multipoint visualization of ulf oscillations using the super dual auroral radar network we present a technique for the automatic detection of pc5 150 s to 600 s period ultralow frequency ulf pulsations in ground and ionospheric backscatter from the super dual auroral radar network superdarn this new technique enables rapid identification and visualization of ulf oscillations over the very wide geographical coverage of superdarn the technique detects ulf oscillations using the lomb scargle periodogram method which provides a natural test for periodic behavior against the null hypothesis of a pure noise distribution this does not require any interpolation across data gaps and is thus appropriate for superdarn data we demonstrate the application of the technique to superdarn data for march 2014 and find that pc5 pulsations are frequently observed by multiple radars simultaneously a preliminary investigation using data from all northern hemisphere superdarn radars indicates that pc5 pulsation activity occurs most often on the nightside of the magnetosphere with a mean frequency of about 2 mhz

extension and partitioning in an oblique subduction zone new zealand constraints from three dimensional numerical modeling 1 contraction strike slip and extension displacements along the hikurangi margin northeast of the north island of new zealand coincide with large lateral gradients in material properties we use a finite difference code utilizing elastic and elastic plastic rheologies to build large scale three dimensional numerical models which investigate the influence of material properties on velocity partitioning within oblique subduction zones rheological variation in the oblique models is constrained by seismic velocity and attenuation information available for the hikurangi margin we compare the effect of weakly versus strongly coupled subduction interfaces on the development of extension and the partitioning of velocity components for orthogonal and oblique convergence and include the effect of ponded sediments beneath the raukumara peninsula extension and velocity partitioning occur if the subduction interface is weak but neither develops if the subduction interface is strong the simple mechanical model incorporating rheological variation based on seismic observations produces kinematics that closely match those published from the hikurangi margin these include extension within the taupo volcanic zone uplift over ponded sediments and dextral contraction to the south

microseismicity and velocity structure in the hawkes bay region new zealand fine structure of the subducting pacific plate summary hawkes bay overlies part of the shallow zone of subduction of the pacific plate beneath the australian plate in the north island of new zealand a temporary dense network of 11 seismograph stations placed in the region recorded more than 350 micro earthquakes in two months providing a data set for velocity and structure inversion the hypocentres define a seismic band 11 12 km thick dipping 6 2 beneath hawke bay and increasing to more than 20 towards the west side of the region this seismic band is thought to delineate the crust of the subducting pacific plate s to p converted phases for deeper events are consistent with a fairly large impedance contrast across the plate interface at the top of the band p and s arrival times were inverted to give a simultaneous determination of hypocentres and velocity structure results indicated a thin layer of low 4 7 ksm 1 p wave velocity in the neighbourhood of the plate interface we interpret this layer as a thickness of subducted sediments beneath the micro earthquake station network this is consistent with seismic reflection work to the south of hawkes bay and with sediment volume calculations the stress distribution as given by focal mechanisms shows that the microseismicity represents primarily intraplate rather than interplate deformation with the main band of microseismicity indicating normal faulting throughout at least the top 10 km of the subducted pacific plate the observation of reverse faulting at depth beneath the main band is interpreted as being due to the flexure of the subducted pacific plate

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

upper plate tectonic stress state may influence interseismic coupling on subduction megathrusts we document a correlation between along strike variation in interseismic coupling depth on subduction interfaces and the upper plate tectonic stress state in new zealand vanuatu and southwest japan deep interseismic coupling occurs where the upper plate stress regime is contractional to transpressional whereas a shallowing of interseismic coupling occurs where there is an along strike shift to back arc or intra arc extension to explain this relationship we draw on theoretical studies suggesting that the fl uid pressure state within the upper plate and on the subduction interface has a strong control on the depth of the transition from frictional to viscous behavior lower fl uid pressures e g close to hydrostatic are expected where the over riding plate is undergoing tectonic extension whereas higher fl uid pressures e g close to lithostatic are expected where the over riding plate experiences long term e g 10 5 yr tectonic shortening low fl uid pressures within the upper plate may lead to a shallow frictional to viscous transition compared to an upper plate that is highly overpressured we hypothesize that the state of tectonic stress and structural permeability in the upper plate are yet other variables to consider when evaluating which physical mechanisms control interseismic coupling of subduction megathrusts

tararua broadband array north island new zealand abstract the tararua broadband array consisting of nine three component seismometers was deployed in southern north island new zealand from february 1991 to september 1992 this l shaped array had an approximately 40 km n s arm and 30 km e w arm its primary purpose was to record waveforms from tonga kermadec earthquakes whose propagation paths have spent the majority of their time in the pacific plate that has been subducted beneath the australasian plate along the tonga kermadec hikurangi margin the continuous recording allowed simultaneous acquisition of an excellent teleseismic and regional broadband seismogram dataset for the main experiment broadband three component waveforms with p wave precursive slab phases up to 15 sec early relative to the jeffreys bullen travel times have been recorded for 71 events multiple frequency analysis of the p waveform shows that the high frequencies 5 hz arrive first with coherent dispersion below about 4 5 hz this behavior can be attributed to a high velocity layer 8 to 10 km thick lying above the cold fast lithosphere of the subducted pacific plate polarization analysis of the p wave precursors show them to arrive at steep incidence angles of 20 to 30 suggesting the phase refracts off the subducted plate9s surface to the array we observe exceptionally large pl leaking modes from shallow events in the tonga kermadec region to the north

wide aperture beamforming of depth phases by timescale contraction abstract teleseismic depth phase signals can be combined with constructive interference from stations spaced as much as 1000 km apart each trace is first deconvolved using an operator based on the p phase of that trace then its timescale is stretched or contracted to eliminate the moveout of pp p time with distance traces can then be added with constructive interference of depth phases out to 10 or more angular distance between the stations on the focal sphere depth phases pp or sp representing hypocentral depths ranging from 6 to 204 km were found using canadian national seismograph network cnsn data for 45 events recorded at ranges of 2000 to 10 800 km which had been reported in the reviewed event bulletins reb of the prototype international data center pidc including 23 events for which the reb reported no depth phases for 28 events for which a catalog depth reb or pde from usgs was available for comparison 18 had depths agreeing either within 6 km or within 15 7 can be explained by assuming an sp phase was misassociated as pp 1 is just outside the error bar of the free depth reported in the reb and only 2 disagree significantly thus using only canadian data depth phases were found by beamforming for many events for which no depth phases had been found by the pidc using worldwide data including arrays the depth phase beamforming method has the potential of substantially increasing the number of earthquakes for which teleseismic depth phases can be detected

ulf pc5 6 magnetic activity in the polar cap as observed along a geomagnetic meridian in antarctica 1 latitudinal and diurnal distributions of spectral power and spatial coherency parameters of the geomagnetic variations in the pc5 6 1 6 mhz frequency range are analyzed using data of magnetometer stations in antarctica the available stations give the possibility to form a latitude chain along the geomagnetic meridian 40 e stretching from magnetic latitude 69 s to 86 s long period ulf activity at polar cap latitudes is characterized by lower amplitudes and wider spectra with lower central frequencies as compared with typical auroral pc5 pulsations the meridional distribution of average pc5 6 spectral power is nonmonotonic and has a minimum near 80 in general the low frequency broadband ulf activities in the polar cap and at auroral latitudes seem to be decoupled this long period ulf activity in the polar cap could be an image of wave activity in the tail lobes or the manifestation of turbulent component of the ionospheric convection at very high latitudes but this requires further investigation

numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

analysis of strong phases other than p and s from a microearthquake survey in the wellington region new zealand a few subcrustal earthquakes located by means of a microearthquake survey in the wellington region at the indian and pacific plate boundary exhibit strong phases between first p and first s arrivals these phases have been identified as having been converted from s to p sp or p to s ps at the mohorovicic discontinuities of the indian and pacific plates examples of these phases are shown here two events are selected and simple models are assumed to draw ray paths of these phases as possible explanations of mode conversion

a multiphase seismic investigation of the shallow subduction zone southern north island new zealand summary the shallow structure of the hikurangi margin in particular the interface between the australian plate and the subducting pacific plate is investigated using the traveltimes of direct and converted seismic phases from local earthquakes mode conversions take place as upgoing energy from earthquakes in the subducted slab crosses the plate interface these ps and sp converted arrivals are observed as intermediate phases between the direct p and s waves they place an additional constraint on the depth of the interface and enable the topography of the subducted plate to be mapped across the region 301 suitable earthquakes were recorded by the leeds tararua broad band seismic array a temporary line of three component short period stations and the permanent stations of the new zealand national network this provided coverage across the land area of southern north island new zealand at a total of 17 stations rays are traced through a structure parametrized using layered b splines and the traveltime residuals inverted simultaneously for hypocentre relocation interface depth and seismic velocity the results are consistent with sediment in the northeast of the study region and gentle topography on the subducting plate this study and recent tectonic reconstructions of the southwest pacific suggest that the subducting plate consists of captured oceanic crust the anomalous nature of this crust partly accounts for the unusual features of the hikurangi margin e g the shallow trench in comparison with the subducting margin further north

shallow subduction tectonics in the raukumara peninsula new zealand as illuminated by earthquake focal mechanisms the raukumara peninsula affords an excellent opportunity to study the subduction process as subduction of the buoyant hikurangi plateau on the pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust here we report on the focal mechanisms of 117 earthquakes of m l 2 4 4 9 and shallower than 80 km recorded during a 5 month deployment of 36 portable seismographs on the peninsula mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes downdip tensional strain predominates in the subducted plate with taxes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone trenchward extensional strain is seen in the uppermost part of the overlying australian plate in line with geodetic and geological results this can be related to extension and gravity sliding of surficial rocks due to uplift of the raukumara range resulting from underplating of subducted sediment there is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface a concentration of low angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest this along strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically

the use of instantaneous polarization attributes for seismic signal detection and image enhancement this research has been supported by the spanish ministry of education culture and sport through grant sb2000 0120 and by a marie curie fellowship of the european community programme ihp hpmf ct 2001 01322

on the duration of west antarctic ice sheet grounding events in ross sea during the quaternary abstract a back stepping succession of three seismically defined grounding zone wedges gzws in the glomar challenger basin palaeo ice stream trough is usually assigned to the short time that elapsed since the west antarctic ice sheet retreat began at 11 ka 14c bp recent radiocarbon dates have however suggested an alternate interpretation in which the youngest of these three gzws the gray unit gzw on the middle shelf corresponds to deposition during the last glacial maximum lgm if so then the gray unit must represent an amalgamation of erosion and deposition spanning a much longer time interval i e the 100 ky interval between the last interglacial and the lgm to test these conflicting interpretations the gray unit sediment volume was mapped from seismic and multibeam data two end member durations were calculated because flux during ice sheet retreat is significantly higher than flux during ice sheet advance using the retreat mode flux the 1 47 ky grounding event estimate shows that the middle shelf gzw could have been deposited during the post lgm retreat however the 147 34 ky grounding event estimate based on the advance mode flux also demonstrates that the gzw might reasonably represent an amalgamation of erosion and deposition as grounded ice gradually advanced between mis5e and mis2 i e from the last interglacial to the last glacial maximum these data thus require that both interpretations of how the near surface stratigraphy relates to grounding line translations in the last glacial cycle be considered feasible working hypotheses

seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern sierra nevada and vicinity california 21 well distributed teleseisms 30 to 100 distance were recorded by mixed broad band bb and short period sp seismic arrays at mineral king mk and horseshoe meadow hm in the southern sierra nevada and at darwin plateau dp between the inyo and argus ranges these arrays permit identification and separation of direct p and s arrivals reflections from topography scattered energy and arrivals from different back azimuths multipath arrivals p to s conversions can be identified from beamed bb or sp seismograms least squares time domain processing recovered single event receiver functions from these beamed seismograms converted phases attributable to the moho are clear and uncomplicated at dp ps p times of 3 9 4 2 s correspond to crustal thicknesses of 31 33 km assuming a mean p crustal velocity of 6 2 km s and a poisson s ratio of 0 255 ps p times at hm eastern sierra are 3 9 4 6 s 32 37 km excepting some unusual seismograms from events at back azimuths of 225 239 mk western sierra times for ps p show a strong e w asymmetry 3 9 4 1 s 31 33 km from the east and 4 8 5 3 s 39 42 km from the west the mk arrivals from the east are multiple and substantially weaker by comparison with hm and dp these results confirm the absence of a thick crust under the southern high sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies at dp the moho event follows an intracrustal negative polarity event defining the top of an s wave low velocity zone this feature dips west under the sierran crest at hm but is absent farther west at mk this feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed sierra overlies a tectonized lower crust sub moho energy is absent under the basin and range dp but is conspicuous under the high sierra at positive arrivals 7 3 s mk and 9 s hm after the p these arrivals might be from the base of a low velocity low density upper mantle body supporting the sierra

static grounding lines and dynamic ice streams evidence from the siple coast west antarctica 1 grounding line dynamics are key to our understanding of marine ice sheets such as the west antarctic ice sheet wais we present an accurate and rapid method for locating grounding lines using glas icesat laser altimetry data we then apply this technique to the siple coast region of the wais our technique exploits the high surface slope at the grounding line relative to the adjacent flat ice streams and ice shelf the technique is validated by ground based observations comparisons between the current grounding line and earlier measured locations demonstrate that the siple coast s grounding line has been largely static over the last two decades this is important as the ice streams have been undergoing considerable variations over this period we conclude that ice stream flow speeds can vary while the grounding line remains stable we suggest that grounding lines may migrate abruptly and then stabilize rather than undergoing continuous retreat

review of hydromagnetic wave studies in the antarctic high latitude ulf waves at all frequencies are reviewed with emphasis on simultaneous measurements in both hemispheres the seasonal variation of interhemisphere high latitude data is valuable in helping to understand the topology of the magnetosphere the frequent occurance of pc 1 2 pulsations at high latitudes and in the polar cap is discussed evidence for ground detection of flux transfer events via magnetic pulsations is reviewed the correlation of high latitude pulsation data with other ground techniques of measuring particle precipitation is presented to better define the subset of high latitude ulf that is produced by local current systems fed by particle precipitation and field aligned currents finally simultaneous ground and spacecraft measurements of ulf are reviewed which provide information on the propagation of ulf across such boundaries as the magnetopause and the ionosphere

upper mantle seismic discontinuities in a subduction zone japan investigated from p to s converted waves abstract the lateral variations of the upper mantle discontinuities in a subduction context are investigated by the use of broadband wave data provided by the geoscope network at the inuyama station japan differential arrival times of p and p to s conversions on the discontinuities are used these converted phases though of weak amplitude can be observed by stacking many seismograms a stack of 50 data with high signal to noise ratio is performed by different techniques the dominant period is around 7 10 s in this study and the lateral resolution is estimated to be of the order of 300 km this is an intermediate scale between short wavelength studies which can resolve very small details and long wavelength studies for which the fresnel zone is wider than about 1000 km the first stacking technique allows to retrieve the mean depth of the discontinuities below the station the main result is that the well known discontinuity close to 660 km is depressed down to about 700 km while the 410 km discontinuity is observed close to its usual depth and that a discontinuity close to 520 km is visible however in such a heterogeneous region the different traces are not coherent and it is useful to account for the variability of arrival times of converted phases using alternative stacking techniques we show that the 410 km discontinuity is elevated in the slab area of about 60 km and the 660 km discontinuity is depressed by more than 50 km this depression of more than 50 km found for the 660 km discontinuity yield to a value of 3 gpa k 1 of the clapeyron slope for the phases change olivine magnesiowustite perovskite although the 520 km discontinuity clearly shows up with all the techniques used it is difficult to reliably access its depth variation in the slab area the evidence for a 520 km discontinuity in this period range while it is not observed at shorter periods suggest that this discontinuity has a thickness between 15 and 25 km the large elevation of the 410 km discontinuity suggests that the thermally controlled depth of the phase change may affect the dynamic behaviour of the slab

velocity and anisotropy structure at the hikurangi subduction margin new zealand from receiver functions summary we examine seismic velocity and anisotropy at the interface between the australian and the subducting pacific plate in southern north island new zealand using receiver functions from teleseismic earthquakes recorded on two l shaped seismic arrays iris station snzo makes up the corner of the southern array velocity models are tested with a ray theoretical method that allows both dipping boundaries and anisotropy assuming a 2 d structure beneath each array the strongest arrival on the northern array consistently appears at 2 s the arrival is modelled by a jump in velocity at a flat boundary at 12 km which may be caused by a change in metamorphic grade this prominent arrival correlates with a weaker arrival at 1 5 s 7 km at station snzo isotropic layers dipping with the slab can explain many features in the first 20 s of the records but anisotropy is needed to explain both the transverse receiver functions and a change in the polarity of a prominent ps arrival at 3 s delay relative to p o nthe radial component this arrival is consistent with a change between an upper layer that is either isotropic or anisotropic with a horizontal symmetry axis and a supraslab metasediment layer having anisotropy with av ertical slow axis of symmetry there is tradeoff between boundary dip the plunge of the axis of symmetry layer thickness and the amount of anisotropy a dip of 20 shallowing to 15 under the tararua array thickness of 4 km and a vertical symmetry axis fits the backazimuth variation of the amplitudes and polarities of the arrivals with anisotropy of 4 6 per cent observed layer multiples are smaller than synthetic predictions to explain this discrepancy either layer boundaries are rough attenuation is strong or the layer boundaries are more gradational at the periods 1 s used in this study compared to previous studies 5 s

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

the paleo ice stream in vestfjorden north norway over the last 35 k y glacial erosion and sediment yield the basal erosion rate and sediment yield of the vestfjorden paleo ice stream a marine based part of the northwestern fennoscandian ice sheet were elucidated during a complete advance retreat cycle using high resolution seismic data and multi beam bathymetry in most of the area there is a semitransparent continuous upper seismic unit showing a southwestward down fjord progradational pattern with megascale glacial lineations on top interpreted to represent a subglacial deformable till the till formed a continuous bed beneath most of the vestfjorden paleo ice stream except for the upstream parts where ice streaming is inferred to have occurred above bedrock the till thickness is up to an order of magnitude larger than those reported from modern ice streams due to a more efficient transfer of material from the source area to the ice streambed the till is draped by glaciomarine and marine sediments the average sedimentation rate in the paleo ice stream drainage route during the period from ca 35 to 11 ka was 2 6 m k y this equals an average sediment discharge of 35 106 t yr and an average sediment yield of 6 4 103 t km 2 yr 1 from this we estimate an average erosion depth of 40 m or rate of 1 7 mm yr in the sediment source area the encircling fjord and valley landscape these values are in agreement with the results reported from similar settings in southern norway but they are nearly twice the figures reported for arctic ice stream erosion they are also significantly higher than the rates estimated from the british ice sheet in conclusion glacial erosion beneath paleo ice streams seems to have been more efficient in subarctic regions

fine structure of deep wadati benioff zone in the izu bonin region estimated from s to p converted phase abstract mechanism of deep earthquakes is one of the most important and still unsolved problems in the field of solid geophysics determining precise location of the deep earthquakes can provide us with clues to approach this problem in this study we address the problem by estimating the separation distances between hypocenters of the deep earthquakes and slab asthenosphere interface we use later arrivals immediately after initial p arrivals from the deep earthquake events in the izu bonin region we interpret these later arrivals as an sp phase which is converted s to p at the upper slab interface our results prove that the hypocenters underlie the subducting upper slab interface using time delays between p and sp phases we find that the interface is located 20 km above the earthquake hypocenters moreover our results also indicate that the almost all deep earthquake hypocenters are distributed within a thin zone about 7 km wide rather than being scattered throughout the slab this observation fits a hypothesis that deep earthquakes are caused by transformational faulting of metastable olivine model

nature of the plate contact and subduction zones diversity abstract in recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision we investigated two fundamental states of the plate contact one based on a fault and the other based on a subduction channel using geodynamic modeling we determined the speci c signatures of both states of the subduction contact we established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate and is one of the controlling factors whether back arc extension develops or not in the present study we combine results of our previous numerical experiments with a re analysis of published observations overall our synthesis connects seismic moment release with back arc deformation and tectonic processes at the margin it leads us to identify four classes of subduction zones the rst two classes result directly from our numerical experiments in class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin in class 2 the plate contacts are largely channel type and have an erosive margin class 3 where the plate contact is entirely channel like consists of accretionary margins with a high sediment supply subduction zones of class 4 mostly characterized by an erosive convergent margin northern chile peru honshu and kuril are more complicated they can be explained by incorporating regional observations

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

seismic studies of the upper mantle and transition zone in the last few years seismic studies of the upper mantle and transition zone have achieved significant new results in several areas progess has been driven largely by the increasing size quality and accessibility of digital seismic datasets from global and regional networks and from experiments using portable instruments this paper focuses on a few of the most interesting and important developments these include 1 measurements of upper mantle anisotropy obtained using a variety of techniques which can reveal details of the flow regimes and tectonic history of the upper mantle 2 improvements in models of upper mantle velocity and attenuation structure with some controversy arising regarding how deep slow velocity anomalies extend beneath oceanic ridges 3 direct seismic observations of topography on the 410 and 660 km discontinuities in the upper mantle providing constraints on mantle temperatures and dynamics and 4 observations suggesting secondary seismic discontinuities in some regions

ice sheet stability and sea level rise a wedge of sediments appears to stabilize the whillans ice stream suggesting that sea level rise may not destabilize ice sheets as much as previously feared

sedimentological signature of a deformable bed preserved beneath an ice stream in a late pleistocene glacial sequence ross sea antarctica abstract subglacial processes are described using a nested set of samples from and 1b drill core collected from beneath the ross ice shelf as part of the antarctic geologic drilling program andrill an interval from the late pleistocene was chosen for study because it is preserved in the sedimentary record yet the depositional processes are applicable to the widely studied glacial conditions at the last glacial maximum a complete glacial interglacial sequence between two glacial surfaces of erosion gses at 41 9 and 47 7 meters below sea floor was studied by centimeter scale core logging based on x radiographs of the archive core halves in addition to core description bulk samples and 45 mm 65 mm thin sections of diamictites for micromorphology analysis above the lower gse 4 6 m of subglacial till occurs including a sequence of thin muddy conglomerate with diverse pebble lithologies massive clast rich muddy diamictite and stratified diamictite with clast rich and clast free beds the sand size fraction of these deposits is dominated by aggregate grains termed till pellets following terminology used for ross sea deposits we present the first detailed description and two different conceptual models of tillpellet formation beneath an ice stream one is related to mechanical processes and the other is related to thermodynamics till pellets are rounded spherical to prolate in form and are associated with turbate structures and aligned grains in thin sections the core of the pellet is either a lithic grain or stiff till with additional clay plastered on the outside forming rounded grains from originally angular ones galaxy structures skelsepic fabric and pressure shadows around grains are indicative of rotational deformation within the till till pellets form in situ within a highly porous deformable bed under an ice stream either by mechanical shearing or by alternating thermal conditions in the till in the first model a slow glacial advance under dry subglacial conditions fractures the till increasing the porosity as shear induces dilation basal melting and high pore water pressure develop as the ice stream thickens decreasing the intergranular effective stress and inducing rotation of the angular till aggregates an alternative model calls for basal freezing with preferential ice growth in larger pore spaces causing rotation of fine grained till aggregates due to concentration of shear stresses in a thin zone beneath the glacier sole survival of till pellets is enhanced in subglacial diamictite as the thin zone of deformation moves upward toward the base of the ice stream a thin granular bed enriched in till pellets is melted out in the grounding zone and this deposit marks the transition from subglacial till to stratified silty claystone representing sub ice shelf deposits

stress orientations in subduction zones and the strength of subduction megathrust faults subduction zone megathrust faults produce most of the world s largest earthquakes although the physical properties of these faults are difficult to observe directly their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them a global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward consistently making an angle of 45 to 60 with respect to the subduction megathrust fault these angles indicate that the megathrust fault is not substantially weaker than its surroundings together with several other lines of evidence this implies that subduction zone megathrusts are weak faults in a low stress environment the deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface

earthquake relocation and three dimensional vp and vp vs models along the low angle alto tiberina fault central italy evidence for fluid overpressure summary the active extension in the central portion of the northern apennines is believed to be accommodated by an ene dipping low angle normal fault cutting the upper crust called alto tiberina fault atf the physical properties and mechanics of such a severely misoriented fault are still unknown and debated in this study we constrain the deep geometry of the atf and investigate the material properties of the fault system by using microseismicity recorded by a dense local seismic network the inversion of local earthquake p and s wave arrival times yields high resolution v p and v p v s images of the fault system we find positive v p anomalies that define the eastward verging thrust and fold belt developed in the sedimentary cover which inner bulk consists of triassic evaporites remains of the older compression microseismicity reveals the 20 ne dipping atf nearly planar from 3 to 14 km depth no sharp velocity contrasts are observed across the fault but only a gentle warping of the velocity anomalies and the dislocation of the triassic evaporites top at shallow depth on the fault hangingwall normal v p and strong high v p v s indicate the presence of a fractured high fluid pressure volume within which steep normal fault segments are located this evidence suggests that fluid pressure plays a dominant role in triggering the shallow background seismicity along the atf low v p v s anomalies are observed suggesting a different level of cracking and pore saturation and that the fault acts as impermeable barrier we speculate that the microseismic activity on the atf is favoured by the entrapped and locally overpressured co2 testified by the low v p v s volumes

multimode migration of scattered and converted waves for the structure of the hikurangi slab interface new zealand reflectivity imaging of local earthquake seismograms has revealed the structure of the hikurangi subduction interface at the location of two strong earthquakes that occurred in 1990 the earthquakes originated within the continental plate of the north island of new zealand and below in the subducting pacific slab we used seismograms from 500 well located events in two earthquake sequences recorded by a small temporary seismograph deployment to directly image the structure and multiphase reflectivity of the plate interface synthetic tests of the imaging method show the effects of the poor 3 d geometric coverage afforded by the seismometer array kirchhoff summation image sections computed from synthetics show accurate depth imaging of backscattering interfaces phase converting interfaces imaged with forward scattered waves are smeared by poor ray coverage to 5 km depth inaccuracy and are only imaged over a small range of their horizontal extent from the data we computed image sections for p p p s s p and s s scattering we mitigated imaging artifacts due to poor ray coverage with an obliquity factor an antialiasing criterion and enhancement by resampling statistics imaging used a sharply layered velocity model we tested for the effects of imaging with first arriving headwaves by imaging through smoothly varying velocity models for our ray geometry early arrival headwaves contribute little to the images the plate interface appears as a 3 5 km thick p p and possibly s s backscatterer with 5j nw dip offset 5 km down to the nw above a normal fault in the slab when illuminated from below a wedge of the interface on the downdip side of the slab fault forms a very prominent p p forward scatterer the edges of the wedge forward scatter some s p and s s energy but an order of magnitude less than the p p forward scattering the imbalances between forward scattering of p and s energy suggest a wedge of subducted sediment retaining significant porosity but with rigidity close to that of surrounding rocks d 2002 elsevier science b v all rights reserved

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

tectonic tremor along the northern hikurangi margin new zealand between 2010 and 2015 we present the first comprehensive tectonic tremor catalog for the northern hikurangi margin obtained from the systematic analysis of data from the new zealand seismic network between 2010 and 2015 the recent densification of this array along the northern hikurangi margin has allowed for the detection and location of tectonic tremor associated with 20 out of 26 identified transients in the continuous gps record along the east coast of the raukumara peninsula eighteen of these transients have never been reported in the literature and are likely to be slow slip events with magnitudes smaller than those previously studied we also investigate the spatiotemporal relationships between shallow offshore slow slip tectonic tremor and earthquakes for the duration of the study period

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

plate interface properties in the northeast hikurangi subduction zone new zealand from converted seismic waves arrival time inversion of local earthquakes in the northeast hikurangi subduction zone has indicated high vp vs at the plate interface but the thickness of the anomalous zone is poorly constrained by the p and s travel times we investigate this plate interface zone further by modeling s to p converted phases using synthetic seismograms for upgoing rays of earthquakes in the subducting plate mantle the modeling indicates a low velocity layer at the plate interface which is generally 1 2 km thick and has vp of 5 0 5 35 km s and vp vs of 2 this is consistent with a subducting sediment channel with near lithostatic fluid the low rigidity indicates a weak zone which will have a strong influence on the distribution of deformation the modeling approach shows promise in defining rheological parameters at the plate interface and mapping their variation across the seismogenic zone of the subduction thrust

polarization analysis of high frequency three component seismic data abstract we present a method to determine the polarization of body waves from three component high frequency data and examples of its application the method is based on the principal component approach one advantage of this approach is that the polarization state can be determined for small time windows compared with the predominant period of the wave this is particularly useful for identifying converted waves within the crust the stability of the result is analyzed with synthetic cases by adding simultaneous arrivals from waves and random noise the method works well with both synthetic and local data in the detection of the polarization of the wave by separating arrivals from different directions from the local data some seismic phases related to crustal conversions are observed that require strong lateral variations

pc5 micropulsation power at conjugate high latitude locations 1 the micropulsation power integrated over the pc5 frequency range has been calculated for the horizontal component of the geomagnetic field at two high latitude conjugate locations dumont d urville corrected geomagnetic coordinates 80 61 s 235 76 e and mould bay corrected geomagnetic coordinates 80 85 n 272 65 e because of the different distances between the geographic and the geomagnetic poles in each hemisphere the comparison between the pc5 power observed at dumont d urville and at mould bay shows the relative importance of geomagnetic and solar illumination effects in driving low frequency micropulsation activity in particular similarities observed at the two sites can be explained in terms of their common geomagnetic characteristics while differences can be attributed to the different sunlight or solar zenith angle configurations results show that the local summer pc5 power is statistically higher in the northern hemisphere than in the southern one this hemispherical difference is smaller for the local equinoxes and it is only very slight or absent for local winters these findings are interpreted in terms of the proportionality between the pc5 power and the ionospheric conductance which is higher at mould bay owing to more permanent and direct sunlight conditions during local summers and equinoxes thus the different geographic coordinates affect the pc5 power at the two considered sites so much so that their effect is visible regardless of the geomagnetic similarities however the influence of the geomagnetic activity on pc5 power is found to be more significant than these geographical effects or than the seasonal effects in fact for kp 70 the magnetic local time modulation of pc5 power is similar in both hemispheres since it is determined by the equivalent geomagnetic characteristics and regardless of possible geographical differences in particular the occurrence of higher pc5 power observed at mould bay than at dumont d urville is not localized in one specific magnetic local time sector the present observations are compared with previous results regarding solar illumination effects on geomagnetic activity and on auroral brightness finally no solar cycle effect is observed on either the pc5 power level on its hemispherical dependence or on its daily modulation

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

multistation measurements of pc5 geomagnetic power amplitudes at high latitudes geomagnetic data from four antarctic stations located at 80 corrected geomagnetic cgm latitude have been analyzed the magnetic local time mlt occurrence distributions and seasonal dependence of geomagnetic power levels in the pc5 frequency band are studied it is found that the pulsation power can be significantly influenced by the mlt location of the station at the specific ut time considered regardless of interplanetary conditions or geomagnetic activity a shift of power amplitudes toward higher values is found during local summer likely in association with the increased ionospheric conductance the local dayside summer increase of power relative to the winter condition is higher than on the nightside this finding is explained by the magnetospheric cusp influence on the production of geomagnetic power

seismic anisotropy in the fore arc region of the hikurangi subduction zone new zealand abstract shear waves from local earthquakes recorded on an array of nine broad band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying australian plate and within and below the subducting slab underlying the southern north island new zealand earthquakes near the plate interface were used to image the crustal anisotropy whereas deeper down dip subduction zone events were used to sample the subducting slab and the mantle below apart from a small amount of variable near surface anisotropy top few kilometres of the crust the measured fast shear wave directions of both the crust of the overlying plate 51 18 and the mantle 41 15 are similar and are nearly parallel to the strike of the subduction zone and the dominant geological features of the region sks measurements which mainly sample the mantle beneath the subducting plate give a value of 28 11 and measurements made elsewhere in the new zealand region using sks and scs2 produce similar results the measurements of crustal anisotropy are similar to those found further south on the wellington peninsula and confirm the presence of near surface anisotropy the maximum delay times for crustal earthquakes indicate a pervasive shear wave velocity anisotropy of 4 in the crust of the overlying plate and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear wave velocity anisotropy of 1 4 in the mantle below the subducting slab these results when combined with the sks measurements suggest trench parallel flow in the mantle below the subducted slab and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab

flow and retreat of the late quaternary pine island thwaites palaeo ice stream west antarctica multibeam swath bathymetry and sub bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo ice stream that carried the combined discharge from the parts of the west antarctic ice sheet now occupied by the pine island and thwaites glacier basins sets of highly elongated bedforms show that at the last glacial maximum the route of the pine island thwaites palaeo ice stream arced north northeast following a prominent cross shelf trough in this area the grounding line advanced to within similar to 68 km of and probably reached the shelf edge minimum ice thickness is estimated at 715 m on the outer shelf and we estimate a minimum ice discharge of similar to 108 km 3 yr 1 assuming velocities similar to today s pine island glacier similar to 2 5 km yr 1 additional bed forms observed in a trough northwest of pine island bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior the style of ice retreat is also evident in five grounding zone wedges which suggest episodic deglaciation characterized by halts in grounding line migration up trough stillstands occurred in association with changes in ice bed gradient and phases of inferred rapid retreat correlate to higher bed slopes supporting theoretical studies that show bed geometry as a control on ice margin recession however estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat our findings show that the ice stream channeled a substantial fraction of west antarctica s discharge in the past just as the pine island and thwaites glaciers do today

geophysical constraints on geodynamic processes at convergent margins a global perspective abstract convergent margins being the boundaries between colliding lithospheric plates form the most disastrous areas in the world due to intensive strong seismicity and volcanism we review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure seismicity and geometry of subducting slab we present global maps of free air and bouguer gravity anomalies heat flow seismicity seismic vs anomalies in the upper mantle and plate convergence rate as well as 20 profiles across different convergent margins a global analysis of these data for three types of convergent margins formed by ocean ocean ocean continent and continent continent collisions allows us to recognize the following patterns 1 plate convergence rate depends on the type of convergent margins and it is significantly larger when at least one of the plates is oceanic however the oldest oceanic plate in the pacific ocean has the smallest convergence rate 2 the presence of an oceanic plate is in general required for generation of high magnitude m 8 0 earthquakes and for generating intermediate and deep seismicity along the convergent margins when oceanic slabs subduct beneath a continent a gap in the seismogenic zone exists at depths between ca 250 km and 500 km given that the seismogenic zone terminates at ca 200 km depth in case of continent continent collision we propose oceanic origin of subducting slabs beneath the zagros the pamir and the vrancea zone 3 dip angle of the subducting slab in continent ocean collision does not correlate neither with the age of subducting oceanic slab nor with the convergence rate for ocean ocean subduction clear trends are recognized steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate with slab rotation towards a near vertical dip angle at depths below ca 500 km at very high convergence rate 4 local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs however near isostatic equilibrium may exist in broad zones of distributed deformation such as tibet 5 no systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation magmatic activity crustal faulting horizontal heat transfer and also due to low number of heat flow measurements across many margins 6 low upper mantle vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels

reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents we use seismic reflection and refraction data to determine crustal structure to map a fore arc basin containing 12 km of sediment and to image the subduction thrust at 35 km depth seismic reflection megasequences within the basin are correlated with onshore geology megasequence x late cretaceous and paleogene marine passive margin sediments megasequence y a similar to 10 000 km 3 submarine landslide emplaced during subduction initiation at 22 ma and megasequence z a neogene subduction margin megasequence the moho lies at 17 km beneath the basin center and at 35 km at the southern margin beneath the western basin margin we interpret reflective units as deformed gondwana fore arc sediment that was thrust in cretaceous time over oceanic crust 7 km thick raukumara basin has normal faults at its western margin and is uplifted along its eastern and southern margins raukumara basin represents a rigid fore arc block 150 km long which contrasts with widespread faulting and large neogene vertical axis rotations farther south taper of the western edge of allochthonous unit y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward as proposed for the correlative onshore allochthon spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore arc crustal thickness the trench slope has many small extensional faults and lacks coherent internal reflections suggesting collapse of indurated rock rather than accretion of 1 km of sediment from the downgoing plate the lack of volcanic intrusion east of the active arc and stratigraphic evidence for the broadening of east cape ridge with time suggests net fore arc accretion since 22 ma we propose a cyclical fore arc kinematic rock moves down a subduction channel to near the base of the crust where underplating drives rock uplift oversteepens the trench slope and causes collapse toward the trench and subduction channel cyclical rock particle paths led to persistent trench slope subsidence during net accretion existing global estimates of fore arc loss are systematically too high because they assume vertical particle paths citation sutherland r et al 2009 reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents tectonics 28 tc5017 doi 10 1029 2008tc002356

reflected and mode converted seismic waves within the shallow aleutian subduction zone southern kenai peninsula alaska pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern kenai peninsula alaska are identified as reflected p and s and converted s to p phases originating within four depth ranges in the upper few kilometers of the cook inlet tertiary basin at midcrustal depths within the overthrust north american plate at about 35 km depth near the top of the wadati benioff seismic zone in proximity to the inferred interplate megathrust and at about 5 10 km below the megathrust in the subducted pacific plate beneath the landward margin of the accreted chugach terrane the mid upper plate mup discontinuity dips as steeply as 20 degree 30 degree to the west northwest over the depth range 12 18 km at shallower depths it flattens and possibly arches with the crest at about 10 km depth similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the chugach terrane the extensive reflectors may have a common origin possibly caused by the presence of underplated rocks relative amplitudes and polarities of the secondary phases originating at mup depths provide few constraints on the nature of the discontinuity it appears that the mupmore discontinuity is seismically inactive and does not represent a brittle ductile transition zone within the upper plate the two converted s to p phases generated near the top of the subducted plate could indicate a low velocity zone associated with subducted oceanic crust less

along strike variation in subducting plate seismicity and mantle wedge attenuation related to fluid release beneath the north island new zealand abstract we analyze seismicity in the subducted slab of the hikurangi subduction zone along a 600 km length beneath the north island new zealand the volcanic character changes along strike with extremely productive rhyolitic volcanism in the central taupo volcanic zone moderate andesitic volcanism in the northern and south central zones and subduction without volcanism in the southernmost hikurangi zone we have relocated slab earthquakes with 3 d velocity models the relocated seismicity shows more detail of the varied distribution and abundance of slab seismicity which below 50 km depth may be related to embrittlement from high fluid pressure the depth of the buoyant subducted hikurangi plateau ranges from 40 to 140 km depth at depths where the subducting slab interacts with the mantle wedge along arc variation in slab seismicity is a fundamental characteristic with patches of abundant seismicity separated by low seismicity zones the largest most numerous patch at 150 220 km depth underlies a pronounced low qp zone in the mantle wedge which is associated with the rhyolite dominant taupo caldera extensive melt in the region of low qp requires high h 2 o flux from the underlying slab the abundant taupo seismicity suggests a correlation between melt production and regions of earthquake fracture permeability following embrittlement which promote migration of dehydration fluid the hydration history of the incoming slab may be a key factor in producing variations in dehydration and intraslab fluid migration broader extensive outer rise yielding and hydration may have occurred near the approach of the re entrant hikurangi plateau forming the slab section that currently has high seismicity slab seismicity deepens from 240 to 330 km along arc as the subduction rate increases from 40 mm yr the southwestern slab seismicity is bounded by an unusually narrow zone with 110 km depth extent this is inferred to be a dehydration front related to heating at a slab edge that is located 70 km further southwest

a seismic crustal section across the east cape convergent margin new zealand abstract crustal seismic reflection measurements were made along a 250 km profile across the pacific australian plate boundary north of new zealand data were recorded to 16 s two way time and have defined a major sedimentary basin corresponding to the southern end of the raukumara basin a fore arc basin reaching a depth of about 13 km this basin contains a lower generally conformable sequence of sediments and an upper sequence dominated by several major uncomformities the eastern part of this basin is cut by several recently active eastward dipping high angle thrust faults which are interpreted to be caused by the recent convergence at the adjacent pacific australian plate boundary the eastern margin of the basin is marked by a sharp change in character on the seismic data we interpret this feature as a terrane boundary coinciding with a major transcurrent fault which has juxtaposed a detached fragment of an island arc fore arc with a thin 15 km crust to the east against a passive margin sedimentary sequence to the west the southern raukumara basin is floored by cretaceous age oceanic crust fragments of which were obducted onto the north island about 22 ma ago the area has had a complex history a schematic model for the evolution of this margin during the past 22 ma is developed suggesting that the raukaumara basin has developed as a combination of fore arc basin processes and oblique crustal extension

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

transdimensional inversion of ambient seismic noise for 3d shear velocity structure of the tasmanian crust abstractambient seismic noise tomography has proven to be a valuable tool for imaging 3d crustal shear velocity using surface waves however conventional two stage inversion schemes are severely limited in their ability to properly quantify solution uncertainty and account for inhomogeneous data coverage in response to these challenges we developed a two stage hierarchical transdimensional bayesian scheme for inverting surface wave dispersion information for a 3d shear velocity structure and apply it to ambient seismic noise data recorded in tasmania southeast australia the key advantages of our bayesian approach are that the number and distribution of model parameters are implicitly controlled by the data and that the standard deviation of the data noise is treated as an unknown in the inversion furthermore the use of bayesian inference which combines prior model information and observed data to quantify the a posteriori probability distribution means that model uncertainty information can b

on factors controlling the depth of interseismic coupling on the hikurangi subduction interface new zealand abstract we apply a finite difference numerical thermal model to the shallow region of the hikurangi subduction margin north island new zealand this subduction zone is characterised by subduction of thick and buoyant crust of the hikurangi plateau and along strike variations in subduction obliquity onshore surface heat flow low in the south high in the north inferred pore fluid pressure and seismic style locked in the south aseismic in the north the model reproduces low onshore surface heat flow observed in the southern hikurangi margin we propose that greater surface heat flow in the north is caused by shear heating and convective heat flow by fluids escaping from the interface the model predicts that the hikurangi megathrust is comparatively cold so that if the lower limit of the interface seismogenic zone is thermally controlled it should occur near the intersection with the hangingwall moho this agrees with observation in the southern locked segment but thermal control alone cannot explain the shallow interseismic locking depth in the central and northern margin in crustal strength curves changing fluid pressure is capable of moving the brittle viscous transition vertically by tens of kilometres we propose that subduction zone seismic style depends on the fluid pressure regime both in the hangingwall and along the interface and define four end member types of megathrust fault segments contrary to many previous studies we suggest that the high fluid pressure low shear strength end member is fully coupled with potential for large interplate earthquakes in the hydrostatic case the fault is stronger but the brittle viscous transition occurs significantly shallower than in the near lithostatic case inferred to promote aseismic creep at relatively shallow levels along strike variation in seismic style on the hikurangi megathrust can therefore be explained by variation in fluid pressure state along the margin

slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip hikurangi subduction margin new zealand abstract travel times recorded onshore from active source marine seismic surveys were used to determine hikurangi forearc wavespeeds ray path midpoints sample forearc crust above the shallow v p from the seabed to 4 km depth consistent with rapidly deposited clastic sediments and near constant v p 5 0 0 2 km s at depths of 4 10 km within the forearc the northern region has slow slip events at shallow depths of c 5 15 km beneath the volume sampled by our analysis average travel times at offsets of 20 80 km are 1 s more than at equivalent offsets in the south and v p increases with depth within the forearc from 3 5 0 1 km s at 4 km depth to 4 5 0 2 km s at 10 km depth we interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously high porosity 10 with hydrostatic reference compaction models to show effective stress is low in both the north 27 10 mpa and south 36 14 mpa in the south pore space and conduits have a high chance of being localized on faults or sand layers the wedge is clearly compressionally critical and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework the observed wedge geometry and our estimate of fluid pressure within the wedge 0 87 0 05 suggest a weak overpressured subduction thrust in the north we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment a pore space geometry characterized by pervasively fractured rock and mudstone and a lower mean stress due to its stable and non critical wedge geometry we consider both end member assumptions of convergent and tensile failure the latter of which reveals the possibility of near lithostatic fluid pressure on the north hikurangi subduction interface although this remains unproven our observations are consistent with high fluid pressures encountered in petroleum wells at depths

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

dynamics of continental collision influence of the plate contact summary observations shows that continental collision may evolve in different ways resulting in a wide range of tectonic responses in search of the controlling conditions and parameters we start from the results of our previous work which demonstrated that the properties of the plate contact are important for the overall dynamics of convergent plate margins two fundamental types of subduction plate contact can be distinguished one based on a fault and the other based on a weak subduction channel in this study we investigate how the plate contact affects the initial stage of continental collision we use a finite element method to solve the heat and the time dependent momentum equations for elastic power law viscous and plastic rheologies for the same rheological properties and driving forces varying the nature of the plate contact leads to three types of responses the presence of a subduction channel promotes coherent and when the boundary conditions allow it plate like subduction of the continental margin in models with a subduction fault coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle slope the approaching continental sliver starts to subduct and the subduction is characterized by a non plate like behaviour slower subduction velocity than in channel models and strong slab deformation if the continental margin is steep and the strength of the incoming continental crust is high fault models result in locking of the trench eventually leading to slab break off if the crustal strength is relatively low shear delamination of part of the crust is expected in the channel model this type of delamination never occurs the tectonic settings used in our experiments prescribed plate velocity of the subducting plate versus fixed subducting plate corresponding to a landlocked basin setting do not significantly influence the nature of the model response we conclude that initial stages of continental collision are strongly affected by whether the subduction contact is a fault or a channel neither the slab pull magnitude nor the tectonic setting is very important to the overall geodynamics at this stage the plate contact type along with the slope of the incoming passive margin and the rheology of the continent controls whether the incoming crust 1 subducts entirely 2 separates partially or entirely from the lithospheric mantle or 3 blocks the trench likely leading to slab break off

expansion and rapid retreat of the west antarctic ice sheet in eastern ross sea possible consequence of over extended ice streams abstract marine geological data from the eastern ross sea provide compelling evidence that the west antarctic ice sheet extended to the continental shelf break during the last glacial maximum swath bathymetry records show mega scale glacial lineations within each of the four large glacial troughs on the shelf these features extend virtually uninterrupted across the shelf and have sharp lateral boundaries sediment cores that sampled the lineations recovered diamicton that is interpreted as deformation till the combined data indicate streaming ice the great extent of the paleo ice streams up to 1000 km in length is attributed to the fact that they flowed across unconsolidated sedimentary strata geomorphic features that are known to form at the retreating grounding line such as recessional moraines and ridges are restricted to the southeastern portion of the shelf iceberg furrows that occur at depths of up to 650 m overprint mega scale lineations these deep furrows are interpreted as having been formed by icebergs calved from the grounding line of the ice sheet which indicates the absence of a fringing ice shelf sediment cores sampled a thin layer of glaciomarine sediments resting sharply on till sub ice shelf deposits similar to those sampled in the western ross sea were not recovered in the eastern ross sea these combined results imply rapid retreat of the ice sheet from the shelf crosscutting lineations with different orientations indicate changes in flow direction that probably occurred as the ice profile was lowered and the physiography of the sea floor across which the ice sheet was retreating changed grounding zone wedges vary in number and location within individual troughs which indicates diachronous retreat of ice streams due to a lack of carbonate material for radiocarbon dating all radiocarbon ages were derived from acid insoluble organic aio fractions these dates indicate early retreat of the ice sheet relative to the western ross sea and other areas of west antarctica however the older dates must be viewed as suspect due to a paucity of diatomaceous material and contamination from old carbon while we were not successful at constraining the timing of ice sheet retreat from the continental shelf the data do indicate that the ice sheet was grounded on the shelf during the lgm

boundary condition of grounding lines prior to collapse larsen b ice shelf antarctica grounding zones where ice sheets transition between resting on bedrock to full floatation help regulate ice flow exposure of the sea floor by the 2002 larsen b ice shelf collapse allowed detailed morphologic mapping and sampling of the embayment sea floor marine geophysical data collected in 2006 reveal a large arcuate complex grounding zone sediment system at the front of crane fjord radiocarbon constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12 000 years ago previous studies and morphologic mapping of the fjord suggest that the crane glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse this implies that the 2002 larsen b ice shelf collapse likely was a response to surface warming rather than to grounding zone instability strengthening the idea that surface processes controlled the disintegration of the larsen ice shelf

subducting plate boundary beneath the northeastern japan arc estimated from sp converted waves abstract in the tohoku district the northeastern part of honshu japan a conspicuous phase is frequently observed between p and s phases in seismograms for events in the lower seismic plane of the double planed deep seismic zone this phase is dominant in the vertical component and has an apparent velocity almost equal to or slightly smaller than the p phase we have never found this phase in seismograms for events in the upper seismic plane from these characteristics the phase is interpreted as sp converted from s to p at the upper boundary of the descending oceanic plate ubp using the travel times of the sp waves we estimated the location of the ubp beneath the tohoku district to be just above the upper seismic plane of the double planed deep seismic zone this position is consistent with those determined from the analyses of the travel times of the scsp and ps waves which are respectively the seismic wave converted from scs to p and that from p to s at the ubp

estimation of arrival times in intense seismic sequences using a master events methodology based on waveform similarity summary intense seismic sequences involve a large number of earthquakes densely clustered in space and time their detailed analysis is important for the geometry of the activated faults which contributes to studies of the seismotectonic characteristics of an area the sheer number of small events as well as their low energy content renders their processing problematic in this work we have developed and applied a methodology to automatically pick the arrival times of p and s waves using a correlation detector event detection is performed using the waveform recordings of a reference station located close to the epicentral area of an intense seismic sequence such as aftershocks or swarms cross correlation matrices are constructed followed by nearest neighbour clustering and the formation of multiplets a master event is chosen from each cluster and its arrival times are picked manually the automatic algorithm uses the p o rs wave of each master event as a correlation detector searches the waveforms of the other events of the same multiplet and imposes the corresponding arrival time when the bestfit is achieved the picks are characterized by observation weights which derive from the quality of thefit the type of the available waveform components and the consistency between multiple measurements the proposed methodology was applied to an important seismic sequence that occurred between 2010 january 18 and 26 near the city of efpalio greece this procedure has the potential to increase 10 fold the amount of information and provide sufficient detail for a subsequent analysis of the spatiotemporal distribution of a seismic series

a new three component detector and automatic single station bulletin production a new three component 3c seismic signal detector is introduced that has enabled us to produce a preliminary single station bulletin in an automatic mode we consider automatic station bulletins as very important because in the high frequency range local events mostly chemical explosions like quarry blasts are truly numerous say 20 to 50 per day and cannot be handled efficiently by an analyst in the conventional way a short term average long term average sta lta test statistic is used either alone or in combination with 3c signal polarization for p phases which allows a lowered threshold an added advantage with the polarity estimate is that a corresponding slowness vector estimate is obtained the detector is operated in three consecutive steps 1 the detector is run on a suite of bandpass filtered records 2 the initial detection log is compressed while retaining parameters from the best filter and 3 the detected and identified phases are grouped into event families and epicenter parameters are estimated the 3c detector has been tested on 6 days of recordings from the station c4 in the noress array located in se norway and results were compared to the array bulletin for teleseismic signals the array has an advantage in snr of about 15 db for local and regional events distance 1500 km the 3c snr in the frequency range 5 to 15 hz is about 6 to 8 db below that of the array the 3c detector picks practically all s phases detected by the array but occasionally misses the preceding pn however some new events missed by the array were also found at local and regional distances the 3c azimuth and distance estimates are generally within 20 and 20 km respectively of the noress solution and the origin time estimates are within a few seconds the 3c epicenter solutions are less stable than that of the array mainly due to phase identification problems and pn azimuth errors at low snr naturally the benefit of analyst screening and re processing would improve the 3c bulletin quality although the performance is considered satisfactory at the present stage of development

high spatial resolution radar observations of ultralow frequency waves in the southern polar cap we present an analysis of ultralow frequency ulf waves detected in the southern polar cap using the super dual auroral radar network superdarn these waves manifest as quasi sinusoidal oscillations in the doppler velocity which arise due to the oscillating ulf wave electric field in the f region ionosphere the event reported in this study occurred during the southern polar winter under quiet geomagnetic conditions the 1 1 1 3 mhz oscillations were observed throughout the polar cap by the mcmurdo dome c east and south pole station superdarn radars and also by the dome concordia and scott base magnetometers at corrected geomagnetic latitudes of 89 s and 80 s respectively in contrast to large scale toroidal field line resonances observed at auroral latitudes we find that the magnetic perturbation vector of the polar cap oscillations did not undergo a 90 rotation upon transmission from the ionosphere to the ground and was instead rotated by about 30 the polar cap oscillations were also accompanied by simultaneous field line resonance flr activity in the dayside auroral zone in the field of view of the zhongshan superdarn radar this flr occurred at the same frequency and exhibited a similar azimuthal phase velocity to the polar cap oscillations indicating that they may have been driven by the same source mechanism

upper mantle anisotropy beneath the seychelles microcontinent 1 the seychelles plateau is a prime example of a microcontinent yet mechanisms for its creation and evolution are poorly understood recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the seychelles are analyzed to study upper mantle seismic anisotropy using sks splitting results strong microseismic noise is attenuated using a polarization filter results show significant variation in time delays t 0 4 2 4 s and smooth variations in orientation 15 69 where is the polarization of the fast shear wave the splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions recent work has suggested that anisotropy measurements for oceanic islands surrounding africa can be explained by mantle flow due to plate motion in combination with density driven flow associated with the african superswell such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle it has been suggested that the seychelles are underlain by a mantle plume predictions of flow induced anisotropy from plume lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau finally we consider lithospheric anisotropy associated with rifting processes that formed the seychelles the large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy fast directions align parallel to an area of transform faulting in the amirantes farther from this area the orientation of anisotropy aligns in similar directions as plate motions this supports suggestions of transpressive deformation during the opening of the mascarene basin

visualization of ulf waves in superdarn data 1 measurements of ionospheric e b drifts obtained with hf radars from the superdarn super dual auroral radar network sometimes show signatures of ulf few mhz waves we present a new data display technique that facilitates the detection of ulf waves in both ground and sea scatter returns statistical study of high time resolution data from the superdarn tiger radar in tasmania australia revealed that ulf wave signatures occur on an everyday basis with ground scatter accounting for about 60 of wave events about half of these events exhibit high coherence across large spatial distances and are associated with ulf pulsations recorded by a ground magnetometer these results show that superdarn radars may be used to routinely monitor ulf waves in the high latitude ionosphere

polarization filtering for automatic picking of seismic data and improved converted phase detection summary data adaptive polarization filtering is used to improve the detection of converted seismic phases both direct waves and mode converted ps and sp arrivals may be more easily picked on the filtered records an autopicking routine is applied that cuts the polarization filtered traces according to the modelled traveltime of each phase through an initial structure use of forward modelled source receiver times reduces the likelihood of an automatic pick being incorrectly made on spurious spikes in the polarization filtered trace it is therefore a realistic way of automatically picking multiphase data sets or more generally linearly polarized phases where low signal to noise ratios may be encountered the method is suitable for any three component seismic data and is here applied to local earthquakes recorded in north island new zealand intermediate energy is observed between the direct p and s arrivals due to phase conversion at the interface between the indo australian and subducting pacific plates the amplitudes of these converted arrivals are often too low for them to be identified above the p wave coda but polarization filtering of the records enables the yield of converted phase picks to be greatly increased

changes in electrical resistivity track changes in tectonic plate coupling 1 interplate coupling on the hikurangi subduction margin along the east coast of new zealand s north island changes north to south from almost uncoupled to locked clay rich sediments and aqueous fluids at the subduction interface have been invoked as key factors in the frictional processes that control interplate coupling here we use magnetotelluric data to show that the subduction interface in the weakly coupled region is electrically conductive but is resistive in the locked region these results indicate the presence of a layer of fluid and clay rich sediments in the weakly coupled region and support the idea that the presence of fluid and hydrated clays at the interface is a major factor controlling plate coupling

study of the structure of the slab mantle interface using reflected and converted seismic waves summary seismic waves reflected and converted at the interface between a downgoing slab and the mantle above it can provide important constraints on the velocity contrast at the interface and hence on the properties of the downgoing lithosphere modelling studies suggest that different physical mechanisms may contribute to the velocity contrast at different depths along the interface to explore the possible mechanisms we examine the amplitudes of seismic waves reflected and converted at the slab mantle interface in japanese subduction zones slab face p reflection amplitudes suggest a velocity contrast ranging upwards to 10 per cent at depths of between 300 and 400 km pis amplitude ratios from scs to p conversions at the slab mantle interface depend on the depth of conversion and are large 0 14 and 0 12 at stations shk and mat the size of the velocity contrasts and amplitude ratios suggests that a model of the interface more complicated than one involving merely temperature and or compositional effects is required we use these observations to constrain layered models of the slab mantle interface using thomson haskell matrix modelling comparing observations with calculated amplitude ratios the results suggest that the crustal layer of basaltic composition persisting to depth during subduction plays a role in the generation of the shallow converted waves the deeper conversions appear to require layering as well perhaps involving eclogite at the slab surface in addition to oriented olivine in the overlying mantle and the a p and a y phase transformations in olivine within the slab

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

survey of ulf wave signatures seen in the tasman international geospace environment radars data ultralow frequency ulf plasma waves propagate through the magnetosphere and ionosphere where they can alter the doppler velocity of hf radar echoes data from the two tasman international geospace environment radars and the fluxgate and induction coil magnetometers located on macquarie island 54 5 s 158 95 e geographic over 2006 2009 show that ulf wave signatures are common using coincident radar and magnetometer data selection criteria 194 events representing a total of 233 4 h were identified the majority of ulf signatures seen in the radar data were detected between 06 and 12 ut 15 and 21 lt using the maximum entropy method the spectral content showed favored frequencies of 1 6 2 1 2 9 and 3 3 mhz but no obvious variation of frequency with latitude most of the observed frequencies were in the range 1 4 mhz a class of doppler velocity signatures that appeared as a zigzag shape in the radar range time plots were identified and may be related to the expected plasmapause latitudes regularly spaced favored frequencies appeared in the ground magnetometer data during the afternoon night and morning sectors for those days where ulf wave events were observed in the radar data

scsp observed on north island new zealand implications for subducting plate structure seismic phase conversions provide important constraints on the layered nature of subduction zone structures recordings from digital stations in north island new zealand have been examined for converted scs to p scsp arrivals from deep 150 km tonga kermadec earthquakes to image layering in the underlying hikurangi subduction zone consistent p wave energy prior to scs has been identified from stations in eastern and southern north island where the subducted plate interface is at a depth of between 15 and 30 km two scs precursors are observed ray tracing indicates that the initial precursor scsp1 corresponds to conversion from the base of an 11 14 km thick subducting pacific crust the second precursor is interpreted as a conversion from the top of the subducting plate the amplitude ratio scsp1 scs increases from 0 10 to 0 19 from northern to southern north island this is within the range expected from a simple first order velocity discontinuity at an oceanic moho a 1 2 km thick layer of low velocity sediment at the top of the subducting plate is required to explain the remaining scsp waveform our results imply that the abnormally thick hikurangi chatham plateau has been subducting beneath new zealand for at least 2 9 myr thus explaining the high uplift rates observed across eastern north island

effect of sedimentation on ice sheet grounding line stability sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding line retreat in response to a rise in relative sea level of at least several meters recent antarctic changes thus cannot be attributed to sea level rise strengthening earlier interpretations that warming has driven ice sheet mass loss large sea level rise such as the 100 meter rise at the end of the last ice age may overwhelm the stabilizing feedback from sedimentation but smaller sea level changes are unlikely to have synchronized the behavior of ice sheets in the past

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

heterogeneous structure around the rupture area of the 2003 tokachi oki earthquake mw 8 0 japan as revealed by aftershock observations using ocean bottom seismometers abstract large earthquakes have repeatedly occurred in the area off southeastern hokkaido island japan as the pacific plate subducts beneath the island which is on the north american plate the most recent large earthquake in this area the 2003 tokachi oki earthquake mw 8 0 occurred on september 26 2003 in order to investigate aftershock activity in the rupture area 47 ocean bottom seismometers obss were quickly deployed after the main shock in the present study we simultaneously estimate the hypocenters and 3 d seismic velocity models from the p and s wave arrivals of the aftershocks recorded by obss the subducting plate is clearly imaged as a northwest dipping zone in which vp is greater than 7 km s and the relocated hypocenters also show the subducting pacific plate the aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the kuril trench the bending of the subducting plate corresponds to the southeastern edge of the rupture area the island arc crust on the overriding plate has p wave velocities of 6 7 km s and a vp vs of 1 73 a region of vp vs greater than 1 88 was found north of the epicenter of the main shock the depth of the high vp vs region extends about 10 km upward from the plate interface the plate boundary just below the high vp vs region has the largest slip at the main rupture a high vp anomaly 7 5 km s is found in the island arc crust in northeast part of the study area which we interpret as a structural boundary related to the arc arc collisional tectonics of the hokkaido region as the rupture of the main shock terminated at this high vp region we suggest that the plate interface geometry and the trench parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock

forced return flow deep in the subduction channel syros greece we present the results of a detailed structural study in the cycladic blueschist unit at fabrika on syros island greece and discuss their significance for tectonic processes at the subduction int

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

discovery of till deposition at the grounding line of whillans ice stream we report on the discovery of a grounding line sedimentary wedge till delta deposited by whillans ice stream west antarctica our observation is that grounding line deposition serves to thicken the ice and stabilize the position of the grounding line the ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium thus the grounding line will tend to remain in the same location despite changes in sea level until sea level rises enough to overcome the excess thickness that is due to the wedge further our observation demonstrates the occurrence of rapid subglacial erosion sediment transport by distributed subglacial till deformation and grounding line sedimentation which have important implications for ice dynamics numerical modeling of ice flow and interpretation of the sedimentation record