origin of high mountains in the continents the southern sierra nevada active and passive seismic experiments show that the southern sierra despite standing 1 8 to 2 8 kilometers above its surroundings is underlain by crust of similar seismic thickness about 30 to 40 kilometers thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range but little subcrustal lithosphere is present beneath the eastern high sierra and adjacent basin and range these and other data imply the crust of both the high sierra and basin and range thinned by a factor of 2 since 20 million years ago at odds with purported late cenozoic regional uplift of some 2 kilometers

lower crustal deformation beneath the central transverse ranges southern california results from the los angeles region seismic experiment 1 we present a p wave velocity model derived from active source seismic data collected during the 1994 los angeles region seismic experiment our model extends previously published upper crustal velocity models to mantle depths our model was developed by both ray tracing through a layered model and calculating travel times through a gridded model it includes an 8 km thick crustal root centered beneath the surface trace of the san andreas fault north of the highest topography in the san gabriel mountains a simple mass balance calculation suggests that 36 km of north south shortening across the san andreas fault in the central transverse ranges could have formed this root if north south compression began when the big bend in the san andreas fault formed at 5 ma 36 km of shortening implies a north south contraction rate of 7 1 mm yr across the central transverse ranges if instead north south compression began when the transverse ranges formed at 3 4 3 9 ma 36 km of shortening implies a contraction rate of 9 2 10 6 mm yr north of the san andreas fault the mojave desert crust has a low velocity 6 3 km s mid and lower crust and a 28 km deep moho south of the san andreas fault beneath the los angeles and san gabriel valley basins there is a fast 6 6 6 8 km s thick 10 12 km lower crust with a 27 km deep moho farther south still the lower crust of the continental borderland is fast 6 6 6 8 km s and thin 5 km with a shallow 22 km deep moho

long distance fluid and heat transport in the oceanic crust entering the nankai subduction zone nantroseize transect abstract i examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the nankai trough seismogenic zone experiment nantroseize transect offshore southern japan the most prominent anomaly is a 50 mw m 2 change in heat flux between integrated ocean drilling program sites c0011 and c0012 over a distance of 30 km wide transition at the seafloor the observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate for a 600 m thick aquifer the permeability is likely 7 10 11 m 2 and hydrothermal circulation transports at least 300 times more heat than conduction alone the heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to 100 km landward of the deformation front vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment basement interface

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

thermal evolution of the new zealand hikurangi subduction margin impact on natural gas generation and methane hydrate formation a model study abstract we present an integrated 2d model of thermal and microbial generation of methane migration into the gas hydrate stability zone hsz and formation of methane hydrates the model reconstructs the shallow 0 20 km thermal structure of the subduction interface between the australian plate and the subducting pacific plate and the trench basin pegasus basin modelled temperatures of less than 110 c within pegasus basin constrain the generation of oil and gas whilst a cool thermal regime is predicted to limit thermogenic generation of gas to a burial depth of 10 km it extends the interval where prolific microbial gas generation occurs the modelled rate of microbial generation of methane increases beneath the hsz and peaks at 1600 m below seafloor diffusive upward migration of microbially generated methane is interpreted to lead to widespread methane hydrate formation and the presence of a semi continuous bottom simulating reflector bsr predicted average hydrate saturation within the hsz is 0 9 for a modelled sedimentary organic matter content of 0 5 and 1 6 for 1 organic matter in fine grained pegasus basin sediments considerably higher concentrations of methane hydrate of up to 20 70 are predicted to occur where gas migration is focussed within the frontal anticline and proto thrust zone southeast of the modern accretionary wedge and in channel and basin floor sandstones related to the hikurangi channel the hikurangi channel sedimentary system transported coarse clastic sediments eroded from the rising southern alps along the eastern margin of the pegasus basin since the miocene it provides carrier beds specifically for transport of thermogenic gas generated close to the subduction interface a buried mesozoic accretionary wedge originating from subduction of the pacific plate beneath gondwana further focusses the migration of gas focussed migration of thermogenic gas leads to the highest predicted hydrate concentrations in potential channel sand reservoirs

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

effect of hydrothermal circulation on slab dehydration for the subduction zone of costa rica and nicaragua abstract dehydration of subducting oceanic plates is associated with mantle wedge melting arc volcanism intraslab earthquakes through dehydration embrittlement and the flux of water into the mantle in this study we present two dimensional thermal models of the costa rica nicaragua subduction zone to investigate dehydration reactions within the subducting cocos plate seismic and geochemical observations indicate that the mantle wedge below nicaragua is more hydrated than that below costa rica these trends have been hypothesized to be due to a variation in either the thermal state or the hydration state of the subducting slab despite only small variations in plate age along strike heat flow measurements near the deformation front reveal significantly lower heat flow offshore nicaragua than offshore costa rica these measurements are interpreted to reflect an along strike change in the efficiency of hydrothermal circulation in the oceanic crust we parameterize thermal models in terms of efficient and inefficient hydrothermal circulation and explore their impact on slab temperature in the context of dehydration models relative to models without fluid flow efficient hydrothermal circulation reduces slab temperature by as much at 60 c to depths of 75 km and increases the predicted depth of eclogitization by 15 km inefficient hydrothermal circulation has a commensurately smaller influence on slab temperatures and the depth of eclogitization for both regions the change in eclogitization depth better fits the observed intraslab crustal seismicity but there is not a strong contrast in the slab thermal structure or location of the main dehydration reactions consistent with other studies these results suggest that observed along strike differences in mantle wedge hydration may be better explained by a northwestward increase in the hydration state of the cocos plate before it is subducted

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

estimating crustal thickness in southern california by stacking pmp arrivals we use observations of the moho reflected phase pmp to constrain crustal thicknesses and upper mantle velocities in southern california we stack normalized absolute values of seismograms from local events in time and range bins after aligning on the initial p arrival and applying a range correction to adjust the various source depths to the surface although most individual seismograms do not allow accurate determination of a pmp arrival time imaging the whole data set in this way shows clear pmp arrivals at ranges from about 90 km to over 250 km pmp pg and pmp pn differential times can be measured from the image and used to estimate the moho depth and upper mantle velocity for southern california we obtain an average crustal thickness of 28 km and an upper mantle velocity of 7 8 km s next we map lateral variations by repeating this procedure for stacks of subsets of the data in which the traces are grouped in caps by moho reflection point estimates of moho depth range from 18 km in the salton trough to 33 km beneath the eastern transverse ranges and 36 km beneath the southernmost sierra nevada the upper mantle velocities generally increase from southwest to northeast across the region we also map postcritical pmp pg amplitudes which vary by a factor of 4 with the highest amplitudes in the northwest mojave desert preliminary experiments with stacking sms arrivals indicate strongly correlated sms and pmp amplitude variations these results provide a guide to source receiver paths that may produce anomalously strong moho reflected phases during future earthquakes

