close to the static friction over most of the fault. At many locations the stress drop rotates significantly during sliding. As was originally proposed by Spudich [1992], we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake. The pre‐earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 MPa. Average values of the initial and final shear stresses over the fault are 3.3 and 1.6 MPa, respectively, indicating that about half of the pre‐earthquake tectonic stress was released during the earthquake. There is a relatively strong correlation between the initial and final stress distributions, which suggests that intrinsic fault properties, not modified by the earthquake, control the spatial distribution of tectonic stress over the fault. We show that on average over the fault the dynamic friction coefficient is equal to about 40% of the static friction coefficient. Diagrams depicting the evolution of shear stress as a function of slip are consistent with slip‐weakening behavior, but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion.

Use of fault striations and dislocation models to infer tectonic shear stress during the 1995 Hyogo-Ken Nanbu (Kobe) earthquake

AbstractDislocation models of the 1995 Hyogo-ken Nanbu (Kobe) earthquake derived by Yoshida et al. (1996) show substantial changes in direction of slip with time at specific points on the Nojima and Rokko fault systems, as do striations we observed on exposures of the Nojima fault surface on Awaji Island. Spudich (1992) showed that the initial stress, that is, the shear traction on the fault before the earthquake origin time, can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points. From Yoshida's slip model, we calculated dynamic stress changes on the ruptured fault surfaces. To estimate errors, we compared the slip velocities and dynamic stress changes of several published models of the earthquake. The differences between these models had an exponential distribution, not gaussian. We developed a Bayesian method to estimate the probability density function (PDF) of initial stress from the striations and from Yoshida's slip model. Striations near Toshima and Hirabayashi give initial stresses of about 13 and 7 MPa, respectively. We obtained initial stresses of about 7 to 17 MPa at depths of 2 to 10 km on a subset of points on the Nojima and Rokko fault systems. Our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near Hirabayashi and Ogura on Awaji Island. Our results indicate that the Nojima fault slipped at very low shear stress, and fractional stress drop was complete near the surface and about 32% below depths of 2 km. Our results at depth depend on the accuracy of the rake rotations in Yoshida's model, which are probably correct on the Nojima fault but debatable on the Rokko fault. Our results imply that curved or cross-cutting fault striations can be formed in a single earthquake, contradicting a common assumption of structural geology.

Ambient tectonic shear stress field in Southern California and seismic hazard regions

According to the fracture mechanics rupture model of earthquakes put forward by us, several equations to compute tectonic ambient shear stress value τ0 have been derived [equations (1), (2), (3), (5)]. τ0 values for intermediate and small earthquakes occurred in Chinese mainland and Southern California have been calculated by use of these equations. The results demonstrate that the level and distribution of τ0 are closely related to the location where large earthquakes will occur, i.e. the region with higher level of τ0 will be prone to occur large earthquakes and the region with lower level will usually occur small earthquakes. According to the spatial distribution of τ0, the seismic hazard regions or the potential earthquake source regions can in some degree be determined. According to the variation of τ0 with time, the large earthquake occurrence time can be roughly estimated. According to the distribution of τ0 in Southern California and variation with time, three high stress level regions are determined, one (Goldfield area) of them is the present seismic hazard region.

Modelling of earthquake rupturing as a stochastic process and estimation of its distribution function from earthquake observations

SUMMARY The effect on earthquake rupturing of heterogeneities in tectonic stress and in material strength along a large fault zone is incorporated in the potential dynamic stress drop, defined as the difference between the tectonic shear stress and the dynamic frictional strength according to a slip-weakening model. The distribution of the potential dynamic stress drop Az,(x) along the strike of the fault plane is modelled as a 1-D stochastic process. Using a simple dynamic fracture criterion, a relation is established between earthquake rupturing and potential dynamic stress drop, by which any earthquake rupture process can be regarded as a segment of a realization of the process At&) where Azd(x) > 0. Since dynamic slip varies approximately linearly with dynamic stress drop, it has the same distribution function as Azd(x), provided that Azd(x) is a Gaussian process. Three independent earthquake observations, i.e. the average stress drop, the Gutenberg-Richter relation and the surface slip along earthquake faults, are used to estimate the distribution function of Azd(x). An analytical solution is derived for the distribution function of Azd(x), which shows that, among all known distribution models, only the fractional Brownian motion with index H -+ 0 (fractal dimension D = 2 in the 1-D case) can give rise to the observed approximately constant stress drop independent of earthquake size. The probability distribution of the size of zerosets of the fractional Brownian motion shows a power-law relation with frequency, which resembles the frequency-seismic-moment relation. Using an average b value of 1 .O for small earthquakes, an index H-+O of the fractional Brownian motion is obtained. The model predicts that the b value for large earthquakes is smaller than that for small earthquakes along the same fault zone, which is in agreement with observations. The surface slip data of two strike-slip-dominated earthquake faults with rupture lengths larger than 100 km are inverted using power spectral analysis. Both data sets display a power-law relation between the sample power spectrum and the spatial frequency, which implies a fractional Brownian distribution. The estimated index H is close to zero for both earthquake faults. Stress drops, b values, and surface slips all independently suggest that the earthquake rupturing process can be modelled stochastically as a fractional Brownian motion with index H -+ 0.

