chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

thermomechanical evolution of the crust during convergence and deep crustal pluton emplacement in the western province of fiordland new zealand 1 fiordland new zealand contains exposures of high pressure p 12 14 kbar granulite facies rocks that form one of earth s largest exposed lower crustal roots of an early cretaceous magmatic arc these exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system we present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust the imbricate thrust zones form part of a well exposed 12 km wide two sided fold thrust belt that preferentially developed in crust that was thermally softened by magmatism changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the western fiordland orthogneiss record thrust and pluton emplacement conditions of p 7 9 kbar paleodepths of 25 30 km these data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs this mechanism produced a characteristic up pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide

interseismic strain accumulation and the earthquake potential on the southern san andreas fault system the southern section of the san andreas fault is the only part of the fault that hasn t ruptured in historic times for this reason it is believed to pose the largest seismic risk in california although whether tectonic stress is actually accumulating there is a topic of heated debate using data from radar interferometry satellites yuri fialko has produced a detailed map of surface deformation in the area that shows that elastic stress is accumulating at a fairly high rate during the past 300 years a displacement of 6 to 9 metres has built up this suggests that the southern san andreas fault is nearing the end of the interseismic phase of the earthquake cycle and that increased seismic activity of some kind can be expected the san andreas fault in california is a mature continental transform fault that accommodates a significant fraction of motion between the north american and pacific plates the two most recent great earthquakes on this fault ruptured its northern and central sections in 1906 and 1857 respectively the southern section of the fault however has not produced a great earthquake in historic times for at least 250 years assuming the average slip rate of a few centimetres per year typical of the rest of the san andreas fault the minimum amount of slip deficit accrued on the southern section is of the order of 7 10 metres comparable to the maximum co seismic offset ever documented on the fault1 2 here i present high resolution measurements of interseismic deformation across the southern san andreas fault system using a well populated catalogue of space borne synthetic aperture radar data the data reveal a nearly equal partitioning of deformation between the southern san andreas and san jacinto faults with a pronounced asymmetry in strain accumulation with respect to the geologically mapped fault traces the observed strain rates confirm that the southern section of the san andreas fault may be approaching the end of the interseismic phase of the earthquake cycle

variability in the greenland sea as revealed by a repeated high spatial resolution conductivity temperature depth survey this paper deals with the mesoscale variability of water masses in the greenland sea and its implications for bottom water formation the results are based on two conductivity temperature depth surveys performed in successive years the same section in the central greenland basin was sampled in 1989 and 1990 an additional transect across fram strait was carried out in 1990 the transects extended from shelf to shelf with a station spacing of 18 km throughout the data set reveals a surprisingly strong horizontal variability of space scales between 20 and 60 km in the entire greenland sea not only in the frontal zones the area investigated is subdivided into four hydrographical regimes for which mesoscale variability is discussed in detail a noteworthy result is the major change of the deep sea thermal structure within 1 year the classical pattern with upward doming cold waters in the central basin was found in 1989 but was replaced by a capped structure with a warm intermediate layer in 1990 the implications of the observed changes are discussed with respect to deep water formation a mechanism based on differential compressibility is proposed which is able to introduce negative heat input selectively into the bottom layer it is shown that the fine structure of temperature profiles observed in summer can be used as a tracer for the occurrence of deep convection during the preceding winter convective depths are concluded of about 2200 m for 1989 and of only about 250 m for 1990

laboratory earthquakes along inhomogeneous faults directionality and supershear we report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch rupture is always found to be asymmetric bilateral in one direction rupture always propagates at the generalized rayleigh wave speed whereas in the opposite direction it is subshear or it transitions to supershear the lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in parkfield california and in north anatolia

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

variations of the velocity contrast and rupture properties of m6 earthquakes along the parkfield section of the san andreas fault summary we investigate the seismic velocity contrast across the san andreas fault saf in the parkfield area using fault zone head waves fzhw that propagate along the bimaterial fault interface and direct p waves we systematically analyse large data sets of near fault waveforms recorded by several seismic networks over the period 1984 2005 clear fzhw are observed at many stations on the ne side of the fault in the creeping section of the saf north of middle mountain mm this indicates the presence of a sharp bimaterial interface and that the ne side of the fault has lower seismic velocities in that region the obtained p wave velocity contrast is about 5 10 per cent north of mm and it systematically decreases to 0 2 per cent near gold hill gh the along strike variations of the velocity contrast are consistent with geological observations of a sliver of high velocity rock immediately to the ne of the saf near gh associated with the gh fault and existing 3 d seismic tomography results the obtained imaging results offer an explanation for the mixed rupture directions of the m6 type parkfield earthquakes the strong velocity contrast around mm is expected to produce a preferred propagation direction to the se for earthquakes that nucleate near mm e g the 1934 and 1966 parkfield earthquakes in contrast the near zero velocity contrast and multiple fault branches near gh imply that earthquakes that nucleate near gh e g the 2004 parkfield earthquake are not expected to have a preferred propagation direction to the se and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities the observed systematic reduction of the velocity contrast along the saf from nw of mm to se of gh provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the parkfield section and propagate to the se and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the nw

rupture directivity of small earthquakes at parkfield 1 theoretical modeling of strike slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction we test this model with 450 small earthquakes 2 3 exhibiting southeast rupture characteristics some spatial and temporal variability in rupture directivity is also apparent we observe a higher proportion of northwest directivity ruptures following the 2004 m 6 parkfield earthquake which ruptured toward the northwest our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors

numerical simulation of microseismicity induced by deep longwall coal mining abstract to find an effective numerical simulation method for microseismicity induced by deep mining activity results obtained by using four simulation methods namely 1 the energy release rate method based on cook s study 2 the strain energy release rate method developed by the authors 3 the volume excess shear stress index method based on spottiswoode s study and 4 the maximum shear seismic moment release rate method developed here by the authors were compared with the microseismicity in a japanese deep longwall coal mine the maximum shear seismic moment release rate method was discovered to be the best method since the intensity of microseismicity in a longwall coal panel its variation with the face advance and the distribution of microseismic events were well simulated

importance of bimaterial interfaces for earthquake dynamics and strong ground motion summary we perform a numerical investigation of dynamic ruptures on a bimaterial interface in 3 d with regularized slip weakening friction and a heterogeneous initial shear stress and discuss the resulting strong ground motion to isolate effects introduced by the material contrast we perform pairs of simulations with opposite material orientations we show that for many parameter sets the dynamics of rupture propagation are significantly influenced by the broken symmetry due to the material contrast during rupture propagation the resulting slip histories of two events with reversed orientations of the material contrast can deviate such that the emanating waves lead to large differences in peak ground motion peak ground velocity and peak ground acceleration even when slip distribution of the individual events are very similar and therefore their moment magnitudes are basically identical we also show that the wrinkle like slip pulse specific to the bimaterial mechanism can be nucleated naturally from an initially crack like mode of rupture when the initial stress allows for large propagation distances once such a pulse has been nucleated it travels at a dominant propagation speed close to the generalized rayleigh velocity the dynamic weakening of the fault due to the normal stress alteration during slip allows nucleating ruptures to overcome asperities of low initial shear stress in the preferred direction which is the direction of displacement on the seismically slower side of the fault in such situations the orientation of the material contrast determines rupture extent and therefore the size of the earthquake potentially by orders of magnitude

aftershock asymmetry on a bimaterial interface to better understand the asymmetric distribution of microearthquake aftershocks along the central san andreas fault we study dynamic models of slip weakening ruptures on an interface separating differing elastic half spaces subshear ruptures grow as slightly asymmetric bilateral cracks with larger propagation velocities slip velocities and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed the southeast front in the context of the san andreas when the se front encounters a stress barrier the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse this slip pulse smooths the stress field and reduces the static stress change beyond the se front furthermore because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon the se rupture front can be left far below the failure threshold while the nw front remains quite close to failure both mechanisms could contribute to the observed aftershock asymmetry formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip weakening law to achieve this while minimizing off fault damage requires either substantial velocity contrasts or small reductions in friction the simulations also show a pronounced asymmetry in the timescales over which barriers to the se and nw experience increasing stresses a result that has implications for the asymmetric distribution of subevents in compound earthquakes

structural boundaries and biotite and garnet isograds in the otago and alpine schists new zealand the otago and alpine schist belts of southern new zealand have traditionally been treated as structurally continuous metamorphic belts with minor modification by brittle faulting mapping of biotite and garnet isograds has been hindered by rock types unfavourable for index mineral growth closer examination of well exposed boundaries between metamorphic zones shows that they juxtapose rocks of different type and structural history apparent structural continuity across these zones is due to development of a locally pervasive boundary parallel foliation on both sides of the boundary in a broad boundary zone up to 2 km wide this feature has implications for mapping and metamorphic petrology in other metamorphic belts where structural continuity has traditionally been assumed true metamorphic isograds may be rare and metamorphic zones may more commonly represent structural slices of complex tectonically disrupted metamorphic piles

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

asymmetry in elastic properties and the evolution of large continental strike slip faults 1 we use geodetic studies to quantify several cases of significant asymmetry in interseismic and coseismic effects along large continental strike slip faults using simple two dimensional edge dislocation models we first show that asymmetric elastic loading characterizes the present main marmara fault a portion of the north anatolian fault along the northern margin of the sea of marmara the ratio of asymmetry there is about 10 this ratio is even larger about 30 along the northern sumatra fault near lake toba caldera we then examine two profiles near point reyes and point arena across the northern san andreas fault that have been previously proposed as affected by asymmetry both in interseismic and coseismic effects we show that an asymmetry ratio of 1 6 in interseismic loading exists near point arena with the southwest side of the fault being more rigid than the northeast one on the other hand we do not find significant asymmetry for the point reyes profile that was previously described as highly asymmetric we examine coseismic motion during the 1906 earthquake along the same two profiles ratios of 1 2 and 1 7 are found for the point arena and point reyes profiles respectively we discuss the possible causes of asymmetry contrasts in seismic velocity in the brittle portion suggest ratios generally not exceeding 2 5 for the dynamic rigidity in the upper brittle section larger ratios may involve other complex causes such as differences between static and dynamic rigidities contrasts in rheology in the deeper creeping sections and postseismic transients we conclude that asymmetry should be systematically included within the parameters to be inverted when dealing with the mechanics of large scale strike slip faults

role of nappe boundaries in subduction related regional deformation spatial variation of meso and microstructures in the seba eclogite unit the sambagawa belt sw japan abstract the seba eclogite unit of the sambagawa high p t metamorphic belt has a kilometre scale synformal structure and its base is the boundary between an overlying eclogite nappe and an underlying non eclogitic nappe the formation of the nappe boundary is related to a phase of strong penetrative ductile deformation d a spatial variations of meso and microstructures of schistose eclogite in the seba unit reveal that there is i a decreasing intensity of d a and ii a time dependent widening of the d a shear zone away from the nappe boundary these features indicate that the nappe boundary behaved as a core zone during the d a phase and furthermore that development of the regional deformation d a is strongly controlled by the presence of the nappe boundary a new model is proposed where each phase of regional deformation in subduction type metamorphic belts is related to ancient activity of a particular nappe boundary in this model a single phase of deformation may be related to exhumation in one nappe and subduction in an adjacent one

an efficient numerical method for earthquake cycles in heterogeneous media alternating subbasin and surface rupturing events on faults crossing a sedimentary basin we present an efficient numerical method for earthquake cycle simulations that employs a finite difference discretization of the off fault material to accommodate spatially variable elastic properties the method is developed for the two dimensional antiplane shear problem of a vertical strike slip fault with rate and state friction we compare earthquake cycles in a homogeneous half space with those in which the upper portion of the fault cuts through a sedimentary basin in both cases we assume velocity weakening behavior over the full seismogenic depth even in the basin to isolate the influence of elastic heterogeneity in a homogeneous half space events rupturing the full seismogenic depth occur periodically event sequences are more complex in basin models with one or several subbasin events confined to the lower section of the fault followed by a much larger surface rupturing event that breaks through the basin this phenomenology emerges only for sufficiently compliant and deep basins predicted surface velocities are essentially identical before subbasin events and surface rupturing events suggesting that geodetic observations would not be useful in predicting the rupture mode the alternating sequence of subbasin and surface rupturing events would complicate interpretation of paleoseismic data our results also offer one potential explanation for the shallow slip deficit observed in many recent earthquakes namely that these events which lack appreciable surface slip are simply one style of rupture subsequent events on these faults might be larger with slip extending all the way to the surface the 1940 mw 7 0 and 1979 mw 6 5 imperial valley events might be considered as examples of these two rupture styles

quasi dynamic versus fully dynamic simulations of earthquakes and aseismic slip with and without enhanced coseismic weakening physics based numerical simulations of earthquakes and slow slip coupled with field observations and laboratory experiments can in principle be used to determine fault properties and potential fault behaviors because of the computational cost of simulating inertial wave mediated effects their representation is often simplified the quasi dynamic qd approach approximately accounts for inertial effects through a radiation damping term we compare qd and fully dynamic fd simulations by exploring the long term behavior of rate and state fault models with and without additional weakening during seismic slip the models incorporate a velocity strengthening vs patch in a velocity weakening vw zone to consider rupture interaction with a slip inhibiting heterogeneity without additional weakening the qd and fd approaches generate qualitatively similar slip patterns with quantitative differences such as slower slip velocities and rupture speeds during earthquakes and more propensity for rupture arrest at the vs patch in the qd cases simulations with additional coseismic weakening produce qualitatively different patterns of earthquakes with near periodic pulse like events in the fd simulations and much larger crack like events accompanied by smaller events in the qd simulations this is because the fd simulations with additional weakening allow earthquake rupture to propagate at a much lower level of prestress than the qd simulations the resulting much larger ruptures in the qd simulations are more likely to propagate through the vs patch unlike for the cases with no additional weakening overall the qd approach should be used with caution as the qd simulation results could drastically differ from the true response of the physical model considered

prediction of coal face rockbursts and microseismicity in deep longwall coal mining a new method is proposed for predicting rockbursts as well as microseismicity in deep longwall coal mining the method is able to predict failure in the coal seam and in the rock mass by using 3 d elastic numerical stress analyses these locations were compared with the results of microseismic monitoring fracturing intensity and its variation with respect to face advance were represented by the maximum shear seismic moment release rate the seismic moment method was applied to three cases in the horonai coal mine and in the miike coal mine japan the first case considered panels w8 5u and ws 5 in the horonai coal mine at 1055 and 1125m deep even though no rockburst occurred areas where microseismic events were concentrated were predicted it was also clear by comparing calculated maximum shear seismic moment release rate to observed data that the magnitude of the seismicity and its variation with respect to the face advance were also successfully predicted the second case corresponded to panel w6 3 in the same mine from 915 to 985m deep microseismic monitoring had not yet been introduced however the rockburst in this panel was successfully predicted as fracturing of coal seam elements at the coal face the last case considered panels e3 e6 in the miike coal mine at 600 and 650 m deep respectively the location and intensity of microseismicity were again predicted in addition toppling as well as rockbursts were also predicted relations between the extent of the relief zone ahead of the coal face and the occurrence of rockbursts are discussed an effective method to prevent rockburst hazard is also proposed

geological and geomorphologic asymmetry across the rupture zones of the 1943 and 1944 earthquakes on the north anatolian fault possible signals for preferred earthquake propagation direction summary the east and west rupture directions of the 1943 and 1944 earthquakes on the north anatolian fault naf are hypothesized to represent respectively long term preferred propagation directions on the corresponding sections of the naf fault sections with preferred rupture direction are expected to have an asymmetric damage structure with respect to the slipping zone to test the above hypothesis we study geological and geomorphologic manifestations of structural asymmetry with respect to the active trace of the naf along the 1943 and 1944 sections the following fault zone elements are mapped gouge fabric in the cm scale fault core structure in a metre scale and secondary faults and fault rocks in tens of metres scale mapping results at three sites on the 1943 rupture and one site on the 1944 rupture are consistent with accumulation of more rock damage on the south side of the 1943 section and on the north side of the 1944 section erosion patterns adjacent to the fault that are not correlated with the distribution of intrinsic and extrinsic erosion controlling variables e g rock type are interpreted as morphologic responses to the damage content of rocks and its impact on rock erodibility the valleys of 11 rivers are parallel to the studied fault sections about 75 per cent of the total river valleys length along the 1943 rupture is on the south side of the fault and about 89 per cent of the total river valleys length along the 1944 rupture is on the north side of the fault morphometric analysis of watersheds in two correlative terrains displaced along the 1944 rupture section shows that stream erosion is considerably more intense in the terrain north of the fault with drainage density values almost double in the north compare to the south badland topography at two sites along the 1943 rupture section is substantially more developed at the 100 m scale on the south side of fault our observations along the 1943 1944 rupture sections including various types of signals that span a large range of scales are systematically compatible with an opposite sense of damage asymmetry between the two fault sections these observations are consistent with opposite preferred direction of ruptures for the two sections similar to the propagation directions of the two recent earthquakes if those rupture directions are dictated by the velocity structure at depth we infer that the south side of the 1943 rupture has faster seismic velocity at seismogenic depth than the north side and that the sense of velocity contrast is reversed along the 1944 rupture zone

influence of plastic deformation on bimaterial fault rupture directivity 1 material juxtapositions across mature faults are a common occurrence previous work has found that this elastic mismatch results in a rupture that will preferentially propagate in the direction of slip displacement on the more compliant side of the fault with more off fault damage in the stiffer material this result has implications for inferring preferred rupture directions based on observations of damage zone asymmetry we perform a complete numerical investigation of the role of the stress state on the distribution of plastic deformation and the direction of preferred rupture propagation we show that there are important factors in addition to the elastic mismatch which control the preferred direction of propagation as well as the side of the fault in which damage predominately accumulates the orientation of the most compressive principal stress is the controlling factor in determining the location of plastic deformation for different orientations plastic deformation can accumulate in either the stiffer or the more compliant material for high angles of most compressive stress the aforementioned preferred rupture direction prediction holds true however the off fault plastic response can reverse that direction for low angles of most compressive stress so that rupture will preferentially propagate in the direction of slip displacement in the stiffer material

40ar 39ar thermochronologic constraints on deformation metamorphism and cooling exhumation of a mesozoic accretionary wedge otago schist new zealand abstract structural thickening of the torlesse accretionary wedge via juxtaposition of arc derived greywackes caples terrane and quartzo feldspathic greywackes torlesse terrane at 120 ma formed a belt of schist otago schist with distinct mica fabrics defining i schistosity ii transposition layering and iii crenulation cleavage thirty five 40ar 39ar step heating experiments on these micas and whole rock micaceous fabrics from the otago schist have shown that the main metamorphism and deformation occurred between 160 and 140 ma recorded in the low grade flanks through 120 ma shear zone deformation this was followed either by very gradual cooling or no cooling until about 110 ma with some form of extensional tectonic exhumation and cooling of the high grade metamorphic core between 109 and 100 ma major shear zones separating the low grade and high grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo tectonic histories apparent ages on the low grade north flank hanging wall to the hyde macraes and rise and shine shear zones range from 145 to 159 ma n 8 whereas on the low grade south flank hanging wall to the remarkables shear zone or caples terrane range from 144 to 156 ma n 5 most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica several of the hanging wall samples with ages of 144 147 ma show no evidence for detrital contamination in thin section or in the form of the age spectra apparent ages from the high grade metamorphic core garnet biotite albite zone range from 131 to 106 ma n 13 with a strong grouping 113 109 ma n 7 in the immediate footwall to the major remarkables shear zone most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica 430 330 c samples with apparent ages of about 110 109 ma tend to give concordant plateaux suggesting more rapid cooling the youngest and most disturbed age spectra come from within the alpine chlorite overprint zone where samples with strong development of crenulation cleavage gave ages 85 107 and 101 ma due to partial resetting during retrogression the bounding remarkables shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127 122 ma whereas overprinting shear zones within the core of the schist show apparent ages of 112 109 and 106 ma these data when linked with extensional exhumation of high grade rocks in other parts of new zealand indicate that the east gondwana margin underwent significant extension in the 110 90 ma period

bimaterial effects in an earthquake cycle model using rate and state friction we have developed a computational framework to study earthquake cycles in 2 d plane strain and apply it to faults separating dissimilar material we consider a planar strike slip fault governed by rate and state friction where quasi dynamic events nucleate spontaneously due to remote tectonic loading we investigate the influence of material contrast over the course of many hundreds of years for the parameters we consider we find that the presence of bimaterial properties influences the earthquake nucleation site such that rupture in the preferred direction that is in the direction of particle motion of the side of the fault with lower shear wave velocity is favorable for large values of the critical slip distance dc events propagating in the preferred rupture direction occur for a wide range of material contrasts for smaller values of dc small events emerge even in the monomaterial case with material mismatch present some of these small events propagate in the nonpreferred direction made possible by a favorable stress distribution left on the fault from previous ruptures these results may shed light on our understanding of rupture directivity on large strike slip faults like the san andreas fault in california which occasionally host events rupturing in the nonpreferred direction

material contrast does not predict earthquake rupture propagation direction 1 earthquakes often occur on faults that juxtapose different rocks the result is rupture behavior that differs from that of an earthquake occurring on a fault in a homogeneous material previous 2d numerical simulations have studied simple cases of earthquake rupture propagation where there is a material contrast across a fault and have come to two different conclusions 1 earthquake rupture propagation direction can be predicted from the material contrast and 2 earthquake rupture propagation direction cannot be predicted from the material contrast in this paper we provide observational evidence from 70 years of earthquakes at parkfield ca and new 3d numerical simulations both the observations and the numerical simulations demonstrate that earthquake rupture propagation direction is unlikely to be predictable on the basis of a material contrast

effects of a low velocity zone on a dynamic rupture dynamic crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous tomographic and near fault seismic studies indicate that the crust near faults is instead comprised of rocks of varying material velocities we have tested the effects of adding material velocity variation to simulations of spontaneously propagating earthquakes we used two dimensional plane strain conditions coupled with a slip weakening fracture criterion and examined earthquakes on faults that bisect finite width low velocity zones embedded in country rock and earthquakes on faults that bound two different velocity materials when a fault bisects a low velocity zone the normal stress remains unchanged but both the rupture velocity and slip velocity pulse shape are perturbed the presence of the low velocity zone induces high frequency oscillations in the slip function near the rupture front when the fault is on the edge of the low velocity zone the oscillations are more pronounced and repeated sticking and slipping can occur near the rupture front for the slip weakening velocity independent friction model however the temporary sticking does not lead to permanent arrest of slip and slip duration is still controlled by the overall rupture dimension when an earthquake ruptures a fault juxtaposing a lower velocity material against a higher velocity material the normal stress across the fault near the crack tip is perturbed the sign of the normal stress perturbation depends on the direction of rupture leading in some cases to a directional dependence of rupture velocity when slip is accompanied by stress reduction a positive feedback develops between the normal and shear stress changes as previously noted by andrews and ben zion 1997 resulting in an apparently unavoidable grid size dependence in computation of stress change near the rupture front numerical experiments indicate however that the rupture velocity is insensitive to this zone size dependence which is highly localized immediately behind the crack tip the factors controlling the rupture velocity in the simulations including directional dependence are further elucidated by a new analytical solution for rupture of an asperity on a frictionless interface

incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography application at parkfield california 1 we present a three dimensional 3d p wave velocity vp model of the parkfield region that utilizes existing p wave arrival time data including fault zone head waves fzhws and data from direct wave secondary arrivals dwsas the first arrival and dwsa travel times are obtained as the global and local minimum travel time paths respectively the inclusion of fzhws and dwsas results in as much as a 5 and a 10 increase in the across fault velocity contrast respectively for the vp model at parkfield relative to that of thurber et al 2006 viewed along strike three pronounced velocity contrast regions are observed a pair of strong positive velocity contrasts sw fast one nw of the 1966 parkfield earthquake hypocenter and the other se of the 2004 parkfield earthquake hypocenter and a strong negative velocity contrast ne fast between the two hypocenters the negative velocity contrast partially to entirely encompasses peak coseismic slip estimated in several slip models for the 2004 earthquake suggesting that the negative velocity contrast played a part in defining the rupture patch of the 2004 parkfield earthquake following ampuero and ben zion 2008 the pattern of velocity contrasts is consistent with the observed bilateral rupture propagation for the 2004 parkfield earthquake although the velocity contrasts also suggest bilateral rupture propagation for the 1966 parkfield earthquake the fault is creeping to the nw here i e exhibiting velocity strengthening behavior thus it is not surprising that rupture propagated only se during this event

seismic monitoring and analysis of excessive gas emissions in heterogeneous coal seams abstract uncontrolled and excessive gas emissions pose a serious threat to safety in underground coal mining in a recently completed research project a suite of monitoring techniques were employed to assess the dynamic response of the coal seam being mined to longwall face advance at coal mine velenje in slovenia together with continuous monitoring of gas emissions two seismic tomography measurement campaigns and a microseismic monitoring programme were implemented at one longwall top coal caving panel over 2000 microseismic events were recorded during a period of four months over the same period there also was a recorded episode of relatively high gas emission in the same longwall district in this paper a detailed analysis of the processed microseismic data collected during the same monitoring period is presented specifically the analysis includes the spatial distribution of the microseismic events with respect to the longwall face advance the magnitude of the energy released per week and its temporal evolution examination of the spatial distribution of the recorded microseismic events has shown that most of the microseismic activity occurred ahead of the advancing face furthermore the analysis of the gas emission and microseismic monitoring data has suggested that there is a direct correlation between microseismicity and gas emission rate and that gas emission rate tends to reach a peak when seismic energy increases dramatically it is believed that localised stress concentration over a relatively strong xylite rich zone and its eventual failure which was also identified by the seismic tomography measurements may have triggered the heightened microseismic activity and the excessive gas emission episode experienced at the longwall panel monitored

repeating earthquakes from rupture of an asperity under an antarctic outlet glacier the inner regions of the antarctic continent are generally regarded as nearly aseismic although microseismicity is known to occur beneath some outlet ice streams related to the interaction between the fast flowing ice and the bedrock here we show the occurrence of unusual earthquakes beneath an antarctic outlet glacier that share almost the same magnitude pointing to the repeated rupture of a single asperity these seismic events produce waveforms with very high similarity and uncommon spectrum and are tightly clustered in space but unlike other reported instances of repeating earthquakes on a patch of the san andreas fault they occur in frequent irregular swarms evidence locates these events at the rock ice interface under the glacier and shows the existence of stick slip motion on a smaller scale than the large slow slip events detected by global seismographs seismic behaviour of large glaciers can presumably be connected to surges in ice motion this study determines a little known environment for fracture dynamics studies while also contributing to the understanding of the coupling processes between fast flowing glaciers and bedrock that influence ice stream evolution and stability

aftershock asymmetry rupture directivity among central san andreas fault microearthquakes using a waveform cross correlation technique we have obtained precise relative locations for nearly 75 of the northern california seismic network catalog 4300 earthquakes occurring between 1984 and 1997 along 50 km of the san andreas fault errors in relative location are meters to tens of meters for events separated by tens to hundreds of meters we find that consecutive earthquakes in the relocated catalog occur no closer than a distance approximately equal to the radius of the first rupture as estimated from the moment magnitude relationship of abercrombie 1996 assuming a 10 mpa stress drop when the relative position vectors between consecutive events are normalized by this distance and projected onto the fault surface they define a hole whose shape suggests that typical microearthquakes are elongate in the mode ii slip parallel direction by several tens of percent moreover of the 100 immediate aftershocks occurring closest to the mode ii edges of the prior rupture more than twice as many occur to the northwest than to the southeast we interpret this asymmetry as resulting from the large contrast in material properties across the fault models of dynamic rupture between dissimilar media predict that ruptures in this region may run preferentially to the southeast in the direction of motion of the lower velocity material if so then the barriers that stop rupture fronts moving to the southeast should initially be farther from failure on average than the barriers that stop rupture fronts moving to the northwest once the rupture stops the induced stress change is more symmetric but the fault remains farther from failure on average to the southeast this interpretation receives some support from pulse width measurements on a localized set of 72 magnitude 0 6 to 3 6 earthquakes

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

nucleation and early seismic propagation of small and large events in a crustal earthquake model 1 earthquake nucleation and early seismic propagation are studied in a two dimensional strike slip fault model with depth variable properties the fault is governed by the dieterich ruina rate and state friction law we use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by lapusta et al 2000 we find that for decreasing values of the characteristic slip distance of the friction law small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone small and large events have very similar nucleation phases in our simulations here by nucleation phase we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic seismically detectable event moment acceleration to which velocity seismograms are proportional in early stages of seismic propagation exhibits irregular fluctuations in the form of speedups and slowdowns in the moment release rate consistently with observations as reported by ellsworth and beroza 1995 our simulations show that such irregular moment acceleration can at least in part be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity strengthening fault regions which inhibit seismic slip index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 3220 mathematical geophysics nonlinear dynamics 7230 seismology seismicity and seismotectonics 3230 mathematical geophysics numerical solutions 8164 tectonophysics stresses crust and lithosphere keywords earthquake nucleation phase event clustering irregular moment release stress concentrations rate and state friction earthquake sequences

rupture dynamics and ground motion from 3 d rough fault simulations 1 we perform three dimensional 3 d numerical calculations of dynamic rupture along non planar faults to study the effects of fault roughness on rupture propagation and resultant ground motion the fault roughness model follows a self similar fractal distribution over length scales spanning three orders of magnitude from 102 to 105 m the fault is governed by a strongly rate weakening friction and the bulk material is subject to drucker prager viscoplasticity fault roughness promotes the development of self healing rupture pulses and a heterogeneous distribution of fault slip at the free surface and at depth the inelastic deformation generated by the large dynamic stress near rupture fronts occurs in a narrow volume around the fault with heterogeneous thickness correlated to local roughness slopes inelastic deformation near the free surface however is induced by the stress waves originated from dynamic rupture at depth and spreads to large distances 10 km away from the fault the present simulations model seismic wave excitation up to 10 hz with rupture lengths of 100 km permitting comparisons with empirical studies of ground motion intensity measures of engineering interest characteristics of site averaged synthetic response spectra including the distance and period dependence of the median values absolute level and intra event standard deviation are comparable to appropriate empirical estimates throughout the period range 0 1 3 0 s this class of model may provide a viable representation of the ground motion excitation process over a wide frequency range in a large spatial domain with potential applications to the numerical prediction of source and path specific effects on earthquake ground motion

comment on material contrast does not predict earthquake rupture propagation direction by r a harris and s m day 1 harris and day 2005 hereinafter referred to as hd concluded from some numerical simulations and low resolution analysis of 8 m4 m6 earthquakes near parkfield ca that material contrast in fault zone structures is not important for earthquake propagation direction the limited theoretical and observational results of hd do not justify such a general conclusion considerations of a larger body of results suggest that material contrast may produce a statistically preferred direction of earthquake ruptures the inferred diversity of the 8 discussed events is consistent with their small number and relatively small size in addition the behavior has a natural explanation in a structure with two material interfaces the san andreas fault saf and the southwest fracture zone sfz on the opposite sides of a deformation damage region large earthquakes propagating predominantly as mode ii ruptures and affected primarily by properties of the bounding crustal blocks rather than the local structure are likely to exhibit a smaller amount of diversity

seismic imaging of a bimaterial interface along the hayward fault ca with fault zone head waves and direct p arrivals we observe fault zone head waves fzhw that are generated by and propagate along a roughly 80 km section of the hayward fault in the san francisco bay area moveout values between the arrival times of fzhw and direct p waves are used to obtain average p wave velocity contrasts across different sections of the fault the results are based on waveforms generated by more than 5 800 earthquakes and recorded at up to 12 stations of the berkeley digital seismic network bdsn and the northern california seismic network ncsn robust identification of fzhw requires the combination of multiple techniques due to the diverse instrumentation of the bdsn and ncsn for single component short period instruments fzhw are identified by examining sets of waveforms from both sides of the fault and finding on one the slow side emergent reversed polarity arrivals before the direct p waves for three component broadband and strong motion instruments the fzhw are identified with polarization analysis that detects early arrivals from the fault direction before the regular body waves which have polarizations along the source receiver backazimuth the results indicate average velocity contrasts of 3 8 along the hayward fault with the southwest side having faster p wave velocities in agreement with tomographic images a systematic moveout between the fzhw and direct p waves for about a 80 km long fault section suggests a single continuous interface in the seismogenic zone over that distance we observe some complexities near the junction with the calaveras fault in the se most portion and near the city of oakland regions giving rise to variable fzhw arrival times can be correlated to first order with the presence of lithological complexity such as slivers of high velocity metamorphic serpentinized rocks and relatively distributed seismicity the seismic velocity contrast and geological complexity have important implications for earthquake and rupture dynamics of the hayward fault including a statistically preferred propagation direction of earthquake ruptures to the se

geochronology and geochemistry of high pressure granulites of the arthur river complex fiordland new zealand cretaceous magmatism and metamorphism on the palaeo pacific margin the arthur river complex is a suite of gabbroic to dioritic orthogneisses in northern fiordland new zealand the arthur river complex separates rocks of the median tectonic zone a mesozoic island arc complex from palaeozoic rocks of the palaeo pacific gondwana margin and is itself intruded by the western fiordland orthogneiss new shrimp u pb single zircon data are presented for magmatic metamorphic and deformation events in the arthur river complex and adjacent rocks from northern fiordland the arthur river complex orthogneisses and dykes are dominated by magmatic zircon dated at 136 129 ma a dioritic orthogneiss that occurs along the eastern margin of the complex is dated at 154 4 3 6 ma and predates adjacent plutons of the median tectonic zone rims on zircon cores from this sample record a thermal event at c 120 ma attributed to the emplacement of the western fiordland orthogneiss migmatitic palaeozoic orthogneiss from the arthur river complex 346 6 ma is interpreted as deformed wall rock very fine rims 5 20 lm also indicate a metamorphic age of c 120 110 ma a post tectonic pegmatite 81 8 1 8 ma may be related to phases of crustal extension associated with the opening of the tasman sea the arthur river complex is interpreted as a batholith emplaced at mid crustal levels and then buried to deep crustal levels due to convergence of the median tectonic zone arc and the continental margin

three dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip 1 fault processes involve complex patterns of seismic events and aseismic slip this work develops a three dimensional 3 d methodology for simulating long term history of spontaneous seismic and aseismic slip on a vertical planar strike slip fault subjected to slow tectonic loading our approach reproduces all stages of earthquake cycles from accelerating slip before dynamic instability to rapid dynamic propagation of earthquake rupture to postseismic slip and to interseismic creep including aseismic transients we use the developed 3 d methodology to study interaction of fault slip with a small patch of higher normal stress over long term slip history for uniform initial prestress dynamic rupture is significantly affected by the stronger patch in the first simulated event but not in subsequent ones the change in behavior is due to redistribution of shear stress by prior slip which demonstrates that distributions of fault strength and stress are related and illustrates the importance of simulating long slip histories even in studies of dynamic rupture despite no long term effect on dynamic rupture the small patch of higher normal stress influences nucleation processes and hence long term slip patterns in the model comparison of the fully dynamic simulations and a widely used quasi dynamic approach shows that the quasi dynamic approach modifies long term slip patterns in addition to resulting in much smaller slip velocities and rupture speeds during dynamic events we show that the response of quasi dynamic formulations with reduced radiation damping terms can be scaled to match the results of the standard quasi dynamic formulation and hence cannot improve the comparison