wedge geometry mechanical strength and interseismic coupling of the hikurangi subduction thrust new zealand the hikurangi subduction thrust interface east coast north island new zealand exhibits along strike variations in seismic style the southern segment is interseismically locked while the plate boundary thrust slips aseismically in the north the geometry of the offshore accretionary prism is also heterogeneous changing from small to large taper angle fault dip plus surface slope from south to north along strike variations in accretionary prism geometry generally reflect changes in coulomb wedge critical taper angle such variations are controlled by the relative strengths of the subduction megathrust and the wedge material in the southern hikurangi margin the small taper angle indicates a relatively weak subduction thrust interface where strong interseismic locking is inferred in the northern margin high taper angle reflects a strong shallow decollement relative to the south thus based on critical coulomb wedge theory the shallow megathrust appears relatively weak in the locked segment and stronger in weakly coupled regions

structure of a cold front over the ocean the kinematic and thermodynamic structure of a strong cold front over the gulf of alaska has been determined using aircraft dropsonde and radiosonde data obtained during the storm transfer and response experiment synoptic scale analysis using the sawyer eliassen secondary circulation equation shows that friction and diabatic heating as well as the geostrophic forcing are important in accounting for the secondary circulation at the front a fine scale analysis with horizontal resolution of 700m and vertical resolution of 100 m shows strong relative inflow of warm boundary layer air toward the front from the east and a weaker inflow of cold air from the west updraught velocities of greater than 6 m s 1 over a 2 km width were measured at 875 mb near the leading edge of the front the frontogenetical effects of confluence turbulent mixing and tilting of isentropic surfaces are evaluated in the region of the front and compared with the results of previous studies the high resolution of the thermodynamic fields provides new data for comparison with high resolution numerical models

idealized modelling of landfalling cold fronts the effects of land surfaces sea surfaces and coastlines on the dynamics of cold fronts are examined in an idealised two dimensional confluent deformation model of frontogenesis when the deformation field is stationary increasing the surface roughness weakens the fronts while increasing the cross front ageostrophic wind and frontal updraughts the frontogenetic effect of deformation balances the frontolytic effect of turbulent mixing resulting in near steady state fronts in steady state the surface fronts are located near the point at which the cross front flow vanishes this equilibrium point moves towards the warm air as the surface roughness is increased because the cross front ageostophic wind also increases with increasing surface roughness surface sensible heating weakens and slows the fronts during the day whereas surface sensible cooling strengthens and accelerates them at night adding a coastline to the model results in very strong gravity current like fronts in the afternoon with a relative flow of cool air towards their leading edge above the boundary layer the synoptic fronts remain unaffected by the coastline as the turbulent mixing weakens in the late afternoon the coastal fronts surge inland advancing faster than the wind speed in the boundary layer when the deformation field is allowed to translate the fronts advance across the ocean towards land if the fronts reach the coastline between mid morning and late afternoon the daytime heating over the land opposes the onshore advection of cold air retarding the fronts at the coastline if the fronts reach the coastline in the evening or early morning they advance onshore relatively unimpeded hence the fronts arrive onshore preferentially in the late afternoon or evening copyright 2010 royal meteorological society

geometry of the hikurangi subduction thrust and upper plate north island new zealand 1 we use 2800 line km of seismic reflection data to map the offshore character and geometry of the hikurangi subduction thrust and outer forearc wedge to depths of 15 km for 200 km along strike south of hawke bay the subduction thrust is relatively smooth dips less than 8 and the wedge is characterized by accretion of young sediment and topographic slopes of less than 3 in hawke bay and north for 200 km a kink in the subduction thrust is apparent with a downdip increase in dip to angles greater than 8 at depths of 10 15 km there is a corresponding steepening of the topographic slope to greater than 3 outboard of the kink and the wedge is characterized by lithified sedimentary rock and slope failure the kink in the subduction thrust is a locus of inherent weakness in the subducting slab we suggest its occurrence relates to a northward increase in subduction rate that controls initial slab dehydration and fluid release rates and hence intraslab deformation patterns the subduction thrust geometry in combination with a northward increase in subducting plate roughness and decrease in the amount of sediment accreted causes the observed spatial change in character of the subduction thrust and forearc wedge

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

airborne doppler radar and sounding analysis of an oceanic cold front abstract an analysis of a cold front over the eastern atlantic ocean based on airborne doppler wind syntheses and dropsonde data is presented the focus and unique aspect of this study is a segment of the front that was near the center of the cyclone the dual doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of 30 km covers a total distance of 450 km in the cross frontal direction the finescale resolution and areal coverage of the dataset are believed to be unprecedented the cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient the latter was most intense aloft and not at the surface in contrast to the classical paradigm of surface cold fronts the shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone this observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface the surface convergence within the frontal zon

evolution and structure of a cold front in an alpine valley as revealed by a doppler lidar the propagation of a cold front and its interaction with foehn winds is investigated in an alpine valley based on observations collected during the field campaign of the mesoscale alpine programme map on 6 november 1999 the key instrument of this study is a doppler lidar that had been operated in the wipp valley austria the cold front approached the european alps from the northwest became distorted at the mountain barrier and entered the east west aligned inn valley near the town of innsbruck primarily via two passes it continued to propagate towards innsbruck from both valley directions as two separate fronts that eventually collided east of innsbruck after part of the cold air had entered the adjacent north south aligned wipp valley a synthesis of doppler lidar measurements with conventional meteorological data including automatic weather stations and radiosondes leads to the conclusion that the cold front in the wipp valley was an atmospheric density current characterized by an elevated head a front relative feeder flow and a typical propagation speed of 7 m s 1 the foehn flow on top of the density current caused strong wind shear and triggered shear flow instability that led to the formation of a turbulent wake behind the head as the density current propagated towards the brenner pass it slowed down the shape of the frontal surface varied in time its inclination of about 10 20 is steeper than previously reported for the inn valley but is consistent with other observations of atmospheric density currents copyright 2010 royal meteorological society

