deep seismic structure of the seismogenic plate boundary in the off sanriku region northeastern japan we determined the deep seismic structure of the seismogenic plate boundary zone of off sanriku the northern part of the japan trench inner slope area travel time data from aftershocks of the 1994 far off sanriku earthquake m 7 5 recorded both by ocean bottom seismographs and the local land seismic network enable us to image the three dimensional seismic velocity structure of the region along the plate boundary there exists a p wave low velocity layer of ca 7 km s 1 and with thickness 10 km as the topmost layer of the subducting pacific plate the present analysis indicates that this layer interpreted as the subducting oceanic crust continues without any velocity variations at least down to ca 40 km depth in the overriding plate we found a zone of considerable high p wave velocity ca 8 km s 1 between the pacific coastal line of the northeastern japan arc at the longitude of 142 5 e this manifests that the trenchward toe of the mantle wedge has significantly higher velocity than that beneath the land area of the arc the landward limit of the high p wave velocity toe corresponds to the down dip limit of the seismogenic plate boundary further to the east the p wave velocity is ca 6 km s 1 the structure image obtained indicates that the nature of the overriding plate is significantly different along the plate boundary in the eastern half of the seismogenic zone the island arc crust and subducting oceanic crust are in contact with each other whereas the high velocity mantle wedge makes up the hanging wall of the deeper part of the seismogenic zone

upper mantle electrical resistivity structure beneath the central mariana subduction system this paper reports on a magnetotelluric mt survey across the central mariana subduction system providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back arc spreading ridge after calculation of mt response functions and their correction for topographic distortion two dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab the resultant isotropic electrical resistivity structure contains several key features there is an uppermost resistive layer with a thickness of up to 150 km beneath the pacific ocean basin 80 100 km beneath the mariana trough and 60 km beneath the parece vela basin along with a conductive mantle beneath the resistive layer a resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge there is no evidence for a conductive feature beneath the back arc spreading center sensitivity tests were applied to these features through inversion of synthetic data the uppermost resistive layer is the cool dry residual from the plate accretion process its thickness beneath the pacific ocean basin is controlled mainly by temperature whereas the roughly constant thickness beneath the mariana trough and beneath the parece vela basin regardless of seafloor age is controlled by composition the conductive mantle beneath the uppermost resistive layer requires hydration of olivine and or melting of the mantle the resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three dimensional and of limited size in contrast the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab the resistive region beneath the back arc spreading center can be explained by dry mantle with typical temperatures suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge evidence for electrical anisotropy in the central mariana upper mantle is weak

reply to the discussion by chen et al on wetting front advancing column test for measuring unsaturated hydraulic conductivity appears in canadian geotechnical journal 47 10 1159 1161 the authors appreciate r chen z h li and j h li for their interest in and valuable thoughts about the paper our paper presents an efficient method for measuring unsaturated hydraulic conductivity of soil the proposed method utilizes water contents recorded at one monitoring section to calculate the hydraulic conductivity with the help of instantaneous wetting front advancing velocity in the new method a wetting zone i e the zone between the monitoring section and the wetting front is assumed to advance a distance of dh in a small time step of dt in the wetting zone the water content contours are assumed to be stationary in this small time step the water content change in the wetting zone is assumed to be very small compared with the water content change due to the advancement of the wetting zone namely qb calculated using eq 8 in li et al 2009 as mentioned in the discussion the water content profile advances along the soil column in a way similar to wave propagation such propagation is under constant boundary conditions both at the front and back of the wetting zone in our experiments and the simulations in the discussion the water content at the front of the wetting zone is lower than the residual water content and the hydraulic conductivity at the front of the wetting zone is extremely low e g lower than 10 13 m s hence the boundary conditions i e the water content suction and hydraulic conductivity along the advancing path at the front of the wetting zone are considered to be unchanged in the small time step however the boundary conditions at the back of the wetting zone vary during the wetting process consider a wetting zone with a suction equivalent to the air entry value aev at the back if the wetting front advances a distance of dh in dt the change in suction at the back of the wetting zone will be du gwdh where gw is the unit weight of water the relative change of suction will then be du aev if the testing material is a coarse sand or gravel the aev is usually small and du aev could be large such a large change induced by gravity in the boundary condition at the back of the wetting front will change the wetting front advancing process for example in the capillary experiment conducted on gw gm with sand the observed wetting front could only reach 113 mm above the water table this is because the dominant pores in the gw gm with sand are large i e above 1 mm zhang and li 2010 similar problems should be considered in the numerical simulation of flow in uniform sand referring to fig d1 in the discussion the aev of the uniform sand is about 3 kpa i e 300 mm water head during the capillary rise process the final water content in a wetted section of the sand column will decrease gradually when the height is above 300 mm approaching the water content at the hydrostatic state hence the boundary condition at the back of the wetting zone changes during the capillary process for such a soil the wetting front advancing experiment should be carried out in a horizontal soil column in which the effect of gravity on the boundary condition is less significant for clays the aev values are likely high say above 100 kpa or 10 000 mm head the change of suction du at the back of the wetting zone is negligible in this case the boundary condition at the back of the wetting zone can be considered to be constant referring to fig d2 in the discussion the wetting front advancing velocities are indeed different if different water contents are used to define the wetting front let us consider the wetting front advancing velocities in silty clay in fig d2 at any instant the slopes of the three curves may differ by 2 times at 3 104 s that is to say the calculated hydraulic conductivity may differ within 2 times when different definitions of the wetting front are used the error is in reality much smaller when a consistent set of definitions is followed considering the high variability of soil hydraulic conductivity such an error may be considered acceptable in engineering practice received 31 may 2010 accepted 16 august 2010 published on the nrc research press web site at cgj nrc ca on 1 october 2010

is the electrical conductivity of the northwestern pacific upper mantle normal 1 the normal oceanic mantle project based on ocean bottom geophysical observations has been underway since 2010 to investigate the physical state of the oceanic lithosphere and asthenosphere we have conducted electromagnetic surveys on old 130 ma seafloor in the northwestern pacific region where no active tectonic processes have been identified in order to image the electrical conductivity structure beneath the region so far data have been collected at four sites through a pilot survey conducted from june 2010 to august 2012 a one dimensional electrical conductivity model was obtained by preliminary analysis of the data by using the magnetotelluric method the model shows that the resistive 0 01 s m 1 lithospheric mantle is as thick as 80 km and that the asthenospheric mantle below has a conductivity of 0 03 s m 1 the resistive layer is slightly thicker than that beneath the philippine sea but significantly thinner than that beneath the area off the bonin trench in the pacific ocean there is a greater difference in age between the survey area and the philippine sea 0 60 ma than between the survey area and the area off the bonin trench 140 155 ma this comparison suggests that the relation between age and lithospheric thickness is not as simple as that predicted by the concept of lithospheric cooling it also suggests that the lithosphere beneath the area off the bonin trench in the pacific ocean is abnormally thick if the mantle beneath the survey area in this study is normal as expected from the plate cooling model

marine magnetotellurics imaged no distinct plume beneath the tristan da cunha hotspot in the southern atlantic ocean the tristan da cunha tdc is a volcanic island located above a prominent hotspot in the atlantic ocean many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot however the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area marine magnetotelluric and seismological observations were conducted in 2012 2013 to examine the upper mantle structure adjacent to tdc the electrical conductivity structure of the upper mantle beneath the area was investigated in this study three dimensional inversion analysis depicted a high conductive layer at 120 km depth but no distinct plume like vertical structure the conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the tdc islands are located compared to younger segment south of the tdc fracture zone while the bathymetry is rather deeper than prediction for the northern segment the apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area other observations suggest that 1 the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge 2 the potential temperature of the segments north of the tdc fracture zone is lower than that of the southern segment at least during the past 30 myr

