flow fields within the tonga slab determined from the moment tensors of deep earthquakes the seismic strain rates and instantaneous velocity fields within the plane of the tonga slab are investigated using 450 harvard centroid moment tensor cmt solutions from the period 1977 1992 down dip seismic strain rates at the bottom of the slab are around 1 10 15 s 1 spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab solutions show a northward motion of the 200 km depth contour in the tonga slab at rates of 8 2 mm yr relative to the 670 km depth contour of the slab relative to the deep southern portion of the tonga slab down dip velocities increase from near zero in southern tonga to 10 2 5 mm yr in northern tonga consistent with the increased subduction rate from south to north described by the pacific australia pole of rotation moreover patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1 1025 dyn cm along strike extensional strain rates in the deep zone of northern tonga are approximately 0 5 10 15 s 1 this is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate suggesting that the seismic strain rates within tonga represent a large fraction of the total strain rate budget within the seismogenic zone an estimated maximum of 60 of the relative motion between the surface and 670 km is absorbed as strain within the slab indicating considerable slab thickening and penetration of the slab into the lower mantle

very high strains recorded in mylonites along the alpine fault new zealand implications for the deep structure of plate boundary faults abstract oblique displacement on the alpine fault which forms the principal structure along the australian pacific plate boundary in south island new zealand has resulted in exhumation of a kilometre wide mylonite zone in the hanging wall adjacent to the current brittle fault trace the mylonites formed under amphibolite facies conditions at depths of ca 25 km and have been uplifted during the past 5 ma a suite of 65 70 ma pegmatite veins in the hanging wall alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca 100 km measurements of the thickness distribution of the pegmatite veins within the non mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites the thicknesses approximate a log normal distribution with a mean value that is progressively reduced through the protomylonites mylonites and ultramylonites by assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions shear strains calculated range from 12 to 22 for the protomylonites 120 to 200 for the mylonites and 180 to 300 for the ultramylonites corresponding to pure shear values of 1 3 in each case these values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 ma were accommodated within a 1 2 km wide mylonite zone through the middle and lower crusts the results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth

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

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

rheology of two phase systems a microphysical and observational approach abstract ductile shear zones commonly contain distinctive bands of high strain rock characterized by intimately mixed fine grained two phase or polyphase material these ultramylonite bands are weaker than the surrounding material and may play a critical role in strain localization how such zones develop how the phases become evenly dispersed the bulk rheology and the controls on grain size are all unclear the following generic scenario may resolve some of these questions 1 dislocation creep and dynamic recrystallization cause grain size reduction commonly the recrystallized grain sizes of the two phases differ 2 grain size reduction causes a switch to grain boundary diffusion creep which requires grain boundary sliding diffusion allows one phase to fill spaces that open between grains of the other this will happen most rapidly in the finer grained phase the grain size of the resulting mixture is therefore controlled by that of the finer grained phase this leads to mixing and dispersion of the two phases producing a fine grained evenly dispersed two phase aggregate 3 the bulk rheology will be controlled by grain boundary diffusion creep of the two phases with the grain size controlled by the finer grained phase bulk flow laws can be developed for quartz feldspar and olivine orthopyroxene ultramylonites based on these concepts using appropriate mixing laws

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

block kinematics of the pacific north america plate boundary in the southwestern united states from inversion of gps seismological and geologic data 1 the active deformation of the southwestern united states 30 41 n is represented by a finite number of rotating elastic plastic spherical caps gps derived horizontal velocities geologic fault slip rates transform fault azimuths and earthquake derived fault slip vector azimuths are inverted for block angular velocities creep on block bounding faults permanent strain rates within the blocks and the rotations of 11 published gps velocity fields into to a common north american reference frame gps velocities are considered to be a combination of rigid block rotations recoverable elastic strain rates resulting from friction on block bounding faults and nonrecoverable strain rates resulting from slip on faults within the blocks the resulting pacific north america angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the gulf of california earthquake slip vectors in the gulf of california and alaska and gps velocities along coastal california and within the pacific basin published fault slip rates are satisfied except in the southern mojave desert where the motion of the mohave block relative to north america is faster than can be explained by mapped faults the largest blocks the sierra nevada great valley and the eastern basin and range show permanent strain rates after removing elastic strain of only a few nanostrain per year demonstrating approximately rigid behavior observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short term strains evident in gps velocities are largely elastic in only about 20 of the region is distributed deformation needed to match the data indicating that a plate tectonic style description of the deformation of the western united states is plausible most blocks rotate about vertical axes at approximately the same rate as the pacific relative to north america suggesting that locally spin rates are communicated from block to block arguing against both floating block and ball bearing mechanisms of block rotation the similarities of the blocks spin rates to that of the pacific suggests that the pacific strongly influences their motions through edge tractions however it is shown that the blocks cannot rotate about the pacific north america pole without spinning counter to the sense of pacific north america shear unlike some other broad plate boundaries in the western united states vertical axis rotations take up very little of the slip rate budget across the region

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

broadscale interseismic deformation and fault slip rates in the central tibetan plateau observed using insar 1 knowledge of the number location and interseismic slip rate of crustal faults is key to our understanding of the bulk mechanics of continental deformation however at present the sparsity of geodetic velocity observations is a major limiting factor to improving our knowledge of the fault network in this study we combine long swath interferometric synthetic aperture radar insar data to form an interseismic displacement rate map of over 1000 km length spanning the central tibetan plateau our dense geodetic observations indicate that strain varies smoothly in space without major concentration at previously mapped faults discrepancy between line of sight insar and gps observations between 33 n 35 n could indicate 4 mm yr of surface uplift in this region derived strain rates are less than 5 10 8 yr 1 across the plateau at the e w left lateral kunlun fault postseismic deformation following the 2001 kokoxili earthquake results in an observed strain rate of 18 4 10 8 yr 1 across a 100 km wide zone following removal of a viscoelastic model we estimate an interseismic slip rate of 12 2 mm yr for the kunlun fault in the central tibetan conjugate fault zone the ne sw striking amdo sewa and nw se striking beng co faults have right lateral slip rates of 6 1 mm yr and 1 4 1 mm yr respectively meanwhile the ne sw striking dongqiao fault has a left lateral slip rate of 1 2 1 mm yr the low slip rates we estimate for these faults indicate that tectonic strain is more broadly distributed across the plateau interior perhaps due to the lithosphere being weak and without the abrupt strength contrasts that exist at the plateau margins

postseismic and interseismic displacements near a strike slip fault a two dimensional theory for general linear viscoelastic rheologies 1 we present an analytic solution for the deformation near an infinite strike slip fault in an elastic layer overlying a linear viscoelastic half space the theory is valid for any linear viscoelastic rheology and any earthquake sequence this is a generalization of the work of j c savage and colleagues which only holds for models with a uniform shear modulus maxwell viscoelastic lower half space and periodic rupture recurrence we demonstrate the theory for models of an elastic layer over a maxwell standard linear solid burgers and triviscous half space for each of these models we calculate example postseismic displacements interseismic displacements in a periodic earthquake sequence and interseismic displacements for a nonperiodic earthquake sequence our solution is for a simple geometry however the model presents an elegant tool to explore the evolution of displacements for relatively complex rheologies and rupture recurrence histories

continuous monitoring of an active fault in a plate suture zone a creepmeter study of the chihshang fault eastern taiwan abstract data from continuously monitored creepmeters across the active chihshang fault in eastern taiwan are presented the chihshang fault is an active segment of the longitudinal valley fault the main suture between the converging philippine and eurasian plates in taiwan since the 1951 earthquake mw 7 0 no earthquake larger than magnitude 6 0 occurred in the chihshang area at least during the last 20 years the chihshang fault underwent a steady creep movement resulting in numerous fractures at the surface five creepmeters were installed in 1998 at two sites tapo and chinyuan within the chihshang active fault zone one year results from august 1998 to july 1999 show a horizontal shortening of 19 4 0 3 mm and 17 3 0 7 mm at tapo and chinyuan respectively these annual shortening rates are in a good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation the creepmeter measurements were made on a daily basis providing accurate information on the previously unknown evolution of creep during the year the records of fault creep at the tapo site thus revealed close seasonal correlation with average rainfall the period of high creep rate coincides with the wet season whereas that of low creep rate coincides with the dry season also in comparison with the tapo site the creep behaviour as a function of time is complex at the chinyuan site possible factors of irregularity are under investigation thermal effect acting on the concrete basement of the creepmeters earth tide effect water table variations in a nearby rice field and rainfall the comparison between gps measurements across the longitudinal valley 31 mm year of horizontal displacement and the creepmeter measurement across the chihshang fault zone 17 19 mm year of horizontal displacement suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters

paradoxical pseudotachylyte fault melt outside the seismogenic zone abstract fault generated melt pseudotachylyte is an established indicator of palaeoseismic faulting the existing consensus that frictionally induced melting occurs within the classic seismogenic zone contrast the contention over how pseudotachylyte forms within the ductile regime central to this issue is whether all pseudotachylyte originates as pressure dependent frictional melt along slip surfaces or if pressure independent processes have roles in its formation propagation of high velocity slip into deeper crustal levels provides a satisfactory explanation for pseudotachylyte at depth but does not of itself rationalize earthquake nucleation outside the classic seismogenic zone pseudotachylyte from the minas fault zone nova scotia canada is used to demonstrate the formation and preservation of fault related melt under lower crustal conditions microstructures retain evidence of intense dislocation glide with minimal climb and ductile disaggregation of the host the latter are consistent with intracrystalline deformation in the peierls stress controlled glide regime it remains unclear whether the crystal plasticity serves only as a precursory stage to rupture and high velocity slip or is itself responsible for both instability and the thermal transient there are similarities between accelerating plastic slip leading to rupture and aseismic creep bursts tremor that emphasize the mechanistic complexity of deep faulting and the need to extend consideration beyond that of a simple brittle ductile response the occurrence of tremor bursts fall within the depth range of paradoxical pseudotachylyte and provides a circumstantial link between active tectonics and the geologic record that merits examination

seismic slip and down dip strain rates in wadati benioff zones the rate of accumulation of seismic moment in wadati benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere between depths of 75 and 175 kilometers a typical down dip strain rate is about 10 15 per second which implies that slabs in this depth range typically accumulate strain of order 10 1 this result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes mantle seismicity repeated catastrophic shear failure is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs slabs are penetratively deformed and they have low flexural rigidity compared to oceanic plates at the earth9s surface

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

evaluation of radargrammetric dem from radarsat images in high relief areas this paper addresses digital elevation model dem generation with radarsat synthetic aperture radar sar fine mode images f2 f4 and f1 f5 over high relief areas shallow look angles and small intersection angles were chosen to reduce layover and radiometric differences in the stereo images and thus increase the performances of the image matching the radargrammetric dems are thus evaluated as functions of processing methods stereo viewing and editing and different geometric parameters intersection angle and derivative topographic surfaces the first results show that the ground control points ccps must be acquired with stereoscopic viewing instead of monoscopic viewing to avoid an error increase of over 30 in the dem an error of one pixel for the image matching has been estimated the dems have also to be edited with semi automatic and stereo viewing to increase their coherence and consistency mainly at mountain ridges and lakes the results also show that a 50 decrease in the intersection angle 80 versus 40 resulted in only a 15 decrease in dem accuracy 24 m versus 28 m in contradiction with the theory finally the dem accuracy is almost linearly correlated with the terrain slopes but not with the terrain aspect expect better results for the foreslopes

new perspectives on geological strain rates calculated from both naturally deformed and actively deforming rocks abstract a value of 10 14 s 1 is commonly cited as an average geological strain rate this value was first suggested for finite strain across an orogen but based on more limited information than the combined geophysical geological and experimental data now available on active and ancient rock deformation thus it is timely to review the data constraining strain rates in the continents and to consider the quantifiable range of crustal strain rates here where resolution allows both spatial and temporal strain rate variations are explored this review supports that a strain rate of 10 14 1 s 1 arises from geological estimates of bulk finite strains microstructural arguments combining laboratory derived piezometers and viscous flow laws however imply local rates that are orders of magnitude faster geodetic rates in contrast are typically 10 15 s 1 in actively deforming areas about an order of magnitude slower than the bulk rates estimated from geological observations this difference in estimated strain rates may arise from either low spatial resolution or the fact that surface velocity fields can not capture strain localisation in the mid to lower crust integration of geological and geodetic rates also shows that strain rates can vary in both space and time over both single and multiple earthquake cycles overall time averaged geological strain rates are likely slower than the strain rates in faults and shear zones that traverse the crust or lithosphere

rapid afterslip following the 1999 chi chi taiwan earthquake 1 postseismic displacements of as much as 14 cm were recorded by gps measurements in the 3 months following the mw 7 6 1999 chi chi taiwan earthquake data from 35 continuous and 90 campaign surveyed stations which show continued east over west thrusting are analyzed to estimate the postseismic slip distribution and fault geometry assuming the shallow fault dips 24 e as determined by numerous studies of the mainshock we invert for the deeper fault structure our results show that the fault dip shallows with depth below the hypocenter merging into a nearly horizontal decollement at a depth of 8 12 km the afterslip distribution shows a maximum slip of 25 cm in the hypocentral region at 7 12 km depth as well as significant slip on the lower decollement afterslip is notably absent in the region of maximum coseismic slip consistent with the afterslip being driven by the mainshock stress change

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

theories and applicability of grain size piezometers the role of dynamic recrystallization mechanisms abstract the average grain size d arising from dynamic recrystallization drx is often used as an indicator of flow stress however a theoretical basis for the scaling relation between d and has yet to be well established in this paper theories for the development of recrystallized grain size are reviewed and their applicability is examined special attention is paid to the dependence of the d relation on drx mechanisms steady state drx is classified into discontinuous drx with bulging blg nucleation grain boundary migration gbm and continuous drx with subgrain rotation sgr nucleation gbm the nucleation and growth model derived from derby ashby theory describes the former case whereas that derived from shimizu theory applies to the latter a static energy balance model derived from twiss theory is applicable to subgrain size but not to recrystallized grain size the lower limit of grain size is possibly constrained by a change in deformation mechanism from dislocation creep to diffusion creep because deformation induced grain size reduction ceases in the diffusion creep field scaling relations determined in the laboratory support the shimizu model in the case of sgr gbm the theoretical piezometer calibrated for quartz suggests significant temperature effects under low temperature metamorphic conditions

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

consistency of geologic and geodetic displacements during andean orogenesis displacement and convergence rates inferred from gps and marine magnetic data suggest that this trend may be continuing at present hence an increasing fraction of convergence is being absorbed by mountain building this change may reflect increased plate coupling due to decreased sediment supply younger subducting lithosphere or increased normal stress at the interface from the effects of uplift index terms 8102 tectonophysics continental contract orogenic belts 8157 evolution of the earth plate motions past 3040 8158 evolution of the earth plate motions present and recent 3040 8099 structural geology general or miscellaneous

quantitative analysis of surface coseismic faulting and postseismic creep accompanying the 2003 mw 6 5 chengkung earthquake in eastern taiwan 1 on the basis of field observations and geodetic measurements we analyzed coseismic and postseismic deformation of the 2003 mw 6 5 chengkung earthquake across the rupture trace of the chihshang fault which lies along the suture zone between the philippine sea plate and the eurasian plate in eastern taiwan at three of our investigation sites along the chihshang fault the earthquake deformation was exhibited by fresh fractures within the 150 to 200 m wide surface fault zone reactivating preexisting fractures in most cases in addition to daily recorded creep meter data geodetic measurements including leveling distance and angle electronic distance meters and gps measurements were carried out during surveys 20 25 days before 20 25 days after and 120 125 days after the earthquake the near fault surface deformation is mainly characterized by anticlinal folding in the hanging wall and minor gentle synclinal folding in the footwall the geodetic data show that within our 150 to 250 m wide networks the coseismic deformation of the main shock produced only about 1 2 cm of horizontal shortening and vertical offset across the fault zone a larger additional displacement of about 7 9 cm for both the horizontal shortening and the vertical offset occurred as postseismic creep during the 120 125 days following the main shock we interpret the predominant folding and the rather large postseismic creep as a result of strong velocity strengthening along the fault plane near the surface which caused locking or coupling effect during the coseismic rupturing the depth of locked segment along which the coseismic and postseismic slip decreased dramatically upward is estimated to be 25 100 m depending on the site we interpreted the velocity strengthening and coupling effect at the shallow level as viscoelastic behavior of unconsolidated deposits in the footwall and melange mudstone in the hanging wall by incorporating far fault continuous gps data it was found that minor but significant deformation occurred outside of the surface ruptures zone of the chihshang fault in addition a possible back thrust or back fold might have occurred in the hanging wall we note that the chengkung earthquake occurred during the dry season this is consistent with the fact that the mechanical coupling of the fault at shallow depth is higher during drier periods inferred from the previous seasonal creep data at outcrop scale regarding damage of construction such as a concrete retaining wall most of the coseismic and postseismic horizontal shortening was essentially absorbed by multiple distributed brittle ruptures which deserves attention in terms of earthquake hazard mitigation

shear zones at the laxford front nw scotland and their significance in the interpretation of lower crustal structure the lewisian rocks in the scourie laxford region nw scotland show variations in shear strains and associated structures on the southern upper margin of a zone of intense laxfordian reworking the scourian block was thrust a large distance obliquely northwards over the northern laxfordian gneisses as part of a laxfordian episode of crustal thickening this was followed by crustal extension during which time the scourian block moved down to the south granite sheets were intruded along the main shear zone in a dilational region at the base of a rotational extensional block the laxfordian contractional and extensional deformation occurred under amphibolite facies conditions and analogies can be made with middle crustal structures inferred from deep seismic data for example structures produced during late laxfordian crustal spreading can be compared to those seen on the birps offshore scottish data which formed during devonian basin development following caledonian crustal thickening the scourie laxford structures give an idea of fault kinematics at the brittle ductile transition in the middle to lower crust steep faults pass down into more gently dipping ductile shears and then to a zone of anastomosing shears which accommodate the strains and rotations at the base of the rigid upper crust the shears may concentrate along a more planar zone of deformation forming a detachment or decoupling zone below which the extension may be variable depending on the mode of lithospheric stretching

space time distribution of afterslip following the 2009 l aquila earthquake 1 the inversion of multitemporal dinsar and gps measurements unravels the coseismic and postseismic afterslip slip distributions associated with the 2009 mw 6 3 l aquila earthquake and provides insights into the rheological properties and long term behavior of the responsible structure the paganica fault well resolved patches of high postseismic slip 10 20 cm appear to surround the main coseismic patch maximum slip 1 m through the entire seismogenic layer above the hypocenter without any obvious depth dependent control time series of postseismic displacement are well reproduced by an exponential function with best fit decay constants in the range of 20 40 days a sudden discontinuity in the evolution of released postseismic moment at 130 days after the main shock does not correlate with independent seismological and geodetic data and is attributed to residual noise in the insar time series the data are unable to resolve migration of afterslip along the fault probably because of the time interval six days between the main shock and the first radar acquisition surface fractures observed along the paganica fault follow the steepest gradients of postseismic line of sight satellite displacements and are consistent with a sudden and delayed failure of the shallow layer in response to upward tapering of slip the occurrence of afterslip at various levels through the entire seismogenic layer argues against exclusive depth dependent variations of frictional properties on the fault supporting the hypothesis of significant horizontal frictional heterogeneities and or geometrical complexities we support the hypothesis that such heterogeneities and complexities may be at the origin of the long term variable behavior suggested by the paleoseismological studies rupture of fault patches with dimensions similar to that activated in 2009 appears to have a 500 year recurrence time interval documented by paleoseismic and historical studies in addition to that paleoseismological evidence of large 0 5 m coseismic offsets seems to require seismic events recurring every 1000 2000 years characterized by 1 multisegment linkage 2 surface ruptures larger than in 2009 and 3 complete failure of the 2009 coseismic and postseismic patches

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

coseismic and postseismic fault slip for the 17 august 1999 m 7 5 izmit turkey earthquake we use global positioning system gps observations and elastic half space models to estimate the distribution of coseismic and postseismic slip along the izmit earthquake rupture our results indicate that large coseismic slip reaching 5 7 meters is confined to the upper 10 kilometers of the crust correlates with structurally distinct fault segments and is relatively low near the hypocenter continued surface deformation during the first 75 days after the earthquake indicates an aseismic fault slip of as much as 0 43 meters on and below the coseismic rupture these observations are consistent with a transition from unstable episodic large earthquakes to stable fault creep sliding at the base of the seismogenic zone

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

strain distribution within a km scale mid crustal shear zone the kuckaus mylonite zone namibia the subvertical kuckaus mylonite zone kmz is a km wide crustal scale proterozoic dextral strike slip shear zone in the aus granulite terrain sw namibia the kmz was active under retrograde amphibolite to greenschist facies conditions and deformed felsic and minor mafic gneisses which had previously experienced granulite facies metamorphism during the namaqua orogeny lenses of pre to syn tectonic leucogranite bodies are also deformed in the shear zone pre kmz deformation d1 is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering shear strain within the kmz is heterogeneous and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges strain is localized along the edge of leucogranites and between gneissic lozenges preserving d1 migmatitic foliations strain localization appears controlled by pre existing foliations grain size and compositional anisotropy between leucogranite and granulite the local presence of retrograde minerals indicate that fluid infiltration occurred in places but most ultramylonite in the kmz is free of retrograde minerals in particular rock composition and d1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space with deformation localization along planes of rheological contrast and along pre existing foliations therefore the spatial distribution of strain in crustal scale ductile shear zones may be highly dependent on lithology and the orientation of pre existing fabric elements in addition foliation development and grain size reduction in high strain zones further localizes strain during progressive shear maintaining the anastomosing shear zone network established by the pre existing heterogeneity

deformation cycles of subduction earthquakes in a viscoelastic earth subduction zones produce the largest earthquakes over the past two decades space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes the short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle piecing together geodetic snapshots from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short term years and long term decades and centuries viscous behaviour of the mantle traditional views based on elastic models such as coseismic deformation being a mirror image of interseismic deformation are being thoroughly revised snapshots of subduction zones using space geodesy reveal that the viscous behaviour of the mantle controls crustal deformation requiring the revision of traditional elastic models for earthquake risk assessment space geodesy the use of satellites to monitor earth has revolutionized our view of crustal deformation between consecutive earthquakes however the brevity of such measurements means that studies must be done by comparing multiple subduction zones at different stages of the earthquake cycle by piecing together geodetic snapshots from the sumatra chile and cascadia subduction zones this review presents a unifying picture in which deformation is controlled by the viscous behaviour of the mantle in both the short term years and the long term decades to centuries

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

preparation and deformation of synthetic aggregates of quartz synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder impure silica gel and high purity silicic acid by hydrothermal isostatic pressing at 300 mpa the resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests the flow stress at 1200 1300 k was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity it is questionable that the natural quartz origin specimens were fully equilibrated in respect to the activity of the water but equilibration has probably been achieved in the amorphous silica origin specimens which crystallized under the experimental conditions in the deformation of the latter materials the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kj mol 1 for both however the impure gel origin material gave a stress exponent of about 2 3 whereas the high purity silicic acid origin material gave a stress exponent of about 4 in spite of the grain sizes being about 90 m and 20 m respectively it is concluded that while the impure specimens have higher intragranular strengths there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high purity specimens

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

high resolution interseismic velocity data along the san andreas fault from gps and insar 1 we compared four interseismic velocity models of the san andreas fault based on gps observations the standard deviations of the predicted secular velocity from the four models are larger north of the san francisco bay area near the creeping segment in central california and along the san jacinto fault and the east california shear zone in southern california a coherence spectrum analysis of the secular velocity fields indicates relatively high correlation among the four models at longer wavelengths 15 40 km with lower correlation at shorter wavelengths to improve the short wavelength accuracy of the interseismic velocity model we integrated interferometric synthetic aperture radar insar observations initially from advanced land observing satellite alos ascending data spanning from the middle of 2006 to the end of 2010 totaling more than 1100 interferograms with gps observations using a sum remove filter restore approach the final insar line of sight data match the point gps observations with a mean absolute deviation of 1 5 mm yr we systematically evaluated the fault creep rates along major faults of the san andreas fault and compared them with creepmeters and alignment array data compiled in uniform california earthquake rupture forecast version 2 ucerf2 moreover this insar line of sight dataset can constrain rapid velocity gradients near the faults which are critical for understanding the along strike variations in stress accumulation rate and associated earthquake hazard

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

ductile strain rate measurements document long term strain localization in the continental crust quantifi cation of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust quartz strain rate metry qsr is a convenient tool for performing such measurements once calibrated we achieve this calibra tion by identifying the best piezometer rheological law pairs that yield a strain rate in agree ment with that measured on the same outcrop by a more direct method taken as a reference when applied to two major continental strike slip shear zones the ailao shan red river asrr southwest china and the karakorum northwest india the calibrated qsr high lights across strike strain rate variations from 1 10 13 s 1 in zones where it is localized strain rates integrated across the shear zones imply fast fault slip rates on the order of 1 1 cm yr 1 karakorum and 4 cm yr 1 asrr proving strong strain localization in these strike slip continental shear zones

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

identifying deformed pseudotachylyte and its influence on the strength and evolution of a crustal shear zone at the base of the seismogenic zone abstract on a strand of the norumbega fault system a paleozoic subvertical seismogenic fault system in northeastern new england usa we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite phyllonite layers we consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone the norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of 10 15 km some of which preserve evidence for mutually overprinting pseudotachylyte and mylonite along one of these the sandhill corner shear zone all stages of the pseudotachylyte to ultramylonite phyllonite transformation are preserved from 1 primary pseudotachylyte structures to 2 initial mineral crystallization 3 grain coarsening and reactions and 4 viscous deformation our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin using these features we estimate that 5 50 of the rock volume in the sandhill corner shear zone mean 30 locally 50 is deformed pseudotachylyte suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone the sandhill corner shear zone localized along the contact between two rheologically contrasting units mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core indicating that seismic rupture also localized there a decrease in grain size promoting grain size sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers the formation of multiple generations of weak deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

postseismic deformation due to the mw 6 0 2004 parkfield earthquake stress driven creep on a fault with spatially variable rate and state friction parameters 1 we investigate the coseismic and postseismic deformation due to the m w 6 0 2004 parkfield california earthquake we produce coseismic and postseismic slip models by inverting data from an array of 14 continuous gps stations from the scign network kinematic inversions of postseismic gps data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip predominantly at a depth of 5 km inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane the kinetics of afterslip is consistent with a velocity strengthening friction generalized to include the case of infinitesimal velocities we performed simulations of stress driven creep using a numerical model that evaluates the time dependent deformation due to coseismic stress changes in a viscoelastoplastic half space starting with a coseismic slip distribution we compute the time dependent evolution of afterslip on a fault plane and the associated displacements at the gps stations data are best explained by a rate strengthening model with frictional parameter a b 7 x 10 3 at a high end of values observed in laboratory experiments we also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks the rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep the san andreas fault around parkfield is deduced to have large along strike variations in rate and state frictional properties velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity weakening patches after the 2004 rupture

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

zooming on northern cap de creus shear zones abstract the northern cap de creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre existing penetrative foliations the shear zones developed under retrograde metamorphic conditions and they are preferentially located in crystalline schists lower grade metasedimentary rocks accommodated deformation by folding the shear zones are linked in an anastomosing framework with self similar properties they are interpreted to have formed as the result of a progressive wrench dominated deformation affecting anisotropic rocks a progressive non coaxial deformation regime is considered responsible for the development of the complex kinematic pattern wherein shear zones with opposite senses of shear may come to lie in close parallelism in such a framework individual shear zones are characterised by marked strain gradients not only across the shear zone but also along it these features together with the geometrical analysis of shear zone marginal domains indicate that these structures developed with a component of shortening across the shear zones the shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model microstructural and quartz c axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales

using microstructures and titaniq thermobarometry of quartz sheared around garnet porphyroclasts to evaluate microstructural evolution and constrain an alpine fault zone geotherm abstract interpretations of deformation processes within ductile shear zones are often based on the characterisation of microstructures preserved in exhumed rocks however exhumed microstructures provide only a snapshot of the closing stages of deformation and we need ways of understanding how microstructures change through time and at what rate this occurs to address this problem we study optical microstructures and electron backscatter diffraction ebsd data from samples of quartz layers deflected around garnet porphyroclasts which generate local stress and strain rate perturbations during mylonitic deformation in the alpine fault zone of new zealand during shearing around rigid garnet porphyroclasts quartz undergoes grain size reduction in response to locally increased stresses while c axes reveal increasing components of rhomb and prism slip reflecting a local increase in shear strain and strain rate titaniq thermobarometry and quartz microstructures suggest a rather narrow range of recorded quartz deformation temperatures around 450 500 c which we propose reflects the cessation of grain boundary migration driven deformation given that temperatures well above the brittle ductile transition for quartz 350 c are preserved we anticipate that rapid cooling and exhumation must have occurred from the 500 c isotherm ultimately we propose a modified geotherm for the central alpine fault zone hanging wall which raises the 500 c isotherm to 11 km depth near the brittle ductile transition our updated alpine fault zone geotherm implies a hotter and weaker middle to lower crust than previously proposed

gravitational potential energy and active deformation in the apennines we use velocity measurements from a network of continuous gps sites spanning the apennines of peninsular italy to test the hypothesis that the active deformation of the region is explained by variations in gravitational potential energy of the lithosphere the simple geometry of the mountain chain allows us to treat the deformation as two dimensional neglecting gradients of velocity along the strike of the chain under this assumption the integral of gravitational potential energy per unit area of the lithosphere gpe in the direction perpendicular to the chain is related by a simple expression to the velocity in the same direction we show that the observed velocities match this expression with an rms misfit of 0 5mm yr this agreement suggests that deformation of the apennines reflects a balance within the mountain chain itself between lateral variations in gpe and the stresses required to deform the lithosphere forces arising from processes external to the belt are not required to explain the observations

temporal stability of deformation rates comparison of geological and geodetic observations hikurangi subduction margin new zealand abstract we examine how well contemporary horizontal velocity fields from global positioning system gps geodetic measurements describe long term deformation rates million year rates of extension shortening strike slip and vertical axis rotations are compared with modelling of gps velocities from the last 10 15 years in the hikurangi subduction margin new zealand within measurement uncertainties of mainly 50 the rates of deformation and vertical axis rotations for each time interval are usually indistinguishable the comparable deformation rates are consistent with the notion of near constant rates since 1 5 ma during this time oblique relative plate motion was partitioned with the majority 85 of margin normal motion accommodated on the plate interface and most 60 margin parallel motion by strike slip faulting and vertical axis rotations in the upper plate prior to 1 5 ma gps velocities cannot be reliably used to describe deformation and rotations of the hikurangi margin because at this time the taupo rift commenced opening the boundary between raukumara and wairoa rotational domains was abandoned and the rates of upper plate shortening increased the broad compatibility of gps and geological deformation and rotation rates since 1 5 ma suggests that the build up of contemporary elastic strains in the north island measured by gps is largely uniform for much of the duration of each seismic cycle the good agreement between the gps and geological deformation rates in the north island is consistent with the view that contemporary elastic strains will mainly be converted to permanent dip slip on the subduction thrust and to displacement on upper plate faults during future large magnitude earthquakes

modeling afterslip and aftershocks following the 1992 landers earthquake one way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake in such a case the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy here we model the postseismic response of a fault zone that is assumed to obey a rate strengthening rheology where the frictional stress varies as a ln e e being the deformation rate and a 0 a rheological parameter the model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data we apply this approach to the analysis of geodetic displacements following the m w 7 3 1992 landers earthquake the model adjusts well the measured displacements and implies a 0 47 0 53 mpa in addition we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms

satellite geodetic imaging reveals internal deformation of western tibet 1 it is often assumed that the majority of the interseismic strain accumulates around the major mapped geological block boundaries however numerous recent earthquakes in the continents have occurred on faults that were previously unidentified existing deformation data from tibet are insufficiently dense to map the distribution of interseismic strain here we combine 265 interferograms formed from 166 radar images with gps data to constrain a high resolution velocity field covering 200 000 km2 of western tibet we confirm that the slip rate of the karakoram fault is low 6 mm yr but show that areas of focused strain do not coincide with the major fault structures some of this strain is due to postseismic relaxation after a mw 6 8 earthquake that occurred in 1996 on a structure that is difficult to identify in satellite imagery models of seismic hazard that rely on imperfect knowledge of the boundaries of crustal blocks can therefore underestimate hazard from unknown faults

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event