a review of cold fronts with prefrontal troughs and wind shifts the conceptual model of a classical surface based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface many cold fronts however do not conform to this model time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front hereafter called a prefrontal trough and prefrontal wind shift respectively although many authors have recognized these prefrontal features previously a review of the responsible mechanisms has not been performed to date this paper presents such a review ten disparate mechanisms with different frontal structures have been identified from the previous literature these mechanisms include those external to the front i e those not directly associated with the cold front itself synoptic scale forcing interaction with lee troughs drylines interaction with fronts in the mid and upper troposphere and frontogenesis associated with inhomogeneities in the prefrontal air mechanisms internal to the front i e those directly associated with the structure and dynamics of the front include the following surface friction frontogenesis acting on alongfront temperature gradients moist processes descent of air ascent of air at the front and generation of prefrontal bores gravity waves given the gaps in our knowledge of the structure evolution and dynamics of surface cold fronts this paper closes with an admonition for improving the links between theory observations and modeling to advance understanding and develop better conceptual models of cold fronts with the goal of improving both scientific understanding and operational forecasting

the role of forcing in cell morphology and evolution within midlatitude squall lines abstract this study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines the forcing chosen was a cold front simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front use of a realistic balanced and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing three dimensional squall lines were modeled with and without the front present in low and high bulk richardson number environments the forced convection evolved in significantly different ways than their isolated unforced counterparts in low shear conditions the line of isolated convective cells split with the adjacent split cells interfering destructively with neighboring cells in the line with forcing present differences in anticyclonic cell intensity and propagation prevented this interaction from occurring leading to longe

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

rates and mechanics of rapid frontal accretion along the very obliquely convergent southern hikurangi margin new zealand analysis of seismic reflection profiles swath bathymetry side scan sonar imagery and sediment samples reveal the three dimensional structure morphology and stratigraphic evolution of the central to southern hikurangi margin accretionary wedge which is developing in response to thick trench fill sediment and oblique convergence between the australian and pacific plates a seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby by coccolith and foraminifera biostratigraphy of three core and dredge samples and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds on the growth and mechanics of frontal accretion under variable convergence obliquity and on the amounts and rates of horizontal shortening the data place constraints on the partitioning of geological strain across the entire southern hikurangi margin the principal deformation front at the toe of the wedge is discontinuous and represented by right stepping thrust faulted and folded ridges up to 1 km high which develop initially from discontinuous protothrusts in the central part of the margin near 41 s where the convergence obliquity is 50 orthogonal convergence rate is slow 27 mm yr and about 75 of the total 4 km of sediment on the pacific plate is accreted frontally the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening at least 80 of this shortening has occurred within the last 0 4 0 1 m y at an average rate of 12 3 mm yr this rate indicates that the frontal 30 km of the wedge accounts for about 33 55 of the predicted orthogonal contraction across the entire plate boundary zone despite plate convergence obliquity of 50 rapid frontal accretion has occurred during the late quaternary with the principal deformation front migrating seaward up to 50 km within the last 0 5 m y i e at a rate of 100 km m y the structural response to this accretion rate has been a reduction in wedge taper and consequently internal deformation behind the present deformation front near the southwestern termination of the wedge where there is an along the margin transition to continental transpressional tectonics the convergence obliquity increases to 56 and the orthogonal convergence rate decreases to 22 mm yr the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward verging folds these structures have accommodated not more than 0 5 km of horizontal shortening at a rate of 1 mm yr which represents 5 of the predicted orthogonal shortening across the entire plate boundary in southern north island the landward vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin to the northern south island region where the upper and lower plates are locked or at least very strongly coupled

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

an observational and numerical study of an intense landfalling cold front along the northwest coast of the united states during coast iop 2 abstract this paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the united states during iop 2 3 4 dec 1993 of the coastal observation and simulation with topography experiment observations suggest that the offshore frontal zone was associated with two transitions the first characterized by a substantial temperature gradient a nearly vertically oriented wind shift from southwesterly ahead to westerly behind and a convective rainband while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind the frontal zone was quite narrow 5 km wide and nearly vertical below 850 mb and its width increased by 1 2 orders of magnitude above 850 mb well before the frontal landfall low level flow to the west of the olympics was associated with geostrophic balance in the cross shore direction and downgradient acceleration in the alongshore direction which contributed to the formation

finescale vertical structure of a cold front as revealed by an airborne doppler radar abstract in the afternoon of 24 may 2002 a well defined and frontogenetic cold front moved through the texas panhandle detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary this paper primarily uses reflectivity and doppler velocity data from an airborne 95 ghz radar as well as flight level thermodynamic data to describe the vertical structure of the cold front as it intersected with the dryline the prefrontal convective boundary layer was weakly capped weakly sheared and about 2 5 times deeper than the cold frontal density current the radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail 40 m the echo structure and dual doppler inferred airflow in the vertical plane reveal typical features such as a nose a head a rear inflow current and a broad current of rising prefrontal air that feeds the accelerating front to rear current over the head the 2d cross fro

surface fluctuations associated with tropical cyclone rainbands observed near taiwan during 2000 08 with radar measurements and temporally high resolution surface observations this study investigates surface fluctuations associated with tropical cyclone rainbands tcrs observed in the vicinity of taiwan during 2000 08 a total of 263 tcrs identified from 37 typhoon eventsduring the studyperiod were analyzed to show the mean and common nature of perturbations of various meteorological variables associated with the passage of tcrs the main patterns of surface thermodynamic fluctuations as revealed from the composite analysis of all identified tcrs include a persistent decrease in temperature dewpoint temperature and equivalent potential temperature ue from the outer to inner edge of the rainband a wavelike variation of pressure perturbations associated with the rainband was evident with a minimum coincident with the outer edge and a maximum located inside the inner edge the kinematics of the rainband was characterized by an obvious decrease in cross band wind component relatively minor variations in along band wind component and the windveering quantitative analysesindicatethatthemajorityofthetcrs 80 90 exhibitedvariations in surface temperature pressure wind speed and wind direction less than 28c 1 5 mb 5 m s 21 and 208 respectively however acleartrendofthemagnitudeoftcrthermodynamicfluctuationsincreasingwiththe radial distance from the tropical cyclone center was observed the tcrs identified in this study were also classified into the outer and inner rainbands which are distinguishedbyaradialdistanceof3timestheradiusofmaximumwind thecompositeandmagnitudeanalyses of their surface fluctuations indicate that the outer rainbands had a higher potential than the inner rainbands to reduce the near surface ue values this observed characteristic is likely related to more pronounced evaporative cooling taking place in drier subcloud regions and the downward transport of low ue air aloft by more vigorous convective downdrafts for the outer rainband fundamentally different features of surface pressure fluctuations and mean frictional vertical velocity and relative vorticity between the outer and inner rainbands were also documented these results reflect a possibly different origin nevertheless there was no dramatic difference in the pattern of kinematic fluctuations between the outer and inner rainbands and their mean magnitudes were also found to be statistically identical which suggests that there is not an entirely clear distinction of surface characteristics for these two types of rainbands

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

diagnostic studies of a two dimensional simulation of frontogeneiss in a moist atmosphere abstract the two dimensional model simulation of frontogenesis in a moist atmosphere discussed by hsie and others is used to provide dynamically consistent data for diagnostic calculations to quantify the physical process important in the frontogenesis the terms in the prognostic equations for the horizontal gradient of potential vorticity relative vorticity and static stability are evaluated to indicate the relative importance of confluence shear tilting and frictional and diabatic process in generating these properties of the front an evaluation of the potential vorticity and equivalent potential vorticity fields and their budgets indicates that symmetric instability is not present in the dry simulation but occurs in the moist simulation and is associated with the formation of rainbands the effect of condensation on the energetics of the frontgenesis is studied by evaluation of the kinetic energy budget the latent heat of condensation generates available potential energy on the mesoscale and e

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

episodic growth of fold thrust belts insights from finite element modelling abstract the sequential development of a fold thrust belt was investigated using 2d finite element modelling fem the new model results show that a thrust system is typically composed of three distinct regions the thrust wedge pre wedge and undeformed region the thrust wedge involves growth that repeats episodically and cyclically a cycle of wedge building starts as frontal accretion occurs which is accompanied by a rapid increase in wedge width reducing the taper angle below critical in response to this the wedge interior tracked here by the 50 m displacement position rapidly propagates forwards into a region of incipient folding the taper angle progressively increases until it obtains a constant apparent critical value 10 during this period the wedge experiences significant shortening after a new thrust initiates at the failure front leading to a decrease in wedge width successive widening of the wedge and subsequent shortening and thrusting maintain a reasonably constant taper angle the fold thrust belt evolves cyclically through a combination of rapid advancement of the wedge and subsequent gradual slow wedge growth the new model results also highlights that there is clear although minor deformation 0 10 m horizontal displacement in front of the thrust wedge

review of atmospheric boundary layer research 1983 1986 the atmospheric boundary layer comprises the lower portion of the atmosphere that is dynamically or thermodynamically affected by the presence of and interactions with the underlying surface the momentum heat water vapor and species exchanges at the surface have important effects on the boundary layer and a large portion of man s activities the height of the boundary layer can range from only several tens of meters in extreme inversion situations to many kilometers boundary layer sub specialties are often divided along the lines of the nature of the underlying surface ocean air sea interaction ice smooth terrain urban vegetation of significant height canopies and large scale features such as hills and valleys

crustal dynamics and active fault mechanics during subduction erosion application of frictional wedge analysis on to the north chilean forearc abstract the forearc region of the non accreting south american plate margin in northern chile is characterised by subduction erosion and regional uplift neotectonic deformation structures reflect simultaneous extensional and contractional fault kinematics in the outer forearc where the brittle crust directly overlies the subducting nazca plate the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as seen in neotectonic surface structures and seismic data in the inner forearc surface structures indicate a compressional stress regime also affecting the western rim of the magmatic arc this stress regime is limited to a brittle crustal wedge segment which overlies the ductile part of the inner forearc lithosphere the shape of the two brittle forearc wedges at the leading edge of the south american plate is controlled by exogenetic surface processes internal deformation processes contemporaneous basal tectonic erosion and underplating mechanical parameter sets controlling and reflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated and applied to general frictional wedge models the states of stress within the crustal wedges are controlled by spatial variations of the basal mechanical parameters in the down dip direction of the forearc wedge base the new models illustrate the fundamental kinematics and dynamic processes of coulomb type basal tectonic erosion and mass transfer modes along active non accretive convergent margins the frictional wedge models explain the dynamics of the simultaneous and contrary deformation processes affecting the forearc crust at the north chilean convergent margin shaped by active basal tectonic erosion

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone accretionary prism and incoming philippine sea plate new insights into episodic deformation at the nankai trough subduction zone are provided by data from two ocean drilling program borehole hydrologic observatories drilled into the philippine sea plate site 1173 and the seaward part of the nankai accretionary prism off southwestern japan site 808 and from an array of high sensitivity borehole accelerometer and velocity seismometers on shikoku island hi net fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site and relaxation of the accretionary prism toe the rate of strain ca 10 6 yr 1 inferred from the rates of pressure rise at site 1173 up to 5 kpa yr 1 is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide opposing these trends are transient pressure anomalies observed in late june early july 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism concurrent with these transients a swarm of very low frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust reverse fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism we suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about june 26 2003 at or near the up dip limit of the locked portion of the subduction thrust and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas in addition to demonstrating that the prism is far from inactive during interseismic intervals the observations may also provide a small amplitude analog for strain and hydrologic response at the time of great subduction earthquakes

control of forearc density structure on megathrust shear strength along the chilean subduction zone abstract unveiling how the interaction of friction pore pressure p and normal stress n controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains an existing density model of the chilean margin 18 45 s allows the computation of the so called vertical stress anomaly vsa that accounts for the component of normal stress due to the weight of the overlying crustal column a spatial correlation between vsa surface geology and several earthquake parameters suggests a significant control exerted by the geologically dominated density structure of the crust on megathrust shear strength the very seismogenic forearc in front of the high central andes 18 34 s has positive vsa values up to 60 mpa and a mafic dominated crust which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column in addition to a likely dry subduction channel characterized by high and low p in contrast the southern andean forearc 34 45 s shows neutral to negative vsa down to 30 mpa correlated with a felsic dominated crust which added to a presumably wet subduction channel low and high p configures a rather weak megathrust below a light forearc earthquakes here are concentrated around the arauco peninsula a first order morpho structural anomaly characterized by comparatively higher values of vsa than the rest of this segment which is the nest of the giant mw 9 5 valdivia 1960 earthquake and the southern rupture termination of the recent mw 8 8 maule earthquake the ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative vsa values the correlation between vsa and the along strike segmentation of the andean orogen indicates that the geologically inherited density structure of the forearc also rules long term mechanical coupling between converging plates and prescribes the possible structure of the orogen because central and southern andes represent two extremes in terms of orogenic structure and seismogenic behavior results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

airborne observations of a front near a col during fastex abstract an analysis of an oceanic front situated near a col defined by the surface pressure field is presented there have been few observational examples of this type of front presented in the literature the primary source of information for this study was data recorded by an aircraft equipped with a doppler radar the front was approximately two dimensional and the cross frontal scale at low levels was 30 40 km a prefrontal low level jet was identified in the high resolution analyses and was shown to be supergeostrophic surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance twi in the alongfront direction large negative values of twi the vertical shear is less than predicted for the given horizontal temperature gradient were located near a region of frontogenesis the strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in g

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

airborne doppler observations of a cold front in the vicinity of vancouver island abstract this study uses airborne doppler radar and flight level measurements from the coastal observations and simulations with topography experiment to examine the detailed mesoscale structure of an oceanic cold front upstream of vancouver island on 13 december 1993 these aircraft observations show that there were dramatic differences in frontal structure and movement between nearshore and offshore regions presumably due to the effects of blocking by the terrain the aircraft observations are considered in two parts since the behavior of the front evolved over the flight period during the early and middle portion of the flight the low level flow east of the front was out of the south southeast in the nearshore region rather than southerly as found farther offshore the nearshore segment of the front was oriented south north and appeared to be stationary the zone of orographic influence was found to extend to a region 20 km offshore and confined primarily to the lowest 1 5 km msl in distinct c

thermal and metamorphic environment of subduction zone episodic tremor and slip 1 episodic tremor and slip ets have been detected in the cascadia and southwest japan subduction zones where the subducting crust is relatively warm because of the young incoming lithosphere 20 ma and modest plate convergence rates 40 60 mm a in the southwest japan subduction zone low frequency earthquakes occur on the plate interface at depths of 30 35 km beneath shikoku where finite element thermal models predict temperatures of 425 c in the subducting oceanic crust and at depths of 30 40 km beneath the kii peninsula where predicted temperatures are 325 c warmer temperatures of 575 c are predicted at ets depths beneath southern vancouver island in the cascadia subduction zone but here tremor also occurs within the overlying fore arc crust where temperatures are lower in the southwest japan and cascadia subduction zones subducting oceanic crust passes through the blueschist greenschist and amphibolite metamorphic facies where mineral dehydration reactions are complex the different temperatures predicted for the two subduction zones suggest that ets does not coincide with a specific temperature or metamorphic reaction several lines of evidence indicate that a free h2o rich fluid is present at least transiently in subducting oceanic crust and fluids released by prograde metamorphic dehydration reactions may help trigger or enable ets within the subducting plate less clear is the role h2o may play in tremor observed in the cascadia fore arc crust where free h2o may exist locally in faults and fractures but retrograde hydration reactions are expected to consume h2o

orogen parallel variation in exhumation and its influence on critical taper evolution the case of the emilia romagna apennine italy abstract the northern apennine prowedge exposes two adjacent sectors showing a marked along strike change in erosion intensity namely the emilia apennine to the northwest and the romagna apennine to the southeast this setting has resulted from pliocene erosion 5 ma and exhumation which have affected the whole romagna sector and mostly the watershed ridge in emilia such an evolution has conceivably influenced the equilibrium of this fold and thrust belt which can be evaluated in terms of critical coulomb wedge theory the present state of the thrust wedge has been assessed by crosschecking wedge tapers measured along transverse profiles with fluid pressure values inferred from deep wellbores the interpretation of available data suggests that both emilia and romagna are currently overcritical this condition is compatible with the presence in both sectors of active ne dipping normal faults which would work to decrease the surface slope of the orogenic wedge however the presence of late miocene pliocene passive roof and out of sequence thrusts in romagna may reveal a past undercritical wedge state ensuing during the regional erosion phase thereby implying that the current overcritical condition would be a recent feature the setting of the emilia apennine i e strong axial exhumation and limited erosion of the prowedge suggests instead a long lasting overcritical wedge which was probably contemporaneous with the pliocene undercritical wedge in romagna the reasons for this evolution are still unclear although they may be linked to lithosphere scale processes that have promoted the uplift of romagna relative to emilia the lessons from the northern apennine thus suggest that erosion and exhumation have the ability to produce marked along strike changes in the equilibrium of a fold and thrust belt

small earthquakes provide insight into plate coupling and fluid distribution in the hikurangi subduction zone new zealand abstract to better understand the nature of interplate coupling in the southern north island of new zealand we compare the distribution of relocated small earthquakes with interseismic geodetic locking of the plate interface from gps and structure within the overlying plate from tomographic inversions relocated seismicity shows a good correlation with both the distribution of strong geodetic locking and geological terranes in the overlying plate it appears that the strong permian triassic rakaia geological terrane controls coupling at the plate interface this relatively impermeable terrane also modulates the small earthquake distribution within the underlying slab promoting clustered seismicity in the slab crust and suppressing lower plane seismicity in the mantle the seismicity and structural data suggest a model where plate coupling is controlled by the ability of fluid to cross the plate interface when an impermeable terrane in the overlying plate prevents such fluid flow plate coupling appears to be strong

comparison of fold thrust belts driven by plate convergence and gravitational failure abstract deepwater fold thrust belts ftbs are predominantly formed by the deformation of sedimentary sequences as a result of subduction of oceanic plates at active margins gravitational failure at passive margins or a combination of these two driving mechanisms at both types of margin a key question is is the ftb driven by gravitational failure basically the same as the ftb driven by plate convergence or are there fundamental differences to address this we reviewed ftb examples from different tectonic settings end members and hybrid systems in terms of geometry structure deformation shortening rate and tectonic process the energy source in gravity driven systems lies within the sedimentary material itself that is being deformed by updip extension and downdip contraction whereas in the plate convergence driven systems it lies outside the local sediment pile and within the broader undeformed crust and lithosphere the energy in gravity driven systems is resupplied by sediment input from large river deltas and therefore deformation tends to be episodic linked to episodes of major sediment input whereas in a system driven by plate motions the energy is resupplied by movement of a boundary upon which force is acting and tends to be stable and less episodic therefore the sedimentation plays an important role in the gravity driven deformation but a less important role in the plate convergence driven deformation in gravity driven systems it promotes upward propagation of normal faulting at upslope by increasing maximum principal stress but hinders upward propagation of thrusting i e mostly buried structures by increasing minimum principal stress at downslope while in plate convergence systems thrusts and folds tend to deform seafloor as a result of large compression with less effect from sedimentation despite the varying basal strength the observed thrust faults in both systems are mostly basinward verging in slight contrast to the theoretical model predication with homogeneous material implicating the control of likely variability of stratigraphic strength on thrust vergence the shortening rate across plate convergence driven systems is high several tens mm yr and continuous on a long timescale whereas across the gravity driven fold thrust belts it is generally slow several mm yr and more variable or even catastrophic through time despite overall seaward younging of thrusting fault activity differs slightly among different systems focused activity at the frontal structures of ftbs is more common at the plate convergence system while activity focused in the rear to middle of ftbs is more common in the gravity driven system plate convergence driven system is primarily limited by rate of plate motion i e the rate at which the plate is fed into the ftb whereas in gravity driven system the movement is limited resisted by the strength of sediments and detachment

reexamining the dynamics of short scale diabatic rossby waves and their role in midlatitude moist cyclogenesis abstract as a step toward a more comprehensive study of the physical processes that underlie explosive cyclogenesis a two dimensional semigeostrophic model with a now standard parameterization of latent heat release is used to diagnose the structure energetics and propagation characteristics of short scale diabatic normal modes in a moist baroclinic atmosphere with the eady basic state upon revisiting the inviscid problem it is found that when a thermodynamically consistent vertical profile of latent heat release is used the short wave cutoff vanishes and growth rates become independent of zonal wavelength for zonal wavelengths shorter than approximately 1900 km the destabilized short scale modes identified previously as diabatic rossby waves owe their existence to the continuous generation of potential vorticity by moist processes associated with warm air advection rising motion and latent heat release to determine if these short scale diabatic rossby wave modes continue to grow at small

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

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

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

large trench parallel gravity variations predict seismogenic behavior in subduction zones we demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench parallel gravity anomaly tpga whereas regions with strongly positive tpga are relatively aseismic these observations suggest that over time scales up to at least 1 million years spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore arc the correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench parellel variations in fore arc topography gravity and seismogenic behavior

an observational and numerical study of a cold front interacting with the olympic mountains during coast iop5 abstract this paper documents the three dimensional flow and precipitation structures associated with a weak cold front interacting with the olympic mountains and the subsequent development of a puget sound convergence zone this study utilizes data collected during coast iop5 the fifth intensive observing period of the coastal observation and simulation with topography field experiment that took place on 11 12 december 1993 one of the most important data sources was a noaa p 3 aircraft which provided flight level data radar reflectivity and doppler winds as it circumnavigated the olympics initially frontal passage along the western foothills of the olympics was accompanied by a 2 3 c temperature drop a rapid wind shift to northwesterlies and an intense line of precipitation 35 45 dbz however the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the olympics surface and p 3 observations document the deformation of the front

variation of frontal and precipitation structure along a cold front a diagnostic study is presented to illustrate the sequence of events in which a warm conveyor belt wcb flow is drawn into the circulation of a developing extratropical cyclone and interacts with dry air aloft to produce distinctive precipitation structures the poleward boundary of the upstream end of the wcb was associated with a classical ana cold frontal structure the other end of the wcb flow closer to the cyclone centre was overrun by a dry intrusion of low wet bulb potential temperature air to give a split kata cold front the orientation of the surface cold front i e the wcb boundary changed significantly between these two parts but the orientation of the surface geostrophic flow within the wcb changed relatively little between the two parts the surface cold front was characterized in most places by a narrow cold frontal rainband composed of shallow line convection elements precipitation cores aligned nearly parallel to the surface geostrophic flow in the wcb the changing relationship between the frontal orientation and the direction of the surface flow accounts for the fact that the line convection elements occurred end to end to give almost two dimensional line convection along the classical ana part of the cold front but were non aligned and widely spaced along the split cold front part

the distribution of surface fluxes and boundary layer divergence in midlatitude ocean storms abstract surface meteorological observations have been used to calculate the distributions of surface fluxes of momentum sensible heat and latent heat and the distributions of surface divergence and curl of surface stress which are characteristic of ocean storms in the gulf of alaska flux calculations were based on aerodynamic equations which include the effects of high wind speed and stratification of the surface layer distributions of divergence determined directly from surface winds were compared with those calculated using the curl of the surface stress results indicate that heat fluxes were weak or oven negative for the most intense storms in a band about 300 km wide immediately east of strong cold or occluded fronts maximum upward latent heat fluxes occurred to the west of the front with secondary maxima along the eastern edge of the storm beyond the 300 km band of weak flux surface convergence occurred to the east and divergence to the west of cold or occluded fronts vertical velocities c

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

numerical simulations of maritime frontogenesis abstract a hydrostatic primitive equation model initialized in a highly baroclinically unstable state was used to simulate maritime cyclogenesis and frontogenesis in order to identify boundary layer physical processes important in maritime frontogenesis several different simulations were performed in an effort to isolate impacts due solely to the boundary layer moist processes were not included an adiabatic and inviscid simulation provided the control for these experiments two different boundary layer parameterizations were used a k theory parameterization featuring richardson number dependent eddy diffusivity and a second order closure scheme with prognostic equations for the turbulence quantities results indicated that strong warm and cold fronts formed in the adiabatic and inviscid case but that the vertical motion fields were weak in the k theory simulation the results were somewhat more realistic with stronger vertical motion in both the k theory and second order closure simulations the b

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

structure of the cold front observed in sesame ave iii and its comparison with the hoskins bretherton frontogenesis model abstract a cold front which passed through the dense network of the sesame ave severe environmental storms and mesoscale experiment atmospheric variability experiment on 25 26 april 1979 was investigated rawinsonde data collected from 23 special stations and 19 national weather service stations at three hour intervals for a 24 hour period were used along with hourly surface data radar summary charts and goes east satellite images severe storms formed along the surface front during this period the analysis focused on the vertical circulation across the frontal surface at low levels the major features of the cold frontal system that emerged from an analysis of this unique data set include a familiar direct vertical circulation with moist warm air ascending just above the surface front however the upgliding motion was intercepted by a secondary circulation at middle levels the analysis result was compared with model predictions of hoskins and bretherton 1972 as calculated by blumen 1980 sever

modelling precipitation in frontal rainbands the main aim of this study is to establish the ability of a simple parametrization to represent adequately the microphysical processes acting in frontal rainbands in order to do this we compare it with a more detailed parametrization in conditions typical of warm frontal and narrow cold frontal rainbands simulations using a 2 dimensional diagnostic model indicate that a mixed phase parametrization is required in dynamical models of high resolution

thrust mechanics in highly overpressured accretionary wedges rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid dilatant fracturing will create escape routes so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile dilatant fracture which is slightly in excess of the load pressure if the maximum compression is nearly horizontal this in turn buffers the strength of the wedge at the cohesive strength which is not pressure dependent and will not vary greatly throughout the wedge near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge the shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment these assumptions allow the application of a simple plastic continuum model which successfully predicts the observed gently convex taper of accretionary wedges they also allow the use of a mechanical model for frontal imbrication which predicts the spacing of imbricate thrusts at the front of the wedge from the strength and thickness of the imbricated layer the ramp angle and the factional resistances on the bounding faults and decollement surfaces reconstruction of thrust geometries from seismic data across the makran and other accretionary wedges allows the determination of the geometrical parameters and the thrust imbrication model allows the mechanical parameters to be calculated in terms of an assumed cohesive strength for the incoming sediment layer the calculated values are reasonable and successful forward modelling of the profile of the makran shows that the postulate of a buffered value of fluid pressure in excess of the load pressure is consistent with observation

the structure water budget and radiational features of a high latitude warm front abstract on 30 september 1994 an arctic low pressure system passed over the southern beaufort sea area of northern canada and research aircraft observations were made within and around the warm front of the storm this study is unique in that the warm front contained subzero centigrade temperatures across the entire frontal region the overall structure of the warm front and surrounding region was similar to midlatitude storms however the precipitation rates liquid water content magnitudes horizontal and vertical winds vertical wind shear turbulence and thermal advection were very weak in addition a low level jet and cloud bands were aligned parallel to the warm front near neutral stability occurred within and around the front and conditional symmetric instability was likely occurring a steep frontal region resulted from strong coriolis influences that in turn limited the amount of cloud and precipitation ahead of the system the precipitation efficiency of the storm was high 60 but is beli

density depth model of the continental wedge at the maximum slip segment of the maule mw8 8 megathrust earthquake abstract complexities in the rupture process during a megathrust earthquake can be attributed to the combined effect of inhomogeneous distribution of stress accumulated during the interseismic period and inhomogeneous rheology of the seismogenic contact we modeled the free air gravity field of the southern central chile convergent margin along five 2 d profiles that cross the patch of highest slip during the chilean 2010 megathrust earthquake in order to analyze variability in the density and shape of the continental wedge and its relationship with seismotectonics we also analyzed the bathymetry to derive the long term interplate friction coefficient the results show that the high slip patch during the maule earthquake corresponds to a segment of the margin characterized by 1 low densities in the continental wedge 2 low vertical loading over the inter plate contact 3 a well developed shelf basin and 4 low taper angles consistent with a low effective basal friction coefficient we interpret the correlation between these parameters in terms of the total potential energy change during the earthquake and conclude that if the normal stress or frictional coefficient are low then a large slip does not necessarily imply a large amount of coseismic work heterogeneities in density of the continental basement can therefore be related to complexities in the pattern of coseismic slip and in the aftershock distribution locally a subducted seamount or seaward spur of high density continental crust may be present near the high slip patch

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones part v the substructure of narrow cold frontal rainbands abstract the organization and structure of a narrow cold frontal rainband ncfr on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity doppler radar observations airborne observations and surface measurements the ncfr was composed of small mesoscale regions of heavy precipitation called precipitation cores pcs oriented at an angle to the synoptic scale cold front in horizontal cross section the pcs were roughly elliptical in shape areas of lighter precipitation called gap regions grs separated the pcs the pcs were so oriented that their loading edges were regions of strong low level convergence the weather associated with the passage of a pc resembled that of a squall line gust front with concurrent windshifts and pressure checks occurring 5 min before heavy precipitation and a fall in temperature the changes in surface weather that accompanied the passage of a gr were more variable but tended to be less marked than for pcs thus the se

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

changes in dip and frictional properties of the basal detachment controlling orogenic wedge propagation and frontal collapse the external central betics case thin skinned fold and thrust belts ftbs have been extensively studied through both field examples and modelling the overall dynamics of ftbs are therefore well understood one less understood aspect is the combined influence of across strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge in this paper we use field data together with reflection seismic interpretation from the external zones of the central betics ftb s spain to identify a significant increase in the wedge basal dip a basement threshold coinciding with the pinch out of a weak substrate this induced both changes to the wedge geometry and to the basal friction which in turn influenced the wedge dynamics the changing dynamics led to a transient stagnation of the ftb propagation topographic build up and subsequent collapse of the ftb front this in turn fed an important langhian depocenter made up of mass transport deposits coevally with the ftb propagation extension took place both parallel and perpendicular to the orogenic trend this case study illustrates how across strike changes in wedge basal properties can control the detailed behaviour of a developing ftb front but questions remain regarding the time space interaction and relative importance of the basal parameters

tectonic and geological framework for gas hydrates and cold seeps on the hikurangi subduction margin new zealand the imbricated frontal wedge of the central hikurangi subduction margin is characteristic of wide ca 150 km poorly drained and over pressured low taper not vert similar 4 thrust systems associated with a relatively smooth subducting plate a thick trench sedimentary sequence not vert similar 3 4 km weak basal decollement and moderate convergence rate not vert similar 40 mm yr new seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures and to establish the geological framework for gas hydrates and fluid seeps we discuss the stratigraphy of the subducting and accreting sequences characterize stratigraphically the location of the interplate decollement and describe the deformation of the upper plate thrust wedge together with its cover sequence of miocene to recent shelf and slope basin sediments we identify approximately the contact between an inner foundation of deforming late cretaceous and paleogene rocks in which widespread out of sequence thrusting occurs and a 65 70 km wide outer wedge of late cenozoic accreted turbidites although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1 2 myr this differs from the offshore hawkes bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin resulting in uplift and complex deformation of the lower slope and a narrow 10 20 km active frontal wedge five areas with multiple fluid seep sites referred to informally as wairarapa uruti ridge omakere ridge rock garden and builders pencil typically lie in 700 1200 m water depth on the crests of thrust faulted anticlinal ridges along the mid slope uruti ridge sites also lie in close proximity to the eastern end of a major strike slip fault rock garden sites lie directly above a subducting seamount structural permeability is inferred to be important at all levels of the thrust system there is a clear relationship between the seeps and major seaward vergent thrust faults near the outer edge of the deforming cretaceous and paleogene inner foundation rocks this indicates that thrust faults are primary fluid conduits and that poor permeability of the cretaceous and paleogene inner foundation focuses fluid flow to its outer edge the sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the decollement within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector bsr and commonly a seismically resolvable shallow fault network through which fluids and gas percolate to the seafloor no active fluid venting has yet been recognized over the frontal accretionary wedge but the presence of a widespread bsr an extensive protothrust zone 200 km by 20 km in the hikurangi trough and two unconfirmed sites of possible previous fluid expulsion suggest that the frontal wedge could be actively dewatering there are presently no constraints on the relative fluid flux between the frontal wedge and the active mid slope fluid seeps article outline

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

hikurangi margin tsunami earthquake generated by slow seismic rupture over a subducted seamount abstract tsunami earthquakes generate much larger tsunami than their surface wave magnitude would suggest and are a problem for tsunami warning systems they are often not accompanied by intense or even strong ground shaking and hence do not provide a natural warning for self evacuation the lesser known 1947 offshore poverty bay and tolaga bay earthquakes along the east coast of the north island new zealand share many characteristics with other well known tsunami earthquakes including low amplitude shaking long durations and anomalously large tsunami however these two new zealand events are rare in that their source area has been imaged directly by long offset 2d seismic reflection profiles in this contribution we propose a source model for the 1947 offshore poverty bay tsunami earthquake recognising that the hypocentre occurs in a region where seismic reflection and magnetic data support the existence of a shallow m 7 earthquakes with complex low velocity rupture scenarios that enhance tsunami waves and their role in seismic hazard should not be under estimated

precipitation banding in idealized baroclinic waves abstractmoist idealized baroclinic wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet for a frictionless lower boundary and with no latent heat release or surface heat and moisture fluxes warm advection is strong and a bent back warm front forms although a narrow vertical velocity maximum forms within the area of synoptic scale ascent near the triple point only a wide warm frontal band forms as surface roughness length increases between simulations to that of an ocean then a land surface warm advection is reduced and the cold front becomes stronger relative to the warm front a separate narrow rainband forms along the cold front which is more intense and farther removed from the wide warm frontal band when roughness length is greater in the simulation with roughness length appropriate to the ocean after the narrow band decays the precipitation becomes oriented along the warm conveyor belt in the warm sector when latent heat release is in

