fault reactivation in brittle viscous wrench systems dynamically scaled analogue models and application to the rhine bresse transfer zone abstract analogue experiments on oblique rifting and subsequent transpressional reactivation were performed with two layer slabs of sand and silicone in this brittle viscous system transtensional and transpressional wrench faulting was induced by movements of a basal rigid plate the dynamically scaled analogue models are confronted with the structural evolution of the rhine bresse transfer zone rbtz that linked palaeogene rifting in the upper rhine and bresse grabens and that appears to have been transpressively reactivated in neogene to recent times fault patterns produced in the sand layer above the basal silicone layer are compared with structural elements in the sedimentary cover separated from the basement by an evaporitic decollement layer in order to investigate strain rate dependence of fault reactivation and graben inversion transpressional shortening was performed under different displacement rates experimental results suggest that the reactivation of pre existing structures in a brittle cover above a viscous decollement is strongly dependent on the strain rate within the viscous layer under low to intermediate displacement rates 2 6 cm h 1 deformation concentrates within the basal viscous layer and former normal faults within the cover are not reactivated the reactivation at higher displacement rates 5 cm h 1 results in a complete inversion of graben structures within the cover ongoing shortening produces lobed thrust fronts which crosscut pre existing normal faults late pliocene to recent en echelon aligned folds and isolated thrust faults in the cover of the rbtz are attributed to thick skinned reactivation of basement faults a comparison of natural and experimentally obtained structures suggests that fault reactivation occurred under low displacement rates

links between long term and short term rheology of the lithosphere insights from strike slip fault modelling the classical interseismic mechanical models of the lithosphere assume that the earth s lithosphere is a horizontally layered medium recent more sophisticated models include lateral variations of physical parameters and are shown to be as effective and sometimes more at reproducing the geodetic data during the seismic cycle than the stratified models these new models have in common to require a finite width low viscosity zone underneath the fault zone proposed explanations for the formation of these localised low viscosity zones include grain size reduction shear heating and localised presence of fluids all these mechanisms are valid in order to locally weaken an initially strong lower crust however they are not efficient to localise strain across strike slip faults in post orogenic settings when the lower crust is already weak a previous numerical modelling study showed that metamorphic core complexes are exhumed underneath transtensional step over in post orogenic setting here we designed a parametric study of the impact of the initial thickness of the weak crust and the width of the step over on the intensity of strain localisation we find that the smaller the width of the step over and the thicker the lower crustal low viscosity channel is the most intense is the localisation in a second round of experiments we do not impose the step over kinematic and find that transtensional bound ary conditions are sufficient to trigger long lasting weakening of the crust through the exhumation of lower crustal material using viscoelastic models intended to mimic the interseismic behaviour of a strike slip fault we find that the modified geometry of the brittle ductile transition is sufficient to explain the interseismic strain localisation across a strike slip fault zone we conclude that a long time after an earthquake has occurred the interseismic geodetic signal is more prone to reflect the rheology of the crust than postseismic relaxation

porous fluid flow enables oceanic subduction initiation on earth 1 although most of the presently active intra oceanic subduction zones are relatively young and initiated during the cenozoic subduction initiation process remains poorly understood previous models of subduction initiation assumed excessive weakening of tectonic plate boundaries that does not reconcile with laboratory rock strength measurements the weakening was assumed to be caused by fluids present along tectonic fractures however no self consistent solid fluid model of subduction initiation has been developed so far here we present new numerical hydro thermo mechanical model of spontaneous intra oceanic subduction initiation where solid rock deformation and fluid percolation are fully coupled based on 2 d numerical experiments we demonstrate that although subduction fails to initiate under fluid absent conditions it can naturally start when porous fluid is present inside oceanic crust and along the plate boundaries fluid percolation is localized along spontaneously forming faults where high fluid pressure compensates lithostatic pressure thus dramatically decreasing friction along the incipient subduction zone through the parametric study we conclude that the most important parameter for subduction initiation is the solid matrix permeability paradoxical at first lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along active faults

earthquake cycle deformation in the tibetan plateau with a weak mid crustal layer 1 geodetic observations of interseismic deformation across the tibetan plateau contain information about both tectonic and earthquake cycle processes time variations in surface velocities between large earthquakes are sensitive to the rheological structure of the subseismogenic crust and in particular the viscosity of the middle and lower crust here we develop a semianalytic solution for time dependent interseismic velocities resulting from viscoelastic stress relaxation in a localized midcrustal layer in response to forcing by a sequence of periodic earthquakes earthquake cycle models with a weak midcrustal layer exhibit substantially more near fault preseismic strain localization than do classic two layer models at short 50 mm yr postseismic velocities in the years following the coseismic ruptures we suggest that earthquake cycle models with a localized midcrustal layer can simultaneously explain both preseismic and postseismic geodetic observations with a single maxwell viscosity while the classic two layer model requires a rheology with multiple relaxation time scales

mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

progressive localization of deformation during exhumation of a major strike slip shear zone norumbega fault zone south central maine usa detailed structural studies along with 40ar 39ar thermochronology provide important constraints on the temporal spatial and structural evolution of a major orogen parallel strike slip shear zone the norumbega fault zone in maine detailed structural analysis in south central maine reveals two very different styles of dextral noncoaxial deformation a wide zone 25 km of heterogeneously distributed ductile shear structures and a relatively narrow zone 1 km of lower greenschist facies high strain mylonitization 40ar 39ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following middle devonian amphibolite facies metamorphism the wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature post amphibolite facies metamorphism but pre regional cooling below 320 c late devonian to early carboniferous transcurrent tectonism the narrow zone of dextral shear represents both a younger latest carboniferous and significantly cooler 320 c deformational event occurring at much higher structural levels a general model of increasingly narrow but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones

viscoelastic coupling model of the san andreas fault along the big bend southern california the big bend segment of the san andreas fault is the 300 km long segment in southern california that strikes about n65 w roughly 25 counterclockwise from the local tangent to the small circle about the pacific north america pole of rotation the broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation continuous slip on the fault below the locking depth at the rate of relative plate motion whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km the discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust thus the broad distribution of deformation observed across the big bend segment can be largely associated with the san andreas fault itself not subsidiary faults distributed throughout the region the working group on california earthquake probabilities 1995 in using geodetic data to estimate the seismic risk in southern california has assumed that strain accumulated off the san andreas fault is released by earthquakes located off the san andreas fault thus they count the san andreas contribution to total seismic moment accumulation more than once leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their type c zones

the rift like structure and asymmetry of the dead sea fault abstract whereas the dead sea fault is a major continental transform active since ca 13 18 ma ago it has a rift like morphology along its southern part it has been argued that this results from a transtensional component active since 5 ma ago due to a regional plate kinematics change we present the results of 3d laboratory experiments carried out to test this hypothesis and to explore its consequences for the structure and morphology of the dead sea fault we conclude that a two stage tectonic history invoking a first stage of pure strike slip and a second stage marked by the addition of a minor transtensional component is consistent with most of the striking geological and geophysical features of the dead sea fault the structural and morphological asymmetry of the dead sea fault can be explained by a transverse horizontal shear in the ductile lower crust below the transform zone a large scale heating event of the arabian mantle is not required to explain these features of the dead sea fault

morphotectonic characterization of major bathymetric lineaments in gulf of cadiz africa iberia plate boundary insights from analogue modelling experiments abstract new high resolution multi beam bathymetry data allowed the recognition of several bathymetric lineaments ca 100 km long trending wnw ese in seafloor sediments of the gulf of cadiz offshore sw iberia the interpretation of multi channel mcs profiles crosscutting these lineaments showed that they are controlled by underlying deep seated faults which have endured a polyphase reactivation history to get insights on the recent tectonic evolution of these structures we performed two sets of analogue modelling experiments assuming 1 right lateral strike slip basement faulting and coupled passive shearing affecting an overlying soft cover and 2 low angle transpressive deformation along a narrow shear band overlying the fault our results show a good correlation between the experimentally obtained structural patterns and the natural morphotectonic lineaments allowing the use of some of the observed natural features as strain gauges based on this we conclude that the study lineaments correspond to the bathymetric expression of ongoing dextral wrenching reactivation of wnw ese pre existing faults and we estimate the age of this tectonic reactivation as being ca 1 8 ma i e form late pliocene to present day these characteristics agree with the most recent kinematic models derived from geodetic observations indicating that present day convergence between nubia and iberia is subparallel to the newly identified lineaments and occurs at a 4 mm yr rate

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

u pb dating of a terminal pliocene coral from the indonesian seaway abstract pristine detrital platygyra corals were discovered in an exhumed package of syn orogenic marine sediments on the island of timor in the eastern indonesian region and dated using u pb techniques a single coral from the upper part of the sequence yields a 238 u 206 pb 207 pb 206 pb concordia age of 2 66 0 14 2 ma that is supported by coral 87 sr 86 sr chemostratigraphy and foraminiferal biostratigraphy from bounding strata minor u series disequilibrium is best explained by u mobility within the last 150 ka as pore water chemistry was altered during exhumation and is unlikely to have affected 238 u 206 pb and the apparent sample age by more than 1 2 the ability to date corals beyond the limits of 14 c and u th techniques provides the opportunity to improve the temporal resolution of associated marine chronostratigraphic records in this instance we refine the timing of timor s emergence from beneath the waters of the indonesian seaway is and the initiation of turbiditic deposition at the study site to between ca 3 35 and 2 66 ma these results have implications for the evolution of topography and is oceanic pathways in the active orogenic belts along the northern fringes of the australian plate

modeling suggests that oblique extension facilitates rifting and continental break up 1 in many cases the initial stage of continental break up was and is associated with oblique rifting that includes break up in the southern and equatorial atlantic separation from eastern and western gondwana as well as many recent rift systems like gulf of california ethiopia rift and dead sea fault using a simple analytic mechanical model and advanced numerical thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three dimensional setting while magmatic processes have been already suggested to affect rift evolution we show that additional mechanisms emerge due to the three dimensionality of an extensional system focusing on non magmatic rift settings we find that oblique extension significantly facilitates the rift process this is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift perpendicular extension the model shows that in the case of two competing non magmatic rifts with one perpendicular and one oblique to the direction of extension but otherwise having identical properties the oblique rift zone is mechanically preferred and thus attracts more strain

seismic and gps evidence for the kinematics and the state of stress of active structures in south and south central tibetan plateau abstract since the middle miocene widely distributed n s striking rifts or rift depressions and ne nw striking strike slip faults have developed as major structures in southern and south central tibet active structures seismic mechanisms and directions and rates of movement determined from gps data are different in south and south central tibetan plateau in southern tibet n s striking normal faults and a few transtensional transpressional faults are the main structures but none of the gps measurements show eastward movement in south central tibet ne and nw striking transpressional transtensional faults and several large scale e w and wnw striking dextral and sinistral strike slip faults are the main structures and are consistent with the movement direction indicated by the gps data on the other hand the pattern of earthquake data from the south central tibetan plateau and especially from the eastern and western syntaxes are very complicated these structures may be produced by n s contraction which has resulted in the n s striking linear structures parallel to the direction of maximum compressive stress and conjugate ne and nw striking transpressional transtensional faults on the southern side of the qiangtang block and in the eastern and western syntaxes but the deformation is not continuous from the south to the south central tibetan plateau underthrusting of the indian plate under the southern tibetan plateau after the middle miocene may be the main cause of these structural features

postseismic relaxation driven by brittle creep a possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks application to the chi chi earthquake taiwan we evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth assuming a velocity strengthening brittle creep rheology we show that this model can help reconcile the time evolution of afterslip as measured from geodesy with aftershocks decay we propose an analytical expression for slip of the brittle creeping fault zone bcfz that applies to any dynamic or static stress perturbation including shear stress and normal stress changes the model predicts an initial logarithmic increase of slip with time postseismic slip rate decays over a characteristic time t r a that does not depend on the amplitude of the stress perturbation and it asymptotically joins the long term creep imposed by interseismic stress buildup given that the seismicity rate might be considered proportional to the sliding velocity of the bcfz the model predicts a decay rate of aftershocks that follows omori s law with a mathematical formalism identical to that of dieterich 1994 although based on a different mechanical rationale our model also differs from dieterich s model in that it requires that aftershock sequences and deep afterslip as constrained from geodetic measurements should follow the same temporal evolution we test this for the 1999 chi chi earthquake m w 7 6 and find that both sets of data are consistent with a model of afterslip due to the response of the bcfz the inferred relaxation time t r 8 5 years requires a value for a log v being the coefficient of friction in the range between 1 3 10 3 and 10 2

kinematic interpretation of the 3d shapes of metamorphic core complexes 1 metamorphic core complexes form dome shaped structures in which the ductile crust is exhumed beneath a detachment fault the 3d dome geometry inferred by mapping the schistosity in the exhumed crust can be either elongated normal to the stretching direction or along it in the first case the domes are interpreted as having formed during extension however in the second case they are interpreted either as strike slip transpressive or constrictive extensional structures depending on the geodynamic context numerical models of metamorphic core complexes published to date are all two dimensional and therefore theoretically only apply to domes which are elongated normal to the stretching direction here we explore by means of 3d thermomechanical modeling the impact of 3d kinematic extensional boundary conditions on the shape of metamorphic core complexes we examine the impact of a transtensional step over and of horsetail splay fault kinematics on the dynamics of exhumation finite strain and p t paths and compare them to cylindrical 3d models we show for the first time that domes formed in transtensional step over or at the tip of propagating strike slip faults display a finite strain field which can be interpreted as characteristic of a transpressive domes although no shortening was applied in the far field applying our models to the cyclades we propose that the coeval formation of domes elongated normal and parallel to the stretching during the miocene can be the result of horsetail splay fault kinematics which could correspond to the formation of a tear in the aegean slab

thermo mechanical model of the dead sea transform abstract we employ finite element thermo mechanical modelling to study the dynamics of a continental transform boundary between the arabian and african plates marked by the dead sea transform dst that accommodated ca 105 km of relative transform displacement during the last 20 myr we show that in the initially cold lithosphere expected at the dst shear deformation localizes in a 20 40 km wide zone where temperature controlled mantle strength is minimal the resulting mechanically weak decoupling zone extends sub vertically through the entire lithosphere one or two major faults at the top of this zone take up most of the transform displacement these and other modelling results are consistent with geological observations and lithospheric structures imaged along the desert seismic line suggesting that the arava valley segment of the dst is an almost pure strike slip plate boundary with less than 3 km of transform perpendicular extension modelling suggests that the location of the arava valley segment of the dst has been controlled by the minimum in lithospheric strength possibly associated with margin of the arabian shield lithosphere and or by regionally increased crustal thickness models also show that heating of the arabian shield mantle adjacent to the dst inferred independently by petrological and seismological studies is required to explain asymmetric late cenozoic uplift in the area

strain modelling seismic anisotropy and coupling at strike slip boundaries applications in new zealand and the san andreas fault despite similar surface transform faulting behaviour observed shear wave splitting patterns in the california and new zealand plate boundary regions are markedly different to better understand the origin of the anisotropy in these regions we model mantle flow and strain for a variety of strike slip plate boundary scenarios simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths straincontrolled models fit the observations in new zealand and california better than simplified flow controlled models fast shear polarizations are progressively rotated toward the shear plane over time and even a constant viscosity model provides a good fit to the fast directions in new zealand and southern california the constant viscosity implies strong coupling between the surface and the deeper mantle to fit the lack of decrease in delay times with distance from the fault the relationship between delay time and strain must saturate at small strains if this is the case then southern new zealand and southern california both fit models in which strain is smaller than in nearby regions equivalent to that achieved along an infinite fault by about 3 10 my of their present motion stratified viscosity allows more rapid rotation of fast directions toward fault parallel than occurs in isoviscous models and can explain the nearly fault parallel fast directions in the central south island different aspects of the northern california results are fit with different models but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy noted in the area suggesting vertical decoupling

the mechanical interaction between the propagating north anatolian fault and the back arc extension in the aegean we use fracture mechanics concepts to analyse the large scale deformation of anatolia and the aegean our purpose is to characterize the process of propagation of the north anatolian fault naf our approach incorporates long term geological constraints in dislocation modelling of present day gps velocities that allows for internal deformation in regions between major structures unravelling the superposition of two deformation fields now interacting in the aegean propagating naf and back arc extension permits us to characterise the large scale damage zone at the western end of the naf and to determine slip rates for the main structures the contemporary slip rate profile of the naf shows that it apparently behaves like a transform fault for 75 of its length the process zone corresponds to a rapid southwestward tapering of the naf slip rate modelling slip rate distributions for the naf under stress free conditions shows that the development of the process zone depends on the boundary conditions imposed at the hellenic arc our stress free modelling suggests a scenario for the evolution of the naf 1 at an earlier stage before the naf had reached the aegean it was not under the influence of hellenic arc pull its slip rate profile is modelled to have a half elliptical shape corresponding to a crack in elastic solid and an elastically strained anatolian lithosphere 2 the present day naf penetrates the aegean and a large process zone has developed due to interaction with the aegean extension and the hellenic arc pull elastic strains in the anatolian lithosphere are relaxed and the naf resembles a transform fault the models that we present suggest that changes in subduction zone behaviour should influence all of the boundaries associated with anatolian extrusion

present day crustal deformation around sagaing fault myanmar 1 global positioning system gps measurement campaigns in myanmar conducted in 1998 and 2000 allow quantifying the present day crustal deformation around the sagaing fault system in central myanmar both a regional network installed at four points within the country and a local 18 station network centered on the city of mandalay across the sagaing fault demonstrate that active deformation related to the northward motion of india is distributed across myanmar in a platelet that extends from the western edge of the shan plateau in the east to the andaman trench in the west in this platelet deformation is rather diffuse and distributed over distinct fault systems in the east the sagaing shan scarp fault system absorbs 10 mm yr this gps study combined with an on land geotectonic survey demonstrate that oblique slip of india along the rigid sundaland block is accommodated by a partitioned system characterized by distribution of deformation over a wide zone index terms 1206 geodesy and gravity crustal movements interplate 8155 1243 geodesy and gravity space geodetic surveys 8110 tectonophysics continental tectonics general 0905 8158 tectonophysics plate motions present and recent 3040 keywords tectonics gps fault southeast asia

analogue modelling of fault reactivation tectonic inversion and oblique remobilisation of grabens abstract analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation models were first subjected to extension which lead to the formation of two linear grabens each model was then subjected to a second phase deformation either parallel or oblique to the previous initial extension and either extensional or contractional the influence of sedimentation has been evaluated using experiments with and without sedimentation in cases of oblique secondary deformation all newly formed faults were parallel to the older grabens thus they were oblique to the direction of the principal stress directions during the second deformation phase in experiments without sedimentation all older faults were reactivated whereas in experiments with sedimentation some of the older faults were not reactivated in the case of an oblique compressional secondary deformation phase with post rift sedimentation strain partitioning occurred between reactivated older normal faults and new reverse faults 2 was vertical on reactivated normal fault planes whereas 3 was vertical on reverse fault planes in the case of oblique compressional secondary deformation phase without post rift sedimentation no strain partitioning was observed in this model 3 was vertical on every fault plane it is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

simulations of tremor related creep reveal a weak crustal root of the san andreas fault 1 deep aseismic roots of faults play a critical role in transferring tectonic loads to shallower brittle crustal faults that rupture in large earthquakes yet until the recent discovery of deep tremor and creep direct inference of the physical properties of lower crustal fault roots has remained elusive observations of tremor near parkfield ca provide the first evidence for present day localized slip on the deep extension of the san andreas fault and triggered transient creep events we develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near parkfield is consistent with triggered fault creep governed by laboratory derived friction laws between depths of 20 35 km on the fault simulated creep and observed tremor northwest of parkfield nearly ceased for 20 30 days in response to small coseismic stress changes of order 104 pa from the 2003 m6 5 san simeon earthquake simulated afterslip and observed tremor following the 2004 m6 0 parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust these creep responses require very low effective normal stress of 1 mpa on the deep san andreas fault and near neutral stability frictional properties expected for gabbroic lower crustal rock

influence of lithosphere viscosity structure on estimates of fault slip rate in the mojave region of the san andreas fault system 1 it is well known that slip rate estimates from geodetic data are nonunique because they depend on model assumptions and parameters that are often not known a priori estimates of fault slip rate on the mojave segment of the san andreas fault system derived from elastic block models and gps data are significantly lower than estimates from geologic data to determine the extent to which the slip rate discrepancy might be due to the oversimplified models of the rheology of the lithosphere we develop a two dimensional linear maxwell viscoelastic earthquake cycle model and simultaneously estimate fault slip rates and lithosphere viscosity structure in the mojave region the model consists of episodic earthquakes in an elastic crust overlying layers with different viscosities that represent the lower crust uppermost mantle and upper mantle we use gps measurements of postseismic relaxation following the 1992 landers earthquake triangulation measurements spanning 1932 1977 gps measurements of the contemporary velocity field and paleoseismic data along the san andreas fault we infer lower crustal 15 30 km depth viscosity of 1019 10 20 pa s uppermost mantle 30 60 km viscosity of 10 20 22 pa s and underlying upper mantle viscosity of 10 18 10 19 pa s consistent with inferences from laboratory experiments of relatively high viscosity lithospheric mantle and lower viscosity lower crust and underlying asthenospheric mantle we infer a 20 30 mm yr slip rate on the san andreas fault in agreement with the lower end of geologic estimates inversions of geodetic data with models that do not incorporate layered viscosity structure may significantly misestimate slip rates

dynamic models of interseismic deformation and stress transfer from plate motion to continental transform faults thermal regime on geologic timescales 10 6 10 7 years as well as on timescales of the order of the earthquake recurrence 10 2 years strain localization in the viscoelastic medium results from thermomechanical coupling and power law dependence of strain rate on stress for conditions corresponding to the san andreas fault saf the predicted width of the shear zone in the lower crust is 3 5 km this shear zone accommodates more than 50 of the far field plate motion coupled thermomechanical models predict a single layer lithosphere in case of dry composition of the lower crust and upper mantle and a jelly sandwich lithosphere in case of wet composition deviatoric stress in the lithosphere in our models is relatively insensitive to the water content the far field loading rate and the fault strength and is of the order of 10 2 mpa thermomechanical coupling gives rise to an inverse correlation between the fault slip rate and the ductile strength of the lithosphere we show that our models are broadly consistent with geodetic and heat flow constrains from the saf in northern california models suggest that the regionally elevated heat flow around the saf may be at least in part due to viscous dissipation in the ductile part of the lithosphere

a continuum mechanics approach to quantify brittle strain on weak faults application to the extensional reactivation of shallow dipping discontinuities a number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field the existence of active low angle normal faults is much debated because 1 the classical theory of fault mechanics implies that faults are locked when the dip is less than 30 and 2 shallow dipping fault planes do not produce large earthquakes m 5 5 to reconcile observations and theory we propose a new model for fault reactivation by introducing an elasto plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties contrary to the classical model which implies that the dilation angle equals the friction angle our model accounts for and permits 0 in the fault gouge as deduced from laboratory and field observations whilst the predicted locking angles differ in most cases by less than 10 from the classical model a significant amount of plastic strain strain occurring in elasto plastic regime is predicted to occur on badly oriented faults prior to locking in a strain hardening regime we describe four modes of reactivation which include complete partial reactivation with without tensile failure in the surrounding medium this paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking we conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

interseismic strain accumulation spin up cycle invariance and irregular rupture sequences using models of infinite length strike slip faults in an elastic layer above linear viscoelastic regions we investigate interseismic deformation in the models we investigate interseismic strain accumulation on mature faults is the result of the cumulative effects of all previous ruptures and is independent of the fault loading conditions the time for a fault to spin up to a mature state depends on the rheologies and the fault loading conditions after the model has spun up the temporal variation of shear stresses is determined by the fault slip rate and model rheologies the change in stress during spin up depends on the slip rate rheologies and fault loading conditions but is independent of the magnitude of the initial stress over enough cycles such that the cumulative deformation is block like the average mature interseismic velocities are equal to the interseismic velocities of an elastic model with the same geometry and distribution of shear moduli in a model that has spun up with the fault rupturing periodically the cumulative deformation is block like at the end of each seismic cycle and the interseismic deformation is cycle invariant i e the same in all cycles when the fault ruptures nonperiodically the fault spins up to a mature state that is the same as if the fault had ruptured periodically with the mean slip rate when the fault slip rate within each cycle varies the interseismic deformation evolves toward the cycle invariant deformation determined by the most recent fault slip rate around a fault whose slip rate has been faster slower than average interseismic velocities are larger smaller than the cycle invariant velocities and increase decrease from cycle to cycle

does interseismic strain localization near strike slip faults result from boundary conditions or rheological structure summary geodetic observations across most of the major strike slip faults show an interseismic strain rate which presents a sharp localization of elastic shear strain in the fault vicinity 20 60km the screw dislocation model of savage burford is commonly used to fit these geodetic data and to retrieve the far field velocity and the locking depth this model is very popular because of its inherent simplicity to derive fault slip rates and mostly because it predicts locking depths which are of the same order of magnitude as the base of the seismogenic zone 5 20km a first issue with the screw dislocation model is that localization is paradoxically introduced by imposing a step function in the velocity field at a depth where the crust is otherwise recognized to behave following a viscous rheology a second issue with this model is that it is not consistent with the rheological model of the crust that is valid for both postseismic 1 10yr interseismic several thousand years and long term geodynamic several thousand to several million years timescales here we use numerical models to study how alternative and more geologically realistic boundary conditions and rheological structures can lead to the localization of elastic strain at the earth s surface during the interseismic period we find that simple elastic models resembling the savage burford model but driven by far field plate velocity are inefficient at localizing strain unless this driving velocity is transmitted by a rigid indentor we also find that models including a weak viscous heterogeneity beneath the fault zone are able to produce appropriate localization of the deformation near the fault this alternative class of model is shown to be pertinent in regards to the boundary conditions and geological observations along exhumed ductile strike slip shear zone

the seismic cycle at subduction thrusts 2 dynamic implications of geodynamic simulations validated with laboratory models 1 the physics governing the seismic cycle at seismically active subduction zones remains poorly understood due to restricted direct observations in time and space to investigate subduction zone dynamics and associated interplate seismicity we validate a continuum visco elasto plastic numerical model with a new laboratory approach paper 1 the analogous laboratory setup includes a visco elastic gelatin wedge underthrusted by a rigid plate with defined velocity weakening and strengthening regions our geodynamic simulation approach includes velocity weakening friction to spontaneously generate a series of fast frictional instabilities that correspond to analog earthquakes a match between numerical and laboratory source parameters is obtained when velocity strengthening is applied in the aseismic regions to stabilize the rupture spontaneous evolution of absolute stresses leads to nucleation by coalescence of neighboring patches mainly occurring at evolving asperities near the seismogenic zone limits consequently a crack or occasionally even pulse like rupture propagates toward the opposite side of the seismogenic zone by increasing stresses ahead of its rupture front until it arrests on a barrier the resulting surface displacements qualitatively agree with geodetic observations and show landward and from near the downdip limit upward interseismic motions these are rebound and reversed coseismically this slip increases adjacent stresses which are relaxed postseismically by afterslip and thereby produce persistent seaward motions the wide range of observed physical phenomena including back propagation and repeated slip and the agreement with laboratory results demonstrate that visco elasto plastic geodynamic models with rate dependent friction form a new tool that can greatly contribute to our understanding of the seismic cycle at subduction zones

asymmetric slip partitioning in the sea of marmara pull apart a clue to propagation processes of the north anatolian fault between 1939 and 1999 the north anatolian fault naf experienced a westward progression of eight large earthquakes over 800 km of its morphological trace the 2000 km long north anatolian transform fault has also grown by westward propagation through continental lithosphere over a much longer timescale 10 myr the sea of marmara is a large pull apart that appears to have been a geometrical mechanical obstacle encountered by the naf during its propagation the present paper focuses on new high resolution data on the submarine fault system that forms a smaller pull apart beneath the northern sea of marmara between two well known strike slip faults on land izmit and ganos faults the outstandingly clear submarine morphology reveals a segmented fault system including pull apart features at a range of scales which indicate a dominant transtensional tectonic regime there is no evidence for a single continuous purely strike slip fault this result is critical to understanding of the seismic behaviour of this region of the naf close to istanbul additionally morphological and geological evidence is found for a stable kinematics consistent both with the long term displacement field determined for the past 5 myr and with present day anatolia eurasia motion determined with gps however within the sea of marmara region the fault kinematics involves asymmetric slip partitioning that appears to have extended throughout the evolution of the pull apart the loading associated with the westward propagation process of the naf may have provided a favourable initial geometry for such a slip separation

a physical model for strain accumulation in the san francisco bay region summary strain accumulation in tectonically active regions is generally a superposition of the effects of background tectonic loading steady state dislocation processes such as creep and transient deformation in the san francisco bay region sfbr the most uncertain of these processes is transient deformation which arises primarily in association with large earthquakes as such it depends upon the history of faulting and the rheology of the crust and mantle which together determine the pattern of longer term decade scale post seismic response to earthquakes we utilize a set of 102 gps velocity vectors in the sfbr in order to characterize the strain rate field and construct a physical model of its present deformation we first perform an inversion for the continuous velocity gradient field from the discrete gps velocity field from which both tensor strain rate and rotation rate may be extracted the present strain rate pattern is well described as a nearly uniform shear strain rate oriented approximately n34 w 140 nanostrain yr 1 plus a n56 e uniaxial compression rate averaging 20 nanostrain yr 1 across the shear zone we fit the velocity and strain rate fields to a model of time dependent deformation within a 135 km wide arcuate shear zone bounded by strong pacific plate and sierra nevada block lithosphere to the sw and ne respectively driving forces are purely lateral consisting of shear zone deformation imposed by the relative motions between the thick pacific plate and sierra nevada block lithospheres assuming a depth dependent viscoelastic structure within the shear zone we account for the effects of steady creep on faults and viscoelastic relaxation following the 1906 san francisco and 1989 loma prieta earthquakes subject to constant velocity boundary conditions on the edges of the shear zone fault creep is realized by evaluating dislocations on the creeping portions of faults in the fluid limit of the viscoelastic model a priori plate boundary pb parallel motion is set to 38 mm yr 1 ag rid search based on fitting the observed strain rate pattern yields a mantle viscosity of 1 2 10 19 p as and a pb perpendicular convergence rate of 3m m yr 1 most of this convergence appears to be uniformly distributed in the pacific sierra nevada plate boundary zone

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

weak ductile shear zone beneath a major strike slip fault inferences from earthquake cycle model constrained by geodetic observations of the western north anatolian fault zone gps data before and after the 1999 izmit duzce earthquakes on the north anatolian fault zone turkey reveal a preseismic strain localization within about 25 km of the fault and a rapid postseismic transient using 3 d finite element calculations of the earthquake cycle in an idealized model of the crust comprising elastic above maxwell viscoelastic layers we show that spatially varying viscosity in the crust can explain these observations depth dependent viscosity without lateral variations can reproduce some of the observations but cannot explain the proximity to the fault of maximum postseismic velocities a localized weak zone beneath the faulted elastic lid satisfactorily explains the observations if the weak zone extends down to midcrustal depths and the ratio of relaxation time to earthquake repeat time ranges from 0 005 to 0 01 for weak zone widths of 24 and 40 km respectively in the weakened domain and greater than 1 0 elsewhere corresponding to viscosities of 1018 0 3 pa s and greater than 1020 pa s models with sharp weak zone boundaries fit the data better than those with a smooth viscosity increase away from the fault implying that the weak zone may be bounded by a relatively abrupt change in material properties such a change might result from lithological contrast grain size reduction fabric development or water content in addition to any effects from shear heating our models also imply that viscosities inferred from postseismic studies primarily reflect the rheology of the weak zone and should not be used to infer the mechanical properties of normal crust

post orogenic extension and metamorphic core complexes in a heterogeneous crust the role of crustal layering inherited from collision application to the cyclades aegean domain summary the development of metamorphic core complexes mcc corresponds to a mode of lithospheric continental stretching that follows collision in most of the models that explain the formation of the mcc high thermal gradients are necessary to weaken the lower crust and to induce its ascent such models fail to explain the exhumation of high pressure low temperature metamorphic rocks in metamorphic core complex structures as observed in the cycladic blueschists in the aegean domain besides account for the lithological crustal stratification induced from collision has never been tested in this paper we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post orogenic continental lithosphere the models are designed and validated by petrological structural and time data from the cyclades as a result high thermal gradients moho temperature higher than 800 c are neither necessary nor always sufficient to induce the development of a metamorphic core complex at the contrary the rheological layering of the crust inherited from collision is a first order parameter controlling the development of extensional structures in post orogenic settings cold mcc can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units as observed in the cyclades

bias in estimates of lithosphere viscosity from interseismic deformation 1 the estimation of uniform viscosities representing the lower crust and uppermost mantle from postseismic or interseismic deformation i e apparent viscosities is inherently biased with respect to a depth dependence of the viscosities within each layer estimates are biased toward a more viscous lower crust or a less viscous lithospheric mantle depending on the relative geometric mean viscosities of the two layers when there is a low viscosity shear zone beneath the fault apparent viscosities are close to that of the shear zone immediately after the earthquake although the apparent viscosities increase significantly during the later interseismic period inferences made from interseismic deformation that the lower crust is more viscous than the upper mantle may be entirely consistent with depth dependent viscosity profiles that have a significant increase in viscosity from the lowermost crust to the uppermost mantle

locking depths estimated from geodesy and seismology along the san andreas fault system implications for seismic moment release 1 the depth of the seismogenic zone is a critical parameter for earthquake hazard models independent observations from seismology and geodesy can provide insight into the depths of faulting but these depths do not always agree here we inspect variations in fault depths of 12 segments of the southern san andreas fault system derived from over 1000 gps velocities and 66 000 relocated earthquake hypocenters geodetically determined locking depths range from 6 to 22 km while seismogenic thicknesses are largely limited to depths of 11 20km theseseismogenicdepthsbestmatchthegeodeticlockingdepthswhenestimated at the 95 cutoff depth in seismicity and most fault segment depths agree to within 2 km however the imperial coyote creek and borrego segments have significant discrepancies in these cases the geodetically inferred locking depths are much shallower than the seismogenic depths we also examine variations in seismic moment accumulation rate per unit fault length as suggested by seismicity and geodesy and find that both approaches yield high rates 1 5 1 8 1 0 13 nm yr km along the mojave and carrizo segments and low rates 0 2 10 13 nm yr km along several san jacinto segments the largest difference in seismic moment between models is calculated for the imperial segment where the moment rate from seismic depths is a factor of 2 5 larger than that from geodetic depths such variability has important implications for the accuracy to which future major earthquake magnitudes can be estimated

thermal mechanical controls on seismicity depth distributions in the san andreas fault zone earthquake depth distributions in continental crust can be related to factors which determine the depth of the transition from brittle to ductile failure these factors include temperature strain rate and lithology the san andreas fault zone of northern and central california is an ideal place to test the relative importance of these factors since there is abundant earthquake data and our understanding of the tectonic interaction of the pacific and north american plates provides constraints for several parameters in the san andreas fault zone we have found an approximately 3 km increase from northern to central california of the median depth of events and the depth above which 90 of events occur in addition depth distributions of earthquakes vary systematically from northern to central california both the maximum depth of earthquakes and the depth of the peak of seismic activity are shallower in northern california utilizing critical strength envelopes we have found that unlike the oceanic case this variation is not thermally controlled rather a combination of low strain rate and low stress level cause the seismicity in northern california to be shallow relative to central california

lithospheric convective instability could induce creep along part of the san andreas fault along the western border of the sierra nevada microplate the san andreas fault california united states is comprised of three segments two north and south segments are locked and support large earthquakes e g the m 7 7 1906 san francisco and the m 7 8 1857 fort tejon earthquakes while the central segment from parkfield to san juan bautista is creeping based on mechanical models we show that the late pliocene quaternary convective removal delamination of the southern sierra nevada mantle lithosphere and associated uplift of the sierra nevada mountains causes the great valley upper crust to deform by flexure and buckling additional three dimensional flexural models imply that the local flexural bulge overlaps with the creeping segment of the fault system while geological observations indicate that the local weakening of the san andreas fault started at the same time that the sierra nevada started its recent phase of uplift we argue that bending stresses promote lithostatic pore pressure to occur in the depth range of 7 15 km causing the effective strength of the fault to vanish and locally favoring creep our results suggest for the first time that earthquake cycles along a major plate boundary may be influenced by convective instabilities in the adjacent upper mantle

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

three dimensional models of deformation near strike slip faults we use three dimensional elastic models to help guide the kinematic interpretation of crustal deformation associated with strike slip faults deformation of the brittle upper crust in the vicinity of strike slip fault systems is modeled with the assumption that upper crustal deformation is driven by the relative plate motion in the upper mantle the driving motion is represented by displacement that is spectied on the bottom of a 15km thick elastic upper crust everywhere except in a zone of finite width in the vicinity of the faults which we term the shear zone stress free basal boundary conditions are specified within the shear zone the basal driving displacement is either pure strike slip or strike slip with a small oblique component and the geometry of the fault system includes a single fault several parallel faults and overlapping en echelon faults we examine the variations in deformation due to changes in the width of the shear zone and due to changes in the shear strength of the faults in models with weak faults the width of the shear zone has a considerable effect on the surficial extent and amplitude i c1 j l c i rl l b blhi aill i srizcxi ld 3 rr ij i n flail 3il i dilitiiiat df ti ijtluii clicili c i i zor tz a j t s ii acmes strong fault lnnlds havs lnore luc tlixd dxz a ac tip of tne faults and the deformation is partly distributed outside the fault zone the dimensions of large basins along strike slip faults such as the rukwa and dead sea basins and the absence of uplift around pull apart basins fit models with weak faults better than models with strong faults our models also suggest that the length to width ratio of pull apart basins depends on the width of the shear zone and the shear strength of the faults and is not constant as previously suggested we show that pure strike slip motion can produce tectonic features such as elongate half grabens along a single fault rotated blocks at the ends of parallel faults or extension perpendicular to overlapping en echelon faults which can be misinterprete d to indicate a regional component of extension zones of subsidence or uplift can become wider than expected for transform plate boundaries when a minor component of oblique motion is added to a system of parallel strike slip faults

viscoelastic shear zone model of a strike slip earthquake cycle i examine the behavior of a two dimensional 2 d strike slip fault system embedded in a 1 d elastic layer schizosphere overlying a uniform viscoelastic half space plastosphere and within the boundaries of a finite width shear zone the viscoelastic coupling model of savage and prescott 1978 considers the viscoelastic response of this system in the absence of the shear zone boundaries to an earthquake occurring within the upper elastic layer steady slip beneath a prescribed depth and the superposition of the responses of multiple earthquakes with characteristic slip occurring at regular intervals so formulated the viscoelastic coupling model predicts that sufficiently long after initiation of the system 1 average fault parallel velocity at any point is the average slip rate of that side of the fault and 2 far field velocities equal the same constant rate because of the sensitivity to the mechanical properties of the schizosphere plastosphere system i e elastic layer thickness plastosphere viscosity this model has been used to infer such properties from measurements of interseismic velocity such inferences exploit the predicted behavior at a known time within the earthquake cycle by modifying the viscoelastic coupling model to satisfy the additional constraint that the absolute velocity at prescribed shear zone boundaries is constant i find that even though the time averaged behavior remains the same the spatiotemporal pattern of surface deformation particularly its temporal variation within an earthquake cycle is markedly different from that predicted by the conventional viscoelastic coupling model these differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened application to the interseismic velocity field along the mojave section of the san andreas fault suggests that the region behaves mechanically like a 600 km wide shear zone accommodating 50 mrn yr fault parallel motion distributed between the san andreas fault system and eastern california shear zone in the viscoelastic coupling model savage and prescott 1978 time dependent deformation around a strike slip fault zone is realized by loading subsequent rupture and relaxation of a fault present in a schizosphere elastic upper crust overlying a ductile plastosphere assuming a two dimensional 2 d geometry periodic rupture of the fault and identical slip interseismic deformation is obtained as the superposition of viscoelastic relaxation of the plastosphere summed over the infinity of past periodic earthquakes this model yields far field interseismic motions equal to 1 2 of the average slip rate v0 imposed on the fault thus even when no external mechanism is invoked to load the fault i e when the locked portion of the fault penetrates the entire schizosphere the model satisfies the expectation of rigid plate like behavior in the far field in addition when periodic coseismic offsets are factored in the model predicts that the time averaged fault parallel velocity of each point is vol2 depending on location

factors controlling the mode of rift interaction in brittle ductile coupled systems a 3d numerical study 1 the way individual faults and rift segments link up is a fundamental aspect of lithosphere extension and continental break up little is known however about the factors that control the selection of the different modes of rift interaction observed in nature here we use state of the art large deformation 3d numerical models to examine the controls on the style and geometry of rift linkage between rift segments during extension of crustal brittle ductile coupled systems we focus on the effect of viscosity of the lower layer the offset between the rift basins and the amount of strain weakening on the efficiency of rift linkage and rift propagation and the style of extension the models predict three main modes of rift interaction 1 oblique to transform linking graben systems for small to moderate rift offset and low lower layer viscosity 2 propagating but non linking and overlapping primary grabens for larger offset and intermediate lower layer viscosity and 3 formation of multiple graben systems with inefficient rift propagation for high lower layer viscosity the transition between the linking mode 1 and non linking mode mode 2 is controlled by the trade off between the rift offset the strength of brittle ductile coupling and the amount of strain weakening the mode transition from overlapping non connecting rift segments mode 2 to distributed deformation mode 3 is mainly controlled by the viscosity of the lower layer and can be understood from minimum energy dissipation analysis arguments

poroelastic rebound along the landers 1992 earthquake surface rupture maps of surface displacement following the 1992 landers california earthquake generated by interferometric processing of ers 1 synthetic aperture radar sar images reveal effects of various postseismic deformation processes along the 1992 surface rupture the large scale pattern of the postseismic displacement field includes large lobes mostly visible on the west side of the fault comparable in shape with the lobes observed in the coseismic displacement field this pattern and the steep displacement gradient observed near the emerson camp rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture models show that horizontal slip occurring on a buried dislocation in a poisson s material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10 20 km from the fault yet this pattern is not observed in the postseismic interferograms as previously proposed to explain local strain in the fault step overs peltzer et al 1996b we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault compensating the vertical ground shift produced by fault afterslip such a rebound is explained by the gradual change of the crustal rocks poisson s ratio value from undrained coseismic to drained postseismic conditions as pore pressure gradients produced by the earthquake dissipate using the poisson s ratio values of 0 27 and 0 31 for the drained and undrained crustal rocks respectively elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the sar data and the horizontal displacement measured at global positioning system gps sites along a 60 km long transect across the emerson fault savage and svarc 1997 using a detailed surface slip distribution on the homestead valley kickapoo and johnson valley faults we modeled the poroelastic rebound in the homestead valley pull apart a poisson s ratio value of 0 35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3 5 years following the earthquake this large value implies a seismic velocity ratio vp vs of 2 1 consistent with the observed low vs values of fault zone guided waves at shallow depth li et al 1997 the sar data also reveal postseismic creep along shallow patches of the eureka peak and burnt mountain faults with a characteristic decay time of 0 8 years coseismic dilatant hardening locking process followed by post seismic pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault

the seismic cycle at subduction thrusts insights from seismo thermo mechanical models 1 the underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics to unravel their relation to associated subduction dynamics and long term deformation we developed a 2 d continuum viscoelastoplastic model that uses an eulerian lagrangian finite difference framework with similar on and off fault physics we extend the validation of this numerical tool to a realistic subduction zone setting that resembles southern chile the resulting quasi periodic pattern of quasi characteristic m8 m9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters albeit at very slow coseismic speeds without any data fitting surface displacements agree with gps data recorded before and during the 2010 m8 8 maule earthquake including the presence of a second order flexural bulge these surface displacements show cycle to cycle variations of slip deficits which overall accommodate 5 of permanent internal shortening we find that thermally and stress driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of 350 c and 450 c ruptures initiate above it and below the forearc moho propagate within it interspersed by small intermittent events and arrest below it as ductile shearing relaxes stresses ruptures typically propagate upward along lithological boundaries and widen as pressures drop the main thrust is constrained to be weak due to fluid induced weakening required to sustain regular subduction and to generate events with natural characteristics fluid pressures of 75 99 of solid pressures the agreement with a range of seismological geodetic and geological observations demonstrates the validity and strength of this physically consistent seismo thermo mechanical approach

upper mantle deformation beneath the north american pacific plate boundary in california from sks splitting in order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central california we present evidence for the presence of two anisotropic domains 1 one with clear e w trending fast directions and delay times in the range 1 5 to 2 0 s and 2 the other closely associated with the san andreas fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers the upper of the two layers provides fast directions close to the strike of the main californian faults and averaged delay times of 0 7 s the lower layers show e w directions and delay times in the range 1 5 to 2 5 s and thus can be compared to what is observed in stations that require a single layer we propose the e w trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the north american lithosphere the shallower anisotropic layer ought to be related to the dynamics of the san andreas fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere we also propose that the anisotropic layer associated with each fault of the san andreas fault system is about 40 km wide at the base of the lithosphere

mechanics of the transition from localized to distributed fracturing in layered brittle ductile systems abstract the mechanical coupling between brittle and ductile layers in the continental lithosphere produces rheological contrasts which are supposed to trigger localized or distributed mode of faulting a plane strain 2d finite element model is used to highlight the mechanical role of the brittle ductile coupling in defining the patterns of fracturing the coupling is performed through the shortening of a von mises elasto viscoplastic layer rimmed by two ductile layers behaving as newtonian incompressible fluids by varying the viscosity of the ductile layers or the amount of softening in the brittle layer the fracturing mode evolves from localized to distributed the mechanics of brittle ductile coupling is explained by the limitation of the fault displacement rate imposed by both brittle and ductile rheologies on these bases an analytical approach is presented in order to estimate the maximum velocity along each fault permitted by both brittle and ductile media this velocity is then compared to the velocity required by the boundary shortening rate if the velocity in the fault is not large enough the development of new faults is necessary from this analysis we define four fracturing modes in a brittle ductile media the localized mode with the onset of a few large faults the distributed mode with very dense fault patterns and finally the ductile control mode and the brittle control mode where the number of faults increases with an increase in the ductile viscosity and a decrease in the brittle softening respectively

strain weakening enables continental plate tectonics much debate exists concerning the strength distribution of the continental lithosphere how it controls lithosphere scale strain localization and hence enables plate tectonics no rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid like behaviour of plate interiors here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle respectively the positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain

mechanical basis for slip along low angle normal faults 1 the existence of active low angle normal faults is much debated because 1 the classical theory of fault mechanics implies that normal faults are locked when the dip is less than 30 and 2 shallow dipping extensional fault planes do not produce large earthquakes m 5 5 however a number of field observations suggest that brittle deformation occurs on low angle normal faults at very shallow dip to reconcile observations and theory we use an alternative model of fault reactivation including a thick elasto plastic frictional fault gouge and test it at large strain by the mean of 2d mechanical modeling we show that plastic compaction allows reducing the effective friction of faults sufficiently for low angle normal faults to be active at dip of 20 as the model predicts that these faults must be active in a slip hardening regime it prevents the occurrence of large earthquakes however we also evidence the neoformation of riedel type shear bands within thick fault zone which we believe may be responsible for repeated small earthquakes and we apply the model to the gulf of corinth greece

postseismic relaxation theory on the spherical earth abstract postseismic displacements and strains observed at the earth9s surface can be explained through the relaxation of a viscoelastic asthenosphere underlying a purely elastic crust a theory is presented for modeling the effects of postseismic relaxation on a spherically symmetric earth for an earthquake point source located in the upper elastic layer the displacement field is decomposed into its toroidal and spheroidal components a linear maxwell rheology in the viscoelastic layers is assumed enabling the use of the correspondence principle for the solutions of the equations of static equilibrium displacement and strain fields are then calculated using normal mode summation computational results are presented for two simple earthquake sources a strike slip fault and a uniaxial thrust fault the patterns of postseismic displacements and strains are found to depend strongly on both the earth model and earthquake source geometry in particular the elastic plate thickness asthenosphere thickness fault type and fault length each play a major role in determining the spatial pattern of postseismic relaxation effects asthenospheric viscosity controls the temporal pattern of relaxation

oligo miocene midcrustal subhorizontal shear zone in indochina new structural observations of oligo miocene deformation in north vietnam along the red river shear zone suggest that left lateral strike slip shear was restricted to the upper and middle crust above a horizontal shear zone left lateral shear deformation is associated with low pressure low temperature parageneses high temperature deformation is restricted to zones where the foliation has a low dip in the core of the dai nui con voi antiformal dome these observations complete those made earlier in the bu khang extensional dome farther south above the horizontal shear zone left lateral transpression was active during the first stage and changed to transtension some 33 myr ago purely extensional metamorphic domes bu khang or transtensional domes dai nui con voi were exhumed during this younger stage our observations plead for caution when interpreting strike slip structures at lithospheric scale

the relationship between surface kinematics and deformation of the whole lithosphere the variation of mechanical properties with depth in the lithosphere determines the relationship between surface deformation and whole lithosphere deformation hence between surface deformation and whole lithosphere dynamics where viscosity or elastic strength is a continuous function with depth surface deformation can be used to constrain both force balance and rheological parameters where viscosity is discontinuous but the upper crust and mantle lithosphere have comparable maximum values surface deformation can be used to approximate force balance and rheological parameters but tradeoffs mean that estimates of stress and viscosity are effective equivalent values rather than actual values where viscosity is both discontinuous and differs by much more than an order of magnitude between the upper crust and mantle lithosphere information about both force balance and rheology are absent from the surface deformation so surface observations alone are insufficient to estimate either the dynamic or mechanical state of the lithosphere

a free plate surface and weak oceanic crust produce single sided subduction on earth 1 earth s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection subduction zones on present day earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle while global self consistent numerical models of mantle convection have reproduced some aspects of plate tectonics the assumptions behind these models do not allow for realistic single sided subduction here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates 1 the presence of a free deformable upper surface and 2 the presence of weak hydrated crust atop subducting slabs we show that assuming a free surface rather than the conventional free slip surface allows the dynamical behavior at convergent plate boundaries to change from double sided to single sided a weak crustal layer further improves the behavior towards steady single sided subduction by acting as lubricating layer between the sinking and the overriding plate this is a first order finding of the causes of single sided subduction which by its own produces important features like the arcuate curvature of subduction trenches citation crameri f p j tackley i meilick t v gerya and b j p kaus 2012 a free plate surface and weak oceanic crust produce single sided subduction on earth geophys res lett 39 l03306 doi 10 1029 2011gl050046

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

3d mechanical modeling of the gps velocity field along the north anatolian fault the north anatolian fault naf extends over 1500 km in a complex tectonic setting in this region of the eastern mediterranean collision of the arabian african and eurasian plates resulted in creation of mountain ranges i e zagros caucasus and the westward extrusion of the anatolian block in this study we investigate the effects of crustal rheology on the long term displacement rate along the naf heat flow and geodetic data are used to constrain our mechanical model built with the three dimensional finite element code adeli the fault motion occurs on a material discontinuity of the model which is controlled by a coulomb type friction the rheology of the lithosphere is composed of a frictional upper crust and a viscoelastic lower crust the lithosphere is supported by a hydrostatic pressure at its base representing the asthenospheric mantle we model the long term deformation of the surroundings of the naf by adjusting the effective fault friction and also the geometry of the surface fault trace to do so we used a frictional range of 0 0 0 2 for the fault and a viscosity varying between 10 19 and 10 21 pa s one of the most striking results of our rheological tests is that the upper part of the fault is locked if the friction exceeds 0 2 by comparing our results with geodetic measurements mcclusky et al j geophys res b 105 2000 5695 5719 and tectonic observations we have defined a realistic model in which the displacement rate on the naf reaches v17 mm yr for a viscosity of 10 19 pa s and a fault friction of 0 05 this strongly suggests that the naf is a weak fault like the san andreas fault in california adding topography with its corresponding crustal root does not induce gravity flow of anatolia rather it has the counter intuitive effect of decreasing the westward anatolian escape we find a poor agreement between our calculated velocity field and what is observed with gps in the marmara and the aegean regions we suspect that the simple lithosphere model is responsible for this discrepancy taking into account the weaknesses of these deforming regions should allow us to build a more realistic model that would match ground observations more appropriately on the other hand our results fit well gps measurements in central anatolia setting the basis of modeling crustal strain in turkey b 2003 elsevier science b v all rights reserved

modeling evolution of the san andreas fault system in northern and central california 1 we present a three dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the san andreas fault system safs in northern and central california over the past 20 myr more specifically we investigate the mechanisms responsible for the eastward landward migrationof the san andreas plate boundary over time a process thathas largely determined the evolution and present structure of safs two possible mechanisms had been previously suggested one mechanism suggests that the pacific plate first cools and captures uprising mantle in the slab window subsequently causing accretion of the continental crustal blocks an alternative scenario attributes accretion to the capture of plate fragments microplates stalled in the ceased farallon north america subduction zone here we test both these scenarios numerically using a recently developed lithospheric scale code slim3d that employs free surface nonlinear temperature and stress dependent elastoviscoplastic rheology and allows for self generation of faults modeling suggests that microplate capture is the primary driving mechanism fortheeastwardmigrationoftheplateboundary whiletheslabwindowcoolingmechanismaloneisincapable of explaining this phenomenon we also show that the system evolves to the present day structure of safs only if the coefficient of friction at mature faults is low 0 08 for the best fit model thus our model provides an independent constraint supporting the weak fault in a strong crust hypothesis for safs

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

fault slip rates effects of elastic heterogeneity on geodetic data and the strength of the lower crust in the salton trough region southern california 1 in southernmost california the salton trough sedimentary basin lies between the subparallel san jacinto and san andreas faults that together with the elsinore fault accommodate 80 of the 50 mm yr of pacific north american relative motion the slip rates on the san andreas and san jacinto faults have been a matter of recent debate and one source of uncertainty has been the influence of crustal strength heterogeneity due to elastically weak basin sediments on geodetic data to evaluate these effects we have modeled regional kinematics with elastic and viscoelastic finite element models that incorporate laterally and vertically varying crustal material properties we find that in general the effects of the sedimentary basin on surface deformation are relatively small and that to explain the geodetic velocity data the slip rate on the san andreas must be higher than that on the san jacinto fault consistent with traditional geologic estimates we also conclude that a relatively strong lower crust of viscosity 5 x 10 19 pa s exists in this region

viscoelastic coupling model for the earthquake cycle driven from below in a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself viscoelastic coupling model and a solution that provides the driving force we consider two cases one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle i e the schizosphere and upper mantle respectively act as stress guides in the lithosphere in both cases the driving stress is attributed to steady motion of the stress guide and the upper crust is assumed to be elastic the surface deformation that accumulates during the interseismic interval depends solely upon the earthquake cycle solution viscoelastic coupling model not upon the driving source solution thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system in particular the suggestion of bourne et al 1998 that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems

width of mantle deformation across a continental transform evidence from upper mantle pn seismic anisotropy measurements seismic anisotropy v p of at least 6 5 2 5 in the upper mantle pn phase is reported 200 km east of the alpine fault in south island new zealand when these new data are combined with a previous pn study and published sks splitting results two distinct domains of mantle deformation are inferred within south island in southern south island the deforming zone in the mantle lithosphere is as wide as 335 km whereas in mid south island the deforming zone narrows to 200 km both regions display wider zones of seismic anisotropy than found at other continental transforms of the world the width and orientation of mantle anisotropy in southern south island are consistent with a model of simple shear in a zone that has undergone 800 200 km of right lateral shear displacement a value close to that predicted by geological reconstructions hence displacement can be accommodated by distributed shear without faulting being required in the mantle

fault creep along the southern san andreas from interferometric synthetic aperture radar permanent scatterers and stacking 1 interferometric synthetic aperture radar insar provides a practical means of mapping creep along major strike slip faults the small amplitude of the creep signal 10 mm yr combined with its short wavelength makes it difficult to extract from long time span interferograms especially in agricultural or heavily vegetated areas we utilize two approaches to extract the fault creep signal from 37 ers sar images along the southern san andreas fault first amplitude stacking is utilized to identify permanent scatterers which are then used to weight the interferogram prior to spatial filtering this weighting improves correlation and also provides a mask for poorly correlated areas second the unwrapped phase is stacked to reduce tropospheric and other short wavelength noise this combined processing enables us to recover the near field 200 m slip signal across the fault due to shallow creep displacement maps from 60 interferograms reveal a diffuse secular strain buildup punctuated by localized interseismic creep of 4 6 mm yr line of sight los 12 18 mm yr horizontal with the exception of durmid hill this entire segment of the southern san andreas experienced right lateral triggered slip of up to 10 cm during the 3 5 year period spanning the 1992 landers earthquake the deformation change following the 1999 hector mine earthquake was much smaller 1 cm and broader than for the landers event profiles across the fault during the interseismic phase show peak to trough amplitude ranging from 15 to 25 mm yr horizontal component and the minimum misfit models show a range of creeping locking depth values that fit the data

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr