Tectonic Shear Research Articles

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.

Discussion: Tectonic shear zones in the London Clay Formation

Discussion: Tectonic shear zones in the London Clay Formation

P - T paths and tectonic evolution of shear zones separating high-grade terrains from cratons: examples from Kola Peninsula (Russia) and Limpopo Region (South Africa)

¶The petrology and P-T evolution of mica schists from two regional scale tectonic (shear) zones that separate high grade terrains (“mobile belts”) from cratons are described. These are the 2.4–1.9 Ga Tanaelv Belt, a suture zone that separates the Lapland granulite complex from the Karelian craton (Kola Peninsula–Fennoscandia), and the 2.69 Ga Hout River Shear Zone that separates the > 2.9 Ga Kaapvaal craton from the 2.69 Ga South Marginal Zone of the Limpopo high-grade terrain (South Africa).

On the measurement of the undrained strength of discontinuities in the direct shear box

A series of unconsolidated, undrained shear box tests on a tectonic shear discontinuity in the London Clay Formation yielded unexpectedly high values of undrained strength. Measurement of the effective stress in the block samples prior to shear leads to the suggestion that the measured undrained strength on the discontinuity is controlled by the post-sampling mean effective stress, rather than the original in situ effective stress acting across the discontinuity. This observation has the result that in any soil where K0 ≠ 1·0, a rigid-boundary test apparatus (such as the shear box) that does not re-establish the in situ stresses cannot measure the correct undrained strength on a discontinuity. Keywords: clays; failure; laboratory tests; shear strength. Une série dessais utilisant une boîte de cisaillement non drané et non consolidé sur une discontinuité de cisaillement tectonique dans la formation dargile de Londres a donné des valeurs étonnamment élevées de résistance non drainée. Les mesures de la contrainte effective dans les échantillons blocs avant cisaillement laissent penser que la résistance non drainée mesurée sur la discontinuité est tributaire de la contrainte effective moyenne après échantillonnage plutôt que de la contrainte effectivein situ ´origine agissant à travers la discontinuité. Cette observation montre que dans tout sol où K0 ≠ 1.0, un appareil ´essai à limite rigide (comme la boîte de cisaillement) qui ne reproduit pas les contraintes in situ ne pent mesurer correctement la résistance non drainée sur une discontinuité.

The geometry of folded tectonic shear sense indicators

The geometry of refolding of early stretching lineations and associated sense of shear indicators is often not fully considered and can potentially lead to grossly incorrect tectonic interpretations of orogens and orogenic events. Sense of shear is not inverted or ‘flipped’ in all cases of refolding by folds with closure angle <90°. The geometry of inversion of ‘flipping’ of shear sense is determined entirely by the angle ( φ) between the stretching lineation and the latter fold axes. Folding around an axis orthogonal to the stretching lineation ( φ=90°) does not result in shear sense inversion. This is a special case; if there is any degree of obliquity ( φ<90°) between fold axis and stretching lineation, there is a change in the orientation of the stretching lineation on alternate limbs and if φ<45° the sense of shear is inverted in alternate limbs.

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.

Tectonic shear zones in the London Clay Formation

Subhorizontal shear zones observed in the London Clay Formation are described. The physical properties of the shear zones are examined, and it is demonstrated both that they are of tectonic origin, the result of bedding plane slip during minor folding, and that they were probably formed before there had been any significant erosion of the London Clay or later overlying formations. The occurrence of the shear zones is reviewed, and it is shown that similar features have been recorded in other areas to the west of London, and it seems likely that they will be encountered elsewhere in the London basin, wherever minor folding occurs. The shape of the London Clay Formation—Reading Formation interface appears to give an indication of the degree of folding, and hence of the possible occurrence of shear zones. The displacements of the shear zones examined are believed to be small, perhaps only a few centimetres, and consequently the strength along the shear zones is probably slightly above the true large-displacement residual, although it is suggested that the drained strength of the shear zone should be assumed to be close to the field residual strength. L'article décrit les zones de cisaillement subhorizontales observées dans la formation argileuse de Londres. Les auteurs examinent les pro-priétés physiques des zones de cisaillement et montrent qu'elles sont d'origine tectonique, étant le résultat d'un glissement intercouche survenu pendant un plissotement, et qu'elles se sont probablement formées avant toute érosion sensible de Pargile londonienne on de formations sus-jacentes ultérieures. Les auteurs examinent la venue des zones de cisaillement et montrent que des caracteristiques semblables ont été relevées dans d'autres régions à 1'ouest de Londres et qu'il est probable qu'on en trouvera d'autres dans toute autre zone de plissotement du bassin de Londres. La configuration de 1'interface entre la formation argileuse de Londres et la formation de Reading semble donner une indication du degré de plissement et, par conséquent, de la venue possible de zones de cisaillement. On pense que les déplacements des zones de cisaillement examinées sont faibles, pas plus de quelques centimétres peut-être. En conséquence, it et probable que la résistance le long des zones de cisaillement est légérement supérieure au véritable résidu des grands déplacements, bien que certaines indications permettent de supposer que la résistance drainée de la zone de cisaillement se rapproche de la résistance de champ résiduelle.

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.