In search of potential earthquake source regions in the Chinese mainland in the light of ambient shear stress field

Earth media are incomplete media. There exist many cracks in it. The achievements of fracture mechanics show that the strength of the incomplete materials will be much lower than that of the complete materials. We consider that earthquake occurrence is the result of unstable propagation of a crack in crust media in proper condition and the earthquake rupture is the phenomenon of a failure by fast fracture under applied low shear stress. It has already been explained by fracture mechanics. The occurrence of failure by fast fracture is necessarily associated with the presence of high level concentration of local stress and strain. The elastic/plastic stress analysis in cracked pieces by Dugdale indicates that the state of stress at the tip of a crack takes a very important role to crack propagation. A plastic zone has necessarily formed in the tip of a crack due to stress concentration. Therefore, the dislocations at the tip of a crack are naturally a plastic displacement, rather than elastic one. The plastic displacementδ=πaτ 0 2 /2µτ y , whereτ 0 is applied shear stress which is equivalent to initial or tectonic shear stress when the quake occurs,a is the half length of a crack,μ is the rigidity,τ y is the yield stresses in shear. The main seismic dislocations take place exactly at the ends of the crack where the plastic zone had been formed. So, a critical assumption is adopted, i. e. we assume the dislocationD(ξ 1,t) as formula (5) in text. The maximum earthquake dislocationD max=πLτ 0 2 /4πτ y , whereL is the fault length. Ifμ is taken the value in the upper crust,μ=33 GPa; andτ y is taken the average value given from laboratories,τ y =30 MPa. Thus, according to observation values ofD max andL, using the formula, one can estimate the initial shear stresses for large earthquakes. Computations show that the initial shear stresses for large earthquakes all over the world are about 5–20 MPa which have some differences between regions. We further research the characteristics of source spectra and have derived the dependent relation of body wave magnitudem b on the shear stressτ 0 and seismic momentM 0 as formula (11) in text. Thus, the formula provides a possibility of computation of large amount of tectonic shear stress values from seismic data. We consider that the tectonic shear stress field is a main factor which controls the earthquake occurrence. The regions with high tectonic shear stress values are considered to be prone to occur great earthquakes (M s⩾6) and called earthquake hazard regions. Based on this criterion,τ 0 values for all earthquakes withm b⩾3.8 all over China since 1987 have been computed, and the great earthquake hazard regions with magnitude ranges have been zoned in the Chinese mainland. During April 1992–January 31, 1994, there were 9M s⩾6 earthquakes which occurred in the Chinese mainland, 8 earthquakes of the 9 had fallen into the regions delineated by us prior to the earthquake occurrence, with only one failure. This new approach as a method for medium-term prediction of strong earthquakes has been proved by practice to be an efficient one. It has good physical bases and bright prospect and worth further research.

Incremental Stress and Earthquakes

SUMMARY We use the Harvard catalogue of seismic moment tensor solutions to investigate the statistical properties of incremental static stress caused by earthquakes. Using Okada’s (1992) program for a point dislocation in a half-space, we compute normal and shear stress on nodal planes for each earthquake in the catalogue, as well as stress invariants in the focal zone of each event. These accumulated stress values are calculated at the location of any future reference earthquake (pre-stress) and then compared with the stress level measured at the same point after the event (post-stress). Comparison of the statistical distributions for pre- and post-stress indicates that the normal stress level has little influence on an earthquake occurrence. On the other hand, the shear stress in the focal zone of an event is significantly higher before an earthquake than after that earthquake (the stress caused by the reference earthquake is not taken into account in this comparison). Earthquakes are more likely to be induced by incremental shear stress if the stress and the moment tensor of an ensuing event are consistent, i.e. the incremental stress has the same sign as the seismic moment tensor of the reference earthquake. Similar results are obtained if we compare the first and second pre- and post-stress invariants: earthquake triggering is not influenced by the average normal stress (first invariant); contrary to that, the average shear stress (second deviatoric invariant) is significantly larger before an earthquake than after it. The distribution of hypocentres from the PDE catalogue confirms the above conclusions: the incremental stress caused by events in the Harvard list and measured at hypocentre locations of PDE events, is higher for the pre-stress shear component, but shows no significant difference for compressional/dilatational stresses. These findings, if interpreted in a typical framework of the Coulomb failure criterion, would suggest that the effective friction coefficient p is close to zero. Since this effect is observed in various seismotectonic regions for shallow and intermediate earthquakes (with a depth of as much as 300 km), the conventional explanation for the low value of p-high pore pressure-is less plausible. We conjecture that these results can be explained by the fractal pattern of earthquake fault geometry which is due to the fault selforganization in conditions of high lithostatic and tectonic shear stress.

Load-strengthening versus load-weakening faulting

Increases in shear stress (τ) along a fault during loading to failure cannot generally occur without changes in the normal stress across the fault (σ n). The fault loading parameter ( ∂δ′ n/ ∂τ = ∂δ n/ ∂τ − ∂P f/ ∂τ) distinguishes situations of load-strengthening ( ∂δ′ n/ ∂τ > 0), where the frictional shear strength of faults increases as tectonic shear stress rises, from load-weakening environments ( ∂δ′ n/ t6 τ < 0) where it decreases. Compressional faulting in tectonic regimes with δ v = δ 3 is always load-strengthening unless fluid pressure is rapidly increasing. Extensional faulting in regimes where δ v = δ 1 is load-weakening unless fluid pressure is dropping rapidly. Strike-slip faulting in terrains where δ v = δ 2 can be either load-weakening or load-strengthening. The particular case where ∂δ′ n/ ∂τ = 0, so that frictional shear strength stays constant during fault loading, is a very special situation corresponding to direct shear. Load-strengthening strike-slip faulting appears to correlate with tectonic transpression and load-weakening with transtension. Differing loading characteristics of faults in different tectonic regimes must induce varying patterns of cyclic fluid redistribution accompanying the seismic cycle, with implications for earthquake recurrence and precursory groundwater phenomena.

Dynamic fault rupture process during the 1978 Thessaloniki earthquake, northern Greece

Recent observations of strong earthquakes that occurred in the Aegean and surrounding areas during the last several years indicate rather low stress drop values. An interesting example is the 1978 Thessaloniki earthquake (northern Greece), with M s = 6.5. Values between 6.6 and 12 bars have been determined for this earthquake by assuming unilateral or bilateral rupture propagation and circular or rectangular fault models. These values are considerably lower than the average values calculated not only for intraplate earthquakes (about 60 bars) but also for interplate earthquakes (about 30 bars). In this paper an attempt is made to investigate the cause of this fact by studying the dynamic rupture process of this earthquake. A three-dimensional, spontaneous, shear crack model on a dipping fault in a layered medium was assumed, and a finite-difference approach has been adopted for numerically solving the wave equations in a three-dimensional space. It was found that such a low stress drop may be obtained in cases of small differences between ambient stress and fault strength, and between tectonic shear stress and dynamic frictional stress. Such physical conditions in the focal region may result in an average fault displacement which is close to the observed one for this earthquake.

Implications of fault-valve behaviour for rupture nucleation and recurrence

Sibson, R.H., 1992. Implications of fault-valve behaviour for rupture nucleation and recurrence. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophyslcs, 211: 283–293. Earthquakes in the shallow crust are generally thought to arise from the frictional instability of existing faults under shear stress. In simple recurrence models, failure on such faults is assumed to occur when tectonic shear stress (τ) rises to some constant critical level. However, frictional strength (τ f) may also vary substantially through the interseismic period, in which case recurrence intervals between successive earthquakes are related to the time-dependence of τ f as well as τ . In part, the manner in which τ f changes with τ depends on the coupling between normal stress and shear stress on the fault, and is related to the mode of faulting and the manner of fault loading. Time-dependence of cohesive strength and static friction may also play a role. However, fluid pressure cycling as a consequence of fault-valve behaviour (where the fault transects a suprahydrostatic gradient in fluid pressure) may give rise to very large variations in fault strength. Geological evidence suggests that value-action may be especially important in the lower regions of the seismogenic zone, where large ruptures tend to nucleate, the largest fluid pressure fluctuations being associated with faults that remain active as a consequence of fluid overpressure though severely misoriented for reactivation in the prevailing stress field. Extensive hydrofracture dilatancy is likely to develop prefailure in the vicinity of such faults. Changes in frictional strength over the interseismic period as a result of fault-valve activity (Δτ f) may greatly exceed the shear stress drop at failure (1 < Δτ < 10 MPa, typically). In such circumstances, recurrence intervals between successive events can be highly variable. A rich variety of recurrence behaviour becomes possible, depending on the relative magnitudes of Δτ f and Δτ, and the coupling between shear stress accumulation and changing fluid pressure levels through the interseismic period.