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

coast effect distortion of marine magnetotelluric data insights from a pilot study offshore northeastern japan abstract we report on strong coast effect distortions observed for broadband marine magnetotelluric mt data collected on the forearc offshore northeastern japan eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple station algorithm yielding good mt responses and inter station transfer functions at periods of 7 10 000 s transverse electric te mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s while the transverse magnetic tm mode responses are galvanically depressed below the te responses an analysis of inter station transfer functions confirms that the apparent resistivity cusps are a magnetic field rather than electric field phenomenon consisting of an amplitude minimum and rapid phase change around a characteristic frequency poynting vectors for a te coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin we show that the characteristic frequency and position of the te mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor the depth of the ocean and the distance from the coastline by including coastlines and bathymetry in 2d inversion we recover the seafloor conductivity structure along the forearc demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere

sea effect correction in magnetotelluric mt data and its application to mt soundings carried out in jeju island korea summary when magnetotelluric mt data are obtained in the vicinity of the coast the surrounding sea makes it difficult to interpret subsurface structures especially at the deep parts of the subsurface we apply an iterative method to remove the sea effect the iterative method was originally developed to remove the distortion due to topographic changes from mt data recorded on the seafloor the iterative sea effect correction method is carried out in two steps the first corrects the sea effect whereas the second inverts the sea effect corrected responses the two steps are alternatively carried out until the criterion for either the inversion or the seaeffect correction is satisfied because the surrounding 3 d sea bathymetry is only incorporated into forward modelling for the sea effect correction it can be more robust than the method that directly incorporates 3 d sea bathymetry into a model space for inversion the synthetic examples show that the sea effect correction method yields an inverted model comparable to the true model by applying the sea effect correction method to real field data acquired in jeju island korea we also demonstrate that the sea effect correction method effectively removes the sea effects from the 1 d and 2 d real field data which helps enhance the inversion results on the basis of these results it may be concluded that the iterative sea effect correction method can be used as a promising technique for recovering the true response of the subsurface in mt data suffering from sea effects

mapping 3d salt using the 2d marine magnetotelluric method case study from gemini prospect gulf of mexico the dominant salt body at gemini prospect gulf of mexico has been analyzed by seismic methods revealing a complex 3d salt volume at depths 1 to 5 km beneath the mud line because of the high contrast in electrical conductivity between the salt and surrounding sediments gemini is an attractive target for electromagnetic interrogation using a broadband magnetotelluric mt sensor package developed at the scripps institution of oceanography data in the period band of 1 to 3000 s were collected at 42 sites in a series of profiles over gemini one of which was directly over a linear ridgelike salt feature striking roughly northwest southeast and another orthogonal to it these two profiles reveal that the strongest mt response arises when the electric field is oriented northeast southwest we test the suitability of 2d inversion of these data for recovering the true salt structure by examining inversions of both actual data and synthetic 3d mt responses derived from the seismically inferred salt volume occam inversions of the northeast southwest component result in resistivity images that generally agree with the seismic data whereas inversions of the complementary component yield significantly poorer fidelity disagreement is greatest 1 2 km along the salt sides and base depth errors for top of salt are less than 500 m although thin deep salt 1k m thick at 5 km depth is not well resolved the inversions reveal a resistive basement and a shallow subseabed environment rich in electrical heterogeneity that is weakly if at all suggested by the seismic data a notable exception is a correlation between a previously uninterpreted seismic reflector and the base of a shallow resistivity anomaly whose presence is consistent with gas accumulation near the hydrate stability zone

evidence for an extensive intrusive component of the deccan large igneous province in the narmada son lineament region india from three dimensional magnetotelluric studies abstract the crustal electrical structure beneath the western segment of narmada son lineament zone in central india has been imaged using magnetotelluric mt data set of 153 stations distributed over a grid 7 8 km site spacing three dimensional modeling of this data set brought out several major crustal conductors with different geometries at different depth levels in the crustal column the conductive features correlating with gravity high anomalies and high seismic velocity zones are interpreted to be mafic ultramafic bodies derived from mantle we infer that these bodies represent the intrusive component of the large igneous province lip of the deccan volcanic episode triggered by the passage of the indian continent over the reunion hot spot during the late cretaceous the disposition and the geometry of the subsurface magmatic bodies which must have served as magma chambers for outpouring of the deccan lavas suggest that they are closely related to the plumbing geometry of the lip of the deccan volcanic episode

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

constraints on the configuration of andaman nicobar subduction zone from em modeling abstract the electrical conductivity structures using long period magneto telluric data at three sites nbg nabagram hav havelock in andaman and cby campbell bay in nicobar region are investigated after obtaining 1d responses 2d forward models of electrical structure are constructed along profiles across north andaman middle andaman and nicobar islands the initial configuration along each profile is constrained from models based on seismic reflection data the responses obtained after iterative forward computations match the observed data at respective sites reasonably well new structures within the forearc high are discerned from north to south we delineate a moderately conductive body beneath north andaman which is associated with the presence of ophiolites around this location in the middle andaman a highly conductive body below the island is interpreted as presence of fluids along the active diligent fault facilitated by the barrier of the invisible bank a complex geometry of resistive material embedded in a highly conductive body inferred beneath at great nicobar at cby is commensurate with the presence of a layered magmatic body possibly linked to the volcanic seamount to the east near the tear of the subducting plate

approximations for the 2 d coast effect on marine magnetotelluric data marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest however the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted for a 2 d coastal model this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast a maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions based on a modelling study we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity based on a simple analytical approach we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest while the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor

lower cretaceous provenance and sedimentary deposition in the eastern carpathians inferences for the evolution of the subducted oceanic domain and its european passive continental margin

classification of large scale subaqueous bedforms a new look at an old problem sepm bedforms and bedding structures abstract a symposium entitled classification of large scale flow transverse bedforms was convened at the 1987 mid year meeting of sepm in austin texas with the purpose of examining the problems involved in classifying large subaqueous flow transverse bedforms developed in fluvial intertidal and marine environments and recommending changes in nomenclature the consensus of the participants is that despite the wide spectrum of morphologies of large scale flow transverse bedforms excluding antidunes they all occupy a similar position in the lower flow regime sequence between ripples and upper plane bed the wide variety of forms is a reflection of secondary effects such as channelization fluctuating water levels and unsteady and reversing flows the bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1 000 m the symposium panel proposes therefore that they should have only one name dune dune is preferred as it has historical precedence over other terms in use such as megaripple and sand wave the term dune should be modified by rimary descriptors of shape i e 2 d or 3 d and size based on spacing small 0 6 5 m medium 5 10 m large 10 100 m or very large 100 m and the adjective subaqueous when it is important to distinguish them from eolian dunes the panel recommends a morphologically based classification that is descriptive with an underlying genetic rationale second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature

inversion of magnetotelluric data in the presence of strong bathymetry topography we describe an algorithm for inverting magnetotelluric mt data in the presence of strong bathymetry or topography instead of correcting distortions due to bathymetry or topography we incorporate them directly into the inversion to achieve a high accuracy in computing mt responses we use finite difference approximations that permit sloping discrete boundaries inside model elements the same approach is applicable to any seafloor electromagnetic calculation and can also be used to incorporate steep topography on land we test our approach on various topographic features and compare our results to that of a finite element approach finally we present inversion examples that illustrate the effectiveness of our inversion algorithm in recovering true subsurface structures in the presence of strong bathymetry and topography

confirming sharp bending of the pacific plate in the northern japan trench subduction zone by applying a traveltime mapping method abstract many destructive earthquakes have occurred in the northern japan trench region owing to the subduction of the pacific plate two plate bending points where the dip of the subducting plate changes markedly have been found and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off aomori 40 n and off miyagi 38 n in this study we investigate whether sharp plate bending is also present in the region off iwate 39 n located approximately halfway between the off aomori and off miyagi regions we analysed the existing off iwate wide angle seismic experimental data by applying a new method in which observed reflection phases are directly projected onto the depth distance domain from the time distance domain as a result we found a plate bending point in the region off iwate this is the first time such a point has been found there distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern japan trench subduction zone the bending axis which is defined by connecting bending points approximately coincides with the updip limit of the rupture zones of large interplate earthquakes seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis and we infer that some of these activities are caused by bending of the subducting plate in addition p wave velocities of the upper plate vary across the bending axis implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities

heterogeneous structure across the source regions of the 1968 tokachi oki and the 1994 sanriku haruka oki earthquakes at the japan trench revealed by an ocean bottom seismic survey abstract to study the physical properties along the subducting plate boundary at the japan trench a seismic study was carried out using ocean bottom seismometers obss and artificial sources was carried out the 250 km long survey line with an almost n s strike crosses the two major focal zones of the 1968 tokachi oki earthquake and the 1994 sanriku haruka oki earthquake ray tracing and non linear inversion were used to analyze the data obtained by the obss p wave velocity structure down to the island arc moho was obtained the result shows distinct heterogeneities along the plate boundary at 40 10 n which are located exactly at the southern boundary of major moment release regions of the above two earthquakes p wave velocities for the crust north of 40 10 n are approximately 7 slower than those for the south the depths of the island arc moho vary from 15 km in the south to 21 km in the north below the sea surface three explanations of the velocity heterogeneities are presented in terms of the migration of water

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

electrical structure of the central cascadia subduction zone the emslab lincoln line revisited abstract the emslab experiment was an ambitious onshore offshore magnetotelluric mt transect of the cascadia subduction zone when completed 1985 1988 it was the largest experiment of its kind modeling and inversion capabilities at the time were however not sufficiently sophisticated to handle a fully regularized inversion of the data including the seafloor data and bathymetric constraints with the main final model presented based on trial and error forward modeling of the responses moreover new data collected as part of the earthscope usarray program are of higher quality due to improvements in instrument technology and augment the original emslab data set presenting an opportunity to revisit the structure in this part of the subduction system we have integrated the original wide band mt data as well as several long period stations from the original emslab data set and invert these in conjunction with emslab seafloor responses and new earthscope data on land this new composite data set has been analyzed in several ways within a two dimensional geometry in which conductivity is assumed to be invariant along a strike direction roughly coincident with that of the subduction zone we have solved for fully smooth regularized models as well as solutions that allow discontinuities in conductivity along the top surface of the descending slab finally we have tested specific features in the emslab model notably a moderately shallow 30 km depth forearc conductor a feature similar to this shallow conductor is a consistent and required feature in our new inversion models but the new models highlight the connection between the slab and what is interpreted to be an accumulation of aqueous fluids in the deep crust the depth 40 km at which the conductor intersects the slab suggests that the fluids are released by the transition of hydrous basalt to eclogite at upper greenschist facies and higher metamorphic grade the nose of the mantle wedge has a conductivity consistent with a dry peridotite composition and thermal models of the system at a depth of around 80 km the mantle intersecting the slab shows a slight increase in conductivity this increase is not sufficient to require the presence of melt but a conductor indicative of melt can be inserted into the model at this depth without compromising the fit

adaptive finite element modelling of two dimensional magnetotelluric fields in general anisotropic media summary an adaptive unstructured mesh finite element fe procedure is presented for improving the quality of numerical solutions to the magnetotelluric forward problem in a general 2 d anisotropic conductivity structure we implement a self adaptive goal oriented grid refinement algorithm in which a finite element analysis is performed on a sequence of refined meshes the mesh refinement process is guided by a dual error estimate weighting to bias refinement towards elements that affect the solution at the em receiver locations we validate the finite element code against a layered 1 d model with a sea water layer further we compare the fe results with those obtained by a finite difference fd scheme for both a block seamountain and a sea bottom hill model both fe and fd schemes show very good agreement for the block seamountain model for the sea bottom hill model however only on the flat seafloor segments both the fe and fd solutions fit very well but on the seafloor slope fd results are oscillating due to a simplistic staircase approximation of the bathymetric undulations the fd scheme for 2 d anisotropic conductors developed primarily for the modelling of magnetotelluric data on a flat earth surface is thus not an adequate tool for dealing with structures with sloping bathymetry and topography whereas the fe method with adapting mesh can easily handle such structures at almost any level of complexity

three dimensional inversion of seafloor magnetotelluric data collected in the philippine sea and the western margin of the northwest pacific ocean we report a result of three dimensional 3 d upper mantle electrical conductivity inversion of seafloor magnetotelluric data we used existing data at 25 sites in the philippine sea and the western margin of the pacific ocean in order to obtain a reliable model by 3 d inversion we evaluated the large and small scale topographic effects we also conducted a comprehensive search of the one dimensional 1 d profiles of the study area in order to determine the best initial and prior models a two phase inversion method was applied so that the error floors for the diagonal and off diagonal elements of the impedance tensor could be separately controlled through this first attempt at inverting real data we obtained basic knowledge about tuning the inversion parameters and conditions we also proposed a procedure to evaluate the reliability of the 3 d conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests after the iterations converged 13 distinct anomalies were found in the inverted 3 d conductivity model four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant this paper presented an example of possible approach in 3 d seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle which will be useful in understanding dynamics and evolution of solid earth

electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data abstract we performed a three year seafloor electromagnetic survey in the philippine sea including the western edge of the pacific ocean to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions 3 d the project iterated one year deployment of ocean bottom electromagnetometers obems using a total of 37 instruments installed at 18 sites the data obtained have been analyzed in the order of their recovery based on a magnetotelluric mt method in this study we attempt to obtain a one dimensional 1 d electrical conductivity model beneath the philippine sea and the pacific region separately that can be used as a reference model in the first step toward the 3 d analysis the resultant 1 d models show three main features 1 the conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply which is qualitatively consistent with the large difference in lithospheric age 2 the conductivity at 200 300 km depth in both regions is more or less the same at approximately 0 3 s m 1 3 the conductivity just below 400 km depth is higher for the philippine sea mantle than for the pacific mantle the conductivity structure can be interpreted in terms of the thermal structure mantle hydration and existence of partial melt using experimental results for the conductivity of mantle minerals if the conductivity is interpreted simply as the effect of temperature the mantle beneath the philippine sea could be hotter than the dry solidus of mantle peridotite and thus partially molten however beneath the pacific region the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration

tertiary tectonic evolution of the external east carpathians romania abstract paleostress calculation and analysis of mesoscopic structures are integrated with depth interpreted geological profiles based on seismic studies and well correlation to derive a tertiary tectonic model for the east carpathians following early miocene and older orogenic phases the first tectonic event that affected the studied area is characterised by a wsw ene oriented shortening of middle miocene late burdigalian in age resulting deformations induced ene ward thrusting of tarcau and marginal units as well as the internal part of the subcarpathian nappe a second shortening event with an e w to wsw ene contraction direction took place in the late miocene sarmatian characterised by further foreland thrusting of the subcarpathian nappe and out of sequence deformation in the tarcau and marginal folds nappes along strike differences in deformation mechanisms are controlled by the friction coefficients along the main detachment layers by the lateral variations in the wedge thickness and by the involvement in the northern part of the thrusting system of the thick competent east european platform tear faulting occurred in both tectonic events the main resulting structure being the triangle zone developed south of the trotus valley the latest miocene latest sarmatian early pliocene is characterised by a strike slip stress field with nne ssw compression and wnw ese tension axis left lateral faults being dominant the last deformation which affected the studied area is characterised by nnw sse shortening during the pliocene major deformations taking place mainly in the sw most bending zone

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

comment on the electrical conductivity of the oceanic upper mantle by g heinson and s constable in a recent paper heinson constable 1992 hereafter hc have attempted to construct an oceanic upper mantle electrical conductivity model by combining laboratory measurements on dry olivine as a function of temperature with a geotherm inferred from a variety of geophysical and petrological data hc argue that the 1 d magnetotelluric mt response computed from their model structure is not compatible with three sets of sea floor electromagnetic em data collected in the late 1970 s and proceed to examine those data in more detail they infer that some of the em data are not consistent with a 1 d structure on the basis of statistical tests and further conclude that the presence of an upper mantle high conductivity zone is not required by the data finally hc attempt to reconcile these disparate observations by suggesting that a high resistive upper mantle which is required by their model combined with the presence of resistive coastlines will result in significant distortion of the em response at points in the interior of the ocean they construct sample mt responses

tectonic features of the japan trench convergent margin off sanriku northeastern japan revealed by multichannel seismic reflection data near the japan trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate we report the results of a multichannel seismic survey in the northern japan trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector the oceanic plate also undergoes deformation as the two plates interact in the subduction processes resulting in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust this may eventually lead to the formation of a smooth plate boundary at the greater depth more than 45 km landward from the trench axis a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust our data indicate that the deformed zone pinches out landward 60 km from the axis at 13 km depth and the slip plane becomes less obvious there seismicity of interplate earthquakes rapidly increases landward from this location leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur we conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust

characteristics of collisional orogens with low topographic build up an example from the carpathians sequence stratigraphy in the hinterland kinematic analysis of thin skinned thrusting in the foreland and thermochronological tracking of exhumation in the orogenic core are combined to quantify the mechanics of an orogen with low topographic build up the carpathian system demonstrates that collisional deformation can couple and thicken the lower orogenic plate along reverse faults that dip more steeply than the subduction zone defining a foreland coupling type of collision near the surface this is expressed by wide antiforms in the upper plate and the thin skinned orogenic wedge a sequence stratigraphic analysis of the back arc transylvanian basin demonstrates that the sedimentary architecture records orogenic uplift pulses with both short and long wavelengths these correspond to the activation of individual thrust sheets in the thin skinned wedge and to lower plate coupling events respectively

three dimensional topography corrections of magnetotelluric data summary topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric mt data three dimensional 3 d topographic effects have been investigated for a trapezoidal hill model using an edge finite element method the 3 d topography generates significant mt anomalies and has both galvanic and inductive effects in any polarization this paper presents two different correction algorithms which are applied to the impedance tensor and to both electric and magnetic fields respectively to reduce topographic effects on mt data the correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography and improve the quality of subsurface interpretation nonlinear least squares inversion of topography corrected data successfully recovers most of structures including a conductive or resistive dyke

a two dimensional interpretation of the geomagnetic coast effect of southeast australia observed on land and seafloor abstract data from detailed investigations of the geomagnetic coast effect across the southeast australian continental margin are interpreted in terms of a two dimensional numerical model there is both land and seafloor control on this model and both e pol and b pol mode responses are incorporated the good fit to the data of a model comprising ocean water and marine sediments is improved when the sub ocean electrical conductivity profile is allowed to differ from the continental conductivity profile a final model is determined by an inversion procedure based on systematic search the main characteristic of this model is that an increase in conductivity occurs at a shallower depth beneath the ocean than beneath the land some 100 km beneath ocean 200 km beneath land the southeast continental margin of australia is considered to have formed by passive rifting at the time of the opening of the tasman sea a depth of 100 km for the base of the oceanic lithosphere corresponds well to the age of rifting some 80 ma ago the contrast with the continental profile suggests an electrical asthenosphere relatively deeper beneath southeast australia

triple oxygen isotope composition of dissolved o2 in the equatorial pacific a tracer of mixing production and respiration 1 as a contribution to the study of equatorial pacific biogeochemistry we measured the o2 ar ratio and the triple isotope composition 18o 17o and 16o of o2 along six meridional lines in the equatorial pacific 8 n 8 s at 95 w 110 w 125 w 140 w 155 w and 170 w o2 ar ratios and 18o were close to equilibrium values within the mixed layer and followed the general trend of increasing 18o with decreasing o2 ar at greater depths the 17 17o 0 5 18o constrains the fraction of photosynthetic o2 17 was slightly elevated with respect to equilibrium within the mixed layer due to local photosynthetic production in aphotic zone waters above 250 m depth the average 17 values were higher than in the mixed layer there are four sources of this photosynthetic signal in the dark ocean production in the euphotic zone prior to subduction in the distant source regions production below the mixed layer during travel to the equatorial zone diapycnal mixing with shallower waters bearing photosynthetic o2 and accumulation of photosynthetic o2 produced at very low rates below the 1 light level our results also constrain biological production rates within the mixed layer at several locations along 95 w and 110 w our average rate of 14c production 53 34 mmol c m 2 d 1 agreed well with other estimates in the equatorial pacific while our average rate of net c production 6 9 6 2 mmol c m 2 d 1 and f ratio 0 12 0 11 were somewhat lower than other estimates adding 18o and 17 as tracers to three dimensional biogeochemical ocean gcms and comparing results with observations will extend our understanding of metabolic rates in the study region

resistivity cross section through the southern central andes as inferred from magnetotelluric and geomagnetic deep soundings to investigate the upper lithospheric structure and subduction induced processes across an active continental margin magnetotelluric mt and geomagnetic deep sounding gds experiments were done in the southern central andes 21 25 s of northern chile southern bolivia and northwestern argentina using two dimensional modeling we have constructed an e w resistivity cross section at latitude 21 45 s that involves all the andean units from the pacific coast to the lowland plains the crust of the coastal cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted nazca plate the electrical resistivity of the normal andean crust seems to be low having values of only 200 ohm m a wide zone of extremely high electrical conductivity with an e w extent of more than 330 km was detected this high conductivity zone hcz is found at a depth of about 25 km under the western cordillera and has a total conductance of more than 23 000 s here it may be caused by partially melted crust an interpretation which would correlate with those from gravity and seismic data the hcz can be followed in the lower crust eastward below the altiplano where it has a total conductance of about 15 000 s in the western part of the eastern cordillera total conductance first increases and then drops abruptly at about longitude 65 10 w the latter hczs can be explained in terms of thrust tectonics where detachment zones may involve fluids as well as increased mineralizations the lowlands of the sub andean and the chaco are characterized by a cover of anisotropic low resistivity the resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m while the deeper mantle gets unexpectedly much more conductive

the coast effect in terms of deviated electric currents a numerical study abstract a numerical thin sheet model is used to study the coast effect and to determine the magnitude of the near surface part of the anomalous field theoretical induction arrows are computed for australia california japan and the british isles and compared with the observed arrows this comparison shows that the main part of the coast effect and in some cases the whole coast effect is due to the deviation of currents flowing in the ocean by the upper part of the resistive continents it is suggested that the term coast effect should refer to the anomalous field whose source is near surface the coast effect so defined behaves like a regional anomaly superimposed on other anomalous features of the transient geomagnetic field a numerical determination of the geometrical features of the coast effect allows consideration of its elimination this elimination is performed for california and japan a correlation between the resulting residual and heat flow data is apparent in both cases

categorizing the morphologic variability of siliciclastic passive continental margins the basic morphology of the continental shelf slope and rise on passive continental margins has many variations in the modern ocean environment through using a satellite derived bathymetric data set of the global ocean we have developed a classification for the shape of modern siliciclastic dominated passive margins the classification subdivides 50 margins into five distinct shape categories based on each margin s pattern of sea floor slopes as a function of depth by comparing these shape categories with the underlying stratal architecture sediment input and present degree of canyon incision of the margins we find apparent correlations between these factors and basic margin shape in general gently sloped margins tend to occur in regions with high sediment input few modern canyons and unstable substrates higher gradient margins tend to have lower sediment input more modern canyons and a variety of subsurface architectures these results suggest two important conclusions 1 morphology differences among modern siliciclastic passive margins can be objectively and systematically categorized and 2 these shape differences seem to be related to aspects of the modern sedimentary environment despite the fact that the margins have evolved over geologic time

broadband marine mt exploration of the east pacific rise at 9 50 n 1 we present the first use at a mid ocean ridge of a recently developed broadband marine magnetotelluric mt instrument the extended high frequency performance of the instrument allows resolution of electrical resistivity structure at shallower depths than traditional marine mt sensors our two dimensional inversion model from data collected at four mt sites on the east pacific rise epr at 9 50 n demonstrates the viability of the method to image electrical resistivity structure in both the crust and shallow mantle while our pilot experiment falls far short of the coverage needed to provide rigorous constraints on structure a low resistivity zone in the crustal portion of the inversion model agrees well with seismic tomography and seafloor compliance results resistivities beneath the ridge imply a crustal partial melt fraction of 1 20 a total melt volume of about 0 75 km3 per kilometer of ridge implies an average melt residence time of about 1000 years

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

1 d electrical conductivity structure beneath the philippine sea results from an ocean bottom magnetotelluric survey abstract eight months of observation using ocean bottom electro magnetometers obems have allowed us to estimate the regional electrical conductivity structure beneath the philippine sea six obems were deployed along a line crossing the philippine sea from nw to se and five of them recorded useful data the raw time series data were cleaned up before we estimated the magnetotelluric mt impedance tensor conductivity structure at five sites is estimated using 1 d occam s inversion to fit the determinant average of each mt impedance tensor after a correction for the effect of topography we examined effect from two dimensionalities on the 1 d conductivity structure and the robustness of solutions the results of the 1 d conductivity structural model are strongly related to tectonic setting and the crustal age beneath each site the structure beneath the spreading axis of the mariana trough shows a distinct low conductivity structure at depths of 50 150 km and it probably reflects the upwelling dynamics operating beneath the spreading axis these low values are comparable with that of olivine with low hydrogen content implying that 1 the melting process extracts water from minerals such as olivine and 2 the melt beginning depth in the mariana trough is deeper than that of the typical morb source region the off axis conductivity profiles infer the existence of a high conductivity peak or a conductivity gradient change at mid depth the depth level of the peak increases with crustal age suggesting that the conductivity structure is related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change possibly combined with the presence of partial melt our results suggest that further ocean bottom em study has high potential to investigate the temperature gradient change and amount of hydrogen water and melt in the upper mantle

in defence of a resistive oceanic upper mantle reply to a comment by tarits chave and schultz in a previous paper heinson constable 1992 we discussed the effect of coastlines on sea floor magnetotelluric mt data collected deep within the ocean basins in order to quantify the geomagnetic coast effect we developed a model for oceanic upper mantle electrical conductivity independent of previous sea floor mt interpretations based on sea floor controlled source electromagnetic csem soundings laboratory studies of mantle materials and global geomagnetic sounding estimates for lower mantle conductivity we demonstrated that the coast effect of our model was large was not manifest as severe anisotropy in the mt response if the ocean basins were bounded by coastlines on three or more sides and that a qualitative agreement existed between the mt response of our simple coastline model and published sea floor mt data the implication of the latter is that by including the effect of the coastlines additional mechanisms for enhanced upper mantle conductivity such as volatiles carbon melt hydrogen and other factors may not be required by the mt data tarits chave schultz 1993 question the validity of a large number of aspects of our work their principal conclusions are that a horizontal or vertical leakage paths between the ocean and other oceans or the mantle will substantially reduce the magnitude of the ocean wide coast effect b the principle of extrapolating laboratory electrical conductivity measurements to mantle conditions is unreliable from uncertainties in the understanding of pressure temperature and petrological states of the upper mantle c published sea floor mt data sets examined by us were of such vintage that any conclusions drawn were unreliable or incorrect and d the conductivity profile compiled by chave flosadottir cox 1990 based on 1 d interpretations of sea floor mt and csem data that took no account of the coast effect is consequently a more reliable estimator of mantle conductivity structure than our model while all aspects of tarits et d s comments were addressed to some extent in our original paper we did not investigate all details fully either in the interests of conciseness or because the impact on our conclusions was minimal we will take this opportunity to delve further into some of these issues but before considering the complications it will be useful to reiterate the motivation for our work first oldenburg s 1981 and oldenburg whittal parker s 1984 1 d interpretations of three sea floor mt data sets included a high conductivity zone hcz in the oceanic upper mantle that decreased in magnitude and deepened with increasing age the existence of a hcz has become a paradigm not only amongst the electromagnetic community but also with seismologists petrologists and mineral physicists many entire papers are written on the subject attempting to explain the disparities between laboratory measurements of olivine conductivity at upper mantle temperatures and the hcz correlating the hcz with seismic low velocity zones lvz examining the temporal evolution of the hcz etc the impact of oldenburg s work has been so large that it seemed appropriate to re examine the basis of the hcz and question whether it is indeed required by the data note that

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

correction of seafloor magnetotelluric data for topographic effects during inversion 1 the large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields for marine magnetotelluric mt studies correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle previously correction for topography was based on the thin sheet approximation which breaks down at periods under 1000 s in the deep ocean this paper introduces an analysis method for seafloor mt data which combines removal of three dimensional 3 d topographic effects with inversion of the data for 2 d structure the observed mt impedance is first corrected to a flat lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation the corrected mt response is then inverted in a flat seafloor model space because of coupling between topographic effects and deeper structure the correction and inversion steps are iterated until changes in each become small the procedure is verified using synthetic and real data tests for synthetic 3 d topography over a half space show that the method closely recovers the true half space model after a few iterations the procedure is also applied to real data collected in the mantle electromagnetic and tomography melt experiment on the east pacific rise at 17 s

3 d magnetotelluric image of offshore magmatism at the walvis ridge and rift basin highlights we report on marine 3d magnetotelluric study on walvis ridge derived 3d electrical resistivity model shows a large scale resistive zone which we link to crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting smaller scale resistive region is attributed to magma ascent during rifting rift basin is identified by low resistivity region the namibian continental margin marks the starting point of the tristan da cunha hotspot trail the walvis ridge this section of the volcanic southwestern african margin is therefore ideal to study the interaction of hotspot volcanism and rifting which occurred in the late jurassic early cretaceous offshore magnetotelluric data image electromagnetically the landfall of walvis ridge two large scale high resistivity anomalies in the 3 d resistivity model indicate old magmatic intrusions related to hot spot volcanism and rifting the large scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2 d offshore seismic profiles one of the high resistivity anomalies above 500 m has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust one of the arms stretches underneath walvis ridge the shape is suggestive of crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting a second smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting we attribute a low resistivity anomaly east of the continent ocean boundary and south of walvis ridge to the presence of a rift basin that formed prior to the rifting

fault plane geometry in the source region of the 1994 sanriku oki earthquake abstract we present the results of a seismic refraction experiment conducted off eastern aomori northeastern japan forearc region where the detailed crustal structure was poorly known an objective analysis using high quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth and in particular to obtain a well constrained geometry of the plate boundary in the source region of the 1994 sanriku oki earthquake our results show that the subducting pacific oceanic plate increases its dip from about 5 to approximately 15 around the longitude of 143 5 e 80 km landward of the trench axis near the latitude of 40 n the location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution a cluster of aftershocks of the 1994 sanriku oki earthquake occurred around the bending point this is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary furthermore the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies

mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data summary marine magnetotelluric mt and marine controlled source electromagnetic csem soundings can be used to study sedimentary structure offshore in an example of this application we collected mt and csem data in the 1 km deep water of the san diego trough california the trough is a pull apart basin and part of the complex pacific north american tectonic plate boundary and is flanked by the thirtymile bank to the west and the coronado bank to the east our mt data are highly distorted by seafloor topography and the coast effect which is largely 2 d and can be modelled using 2 d finite element codes the distortion includes a strong several orders of magnitude static depression of tm mode resistivities electric field perpendicular to structure upward cusps in the te mode resistivities electric field parallel to structure and negative te mode phases the depressed tm mode resistivity is a well known consequence of galvanic interruption of coastperpendicular electric fields the te mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins steepening the magnetic field and even causing a phase reversal in the horizontal field used for mt impedance calculations and thus generating negative phases the land side enhanced vertical magnetic field is well known as the geomagnetic coast effect but the ocean side consequences have been less well documented although the mt data are dominated by coast effect and topographic distortion inclusion of accurate bathymetry in the inversion model s finite element mesh allows the subseafloor geological structure to be recovered this shows the trough sediments to be about 3 km thick bounded to the west by resistive basement but to the east by conductive clastic sediments forming coronado bank amplitudes and phases of five frequencies of csem data from 0 1 to 1 0 hz collected along the axis of the trough are well fit with a simple 1 d layered model indicating that sediment resistivities increase with depth from 1 5 to 2 3 m and are no more than 3300 m thick thinning to the north in good agreement with the mt model an existing density model generated by fitting surface and deep towed gravity is in good agreement with the em interpretations in particular combining sediment densities and csem resistivities allows us to estimate pore water conductivity and temperature which follows a geothermal gradient of 25 4 8kk m 1

revised 1 d mantle electrical conductivity structure beneath the north pacific summary the 1 d electrical conductivity model in the mid mantle beneath the northern pacific was revised in order to discuss the mean state of the mantle and to obtain a credible starting model for 3 d inversions semi global geomagnetic depth sounding gds responses obtained at 13 stations and submarine cable magnetotelluric mt responses for eight cables in a period range of 1 7 113 d were used to obtain the revised structure it is well known that surface conductivity heterogeneity due to the ocean land conductivity contrast has a large influence on the responses up to a period of 20 d the effect of surface ocean land distribution was removed by performing 3 d electromagnetic induction modelling including a surface conductivity heterogeneity distribution laid above a 1 d mantle structure the corrected responses were averaged in the logarithmic space and the resulting responses quasi 1 d response were inverted to obtain 1 d conductivity models correction of the surface heterogeneity effect requires a 1 d conductivity model and the 1 d model must be similar to the resultant 1 d model for a consistent correction we attained this by making iterations of the surface layer correction and 1 d inversion synthetic tests revealed that the iterative scheme could recover the supposed structure even for a 3 d heterogeneous mantle since electromagnetic sounding is not sensitive to the presence of a sharp discontinuity we examined three different cases of conductivity variation with depth 1 a model with no discontinuity 2 a model with two jumps at 400 and 650 km depths and 3 a model with three jumps at 400 500 and 650 km depths the conductivity of the two jump model in the transition zone is higher than the experimentally determined conductivity of dry wadsleyite and ringwoodite if the difference is entirely due to the effect of water in the transition zone then the conductivity is consistent with a water content in the region of 0 5 wt per cent however if an additional discontinuity of electrical conductivity is allowed at the 500 km depth the obtained conductivity for the upper 100 km of the transition zone is lower and can be explained by a less water content 0 1 wt per cent in wadsleyite

magnetotelluric image of the fluid cycle in the costa rican subduction zone fluids entering the subduction zone play a key role in the subduction process they cause changes in the dynamics and thermal structure of the subduction zone1 and trigger earthquakes when released from the subducting plate during metamorphism fluids are delivered to the subduction zone by the oceanic crust and also enter as the oceanic plate bends downwards at the plate boundary however the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions4 or transfer to the deep mantle implying possible storage of fluids in the crust here we use magnetotelluric data to map the entire hydration and dehydration cycle of the costa rican subduction zone to 120 km depth along the incoming plate bend we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle along the subducting plate interface we document the dehydration of sediments the crust and mantle we identify an accumulation of fluids at 20 30 km depth at a distance of 30 km seaward from the volcanic arc comparison with other subduction zones5 14 indicates that such fluid accumulation is a global phenomenon although we are unable to test whether these fluid reservoirs grow with time we suggest that they can account for some of the missing outflow of fluid at subduction zones

magnetotelluric imaging of the society islands hotspot in april june 1989 seafloor magnetotelluric data across and along the leading edge of the tahiti hotspot were obtained the magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced the corrected response functions are consistent with a two dimensional structure inversion of the data shows a slightly higher conductivity relative to a reference site located away from the hotspot down to 130 km depth beneath the active area southeast of tahiti underlain by a more resistive structure there is a suggestion for a change in conductivity in the 400 450 km depth range which is consistent with elevated temperatures this result is consistent with a mantle plume of limited extent less than 150 km radius located near the leading edge of the tahiti hotspot the magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area as would be required by a superswell model

a new technique for the incorporation of seafloor topography in electromagnetic modelling summary we describe a new technique for incorporating seafloor topography in electromagnetic modelling it is based on a transformation of the topographic relief into a change in electrical conductivity and magnetic permeability within a flat seafloor since the technique allows us to model arbitrary topographic changes without extra grid cells and any restriction by vertical discretization we can model very precise topographic changes easily without an extra burden in terms of computer memory or calculation time the reliability and stability of the technique are tested by comparing the magnetotelluric responses from two synthetic seafloor topography models with three different approaches these incorporate the topographic change in terms of 1 the change in conductance using the thin sheet approximation 2 a series of rectangular block like steps and 3 triangular finite elements the technique is easily applied to any modelling method including 3d modelling allowing us to model complex structure in the earth while taking full account of the 3d seafloor topography

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

structure of the mantle beneath the alboran basin from magnetotelluric soundings we present results of marine mt acquisition in the alboran sea that also incorporates previously acquired land mt from southern spain into our analysis the marine data show complex mt response functions with strong distortion due to seafloor topography and the coastline but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3d inversion model has allowed us to image the structure in the underlying mantle the resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath gibraltar which plunges nearly vertically beneath the alboran our model contains three primary features of interest a resistive body beneath the central alboran which extends to a depth of 150 km at this depth the mantle resistivity decreases to values of 100 ohm m slightly higher than those seen in typical asthenosphere at the same depth this transition suggests a change in slab properties with depth perhaps reflecting a change in the nature of the seafloor subducted in the past two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle or both of these the one in the center of the alboran basin in the uppermost mantle 20 30km depth beneath neogene volcanics and west of the termination of the nekkor fault is consistent with geochemical models which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics

electromagnetic evidence for volatile rich upwelling beneath the society hotspot french polynesia we have conducted a seafloor magnetotelluric survey that images for the first time three dimensional electrical conductivity structure in the upper mantle beneath the society hotspot a striking feature in our model is a high conductivity anomaly a few hundred kilometers in diameter which is continuous from the lowest part of the upper mantle to a depth of approximately 50 km below sea level using theoretical and experimental results from mineral physics we interpret the high conductivity anomaly as evidence of the melt fraction up to 2 2 vol which is robust regardless of assumed temperature and the existence of carbonated silicate melt beneath the hotspot our results suggest that the society hotspot is a pathway for ascending volatiles from the deeper part of the upper mantle to the surface

the tasman project of seafloor magnetotelluric exploration experiment and observations abstract a major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between australia and new zealand simultaneous observations were made by land magnetometers on the australian continent the large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites the parkinson arrows show a strong coast effect near the australian continent and a weaker coast effect over the submerged lord howe rise the occurrence of smoothly varying arrows across the tasman sea suggests a simple large scale pattern for electromagnetic induction in the tasman sea the interpretation of parkinson arrows at seafloor sites is shown to require special care due to the attenuation and phase shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations the magnetotelluric impedances exhibit a strong anisotropy at all sites in the tasman sea the orientation of the impedance axes is constant across the sea and the anisotropy may be attributed to the two dimensional shape of the tasman sea calculations based on the distance scale of the magnetotelluric anisotropy interpreted as an ocean continent boundary effect give a lower bound for the integrated resistivity of the tasman sea oceanic lithosphere of order 107 m2 corresponding to an estimate of 2 102 m for its average resistivity

asymmetric electrical structure in the mantle beneath the east pacific rise at 17 s the magnetotelluric component of the mantle electromagnetic and tomography melt experiment measured the electrical resistivity structure of the mantle beneath the fast spreading southern east pacific rise epr the data reveal an asymmetric resistivity structure with lower resistivity to the west of the ridge the uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles mantle resistivities to the west of the ridge are consistent with a low melt fraction about 1 to 2 percent interconnected melt distributed over a broad region and extending to depths of about 150 kilometers the asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates

geomagnetic induction in the tasman sea and electrical conductivity structure beneath the tasman seafloor summary during the tasman project of seafloor magnetotelluric exploration recordings were made of the natural magnetic and electric field variations along a line of seafloor sites crossing the tasman sea using these data and additional magnetic field recordings made on the australian continent the pattern of geomagnetic induction in the tasman sea region is examined an investigation of the seafloor magnetotelluric impedance tensor indicates the geomagnetic induction is influenced by the large scale 3 d conductivity structure of the region this result is confirmed by a study of impedance estimates obtained using the vertical gradient sounding method in which the seafloor impedance is determined from the attenuation of the horizontal magnetic field variations through the ocean inversion of the e polarization impedance component from sites in the central tasman sea indicates the conductivity structure at depths greater than about 100 km is laterally homogeneous in this region it is probable that the conductivity structure at these sites includes a high conducting layer similar to one observed in the pacific ocean one seafloor site lying on the tasmantid seamount chain exhibits an increased conductivity at depths of less than 100 km the increased conductivity may be due to heating of the lithosphere by a hotspot one proposed source of the seamount chain

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

adaptive unstructured grid finite element simulation of two dimensional magnetotelluric fields for arbitrary surface and seafloor topography summary we present an adaptive unstructured triangular grid finite element approach for effectively simulating plane wave diffusive electromagnetic fields in 2 d conductivity structures the most striking advantage of irregular grids is their potential to incorporate arbitrary geometries including surface and seafloor topography adaptive mesh refinement strategies using an a posteriori error estimator yield most efficient numerical solutions since meshes are only refined where required we demonstrate the robustness of this approach by comparison with analytical solutions and previously published numerical simulations maximum errors may systematically be reduced to for example 0 8 per cent for the apparent resistivity and 0 2 in the phase an additional accuracy study of the thickness of the air layer in e polarization suggests to keep a minimum thickness depending on lateral conductivity contrasts within the earth furthermore we point out the new quality and flexibility of our simulation technique by addressing two marine magnetotelluric applications in the first case we discuss topographic effects associated with a synthetic sinusoidal sea bottom model and in the second case we show a close to reality scenario using real bathymetry data from the east pacific rise at 17 s

a model of late cenozoic transcurrent motion and deformation in the fore arc of northeast japan constraints from geophysical studies abstract we use recently acquired seismic reflection data and reprocessed sections to interpret the structure of the northern part of the ne japan forearc we identify a nnw sse trending deformation zone bounded by large transcurrent faults trends t1 t4 from west to east on the upper continental slope the lower slope is characterized by a frontal prism and reverse faults bathymetric data reveal several en echelon topographic bulges with nw se elongation aligned parallel to the fault trends the bulges are interpreted as a dextral wrench zone that has developed since the neogene based on structural contours of two late cenozoic seismic horizons clockwise rotation associated the wrenching is indicated by the paleomagnetism of paleogene marine sediments obtained from a drill hole within the faulted zone close to trend t1 a previous seismic refraction study shows remarkable contrast in p wave velocity across trend t2 indicating a considerable rearrangement of crust associated with transcurrent motion along this fault we propose a paleogeographic reconstruction assuming 200 km southward trenchward lateral transportation along trend t2 and its northern extension in our model subduction erosion at the northern japan trench was enhanced by trenchward migration of the fore arc sliver but steadied following rearrangement of the sliver along the transect the calculated variation in subduction erosion is consistent with subduction histories based on existing well data

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

electrical conductivity beneath the bolivian orocline and its relation to subduction processes at the south american continental margin 1 a long period magnetotelluric data set was obtained during 2002 and 2004 in the central andes to study the deep electrical conductivity structure in the region of the bolivian orocline between latitudes 17 s and 19 s the profile extends from the coastal cordillera in northernmost chile crosses the volcanic arc and the altiplano high plateau in central bolivia and ends in the eastern cordillera two dimensional inversion revealed several well defined conductivity anomalies in upper crustal levels the conductive sedimentary basins of the central altiplano and the resistive arequipa block beneath the western altiplano are imaged earlier seismological and magnetotelluric investigations on the southern altiplano inferred a large highly conductive partially molten body in the mid to deep crust it was assumed that this structure would be underlying the entire plateau but this is not the case according to the new models instead the most prominent feature in the new investigation area is a high conductivity zone at upper mantle depths below the high plateau which may be interpreted as an image of partial melts and fluids triggered by water supply from the subducting nazca slab this conductor would be in accordance with the standard subduction scenario it is however laterally offset by almost 100 km from the volcanic arc in contrast the deep crust and upper mantle beneath the arc is moderately resistive both observations may hint at an emerging shift of the magmatic fluid system in the central andes

insight into complex rupturing of the immature bending normal fault in the outer slope of the japan trench from aftershocks of the 2005 sanriku earthquake mw 7 0 located by ocean bottom seismometry 1 the distribution of aftershocks of a large mw 7 0 normal faulting earthquake beneath the outer slope of the japan trench in 2005 measured in 2007 using ocean bottom seismographs indicates that the earthquake was involved with a set of conjugate normal faults although the faults reach to the upper mantle the estimated vp and vp vs show no remarkable changes that can be related to extensive hydration in the crust or uppermost mantle the absence of horst graben topographic structure in the rupture area suggests that immaturity of the bending fault system is responsible for the relatively unhydrated lithosphere several earthquakes below the aftershock zone may belong to the lower plane seismicity of the shallow double seismic zone because no earthquakes were recorded in the area for more than 80 years before the 2005 event shallow extensional and deep compressive earthquakes may be activated concurrently in the focal area of this earthquake

reducing motion noise in marine magnetotelluric measurements by means of tilt records the analysis of marine magnetotelluric data is often complicated by disturbing signals that are caused by small scale periodic movements of the instrument the motion induced noise leads to a bias and or severe scattering in the derived magnetotelluric transfer functions both the motion itself and its effects on the magnetic and telluric time series are investigated in this study using an 80 d magnetotelluric data set that includes dynamic tilt records measured in the pacific ocean off costa rica we apply a standard motion removal technique as well as a newly developed method to correct for motion induced noise the resulting magnetotelluric transfer functions are of significantly better quality than the uncorrected ones furthermore the study of the properties of motion noise leads to conclusions about the optimal processing approach even in case of data sets where an explicit correction for that noise is not possible

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

adaptive finite element modeling using unstructured grids the 2d magnetotelluric example existing numerical modeling techniques commonly used for electromagnetic em exploration are bound by the limitations of approximating complex structures using a rectangular grid a more flexible tool is the adaptive finite element fe method using unstructured grids composed of irregular triangles an unstructured grid can readily conform to complicated structural boundaries to ensure numerical accuracy adaptive refinement usinganaposteriorierrorestimatorisperformediterativelytorefinethegridwheresolutionaccuracyisinsufficient tworecently developed asymptotically exact a posteriori error estimators are based on a superconvergent gradient recovery operator the first reliessolelyonthenormeddifferencebetweentherecoveredgradients and the piecewise constant fe gradients and is effective for lowering the global error in the fe solution for many problems an accurate solution is required only in a few discrete regionsandamoreefficienterrorestimatorispossiblebyconsidering the local influence of errors from coarse elements elsewhere in the grid the second error estimator accomplishes this by using weights determined from the solution to an appropriate dual problem to modify the first error estimator application of thesemethodsfor2dmagnetotelluricmtmodelingreveals as expected that the dual weighted error estimator is far more efficientinachievingaccuratemtresponses refiningabout15 of elements per iteration gives the fastest convergence rate for a given refined grid the solution error at higher frequencies varies in proportion to the skin depth requiring refinement about every

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

a newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere asthenosphere boundary the marine magnetotelluric mt method is a useful tool for offshore studies aimed at for example hydrocarbon exploration and the understanding of earth s tectonics marine mt data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land at mid ocean ridges the resistivity of earth s structure can be assumed to be two dimensional which allows mt data to be decomposed into a transverse electrical te mode with electric current flowing approximately along the ridge and a transverse magnetic tm mode with electric current flowing perpendicular to the ridge we collected marine mt data at the middle atlantic ridge which exhibited highly negative tm mode phases as large as 180 at relatively high frequencies 0 1 0 01 hz similar negative phases have been observed in other marinemtdata sets but have not been the subject of study we show here that these negative phases are caused by a newly distinguished coast effect the tm mode coast effect is not only a galvanic effect as previously understood but also includes inductive distortions tm mode negative phases are caused by the turning of the poynting vector the phase change of electromagnetic fields and vertically flowing currents in the seafloor the findings provide a new understanding of the tm mode coast effect which can guide our ability to fit the field data with the inclusion of coastlines and reduce misinterpretation of the data in offshore studies the study also shows that the tmmode coast effect is sensitive to the depth and conductivity of the asthenosphere an important feature of the earth s interior that was the object of our atlantic ocean study

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

genetic significance of an albian conglomerate clastic wedge eastern carpathians romania abstract the impressive 2000 m thick conglomerates of the bucegi formation exposed in the southernmost part of the eastern carpathians were interpreted initially as large alluvial fans and later suggested to be deposited as deepwater submarine slope deposits however the routing system of the coarse sediment transfer from the source area to the deepwater slope source to sink analysis has not been explained and the mechanisms involved in the shelf sediment storage and bypass onto the slope have not been discussed the present research on the albian bucegi formation has provided the following new insights on their source to sink aspect 1 that the upper member of the bucegi formation with its frequent channelized and sheet like fine conglomerates and sandstones contrasts greatly with the middle and lower members of deepwater slope and basin floor origin the upper member is interpreted as fluvial and shallow marine deposits that were temporarily stored and reworked on a shelf albeit a narrow one bridging the area between the deforming hinterland and the deepwater slope deposits 2 the upper and middle members are genetically linked and developed through the basinward migration of a large scale hundreds of metres in amplitude clinoform with relative flat lying topsets and slightly steeper few degrees coarser grained slopes that built out to the south and southeast a configuration that is common along continental margins and also generally along all types of deepwater basin margins 3 the middle member contains a range of submarine sediment density flows that vary from high density mobile debris flows to lower density sandy turbidites the sediment textures sorted grain populations inherited from the shelf sorting factory can to some extent still be recognised in the slope stratigraphy and 4 the large 10 20 m diameter carbonate and metamorphic olistoliths that are ubiquitous on the shelf and to a lesser extent slope reflect the steep gradients and very active tectonic setting of the fractured and thrusted hinterland from which these outsized blocks were transported onto the adjacent shelf the now proposed narrow shelf platform of the albian bucegi basin margin thus functioned to temporarily store sands and gravels to distinctly sort some of this sediment and to eventually bypass both sorted and new flood generated unsorted materials onto the slope compared with other basin margins this albian bucegi margin was extremely coarse grained because of its proximity to the actively deforming mountain range to a fractured basement that produced more gravel than sand to the great sediment flux from steep short rivers and to the narrowness 10 20 km of the shelf

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

electrical structure beneath the northern melt line on the east pacific rise at 15 45 s 1 the electrical structure of the upper mantle beneath the east pacific rise epr at 15 45 s is imaged by inverting seafloor magnetotelluric data obtained during the mantle electromagnetic and tomography melt experiment the electrical conductivity model shows no evidence for a conductive region immediately beneath the ridge in contrast to the model previously obtained beneath the epr at 17 s this observation can be explained by differences in current melt production along the ridge consistent with other observations the mantle to the east of the ridge at 60 100 km depth is anisotropic with higher conductivity in the spreading direction compared to the along strike direction similar to the 17 s region the high conductivity in the spreading direction can be explained by a hydrated mantle with strain induced lattice preferred orientation of olivine or by partial melt preferentially connected in the spreading direction

microporosity structure of coarse granular soils to date the microporosity structures of coarse soils with various coarse fines contents are still not fully understood in this study the pore size distributions psds of five types of soil varying from gravel to clay were characterized using mercury intrusion porosimetry the soil with a coarse content below 70 i e fines content above 30 is found to have a fines controlled microstructure which is sensitive to water content changes such soil forms a dual porosity structure due to compaction in which both intraaggregate pores and interaggregate pores are dominant after saturation the dual porosity structure evolves into a unimodal porosity structure dominated by the intraaggregate pores during drying such soil exhibits a significant reduction of total volume the soil with a coarse content above 70 instead has a coarse controlled microstructure which is stable upon water content changes such soil maintains dual porosity structures no matter if the soil is compacted saturated or dried as an example of application the measured psds are used to predict the soil water characteristic curves swccs for the test soils and the predictions are consistent with the swccs measured in the laboratory

wetting front advancing column test for measuring unsaturated hydraulic conductivity unsaturated hydraulic conductivity is the primary soil parameter required when performing seepage analyses for unsaturated saturated soil systems unsaturated hydraulic conductivity is also one of

the effect of seafloor topography on magnetotelluric fields an analytical formulation confirmed with numerical results summary magnetotelluric fields and impedances are distorted at undulating interfaces an analytical formulation is presented to calculate magnetotelluric effects in the presence of a sinusoidal interface in contrast to previous analytical approaches this formulation is not based on perturbation theory it is applicable to observations both on land and on the seafloor electric and magnetic fields as well as apparent resistivities and phases are calculated on the interface the topographic distortion on land mainly influences the tm mode data where the electric field is perpendicular to the geological strike both the tm mode and the orthogonal te mode data are distorted on the seafloor systematic parameter tests indicate which modes are independent of the period and the conductivity contrast provided that the induction depths are large relative to the amplitude of the topography the differing physics of the seafloor and land surface is illustrated by plotting current streamlines for the land model contour lines diverge below a hill and converge below a valley for the seafloor model electric currents mainly flow in the conductive sea water contour lines converge above a hill and diverge above a valley numerical results derived from finite element modelling support the analytical solutions streamlines of the electric current derived from a model for the central andes illustrate the connection between a graphical display of electromagnetic fields and an algebraic sensitivity analysis of the magnetotelluric impedance tensor