Relationships between pore pressure, stresses, and present-day geodynamics in the Scotian Shelf, offshore eastern Canada : N. A. Yassir & J. S. Bell, American Association of Petroleum Geologists Bulletin, 78(12), 1994, pp 1863–1880

A geomechanical study of the Mesozoic and Tertiary sediments beneath the Scotian Shelf shows that two major weak layers exist at depth: the overpressured unit at 4000 m and the basal Argo salt, which is ductile and diapiric. A consistent relationship is observed between the overpressures and the stress state in the basin. First, the smaller horizontal principal stress increases dramatically just above the onset of overpressuring, and secondly, borehole breakouts--indicators of stress anisotropy--decrease in abundance within the overpressured zone. Neither tectonic shear nor rapid burial can explain these observations; however, overpressuring by fluid generation will result in horizontal stress increase and eventually lead to an isotropic stress state. Compelling evidence is found for hydrocarbon generation being a key cause of overpressuring in the Scotian Shelf. Present-day horizontal stress orientations in the sediments overlying the overpressured zone are aligned with contours of equal overpressure, but not with the traces of Tertiary listric normal faults or with the shelf edge. This implies that contemporary stresses in these sediments are causally related to overpressuring. The upward transmission of lithospheric plate movements and associated stress is unlikely because the ductile Argo salt effectively detaches the sediments from basement. Instead, the rocks of the Scotian Shelf are attempting to slide down the geomechanically weak overpressured zone in the directions indicated by the breakouts, but they are prevented from doing so by the bowl-shaped basement structure in the region.

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.

Raman spectra of coal-based graphite

Graphite formed in response to thermal contact metamorphism of coal bodies with magmatic intrusion is referred to as coal-based graphite. The first-order Raman spectrum of all the coal-based graphite taken from the Lutang area, Hunan Province exhibits a single Raman band near 1585cm-1, which comes directly from in-plane vibration of aromatic layer assigned to the E2g mode. Their Raman band of the structural defect in-plane can be divided into 2 types: one is the defect band (D peak) caused by the primary structural delect of the graphite in graphitization process, which is called D2-peak located at 1 360 cm-1; the other is the defect bank caused by the secondary structural defect in the graphite subject to tectonic shearing stress, which is called D1 peak located at 1370cm-1. The second Raman spectrum of the coal-based graphite shows three-dimensional lattice degree in the coal-based graphite. If the three-dimensional lattice of graphite is not well developed, it exhibits only a band of 2700cm-1; if the three-dimensional lattice of the coal-based graphite has been formed, the band of 2700cm-1 is split into two Raman bands, which are respectively located near 2681 cm-1 and near 2726cm-1, Raman band intensity ratio increases synchronously as the ordering degree of three-dimensional lattice increases regularly.

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.

Relationships Between Pore Pressure, Stresses, and Present-Day Geodynamics in the Scotian Shelf, Offshore Eastern Canada

A geomechanical study of the Mesozoic and Tertiary sediments beneath the Scotian Shelf shows that two major weak layers exist at depth: the overpressured unit at 4000 m and the basal Argo salt, which is ductile and diapiric. A consistent relationship is observed between the overpressures and the stress state in the basin. First, the smaller horizontal principal stress increases dramatically just above the onset of overpressuring, and secondly, borehole breakouts--indicators of stress anisotropy--decrease in abundance within the overpressured zone. Neither tectonic shear nor rapid burial can explain these observations; however, overpressuring by fluid generation will result in horizontal stress increase and eventually lead to an isotropic stress state. Compelling evidence is found for hydrocarbon generation being a key cause of overpressuring in the Scotian Shelf. Present-day horizontal stress orientations in the sediments overlying the overpressured zone are aligned with contours of equal overpressure, but not with the traces of Tertiary listric normal faults or with the shelf edge. This implies that contemporary stresses in these sediments are causally related to overpressuring. The upward transmission of lithospheric plate movements and associated stress is unlikely because the ductile Argo salt effectively detaches the sediments from basement. Instead, the rocks of the Scotian Shelf are attempting to slide down the geomechanically weak overpressured zone in the directions indicated by the breakouts, but they are prevented from doing so by the bowl-shaped basement structure in the region.

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.

Petrology and geochemistry of peridotites and associated vein rocks of Zabargad Island, Red Sea, Egypt

Zabargad (St. John's) Island in the Red Sea contains three ultramafic bodies, one of which bas produced the famous gem olivine (peridot). The ultramafic rock types consist of two major groups—the peridotites and the vein rocks within them. The peridotites are divided into three groups: primitive, depleted and metasomatized. The primitive peridotites are the most abundant and are represented by mainly pristine spinellherzolites which have chemical compositions representative of the subcontinental upper mantle. The depleted peridotites are mainly harzburgites and nome dunites and both are similar to worldwide occurrences. The most depleted peridotites also appear to have the greatest metasomatic additions of incompatible elements, as has been noted at other localities. Metasomatic additions were clearly accompanied by tectonic shearing. Metasomatism included infiltration of incompatible elements and the formation of porphyroblasts of clinopyroxene, amphibole, Al-spinel and plagioclase; il took place under a variety of p-T conditions and with fluids of differing compositions.

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.