tremor genic slow slip regions may be deeper and warmer and may slip slower than non tremor genic regions slow slip events sses are observed worldwide and often coincide with tectonic tremor notable examples of sses lacking observed tectonic tremor however occur beneath k lauea volcano hawaii the boso peninsula japan near san juan bautista on the san andreas fault california and recently in central ecuador these sses are similar to other worldwide sses in many ways e g size or duration but lack the concurrent tectonic tremor observed elsewhere instead they trigger swarms of regular earthquakes we investigate the physical conditions that may distinguish these non tremor genic sses from those associated with tectonic tremor including slip velocity pressure temperature fluids and fault asperities although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions slip velocities of sses at k lauea volcano 10 6 m s and boso peninsula 10 7 m s are among the fastest sses worldwide k lauea volcano the boso peninsula and central ecuador are also among the shallowest sses worldwide and thus have lower confining pressures and cooler temperatures in their respective slow slip zones fluids also likely contribute to tremor generation and no corresponding zone of high vp vs has been noted at k lauea or boso we suggest that the relatively faster slip velocities at k lauea volcano and the boso peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip while different conditions produce slower sse velocities elsewhere and trigger tectonic tremor

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

the first direct measurements of upper oceanic crustal compressional wave attenuation the first direct measurement of compressional wave attenuation of the uppermost 650 m of oceanic crust was performed using data recorded by seafloor hydrophones and large 56 116 kg deep explosive sources the site was 13 km east of the southernmost juan de fuca ridge on crust 0 4 m y old spectral ratios were performed between bottom refracting waves and direct water waves adjusted for spreading losses and transmission coefficient losses several tests of the data were performed demonstrating that attenuation is linearly related to frequency between 15 and 140 hz but frequency independent components of attenuation are also evident values of compressional wave q cluster between 20 and 50 and do not show any systematic variation with depth over 650 m the attenuation results also indicate the presence of heterogeneities within the crust as the solutions for each receiver s data set are significantly different no evidence for azimuthal variations of attenuation are supported by the data although the data do not optimally sample a wide variation of azimuths our attenuation values are judged to be normal to higher than expected for the whole oceanic crust based upon comparisons to results from synthetic seismogram modeling by others and by modeling signal to noise ratios of typical seismic refraction profiles the results are consistent with recent laboratory measurements at ultrasonic frequencies for dry and saturated basalts at seafloor pressures and temperatures

margin architecture and seismic attenuation in the central costa rican forearc seismic attenuation across the central costa rican margin wedge is determined fromamplitude analysis ofwideangle seismic data travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench parallel profiles located 30 km p21 and 35 km p18 landward of the deformation front northeast of quepos plateau tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench a 1 1 5 km thick decollement zone with seismic velocities of 3 5 4 5 km s is sandwiched between the marginwedge and the subducting cocos plate for strike line p21 amplitude modeling indicates a qp value of 50 150 for the upper margin wedge with seismic velocities ranging from 3 9 km s to 4 9 km s along strike line p18 qp values of 50 150 are determined with velocities of 4 3 5 0 km s in the upper margin wedge increasing to 5 1 5 4 km s in the lower margin wedge quantitative amplitude decay curves support the observed upper plate qp values in conjunction with earlier results from offshore nicoya peninsula our study documents landward decreasing attenuation across the margin wedge consistent with a change in lithology from the sediment dominated frontal prism to the igneous composition of the forearc middle prism

a comparison of velocity and attenuation between the nicobar and bengal deep sea fans the compressional wave velocity and attenuation measurements from a deep source deep receiver seismic refraction experiment from the nicobar fan indian ocean are presented and compared to a similar set of measurements in the bengal fan the two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories travel time inversion of the data into a velocity depth model uses the tau zeta linear inversion scheme we present a revised reparameterization of the travel time data and improved error analysis for the inversion process although the velocity structures with depth are morphologically similar there exist discrete differences between the two stations in the nicobar fan the initial velocity assumed is 1 513 km s with an initial gradient of 2 32 s 1 the velocity increases smoothly with depth while the velocity gradient decreases rapidly to a constant 0 81 s 1 the bengal fan station showed the same trend although the velocity gradient at depth was only 0 67 s 1 the attenuation profiles are similar only in gross structure with the nicobar fan values of q 1 being a factor of 3 to 4 less than the bengal station the differences in velocity and attenuation structure can be attributed to a number of causes including porosity and sedimentation rates which are related to the distance from the sediment source and the presence of a normal fault near the nicobar fan site this fault may act as a conduit for water removal in the presence of overburden pressure it is believed that porosity changes due to whatever cause dominate the lateral and vertical variations of velocity velocity gradients and together with intrabed multiples attenuation in thick sedimentary sections

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

seawater sediment basalt interactions stable isotope h o and elemental fluxes within the ordovician volcano sedimentary sequence of erquy brittany france the ordovician volcano sedimentary succession of erquy northern brittany is made of immature sediments thermally metamorphosed at the contact of intruding basic sills pillow lavas constitute the upper part of the sequence the trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin this volcanic sequence was metamorphosed under low to moderate greenschist facies conditions in this study the direction and amplitude of chemical and isotopic fluxes in the basalt sediment water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized cationic thermometry on chlorites and isotopic thermometry on plagioclase chlorite pairs indicate closure metamorphic temperatures in the range 200 250 c for the basaltic sills stable isotope compositions of iron rich chlorites 18o 5 5 d from 60 to 50 and plagioclases 18o from 9 to 10 reveal that the source of the fluid was certainly seawater the 18o variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction clinopyroxene plagioclase ilmenite chlorite albite epidote quartz sphene that produced major element mobility at the scale of the volcanosedimentary sequence calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water rock ratios 1 were at sill sediment boundaries and within pillow lavas at the top of the pile the volcanosedimentary sequence of erquy was a net sink for na and a source for ca no mg uptake could be detected whereas the hydrothermal alteration of the sediments released fe si and k trapped by the volcanic rocks the 18o value of the fluid reacting with sills appears to have shifted no more than 4 after percolation at low temperature through immature sediments 18o 12 the erquy volcano sedimentary sequence represents a marine hydrothermal system dominated by low temperature exchange which allowed a general 18o enrichment of the volcanic rocks and a 18o depletion of sediments

crustal structure along the edge transect beneath the kodiak shelf off alaska derived from obh seismic refraction data summary along the edge alaska transect over the kodiak shelf off alaska a deep crustal seismic refraction experiment was carried out in 1994 using an airgun source with a high shot density and closely spaced ocean bottom hydrophones obhs these data clearly define a gently dipping plate boundary that gradually increases from 2 5d at the trench to 5d 250 km landwards of the trench beneath the shelf a landward increase of the velocities in the subducting oceanic crust was found the sedimentary upper plate beneath the continental slope can be divided into a young neogene accretionary prism beneath the lower slope and a more layered structure below the middle and upper slope where a body with elevated velocities 4 2 5 1 km s 1 in the lower part is interpreted as well consolidated sediment of possibly late eocene age acting as the backstop across the inner shelf between kodiak island and kenai peninsula near surface high velocities 6 0 km s 1 document a strong uplift 10 km of the mesozoic and palaeocene accretionary complex and furthermore suggest its seaward extension beneath the outer shelf the most striking feature along the edge transect is the thick low velocity zone lvz that coincides with the arched reflectors in the lower crust beneath the kodiak island kenai peninsula axis on the edge seismic reflection data this lvz is interpreted as indicating underplated low velocity rocks which contribute the same volume of new material as the frontal accretion to the continent the growth of which is rapid totalling 4200 km2 across the kodiak shelf since eocene times a calculation of the material balance shows that an underplating of underthrust sediment alone may not account for the huge volume of the underplated low velocity rocks we speculate that the underplating of continental fragment or more likely seamounts or plateau may be involved in continental growth

velocity and attenuation profiles in the monterey deep sea fan data obtained during a marine refraction experiment are used to estimate velocity and attenuation profiles in an area of thick sediments 2 5 3 km on the monterey deep sea fan a 20 element vertical hydrophone array was deployed at mid depth in 2800 m of water explosive sources set to detonate at 1830 m were launched at ranges between 3 5 and 37 km from the array estimates of velocity and attenuation as a function of depth are obtained from an analysis of the pressure time series generated by the explosive charges and received at the array the sediment is modeled as a horizontally layered laterally homogeneous medium a least squares solution is found for linear velocity gradients in each layer of the model velocity as a function of depth is obtained from these gradients attenuation coefficients in each layer of the model are found using a method of spectral ratios the results presented here valid for frequencies between 10 and 100 hz show an increase in attenuation to a depth of about 500 m

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

tectonic structure of the convergent pacific margin offshore costa rica from multichannel seismic reflection data the middle america trench between the cocos ridge and a well studied corridor off the nicoya peninsula has a more varied morphology and structure than previously reported the morphological positive features on the lower plate significantly affect the upper plate structure the cocos ridge has uplifted the margin opposite the osa peninsula northwest of cocos ridge numerous seamounts on the oceanic crust sculptured the margin as they subducted a seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted the greater portion of the trench sediment is subducted beneath a lower slope accretionary mass beneath the middle and upper slope is a margin wedge consisting of a high velocity rock with few internal reflections its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area overlying this surface is a low velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred eocene section beneath the shelf the shelf basement is composed of nicoya complex ophiolite with the same acoustic velocity similar structure and no apparent dividing geologic boundary with the margin wedge we favor a seaward continuation of the nicoya complex to the middle slope and emphasize the evidence for a non steady state tertiary tectonic history

low temperature alteration of the upper oceanic crust and the alkalinity budget of seawater abstract while the input of river alkalinity into seawater is relatively well known the complementary acidity production is poorly understood using the major element budget of seafloor alteration of the upper 500 m of 120 ma old oceanic crust at dsdp odp sites 417a 417d and 418a in the central western atlantic we estimate the acidity flux associated with the low temperature weathering of the upper oceanic crust the acidity flux is calculated based on major element fluxes and charge balance considerations the relevant chemical fluxes from seawater to the upper crust are 4 1 0 1 1 4 1 4 2 2 0 6 and 12 2 1011 mol yr 1 for k mg na and silicate ca respectively the associated acidity flux is 3 5 3 1011 eq yr 1 relative to continental weathering these fluxes are significant for k and silicate ca but are minor for na mg and acidity thus riverine fluxes of alkalinity are not significantly balanced by acidity fluxes from low temperature upper ocean crust alteration

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

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

crustal structure of the central costa rica subduction zone implications for basal erosion from seismic wide angle data on the pacific margin off central costa rica an anomalous lens shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front this feature was previously recognized in reflection seismic data when it was termed megalens its origin and seismic velocity structure however could not unambiguously be derived from earlier studies therefore during rv sonne cruise so163 seismic wide angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the megalens intersecting on the middle slope the p wave velocities and structure of the subducting oceanic cocos plate and overriding caribbean plate were determined by modelling the wide angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms the margin wedge is defined by high seismic velocities 4 3 6 1 km s 1 identified within a wedge shaped body covered by a slope sediment drape it is divided into two layers with different velocity gradients the lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the megalens seismic velocities of the megalens are lower 3 8 4 3 km s 1 relative to the margin wedge we propose that the megalens represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate upward migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing thus causing material transfer from the upper plate to the lower plate results from amplitude modelling support that the megalens observed off central costa rica is bound by a low velocity zone documenting fluid drainage from the plate boundary to the upper plate

the shallow attenuation structure of the fast spreading east pacific rise near 9 30 n direct measurements of q values in the shallow crust are difficult to obtain due to the lack of energy turning in this region we analyze a unique data set consisting of seven refraction experiments carried out with both the source and receiver near the seafloor this experimental geometry allows us to resolve the shallow compressional wave attenuation structure of young crust at the east pacific rise all models depict a low q layer 2a apparent q 11 22 and several models resolve a layer 2b with a much higher q apparent q 70 this structure is consistent with a layer 2a composed of extrusives overlying a layer 2b composed of sheeted dikes

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

seismic attenuation in the costa rica margin wedge amplitude modeling of ocean bottom hydrophone data abstract travel time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles located 10 20 km landward of the middle america trench offshore costa rica these two profiles bracket a region of rapid change in physical properties and structural styles the travel time modeling indicates little structural change along the profiles and therefore the data are suitable for amplitude modeling with the reflectivity method for the profile 10 km from the trench amplitude modeling indicates a margin wedge with velocities of 4 4 4 km s and qp 25 50 for the profile 20 km from the trench the amplitude modeling indicates a margin wedge with velocities of 4 4 4 8 km s and qp 50 75 the primary new result of this analysis is the constraint that qp 25 75 within the margin wedge at both locations these high attenuation values are consistent with fracturing of the material making up the margin wedge either ophiolitic nicoya complex rocks dewatered and cemented sediments or a tectonic mixture of both

upper crustal structure of the western colombian basin caribbean sea multichannel seismic reflection profiles show that most of the western colombian basin is underlain by a large oceanic plateau the crust of this plateau has a smooth upper surface an oceanic velocity structure and a thickness at least twice that of typical oceanic crust locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments a mounded reflection pattern within acoustic basement at the crest of the plateau mono rise may represent volcanic knolls smooth surfaced basement of the western colombian basin is probably equivalent to the upper cretaceous basement complex below seismic reflection horizon b in the central venezuelan basin and resembles the basements of some cretaceous oceanic plateaus and aseismic rises in the western pacific an area of apparently typical oceanic crust lies adjacent to the colombian basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed reflections within the flank of the colombian basin plateau appear to overlap this crust indicating that the plateau is younger than the oceanic crust oceanic plateau crust extends to panama and costa rica without apparent structural break and may be continuous with the basement complex of southern central america

seismic structure of the upper oceanic crust revealed by in situ q logs in situ seismic attenuation is computed through 2 1 km of the upper oceanic crust in the vicinity of odp hole 504b the results strongly tie crustal properties to seismic measurables and observed geological structures we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity the in situ attenuation q 1 consists of both intrinsic and scattering contributions but is dominated by the scattering attenuation unless porosities are near zero when it approaches typical estimates from seismic refraction studies the attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic vp log and is found to decrease step wise in relatively homogeneous layers from q 25 to q 300 between the top of seismic layer 2a and a sharp discontinuity at 1 3 km depth these changes correspond with heterogeneities at 1 0 1 3 m and at 5 6 10 0 m wavelengths that we interpret to be associated with fracturing and structure of pillow basalts and lava flows in seismic layers 2a and 2b although seismic velocity studies suggest that the layer 2 3 boundary also occurs at about 1 3 km the large variation in q 140 to 460 below this depth indicates that a seismically homogeneous and uniform layer 3 has not been reached in hole 504b we derive an empirical relationship between attenuation and porosity q 1 q0 1 e where q0 1 0 004 and 25 that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models

benthic foraminiferal distribution and sedimentary structures suggest tectonic erosion at the costa rica convergent plate margin distribution patterns of benthic foraminiferal faunas from odp leg 170 sites 1041 and 1042 show that the costa rican convergent margin subsided from coastal to abyssal depth from middle miocene to present this favours the model of a margin undergoing active subduction erosion we propose that subduction erosion leads to the removal of material from the base of the forearc wedge and as a consequence to progressive subsidence of the forearc a mean subsidence rate is estimated to be approximately 0 4 mm yr 1

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

shallow structure of oceanic crust in the western north atlantic from seismic waveform inversion and modeling seismic reflection data from mesozoic oceanic crust in the western north atlantic in the vicinity of the blake spur fracture zone have imaged a number of short subhorizontal events in the upper 1 km of the crystalline crust wide angle expanding spread profile esp data from the same region indicate the presence of first order velocity discontinuities in this depth interval using esp data reprocessed to enhance precritical reflection events we investigate the relationship between the precritical and postcritical reflections with three different approaches amplitude analysis forward modeling and waveform inversion the precritical reflections have high apparent reflection coefficients 0 1 and may be divided into two types those which can be traced out to critical ranges 5 6 km representing velocity steps and those which fade out at ranges of 3 4 km representing shorter wavelength velocity variations synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval while the shorter wavelength features may correspond to high or low velocity zones with a thickness of 20 m 10 of the dominant seismic wavelength or less we investigate the velocity structure in detail by full waveform inversion of a selected portion of esp data transformed into the intercept time slowness domain the inversion method minimizes the sample by sample misfit between the data and reflectivity synthetics in a least squares sense a conjugate gradient algorithm was used starting from a velocity structure derived from forward modeling of wide angle esp data the inversion tightly constrains short wavelength components of the velocity structure and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity synthetic seismograms generated from sonic log data from deep sea drilling project hole 418a show reflections of similar amplitude and character to those seen in the esp data these reflections probably originate from rather subtle features of the velocity depth profile related primarily to changes in porosity in the pillow lava sequence a deeper reflector 800 ms below oceanic basement in the blake spur fracture zone may correspond to the top of a zone of serpentinized upper mantle

detailed subduction structure across the eastern nankai trough obtained from ocean bottom seismographic profiles to investigate the deep crustal structure of the philippine sea plate at its northern margin we performed a seismic refraction and wide angle reflection survey around the eastern nankai trough by using an ocean bottom seismograph array with an air gun source we derived a crustal structure model across the trough from the shikoku basin to the continental slope that explains not only our seismic data but also previously published gravity data the origin of the zenisu ridge a conspicuous topographic high along the oceanward slope of the nankai trough is still a matter of argument there has been some controversy as to whether the igneous activity of the izu ogasawara arc or the seafloor spreading of the shikoku basin is responsible for the formation of the ridge although the crustal thickness beneath the ridge is between 8 and 11 km slightly thicker than typical oceanic crust our structure model clearly indicates that the zenisu ridge has an oceanic crust and its structure is very similar to that of the shikoku basin beneath the south flank of the zenisu ridge the moho shows an offset of 5 km in depth this offset may correlate with the recently proposed nascent subduction boundary and subduction collision tectonics of this area the velocity structure beneath the continental slope appears characteristic for a well developed accretionary wedge bounded by the continental upper crust of the japan island arc to the northwest and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

seismic attenuation in the crust at the mid atlantic plate boundary in south west iceland summary very little seismic attenuation occurs in the mid to lower crust of south west iceland the lowest path averaged quality factor for a wave turning in the mid to lower crust 12 20 km is qp 110 for p waves and q5 250 for s waves with most of the data having higher values typically qp 200 300 and qr 400 600 attenuation estimates based on a 1 d layered inversion give correspondingly high values qp 800 and q 800 2000 these q values are inconsistent with thermal models that predict a broad 100 km wide region of above solidus temperatures centred on the volcanic zones the observed attenuation implies an upper limit for mid to lower crustal temperature in the 700 775 c range assuming a gabbroic lithology much higher attenuation qp 60 qr 100 occurs in the uppermost 4 km of crust this is most likely apparent attenuation caused by strong near surface seismic heterogeneity resulting from fissures faults and extreme changes in porosity up to 20 30 per cent the quality factor of the near surface layer varies regionally and is lower in volcanic zones than in either the reykjanes peninsula or the south iceland lowland