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

seismic reflection constraints on the structure of the crust beneath the san bernardino mountains transverse ranges southern california a 30 km long n s seismic reflection line was shot by california consortium for crust studies calcrust across the southern mojave desert and onto the northern flank of the san bernardino mountains in southern california on the northern end of the seismic section the reflectivity increases markedly in the midcrust at a depth corresponding to a two way travel time of 4 to 5 s 12 15 km suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust the high reflectivity disappears at about 8 s 24 km and may be correlated with a change in seismic velocity in the lower crust from 6 3 km s to 6 8 km s a band of reflectivity between 9 5 and 10 s 27 30 km is believed to represent the moho the midcrustal relectivity transition and moho both deflect downward toward the san bernardino mountains uplift over the entire length of the profile the deflection of the midcrustal transition 12 appears greater than that of the moho 6 resulting in a thinning of the lower crust to the south beneath the uplift in addition the midcrustal transition coincides with the base of the seismogenic zone brittle ductile transition which is also dipping southward beneath the san bernardino mountains while the moho deflection is consistent with elastic flexure resulting from edge loading by the san bernardino mountains which have been thrust over the mojave block it is suggested that the thinning of the lower crust beneath the san bernardino mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust since a portion of the load is transmitted through the lower crust to the moho the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift 2 m y

regional crustal thickness variations of the peninsular ranges southern california we used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern peninsular ranges california depth to the moho varies from 37 km beneath the western peninsular ranges batholith to 27 km at the western edge of the salton trough an average apparent dip of 10 to the west over a lateral distance of 60 km we previously obtained a similar result for a profile 100 km to the south a moho dip of 20 over 30 km lateral distance in both cases the moho depth variations do not correlate with topography of the eastern batholith but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes these observations suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that compensation is through lateral variations in crustal or upper mantle density rather than through an airy root

a detailed image of the continent borderland transition beneath long beach california new crustal images beneath long beach california show the region of the inner borderland to continent transition the cross sections are obtained from stacked autocorrelations of virtual sources generated from oil industry data recorded in the city of long beach ca they show that the moho is dipping at 65 and obliquely truncates an 10 km thick flat lying lower crustal fabric the moho appears to be fault controlled and an integral part of the extrusion of the catalina schist that underlays the inner borderland the basement interface has significant offsets of up to 2 km none of which correspond to the mapped trace of the newport inglewood fault

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

pn velocity anisotropy in southern california we analyze p n propagation as a function of azimuth across a 28 station 150 km aperture subarray of the scarlet network centered near the central transverse ranges california we selected signals from 81 earthquakes and explosions with epicentral distances ranging from 150 to 400 km covering all azimuths except a 40 gap from the southwest and a lesser gap from the northeast direction for each source the apparent velocity of p n was determined using a one norm measure of misfit the apparent p n velocity does not show any systematic variation with epicentral distance but exhibits a strong azimuthal dependence our preferred interpretation calls for a slightly dipping 2 to n40w planar moho with 3 to 4 per cent anisotropy of subcrustal material transverse isotropy with a nearly horizontal symmetry axis is sufficient to explain the data the direction of sagittal symmetry is n50w the isotropic velocity of p n is 7 8 km sec in contrast a higher 8 1 km sec p n velocity is found in the mojave block with no indication of anisotropy these observations are consistent with a subcrustal model of the pacific north america plate boundary where ductile flow is characterized by simple shear in a vertical plane with strike parallel to the direction of relative plate motion

thermal effects of hydrothermal circulation and seamount subduction temperatures in the nankai trough seismogenic zone experiment transect japan 1 we examine the thermal effects of seamount subduction seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position prior to subduction seamounts provide high permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust as the seamount is subducted the high permeability pathway is closed shutting down the advective transfer of heat if significant fluid flow occurs it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system additionally subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge we apply these features of seamount subduction to a thermal model for the nankai trough seismogenic zone experiment transect on the margin of japan models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench they yield temperatures of 100 c 295 c for the rupture area of the 1944 tonankai earthquake temperatures in the region of episodic tremor and slip are estimated at 290 c 325 c 70 c cooler than a model with no fluid circulation

pn travel times in southern california the pn arrivals recorded from the southern california array have been used to demonstrate regional variations in crustal thickness and pn velocity and to investigate the presence of anisotropy in the mantle over 2800 travel times were used in the analysis the classical time term method was extended to allow the moho velocity to vary in a block type structure the statistical f test shows these effects to be significant crustal delays determined depend on both crustal velocities and moho depth the ventura and los angeles basins have sediment delays which indicate sediment thicknesses near 10 kin the moho depth itself varies by 10 km over southern california with a mean of about 29 kin slightly thicker crust is prevalent in the northwestern corner of the array near the southern coast ranges five kilometers of rapid thinning between the coast and the channel islands is shown by a sharp gradient in the delay times normal crustal thicknesses occur over the rest of the array except for the area east of the salton trough where an anomalously thin crust of 22 km exists this area extends from near the plate boundary into arizona while the peninsular ranges and the san bernadino mountains have small roots the san gabriel mountains have none pn velocities range from 7 6 to 8 2 km s within the array high velocities are found in the mojave and offshore regions low velocities are found in the peninsular ranges and the transverse ranges the analysis supports a mean anisotropy of 0 15 km s with the fast direction at n 75 o w this value represents an average of a parameter that varies considerably in the region

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

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

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

cold seep and oxygen minimum zone associated sources of margin heterogeneity affect benthic assemblages diversity and nutrition at the cascadian margin ne pacific ocean abstract hydrate ridge hr located on the northeastern pacific margin off oregon is characterized by the presence of outcropping hydrates and active methane seepage additionally permanent low oxygen conditions overlay the benthic realm this study evaluated the relative influence of both seepage and oxygen minima as sources of habitat heterogeneity and potential stress inducing features on the bathyal metazoan benthos primarily nematodes at three different seep and non seep hr locations exposed to decreasing bottom water oxygen concentrations with increasing water depth the nematode seep communities at hr exhibited low diversity with dominance of only one or two genera daptonema and metadesmolaimus elevated average individual biomass and 13c evidence for strong dependance on chemosynthesis derived carbon resembling deep sea seeps worldwide although the hr seep habitats harbored a distinct nematode community like in other known seep communities they differed from deep sea seeps in well oxygenated waters based on that they shared the dominant genera with the surrounding non seep sediments overlain by oxygen deficient bottom water the homogenizing effect of the oxygen minimum zone on the seep nematode assemblages and surrounding sediments was constant with increasing water depth and concomitant greater oxygen deficiency resulting in a loss of habitat heterogeneity

isotopic evidence for chemosynthesis based nutrition of macrobenthos the lightness of being at pacific methane seeps the importance of chemosynthetic nutritional pathways was examined for macrofaunal invertebrates 300 mm from methane seeps in the gulf of alaska 4 413 4 443 m on the oregon margin 590 m and on the northern california slope eel river margin 520 m by use of natural abundance stable isotopic data seep macrofauna exhibited lighter d 13 c and d 15 n values than those in nonseep sediments but isotopic signatures varied among seep sites macrofaunal isotopic signatures indicated chemosynthetically fixed carbon sources with a significant contribution from methane derived carbon mdc in macrofauna from sediments of pogonophoran fields averaged 13

viscoelastic stress triggering of the 1999 hector mine earthquake by the 1992 landers earthquake the 1999 mw 7 1 hector mine earthquake occurred only 20 km from the 1992 mw 7 3 landers earthquake fault the close spacing between the two earthquakes suggests that the landers earthquake trigger the hector mine event based on an elastic half space model scientists from the usgs scec and cdmg 2000 have found a negative coulomb stress change at the hector mine hypocenter due to landers this negative stress change is inconsistent with the hypothesis of static stress triggering in this paper i show evidence of stress triggering of the hector mine earthquake by landers due to a process governed by viscoelastic flow in the lower crust this visoelastic flow has produced broad scale postseismic rebound observed by gps and insar measurements the result of this study is that viscoelastic flow has significantly modified the regional stress field in the mojave desert after the landers earthquake the evolving stress field including viscoelastic flow has gradually moved the coulomb stress change at the hector mine hypocenter to a positive level the increase in coulomb stress exceeded 1 bar right before the hector mine earthquake bringing the hector mine ruptures to the proximity of catastrophic failure

origin of elevated sediment permeability in a hydrothermal seepage zone eastern flank of the juan de fuca ridge and implications for transport of fluid and heat elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the juan de fuca ridge consolidation and permeability tests of fine grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site primarily hemipelagic have 10 15 higher porosity and 10 higher permeability at a given depth and 10 100 lower hydraulic impedance for the 40 m thick sediment column the correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result from fluid flow but are intrinsic properties of the hemipelagic sediment on the basis of measured sediment properties fluid pressure at the top of basement is 5 kpa greater than hydrostatic a simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops 4 20 km suggests that local basement permeability is between 3 10 13 and 3 10 11 m2 within the range measured in nearby boreholes the measured sediment properties combined with other published data indicate that a few tens of meters of fine grained terrigenous hemipelagic or calcareous marine sediment effectively seal the basement aquifer whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow the large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

crustal and uppermost mantle structure under southern california the travel times of pg and pn arrivals are used together as a new approach to estimate the station delays lateral velocity variations in the upper crust and upper mantle crustal thickness variations and pn anisotropy in the upper mantle under southern california the main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from pn arrivals significant negative station delays are found in the colorado desert san jacinto mountains san gabriel mountains central mojave desert and from the southern ventura basin to santa cruz island in addition to the positive delays associated with well known sedimentary basins lateral velocity variations in the upper crust exceed a few percent only in the coso ranges these variations show no clear association with surface faulting they also show little resemblance to the upper mantle velocity variations which have fast velocity under the mojave and near the offshore area and slow velocity in most other areas the southern california moho shows large variations large crustal thinning occurs near the salton sea northwest of the white wolf fault in the coso range near san diego and offshore while the san bernardino mountains western granite mountains and eastern mojave south of the ludlow fault are underlain by roots a significant amount 3 of pn anisotropy is present in the uppermost mantle with a fast direction n 75 4 w this could be explained very well by averaging the pn anisotropy estimates east and west of the san andreas

heat flow and fluid flow regime in the western nankai accretionary prism the thermal structure of the western nankai subduction zone has been studied through surface heat flow measurements with conventional geothermal probes and estimation of heat flow from depths of gas hydrate bottom simulating reflectors temperature and thermal conductivity measurements at odp site 808 located at the toe of the nankai accretionary prism added important information on any variation in heat flow with depth and thus on thermal effects of pore fluid flow in the prism surface heat flow is generally higher than 130 mw m 2 on the floor of the nankai trough such high heat flow cannot be explained simply by conductive cooling of the subducting shikoku basin lithosphere recent closely spaced probe measurements revealed that the heat flow on the trough floor shows local variation with particularly high values near the deformation front the heat flow distribution suggests that warm fluid flows seaward possibly along the decollement and locally rises toward the seafloor along some channels resulting in both the regional and local high heat flow anomalies contrary to this inference no active localized fluid flow was detected at site 808 the temperatures measured at six depths down to 347 m below seafloor mbsf just below the frontal thrust can be well explained by conductive heat flow of 125 130 mw m2 no geochemical anomaly associated with present day pore fluid movement was observed in cores recovered from the frontal thrust or the decollement on the other hand the high heat flow at site 808 requires some heat source below 350 mbsf there is a possibility that the fluid flow in the nankai accretionary prism is localized and transient and thus was not detected at this single drill site

earthquake refraction profiles of the root of the sierra nevada we examine the seismic structure of the sierra nevada using records of nine earthquakes and one explosion in and near the sierra recorded on stations in the sierra we first interpret travel times from these paths which are confined to a single tectonic block in terms of one dimensional structures the most nearly reversed pair of earthquakes the 1966 truckee and 1983 durrwood meadows earthquakes share refracted pn arrival times corrected to surface focus along a line t 8 75 0 25 8 0 suggesting that a nearly flat layer of 8 0 km s mantle material lies at depths of 46 48 km first arrivals from these events do not constrain velocities from 30 to 45 km depth secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6 9 and 7 8 km s the presence of this 7 x km s layer can help to explain previous contradictory observations the 7 x km s layer could be interpreted as either the mafic bottom of a silicic mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to cenozoic arc volcanism or basin and range spreading arrivals at stations in the foothills and the crest of the sierra cannot be fit with a single longitudinal structure indicating a lateral variation of velocity structure these variations support previously inferred variations of lithospheric structure with higher velocity thinner crust to the west beneath the sierran foothills and slower velocity crust or possibly upper mantle beneath the high mountains in the eastern sierra rapid changes in arrival times between stations separated by short distances in the eastern sierra suggest that a sharp boundary exists between the sierra and the basin and range at moho depths we also present fresh evidence of the asymmetry of the root of the sierra wherein arrivals from earthquakes on the west of the sierra are delayed within the sierra and return to original values in the basin and range while arrivals from earthquakes and explosions from the sierra into the great valley we suggest that if the 7 x km s material occurs in a wedge above the moho then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8 0 km s moho while those from the east may be entering the root along a 7 x km s layer that is near the depth of the basin and range moho

