seismic reflection imaging of the juan de fuca plate from ridge to trench new constraints on the distribution of faulting and evolution of the crust prior to subduction we present prestack time migrated multichannel seismic images along two cross plate transects from the juan de fuca jdf ridge to the cascadia deformation front df offshore oregon and washington from which we characterize crustal structure distribution and extent of faults across the plate interior as the crust ages and near the df in response to subduction bending within the plate interior we observe numerous small offset faults in the sediment section beginning 50 70 km from the ridge axis with sparse fault plane reflections confined to the upper crust plate bending due to sediment loading and subduction initiates at 120 150 km and 65 80 km seaward of the df respectively and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity most bend faulting deformation occurs within 40 km from the df on the oregon transect bright fault plane reflections that extend through the crust and 6 7 km into the mantle are observed if attributed to serpentinization 0 12 0 92 wt water within the uppermost 6 km of the mantle is estimated on the washington transect bending faults are confined to the sediment section and upper middle crust the regional difference in subduction bend faulting and potential hydration of the jdf plate is inconsistent with the spatial distribution of intermediate depth intraslab seismicity at cascadia a series of distinctive ridgeward dipping 20 40 lower crustal reflections are imaged in 6 8 ma crust along both transects and are interpreted as ductile shear zones formed within the ridge s accretionary zone in response to temporal variations in mantle upwelling possibly associated with previously recognized plate reorganizations at 8 5 ma and 5 9 ma

wave field decomposition of ocean bottom seismograms seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer we examine the feasibility of removing these reverberations through a 1d wave field decomposition based on a reflection transmission formulation of the problem two decomposition schemes are presented the first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure the second approach involves only displacement recordings and knowledge of the water column depth this latter quantity will generally be known a priori from deployment logs or alternatively it may be estimated using vertical displacement and pressure seismograms in a preprocessing step both approaches require prior information on seabed properties in the case of p wave incidence the decomposition depends primarily on the seabed s velocity this quantity can be determined by examining trial decompositions over a range of s and selecting the value that minimizes the energy of the upgoing s wave component at the arrival time of the incident wave we apply this approach to synthetic seismograms for a simple earth structure and to recordings of two large events from the central oregon locked zone array colza on the continental margin of oregon the real data indicate that the wave field decompositions are largely successful at lower frequencies 0 1 hz but that 3d scattering likely originating near the sediment basement contact is manifested at higher frequencies

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

on the cause of the asymmetric distribution of seamounts about the juan de fuca ridge ridge crest migration over a heterogeneous asthenosphere abstract the distribution of non hotspot seamounts in the northeast pacific is highly asymmetric small seamount chains and isolated edifices are numerous on the pacific plate but nearly absent on the juan de fuca plate we propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference plate motion solutions indicate that the juan de fuca ridge is migrating to the west at a ridge perpendicular rate of 20 mm a 1 which is large compared to the half spreading rate of 30 mm a 1 migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre if slightly enriched lower melting temperature heterogeneities are present in the upper mantle they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite seamount volcanism will occur preferentially on the pacific plate because of two factors firstly more asthenosphere and hence more early melt heterogeneities must ascend to supply the pacific plate which is translating more rapidly than the relatively stationary juan de fuca plate secondly the asthenosphere that is required to ascend to supply the thickening of the juan de fuca plate will have been flushed of its significant shallow level heterogeneities by the previous advance and passing of the ridge few should remain to cause volcanism on the juan de fuca plate testable physical and petrologic predictions of this model can be identified for example within a small seamount chain the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis variation in lava chemistry along a small seamount chain which is interpreted to represent magmatism from a single large heterogeneity will provide the major discriminant of this hypothesis systematic variations should be observed with earlier formed edifices being constructed of more enriched lavas and later younger ones of more depleted lavas isotope ratios should be unaffected by varying degrees of partial melting but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant

initiation of strike slip faults serpentinization and methane the nootka fault zone the juan de fuca explorer plate boundary the nootka fault zone is a ridge trench trench transform fault that was initiated 4 ma when the explorer ridge became independent of the juan de fuca ridge multibeam data around the fault zone and a compilation of several seismic reflection surveys provide insight into initiation of strike slip faults previous interpretations assumed that the two faults seen cutting the seafloor are subparallel to shear between the explorer and juan de fuca plates and formed instantaneously at 4 ma increased data density shows that these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults formed at the ridge they are surrounded by numerous buried steeply dipping small offset growth faults at least some of which are likely still active our observations corroborate analogue models of strike slip fault initiation that predict formation of riedel like shears within a zone of faulting and that displacement localizes over time the existence of several long subparallel faults and a very wide zone of faulting has been predicted by models of distributed shear at depth along the nootka fault zone basement has risen by several hundred meters and bright reversed polarity reflectors some of which are interpreted to be methane hydrate reflectors are common hydration likely as serpentinization of the upper mantle could explain both sets of observations serpentinization can result in a 30 50 volume expansion and methane is observed in vents driven by this process biogenic sources of methane are likely to be present and concentrated by currently active fluid flow in the faulted sediments

active deformation of the gorda plate constraining deformation models with new geophysical data the gorda plate the southernmost fragment of the larger juan de fuca plate system is an example of a nonrigidly deforming tectonic accommodation zone or buffer plate absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion here we present a new structural analysis of the plate based on full plate bathy metric coverage augmented by seismic reflection data and earthquake moment tensors and locations we interpret internal deformation of the gorda plate as an asymmetrical flexural slip buckle with a vertical axis utilizing reactivation of spreading ridge fabric normal faults as strike slip faults newly formed second generation faults crosscutting the structural grain overprint the reactivated structures the spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone this model based on fault constraints has allowed investigation of ridge plate subduction interac tions and suggests that spreading rate variations along the gorda ridge may be con trolled by internal deformation of the plate rather than the reverse as previously hypothesized

provenance stratigraphic architecture and hydrogeologic influence of turbidites on the mid ocean ridge flank of northwestern cascadia basin pacific ocean abstract the northwestern edge of cascadia basin north pacific ocean is unusual because late pliocene to holocene turbidites lap onto juvenile oceanic crust of the juan de fuca ridge subsidence of the ridge flank combines with irregular westward progradation of the turbidite facies to create a stratigraphic section that coarsens and thickens upward the sand provenance is mixed individual turbidity currents have funneled into the area through several shelf slope and abyssal floor conduits including vancouver valley juan de fuca channel barkley canyon and nitinat canyon local bathymetric blockage deflection and reflection of flow paths by basement relief remobilization by intrabasinal submarine slides and debris flows episodic channel switching and sporadic overbank flooding have combined to produce erratic recurrence intervals for the turbidity currents only the tallest basement highs have remained isolated from turbidite deposition during the last 500 000 years spatial and temporal differences in sediment accumulation are important because they modulate the three dimensional responses of compaction and consolidation those changes in physical properties govern where and when hydraulic communication with the underlying basement shuts down the basal hemipelagic layer of cascadia basin transforms to an effective hydrologic seal seepage rates 1 mm yr once the sediment basalt interface is buried by 100 150 m of strata rapid accumulation of turbidites therefore accelerates the hydrogeologic conversion of igneous basement from open to sealed

fossilization modes in the chengjiang lagerstatte cambrian of china testing the roles of organic preservation and diagenetic alteration in exceptional preservation abstract one important and poorly understood issue concerning the taphonomy of nonmineralizing organisms in cambrian burgess shale type deposits is the diagenetic pathways by which organisms have become exceptionally preserved scanning electron microscopy sem analyses of exceptionally preserved fossils from the lower cambrian chengjiang deposit of china demonstrate that nonmineralized tissue is preserved in a variety of ways and further suggest that there was some taxonomic control over the precipitation of authigenic minerals during early diagenesis organic preservation is limited to certain decay resistant structures such as heavily sclerotized spines on the claws of anomalocaridids labile organic tissues of nearly all nonmineralizing animals however are preserved by thin films of diagenetic minerals microstructural studies energy dispersive x ray edx analyses and elemental mapping of these minerals indicate that mineral films include apatite pyrite and fe rich aluminosilicates the results support the assumption that early diagenetic mineralization such as phosphatization and pyritization played a key role in the preservation of nonmineralized organisms pyritization seems to be the most important process by which nonmineralizing chengjiang organisms are preserved in exceptional condition precipitation of fe rich aluminosilicates which occurred following pyritization also played a role in the preservation of some chengjiang fossils phosphatization in the chengjiang lagerstatte was evidently rare and limited taxonomically or to animals having large masses of organic material

evolution of seismic layer 2b across the juan de fuca ridge from hydrophone streamer 2 d traveltime tomography how oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the earth s interior and the oceans although considerable work has been done characterizing the evolution of seismic layer 2a there has been little analysis of the processes that affect layer 2b after formation here we present high resolution 2 d tomographic models of seismic layer 2b along 300 km long multichannel seismic transects crossing the endeavour northern symmetric and cleft segments of the juan de fuca ridge these models show that seismic layer 2b evolves rapidly following a different course than layer 2a the upper layer 2b velocities increase on average by 0 8 km s and reach a generally constant velocity of 5 2 0 3 km s within the first 0 5 myr after crustal formation this suggests that the strongest impact on layer 2b evolution may be that of mineral precipitation due to active hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber variations in upper layer 2b velocity with age at time scales 0 5 ma show correlation about the ridge axis indicating that in the long term crustal accretion processes affect both sides of the ridge axis in a similar way below the 0 5 ma threshold differences in 2b velocity are likely imprinted during crustal formation or early crustal evolution layer 2b velocities at propagator wakes 5 0 0 2 km s where enhanced faulting and cracking are expected and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2b analysis of the layer 2b velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2b velocities respectively additionally the distance between present up flow and down flow zones is related to the amount of sediment cover because as sediment cover increases and basement outcrops become covered direct pathways from the igneous basement through the seafloor are cut off forcing convective cells to find alternate paths

seismic evidence for the migration of fluids within the accretionary complex of western canada multichannel deep seismic reflection data from the subduction zone of western canada delineate the wedge of accereted sediments and the principal terranes crescent pacific rim and wrangellia that form the convergent margin the top of the igneous oceanic crust is defined by subhorizontal reflections extending at least 100 km landward of the deformation front upon incorporation into the accretionary wedge the clearly defined stratigraphy of the incoming oceanic sedimentary section is destroyed over a distance of about 10 km initially an unreflective zone which correlates well with maximum fluid expulsion is formed farther landward a predominantly landward dipping reflectivity exists a number of reflections are thrust faults which appear to merge at depth with the subhorizontal reflections but most have another origin these reflections may be related to the movement of fluids generated by the compaction of sediments or possibly by the dehydration of the subducting plate they are strongest i

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

seismic structure of the southern cascadia subduction zone and accretionary prism north of the mendocino triple junction four multichannel seismic reflection profiles collected as part of the mendocino triple junction seismic experiment image the toe of the southern cascadia accretionary prism today 250 600 m of sediment is subducting with the gorda plate and 1500 3200 m is accreting to the northern california margin faults imaged west and east of the deformation front show mixed structural vergence a north south trending 20 km long portion of the central margin is landward vergent for the outer 6 8 km of the toe of the prism this region of landward vergence exhibits no frontal thrust is unusually steep and narrow and is likely caused by a seaward dipping backstop close to the deformation front the lack of margin wide preferred seaward vergence and wedge taper analysis suggests the prism has low basal shear stress the three southern lines image wedge shaped fragments of oceanic crust 1 1 7 3 km in width and 250 700 m thick near the deformation front these wedges suggest shortening and thickening of the upper oceanic crust discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans the southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the gorda basin oceanic crust and overlying sediments these high angle faults bounding basement highs are interpreted as strike slip faults reactivating structures originally formed at the spreading ridge northeast or northwest trending strike slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north south gorda pacific plate convergence

faulting and hydration of the juan de fuca plate system abstract multichannel seismic observations provide the first direct images of crustal scale normal faults within the juan de fuca plate system and indicate that brittle deformation extends up to 200 km seaward of the cascadia trench within the sedimentary layering steeply dipping faults are identified by stratigraphic offsets with maximum throws of 110 10 m found near the trench fault throws diminish both upsection and seaward from the trench long term throw rates are estimated to be 13 2 mm kyr faulted offsets within the sedimentary layering are typically linked to larger offset scarps in the basement topography suggesting reactivation of the normal fault systems formed at the spreading center imaged reflections within the gabbroic igneous crust indicate swallowing fault dips at depth these reflections require local alteration to produce an impedance contrast indicating that the imaged fault structures provide pathways for fluid transport and hydration as the depth extent of imaged faulting within this young and sediment insulated oceanic plate is primarily limited to approximately moho depths fault controlled hydration appears to be largely restricted to crustal levels if dehydration embrittlement is an important mechanism for triggering intermediate depth earthquakes within the subducting slab then the limited occurrence rate and magnitude of intraslab seismicity at the cascadia margin may in part be explained by the limited amount of water imbedded into the uppermost oceanic mantle prior to subduction the distribution of submarine earthquakes within the juan de fuca plate system indicates that propagator wake areas are likely to be more faulted and therefore more hydrated than other parts of this plate system however being largely restricted to crustal levels this localized increase in hydration generally does not appear to have a measurable effect on the intraslab seismicity along most of the subducted propagator wakes at the cascadia margin

crustal scale seismic structure from trench to forearc in the cascadia subduction zone the de hydration process and the amount of hydrated sediment carried by the downgoing oceanic plate play a key role in the subduction dynamics a high resolution shear velocity model from the crust down to the uppermost mantle extending from trench to forearc is constructed in the northern cascadia subduction zone to investigate seismic characteristics related to slab deformation and de hydration at the plate boundary a total of 220 seismic stations are used including the cascadia initiative amphibious array and inland broadband and short period stations the empirical green s functions extracted from continuous ambient noise data from 2006 to 2014 provide high quality rayleigh wave signals at periods of 4 50 s we simulate wave propagation using finite difference method to generate station strain green s tensors and synthetic waveforms the phase delays of rayleigh waves between the observed and synthetic data are measured at multiple period ranges we then invert for the velocity perturbations from the reference model and progressively improve the model resolution our tomographic imaging shows many regional and local scale low velocity features which are possibly related to slab de hydration from the oceanic plate to the overriding plate specifically we observe 1 nw se oriented linear low velocity features across the trench indicating hydration of the oceanic plate induced by bending related faultings 2 w e oriented fingerlike low velocity structures off the continental margins due to dehydration of the juan de fuca plate and 3 seismic lows atop the plate interface beneath the washington forearc indicating fluid rich sediments subducted and overthrusted at the accretionary wedge

thermal models of the middle america trench at the nicoya peninsula costa rica 1 we present thermal models of the 24 ma cocos plate subducting under the caribbean plate at the nicoya peninsula costa rica our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism initial geotherms of the incoming plate representing different effective cooling depths of hydrothermal circulation affect the thermal structure and temperatures along the main subduction thrust excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required conductive plate cooling over predicts heat flow across the frontal forearc with no or very low frictional heating models with hydrothermal cooling to depths of 2 km under predicts the heat flow comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100 150 c but there are large along strike variations in both the seismicity and thermal regime

intraplate deformation of oceanic crust near the rio grande rise in the south atlantic abstract globally oceanic crust has relatively uniform thickness and structure compared to continental crust with typical ocean crust being 5 8 km thick however a substantial proportion of oceanic crust is thickened outside the normal range in the form of oceanic plateaus and crust affected by hotspots the rio grande rise rgr is the most prominent bathymetric feature in the western south atlantic ocean and was thickened by activity of the tristan hotspot we use multichannel seismic data collected along the eastern margin of the rgr to investigate the relationship history between the thickened crust of the rgr and the adjacent normal crust we find numerous normal faults spatially coincident with the region where oceanic crust begins to thicken from normal to rgr crust using crosscutting relationships and superposition our interpretation supports protracted extensional deformation at the eastern margin of the rgr including recent activity that appears to deform the modern seafloor we attribute the faulting to differential subsidence linked to crustal thickness variations between the thickened rgr and normal oceanic crust given the fundamental processes we attribute to the cause of faulting and the proportion of thickened oceanic crust globally it is possible that regions of many oceanic plates experience intraplate deformation after formation

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

the role of hydrous phases in the formation of trench parallel anisotropy evidence from rayleigh waves in cascadia 1 the cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research we provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well localized region of trench parallel fast directions in rayleigh wave phase velocities near the cascade arc at 45 to 66 s periods we constrain the location of the anisotropic material to within or directly above the oceanic plate using the depth sensitivity of rayleigh waves as a function of frequency and the accurate slab imaging available for cascadia from scattered wave studies we infer that the likely source of trench parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones

laboratory numerical model comparisons of canyon flows a parameter study abstract an integrated set of laboratory and numerical model experiments has been conducted to understand the development of residual circulation surrounding a coastal canyon and to explore further the degree to which laboratory experiments can provide useful benchmark datasets for numerical models of the coastal ocean the use of an idealized shear stress boundary condition along the coastal floor in the numerical model gives good quantitative agreement with the laboratory results for the zeroth order time dependent flow and good qualitative agreement for the higher order i e o ro where ro the rossby number is small time mean flow the quantitative agreement for the latter however is not within estimates of laboratory uncertainties it is shown that the use of a no slip condition along the floor improves the model away from the canyon boundaries but the enhanced viscosities needed to obtain numerical stability give boundary layers that are too wide along the coastline the laboratory and nume

internal deformation of the southern gorda plate fragmentation of a weak plate near the mendocino triple junction north south compression across the gorda pacific plate boundary caused by the northward migrating mendocino triple junction appears to reactivate gorda plate normal faults originally formed at the spreading ridge as left lateral strike slip faults both seismically imaged faults and magnetic anomalies fan eastward from n208e near the gorda ridge to n758e near the triple junction near the triple junction the gorda plate is faulted pervasively and appears to be extending east southeast as it subducts beneath north america continuation of northeast southwest oriented deformation in the southern gorda plate beneath the continental margin contrasts with the northwest southeast trending structures in the overlying accretionary prism suggesting partial gorda north american plate decoupling southeast of the triple junction a slabless window is generated by removal of the subducting gorda plate southwest of the triple junction the pacific plate acts as a rigid barrier forcing southern gorda crust to rotate clockwise fragment and flow into the slabless window net clockwise rotation of the southern gorda crust forms a boundary with the nonrotating northern gorda plate which is observed as a bend in the magnetic anomalies this boundary which is compressional on the western end and extensional to the east may separate the stress regime of the southern gorda plate from the remainder of the cascadia subduction zone

identifying the rupture plane of the 2001 nisqually washington earthquake the 2001 nisqually earthquake occurred within the subducting juan de fuca plate previous seismic and geodetic studies could not confidently identify its actual fault plane from the two nodal planes in this study we apply the recently developed source scanning algorithm to local seismic waveforms and show unambiguously that the steeply east dipping plane is the rupture plane the rupture began near the bottom of the subducting crust and propagated downward into the subducting uppermost mantle if intraslab earthquakes are assumed to be due to dehydration embrittlement the source dimension is unlikely to grow any larger because the warm thermal state of the subducting juan de fuca plate limits dehydration to a shallow depth below the slab surface numerical modeling of the thermal structure indicates that dehydration embrittlement can only take place in the top 10 km of the subducting mantle implying that the maximum size of an intraslab earthquake in northern cascadia would be m w 7 or less

ocean currents across the entrance to the gulf of california observations of transport and currents in april may and december 1992 and january 1993 were made across the entrance to the gulf of california with an acoustically tracked dropsonde flow was into the gulf along sinaloa and out of the gulf along baja california the transports were 5 10 sv and the currents were deep with 10 cm s 1 flow extending to depths greater than 1000 m the currents were intensified in the upper 300 m geostrophic flows compared well with observed currents when smoothed to 2 3 times the rossby radius salinity in the upper 300 m was higher on the baja california side of the gulf indicating modification of subtropical subsurface waters within the gulf as well as the presence of surface and near surface gulf waters the salinity minimum associated with california current waters at 50 m had narrow cores that can be resolved only with closely spaced conductivity temperature depth casts mass and heat fluxes for the upper 300 m were estimated as 280 t s 1 and 0 1 1012 w in may and 170 t s 1 and 2 0 1012 w in december

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

influence of the mendocino triple junction on the tectonics of coastal california abstract the migration of the mendocino triple junction through central and northern california over the past 25 30 million years has led to a profound change in plate interactions along coastal california the tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the san andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations the primary driver of this tectonism is a coupling among the subducting juan de fuca gorda north american and pacific plates that migrates with the triple junction this coupling leads to ephemeral thickening of the overlying north american crust associated uplift and subsequent subsidence and a distinctive sequence of fault development and volcanism

crustal structure beneath the galapagos archipelago from ambient noise tomography and its implications for plume lithosphere interactions 1 to constrain the seismic velocity structure of the crust beneath the galapagos archipelago we conducted a tomographic study using high frequency rayleigh waves obtained from cross correlations of ambient noise we analyzed waves with periods between 5 and 8 5 s sensitive to shear wave velocity vs structure between about 3 and 10 km depth after accounting for the effect of water depth crustal velocities are up to 25 lower than those of very young crust at the east pacific rise and are comparable to those of hawaii we attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust on average vs between 3 and 10 km depth beneath the western archipelago is up to 15 lower than beneath the eastern archipelago we attribute the west to east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the galapagos plume the results of this study in combination with previous work indicate that many of the unusual aspects of the galapagos archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres our findings indicate that observed variations in the flexural response to loading observed in the galapagos cannot be explained by the current thermal state of the lithosphere instead the flexural response likely represents varying elastic strength at the time of loading we also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago the darwinian lineations may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume lithosphere interactions

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

three dimensional structure from feathered two dimensional marine seismic reflection data the eastern nankai trough 1 for a typical two dimensional 2 d marine multichannel seismic reflection survey cross currents cause receiver cable side drift and data collection becomes a limited swath 3 d survey processing of swath marine survey data using standard 2 d imaging procedures can create spurious discontinuities and wipeouts in the images of reflection events these anomalies are commonly attributed to geology and tectonics during interpretation we show how discontinuous reflections and wipeouts are created wherever subsurface structures have significant cross profile dip we also show that more accurate imaging can be achieved by cross dip processing which takes into account the 3 d character of the data and of the geology the final products are a much improved stack determinations of structural dips at right angles to the survey line and an improved velocity model we apply this cross dip processing to data from the eastern nankai trough sw japan improved imaging combined with the new cross dip information allows us to constrain better the 3 d character of the deformation in the accretionary prism and oceanic crust in particular the extracted structural cross dip values provide evidence that supports two hypotheses related to the underthrusting of a large oceanic ridge paleo zenisu ridge underneath the accretionary wedge 1 parts of the accretionary wedge basins have been uplifted and deformed due to the subduction of a volcano on the ridge 2 the early phase of oceanic lithosphere shortening which produced the underthrusted paleo zenisu ridge was related to n s compression oblique to the general plate motion in this region

seismic reflection constraints on imbrication and underplating of the northern cascadia convergent margin an interpretation of the deep structure of the continental shelf offshore southern vancouver island subject to constraints from other geophysical data is derived by combining seismic reflection profiles shot in 1989 with those from an earlier 1985 survey accretionary wedge sediments which extend landward beneath the volcanic crescent terrane comprise two primary units both of which have shortened through duplex formation the maximum thickness of the crescent terrane 6 8 km occurs just seaward of its contact with the inboard largely metasedimentary pacific rim terrane the e region of reflectivity first detected dipping landward beneath vancouver island is regionally extensive being observed on all the seismic profiles the e reflectivity thins seaward and splits into two or more strands that probably link into major faults within the accreted sedimentary wedge reflections from the interplate decollement beneath the outer continental shelf separate from the downgoing plate continue into the

spatial variation in the diversity and composition of the lower cambrian series 2 stage 3 chengjiang biota southwest china abstract strata hosting the lower cambrian series 2 stage 3 chengjiang biota maotianshan shale member yu anshan formation occur throughout the eastern part of yunnan province southwest china in this study literature based faunal inventories from 10 areas representing 34 localities together with 22 038 new specimens collected at three localities from three of the 10 areas were analyzed quantitatively to assess large scale spatial variation in taxonomic diversity and composition our analyses show substantial co variation between local paleoenvironmental settings and species diversity and suggest the presence of three general taphofacies in the maotianshan shale member from west to east low diversity 26 species characterizes the wuding area where fossiliferous strata have undergone extensive bioturbation high diversity 215 species characterizes the chengjiang haikou anning areas where fossiliferous strata consist of stacked couplets of thin event and background mudstone layers moderate diversity 55 species characterizes the malong area where fossiliferous strata consist of indistinctly bedded shale showing fewer single event layers but abundant background layers in spite of variations in sampling effort between sites spatial patterns in species diversity and community composition were controlled primarily by variation in depositional environments along a proximal offshore to lower shoreface gradient which influenced both ecological and taphonomic factors the offshore settings haikou chengjiang anning areas show relatively high species diversity and exceptional preservation of soft bodied fossils reflecting rapid burial in distal tempestites relatively high degrees of bioturbation time averaging and hydrodynamic disturbance caused a substantial decrease in the quality of preservation and number of soft bodied species in the wuding and malong areas arthropods are the most diverse group in most localities followed by priapulids poriferans brachiopods lobopods and hyoliths this study shows that the chengjiang biota lends itself very well to high resolution characterization of spatial variation in taxonomic diversity faunal composition and fossil preservation furthermore the chengjiang biota may provide a unique opportunity to assess the roles of environmental factors taphonomy and ecological controls on species diversity at local to regional spatial scales

the 13 january 2001 el salvador earthquake a multidata analysis 1 on 13 january 2001 a large normal faulting intermediate depth event mw 7 7 occurred 40 km off the el salvadorian coast central america we analyze this earthquake using teleseismic regional and local data we first build a kinematic source model by simultaneously inverting p and sh displacement waveforms and source time functions derived from surface waves using an empirical green s function analysis in an attempt to discriminate between the two nodal planes 30 trenchward dipping and 60 landward dipping we perform identical inversions using both possible fault planes after relocating the hypocentral depth at 54 km we retrieve the kinematic features of the rupture using a combination of the neighborhood algorithm of sambridge 1999 and the simplex method allowing for variable rupture velocity and slip we find updip rupture propagation yielding a centroid depth around 47 km for both assumed fault planes with a larger variance reduction obtained using the 60 landward dipping nodal plane we test the two possible fault models using regional broadband data and near field accelerograms provided by universidad centro americana 2001 near field data confirm that the steeper landward dipping nodal plane is preferred rupture propagated mostly updip and to the northwest resulting in a main moment release zone of approximately 25 km 50 km with an average slip of 3 5 m the large slip occurs near the interplate interface at a location where the slab steepens dip significantly the occurrence of this event is well explained by bending of the subducting plate

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

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

relationship between the cascadia fore arc mantle wedge nonvolcanic tremor and the downdip limit of seismogenic rupture great earthquakes anticipated on the cascadia subduction fault can potentially rupture beyond the geodetically and thermally inferred locked zone to the depths of episodic tremor and slip ets or to the even deeper fore arc mantle corner fmc to evaluate these extreme rupture limits we map the fmc from southern vancouver island to central oregon by combining published seismic velocity structures with a model of the juan de fuca plate these data indicate that the fmc is somewhat shallower beneath vancouver island 36 38 km and oregon 35 40 km and deeper beneath washington 41 43 km the updip edge of tremor follows the same general pattern overlying a slightly shallower juan de fuca plate beneath vancouver island and oregon 30 km and a deeper plate beneath washington 35 km similar to the nankai subduction zone the best constrained fmc depths correlate with the center of the tremor band suggesting that ets is controlled by conditions near the fmc rather than directly by temperature or pressure unlike nankai a gap as wide as 70 km exists between the downdip limit of the inferred locked zone and the fmc this gap also encompasses a 50 km wide gap between the inferred locked zones and the updip limit of tremor the separation of these features offers a natural laboratory for determining the key controls on downdip rupture limits

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

internal deformation of the gorda plate observed by hydroacoustic monitoring on august 29 1991 the national oceanic and atmospheric administration vents program began monitoring the northeast pacific ocean plate boundaries using continuously recorded data from the u s navy s sound surveillance system sosus hydrophone arrays routine detection and location of small 2 4 m 3 5 earthquakes revealed a nne ssw trending band of seismicity through the center of the gorda plate the majority of the earthquakes in the band were not detected by land based seismic networks this band of microearthquakes was active from the initiation of sosus monitoring in august 1991 through july 1992 at a rate of 20 events per week since august 1992 however the activity has effectively ceased some of the microearthquakes in the band are likely aftershocks to three large mw 6 mid gorda plate earthquakes that occurred 1 2 months prior to the beginning of sosus monitoring the hydroacoustically detected events in the band however do not occur in the proximity of the mw 6 events and the activity level increased several months afterward suggesting the microseismicity band represents a more general pattern of deformation within the gorda plate the cessation of the midplate activity follows the occurrence of a large earthquake sequence in the adjacent cascadia subduction zone in april 1992 it is proposed that the band of microseismicity and the three large midplate events reflect an accumulation of stress in the gorda plate with the eventual termination of this pattern due to stress reduction associated with movement in the adjacent subduction zone stress field modeling of the region yields results consistent with this interpretation

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

a poorly preserved fish like animal from the chengjiang lagerstatte cambrian series 2 stage 3 abstract specimens of a poorly preserved animal including individuals and body parts have been discovered from the chengjiang lagerstatte these specimens are interpreted to be a fish like animal and are described as an unnamed myllokunmingiid the morphologies and preservation of individuals and body parts are described and the relative decay patterns are briefly discussed these specimens are from the event mudstone beds of the maotianshan shale member but exhibit the classic preservation of organisms in the background mudstone beds slow accumulation and this might indicate that different preservation modes of organisms co exist in the event mudstone beds based on the facts that lenses cannot be discerned on all the specimens of myllokunmingiids their visual organs may have been more like eyespots than true eyes

reflection signature of seismic and aseismic slip on the northern cascadia subduction interface at the northern cascadia margin the juan de fuca plate is underthrusting north america at about 45 mm yr 1 ref 1 resulting in the potential for destructive great earthquakes2 3 the downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake thought to have been in 1700 4 occurred before instrumental recording thermal and deformation studies5 indicate that off southern vancouver island the interplate interface is presently fully locked for a distance of 60 km downdip from the deformation front great thrust earthquakes on this section of the interface with magnitudes of up to 9 4 5 have been estimated to occur at an average interval of about 590 yr ref 3 further downdip there is a transition from fully locked behaviour to aseismic sliding where high temperatures allow ductile deformation with the deep aseismic zone exhibiting slow slip thrust events6 here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked to a broad reflection band at greater depth where aseismic slip is thought to be occurring this change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

multi mode conversion imaging of the subducted gorda and juan de fuca plates below the north american continent abstract receiver function analysis and seismic tomography show tectonic structures dipping eastward in the mantle below the cascadia volcanic arc western us that have been related to the subduction of the gorda and juan de fuca oceanic micro plates inconsistencies in the dip angle and depth extent of the slab between the two methods undermine the interpretation of the structure and processes at work receiver function imaging is biased by multiple reflection phases that interfere with converted phases and produce spurious discontinuities in images here we correct the interference using a multiple mode conversion imaging technique that efficiently removes artifacts under dipping structures the method has the advantage of being applicable to large aperture arrays and can image large scale structures down to the transition zone with this approach the interfaces between the subducting and overriding plates and the oceanic moho are imaged at shallow depths

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

a 2 d tomographic model of the juan de fuca plate from accretion at axial seamount to subduction at the cascadia margin from an active source ocean bottom seismometer survey we report results from a wide angle controlled source seismic experiment across the juan de fuca plate designed to investigate the evolution of the plate from accretion at the juan de fuca ridge to subduction at the cascadia margin a two dimensional velocity model of the crust and upper mantle is derived from a joint reflection refraction travel time inversion to interpret our tomography results we first generate a plausible baseline velocity model assuming a plate cooling model and realistic oceanic lithologies we then use an effective medium theory to infer from our tomography results the extent of porosity alteration and water content that would be required to explain the departure from the baseline model in crust of ages 1 ma and away from propagator wakes and regions of faulting due to plate bending we obtain estimates of upper crustal hydration of 0 5 2 1 wt and find mostly dry lower crust and upper mantle in sections of the crust affected by propagator wakes we find upper estimates of upper crustal lower crustal and upper mantle hydration of 3 1 0 8 and 1 8 wt respectively at the cascadia deformation front we find that the amount of water stored at uppermost mantle levels in the downgoing jdf plate is very limited 0 3 wt with most of the water carried into the subduction zone being stored in the oceanic crust

explorer deformation zone evidence of a large shear zone and reorganization of the pacific juan de fuca north american triple junction recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the explorer juan de fuca ridge systems the boundaries between the pacific and northern juan de fuca plates the bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the sovanco fracture zone to include the heck and heckle seamounts and potentially as far south as the springfield seamounts and cobb offset along the juan de fuca ridge this indicates that the triple junction between the pacific juan de fuca north american plates may be reorganizing southward to establish at the cobb offset

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

fracture alignments in marine sediments off vancouver island from ps splitting analysis alignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction and extension and basement faulting the orientation of these alignments can be estimated through analyses of s wave splitting if sensors in ocean bottom observations are deployed through free fall sensor orientation needs to be determined in order for the recorded data to be used for such analyses here we estimate the sensor orientation from the linear particle motions of p to s ps phases converted at the sediment basement interface and also from t waves that are excited by earthquakes and propagate in the seawater we examine waveforms of local earthquakes recorded by 32 ocean bottom seismometers obss that were deployed through free fall for three months in 2010 off vancouver island where the strike slip nootka fault zone nfz intersects the deformation front of the cascadia subduction zone because the particle motion of the ps wave was corrected by estimating splitting parameters the fast polarization direction which reflects s wave anisotropic structure within the sediment can also be evaluated consequently we could estimate the fast polarization direction at obss deployed near the nfz and west of the deformation front the obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left lateral motion of the fault in the basement along the nfz margin normal cracks due to horizontal compression west of and slightly away from the deformation front and frontal thrust faults within the accretionary prism near the deformation front

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

low potential for large intraslab earthquakes in the central cascadia subduction zone the cascadia subduction zone csz can be divided into three distinct sections based on the characteristics of intraslab seismicity based on a 150 year historical record no moderate to large intraslab earthquakes of moment magnitude m 5 5 or greater have occurred within the subducting juan de fuca plate of the central csz from south of the puget sound in northwestern washington to the oregon california border also very few intraslab earthquakes as small as m 3 have been instrumentally located within the central csz since 1960 and a wadati benioff zone is not apparent in the southern csz beneath northwestern california a wadati benioff zone is present to a depth of about 40 km but no large gorda block earthquakes have been observed in the downgoing slab although large events have occurred near the trench axis in contrast the puget sound region within the northern csz has been repeatedly shaken by large intraslab earthquakes of m 6 5 in the depth range of 40 to 60 km such as the recent 2001 m 6 8 nisqually event a critical question addressed in this article is what is the potential for such large shallow intraslab earthquakes in the central csz beneath western oregon and southwestern washington i have evaluated the available information on the thermal and physical properties geometry and historical and contemporary seismicity of the central csz and performed thermal modeling based on these analyses and comparisons with other subduction zones worldwide the lack of shallow intraslab earthquakes in the central csz is not unusual the hot temperatures 500 c within the juan de fuca plate particularly below a depth of 40 km where large events are expected are not conducive to earthquake generation resulting in either the complete absence of m 6 5 shallow intraslab earthquakes or long recurrence intervals hundreds of years between such events temperatures appear to be sufficiently high in the central csz so that no wadati benioff zone can exist even at shallow depths 40 km the young plate age slower convergence rate and the insulating effect of the siletz terrane above the plate are factors that probably lead to the hot temperatures in this portion of the slab the variability in the maximum depth of the wadati benioff zone along the csz 60 km beneath the puget sound 40 km within the subducting gorda block and essentially zero in the central csz reflect the differing temperature conditions that is the cutoff temperature varies with depth and rock composition and also the potential for large shallow intraslab earthquakes in addition to the effects of temperature the level of tectonic stresses which vary along the length of the csz must also be a factor in controlling the occurrence of large intraslab earthquakes large events can occur in the puget sound region probably because of cooler intraslab temperatures and possibly because of a stress concentration or zone of weakness along the pronounced arch in the juan de fuca plate a previous study has suggested an intraslab subduction zone origin for a m 7 3 earthquake that occurred in 1873 near the town of brookings in southernmost oregon however analysis of its seismotectonic setting and comparison with other historical earthquakes in northernmost california suggest that the event probably had a very shallow origin within the gorda block southern csz and was not a deep intraslab earthquake in the central csz

seismic anisotropy beneath the juan de fuca plate system evidence for heterogeneous mantle flow here we use sks shear wave splitting observations from ocean bottom seismometer data to infer patterns of mantle deformation beneath the juan de fuca plate and its adjoining boundaries our results indicate that the asthenosphere beneath the juan de fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone geographically restricted deviations from this pattern indicate the presence of secondary processes at discrete plate boundaries such as the blanco transform fault seismic anisotropy is attributed to relative plate motion within a narrow zone 50 km beneath the deforming southern gorda plate region a diffuse plate boundary splitting observations similarly suggest deformation dominated by relative motion between the rigid juan de fuca and pacific plates but distributed over a broad zone 200 km our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback as suggested by onshore studies instead reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern cascadia

three dimensional stratified flow over alarcon seamount gulf of california entrance abstract in situ hydrographic measurements at the entrance to the gulf of california gc over the alarcon seamount asm show that isopycnals and isotherms undulate being displaced in an upward direction and generating a dome like structure in the density temperature and salinity fields while only small perturbations in both fields are observed longitudinally on the other hand some along gulf observations show an 80 m amplitude internal lee wave we hypothesize that these perturbations result from the interaction of the mean flow with local topography which generates internal lee waves as the flow passes over the asm geostrophic velocities over the asm at 750 db level show a meandering jet flow and two associated eddies a fully three dimensional boundary fitted curvilinear ocean model in primitive variables is used to simulate and accurately represent the 3d circulation with high resolution in the neighborhood of the asm the simulated flow pattern shows a meandering jet over the topography with increased velocities on the flanks of the asm interaction of current and topography generates a pair of baroclinic eddies similar to those observed during winter over the asm the associated density field consists of an upwelling on the upstream side of the seamount leading to a patch of relatively dense water over the seamount and a downwelling on the lee side leading to a patch of relatively less dense water downstream

effects of the fieberling seamount group upon flow and thermohaline structure in the spring of 1991 the circulation and thermohaline structure around fieberling fieberling ii and hoke seamounts are examined on the basis of two high resolution surveys in april and may 1991 the large scale flow approached the seamounts from the southeast impinging first on hoke then splitting with one branch meandering westward in jetlike fashion over the fieberling ii and fieberling seamounts and the other turning to the northeast the jets were typically 10 20 km wide with core speeds of 0 2 0 5 m s and were accompanied by pairs of cyclonic and anticyclonic eddies with diameters of 10 30 km located near the seamounts while the large scale flow pattern persisted over a week significant changes were observed in the shape of the meanders and the locations of the eddies over this period transient bottom intensified upwelling cones 20 km in diameter and 150 220 m high were encountered on all three seamounts investigated as were pronounced 60 m deep mixed layers above the summits their persistence at a given location was less than a week geopotential height perturbations of the order of 0 8 j kg equivalent to 0 08 m in elevation were observed in the vicinity of the seamounts these perturbations are dominated by mesoscale ridges troughs peaks and depressions the positions of which vary with depth as well as time in general the surface geopotential topographies differ in shape from those observed at depths where the seamount peaks occur which is attributed to the vanishing influence of the california current beneath 200 m

upper crustal evolution across the juan de fuca ridge flanks 1 recent p wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2a doubles or reaches 4 3 km s found in mature crust in 10 ma after crustal formation this velocity change is commonly attributed to precipitation of low temperature alteration minerals within the extrusive rocks associated with ridge flank hydrothermal circulation sediment blanketing acting as a thermal insulator has been proposed to further accelerate layer 2a evolution by enhancing mineral precipitation we carried out 1 d traveltime modeling on common midpoint supergathers from our 2002 juan de fuca ridge multichannel seismic data to determine upper crustal structure at 3 km intervals along 300 km long transects crossing the endeavor northern symmetric and cleft ridge segments our results show a regional correlation between upper crustal velocity and crustal age the measured velocity increase with crustal age is not uniform across the investigated ridge flanks for the ridge flanks blanketed with a sealing sedimentary cover the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks 60 increase versus 28 over the same crustal age range of 5 9 ma extrapolation of velocity age gradients indicates that layer 2a velocity reaches 4 3 km s by 8 ma on the sediment blanketed flanks compared to 16 ma on the flanks with thin and discontinuous sediment cover the computed thickness gradients show that layer 2a does not thin and disappear in the juan de fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust however layer 2a on the fully sedimented ridge flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks 330 80 versus 430 80 m the change in thickness occurs over a 10 20 km distance coincident with the onset of sediment burial our results also suggest that propagator wakes can have atypical layer 2a thickness and velocity impact of propagator wakes is evident in the chemical signature of the fluids sampled by odp drill holes along the east endeavor transect providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

the thermal environment of cascadia basin 1 located adjacent to the ne pacific convergent boundary cascadia basin has a global impact well beyond its small geographic size composed of young oceanic crust formed at the juan de fuca ridge igneous rocks underlying the basin are partially insulated from cooling of their initial heat of formation by a thick layer of pelagic and turbidite sediments derived from the adjacent north american margin the igneous seafloor is eventually consumed at the cascadia subduction zone where interactions between the approaching oceanic crust and the north american continental margin are partially controlled by the thermal environment within cascadia basin basement topographic relief varies dramatically and sediments have a wide range of thickness and physical properties this variation produces regional differences in heat flow and basement temperatures for seafloor even of similar age previous studies proposed a north south thermal gradient within cascadia basin with high geothermal flux and crustal temperatures measured in the heavily sedimented northern portion near vancouver island and lower than average heat flux and basement temperatures predicted for the central and southern portions of the basin if confirmed this prediction has implications for processes associated with the cascadia subduction zone including the location of the locked zone of the megathrust fault although existing archival geophysical data in the central and southern basin are sparse nonuniformly distributed and derived from a wide range of historical sources a substantial n s geothermal gradient appears to be confirmed by our present compilation of combined water column and heat flow measurements

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved