shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

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

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

a two dimensional model simulation of the el chichon volcanic eruption cloud using a two dimensional model of sulfate photochemistry transport and aerosol microphysics we have simulated a 2 year period following the eruptions of el chichon our calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years the model reproduces the observed optical depth lidar backscatter and infrared extinction coefficients if about 10 megatonnes of so2 are injected the major deficiency of the model is a somewhat too rapid transport

subduction induced strain in the upper mantle east of the mendocino triple junction california we observe splitting of teleseismic shear waves at five stations of the berkeley digital seismic network located east of the mendocino triple junction in northeastern california that is dependent on the arrival direction of the seismic phases the observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis we assume that the anisotropy is caused by the preferred alignment of olivine crystals a grid search over possible orientations of the olivine a axes reveals that south of the mendocino triple junction they dip to the east whereas north of the triple junction they dip to the northeast on the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow p wave velocity anomalies in the upper mantle we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast p wave velocity anomaly assuming that the fast p velocity anomaly represents subducted slab material we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid strong down going plate and the surrounding mantle south of the triple junction the fabric may have developed while subduction of the farallon plate was still ongoing in this region prior to 6 ma however we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the north american lithosphere the flow is modulated by the presence of rigid lithosphere to the north and east

structure and tectonics of the upper cenozoic puerto rico virgin islands carbonate platform as determined from seismic reflection studies the puerto rico virgin islands carbonate platform was deposited over an area of 18 000 km2 from early oligocene to holocene on top of an inactive and subsiding cretaceous earliest oligocene island arc regional single channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small 2250 km2 portion of the platform exposed by late neogene tectonic uplift along the north coast of puerto rico seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern puerto rico st croix u s virgin islands and the saba bank limestone thickness variations from a little over 2 km to almost zero are used to subdivide the puerto rico virgin islands platform into five distinct carbonate provinces 1 north puerto rico area including the onshore exposures 2 virgin islands area 3 st croix and saba bank area 4 south puerto rico area and 5 mona passage area carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform the most prominent feature of the platform in the puerto rico virgin islands area is a large east west trending arch the northern limb of this arch exhibits a smoother more uniform dip than the steeper more abruptly faulted southern limb the core of the arch is responsible for the exposure of arc basement rocks on puerto rico the origin of this arch which occurs over a 300 km wide area is best explained by north south shortening and arching caused by interaction at depth of subducted slabs of the north america and caribbean plates other important evidence for this model can be found in the benioff zones observed in the earthquake profiles loading of the caribbean plate results in downward flexing of the north america plate and causes the 4 km subsidence of the carbonate platform north of puerto rico

sks splitting and the seismic anisotropy of the mantle beneath the hikurangi subduction zone new zealand abstract the analysis of 12 sks phases recorded on broadband stations above the hikurangi subduction zone in new zealand shows clear evidence of mantle anisotropy with the fast direction 28 5 almost parallel to the strike of subduction and the dominant geology of the region the slow shear wave delay times show a systematic change with the azimuth of the arrivals which if hexagonal symmetry is assumed indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal or that more than one anisotropic layer is present the magnitude of the delays 1 5 0 4s suggests that the anisotropy is most probably confined to the top 300 km of the mantle

el chichon the genesis of volcanic sulfur dioxide monitoring from space the 1982 eruption of el chichon inspired a new technique for monitoring volcanic clouds data from the total ozone mapping spectrometer toms instrument on the nimbus 7 satellite were used to measure sulfur dioxide in addition to ozone for the first time precise data on the sulfur dioxide mass in even the largest explosive eruption plumes could be determined the plumes could be tracked globally as they are carried by winds magmatic eruptions could be discriminated from phreatic eruptions the data from el chichon are reanalyzed in this paper using the latest version of the toms instrument calibration v8 they show the shearing of the eruption cloud into a globe circling band while still anchored over mexico in three weeks the measured sulfur dioxide mass in the initial march 28 eruption was 1 6 tg the april 3 eruption produced 0 3 tg more and the april 4 eruptions added 5 6 tg for a cumulative total of 7 5 tg in substantial agreement with estimates from prior data versions toms aerosol index absorbing aerosol data show rapid fallout of dense ash east and south of the volcano in agreement with advanced very high resolution radiometer avhrr ash cloud positions

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

seismic anisotropy beneath southern iberia from sks splitting abstract seismic anisotropy of the south iberian upper mantle is investigated using shear wave splitting of sks phases we analyzed teleseismic events recorded by sixteen permanent broadband stations installed on the southern iberian peninsula and in northern africa and we determined fast polarization directions and delay times t between fast and slow components the area of investigation extends across two important geological structures in the variscan iberian peninsula the variscan iberian massif in its center and the gibraltar arc in the southeast that represents the most westerly alpine belt in the western mediterranean shear wave splitting measurements from stations in the betic domain show homogeneous ene wsw fast directions nearly parallel to the trend of the mountain belt and smooth spatial variations stations in the north toward the southern part of the variscan iberian massif show homogeneous fast directions however trending ns to ne sw different from those recorded in the betic these observations may reflect a post hercynian variscan deformation of the ossa morena zone related to the main stages in the tectonic evolution of this part namely transpressional stage transtensional stage and shortening episode or a deformation related to the posterior alpine orogeny along the gibraltar arc we observe a smoothly varying trend changing from ene wsw in the eastern betics to ns in the area of gibraltar and ceuta following more or less the general trend of the mountain belt around the alboran sea and the coastline since a similar rotation is also visible in results from pn anisotropy this suggests that the anisotropy is vertically coherent starting from just below the moho comparing the anisotropy pattern expected from various geodynamic models with the observed sks splitting suggests that the anisotropy is best explained by a model of slab rollback rather than by delamination models

fault interactions and large complex earthquakes in the los angeles area faults in complex tectonic environments interact in various ways including triggered rupture of one fault by another that may increase seismic hazard in the surrounding region we model static and dynamic fault interactions between the strike slip and thrust fault systems in southern california we find that rupture of the sierra madre cucamonga thrust fault system is unlikely to trigger rupture of the san andreas or san jacinto strike slip faults however a large northern san jacinto fault earthquake could trigger a cascading rupture of the sierra madre cucamonga system potentially causing a moment magnitude 7 5 to 7 8 earthquake on the edge of the los angeles metropolitan region

response of the san andreas fault to the 1983 coalinga nunez earthquakes an application of interaction based probabilities for parkfield 1 the parkfield cholame section of the san andreas fault site of an unfulfilled earthquake forecast in 1985 is the best monitored section of the world s most closely watched fault in 1983 the m 6 5 coalinga and m 6 0 nunez events struck 25 km northeast of parkfield seismicity rates climbed for 18 months along the creeping section of the san andreas north of parkfield and dropped for 6 years along the locked section to the south right lateral creep also slowed or reversed from parkfield south here we calculate that the coalinga sequence increased the shear and coulomb stress on the creeping section causing the rate of small shocks to rise until the added stress was shed by additional slip however the 1983 events decreased the shear and coulomb stress on the parkfield segment causing surface creep and seismicity rates to drop we use these observations to cast the likelihood of a parkfield earthquake into an interaction based probability which includes both the renewal of stress following the 1966 parkfield earthquake and the stress transfer from the 1983 coalinga events we calculate that the 1983 shocks dropped the 10 year probability of a m 6 parkfield earthquake by 22 from 54 22 to 42 23 and that the probability did not recover until about 1991 when seismicity and creep resumed our analysis may thus explain why the parkfield earthquake did not strike in the 1980s but not why it was absent in the 1990s we calculate a 58 17 probability of a m 6 parkfield earthquake during 2001 2011 index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 7260 seismology theory and modeling keywords coalinga earthquake parkfield stress change earthquake probability seismicity rate

subsidence history of the bahama escarpment and the nature of the crust underlying the bahamas abstract subsidence of the bahama escarpment determined from deep diving submersible and dredge samples is used to constrain the nature of crust underlying the bahamas horizontal disposition of the hauterivian barremian 125 ma age boundary along the bahama escarpment is inconsistent with an underlying oceanic crust either normal or thickened here and suggests that thinned continental crust underlies the bahamas subsidence curves are then fit based on a stretched lithosphere model to a stratigraphic section 2000 4000 m off cat island this analysis indicates crustal thinning by a factor of 2 0 2 5 resulting in present crustal thicknesses of 10 12 km we propose that rifting beneath the bahamas occurred from middle 175 180 ma to late 160 ma jurassic time the pre extension bahamas fit between north and south america and africa in early jurassic time eliminating overlap of the present bahamas onto africa in reconstructions of the north atlantic

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

synthetic seismicity models for the wellington region new zealand implications for the temporal distribution of large events our previous synthetic seismicity model for multiple interacting faults in a three dimensional half space has been extended and applied to the wellington region new zealand generating long catalogs of earthquakes for studying the effects of elastic interactions on the temporal distribution of large events the region is one of oblique plate convergence with the interface between the subducting and overlying plates at an average depth of 22 km faults included besides the subduction thrust are segments of the four major arc parallel strike slip faults overlying the plate interface we have used 40 different models with geometric and mechanical parameters chosen at random within reasonable ranges to generate catalogs of 200 000 years duration each for comparison each model was rerun with the elastic interactions suppressed considering events of magnitude 7 2 or more characteristic events in the sense that they rupture most of a fault plane the number of short 250 years the relative numbers are again mostly higher experiments with simple models indicate that this pattern requires both interfault enhancement and inhibition of large events mutual enhancement occurs most often between the subduction thrust and the overlying strike slip faults and between the two segments of the wellington fault that almost join end to end mutual inhibition mostly occurs between the subparallel strike slip faults

modeling earthquake cycles in the shumagin subduction segment alaska with seismic and geodetic constraints deformation associated with the earthquake cycle in the shumagin islands segment of the alaska aleutian subduction zone is analyzed with the use of a two dimensional finite element model the model consists of an oceanic plate dipping under an upper plate both of which respond elastically to stress fluctuations in the earthquake cycle and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically it is tailored to the geometry of the shumagin islands region by using seismicity to define the position of the interplate interface and partially coupled region along it the model is preconditioned by forcing this interface to undergo periodically repeated slips up to and including the time of the may 31 1917 event ms 7 4 in that region with each chosen to be consistent with the moment and estimated rupture area of that event we investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface in models with significant relaxation in the wedge or downdip thrust zone results show that as the intraseismic stage matures there is a region of diminished compressional strain rates and even of locally extensional rates on the earth s surface above the downdip end of the seismically coupled zone based on the seismic estimates of the location of the coupled zone this region is in the area of the shumagin islands we find that if approximately 20 of the convergence takes place seismically compatible with the previous seismic history and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and or along the downdip interface then deformations predicted by the model can be made consistent with the measured strain data from the shumagin islands geodetic network lisowski et al 1988 larson and lisowski 1994 as well as uplift and tilt data savage and plafker 1991 beavan 1992 our model simulations here suggest that the shumagin segment is capable of large earthquakes the hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints

shear wave splitting variations across an array in the southern north island new zealand 1 we observe significant spatial variations in shear wave splitting from deep teleseismic s and regional scs recorded on an array in the fore arc region of the hikurangi subduction zone new zealand we also observe variations from different events recorded at the same station the fast polarization directions range between 23 and 72 and the delay times range between 0 75 s and 1 80 s these observations contrast with previous sks studies in new zealand where little spatial variation is seen and highlights the importance of using a variety of phases with different periods our observations are consistent with a model in which a lateral anisotropic boundary runs ne sw in a zone between kapiti island and 40 km east of the tararua mountain range the anisotropic boundary is possibly related to a prominent vertical velocity feature near kapiti island

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

three dimensional euler deconvolution and tectonic interpretation of marine magnetic anomaly data in the puerto rico trench during the summer of 1996 single channel seismic magnetic gravity hydrosweep bathymetric and hmr1 sidescan data were collected north of the island of puerto rico to constrain the geologic and tectonic setting of the puerto rico trench magnetic data from this cruise are merged with other available data then processed and interpreted with the aid of the euler deconvolution method the area north of the island of puerto rico is divided into three magnetic anomaly zones zone 1 is dominated by northwest to southeast trending magnetic anomalies zone 2 consists of an east west region of relatively low amplitude anomalies and occurs south of zone 1 zone 3 is dominated by the highest magnetic values in the study area and the source region is roughly centered under the island of puerto rico respectively these zones approximate three geologic provinces of the puerto rico trench composed of early cretaceous ocean crust with sedimentary cover a blueschist belt and an oligocene cretaceous island arc with a limestone cap also mapped are the main ridge fracture zone and the fourth of july fracture zone both on the north american plate which correlate with the main ridge and fourth of july ridge on the caribbean plate the 3 d euler deconvolution facilitates the identification of new faults as well as the mapping of known faults also evident in the seismic and bathymetric data a model concerning the formation of the puerto rico trench is proposed that incorporates the existence of strike slip faults and the fracture zones associated with the subduction of the north america plate evidence from this study support a tectonic interpretation of subduction followed by more recent strike slip faulting that is accompanied by only a minimal amount of subduction

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

upper plate stressing and seismicity in the subduction earthquake cycle we investigate upper plate stressing during the earthquake cycle in a subduction segment using three dimensional 3 d elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface we show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate oblique subduction segments two from the aleutians andreanof islands 1986 and rat islands 1965 and one from indonesia biak 1996 are studied all examples of upper plate seismicity from the aleutians represent events occurring toward the beginning of a new cycle while in biak indonesia the examined events occur both toward the end of one cycle and the beginning of the next in the majority of cases studied the location and mode of the upper plate seismicity are consistent with space and time dependent stressing as predicted by modeling this confirms earlier observations that seismicity in the vicinity of large great subduction earthquakes toward the outer rise at intermediate depth and now in the upper plate depends in an interpretable manner on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

mantle flow in subduction systems the mantle wedge flow field and implications for wedge processes 1 the mantle wedge above subducting slabs is associated with many important processes including the transport of melt and volatiles our understanding of mantle wedge dynamics is incomplete as the mantle flow field above subducting slabs remains poorly understood because seismic anisotropy is a consequence of deformation measurements of shear wave splitting can constrain the geometry of mantle flow in order to identify processes that make first order contributions to the pattern of wedge flow we have compiled a data set of local s splitting measurements from mantle wedges worldwide there is a large amount of variability in splitting parameters with average delay times ranging from 0 1 to 0 3 s up to 1 0 1 5 s and large variations in fast directions we tested for relationships between splitting parameters and a variety of parameters related to subduction processes we also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge we find that no simple model can explain all of the trends observed in the global data set mantle wedge flow is likely controlled by a combination of downdip motion of the slab trench migration ambient mantle flow small scale convection proximity to slab edges and slab morphology with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology there is also a likely contribution from b type olivine and or serpentinite fabric in many subduction zones governed by the local thermal structure and volatile distribution

the el chichon stratospheric cloud solid particulates and settling rates sampling of the el chichon stratospheric cloud in early may and in late july 1982 showed that a significant proportion of the cloud consisted of solid particles between 2 m and 40 m size in addition many particles may have been part of larger aggregates or clusters that ranged in size from 50 m the majority of individual grains were angular aluminosilicate glass shards with various amounts of smaller adhering particles surface features on individual grains include sulfuric acid droplets and larger 0 5 m to 1 m sulfate gel droplets with various amounts of na mg ca and fe the sulfate gels probably formed by the interaction of sulfur rich gases and solid particles within the cloud soon after eruption ca sulfate laths may have formed by condensation within the plume during eruption or alternatively at a later stage by the reaction of sulfuric acid aerosols with ash fragments within the stratospheric cloud a wilson huang formulation for the settling rate of individual particles qualitatively agrees with the observed particle size distribution for a period at least four months after injection of material into the stratosphere this result emphasizes the importance of particle shape in controlling the settling rate of volcanic ash from the stratosphere

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

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

strain partitioning and fault slip rates in the northeastern caribbean from gps measurements 1 gps data collected in the dominican republic from 1994 to 2001 show that oblique convergence between the north american and caribbean plates is partitioned between 5 2 2 mm yr 1 of dip slip reverse motion on the north hispaniola thrust approximately n s and 12 8 2 5 mm yr 1 and 9 0 9 0 mm yr 1 of approximately e w left lateral strike slip motion on the septentrional and enriquillo faults 95 confidence the agreement between gps and paleoseimological slip rates on the septentrional fault together with the 770 960 years since the last major earthquake and the 800 1200 years maximum repeat time of major earthquakes may indicate that the septentrional fault is currently in the late phase of its rupture cycle

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

seismic anisotropy beneath the lower half of the north island new zealand teleseismic scs and sks events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the north island new zealand this area lies above the hikurangi subduction zone and the array provides ray paths which sample the mantle both above and below the slab shear wave splitting measurements give similar fast polarizations and delay times at each station the average sks fast polarization is approximately ne sw subparallel to the strike of subduction and the major geological features with an average sks delay time of 1 6 0 1 s this lack of variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle the anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence any anisotropy in the slab is probably due to fossil mineral alignment anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow the similar ne sw fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench parallel direction in both regions

late holocene liquefaction features in the dominican republic a powerful tool for earthquake hazard assessment in the northeastern caribbean several generations of sand blows and sand dikes indicative of significant and recurrent liquefaction are preserved in the late holocene alluvial deposits of the cibao valley in northern dominican republic the cibao valley is structurally controlled by the septentrional fault an onshore section of the north american caribbean strike slip plate boundary the septentrional fault was previously studied in the central part of the valley where it sinistrally offsets holocene terrace risers and soil horizons in the eastern and western parts of the valley the septentrional fault is buried by holocene alluvial deposits making direct study of the structure difficult liquefaction features that formed in these holocene deposits as a result of strong ground shaking provide a record of earthquakes in these areas liquefaction features in the eastern cibao valley indicate that at least one historic earthquake probably the moment magnitude m 8 4 august 1946 event and two to four prehistoric earthquakes of m 7 to 8 struck this area during the past 1100 yr the prehistoric earthquakes appear to cluster in time and could have resulted from rupture of the central and eastern sections of the septentrional fault circa a d 1200 liquefaction features in the western cibao valley indicate that one historic earthquake probably the m 8 7 may 1842 event and two prehistoric earthquakes of m 7 8 struck this area during the past 1600 yr our findings suggest that rupture of the septentrional fault circa a d 1200 may have extended beyond the central cibao valley and generated an earthquake of m 8 additional information regarding the age and size distribution of liquefaction features is needed to reconstruct the prehistoric earthquake history of hispaniola and to define the long term behavior and earthquake potential of faults associated with the north american caribbean plate boundary manuscript received 6 september 2002

modelling shear wave splitting observations from wellington new zealand summary frequency dependent anisotropy was previously observed at the permanent broad band station snzo south karori wellington new zealand this has important implications for the interpretation of measurements in other subduction zones and hence for our understanding of mantle flow this motivated us to make further splitting measurements using events recorded since the previous study and to develop a new modelling technique thus in this study we have made 67 high quality shear wave splitting measurements using events recorded at the snzo station spanning a 10 yr period this station is the only one operating in new zealand for longer than 2 yr using a combination of teleseismic sks and s phases and regional scs phases provides good azimuthal coverage allowing us to undertake detailed modelling the splitting measurements indicate that in addition to the frequency dependence observed previously at this station there are also variations with propagation and initial polarization directions the fast polarization directions range between 2 and 103 and the delay times range between 0 75 s and 3 05 s these ranges are much larger than observed previously at snzo or elsewhere in new zealand because of the observed frequency dependence we measure the dominant frequency of the phase used to make the splitting measurement and take this into account in the modelling we fit the fast polarization directions fairly well with a two layer anisotropic model with horizontal axes of symmetry however such a model does not fit the delay times or explain the frequency dependence we have developed a new inversion method which allows for an inclined axis of symmetry in each of the two layers however applying this method to snzo does not significantly improve the fit over a two layer model with horizontal symmetry axes we are therefore unable to explain the frequency dependence or large variation in delay time values with multiple horizontal layers of anisotropy even allowing for dipping axes of symmetry for the upper layer of anisotropy the modelled fast polarization directions lie in the range 30 to 110 subparallel to the major geological features as well as the relative direction of plate motion we suggest that the upper layer of anisotropy consists of crust and subducting slab and that the lower layer consists of subslab mantle for the lower layer of anisotropy the modelled fast polarization directions lie in the range 50 to 30 which are intermediate

volcanoes the stratosphere and climate abstract a list of volcanic eruption plumes observed to ascend into or near the stratosphere since 1883 shows that the volcanoes divide readily into two groups one at low and one at higher latitudes a model for the rise of a buoyant volcanic plume rise as applied to volcanic eruptions is corrected for realistic temperature profiles and for the finite vertical extent of the resultant debris clouds the utility of the model can be questioned however owing to the highly uncertain and variable nature of the efficiency of use of heat energy of buoyant rise the observed correlation of stratospheric plumes with climatic effects indicates that those plumes nearer the equator have the largest impact on surface temperatures analysis of the observations also suggests that injection of debris into the stratosphere is more important in determining the effect on climate than either the total volcanic explosivity of the eruption or the actual height reached within the stratosphere

hydration of the crust and upper mantle of the hikurangi plateau as it subducts at the southern hikurangi margin abstract controlled source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave speeds in the crust and upper mantle near the trench similar studies also have found unusually high p wave speeds vp in the upper mantle under oceanic plateaus onshore offshore seismic profiling at the southern hikurangi margin where the 120 ma old oceanic hikurangi plateau is subducting indicates that a fast vp 8 7 0 2 km s upper mantle layer lies beneath a 25 km thick mantle layer with more regular wave speeds vp 8 0 0 2 km s under the hikurangi trough this is consistent with previous findings of upper mantle vp 8 7 9 0 km s in the margin parallel direction under the north island 100 km northwest of the deformation front at depths 8 10 km below the moho our profiles are margin perpendicular thus we show that the upper mantle lid of the subducting pacific plate is characterized by unusually high p wave speeds along all azimuths we find an area of lowered vp in the 12 1 km thick hikurangi plateau crust beneath the trough this drop in vp is 10 and a similar drop in vp is deduced to depths of 25 2 km into the upper mantle we interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low velocity zone in the faster upper mantle layer this zone formed from serpentinisation by hydration through bending induced normal faults and or due to crack porosity introduced by thermal cracking further enhanced by bending related faulting thus the regular mantle vp 8 km s is not in fact regular but rather the high speed mantle has mechanically bent fractured and altered the absolute depth of fast mantle vp under the hikurangi trough is around 50 km the onset of the lower band of seismicity of the double seismic zone and high upper mantle vp under the north island is observed at similar depths this is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough our study on the hikurangi margin is different as the subducting plate here contains a 120 ma old oceanic plateau with a 12 km thick crust but the results are similar to other subduction margins where regular oceanic crust is subducting

stress interaction between subduction earthquakes and forearc strike slip faults modeling and application to the northern caribbean plate boundary 1 strike slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers we explore the interaction between thrust events on the subduction interface and strike slip faults within the forearc region using three dimensional models of static coulomb stress change model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the coulomb stress on strike slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region in contrast subduction events with slip vectors perpendicular to the trench axis enhance the coulomb stress on strike slip faults farther back in the forearc while reducing the stress adjacent to the trench a significant contribution to coulomb stress increase on strike slip faults in the back region of the forearc comes from unclamping of the fault i e reduction in normal stress due to thrust motion on the subduction interface we argue that although coulomb stress changes from individual subduction earthquakes are ephemeral their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant we use the coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the caribbean subduction zone subduction earthquakes with slip vectors nearly perpendicular to the caribbean trench axis is dominant in the hispaniola segment where the strike slip faults are more than 60 km inland from the trench in contrast subduction slip motion is nearly parallel to the caribbean trench axis along the puerto rico segment where the strike slip fault is less than 15 km from the trench this observed jump from a strike slip fault close to the trench axis in the puerto rico segment to the inland faults in hispaniola is explained by different distributions of coulomb stress in the forearc region of the two segments as a result of the change from the nearly trench parallel slip on the puerto rico subduction interface to the more perpendicular subduction slip beneath hispaniola the observations and modeling suggest that subduction induced strike slip seismic hazard to puerto rico may be smaller than previously assumed but the hazard to hispaniola remains high

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time

satellite limb scanning thermal infrared observations of the el chichon stratospheric aerosol first results the infrared radiometer experiment on the solar mesosphere explorer satellite has been continuously measuring the 6 8 m thermal emission from the stratospheric aerosol from the el chichon volcano since the time of eruption in early april 1982 inversion results for the zonally averaged infrared extinction coefficient in height latitude and time show that the aerosol increased in mass to a maximum of 8 tg 8 1012gm about 15 weeks after the april 4 eruption it descended in height with an average speed consistent with the gravitational settling time of particles with a diameter of about 1 4 m