t phase observations in global seismogram stacks the t phase conversion of acoustic to seismic energy is typically observed as a high frequency wave train at hydrophones or coastal seismic stations here we show that the t phase can be observed in broadband waveform stacks of 5200 earthquakes recorded by the global seismic network to enhance the phase arrivals in stacks we apply short time window average over long time window average filtering to individual traces before stacking although the t phase arrival is visible in stacks from seismograms filtered at 0 5 5 hz it appears much stronger at higher frequencies 2 8 hz and is further enhanced by only stacking seismograms from oceanic paths stacking only subsets of the data depending on continental path lengths on the receiver side shows that the t phase can be observed at stations up to 4 inland from the coast and changes in the t phase arrival time correspond to reasonable crustal velocities

hydrothermal circulation in subducting crust reduces subduction zone temperatures most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment the decollement and wedge likely transports only a minor amount of heat we model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can have a great influence on subduction zone temperatures fractured basaltic basement has permeability several orders of magnitude higher than a typical decollement allowing fluid circulation to redistribute and extract heat from a subduction zone we simulate systems with upper basaltic basement permeability ranging from 10 13 to 10 10 m2 in addition we incorporate the effect of permeability reduction within the basaltic basement as it is subducted the models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport with continuous sediment cover heat is extracted from under the margin wedge to the trench in models where faulted ocean crust exposes high permeability basement to the ocean floor cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models hydrothermally cooled ocean crust also acts to slow thermally controlled diagenetic reaction progress within subducting sediment

simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern california we determined a detailed three dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern california using local and teleseismic data simultaneously we used 131 372 p wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the caltech us geological survey southern california seismic network compared with previous local and teleseismic studies the station coverage is considerably improved and the number of data used is greatly increased in the present study this together with the local and teleseismic joint inversion approach results in a unified three dimensional 3 d p wave velocity model of the crust and mantle in southern california with a higher resolution than the previous models the result obtained shows very heterogeneous structures in the crust and upper mantle in southern california shallow crustal structures correlate well with surface geological features sedimentary basins such as the los angeles basin ventura basin and santa maria basin are imaged well as low velocities and batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities the velocity in the crust is generally low in the mojave desert the crustal velocities change abruptly across major faults such as the san andreas san jacinto elsinore and garlock faults in the upper mantle three major anomalies are found beneath the transverse ranges salton trough and southern sierra nevada the transverse ranges feature appears as a curtain like east trending and north dipping high velocity anomaly with a thickness of approximately 50 km and extends most deeply at its eastern end to about 250 km the salton trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth the southern sierra nevada feature is a slab like high velocity anomaly which extends to about 250 km the mantle velocity is generally low in the upper 100 200 km beneath the volcanic areas in southern california the seismic velocity anomalies in the mantle beneath southern california are considered to result from small scale mantle convection

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

mapping the crustal thickness in southwest japan using moho reflected waves in this work we have tried to detect and collect later phases associated with the moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest sw japan we first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in sw japan taking the advantage of the high quality and great quantity of hi net waveform data now available throughout the entire japan islands we have detected 1659 moho reflected phases pmp and sms from shallow events in sw japan we estimated the crustal thickness in the study area using travel time differences between these later phases and the first p and s arrivals our results show that the moho is deep in chubu district and becomes shallower along the coastlines of the japan sea and the pacific ocean a thinner crust appears in osaka bay and awajii island where the 1995 kobe earthquake m7 2 occurred

methane seepage effects on biodiversity and biological traits of macrofauna inhabiting authigenic carbonates abstract authigenic carbonate rocks at methane seeps are recognized as hosting diverse and abundant invertebrate assemblages with potential forcing from fluid seepage and hydrography mensurative studies of carbonate macrofauna 0 3 mm at hydrate ridge or revealed little effect of water depth and overlying oxygenation at 600 m and 800 m but a large influence of seepage activity on density taxonomic composition diversity and biological traits feeding lifestyle motility size and calcification rocks exposed to active seepage had 3 4 higher total macrofaunal densities than under inactive conditions assemblages exhibited higher species richness and reduced evenness greater dominance under active seepage than inactive conditions but no difference in h or rarefaction diversity actively seeping sites were characterized by errant motile bacterial grazing small and medium sized heavily calcified species whereas inactive sites exhibited a greater diversity of feeding modes and more burrowers sessile large and lightly calcified species active rocks supported more exogonid syllidae ampharetid and cirratulid polychaetes provannid snails pyropeltid limpets nemerteans and sponges whereas inactive rocks supported higher densities of ophiuroids isopods gammarid amphipods hydroids typosyllis syllidae and tanaids transplant experiments in which rocks were transferred between active and inactive sites at hydrate ridge north 600 m revealed that assemblages respond within 13 months to increase or cessation of seepage taking on the feeding size and calcification characteristics of the background fauna at the new site lifestyles and motility patterns shifted more slowly as the sessile attached species did not track seepage as quickly provannid snails and pyropeltid limpets rapidly colonized rocks transplanted to active sites and disappeared when transplanted to inactive sites given the known variability of fluid fluxes and rapid community response a mosaic of communities changing in space and time is hypothesized to generate the relatively high species diversity at methane seeps

regional heat flow variations across the sedimented juan de fuca ridge eastern flank constraints on lithospheric cooling and lateral hydrothermal heat transport seafloor heat flow has been estimated continuously along a transect on the sedimented eastern flank of the juan de fuca ridge to provide constraints on the scale and rate of lateral heat and fluid transport in the upper igneous crust and on the total flux from the young lithosphere beneath the profile extends from 20 km east of the ridge axis where turbidite sediments lap onto the oceanic crust at an age of less than 1 ma and ends 110 km east of the axis over 3 6 ma crust estimated heat flow is derived from the combination of the depth to igneous basement determined from seismic reflection profiles and basement temperatures measured in nine ocean drilling program boreholes along the transect the strategy is based on the knowledge that in this young area upper basement temperatures are locally homogenized by vigorous hydrothermal circulation and hence vary smoothly between drilling sites at the young end of the transect near the region of extensive outcrop heat flow is only 15 of that predicted for lithosphere of this age heat flow increases systematically to the east reaching the full predicted level about 20 km from the point of sediment basement onlap the anomaly can be accounted for by mixing and or eastward flow of cool water in the upper igneous crust beneath the sediment at a darcian rate of the order of 1 m yr a geochemical signature of fresh seawater in basement is observed many tens of kilometers farther to the east although the thermal signature is no longer present along the remainder of the transect the heat flow conforms very well to that predicted by simple boundary layer cooling theory once the thermal effects of rapid sedimentation on this ridge flank are accounted for

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

stable isotope signatures and methane use by new zealand cold seep benthos the carbon isotopic composition of seep faunal tissue represents a time integrated view of the interaction between biology and the biogeochemical gradients within the environment here we provide an initial description of carbon and nitrogen stable isotope signatures of dominant symbiont bearing megafauna and heterotrophic mega and macrofauna from 10 methane seep sites on the continental margin of the north island of new zealand 662 1201 m water depth isotopic signatures suggest that sulfide oxidation supports symbiont bearing taxa including solemyid and vesicomyid bivalves and methanotrophic symbionts are present in the seep mussel bathymodiolus sp multiple species of frenulata siboglinidae are present and have a range of isotopic values that are indicative of both thiotroph and methanotroph based nutrition isotopic composition of the tubeworm lamellibrachia sp varied by 23 3 among individuals although there was no consistent difference among sites variation in methane use by heterotrophic fauna appears to reflect the availability of hard vs soft substrate macrofauna on hard substrates had high 13c signatures reflecting consumption of photosynthetic derived organic matter two unique biotic assemblages were discovered to be fueled largely by methane a hard substrate multi phyla sponge associated community inhabiting the sponge pseudosuberites sp and a soft sediment assemblage dominated by ampharetid polychaetes isotope signatures yield estimates of 38 100 and 6 100 methane derived carbon in sponge associates and ampharetid bed macrofauna respectively these estimates are comparable to those made for deeper methane seeps at the florida escarpment 3290 m and kodiak alaska seeps 4445 m the overall high use of methane as a carbon source by both symbiont bearing and heterotrophic fauna suggests that new zealand methane seeps are an ideal model system to study the interaction among metazoans bacteria archaea and their resulting effect on methane cycles

a comparative analysis of deformable layer tomography and cell tomography along the larse lines in southern california summary we conducted a comparative analysis of the new deformable layer tomography dlt with the traditional fixed in space cell tomography along three 2 d crustal profiles of p wave velocities in southern california using first arrivals from local earthquakes and two los angeles regional seismic experiment larse seismic lines the dlt method inverts for the depth varying geometry of velocity interfaces which include major velocity discontinuities near the top and bottom of the crust and boundaries between velocity layers in the middle crust in regions characterized by large lateral velocity changes the dlt method may improve the depth resolution of major velocity interfaces compared to traditional cell tomography using fixed in space cells or grids we also tested the use of crustal thickness estimates from a receiver function study for constraining the depth range of the moho geometry tests indicate that in areas of sufficient ray coverage the depth resolution of the dlt for major velocity interfaces is usually less than 1 km in the upper crust and less than 3 km in the lower crust our 2 d dlt models fit the first arrival data significantly better than three published 3 d cell tomography models along the profiles the configurations of basin edges and undulating velocity discontinuities are well delineated in the dlt models and indicative of the crustal architecture of the region

nitrogen cycling in a deep ocean margin sediment sagami bay japan on the basis of in situ no microprofiles and chamber incubations complemented by laboratory based assessments of anammox and denitrification we evaluate the nitrogen turnover of an ocean margin sediment at 1450 m water depth in situ no profiles horizontally separated by 12 mm reflected highly variable no penetration depths no consumption rates and nitrification on average the turnover time of the pore water no pool was 0 2 d net release of nh z during mineralization 0 95 mmol m 22 d21 sustained a net efflux of ammonia 53 nitrification 24 and anammox activity 23 the sediment had a relatively high in situ net influx of no 1 44 mmol m22 d21 that balanced the n2 production as assessed by onboard tracer experiments n2 production was attributed to prokaryotic denitrification 59 anammox 37 and foraminifera based denitrification 4 anammox thereby represented an important nutrient sink but the n2 production was dominated by denitrification despite the fact that no stored inside foraminifera represented 80 of the total benthic no pool the slow intracellular no turnover that on average sustained foraminifera metabolism for 12 52 d contributed only to a minor extent to the overall n2 production the microbial activity in the surface sediment is a net nutrient sink of 1 1 mmol n m 22 d 21 which aligns with many studies performed in coastal and shelf environments continental margin areas can act as significant n sinks and play an important role in regional n budgets

three dimensional thermal model of the costa rica nicaragua subduction zone the thermal structure of a subduction zone controls many key processes including subducting plate metamorphism and dehydration the megathrust earthquake seismogenic zone and volcanic arc magmatism here we present the first three dimensional 3d steady state kinematic dynamic thermal model for the costa rica nicaragua subduction zone the model consists of the subducting cocos plate the overriding caribbean plate and a viscous mantle wedge in which flow is driven by interactions with the downgoing slab the cocos plate geometry includes along strike variations in slab dip which induce along strike flow in the mantle wedge along strike flow occurs primarily below costa rica with a maximum magnitude of 4 cm year 40 of the convergence rate for a mantle with a dislocation creep rheology an isoviscous mantle has lower velocities along margin flow causes temperatures variations of up to 80 c in the subducting slab and mantle wedge at the volcanic arc and backarc the 3d effects do not strongly alter the shallow 35 km thermal structure of the subduction zone the models predict that the megathrust seismogenic zone width decreases from 100 km below costa rica to just a few kilometers below nicaragua the narrow width in the north is due to hydrothermal cooling of the oceanic plate these results are in good agreement with previous 2d models and with the rupture area of recent earthquakes in the models along strike mantle flow is induced only by variations in slab dip with flow directed toward the south where the dip angle is smallest in contrast geochemical and seismic observations suggest a northward flow of 6 19 cm year we do not observe this in our models suggesting that northward flow may be driven by additional factors such as slab rollback or proximity to a slab edge slab window such high velocities may significantly affect the thermal structure especially at the southern end of the subduction zone in this area 3d models that include slab rollback and a slab edge are needed to investigate the mantle structure and dynamics

crustal heterogeneity beneath southwest japan estimated from direct and moho reflected waves abstract in this study we have used later phases reflected at the moho discontinuity in addition to direct arrival time data to study the 3 d crustal velocity structure beneath southwest sw japan we have collected more than 6000 direct and moho reflected phases and used them to obtain p and s wave velocity structures in the study area our results revealed significant low velocity anomalies beneath the active and quaternary volcanoes in sw japan results obtained using each data set p pmp s and sms are generally consistent with each other and all showed low velocity anomalies beneath most of the active and quaternary volcanoes western kii peninsula and eastern shikoku ray path analysis showed that the data set we used has a good ray criss crossing that is sufficient to accurately locate the velocity anomalies in the study area in addition results of the checkerboard resolution test indicate that these structures are generally well recovered and that adding later phases could significantly improve the resolution scale of the obtained tomographic images in comparison with those from first arrivals alone

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

application of stacking and inversion techniques to three dimensional wide angle reflection and refraction seismic data of the eastern alps summary we present new methods for the interpretation of 3 d seismic wide angle reflection and refraction data with application to data acquired during the experiments celebration 2000 and alp 2002 in the area of the eastern alps and their transition to the surrounding tectonic provinces bohemian massif carpathians pannonian domain dinarides data was acquired on a net of arbitrarily oriented seismic lines by simultaneous recording on all lines of seismic waves from the shots which allows 2 d and 3 d interpretations much 80 of the data set consists of crossline traces low signal to noise s n ratio in the area of the young orogens decreases the quality of travel time picks in these seismically heterogeneous areas it is difficult to assign clearly defined arrivals to the seismic phases in particular on crossline record sections in order to enhance the s n ratio signal detection and stacking techniques have been applied to enhance the pg pn and pmp phases further inversion methods have been developed for the interpretation of war r data based on automated 1 d inversion pg and the application of the delay time concept pn the results include a 3 d velocity model of the crust based on pg waves time and depth maps of the moho and a pn velocity map the models based on stacked data are robust and provide a larger coverage than models based on travel time picks from single fold unstacked traces but have relatively low resolution especially near the surface they were used as the basis for constructing models with improved resolution by the inversion of picks from single fold data the results correlate well with geological structures and show new prominent features in the eastern alps area and their surrounds the velocity distribution in the crust has strong lateral variations and the moho in the investigation area appears to be fragmented into three parts

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

along strike variations in underthrust sediment dewatering on the nicoya margin costa rica related to the updip limit of seismicity 1 along the costa rican subduction zone offshore the nicoya peninsula an offset in the updip limit of seismicity coincides with a transition between subduction of warm crust generated at the cocos nazca spreading center and cool crust formed at the east pacific rise we evaluate whether the observed difference in thermal state of incoming crust would result in significant differences in sediment dehydration reaction progress along strike thought to be a control on the updip limit of seismicity we combine thermal models with models of dehydration reaction kinetics for opal and smectite to estimate the distribution of diagenetic fluid sources the modeled distribution of diagenetic fluid sources mimics the pattern of the updip limit of seismicity seismicity begins 15 20 km landward of most of the smectite to illite transition this suggests that the location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of smectite to illite dehydration

deep sea habitat heterogeneity influence on meiofaunal communities in the gulf of guinea to estimate the degree of spatial heterogeneity of benthic deep sea communities we carried out a multiple scale from m s to 200 km investigation in the congo angola margins equatorial west african margin 3150 4800 m in which we examined the metazoan meiofauna at a variety of habitats along the congo channel system and in the associated cold seep we investigate the structure density vertical distribution patterns in the sediment and biomass of meiofaunal communities in the gulf of guinea and how they are controlled by hydrologic and biogeochemical processes the meiofaunal communities in the gulf of guinea were shaped by heterogeneous conditions on the margin and reflect the multiple scale spatial variability that corresponds with the different identified habitats the two control sites located at 100 km away from the canyon were inhabited by very dense and the most diverse meiobenthic communities similar meiobenthic communities inhabited the transition zone between the canyon and the cold seep sites located along the congo channel were obviously affected by the local high velocity bottom currents and unstable sedimentary conditions in this active submarine system extremely low meiobenthic densities and very low proportions in the most surficial sediment layers provided evidence for recently highly disturbed sediments at these sites the remote operated vehicle rov victor 6000 provided images of the cold seep showing a patchy distribution of several types of patchy distributed megafaunal communities dominated by three key symbiotic taxa mytilidae vesicomyidae and siboglinidae these cold seep sediments were colonised by a unique meiobenthic community characterised by a high small scale m s patchiness low species richness and the prominent dominance of two large sized nematode species sabatieria mortenseni which is a cosmopolitan nematode known from littoral habitats and an undescribed desmodora species the high individual body weight of s mortenseni and its dominance at the cold seep site resulted in a significantly higher nematode biomass at the seep compared to the surrounding sites in addition the vertical nematode profiles with maximum proportions in subsurficial layers points to a chemosynthesis based meiobenthic community in this cold seep in contrast to the phytodetritus based communities at the control sites and at the transition zone

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

large amplitude moho reflections sms from landers aftershocks southern california closely spaced aftershocks of the 28 june 1992 landers earthquake m w 7 3 were used to make event record sections that show the transverse components of s and sms arrivals at a distance of 70 to 170 km for the data recorded toward the north in the mojave desert large sms phases are observed with amplitudes 2 to 5 times greater than the direct s for similar distances to the south the sms arrival is comparable to or smaller than the s comparisons to synthetic seismograms indicate that the large amplitude sms phases are produced by the simple crustal structure of the mojave desert that allows a large moho reflection in contrast the more complex geologic structure to the south partitions the seismic energy into a more complicated set of seismic phases so that the moho reflection is diminished in amplitude the large sms phases observed in the mojave enhance the overall ground motions by a factor of 2 to 3 this suggests that when damaging earthquakes occur in other regions of simple crustal structures moho reflections will produce amplified strong motions at distance ranges around 100 km depending on the local structure

the influence of critical moho reflections on strong ground motions recorded in san francisco and oakland during the 1989 loma prieta earthquake the amplitudes of strong ground motions from the loma prieta earthquake recorded in the san francisco and oakland areas exceeded the levels predicted by standard empirical attenuation relations preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust these reflections which are identified on the basis of their arrival times and phase velocity and by comparison with simulated accelerograms were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust these motions were further amplified presumably by impedance contrast effects at soft soil sites in san francisco and oakland the effect of the critical reflections in amplifying peak accelerations of the loma prieta earthquake in the san francisco and oakland regions was as large as the effect of soft soil site conditions focal depth has an important influence on strong motion attenuation at distances beyond about 40 km and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in san francisco and oakland and the conclusions drawn from that explanation

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

forward and inverse three dimensional p wave velocity models of the southern california crust we construct a three dimensional p wave velocity model of the southern california crust by combining existing one dimensional models each describing a region defined by surface geology and calibrate the model with travel times from three explosions the model is expressed as blocks each of a given slowness the variance of the p wave travel time residuals of 1000 earthquakes relocated in and near the los angeles basin where the model is most detailed is half that of the catalog locations in the standard one dimensional model for southern california starting from the forward model we invert 21 000 p wave arrivals from earthquakes for hypocenters and block slownesses using the technique of roecker 1981 the variance of these p wave travel time residuals decreases 47 during the inversion many of the blocks representing the upper crust and midcrust are well sampled and well resolved the resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions for example the velocity structure of the los angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the peninsular ranges moho is between 26 and 32 km depth in contrast the ventura basin has mostly slower sediments above a deeper higher velocity basement compared to catalog locations relocations in the final three dimensional model of 98 ml 4 earthquakes throughout southern california tend to deepen below sediment filled valleys and basins shallow in regions without sedimentary cover and have a 44 lower p wave travel time residual variance

food web structure of seep sediment macrobenthos from the gulf of mexico abstract the slope environment of the gulf of mexico gom supports dense communities of seep megafaunal invertebrates that rely on endosymbiotic bacteria for nutrition seep sediments also contain smaller macrofaunal invertebrates whose nutritional pathways are not well understood using stable isotope analysis we investigate the utilization of chemosynthetically fixed and methane derived organic matter by macrofauna biological sampling was conducted in three lower slope gom seep environs green canyon gc852 1428 m atwater valley at340 2230 m and alaminos canyon ac601 2384 m infaunal 13 c and 15 n exhibited a broad range of values most infauna appeared to be heterotrophic although several taxa had very light 15 n and 13 c values indicating possible reliance on chemoautotrophic symbioses the lightest 13 c and 15 n values were observed in nematodes 13 c 54 6 0 1 15 n 6 1 0 2 and one gastropod 13 c 54 1 15 n 1 1 from green canyon mixing model results indicated that sulfur oxidizing beggiatoa may be an important food source for seep infauna the rate of utilization ranged from 60 to 100 at green canyon and atwater valley the overall range in isotope values was similar across the three sites suggesting that biogeochemical processes may be very similar in these geographically distinct areas

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

crustal structure of the borderland continent transition zone of southern california adjacent to los angeles we use data from the onshore offshore component of los angeles region seismic experiment larse to model the broad scale features of the midcrust to upper mantle beneath a north south transect that spans the continental borderland in the los angeles california region we have developed an analysis method for wide angle seismic data that consists primarily of refractions lacks near offset recordings and contains wide gaps in coverage although the data restrict the analysis to the modeling of broad scale structure the technique allows one to explore the limits of the data and determine the resolving power of the data set the resulting composite velocity model constrains the crustal thickness and location and width of the continent borderland transition zone we find that the mid to lower crust layer velocities of the inner borderland are slightly lower than the corresponding layers in the average southern california crust model while the upper mantle velocity is significantly higher the data require the moho to deepen significantly to the north we constrain the transition zone to initiate between the offshore slope and the southwest los angeles basin if the inner borderland crust is 22 km thick then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest los angeles basin and have a width of 20 25 km the strong coherent and continuous pn phase suggests the moho is coherent and laterally continuous beneath the inner borderland and transition zone the inner california borderland seems to be modified and thickened oceanic crust with the oceanic upper mantle intact beneath it

remarkably consistent thermal state of the south central chile subduction zone from 36 s to 45 s delineating rupture areas of subduction zone earthquakes is necessary for understanding the controls on seismic and aseismic slip for the largest recorded earthquake the 1960 chile event with moment magnitude 9 5 the rupture area is only loosely defined due to limitations in the global seismic network at the time the rupture extends 900 km along strike coastal deformation is consistent with either a constant rupture width of 180 200 km along the entire length or a narrower 115 km rupture in the southern half a southward narrowing of the seismogenic zone has been hypothesized to result from warming of the subduction zone to the south where the subducting plate is younger we present results of thermal models at 36 s 38 s 43 s and 45 s to examine potential along strike changes in thermal state models most consistent with observed surface heat flux include fluid circulation in the oceanic crust that advects heat to the ocean this ventilated hydrothermal circulation preferentially cools transects with young subducting lithosphere frictional heating preferentially warms transects with older subducting lithosphere the combined effects of frictional heating and hydrothermal circulation increase decollement temperatures in the 36 s and 38 s transects by up to 155 c and decrease temperatures in the 45 s transect by up to 150 c in our preferred models decollement temperatures 200 km landward of the trench in all four transects are 350 400 c this is consistent with a constant 200 km wide seismogenic zone for the 1960 mw 9 5 rupture with decreasing slip magnitude in the southern half of the rupture

effect of viscoelastic postseismic relaxation on estimates of interseismic crustal strain accumulation at yucca mountain nevada 1 we estimate the long term crustal strain rate at yucca mountain ym nevada from gps velocities taking into account viscoelastic relaxation following recent earthquakes to remove bias associated with transient deformation the ym data reveal postseismic relaxation in time series non linearity and geographic variation of the transient signal from the data we estimate best fitting lower crust and upper mantle viscosities of 1019 5 pa s and 1018 5 pa s respectively once the relaxation model predictions are subtracted from the data the long term shear strain accumulation rate is between 16 3 and 25 1 nanostrains year ns yr to 99 confidence a range much larger than the formal uncertainties from gps measurement we conclude that 1 a maxwell viscoelastic model cannot explain all the deformation observed at ym 2 uncertainty in viscosities dominates uncertainty in ym strain rates and 3 the effects of large recent earthquakes must be accounted for in seismic hazard studies using gps

variations in bioturbation across the oxygen minimum zone in the northwest arabian sea abstract oxygen minimum zones are expected to alter substantially the nature rates and depths of bioturbation along continental margins yet these effects remain poorly studied using excess 210 pb profiles sediment x radiography and box core samples for macrofauna we examined bioturbation processes at six stations 400 700 850 1000 1250 and 3400 m deep along a transect across the oxygen minimum zone omz on the oman margin bottom water oxygen concentrations ranged from 0 13 ml l 1 at 400 m to 2 99 ml l 1 at 3400 m 210 pb mixed layer depth and bioturbation intensity db exhibited high within station variance and means did not differ significantly among stations however the mean mixed layer depth 4 6 cm for pooled omz stations 400 1000 m depths 0 13 0 27 ml l 1 bottom water oxygen was half that for stations from similar water depths along well oxygenated atlantic and pacific slopes 11 1 cm suggesting that oxygen stress reduced 210 pb mixing depth on the oman margin modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom water oxygen concentration from the edge to the core of the oman omz spearman s rho 0 89 p 0 02 suggesting that these parameters are useful proxies for bottom water oxygen concentrations under dysaerobic conditions in contrast neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen related patterns along the transect reduced 210 pb mixing depth within the oman margin omz appeared to result from a predominance of surface deposit feeders and tube builders within this zone rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass the number of burrow types per station was highly correlated with macrofaunal species diversity suggesting that burrow diversity may be a good proxy for species diversity in paleo dysaerobic assemblages we conclude that bottom water oxygen concentrations of 0 13 0 27 ml l 1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins

global analysis of the effect of fluid flow on subduction zone temperatures 1 knowledge of the controls on temperature distributions at subduction zones is critical for understanding a wide range of seismic metamorphic and magmatic processes here we present the results of 220 thermal model simulations covering the majority of known subduction zone convergence rates incoming plate ages and slab dips we quantify the thermal effects of fluid circulation in the subducting crust by comparing results with and without advective heat transfer in the oceanic crustal aquifer we find that hydrothermal cooling of a subduction zone is maximized when the subducting slab is young slowly converging steeply dipping and the crustal aquifer is ventilated near the trench incoming plate age is one of the primary controls on the effectiveness of advective heat transfer in the aquifer and the greatest temperature effects occur with an incoming plate 50 ma the thermal effects of fluid circulation decrease dramatically with increasing age of the incoming plate temperatures in the cascadia nankai southern chile colombia ecuador mexico and solomon islands subduction zones are likely strongly affected by fluid circulation for these systems only thermal models of cascadia and nankai have included fluid flow in subducting crust

faunal responses to oxygen gradients on the pakistan margin a comparison of foraminiferans macrofauna and megafauna the pakistan margin is characterised by a strong mid water oxygen minimum zone omz that intercepts the seabed at bathyal depths 150 1300 m we investigated whether faunal abundance and diversity trends were similar among protists foraminiferans and gromiids metazoan macrofauna and megafauna along a transect 140 1850 m water depth across the omz during the 2003 intermonsoon march may and late post monsoon august october seasons all groups exhibited some drop in abundance in the omz core 250 500 m water depth o2 0 10 0 13 ml l 1 4 46 5 80 m but to differing degrees densities of foraminiferans 63 m were slightly depressed at 300 m peaked at 738 m and were much lower at deeper stations foraminiferans 300 m were the overwhelmingly dominant macrofaunal organisms in the omz core macrofaunal metazoans reached maximum densities at 140 m depth with additional peaks at 850 940 and 1850 m where foraminiferans were less abundant the polychaete linopherus sp was responsible for a macrofaunal biomass peak at 950 m apart from large swimming animals fish and natant decapods metazoan megafauna were absent between 300 and 900 m o2 0 2 ml l 1 8 92 m the progressively deeper abundance peaks for foraminiferans 63 m linopherus sp and ophiuroids probably represent lower omz boundary edge effects and suggest a link between body size and tolerance of hypoxia macro and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa indicating that the lower part of the omz is also a region of rapid faunal change species diversity was depressed in all groups in the omz core but this was much more pronounced for macrofauna and megafauna than for foraminiferans oxygen levels strongly influenced the taxonomic composition of all faunal groups calcareous foraminiferans dominated the seasonally and permanently hypoxic sites 136 300 m agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were 0 13 ml l 1 5 80 m polychaetes were the main macrofaunal taxon within the omz calcareous macrofauna and megafauna molluscs and echinoderms were rare or absent where oxygen levels were lowest the rarity of larger animals between 300 and 700 m on the pakistan margin compared with the abundant macrofauna in the omz core off oman is the most notable contrast between the two sides of the arabian sea this difference probably reflects the slightly higher oxygen levels and better food quality on the western side

deep structure of southern california abstract we used 214 210 p wave arrival times from 7536 local earthquakes and 16 470 travel time residuals from 332 teleseismic events recorded by the southern california seismic network to determine a detailed three dimensional 3d p wave velocity structure of the crust and mantle down to 600 km depth beneath southern california in this study we have taken into account the moho topography under this region determined by a previous receiver function study we found that the undulations of the moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle when the moho topography is taken into account ray paths and travel times can be computed more accurately the tomographic images show a very heterogeneous structure in the crust and upper mantle under southern california the velocity structure in the shallow depth correlates well with the surface geological features three major anomalies in the upper mantle are revealed clearly beneath the southern sierra nevada salton trough and transverse ranges compared with the previous result both the shape and size of the three anomalies show some differences our result revealed two prominent low velocity low v anomalies above the high velocity high v anomalies in the upper mantle under southern sierra nevada and the transverse ranges we consider that the upper mantle anomaly under the transverse ranges was formed through convergence and sinking of the entire subcrustal lithosphere vertical motions and removal of the dense crust root caused the high v anomaly under the southern sierra nevada the prominent low v anomalies above the two high v anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs the salton trough low is the response to the lithospheric extension when the pacific plate was rifted away from the north american plate

abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the cocos plate 1 new thermal data from 18 24 ma lithosphere on the cocos plate delineate contrasting subsurface thermal conditions in adjacent sections of crust heat flow through seafloor created at the east pacific rise is generally suppressed by 70 relative to conductive lithospheric cooling models whereas heat flow through adjacent similarly aged lithosphere generated at the cocos nazca spreading center is consistent with these models the transition between thermal regimes is remarkably abrupt only 2 5 km wide indicating a shallow hydrothermal origin the transition is more closely associated with differences in the distribution of basement outcrops than with tectonic boundaries demonstrating the importance of the former in extracting heat from the lithosphere on a regional basis