uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

evolution of faulting and plate boundary deformation in the southern taranaki basin new zealand abstract faulting and folding in the southern taranaki basin constrain the evolution of the new zealand plate boundary since 80 ma sedimentary rocks up to 8 km thick record multiple phases of deformation which have been examined using 2d and 3d seismic reflection data resulting in fault displacement time curves and basin wide isopach maps with temporal resolutions of 5 10 myr and 1 4 myr pre and post 23 ma respectively three main phases of tectonic activity have been recognised late cretaceous and palaeocene extension 80 55 ma mainly oligocene and younger contraction and plio pleistocene 3 7 0 ma extension most of the largest faults e g cape egmont fault accrued displacement during the late cretaceous and were reactivated one or more times during subsequent episodes of deformation the oldest phase of extension occurred during gondwana break up and was ubiquitous throughout the basin contraction along the eastern boundary of the basin associated with the onset of hikurangi margin subduction commenced as early as late eocene the zone of contraction widened and migrated westward during the miocene with reverse faults and folds in westernmost parts of the basin formed in the late miocene 7 5 ma initiation and episodic widening of this zone of contraction may have been partly triggered by changes in the rate of plate convergence contraction is now mainly confined to the northern south island and has been succeeded to the north by plio pleistocene extension the present day transition zone between extension in the north and contraction in the south is defined by a wnw trending line across the basin the extension contraction transition migrated southward during the late miocene and pliocene consistent with steepening of the subducting plate and associated southward movement of the southern termination of the hikurangi subduction system

the 1989 macquarie ridge earthquake and its contribution the regional seismic moment budget the macquarie ridge earthquake 1989 may 23 mw 8 2 occurred in a region of complex tectonics that marks the southern continuation of the plate boundary pac aus that runs through new zealand the first motion focal mechanism from wwssn and regional station polarities indicates dominantly strike slip movement strike 025 dip 80 rake 180 probably dextral on the northeast striking nodal plane the plate boundary south of new zealand appears to be segmented and marked by linear bathymetric highs with distinctive trends the distribution of shallow seismicity follows these bathymetric trends the moment tensors for the largest earthquakes ms 6 0 occurring on five distinctive segments of the plate boundary for the period 1964 1989 have been summed and compared with the deformation predicted by the plate motions recurrence times of the largest earthquakes in each segment determined by assuming that all deformation occurs seismically along the macquarie ridge complex mrc generally show longer recurrence times than those determined from the historic record these results suggest that during the last 70 years there has been more seismic moment release along the mrc than predicted by the plate motions the preferred explanation is that this century is a seismically overactive period which is part of a natural episodicity of quiescence and overactivity alternative explanations include complex plate interactions resulting from proximity of the plate boundary to the instantaneous pole of rotation and or the existence of a southeast australian microplate

s wave residuals from earthquakes in the tibetan region and lateral variations in the upper mantle abstract mean s wave residuals from 46 earthquakes within and on the margins of the tibetan plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history the earliest s waves come from earthquakes in western tibet the karakorum and the western himalaya and the latest come from earthquakes in north central tibet although s waves from earthquakes in the himalaya tend to be early the east west variation in residuals across tibet is at least as large as the north south difference between the himalaya and northern tibet if the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection then upwelling beneath north central tibet seems to be flanked by downwelling in western eastern and probably southern tibet this convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath tibet

tectonic rotations about vertical axes during the last 4 ma in part of the new zealand plate boundary zone the declinations of the primary magnetization of sedimentary rocks in the northern part of the new zealand plate boundary zone after thermal or alternating field cleaning have been used to determine tectonic rotations about vertical axes of rigid crustal blocks the pattern of rotations during the last 4 ma combined with structural style and continuity defines seven structural domains each ca 100 100 200 km across which contain crustal blocks up to 100 km across and 20 km thick large crustal blocks tens of km across in two of these domains have rotated clockwise more than 20 relative to one of the margins of the plate boundary zone in the last 4 ma their behaviour appears to be controlled by the nature of the plate boundaries such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge small crustal blocks 10 km across may have rotated clockwise through angles greater than 20 during the last 4 ma floating on an underlying zone of more distributed deformation the tectonic rotations of the large crustal blocks and the nature of the deformation at their boundaries combined with an interpolation of the relative plate positions can be used to reconstruct the plate boundary zone at ca 4 ma

oblique slip on the puysegur subduction interface in the 2009 july mw 7 8 dusky sound earthquake from gps and insar observations implications for the tectonics of southwestern new zealand summary the mw 7 8 dusky sound earthquake of 2009 july 15 was the largest earthquake in new zealand in the past 80 yr and is the only major subduction interface earthquake in the new zealand historical record we have estimated the coseismic and early post seismic slip distribution in the earthquake by inversion of gps and differential interferometric synthetic aperture radar dinsar observations we show that slip during the earthquake was highly obliqueandessentiallyintheaustralia pacificrelativeplatemotiondirection failureoccurred onaca 80by50kmpatchofthesubductioninterfacewithmaximumslip5 6m inthisregion the offshore alpine fault carries up to 30 mm yr 1 of slip before terminating at resolution ridge near the southern end of the 2009 slip patch the southernmost part of the alpine fault trace lies near the up dip end of the 2009 slip patch and the fault probably exists only within the crust of the over riding plate in this region terminating at shallow depth where it meets the subduction interface there is no evidence for rupture of the alpine fault during the 2009 event however coulomb stress changes from the subduction earthquake have brought this section of the alpine fault closer to failure in contrast to many subduction zones undergoing oblique subduction where the slip in major earthquakes is partitioned into more or less pure thrustearthquakesonthesubductioninterfaceandstrike slipearthquakesinthebackarc thisis a case where the majority of the subduction interface slip is not partitioned with only a shallow convergence zone west of the strike slip alpine fault undergoing contraction approximately normal to subduction zone strike north of the 2009 earthquake region the alpine fault lies further east relative to the subduction interface and partitioning between strike slip on the alpine fault and trench normal thrusting on the subduction interface takes place in a more typical fashion our results provide the first clear demonstration of non partitioned oblique slip between the australian and pacific plates offshore of southern new zealand

tectonic and structural setting for active mesothermal gold vein systems southern alps new zealand abstract the southern alps of new zealand is an active oblique collisional mountain belt with extensive regional tectonically driven fluid flow there is no evidence for igneous activity and fluids consist of varying proportions of meteoric water and mid crustal fluid derived from dehydration reactions fluid flow is controlled by fracture porosity particularly in damage zones along faults and fault intersections gold and arsenic bearing veins exposed at the surface indicate two principal zones of gold mineralisation at depth one of these is in the highest mountains near to but not in the region of maximum uplift deformation is dominated by reverse faulting but some normal and strike slip faults occur as well the other zone of gold mineralisation is at and near the intersection of regional oblique dextral reverse faults and regional strike slip faults both zones are characterised by small discontinuous vein systems locally accompanied by ankeritic alteration of host rock veins occur in extensional and shear veins and in dilational jogs with implosion breccias gold mineralisation occurred at many structural levels between the brittle ductile transition and the near surface region the southern alps hydrothermal system represents an active roof zone to a mesothermal gold deposition system at depth as such this is a modern analogue for mesothermal gold terranes elsewhere in new zealand and around the world observations on the regional distribution of fluid flow in active orogens can give insights into fluid flow at depth where gold mineralisation is occurring now comparison of these observations with ancient gold bearing belts allows construction of three dimensional concepts of orogenic fluid flow and gold mineralisation

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

rotation of ductile fabrics across the alpine fault and cenozoic bending of the new zealand orocline this study examines the role and kinematic mechanism of vertical axis rotations and folding during early development phases of a transform plate boundary in continental crust near the alpine fault in new zealand this involved an initial period of widely distributed shear including wrench folding together with vertical axis rotations and orocline development followed by a focusing of slip onto a single fault in the south island the haast schist has been displaced horizontally in the neogene by c 480 km of dextral slip on the alpine fault ls tectonites on either side of the fault lie on different limbs of a profound z shaped orocline rotation of these limbs took place about a vertical axis resulting in 25 30 of clockwise rotation of fabrics to the nw of the alpine fault relative to those to the south foliation anisotropy caused bending to take place locally by folding about an inclined hinge in the limbs of a pre existing synform which tightened during the deformation on both sides of the fault involvement of post oligocene age structures in wrench folding and bending suggests that the bending took place entirely in the cenozoic before or during miocene development of the alpine fault as a through going structure during this early phase of plate boundary deformation shear was distributed across a zone 100 km wide vertical axis rotation affected fault bounded basement slivers that shortened perpendicular to their strike a style of deformation like that in the transverse ranges in california today

fault bounded blocks and their role in localising sedimentation and deformation adjacent to the alpine fault southern new zealand abstract continental transform boundaries in detail consist of zones of fault bounded blocks adjacent to the principal active transform fault s relative movement of these blocks in response to plate boundary motions gives rise to differential vertical displacements of the earth s crust and hence exercises a degree of control on the tectonic evolution of sedimentary basins adjacent to the transform boundary the cenozoic sedimentary basins of southwest new zealand have several features in common that may be attributed directly to the movement on the adjacent plate boundary however their location detailed sedimentary evolution and tectonic development may best be understood in the context of the relative displacements of a number of fault bounded blocks in response to the plate motions in particular movements of the fiordland block bounded by the alpine and moonlight fault zones exercised a large measure of influence on the development of the basins the tectonic model presented is consistent with plate boundary motions inferred from marine magnetic anomalies while at the same time providing an explanation for many of the more detailed features of the basins

earthquakes gravity and the origin of the bali basin an example of a nascent continental fold and thrust belt we infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the bali region that the bali basin is a downwarp in the crust of the sunda shelf produced and maintained by thrusting along the flores back arc thrust zone earthquake source mechanisms and focal depths are inferred from the inversion of long period p and sh waves for all events and short period p waves for two of the events centroidal depths that give the best fit to the seismograms range from 10 to 18 km but uncertainties in depth allow a range from 7 to 24 km the p wave nodal planes that dip south at 13 to 35 7 strike roughly parallel to the volcanic arc and are consistent with thrusting of crust of the bali basin beneath it the positions of the earthquakes with respect to crustal features inferred from seismic and gravity data suggest that the earthquakes occur in the basement along the western end of the flores thrust zone the slip direction for the back arc thrust zone inferred from the orientation of the earthquake slip vectors indicates that the thrusting in the bali basin is probably part of the overall plate convergence as it roughly coincides with the convergence direction between the sunda arc and the indian ocean plate summation of seismic moments of earthquakes between 1960 and 1985 suggests a minimum rate of convergence across the thrust zone of 4 2 mm a the presence of back arc thrusting suggests that some coupling between the indian ocean plate and the sunda arc occurs but mechanisms such as continental collision or a shallow subduction of the indian ocean plate probably can be ruled out the present tectonic setting and structure of the bali basin is comparable to the early forelands of the andes or western north america in that a fold and thrust belt is forming on the continental side of an arc trench system at which oceanic lithosphere is being subducted the bali basin is flanked by the tertiary java basin to the west and the oceanic flores basin to the east and thus provides an actualistic setting for the development of a fold and thrust belt in which structure and timing of deformation can change significantly along strike on the scale a few hundred kilometers

spatial clustering and stress drops of foreshocks of the february 1990 tennyson and weber new zealand earthquakes the ability to distinguish foreshocks from background seismicity is very important in short term earthquake prediction to that end we have looked at spatial clustering using waveform cross correlation and stress drops of foreshocks of two new zealand earthquake sequences that occurred in 1990 the tennyson sequence located in a continental margin type strike slip environment consisted of a group of foreshocks an ml 5 8 mainshock and many aftershocks a cross correlation analysis showed five spatially close clusters of activity prior to the mainshock two were event pairs located within the final aftershock zone two were clusters of four events each located outside the aftershock zone and the fifth was a cluster of eight immediate foreshocks loated within the aftershock zone an analysis of two nearby control regions showed that pairs of identical events were not uncommon but larger clusters were stress drops of three events in the 12 days before the mainshock obtained by deconvolving small events as empirical green s functions were lower than for earlier preshocks and aftershocks source time functions derived from the green s function deconvolution indicated that a unilateral rupture model was more appropriate than a circular source model cross correlation values from the ml 5 9 weber sequence also showed spatial clustering but this was well removed from the mainshock in space and time a control area also showed similar clustering suggesting that it is a normal feature of the seismicity at a convergent margin the weber foreshocks only four in all were not highly correlated for both sequences foreshocks did not correlate with the aftershocks indicating that they occurred in a region of complete coseismic stress relief a stress drop of 1650 bars was obtained for a 44 km deep event that occurred within the upper part of the subducting pacific plate nearby but not related to the tennyson sequence

source parameters for 11 earthquakes in the tien shan central asia determined by p and sh waveform inversion the tien shan mountain belt of central asia was formed in late paleozoic time and reactivated in tertiary time following the collision of india with eurasia more than 1500 km to the south to study the style and distribution of faulting occurring today in the tien shan we digitized long period world wide standard seismograph network p and sh waveforms of 11 of the largest tien shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths four of the earthquakes occurred near the southern edge of the tien shan and two occurred in the intermontane fergana basin these earthquakes occurred at depths of 10 20 km and are associated with thrust faulting on east west to southwest northeast striking fault planes that dip moderately 35 55 the other five earthquakes in the northern tien shan were mostly deeper 15 44 km and the fault plane solutions of four of them are similar to the events farther south the exception an event on june 2 1973 probably occurred on a gently northward dipping east west striking fault plane all events occur within the basement at depths of 10 km or greater waveforms for earthquakes near the edges of the dzungarian fergana and kucha basins are fit best using velocity structures with thick sediment layers implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains we examined only the relatively small mb 5 5 6 2 events after 1965 and did not study any of the largest events ms 8 that occurred in the tien shan earlier in this century still we can conclude that north south shortening is presently occurring in the tien shan with the formation of basement uplifts flanked by moderately dipping thrust faults the present day tectonics of the tien shan seem to be analogous to those of the rocky mountains in colorado wyoming and utah during the laramide orogeny in late cretaceous and early tertiary time

geodetically determined strain across the southern end of the tonga kermadec hikurangi subduction zone summary a method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used but the angles of the old survey have not necessarily been reobserved this method is applied to triangulation in the northern south island at the southern end of the tonga kermadec hikurangi subduction zone from a profile of shear strain across the indian pacific plate boundary the displacement of the indian plate relative to the pacific is calculated to be 54 9 mm yr at an azimuth of 84 f lo in remarkable agreement with the motion predicted by global plate tectonic models most of this motion occurs within a 15okm wide zone bounded on the east by the hikurangi trough within this zone the motion is partitioned near the hikurangi trough no slip is occurring at the upper surface of the subducting pacific plate the subduction thrust and motion is predominantly thrusting normal to the trough axis to the west is a region of predominantly dextral strike slip faulting this pattern is consistent with fitch s model of oblique subduction to the south of the profile a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab indicating the end of the partitioned motion

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

late barremian early aptian dark facies of the subbetic betic cordillera southern spain calcareous nannofossil quantitative analyses chemostratigraphy and palaeoceanographic reconstructions abstract in order to determine the driving mechanism behind the deposition of the aptian dark facies in the pelagic subbetic basin southern spain two latest barremian early aptian complementary sections embracing the oceanic anoxic event 1a were selected for a multidisciplinary study quantitative and statistical analyses of calcareous nannofossil assemblages are complemented with planktonic foraminifera radiolaria and geochemical data c and o isotopes and biomarkers the results indicate that most of the latest barremian was characterized by oligotrophic conditions with a low nutricline and neritic influence which were interrupted by short lived episodes of mesotrophic surface waters in contrast broadly meso eutrophic surface waters with a high nutricline and a probable development of water stratification are inferred for most of the early aptian it is suggested that an increased runoff as a consequence of a more humid climate in the adjacent continental area caused this fertilization neritic dwellers such as nannoconids and pentaliths experienced a drastic decrease in abundance as the result of the combined effects of a biocalcification crisis nannoconid crisis and the deepening of the basin high productivity and enhanced surface water stratification favoured low oxygen content in the bottom waters increasing organic matter preservation a reduction in surface water fertility and a shift towards generally cooler conditions occurred during the late early aptian it is postulated that a cooler and drier climate leads to a reduction in runoff and decreased nutrient flux into the basin the obtained results suggest that the formation of the aptian dark facies was mainly controlled by enhanced production of organic matter in the upper water column and not enhanced preservation

continental extension of the pacific plate at the southern termination of the hikurangi subduction zone the north mernoo fault zone offshore new zealand a complex transition from subduction at the hikurangi margin to transpressive continental collision across the southern alps occurs in the ne south island region of new zealand offshore the southern limit of oceanic subduction is constrained by the chatham rise a submerged continental plateau on the pacific plate which is presumably too buoyant for normal subduction in the nw corner of the rise is the north mernoo fault zone nmfz a 100 by 300 km region of basement involved active normal faulting of late miocene circa 8 6 ma to recent age the nmfz evolved contemporaneously with the rotation and development of the adjacent plate boundary zone growth faulting is characteristic within the sedimentary cover which is generally less than 2 km thick but the distribution of faulting has varied temporally the amount of mid pliocene recent extension is estimated to be not more than a few percent the nmfz consists of an array of overlapping south dipping normal faults which are typically 2 5 km apart and which trend roughly e w at a high angle to the plate boundary zone late quaternary surface traces are widely distributed on the mid to upper continental slope but many surface scarps are poorly preserved due to extensive erosion of the seafloor the western end of the fault zone crosses submarine canyons at the southern end of the hikurangi trough and almost abuts two fold and thrust fault systems on the ne south island continental margin although few data constrain the kinematics of extension a single mw 6 1 earth quake at the western end of the nmfz suggests planar rotating faults with a component of left lateral oblique slip two possible models of regional extension are considered 1 lateral buckling of the upper crust in which faulted slivers of crust are peeled off as the chatham rise slides past the transpressive marlborough fault system toward the southern alps and 2 flexure of the edge of continental pacific plate as the chatham rise is bent downward into the southern end of the hikurangi subduction zone improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

active tectonics of the north and central aegean sea summary in this paper we examine the connection between the westward motion of turkey relative to europe and the extension in and around the aegean sea the principal new data available since the last attempt to synthesize the tectonics of this region by mckenzie 1978 are much improved focal mechanisms of earthquakes constrained by p and sh body wave modelling as well as by first motions these mechanisms show that the faulting in the western part of the aegean region is mostly extensional in nature on normal faults with a nw to wnw strike and with slip vectors directed nnw to nne there is evidence from palaeomagnetism that this western region rotates clockwise relative to stable europe in the central and eastern aegean and in nw turkey distributed right lateral strike slip is more prevalent on faults trending ne to ene and with slip vectors directed ne palaeomagnetic data in this eastern region is more ambiguous but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations relative to europe in the south the strike slip faulting that enters the central aegean from the east appears to end abruptly in the sw against the nw trending normal faults of greece the kinematics of the deformation is controlled by three factors the westward motion of turkey relative to europe the continental collision between nw greece albania and the apulia adriatic platform in the west and the presence of the hellenic subduction zone to the south as the right lateral slip on the north anatolian fault enters the aegean region it splays out becoming distributed on several parallel faults the continental shortening in nw greece and albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed e w right lateral shear and thus leads to e w shortening in the northern aegean which is compensated by n s extension as the southern aegean margin can move easily over the hellenic subduction zone the dynamics of the system once initiated is self sustaining being driven by the high topography in eastern turkey and by the roll back of the subducted slab beneath the southern aegean the geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate in spite of its extreme simplicity a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the aegean region including the slip vectors and nature of the faulting in the eastern and western parts the senses and approximate rates of rotation the overall extensional velocity across the aegean and the distribution of strain rates as seen in the seismicity and topography or bathymetry and using geodetic measurements as part of this study we re examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 gulf of corinth earthquakes and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting an important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves

preliminary geodetic strain measurement from the south fiordland region of new zealand using repeat gps surveys implications for subduction on the puysegur trench comparing 1996 gps data and data collected only one year earlier demonstrates that 50 of the instantaneous relative plate motion is accommodated between the west coast of the south island and the eastern side of the waiau basin a distance of about 75 km the observed relative velocity vectors are nearly parallel to the inferred orientation of the plate boundary and rotated more than 30 clockwise compared to the relative plate motion vector a difference that is significant at the 95 level of confidence modeling shows that the observed deformation rates are consistent with a model where nearly all the relative plate motion occurs on the plate boundary thrust as defined by the benioff zone associated with subduction on the puysegur trench

geoid anomalies over gorringe ridge north atlantic ocean abstract gorringe ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the azores gibraltar plate boundary the geoid over this structure derived from seasat altimeter data exhibits a 9 m height anomaly with a north south lateral extension smaller than 200 km an attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity which has been compiled as a function of depth it is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded as it assumes a continuous unfractured elastic plate models involving local heterogeneities are then tested the comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation uncompensated models previously proposed to explain the gravity anomalies are tested using the geoid one model purdy and bonnin in bonnin 11 which involves an uplift of upper mantle material at depth generates too strong geoid anomalies and must be discarded another model which represents a nascent subduction zone le pichon et al 25 fits both the gravity and geoid anomalies but leads to difficulties in explaining the deep seismicity north of gorringe ridge a model in isostatic equilibrium is also able to fit both gravity and geoid anomalies this model involves a deep root of density 3 0 g cm 3 as has been previously proposed for many oceanic ridges and plateaus this model is compatible with the deep seismicity but the origin of this low density material at great depth is up to now an unresolved question more likely dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered

structure and evolution of the seismically active ostler fault zone new zealand based on interpretations of multiple high resolution seismic reflection profiles abstract to improve our understanding of active faulting away from the main plate boundary on new zealand s south island we have acquired high resolution seismic data across the ostler fault zone twelve 1 2 km long lines perpendicular to fault strike and a 1 6 km long crossline were collected in a region of the mackenzie basin where surface mapping delineates significant complexity in the form of two non overlapping reverse fault strands separated by a transfer zone characterised by multiple smaller strands and increased folding interpretation of the resultant images includes a 45 55 west dipping principal fault and two 25 30 west dipping subsidiary faults one in the hanging wall and one in the footwall of the principal fault the geologically mapped complexities are shown to be caused by shallow variations in the structure of the principal fault which breaks the surface in the north and south but not within the transfer zone where it forms a triangle zone with associated backthrusting and minor faulting these complexities only extend to 300 m depth structures below this level are markedly simpler and much more 2d in nature with the principal fault strand extending over a much longer distance than the individual strands observed at the surface since longer faults are susceptible to larger earthquakes than shorter ones seismic hazard at the study site may be higher than previously thought multiple surface fault strands that give way to a single more major stand at relatively shallow depths may be a common feature of segmented fault systems the deepest layered reflections at our site are consistent with the presence of a late cretaceous tertiary basin underlying the present day mackenzie basin structural restoration of the seismic images back to the base of quaternary fluvioglacial terraces and back to the top of a late pliocene pleistocene fluviolacustrine unit indicate that compression was initiated prior to the late pliocene and that it has continued at a comparatively steady rate of about a millimetre per year to the present day the fluviolacustrine unit has experienced 440 800 m of along fault vertical offset and 870 1080 m of horizontal shortening since that time our study demonstrates that structural reconstructions based on high resolution seismic reflection data provide critical displacement information that can be used to estimate slip rates

observations of infrasound from surf in southern california 1 we observe 10 000 s of infrasound signals annually in the frequency range from 1 5 hz at the i57us infrasound array in southern california 75 of these signals arrive at i57us at azimuths between 260 and 320 degrees there is a good correlation between the amplitudes of signals from 260 320 and wave height measurements offshore southern california providing firm evidence that the signals from these azimuths are caused by surf action by modeling the propagation of infrasound for specific time periods using 3d ray tracing and up to date atmospheric models we show that amplitudes of signals recorded at i57us are also dependent upon atmospheric winds the modeled rays that return in the stratosphere fit observations more closely than modeled rays that return in the troposphere suggesting the signals are from stratospheric returns our findings are the first observations of surf infrasound at a long range from the source region this suggests that surf signals might serve as a probe of the atmosphere much like microbarom signals are used to probe the atmosphere at greater range

structure and kinematics of oblique continental convergence in northern fiordland new zealand abstract southeast of the alpine fault in fiordland new zealand late tertiary faults envelope exposures of early cretaceous high p 12 16 kbar granulite facies orthogneisses these exposures allowed us to examine how pre cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence fault slip data and kinematic modelling show that oblique slip and reverse displacements preferentially were partitioned east of the alpine fault onto curved ne striking surfaces and moderately dipping 30 52 n and nw striking surfaces respectively approximately 3 0 5 5 km of exhumation and 6 5 7 0 km of oblique dextral displacement occurred on faults that display palm tree style geometries and subhorizontal detachments in profile this heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the alpine fault north of milford sound the principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7 9 ma the effects of these pre existing structures and a focusing of contractional deformation along the northeast margin of fiordland resulted in an unusually high degree of strike slip partitioning across the southernmost onshore segment of the alpine fault

stresses and strains in a twisted subduction zone fiordland new zealand summary in the fiordland region of the south island of new zealand the subducted australian plate steepens from south to north becoming near vertical below 75 km depth between doubtful and milford sounds this steepening is accompanied by a sharp change in strike of the subduction zone of about 17 here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three month deployment of 24 portable digital seismographs we determine focal mechanisms of 115 earthquakes of ml 2 5 4 5 using both first motion polarity data and amplitudes of seismogram envelopes we also invert for the stress tensor orientation for regions within both the subducted and overlying plates using a method which invokes the coulomb failure criterion and considers all p wave first motions together regardless of whether or not they are sufficient to define single event focal mechanisms focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths this stress and strain regime is related to the 17 change in strike of the subduction zone which leads to an arching up of the subducted plate at the bend such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend earthquakes in the subducted plate show predominantly normal faulting relative to the plate arrival time inversions for 3 d seismic velocity structure indicate that a zone of high vp 8 5 km s 1 in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone the steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike this mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction

abrupt strike slip fault to subduction transition the alpine fault puysegur trench connection new zealand swath bathymetry and other geophysical data collected over the fiordland margin southwest of new zealand are used to investigate the mechanism of transform subduction transition between the alpine fault and the puysegur trench two segments of the pacific australian plate boundary in this region the cenozoic southeast tasman basin which obliquely underthrusts fiordland at the puysegur trench is separated from the cretaceous tasman basin by the resolution ridge system a major lithospheric discontinuity of the downgoing plate interpretation of seafloor morphology shows that the alpine fault extends offshore along the fiordland coast and splits into west and east branches the west branch cuts obliquely across the margin and connects sharply to the puysegur subduction front at the northeastern tip of the resolution ridge system earthquake and seismic reflection data indicate that the west branch is genetically controlled by downgoing plate structures associated with the resolution ridge system hence the west branch is interpreted as the surface trace of the plate boundary segment extending between the alpine fault and the puysegur trench we conclude that the development of the strike slip segment of the plate boundary and its sharp transition to the puysegur subduction are controlled by inherited structures of the australian plate furthermore according to geophysical data presented here a tearing of the downgoing plate can be interpreted beneath the west branch a review of geophysical data along the region of the alpine fault hikurangi trough northeast new zealand shows a progressive transform subduction transition that is accommodated by motion partitioning between the subduction interface and strike slip faults this transition is accounted for by an interplate coupling that progressively increases toward the alpine fault in relation with a gradual thickening of the downgoing crust the comparison between the fiordland and the hikurangi strike slip subduction transitions show that presence of inherited downgoing plate crustal faults properly oriented with respect to the plate motion facilitates a sharp strike slip subduction transition

outer rise earthquakes and seismic coupling outer rise earthquakes can be separated into two basic categories based on focal mechanism compressional and tensional events reflecting compressional and tensional outer rise stress regimes we propose a model that connects the stress regime in the outer rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer rise earthquakes we find that compressional outer rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones or equivalently just prior to large subduction zone earthquakes the compressional stress regime is probably caused by slip in adjacent subduction zone segments there are two modes of occurrence for tensional outer rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones in coupled subduction zones tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer rise in subduction zones that are inherently uncoupled tensional events can occur at any time therefore in uncoupled subduction zones the outer rise is always in tension and only tensional outer rise events occur there while in coupled subduction zones the outer rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements

focal mechanisms of large earthquakes in the south island of new zealand implications for the accommodation of pacific australia plate motion b the plate motion model nuvel 1 predicts oblique convergence between the pacific and australian plates in the south island of new zealand we used p and sh body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes ms 5 8 that have occurred in this region since 1964 in order to see how the plate motion is accommodated at the southern end of the alpine fault convergence is achieved by oblique slip movement along a concentrated zone of deformation in the southern offshore region one event may be related to thrusting of the australian plate beneath the pacific plate and another strike slip event probably demonstrates movement on an active strike slip fault system parallel to but offset from the southern limit of the alpine fault this geometry provides a possible mechanism for the rapid uplift of the fiordland region deformation in the northern south island is more distributed in the south west marlborough region partitioning occurs between strike slip faulting in the se and reverse faulting farther nw in the buller region we suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre existing normal faults in the buller region shortening in the buller region may have deflected the ne end of the alpine fault towards the nw forming the prominent bend the marlborough fault system with its youngest and most active faults to the se probably developed in an attempt to maintain a through going strike slip structure as each of the strike slip faults was transported towards the north west partitioning of the opposite polarity with reverse faulting se of the strike slip faulting occurs in north east marlborough the boundary between the two different styles of partitioning in ne and sw marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the marlborough fault system a normal faulting earthquake on the northern edge of the chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying marlborough fault system

joint hypocentre determination of intermediate depth earthquakes in fiordland new zealand abstract relocation of well observed intermediate depth earthquakes in the fiordland region by the method of joint hypocentre determination has revealed some fine structure in the benioff zone the earthquakes occur in three groups the central group is the largest and occupies a planar volume less than 15 km thick striking n40 e and dipping at 80 the deepest events in the region at depths of 150 km occur at the northeast end of this group the two smaller groups lie to the northeast and to the south of the main group the focal mechanism of the majority of the main group is that of thrust faulting we suggest that the main group lies within a section of indian plate lithosphere which has been broken off and rotated into its observed position and that the northern edge of the unbroken subducted indian plate is indicated by the southern group we suggest that the small northeastern group has quite a different tectonic origin and is similar to a group of earthquakes further north which are at a similar distance from and presumably related to the alpine fault use has also been made of the travel time information which is a by product of the joint hypocentre method to construct upper mantle velocity models for p and s waves in the south island the features of this model are a high velocity region in the vicinity of the benioff zone and a subcrustal zone of high seismic velocities running east west across the center of the south island in an otherwise normal mantle

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

earthquake source parameters and stress distribution in the adak island region of the central aleutian islands alaska source parameters have been systematically determined for all earthquakes with mw 5 that occurred between 172 w and 179 w longitude in the adak island region of the central aleutian islands during 1977 march 1987 we relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination the first method uses arrival times of direct and prominent reflected phases primarily pwp the second method uses broadband p wave displacement seismograms in an inversion for source depth the analysis of these events provides a well constrained data set for the study of stress release along an active subduction zone three earthquakes which occurred seaward of the trench axis are located just below the crust mantle interface and show extension nearly perpendicular to the trench axis seven events occurred in the wadati benioff zone sixty four events are located in the main thrust zone and except for five unusual events are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle in cross section the thrust zone appears as a thin 10 15 km thickness interplate region which extends from 15 to 50 km depth a 10 15 average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the pacific and north american plates exists for these events the observed slip vectors are oriented more normal to the trench than is predicted by plate motions five earthquakes which occurred as aftershocks to the may 7 1986 earthquake mw 8 0 in the crust of the overriding plate have strike slip mechanisms consistent with right lateral motion on arc parallel fault planes observations of slip vectors along the whole aleutian arc show a similar trend to that observed in the adak island region the largest differences 30 between observed and predicted slip azimuths occur around 175 e we propose a model of plate interaction in which a portion of the along arc motion occurs along a weak strike slip shear zone in the upper plate near the volcanic line the slip azimuths in the main thrust zone fit this model well if the amount of transcurrent slip occurring in the upper plate is 60 of the arc parallel relative plate motion a consequence of the model is along arc extension of the overriding plate between the accretionary wedge and the volcanic line especially in the western part of the aleutian arc calculations based on a tectonic model by geist et al 1988 for the formation of arc summit basins through block rotation and translation suggest that along arc extension has been significant since late miocene or early pliocene

transition from strike slip faulting to oblique subduction active tectonics at the puysegur margin south new zealand abstract south of new zealand the pacific australia pac aus plate boundary runs along the intracontinental alpine fault the puysegur subduction front and the intraoceanic puysegur fault the puysegur fault is located along puysegur ridge which terminates at ca 47 s against the continental puysegur bank in a complex zone of deformation called the snares zone at puysegur trench the australian plate subducts beneath puysegur bank and the fiordland massif east of fiordland and puysegur bank the moonlight fault system mfs represents the eocene strike slip plate boundary interpretation of seafloor morphology and seismic reflection profiles acquired over puysegur bank and the snares zone allows study of the transition from intraoceanic strike slip faulting along the puysegur ridge to oblique subduction at the puysegur trench and to better understand the genetic link between the puysegur fault and the mfs seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 geodynz survey and single beam echo soundings acquired by the nz royal navy the snares zone is the key transition zone from strike slip faulting to subduction it divides into three sectors namely east nw and sw sectors a conspicuous 3600 m deep trough the snares trough separates the nw and east sectors the east sector is characterised by the ne termination of puysegur ridge into right stepping en echelon ridges that accommodate a change of strike from the puysegur fault to the mfs between 48 s and 47 s in the nw sector and the snares trough a series of transpressional faults splay northwards from the puysegur fault between 49 50 s and 48 s thrusts develop progressively at puysegur trench into a decollement north of 48 s the snares trough develops between two splays of the puysegur fault indicating superficial extension associated with the subsidence of puysegur ridge seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the snares fault and terminate at the top of the puysegur trench slope between ca 48 s and 46 30 s the relative plate motion appears to be distributed over the puysegur subduction zone and the strike slip faults located on the edge of the upper plate conversely north of ca 46 s a lack of active strike slip faulting along the mfs and across most of puysegur bank indicates that the subduction in the northern part of puysegur trench accounts for most of the oblique convergence hence active transpression in the snares fault zone indicates that the relative pac aus plate motion is transferred from strike slip faulting along the puysegur fault to subduction at puysegur trench the progressive transition from thrusts at puysegur trench and strike slip faulting at the puysegur fault to oblique subduction at puysegur trench suggests that the subduction interface progressively developed from a western shallow splay of the puysegur fault it implies that the transfer fault links the subduction interface at depth a tectonic sliver is identified between puysegur trench and the puysegur fault its northwards motion relative to the pacific plate implies that is might collide with puysegur bank

modeling stress changes induced by earthquakes in the southern marlborough region south island new zealand between 1888 and 1994 a series of seven earthquakes of m w 5 9 7 2 occurred within a 150 km by 150 km area southern marlborough region directly east of the alpine fault major strike slip faults in this region including the hope kakapo and clarence faults serve to transfer plate motion from the alpine fault northeastward to the hikurangi subduction zone we have modeled induced changes in coulomb failure stress cfs to determine whether stress triggering could have occurred during this sequence and to determine how the earthquakes have affected stress along the north westland segment of the alpine fault which appears to have last ruptured in this region in a d 1717 we included the effects of secular stress accumulation on the four most rapidly slipping faults in the region alpine awatere clarence and hope but neglected the effects of viscoelastic relaxation our results suggest that at least six of seven earthquakes and possibly all seven events when uncertainties in earthquake location and focal mechanism are considered occurred in regions where cfs exceeded 10 kpa indicating stress triggering may have occurred although cfs exceeds 10 kpa along most of the north westland segment of the alpine fault at depths of 10 km or less cfs is less than 10 kpa at depths of 10 km or more if rupture for large alpine fault events initiates at depths of 10 km or more then this result implies that the induced stress since 1848 has not brought this segment significantly closer to failure manuscript received 1 february 2001

seismicity structure and tectonics of the wellington region new zealand summary the wellington region lies on the border between the pacific and australian plates the former being subducted the plate interface lies at shallow depth under the region 10 60 km so that detailed and long term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible hypocentres for events recorded by a 12 station seismic network for the period 1978 1982 7231 events have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival time data from 129 local events for both hypocentres and velocity the hypocentres show that the subducting pacific plate is defined by a nw dipping zone of relatively intense activity the plate interface as given by the upper envelope of this activity is offset vertically by 7 km on average along a nw se line through cook strait parallel to the direction of absolute motion of the pacific plate rather than to the direction of convergence the strike of the interface at deeper levels also changes across this line this distortion may partly reflect effects of the transition to continent continent type convergence further south within the subducting pacific plate the seismic activity defines two parallel zones separated by about 20 km the lower zone being much less active and perhaps merging with the upper zone at a depth of 80 90 km composite focal mechanisms of events in the upper zone imply predominantly down dip tension with a compression axis perpendicular to the plate interface the mechanism in the lower zone also implies down dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate these two types of mechanism correlate well with those found previously for events at depths of 150 km or more thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region the lithosphere acting as a stress guide there is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface the seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust which here is probably considerably thicker than normal there is no evidence for a substantial increase in velocity down

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

determination of fault planes in a complex aftershock sequence using two dimensional slip inversion summary the 1994 arthur s pass earthquake mw 6 7 south island new zealand had a complex aftershock sequence including events aligned with major mapped faults to determine whether the major ne sw trending strike slip faults in the region were activated during this aftershock sequence we investigate the largest well recorded aftershocks the arthur s pass earthquake itself was a reverse faulting event but the majority of the aftershocks were strike slip we use the empirical green s function method to obtain source time functions for four aftershocks ml 4 1 5 1 we then invert for slip on each nodal plane and compare the variance reduction to determine which is the fault plane the two largest earthquakes ml 5 1 and ml 4 2 located close to the mapped trace of the bruce fault both occurred on fault planes striking nnw sse perpendicular to the strike of the bruce and other regional strike slip faults the third earthquake studied ml 4 1 located on a lineation of aftershocks parallel to the regional mapped trend had a preferred fault plane with a ne sw strike the fourth aftershock ml 4 1 was located close to the main shock fault plane and had an oblique reverse mechanism this earthquake exhibited northward directivity but the fault plane could not be identified the earthquake stress drops ranged from 1 to 10 mpa

tsunami hazards along the eastern australian coast from potential earthquakes results from numerical simulations australia is surrounded by the pacific ocean and the indian ocean and thus may suffer from tsunamis due to its proximity to the subduction earthquakes around the boundary of australian plate potential tsunami risks along the eastern coast where more and more people currently live are numerically investigated through a scenario based method to provide an estimation of the tsunami hazard in this region we have chosen and calculated the tsunami waves generated at the new hebrides trench and the puysegur trench and we further investigated the relevant tsunami hazards along the eastern coast and their sensitivities to various sea floor frictions and earthquake parameters i e the strike the dip and the slip angles and the earthquake magnitude rupture length the results indicate that the puysegur trench possesses a seismic threat causing wave amplitudes over 1 5 m along the coast of tasmania victoria and new south wales and even reaching over 2 6 m at the regions close to sydney maria island and gabo island for a certain worse case while the cities along the coast of queensland are potentially less vulnerable than those on the southeastern australian coast

partitioning of strike slip and convergent motion between eurasia and arabia in eastern turkey and the caucasus the plate motion model nuvel 1 de mets et al 1990 predicts a strongly oblique component to the convergence between arabia and eurasia at the longitude of the caucasus earthquake focal mechanisms suggest that this oblique convergence is partitioned into pure right lateral strike slip motion in eastern turkey and pure shortening by thrusting in the greater caucasus such partitioning offers an explanation for the apparent eastward continuation of the right lateral faulting of the north anatolian fault from turkey into iran from a summation of seismic moment tensors between 1911 and 1991 it appears that most of the right lateral strike slip motion in eastern turkey is accommodated by faulting in large earthquakes while most of the shortening in the caucasus appears to occur aseismically

late cenozoic exhumation model of new zealand impacts from tectonics and climate abstract late cenozoic convergence between the australian and pacific plates has led to extensive crustal deformation and exhumation in new zealand over the past five decades the convergence associated exhumation history of the palaeozoic mesozoic new zealand basement has been studied using various thermochronometric techniques here we compile 992 published ages and invert them to estimate the temporal and spatial variation in exhumation rates since 25 ma which we then interpret with a plate tectonic reconstruction model the results reveal widespread increases in the exhumation rates at 6 4 ma a time correlated with accelerations in the relative plate motion the absolute increases in exhumation rates are however not uniformly distributed over new zealand since the plio pleistocene the inferred models suggest variable response of exhumation to global cooling in the non glaciated eastern north island the hikurangi subduction margin the million year average exhumation rates have remained relatively low 5 km myr have been observed in the collision zone of the central southern alps of south island where both tectonic deformation and precipitation rates are highest in fiordland the puysegur subduction margin despite the weak crustal deformation the exhumation rates have been high 0 5 3 km myr which we suggest is due to intensified glacial erosion our findings highlight the determining role of tectonics in setting up the regional exhumation pattern but both strong tectonic deformation and efficient erosion are required to maintain rapid exhumation rates the influence of climate on mountain erosion can only be distinguished in regions under slow to mediate exhumation

potentially active faults in the rapidly eroding landscape adjacent to the alpine fault central southern alps new zealand 1 potentially active faults are exposed in the steep glaciated topography of the central southern alps new zealand immediately adjacent to the alpine fault plate boundary four major faults exposed along the flanks of three of the highest mountain ranges strike 10 23 km potentially 40 km nne oblique to the alpine fault dipping 57 12 nw in the opposite direction youngest discernable motions were reverse dip slip accommodating both margin perpendicular shortening and dextral margin parallel components of plate motion kinematic analysis yields a compression axis 295 10 9 trend or plunge equivalent to the contemporary shortening determined from seismological and geodetic studies suggesting the faults may be active although definitive evidence for recent movement or single event displacements is lacking there are 106 other potentially active faults mapped in central southern alps with strike lengths 4 73 km earthquake parameters were assigned from fault trace lengths and historical earthquake statistics indicating potential for mw 5 5 7 4 earthquakes at recurrence intervals of 1000 10 000 years such long recurrence intervals are consistent with the faults having little surface expression with rapid erosion of these seismically agitated mountains erasing any evidence of surface rupture during periods between earthquakes the central southern alps faults exemplify the difficulty in fully deciphering long term e g holocene or quaternary records of seismicity in tectonically active regions with rapidly evolving landscapes although there may be little evidence of surface ruptures remaining in the landscape the faults are still an important potential source of earthquakes and seismic hazard

andersonian wrench faulting in a regional stress field during the 2010 2011 canterbury new zealand earthquake sequence the initial mw7 1 darfield earthquake sequence was centred west of christchurch city in the south island of new zealand but aftershocks including a highly destructive mw6 3 event eventually extended eastwards across the city to the coast the mainshock gave rise to right lateral strike slip of up to 5 m along the segmented rupture trace of a subvertical fault trending 085 58 across the canterbury plains for c 30 km in agreement with teleseismic focal mechan isms near field data however suggest that the mainshock was composite initiating with reverse slip north of the surface rupture stress determinations for the central south island show maximum compressive stress s1 to be horizontal and oriented 115 58 the principal dextral rupture there fore lies at c 3 08 to regional s1 the classic andersonian orientation for a low displacement wrench fault an aftershock lineament trending c 1458 possibly represents a conjugate left lateral strike slip structure this stress field is also consistent with predominantly reverse slip reactivation of nne ne faults along the southern alps range front the main strike slip fault appears to have a low cumulative displacement and may represent either a fairly newly formed fault in the regional stress field or an existing subvertical fault that happens to be optimally oriented for frictional reactivation

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

stress field and kinematics for diffuse microseismicity in a zone of continental transpression south island new zealand we analyze shallow 0 20 km microseismicity adjacent to the alpine fault in new zealand where there is oblique convergence of the australian and pacific plates focal mechanisms for 155 earthquakes june 2012 october 2013 are inverted to determine the orientation of the stress field this yields a principal horizontal axis of compression shmax 114 10 which cannot be explained in terms of the sum of stress from tectonic loading due to plate convergence indicated by gps observations and gravitational stresses the azimuth of slip vectors for individual focal mechanisms cluster perpendicular and parallel to the plate convergence vector these faults however strike at 45 to shmax from the stress inversion suggesting a very low coefficient of friction the earthquake slip directions may be kinematically controlled accommodating the plate convergence on a limited set of fractures similar to the segmentation for neotectonic faulting along the alpine fault which is partitioned into strike slip and thrust segments at a 1 10 km scale we suggest two possible controls on our calculated shmax azimuths firstly there may be a slight clockwise bias in the estimates of shmax from earthquakes slip may be occurring on a more limited range of fractures than assumed by the stress inversion method although this effect is likely to be relatively small 5 more importantly the components of the stress field may be relieved at different time scales during big earthquakes resulting in a residual stress field that varies significantly 15 on timescales of several large earthquakes

continental crust under compression a seismic refraction study of south island geophysical transect i south island new zealand 1 the 1996 south island geophysical transect sight active source seismic survey was designed to show the style of lithospheric thickening due to late cenozoic oblique convergence across the australian pacific plate boundary in new zealand as part of this study two seismic refraction lines were shot across central south island and offshore extensions of the continental crust in the tasman sea and pacific ocean we present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects a tomographic inversion of 62 563 travel times from crustal and upper mantle refractions and wide angle reflections resulted in a model with a two layer crust upper crustal velocities were between 5 9 and 6 3 km s and lower crustal velocities were between 6 5 and 7 0 km s continental compression has locally reduced the seismic velocities in the pacific plate crust by 0 2 0 3 km s a possible effect of high strain and fluids in the crust the thickening of the crust from 28 km at the east coast of south island to 37 km beneath the southern alps can account for about 25 of the 80 110 km shortening of pacific plate crust while the rest must be accounted for by rapid erosion of mesozoic sedimentary rocks on the west side of the orogen in our model the lower crust forms a continuous 2 6 km thick layer beneath central south island the asymmetric topography of the southern alps is reflected in the crustal root which has a steeper flank at the west coast this observation is consistent with westward underthrusting of pacific lower lithosphere beneath south island that has been suggested in earlier studies

stress changes induced by earthquakes and secular stress accumulation in the buller region south island new zealand 1929 2002 between 1929 and 1968 the buller region located west of the strike slip alpine fault system experienced two mw 7 1 earthquakes along high angle reverse faults we have modeled induced changes in coulomb failure stress cfs to determine whether stress triggering could have occurred during the 1929 and 1968 sequences and to determine how these earthquakes have affected stress along the alpine fault and neighboring strike slip faults located to the east of the alpine fault we have included the effects of secular stress accumulation on the five most rapidly slipping fault systems in the region alpine wairau awatere clarence and hope but have neglected the effects of viscoelastic relaxation our results suggest that larger aftershocks of the 1929 buller mainshock and moderate mw 7 0 events in 1848 and 1888 along strike slip faults to the east of the buller region or by secular stress accumulation since 1848 the 1929 buller earthquake may have delayed the 1968 inangahua mainshock present values of cfs along segments of the major strike slip faults located east of the buller region indicate that every fault segment except the north westlands north segment of the alpine fault contains regions of negative cfs that are related to the coseismic effects of mw 7 0 earthquakes occurring between 1848 and 1968 the complex variation in cfs along most major strike slip faults in the region highlights the difficulty in evaluating which faults may presently be the closest to failure

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

source parameters of seven large australian earthquakes determined by body waveform inversion summary we determine source mechanisms and centroid depths for the seven largest earthquakes in and near the australian continent in the past 20yr using teleseismic body waves six of the earthquakes were thrusts and one was strike slip all five earthquakes that occurred beneath the continent were shallower than 10 km the ranges of centroid depths are constrained to be 1 5 2 4 0 3 5 11 and 6 10km and indicate nearly pure thrust faulting the three shallowest events were associated with surface faulting while the published first motion solutions for these three events included large amounts of strike slip our thrust mechanisms are more consistent with displacements observed at the surface of the earth the range of possible centroid depths for a strike slip earthquake beneath the continental shelf off the northwest coast is 23 29 km and that for a thrust earthquake in oceanic lithosphere beneath the tasman sea is 22 26 km below sea level the pressure axis is consistently near horizontal and in the western half of the continent five earthquakes indicate that the azimuth of the maximum compressive stress is e ene azimuthal range of 66 91 on the basis of one earthquake we conclude that the p axis is oriented nnw beneath the tasman sea

seismic and geodetic constraints on plate boundary deformation across the northern macquarie ridge and southern south island of new zealand summary in the southern new zealand region the boundary between the australian and pacific plates changes from oblique continental convergence in the central south island of new zealand to interoceanic transcurrent motion along the macquarie ridge source parameters of recent post 1964 upper crustal events mw 5 3 in the region 43 53 s 155 175 e are determined using p and sh waveform analysis in order to investigate the nature of strain accommodated seismically throughout the region repeated occupation of three geodetic networks across the southern south island provides estimates of contemporary crustal strain seismicity over the last 35 years shows uneven spatial and temporal distribution with clusters of events occurring in regions along the plate boundary that exhibit structural variation focal mechanisms of individual earthquakes vary widely but are consistent with the accommodation of long term estimates of the relative plate motion across the southern south island geodetically measured principal contraction directions are consistent with p axis orientations the mode of accommodation of the relative plate motion varies throughout the region influenced by the change in crustal composition on either side of the plate boundary the underlying structure as a relic of previous relative motions and the variation of obliquity of the relative motion with respect to these structures

up dip partitioning of displacement components on the oblique slip clarence fault new zealand abstract active strike slip faults in new zealand occur within an obliquely convergent plate boundary zone although the traces of these faults commonly delineate the base of mountain ranges they do not always accommodate significant shortening at the free surface along the active trace of clarence fault in northeastern south island new zealand displaced landforms and slickenside striations indicate predominantly horizontal displacements at the ground surface and a right lateral slip rate of ca 3 5 5 mm year during the holocene the inland kaikoura mountain range occupies the hanging wall of the fault and rises steeply from the active trace to altitudes of ca 3 km the geomorphology of the range indicates active uplift and mountain building which is interpreted to result in part from a vertical component of fault slip at depth these data are consistent with the fault accommodating oblique slip at depth aligned parallel to the plate motion vector and compatible with regional geodetic data and earthquake focal mechanisms oblique slip on the clarence fault at depth is partitioned at the free surface into 1 right lateral displacement on the fault and 2 hanging wall uplift produced by distributed displacement on small scale faults parallel to the main fault decoupling of slip components reflects an up dip transfer of fault throw to an off fault zone of distributed uplift such zones are common in the hanging walls of thrusts and reverse faults and support the idea that the dip of the oblique slip clarence fault steepens towards the free surface

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

seven million years of strike slip and related off fault deformation northeastern marlborough fault system south island new zealand the marlborough fault system mfs south island new zealand is remarkable for the oblique slip nature of its active faulting and can be placed in a framework of pacific australian pac aus plate motions and geodetic strain new zealand illustrates changing degrees of upper plate slip partitioning in an obliquely convergent margin that varies along strike in crustal type and subduction geometry in this paper we focus on the ne part of this transcurrent fault system especially the awatere fault which is the only marlborough fault to offset a major neogene basin strike slip on an ancestral eastern strand of this fault began after 7 4 6 4 ma later vertical axis rotation may have caused it to be abandoned in favor of the currently active strand since 7 ma at least 34 10 km of dextral slip has accrued on the awatere fault 6 km of slip has accrued on two nearby strands and 4 km has accrued on the currently active coastal strand comparison of miocene with late quaternary offsets suggests that rates of strike slip on the fault have been nearly constant or have possibly decelerated we interpret spatial and temporal patterns of strike slip in the mfs to reflect southward migration of transform motion resulting from increased pac aus plate convergence late neogene to present day shortening directions in ne south island are indicative of a partially slip partitioned margin slip partitioning diminishes southward as the underlying subduction interface becomes strongly coupled suggesting a simple mechanical relationship between these processes

source parameters of large historical 1918 1962 earthquakes south island new zealand we have studied 18 earthquakes of m w 5 5 within the north island new zealand region to determine fault parameters for earthquake hazards and regional tectonic studies since most large m w 6 5 earthquakes in the north island occurred prior to 1961 these events provide important information to supplement studies of more recent seismicity our results indicate that no large plate interface earthquakes have occurred in the central and southern north island for the past 80 years although six strike slip earthquakes of m w 6 8 occurred within the australian upper plate during an extremely active 25 year period that began in 1917 only the m w 7 2 strike slip events were associated with surface faulting indicating the difficulties that may arise in attempting to identify active faults within this region two earthquakes m w 6 9 7 1 off the northeastern north island in 1947 appear to have occurred along the plate interface and were associated with local tsunamis having runup heights of up to 10 m two m w 6 8 events also occurred within the pacific lower plate highlighting the hazards related to intraslab events slip vectors for the earthquakes studied suggest that the majority of transcurrent motion along the plate margin is accommodated within the australian plate similar to the results obtained from studies of more recent smaller earthquakes pure thrusting occurs along the plate interface and t axes of intraslab events indicate downdip tension in the pacific plate

effects of near source bathymetry on teleseismic p waveforms teleseismic p waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms this study suggests that considerable misfit results from the non horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water crust interface geometric ray theory synthetics calculated for a double couple source show that p waveforms are extremely sensitive to seafloor orientation because of the large impedance contrast across the boundary seafloor dips as small as 0 5 significantly modify the ray paths affecting the take off angle at the source reflection and transmission coefficients at the interface geometric spreading and travel time to test the applicability of such modeling p waveforms from the march 29 1976 cocos ridge intraplate earthquake were studied the waveforms many of which show prominent water multiples poorly modeled with flat lying bathymetry were inverted for the best fitting planar ocean bottom orientation using a grid search technique synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure and reproduce the observed azimuthal dependence of water multiple amplitudes the derived ocean bottom dip angle and direction are in agreement with bathymetric charts suggesting p waveforms provide constraints on the seafloor orientation in the epicentral region green s functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

seismogenic stress field estimation in the calabrian arc region south italy from a bayesian approach a new high quality waveform inversion focal mechanism database of the calabrian arc region has been compiled by integrating 292 mechanisms selected from literature and catalogs with 146 newly computed solutions the new database has then been used for computation of posterior density distributions of stress tensor components by a bayesian method never applied in south italy before the present study the application of this method to the enhanced database has allowed us to provide a detailed picture of seismotectonic stress regimes in this very complex area where lithospheric unit configuration and geodynamic engines are still strongly debated our results well constrain the extensional domain of calabrian arc and the compressional one of the southernmost tyrrhenian sea in addition previously undetected transcurrent regimes have been identified in the ionian offshore the new information released here will furnish useful tools and constraints for future geodynamic investigations

extension and gold mineralisation in the hanging walls of active convergent continental shear zones abstract orogenic gold bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries the vein systems hosted in extensional structures within compressively deformed rocks are a widespread feature of these orogens in many cases the extensional structures that host gold bearing veins have been superimposed on and locally controlled by compressional structures formed within the convergent orogen exploring these observations within the context of a three dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens our models explore the effect of convergence angle and mid crustal strength on stress states and compare them to the southern alps and taiwan the dilatation zones coincide with the highest mountains in the hanging walls of major plate boundary faults and can extend as deep as the brittle ductile transition extensional deformation is favoured in the topographic divide region of oblique orogens with mid lower crustal rheology that promotes localisation rather than diffuse deformation in the near surface topography influences the stress state to a depth approximately equal to the topographic relief bringing the rock closer to failure and rotating 1 to near vertical the distribution of gold bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens

timing of late holocene paleoearthquakes on the hurunui segment of the hope fault implications for plate boundary strain release through south island new zealand the hurunui segment of the dextral slip hope fault extends for 42 km between the hope and hurunui river catchments and westward to harper pass at the main divide of south island we conducted paleoseismic soil and landscape dating studies in the upper hurunui valley to determine the timing of past earthquake ruptures along this geometric fault segment on a late holocene alluvial surface at matagouri flat a young channel system is displaced dextrally by 4 5 0 6 m which is attributed to the most recent faulting event a trench and soil pits excavated nearby yield evidence for the timing of the last two surface faulting events these events dated from a combination of radiocarbon dates relative soil tree and historical constraints modified within an oxcal analysis occurred at ca a d 1655 1835 and 1425 1625 the occurrence of two rupture events during the last 600 yr is consistent with previous estimates stating a short recurrence interval for the hurunui segment the most recent faulting event in our trenches predates the historic 1888 north canterbury amuri earthquake which ruptured the hope river segment to the east a comparison of the record from the hurunui segment with paleoseismic records from other segments along the hope fault zone indicates evidence for two or three rupture events within the last 700 900 yr within the age resolution of the current dates the data suggest that along strike rupture occurred along the majority of the hope fault zone between 120 and 360 yr ago i e a d 1650 1888 and may all postdate the most recent rupture of the alpine fault in ca a d 1717 in addition the timings of penultimate faulting events along the hurunui hope river and conway segments and the hanmer fault all fall in the range 400 700 yr ago these preliminary observations indicate that rupture of segments of the hope fault and indeed the alpine fault could occur as clusters that are years to decades apart rupture of the major plate boundary faults in the northern south island may therefore occur in sequences that are enhanced or even retarded by stress triggering and fault interactions

a comparison of historical and paleoseismicity in a newly formed fault zone and a mature fault zone north canterbury new zealand the timing of large holocene prehistoric earthquakes is determined by dated surface ruptures and landslides at the edge of the australia pacific plate boundary zone in north canterbury new zealand collectively these data indicate two large m 7 earthquakes during the last circa 2500 years within a newly formed zone of hybrid strike slip and thrust faulting herein described as the porter s pass to amberley fault zone ppafz two earlier events during the holocene are also recognized but the data prior to 2500 years are presumed to be incomplete a return period of 1300 2000 years between large earthquakes in the ppafz is consistent with a late holocene slip rate of 3 4 mm yr if each displacement is in the range 4 8 m historical seismicity in the ppafz is characterized by frequent small and moderate magnitude earthquakes and a seismicity rate that is identical to a region surrounding the structurally mature hope fault of the marlborough fault system farther north this is despite an order of magnitude difference in slip rate between the respective fault zones and considerable differences in the recurrence rate of large earthquakes the magnitude frequency distribution in the hope fault region is in accord with the characteristic earthquake model whereas the rate of large earthquakes in the ppafz is approximated but over predicted by the gutenberg richter model the comparison of these two fault zones demonstrates the importance of the structural maturity of the fault zone in relation to seismicity rates inferred from recent historical and paleoseismic data

variation in fault behaviour in different tectonic provinces of new zealand abstract fault behaviour appears to be different in different tectonic provinces of new zealand uplifted holocene marine terraces along the east coast of the north island suggest time predictable behaviour on reverse faults of the hikurangi subduction margin major dextral faults of the strike slip fault province in central new zealand appear to show characteristic slip behaviour short normal faults in the central volcanic region show highly variable increments of displacement finally reverse faults of the central otago and northwest nelson region may have intermittently characteristic patterns of activity each of these behaviour patterns has been observed elsewhere although where there are good data to constrain the actual dates of palaeoearthquakes a uniform slip or coupled model best explains most situations temporal and spatial clustering of fault rupture and major earthquakes are increasingly being recognized it is crucial to obtain better dates and to determine the extent of individual rupture segments in order to better characterize fault behaviour non characteristic fault behaviour has serious implications for current methods of seismic hazard evaluation seismic hazard studies should utilize specific fault data in preference to assuming a particular fault behaviour model

unfolding the wadati benioff zone in the kermadec new zealand region abstract as the deep seismic zones are associated with subducted oceanic crust it is important to establish the extent and continuity of each of these zones using consistent earthquake locations then the former oceanic crust can be restored to the surface of the earth by unfolding the seismic zone about the trench this unfolded crust should be used in plate reconstructions this process has been carried out for the deep seismic zone in the kermadec new zealand region where the deep seismic zone is defined by a careful selection of i s c hypocenters and the former oceanic crust is established by unfolding plate reconstructions using two sets of rotation poles show that the deepest parts of the observed seismic zone along its length were not subducted at the same time further the currently observable zone is not directly associated with earlier volcanism in northwest new zealand between 6 and 20 ma ago

tearing of the subducted australian plate during the te anau new zealand earthquake of 1988 june 3 summary the ml 6 1 earthquake of 1988 june 3 was widely felt in the south island of new zealand and caused landslides in fiordland it occurred at a depth of 57 km near 45 10 s 167 17 e in a region where the australian plate is subducting beneath the pacific plate immediately after the event portable seismographs were installed in the te anau area data from a well recorded subset of aftershocks have been used to invert for the seismic velocities of the lower crust and upper mantle of the region and for station terms the large range in station terms obtained 3 3 s for p waves emphasizes the structural complexity of the fiordland region the aftershocks as relocated with the new velocity model occur in a cigar shaped zone which extends from about 40 to 70 km in depth and dips southeast at approximately 65 the orientation of the aftershock zone in relation to the focal mechanism of the mainshoek suggests that the earthquake involved down dip tearing of the crust of the subducted australian plate with the ne part of the plate having moved up and to the east southeast relative to the sw part down dip tearing of the crust of the australian plate is also inferred for an earthquake of ml 5 9 which occurred on 1988 july 19 this was located directly down the dip of the subducted plate from the june 3 event at a depth of 122 km taken together with other seismological data these two events suggest the existence of a major tear in the subducted plate and that the shallow part of the plate to the northeast of this tear is resisting subduction the resistance may result from subduction of a region of relatively thick crust this would provide an explanation for the uneven distribution of both shallow and intermediate depth seismicity in fiordland the differing dips of the subducted plate along the margin fault plane solutions and spatial variations in the b value of intermediate depth events subduction of a region of relatively thick crust also provides a mechanism for tilting up the western part of the fiordland block

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

crustal deformation of the marlborough fault zone in the south island of new zealand geodetic constraints over the interval 1982 1994 crustal deformation across the marlborough fault zone in the south island of new zealand has been investigated by resurveying with the global positioning system gps a triangulation and trilateration network across part of the zone the principal strain rates which mostly have errors between 5 and 10 vary systematically across the region the principal axis of horizontal contraction gradually swings from se nw southeast of the hope fault to almost east west in the west the component of velocity parallel to the strike of the faults can account for 95 of the relative motion between the australian and pacific plates this component exhibits little variation west of the alpine wairau fault and shows an almost linear variation from the wairau fault to the east coast the horizontal strain rate northwest of the wairau fault is a small east west uniaxial contraction 0 1 ppm yr 1 if it is assumed that the crustal blocks bounded by the major faults are primarily driven by basal shear tractions with stresses on the faults being relatively insignificant then the slip rates expected on the faults may be derived from the observed velocity variations across the fault zone these predicted slip rates are in good agreement with geologically observed slip rates the larger slip rates on the hope fault are related to the observation that the strain field extends southeast of the fault over a distance large compared with the spacing between the faults the agreement between predicted and geological slip rates lends support to the idea that the observed pattern of deformation reflects that in the lower lithosphere

active tectonics of the eastern sunda and banda arcs the tectonics of the eastern sunda and banda arcs of eastern indonesia are inferred from the centroid depths seismic moments and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984 source parameters are estimated by inverting long period teleseismic p and sh waves earthquakes show that the collision of the australian continent with the banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the timor and seram troughs along the eastern end of the sunda arc and along the southern banda arc earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence by elongating in the direction perpendicular to convergence and by thrusting over the back arc basin shallow thrust and strike slip earthquakes beneath the banda basin the back arc basin demonstrate that deformation within the back arc accommodates some of the northward motion of the australian continent the rate of north south shortening of the entire upper plate at the longitude of timor calculated from the seismic moments of the earthquakes over the 22 year period is roughly 20 of the predicted convergence rate between australia and southeast asia strike slip faulting within the banda basin results also in eastward motion of the banda arc with respect to western indonesia and australia eastward extrusion of the banda arc results in thrusting at the aru trough earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc s polarity has reversed although the wetar back arc thrust zone at present has a high seismic slip rate earthquakes and their mechanisms provide evidence that the lithosphere of the weber basin a 7500 m deep forearc basin rests on the subducting lithosphere without intervening asthenosphere so that the weber basin subsides in response to sinking of the subducting lithosphere closure of the banda basin is geometrically incompatible with the australian plate subducting simultaneously beneath both the timor and seram troughs instead the bird s head which subducts beneath seram is decoupled from the australian plate in western new guinea and probably moves west or southwest with respect to australia in the eastern indonesian example strike slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc continent collision

hope fault jordan thrust and uplift of the seaward kaikoura range new zealand in the northern south island of new zealand displacement at the pacific indian plate boundary is accommodated by the east north east striking right lateral strike slip marlborough fault system the southernmost marlborough fault is the hope fault the late pleistocene holocene horizontal slip rate on this fault is 20 25 mm yr about half of the rate of pacific australian plate motion near the eastern end of the hope fault most displacement is transferred to the north northeast striking jordan thrust but the average dip slip rate at the surface trace of this thrust is less than 4 mm yr we propose that most slip takes place on a blind thrust expressed at the surface by the fault propagation folding of the seaward kaikoura range and that the rate of uplift of this range is as high as that of the southern alps 6 to 10 mm yr the major restraining bend of the alpine fault has the same average slip rate as the wairau fault 4 6 mm yr even though the alpine fault is an east dipping reverse separation fault at the restraining bend this low slip rate results in uplift of the spenser mountains east of the bend at a rate lower than that of the southern alps and seaward kaikoura range

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

comparative tomography of reverse slip and strike slip seismotectonic provinces in the northern south island new zealand abstract actively deforming crust and upper mantle around the obliquely convergent pacific australia plate boundary in the northern south island of new zealand have been investigated by seismic tomography using data from a temporary c 50 station network plus geonet the alpine wairau fault principal component of the plate boundary at the surface transects the study area separating the buller nelson bn province of active compressional inversion involving steep reverse faulting to the northwest from the marlborough fault system ma dominated by dextral strike slip faulting to the southeast in the course of this study ma hosted the 2013 seddon earthquake sequence m5 9 6 6 and the 2016 m7 8 kaikoura earthquake active fault structures and present seismicity are associated with a heterogeneous distribution of low velocity and vp vs anomalies elsewhere there is a general association between crustal seismicity and low velocity zones along major fault structures within both the ma and bn seismotectonic provinces along which high vp vs anomalies are locally conspicuous areas of active crustal seismicity are also generally characterized by high vp vs for instance the hypocentre and aftershocks of the 2016 m7 8 kaikoura earthquake overlie a low velocity region with high vp vs the coincidence of anomalously low vp and vs and anomalously high vp vs with zones of high electrical conductivity defined by a previous mt transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone weakening the overlying crust and promoting seismic rupture along major fault systems it seems possible that the association of mid crustal low velocity zones with anomalously high vp vs may diagnose rupture preparation zones where frictional strength is being lowered by the build up of fluid overpressure concurrent with accumulating shear stress so that eventual fault failure is dual driven

shallow subduction tectonics and fault interaction the weber new zealand earthquake sequence of 1990 1992 a series of four large earthquakes with epicenters very close to one another occurred in 1990 1992 in the weber region of the southeast coast of the north island new zealand the region is one of oblique plate convergence and subduction the plate interface being at about 20 km depth the first event february 19 1990 ml 6 1 mw 6 2 occurred within the upper part of the subducting pacific plate on a steeply northwest dipping normal fault as defined by relocated aftershocks the mechanism of this event reflects downdip tension within the subducting plate due to slab pull forces as is common in new zealand the second event may 13 1990 ml 6 2 mw 6 4 occurred on an imbricate fault in the overlying australian plate dipping shallowly northwest with components of both thrusting and dextral strike slip motion the slip vector is similar to the direction of plate convergence showing no arc normal rotation as observed for subduction thrust events in other areas of oblique convergence it is unlikely that the spatial and temporal closeness of these events is coincidence a similar pair occurred to the south in 1942 but the coupling mechanism is not clear perhaps being related to aseismic slip downdip on the subduction interface two subsequent events both of ml 5 5 occurred on august 15 1990 and march 2 1992 and extended the aftershock zone of the deeper first main shock to the northeast matching the extent of the shallower second main shock mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to the relevant mainshock the time history of the aftershocks of the first deeper event shows a period of quiescence beginning 35 days before the second shallow event the mean magnitude of those events that did occur was high this observation cannot be explained by station biases weather or changes in analysis methods and may be due to stress changes induced by precursory aseismic slip which would discourage aftershock occurrence

slip distribution of the lake tennyson earthquake new zealand as inferred from static stress changes and off fault aftershocks the fault plane of an earthquake can be estimated in many different ways one is to examine the distribution of a well located aftershock sequence to see if a fault plane is evident here we present a new method where we take an earthquake aftershock sequence with distinguishable off fault clusters and vary the slip distribution to achieve a good correlation between regions of increased coulomb failure stress cfs and aftershock occurrence we find with the lake tennyson earthquake that simply taking an area outlined by aftershocks to estimate a fault plane may not be valid if we do this patterns of increased cfs have negative correlation with off fault aftershock occurrence however if we restrict slip to only deeper regions and over a smaller area then the correlation between regions of increased cfs and aftershock off fault occurrence is good

balancing the plate motion budget in the south island new zealand using gps geological and seismological data summary the landmass of new zealand exists as a consequence of transpressional collision between the australian and pacific plates providing an excellent opportunity to quantify the kinematics of deformation at this type of tectonic boundary we interpret gps geological and seismological data describing the active deformation in the south island new zealand by using an elastic rotating block approach that automatically balances the pacific australia relative plate motion budget the data in new zealand are fit to within uncertainty when inverted simultaneously for angular velocities of rotating tectonic blocks and the degree of coupling on faults bounding the blocks we find that most of the plate motion budget has been accounted for in previous geological studies although we suggest that the porter s pass amberley fault zone in north canterbury and a zone of faults in the foothills of the southern alps may have slip rates about twice that of the geological estimates up to 5 mm yr 1 of active deformation on faults distributed within the southern alps 100 km to the east of the alpine fault is possible the role of tectonic block rotations in partitioning plate boundary deformation is less pronounced in the south island compared to the north island vertical axis rotation rates of tectonic blocks in the south island are similar to that of the pacific plate suggesting that edge forces dominate the block kinematics there the southward migrating chatham rise exerts a major influence on the evolution of the new zealand plate boundary we discuss a model for the development of the marlborough fault system and hikurangi subduction zone in the context of this migration

the last 25 ma of rotational deformation in part of the new zealand plate boundary zone abstract the marlborough fault system is a zone of dextral transpression in continental crust and marks the southern end of the hikurangi subduction system between the obliquely convergent australian and pacific plates detailed mapping over an area of 100 km2 has revealed a complex history of deformation during the last 25 ma the development of a thrust belt in the miocene referred to as d1 deformation was synchronous with the deposition of a thick submarine fan complex the beginning of d1 deformation may mark the initiation of the subduction zone in this part of the plate boundary zone subsequently in the plio pleistocene the region formed a zone of dextral shear marked by major strike slip faults and zones of distributed deformation referred to as d2 deformation crustal blocks less than 5 km across may have rotated ca 60 clockwise about vertical axes relative to the pacific plate in the last 4 ma the pattern of deformation during the last 10 ka to 100 years is consistent with the long term d2 deformation and suggests that large crustal blocks resting on the subducted pacific plate have rotated clockwise about vertical axes relative to the pacific plate accommodated by dextral shear further to the sw in the marlborough fault system the overall effect of the deformation has been the rotation of the edge of the southern part of the hikurangi margin from a w to nw trend in the early miocene to a ne trend today

active deformation in the new guinea fold and thrust belt seismological evidence for strike slip faulting and basement involved thrusting the new guinea fold and thrust belt forms a 1000 km long mountain chain running the length of new guinea but its trend is oblique to the predicted convergence direction a large component of left lateral shear across the mountain belt is expected but geological evidence for such motion is ambiguous surface structures in the fold and thrust belt show mountain building by folding and thrusting supposedly thin skinned of pre pliocene continental shelf sediments although some evidence indicates that active faulting penetrates the basement we examine the active deformation of this mountain belt by using teleseismic long period p and sh waveforms short period p waveforms and p wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the new guinea fold and thrust belt since 1964 eight thrust events with seismic moments m0 between 1017 and 2 1018 n m are distributed throughout the thrust belt at depths ranging from 0 5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends six of the eight events are between 11 and 25 km deep the steep dips of nodal planes the depths of these earthquakes and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned i e confined above a shallow detachment surface but penetrates the crystalline basement at high angles most earthquakes in the western half of the thrust belt including the three largest earthquakes examined m0 5 1018 n m show strike slip faulting with the probable fault plane oriented east west so that displacement is left lateral the nine strike slip earthquakes account for almost 90 of the observed seismic moment in the period 1964 1985 and indicate a seismic slip rate of 5 25 mm a all but one of these strike slip earthquakes are shallower than the thrust earthquakes suggesting that east northeast convergence across the fold and thrust belt is accommodated by a decoupled system of strike slip and dip slip faults while the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in new guinea seismological evidence suggests that translation by strike slip faulting may play a much greater role than previously recognized by analogy in older collisional orogens whose motions can be inferred only from remnant structures and not from seismicity strike slip faulting may have been equally important in accommodating convergence we estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the new guinea highlands may account for 5 20 of the total convergence between the pacific plate and australia in new guinea

three dimensional numerical experiments of strain partitioning at oblique plate boundaries implications for contrasting tectonic styles in the southern coast ranges california and central south island new zealand a three dimensional numerical model is used to investigate the nature and distribution of crustal scale deformation and strain partitioning across obliquely convergent transpressive plate boundaries a uniform elastic plastic crustal layer obeying murrell s extension to three dimensions of the griffith failure criterion was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres one of which detaches and subducts beneath the other the results show deformation concentrated in relatively narrow shear zones the character planar versus curved and dip of these shear zones or crustal scale faults depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary in models dominated by transcurrent boundary conditions two separate sets of coexisting structures develop 1 a pair of planar relatively shallow dipping thrust zones arranged in a v structure that accommodate convergent motion and 2 a set of curved near vertical strike slip zones arranged in a flower structure that accommodates the transcurrent motion in models dominated by convergence both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones there is a gradational behavior between these end member partitioned and nonpartitioned modes for intermediate boundary conditions in all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by mohr s incipient faulting hypothesis the two basic modes are interpreted to underlie the contrasting tectonic styles of the southern coast ranges california and central south island new zealand factors other than obliquity of convergence which play a role in strain partitioning are discussed