the evolution of an observed cold front part i numerical simulation abstract the 48 h evolution of an observed cold front is simulated by a three dimensional mesoscale numerical model for a typical springtime synoptic situation over the southeastern united states the model used in this study employs anelastic equations of motion on a limited area domain with locally determined inflow outflow side boundaries both the observed and simulated characteristics of the weather system indicate a mature front which intensifies and then weakens over the 48 h period moist convection occurs in the form of intermittent squall lines in the observed case in the numerical simulation convection develops above and somewhat ahead of the surface front after 24 h as in ensemble of convective cells an investigation is made of the mesoscale and subsynoptic scale features of this solution to determine their sensitivity to the inclusion of moisture and to the magnitude of the eddy viscosity used in the numerical simulation the primary effect of increased eddy viscosity is to reduce somewhat

compositional and fluid pressure controls on the state of stress on the nankai subduction thrust a weak plate boundary abstract we show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the nankai subduction plate boundary off southwest japan ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients r at stress levels between 1 0 and 40 mpa the direct shear r values for purified smectite are 0 14 0 02 for illite 0 25 0 01 and for chlorite 0 26 0 02 for point load velocities of 0 0001 mm s these clay minerals dominate the nankai subduction decollement zone illite plus quartz is mechanically important in the altered incoming muroto section and the predicted decollement r should lie between 0 2 and 0 32 this low residual strength together with elevated fluid pressure limits shear stresses to below 4 mpa within the frontal 50 km of the subduction system consistent with the low wedge taper in this region a higher wedge taper off the ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in r are predicted to occur as a result of variations in smectite and total clay content these variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite however our data suggest the low r values of the clay rich decollement still limit shear stresses to between 17 and 29 mpa within the frontal 50 km of the wedge consistent with other estimates of plate boundary weakness indeed we propose that it should be expected that subduction plate boundaries like nankai will be weak because of the intrinsic presence of clay rich faults and moderate fluid overpressures our data do not support the hypothesis that the smectite to illite reaction directly controls the onset of seismogenic behavior deep in the nankai system because there is already a mechanical dominance of illite rather than smectite in the shallow decollement zone and we find all the clay phases tend to velocity strengthen however temperature activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up dip limit of the seismogenic zone

discussion of structural petrology of the ronda peridotite sw spain deformation history by d van der wal and r l m vissers

critical wedges in three dimensions analytical expressions from mohr coulomb constrained perturbation analysis we present simple analytical expressions for evaluating three dimensional stress fields in critical wedges coulomb failure throughout that are deforming within an oblique convergent zone we assume that the load and geometry variations in the lateral direction and the topographic slope are relatively small the stress state of a two dimensional critical wedge is perturbed by admitting a small lateral shear which is accommodated to maintain criticality by a reduction in the dominant normal compressive stress cohesionless cases are further simplified with taylor series expansions in the topographic slope the analytical expressions distinguish the behavior of strong base wedges associated with steep slopes from weak base wedges with more modest slopes and are reliable for lateral shear values as large as half the lithostatic load in the weak base case where the vertical shear is small the orientations of the principal stresses are very sensitive to the perturbation as the lateral shear increases through relatively small values the roles of the lateral and vertical coordinate axes are interchanged with a resultant transition from predominantly thrust failure to strike slip failure transition to strike slip in the strong base case is more gradual the particular value of horizontal shear for which transition occurs also depends on the difference of the minor normal stresses we produce an analytical expression predicting displacement patterns compatible with those observed in natural and analog orogens displacement partitioning is predicted for weak base wedges due to minor changes in lateral boundary conditions associated with variable orogen geometry

airborne radar observations of a cold front during fastex1 abstract a detailed analysis using airborne doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented the precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core relative winds in contrast to previous schematic models it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation core the southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipitation the peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical vorticity in the boundary layer the overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled cla

the formation of a forward tilting cold front with multiple cloud bands during superstorm 1993 abstract a mesoscale model simulation is used to examine the evolution of the cold front and accompanying cloud bands in eastern mexico associated with superstorm 1993 12 14 march the simulated cold front differed in structure and evolution from a classical cold front in agreement with evidence from observations and european centre for medium range weather forecasts analyses the surface cold front as defined by the leading edge of strong northerlies and cold advection initially possessed a rearward tilt with height over southern texas within 6 h the leading edge of the front moved equatorward and developed a large scale forward tilt of greater than 200 km in the horizontal from the surface to 700 hpa this forward tilt occurred as a mid to upper tropospheric baroclinic zone arrived from over the sierra madre descended into eastern mexico and interacted with the surface cold front embedded within this large scale forward tilt was a locally enhanced horizontal potential temperature gradient tha

seismic reflection character of the hikurangi subduction interface new zealand in the region of repeated gisborne slow slip events summary we use seismic reflection data to map the geometry and character of the subduction interface in the gisborne area of the hikurangi subduction margin new zealand which experiences repeated shallow 15 km slow slip events the reflection character and geometry in this area is highly variable which we interpret to be related to the subduction of seamounts and underthrust sediments three zones of high amplitude interface reflectivity hrz 1 2 and 3 are interpreted to be the result of fluid rich sediments that have been entrained with subducting seamounts the interface above the hrz zones is shallower than the surrounding areas by 2 4 km due to the warping of the interface to accommodate seamount subduction these zones of high amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008 we hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high amplitude reflectivity due to 1 the enhanced underthrusting of fluid rich sediment 2 reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and 3 potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations from our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events

inferences on sediment strength and fault friction from structures at the aleutian trench the structural relations of conjugate thrust faults and the master decollement imaged in a research level processed multichannel seismic reflection record across the eastern aleutian trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone these estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms they suggest the presence of high pore fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment therefore coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope