review of recent developments in tidal hydrodynamic modeling ii turbulence energy models the second part of this review deals with three dimensional tidal models for homogeneous and stratified sea regions in which the vertical eddy viscosity and diffusivity are computed from turbulence energy submodels the formulation of a range of turbulence energy models is presented and the solution of the three dimensional equations using finite difference methods in the vertical on a number of transformed grid log log linear is described the application of turbulence energy models is illustrated using results from a three dimensional model of tides in the irish sea a comparison of computed tidal current profiles determined using a two equation turbulence closure model with those derived using a simple flow dependent eddy viscosity model and with observations did not reveal any significant differences between the models recent work on the effect of stable stratification on turbulence energy and tidal currents is also considered as this is the more critical test of the accuracy of turbulence energy models processes influencing the internal tide generation in regions of steep topography are also examined using the three dimensional turbulence energy model with a prognostic density field the role of the nonlinear terms in the generation of short internal waves at the top of the shelf break and intense surface and bed mixing in this region is examined together with the effects of an upwelling and downwelling favorable wind

application of turbulence energy models to the computation of tidal currents and mixing intensities in shelf edge regions abstract the major steps in the formulation of a three dimensional shelf edge model using a sigma coordinate system in the vertical are briefly described vertical diffusion of momentum is parameterized using a range of turbulence closure models and results are compared with earlier calculations using a simple flow related viscosity the influence of the magnitude of horizontal eddy viscosity upon spatial variability at the shelf edge is examined the model is applied to the calculation of m2 and o1 tidal elevations and currents in the shelf edge region off scotland the hebrides shelf and comparisons are made with data collected in the region computed m2 tidal elevations and currents are shown to be in adequate agreement with observations with no significant differences between tidal profiles computed with the turbulence energy models or the earlier calculations using a simple viscosity model the magnitude of the horizontal eddy viscosity does not appear to affect this component of the tide tidal

the influence of wind direction upon flow along the west coast of britain and in the north channel of the irish sea abstract this paper briefly describes the development of a three dimensional nonlinear hydrodynamic model of the sea region off the west coast of scotland the north channel and the irish sea the model uses a sigma coordinate in the vertical and subgrid scale mixing of momentum is parameterized by a vertical eddy viscosity computed using a turbulence energy submodel numerical calculations illustrate the importance of wind direction and topography in determining flow pathways along the west coast of scotland through the north channel and within the irish sea the role of a northward flow into the irish sea through st george s channel arising from wind events over the celtic sea in controlling the flow in the north channel and deep regions of the irish sea is also considered the series of calculations presented here helps to explain the spatial and temporal variability of the observed flows from both direct measurements and tracer studies found off the west cost of scotland and in the north chan

a vertically averaged circulation model using boundary fitted coordinates abstract a two dimensional vertically averaged circulation model using boundary fitted coordinates has been developed for predicting sea level and currants in estuarine and shelf waters the basic idea of the approach is to use a set of coupled quasi linear elliptic transformation equations to map the physical domain to a corresponding transformed plane such that all boundaries are coincident with coordinate lines and the transformed mesh is rectangular the hydrodynamic equations are then solved by a multi operation finite difference technique in the rectangular mesh transformed grid comparisons of the circulation model predictions for tidally forced flows in a wedge section with both flat and quadratic bottom topography and in a flat channel with exponential variation in width were in excellent agreement with corresponding analytic solutions simulation of steady state wind induced setup in a closed basin formed using elliptic cylindrical coordinates also was in excellent agreement with the analytic

elastic finite difference modeling of the 1971 san fernando california earthquake finite difference seismograms calculated for the 1971 san fernando earthquake show strong effects due to lateral variation in sediment thickness in the san fernando valley and the los angeles basin using basin structure derived mostly from well logs and teleseismically determined source parameters two dimensional sh and p sv finite difference calculations reproduce the amplitude and duration of the strong motion velocities recorded across the los angeles and san fernando basins for the period range of 2 to 10 sec the edges of basins nearest the seismic source show ground motion amplification up to a factor of 3 over the case without the basin and tend to convert direct shear waves into love and rayleigh waves that travel within the basins

modeling the effect of wave current interaction on the three dimensional wind driven circulation of the eastern irish sea abstract a high resolution 0 5 north south by 1 0 west east three dimensional hydrodynamic model of the eastern irish sea is used to examine the influence of enhanced bed friction due to wind wave effects upon the three dimensional wind induced circulation of the eastern irish sea the model uses a mixed finite difference modal approach in which a standard finite difference grid is employed in the horizontal with the galerkin method with an expansion of functions modes in the vertical giving a continuous current profile from sea surface to seabed vertical eddy viscosity within the model is a function of the flow field the model is used to examine the wind induced response of the area to spatially uniform and constant in time northerly and westerly wind stresses of 1 n m 2 the effect upon bed stresses and hence the three dimensional circulation of the region of enhanced bed turbulence due to wind wave effects is also considered using idealized wave fields changes in bed stress particularl

the influence of eddy viscosity parameterization and turbulence energy closure scheme upon the coupling of tidal and wind induced currents a three dimensional model of the irish sea and the shelf and shelf edge region off the west coast of scotland is used to examine the interaction of wind induced surface currents and m 2 tidal currents which can lead to an increased near surface m 4 tidal current in the region calculations are performed using uniform southerly and westerly wind stresses with vertical eddy viscosity computed using either a simple flow dependent form of viscosity or one related to the flow and surface wind stress or derived using a two equation turbulence model resulting m 4 tidal distributions computed with the various viscosity formulations are compared with each other the spatial distribution the m 4 tidal current in the case of the flow dependent viscosity shows a significant enhancement of the surface m 4 tide due to non linear coupling between the surface shear and the tidally dependent viscosity in deep water this is the dominant mechanism giving rise to an enhancement in near surface m 4 current in shallow water however where other non linear processes are important in generating the m 4 tide namely changes in water depth and friction these are significant as well as the non linear surface shear effect in changing the m 4 tidal distribution however when an eddy viscosity formulation depending upon flow and surface wind stress is used the non linear coupling mechanism producing an enhanced m 4 tidal surface current is reduced and no increase in m 4 surface current occurs calculations with the two equation turbulence model do not produce an increase in the near surface m 4 tide as the near surface viscosity in this model is determined principally by the wind stress rather than the tidal current some conclusions concerned with developing a simple eddy viscosity model having similar characteristics to the turbulence energy model and experimental data sets for validation are considered at the end of the paper

order of magnitude increase in subducted h2o due to hydrated normal faults within the wadati benioff zone it is widely proposed that the oceanic mantle is hydrated by outer rise normal faults and carries large amounts of water to the deep mantle however the extent of oceanic mantle hydration is poorly constrained by existing observations and is a major source of uncertainty in determining the total water delivered to the mantle full waveform modeling of dispersed p wave arrivals from events deep within the wadati benioff zone of northern japan shows that hydrated fault zone structures are present at intermediate depths analysis of the p wave coda associated with events 5 35 km below the top of the slab gives an overall indication of the bulk hydration of the subducting oceanic mantle and can be explained by a 40 km thick layer that is 17 31 serpentinized this suggests that the top of the oceanic mantle is 2 0 3 5 wt hydrated subducting 170 318 tg m y of water per meter of arc beneath northern japan this order of magnitude increase in the estimated h 2 o fl ux in this arc implies that over the age of the earth the equivalent of as many as 3 5 present day oceans of water could be subducted along the kuril and izu bonin arcs alone these results offer the fi rst direct measure of the lower lithosphere hydration at intermediate depths and suggest that regassing of the mantle is more vigorous than has previously been proposed

salt dynamics in well mixed estuaries importance of advection by tides abstractunderstanding salt dynamics is important to adequately model salt intrusion baroclinic forcing and sediment transport in this paper the importance of the residual salt transport due to tidal advection in well mixed tidal estuaries is studied the water motion is resolved in a consistent way with a width averaged analytical model coupled to an advection diffusion equation describing the salt dynamics the residual salt balance obtained from the coupled model shows that the seaward salt transport driven by river discharge is balanced by the landward salt transport due to tidal advection and horizontal diffusion it is found that the tidal advection behaves as a diffusion process and this contribution is named tidal advective diffusion the horizontal diffusion parameterizes processes not explicitly resolved in the model and is called the prescribed diffusion the tidal advective diffusion results from the correlation between the tidal velocity and salinity and can be explicitly calculated with

double seismic zone in tonga the clear evidence of a double seismic zone beneath japan1 2 has led seismologists to search for other such zones elsewhere because they have been found in few other areas3 6 double seismic zones are thought to be rare and to depend strongly on a peculiar combination of external conditions such as age of the descending slab and the convergence rate 7 we report here a new double seismic zone which has been found beneath the tonga arc8 where the stress state in the subducting slab has been described as strongly down dip compressional7 9 10 this discovery suggests that the double seismic zone is a general feature of subducting slabs and may be found in many other subduction zones

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

a three dimensional baroclinic model of the irish sea formation of the thermal fronts and associated circulation a high resolution three dimensional baroclinic shelf sea model is developed and applied to the determination of the various processes influencing the generation and position of thermal fronts and the associated circulation in the irish sea the model has a horizontal grid resolution of approximately 3 6 km by 3 0 km and can resolve the thermal fronts in the region in particular in the western irish sea although the grid is not sufficiently fine to resolve small scale features along these fronts produced by baroclinic instabilities using meteorological forcing from dublin airport located close to the center of the region and tidal forcing a series of numerical experiments were performed to examine the processes influencing the location and dynamics of thermal fronts and associated circulation in the irish sea the results show a reasonable qualitative agreement with observations of the thermal stratification and the associated circulation in the western irish sea the ability of the model to reproduce the main features of the frontal structures in the region having been established it is used to examine the interannual variability of the density flow field calculations using meteorological forcing from different years show that the western irish sea fronts and the associated cyclonic circulation are persistent features although exact details of the fronts and their times of formation and breakdown show a large interannual variability model results also reveal a patch of thermal stratification in the northern part of the eastern irish sea with northward thermal density driven currents in this region

the influence of wind effects upon internal tides in shelf edge regions the interaction of the internal tide with wind induced currents in the shelf edge region off the west coast of scotland is studied using a baroclinic shelf edge model the model is used in cross shelf form with a horizontal finite difference grid of the order of 0 6 km and 50 sigma levels in the vertical to study the modification of the internal tide produced by upwelling and downwelling winds horizontal mixing in the model is parameterized using either the laplacian form of the horizontal diffusion or the biharmonic form and the sensitivity of the solution to both forms is examined coefficients for the vertical diffusion of momentum and density are determined using either an algebraic expression involving the richardson number or from a two equation turbulence energy submodel calculations show that in the case of an upwelling favorable wind the density gradient in the near bed region is increased leading to a slight modification compared to the tide only solution of the internal tide at the fundamental frequency with significant increases in amplitude of the higher harmonics due to the increase in the nonlinear terms produced by the increase in the density gradient with a downwelling favorable wind the amplitude of the current and internal displacement of the internal tide at the fundamental frequency are significantly reduced due to the change in the density field in the region of internal tidal production this also leads to a reduction in the amplitude of the higher tidal harmonics by using a fine grid in the horizontal the coefficients in the horizontal diffusion terms were set at a minimum and no significant difference in solutions computed with the laplacian or biharmonic diffusion terms was found similarly there are no differences in the major features of the flow field computed with the various parameterizations of the vertical diffusion although there are some differences in the magnitude of the diffusion coefficients

incoming plate faulting in the northern and western pacific and implications for subduction zone water budgets abstract the greatest uncertainty in the amount of water input into the earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench we explore the maximum depth of extension within the incoming plate at northwestern pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments we relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988 2011 by inverting teleseismic broadband p and sh waveforms we observe that the top 10 15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes 60 of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle 80 occur above 10 km within the mantle and 95 occur above 15 km there is evidence for variation throughout the different regions of study for example extensional earthquakes occur down to 20 km below the crust in the western aleutians and izu bonin we propose that the incoming plate mantle is most strongly hydrated in the upper 5 km and that partial serpentinization exists regionally within the incoming plate mantle to 15 km making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water we calculate that the total water carried into the northern and western pacific subduction zones is generally higher than previous estimates and is approximately 4 6 10 8 tg myr or 45 70 10 3 tg myr per kilometer of subduction zone

spreading rate dependence of melt extraction at mid ocean ridges from mantle seismic refraction data a variety of observations indicate that mid ocean ridges produce less crust at spreading rates below 20 mm yr 1 refs 1 3 reflecting changes in fundamental ridge processes with decreasing spreading rate the nature of these changes however remains uncertain with end member explanations being decreasing shallow melting or incomplete melt extraction each due to the influence of a thicker thermal lid here we present results of a seismic refraction experiment designed to study mid ocean ridge processes by imaging residual mantle structure our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo spreading rate changes in mantle velocity gradient basement topography and crustal thickness all correlate with this spreading rate change these observations can be explained by variations in melt extraction at the ridge with a gabbroic phase preferentially retained in the mantle at slower spreading rates the estimated volume of retained melt balances the approximately 1 5 km difference in crustal thickness suggesting that changes in spreading rate affect melt extraction processes rather than total melting

did paleo tethyan anoxia kill arc magma fertility for porphyry copper formation the tethyan orogen is host to numerous porphyry cu mo au deposits but the majority formed during subduction of the neo tethyan ocean basin in the late mesozoic cenozoic very few deposits have been found associated with paleo tethyan subduction we propose that this sparsity is due to widespread anoxia in the paleo tethyan ocean basin leading to the generation of relatively reduced arc magmas that were infertile for porphyry cu formation a compilation of published geochemical data indicates that neo tethyan arc rocks have higher average cu contents and v sc and sr y ratios compared to paleo tethyan rocks indicating higher magmatic oxidation states and greater fertility for ore formation during neo tethyan subduction subduction of relatively reduced oceanic lithosphere or reduction of normal moderately oxidized arc magmas by interaction with reduced lithosphere can therefore destroy the ore forming potential of arc magmatic suites

numerical study of tidal circulation and nonlinear dynamics in lunenburg bay nova scotia 1 a three dimensional ocean circulation model is used to study the barotropic tidal circulation and nonlinear tidal dynamics in lunenburg bay nova scotia the model performance is first assessed using the observed bottom pressures and currents made in septembers of 1991 and 2003 the model results reproduce reasonably well the observed tidal circulation in the study region the simulated tidal circulation is dominated by the semidiurnal m2 tide and an intense jet like flow through a narrow channel connecting lunenburg bay and two shallow coves to demonstrate the importance of nonlinear tidal dynamics in the region the first two harmonics of m2 i e m4 and m6 are computed from the model results forced by the m2 tide at the model open boundaries both m4 and m6 are relatively large in western lunenburg bay and vicinity indicating the importance of the nonlinear tidal dynamics over these areas to quantify the role of various nonlinear terms in generating the intense jet and associated residual flows in the region of the jet both transient and tidally averaged time mean momentum equations are examined the transient and time mean momentum balances demonstrate that the nonlinear interaction of the tidal currents with local bathymetry is balanced primarily by the horizontal pressure gradients momentum advection and dissipation the coriolis effect plays a minor role in the study region

eddy viscosity and tidal cycles in a shallow sea abstract the very pronounced spring neap tidal cycle in the south australian gulfs leads to an unusually large variation in the magnitude of the tidal currents measurements of the currents in these gulfs show that the non tidal circulation depends on the strength of the tidal currents and hence on the spring neap cycle a simple model is produced in which the vertical eddy viscosity is a function of both wind strength and tidal currents results from the model agree with the observations and suggest a means whereby variations in tidal mixing may be accounted for in hydrodynamic modelling of the circulation in shallow seas

a three dimensional model of diurnal and semidiurnal tides on the european shelf a three dimensional hydrodynamic model of the continental shelf is used to examine the sensitivity of computed diurnal k1 and o1 and semidiurnal m2 s2 and n2 tidal elevations and currents to changes in eddy viscosity profile friction coefficient and water depth the influence of the tide generating potential is also considered the model uses a standard finite difference grid in the horizontal with a functional approach in the vertical thereby giving a continuous tidal current profile from sea surface to seabed the magnitude of the vertical eddy viscosity depends upon the flow field comparisons are made between model computed profiles and harmonic analyses of 278 current time series at a range of locations in varying water depths with particular emphasis on locations where there are current measurements at a number of points in the vertical a detailed examination of these comparisons shows that frictional effects in the near bed region are particularly important in determining near bed shear and that an accurate specification of water depth is critical for tidal current magnitude the spatial distribution of computed current ellipses shows that the semidiurnal currents are strongest in shallow water with the diurnal currents exhibiting an intensification in the shelf edge region the inclusion of the equilibrium tide is particularly important for the correct determination of the magnitude of the k1 tidal current as well as k1 elevations in the north sea the detailed comparisons show that the magnitude of the tidal currents is correctly determined to within 5 cm s 1 at 220 locations with the model accurately to within 2 cm s 1 reproducing the vertical variation the most sensitive test of a three dimensional model of the tidal current at most locations

processes influencing storm induced currents in the irish sea abstract although the problem of predicting storm surge elevations has received significant attention the simulation of currents has suffered because of lack of current observations during surges current measurements made during surge conditions are presented here and are used in combination with three dimensional models to understand processes producing storm currents in the irish sea a coarse grid resolution of order 7 km model of the west coast of britain together with a fine grid of order 1 km model of the eastern irish sea is used to examine the processes namely open boundary forcing of the west coast model and wind fields that produced flows within the eastern irish sea during the storm surge of november 1977 simulations of the surge show that the fine grid model nested within the west coast model can reproduce observed coastal changes in surge elevation however an observed major inflow that was recorded by current meters in the region prior to a storm surge elevation peak is not repr

a high resolution three dimensional model of the m2 m4m6 s2 n2 k1and o1tides in the eastern irish sea abstract the spatial distribution of the m 2 s 2 n 2 m 4 m 6 k 1 and o 1 tidal elevations and currents in the eastern irish sea is examined using a high resolution three dimensional model of the region the model uses a functional representation of the current in the vertical and can resolve the near bed high shear bottom boundary layer comparison of computed and observed tidal elevations and currents shows that the grid is sufficiently fine of order 1 km and the near bed region sufficiently well resolved to enable tidal elevations and currents particularly near bed currents to be accurately reproduced in order to accurately reproduce the near bed currents its is essential to reduce eddy viscosity close to the bed and enhance the bottom friction coefficient above that used in two dimensional models calculations of the m 2 m 4 and m 6 tidal currents with and without the other tidal constituents show that it is essential to include the other constituents if tidal elevations and currents are to be determined with a high level of accuracy with the m 6 tide being significantly influenced by the presence of the other constituents the presence of other tidal constituents not only increases bottom friction in an analogous manner to a two dimensional model but because the model involves a flow dependent viscosity they increase its magnitude which affects current structure the calculation of parameters such as the maximum bed stress which is important in sediment transport can be severely underestimated if the other tidal constituents are not included

a juvenile oceanic island arc origin for the archean ca 2 97 ga fiskenaesset anorthosite complex southwestern greenland evidence from oxygen isotopes abstract the archean ca 2 97 ga fiskenaesset layered intrusion southwestern greenland consists of an association of anorthosite leucogabbro gabbro hornblendite pyroxenite peridotite and dunite the intrusion is characterized by well preserved igneous layering cumulate texture and primary igneous minerals including olivine pyroxene plagioclase hornblende and chromite we use new whole rock n 36 and mineral n 32 oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the fiskenaesset complex at majorqap q va to revisit geodynamic and petrogenetic hypotheses proposed for the origin of archean terranes the fiskenaesset complex has modern mantle like whole rock o isotope compositions o 18 5 8 0 5 average o 18 values increase from peridotite o 18 5 0 through hornblendite o 18 5 7 gabbro o 18 5 8 pyroxene hornblendite o 18 6 0 and leucogabbro o 18 6 3 to anorthosite o 18 6 3 these whole rock isotopic compositions reflect the approximate modal abundances of olivine average o 18 4 9 hornblende average o 18 5 7 clinopyroxene average o 18 6 4 and plagioclase average o 18 6 4 in each rock type as a consequence of mineral fractionation in the magma chamber s field relationships and the absence of crustal contamination suggest that the fiskenaesset complex formed in an oceanic setting subduction zone like whole rock trace element signatures and mantle like o 18 and initial e nd values are consistent with formation of these rocks in a juvenile oceanic island arc setting field and geochemical data from the fiskenaesset region and adjacent terranes suggest that the origin of archean crust in southwestern greenland is consistent with phanerozoic like plate tectonic processes rather than density driven sinking delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre existing continental crust mantle like o 18 values in the ca 2 97 ga fiskenaesset complex are inconsistent with substantial recycling of continental crust indicating that the fiskenaesset rocks originated distal from continental sources

geophysical evidence for hydration of the crust and mantle of the nazca plate during bending at the north chile trench water transported in subducting oceanic plates plays a key role in a number of phenomena including intraslab seismicity and arc magmatism however the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated we present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the nazca plate at the north chile trench crustal seismic velocities at the trench are lower than velocities of mature fast spreading crust and even lower than velocities of highly extended slow spreading crust in addition the nazca plate at the north chile trench may contain an 20 km thick upper mantle layer with 17 serpentine which implies 2 5 wt water these results document pervasive rock alteration by water percolation linked to bending related extensional faulting

zircon age constraints on sediment provenance in the caspian region sensitive high resolution ion microprobe shrimp u pb ages for detrital zircons from the caspian region reveal the age ranges of basement terrains that supplied the sediment one sample from the modern volga river has groupings at c 340 370 ma c 900 1300 ma and c 1450 1800 ma with a small number of older zircons this is consistent with derivation from the precambrian basement of the east european craton and palaeozoic arcs in the urals mid and late proterozoic components may be derived from beyond the present volga drainage basin such as the sveconorwegian orogen a bajocian sandstone from the greater caucasus has 73 zircons that post date 350 ma ages cluster at c 165 185 ma c 220 260 ma c 280 360 ma and c 440 460 ma this pattern suggests derivation from palaeozoic basement of the greater caucasus itself and or the scythian platform and igneous rocks generated at a jurassic arc in the lesser caucasus four samples from the lower pliocene productive series of the south caspian basin have common phanerozoic grains and groups between c 900 1300 ma and 1500 2000 ma each sample contains zircons dated to c 2700 ma the overall age patterns in the productive series samples suggest a combination of east european craton and greater caucasus source components

detailed structure and sharpness of upper mantle discontinuities in the tonga subduction zone from regional broadband arrays 1 recordings of deep tonga earthquakes from two arrays of 12 broadband seismographs each in the fiji and tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the tonga subduction zone the arrays operated as part of the seismic arrays in fiji and tonga saft experiment from july 2001 to august 2002 in comparison with the commonly used teleseismic approaches the short path lengths for the local data provide smaller fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness to enhance the low amplitude discontinuity phases s410p p660p and s660p deconvolved seismograms from each event array pair are aligned on the maximum amplitude of the direct p wave and subsequently slant stacked for the 410 km discontinuity the results show no systematic variations in depth with distance to the cold slab the 660 km discontinuity varies between 656 and 714 km in depth for the southern and central parts of the subduction zone the largest depths occur in the core of the tonga slab for the northern part two separate depressions of the 660 km discontinuity are observed these anomalies are interpreted as being induced by the active steeply subducting tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil vityaz trench respectively interpreting the deflections of the 660 km discontinuity in terms of local temperatures implies a thermal anomaly of 800 k to 1200 k at 660 km depth except for the southern region where it may thicken the width of the depressed 660 km discontinuity region implies that the tonga slab seems to penetrate the 660 km discontinuity with little deformation waveform modeling suggests that both the 410 and 660 km discontinuities are sharp the 660 km discontinuity is at most 2 km thick in many parts of the region and a first order discontinuity cannot be precluded the 410 km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab this suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities

low frequency variability of the scottish coastal current induced by along shore pressure gradients abstract mean currents over much of the continental shelf west of scotland have a northward component low frequency currents measured close to the west coast of scotland in 1983 and 1984 show a mean northward flow but indicate that it is highly variable over time scales corresponding to the passage of weather systems it is shown that coastal along shore pressure gradients are an essential part of the description of the along shore dynamics northward pressure gradient and current pulses are associated with depressions passing to the north of britain whilst less frequent southward pulses are the response to smaller depressions tracking further south across the british isles part of the observed northward mean flow in the region may thus result from a pumping effect arising from the passage of successive depressions to the north of britain

the m2 tide and its residual on the outer banks of the gulf of maine abstract tidal computations are reported for the gulf of maine with emphasis on its seaward banks georges bank browns bank nantucket shoals and the nearshore region off cape sable the model is 3d and nonlinear with quadratic vertical viscosity stratification is ignored the objective is to establish a theoretical baseline for tidal and residual currents on detailed topography against which seasonal changes may be examined two triangular meshes are used the base case corresponds to earlier studies with roughly 6 km resolution on the banks a more refined and extended mesh provides resolution at 2 3 km in critical areas and has boundaries farther removed from the banks both meshes provide very good fits with observed m2 elevations comparison of computed and observed m2 current profiles at 76 sites reveals general agreement that improves with resolution for the refined mesh and the 48 sites with records exceeding 60 days average deviations for m2 ellipse parameters are 6 1 4 7 cm s 1 for the

modeling the nonlinear effect of wind on rectilinear tidal flow 1 observations of near bed alongshore flow in 4 10 m depth at terschelling netherlands show that strong alongshore winds 15 20 m s in the flood direction reduce the tide current range and alter the tidal flow curve by bringing maximum flood flow forward in time and delaying maximum ebb flow these nonlinear effects which are more pronounced in shallower water are reproduced well by a single point model in the vertical driven by large scale o km sea surface gradients and wind stress using a quadratic bottom stress formulation and a time dependent eddy viscosity derived from a k e turbulence closure model subsequent idealized model simulations using a terschelling based m2 sea surface gradient and wind speeds between 0 and 20 m s show that the wind induced modifications of the tidal flow are consistent with the interaction of the wind induced flow with the tidal flow through the quadratic bed stress the wind driven flow enhances friction during the flood phase more than it reduces friction during the ebb flow thereby increasing friction over a tidal cycle and as a consequence reducing the tide current range the predicted increase in asymmetry of bed friction during flood and ebb flow with increasing wind speed also increases decreases the m4 m6 amplitude consistent with the observations the m4 m2 amplitude ratio is predicted to be largest when the flow has just become unidirectional that is when the wind driven flow equals the tidal current

a three dimensional model of diurnal and semidiurnal tides and tidal mixing in the north channel of the irish sea the spatial variability of the m2 s2 n2 k1 and o1 tides are examined using a high resolution model of the north channel of the irish sea the sensitivity of computed currents and elevations to two formulations of the vertical eddy viscosity one involving a turbulence energy submodel and another in which it is computed from the flow field are examined tidal elevation amplitudes and phases are compared at 20 gauges and shown to be in good agreement with observations root mean square rms errors of 7 85 cm 32 turbulence energy model and 7 93 cm 41 flow dependent viscosity model were found for the m2 tide comparable errors were found for the other semidiurnal tides differences in the amplitude errors between the two models are below the uncertainty associated with open boundary forcing and measurement errors and are therefore not statistically significant computed cotidal charts showed good agreement with published cotidal charts derived from observations comparison with observed m2 tidal current profiles suggests that at some locations a more accurate profile is determined with the turbulence model while at others the flow dependent solution is more accurate a comparison of m2 tidal current shear in the vertical showed that the turbulence energy model had a slight bias to underestimate this shear this bias was not found in the flow dependent viscosity model for the other tidal constituents the uncertainty in the accuracy of the currents prevented such a comparison also it is not possible to decide with any confidence which closure method is more accurate due to possible errors in the data and the small number of locations where significant shear is found in the vertical the spatial variability of tidal elevations and currents shows an amphidromic point for the semidiurnal tides but none for the diurnal tides in the northern part of the north channel significant differences in tidal current are found between the north channel and the clyde sea where tidal currents are strong and weak respectively this difference is related to the tidal energy flux in the region which is confined to the north channel the spring neap temporal and spatial variability of tidal turbulence shows a close correlation with tidal current magnitude and water depth this in part explains why the simple flow related viscosity model gives comparable results to those from the turbulence model

guided waves at subduction zones dependencies on slab geometry receiver locations and earthquake sources summary we investigate the geometry of deep subduction zone waveguides depth 100 km the wavefield characteristics for up dip profiles are described and compared with data recorded at the chile peru subduction zone observed distorted p onsets at stations in northern chile near 21 s can be matched by 2 d finite difference simulations of a thin low velocity layer lvl atop the slab in an iasp91 velocity model the replacement of the lvl by simple random velocity undulations in the slab in the same model cannot explain the observations varying slab geometries are investigated and the distribution of guided wave onsets originating in deep waveguides is predicted relative to the slab surface further double couple source position and orientation is explored and found to be closely limited by the guided wave observations sources situated above the layer and at distances more than 2 layer widths below the subducted moho are not suitable for the remaining favourable source locations a strong link between pulse shapes and fault plane dip angle is evident we conclude that up dip guided wave observations at subduction zones follow a simple pattern given by slab geometry and modified by source position the resulting onsets are shaped by layer thickness and velocity contrast and further influenced by the shape of the slab surface

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

the wind induced circulation and the interaction of wind forced and tidally driven currents on the european shelf abstract a three dimensional hydrodynamic model of the continental shelf is used to examine the wind induced flow on the shelf produced by uniform northerly and westerly wind stresses the model has a functional approach in the vertical and a finite difference grid in the horizontal a flow dependent eddy viscosity is used to parameterize vertical mixing of momentum initial calculations of the wind driven flow in the absence of tides are used to examine the spatial variability of the currents in response to uniform northerly and westerly winds and the time taken to reach a steady state results of subsequent calculations including tidal turbulence show that the wind induced flow is modified by the non linear interaction between tidal and wind driven currents arising from the flow dependent eddy viscosity term and bottom friction effects the spatial variability of wind induced currents on the shelf is related to topographic features and used to explain some of the variability found in the flow fields on the shelf that have been deduced from measurements a comparison of tidal current ellipses at the fundamental frequency and higher harmonic derived from a harmonic analysis of tidal and wind driven flows shows that the surface current ellipses particularly those of the higher harmonic are modified by the presence of the wind driven flow the processes giving rise to this modification are the time variability of the eddy viscosity and the near surface shear produced by the wind stress

evidence of upper mantle processes related to continental rifting versus oceanic crust in the gulf of california summary receiver functions from teleseismic events recorded by stations around the gulf of california are used to map the upper mantle seismic discontinuities we observe a mean transition zone thickness comparable to the global average for most of the region a low velocity layer is detected above the 410 discontinuity that varies in thickness along the gulf of california the 660 discontinuity shows complex waveforms south of latitude 30 n as a result of the phase change of garnet to perovskite within the transition zone a complex behaviour of the receiver functions is observed mainly at the southern end of the gulf the north south variations of this zone are likely associated with a slab window at the northern gulf resulting from the cessation of subduction of the farallon plate 12 ma and the subduction of the guadalupe and magdalena microplates at the southern end resulting in a hydrated upper mantle our results suggest that change in rifting styles occurring along the gulf of california mirrors deeper processes in the upper mantle

sensitivity of tidal bed stress distributions near bed currents overtides and tidal residuals to frictional effects in the eastern irish sea abstract a three dimensional nonlinear high resolution hydrodynamic model of the eastern irish sea is used to examine the influence of near bed viscosity and bottom friction upon the m2 tide and overrides in the region initially bed stress in the model is related to the depth mean current the formulation used in a classic two dimensional model the resulting hybrid two dimensional three dimensional model enables changes in current profile due to variations in eddy viscosity to he examined independently of those due to changes in bed stress also results from the model are compared with those from a three dimensional model with bed stress related to bottom current computed tidal elevations and surface currents are found to be fairly insensitive to the parameterization of bottom stress however tidal current profiles in the near bed region and maximum bed stress are found to be sensitive to variations in near bed viscosity and friction coefficient although reducing eddy viscosity in the near bed

a three dimensional model of the m4 tide in the irish sea the importance of open boundary conditions and influence of wind a fully nonlinear three dimensional hydrodynamic model of the irish sea is used to examine the processes determining the spatial variability of the m4 tide in the irish sea the model is based upon a mixed spectral finite difference approach with an expansion of functions in the vertical yielding a continuous current profile and a finite difference grid in the horizontal the model has a sufficiently fine grid to enable the nonlinear advective terms to be resolved in their finite difference form and the spatial variability of the m4 tide to be represented computed m4 elevations and currents determined using a radiation open boundary condition with both m2 mid m4 elevation and current input and a flow dependent viscosity are found to be in satisfactory agreement with observations considering the limited grid resolution of the model and the level of noise in the m4 tide as a percentage of the signal is much higher than for m2 this agreement is good enough to establish the credibility of using the model to examine the processes influencing the m4 tide which are discussed in detail the importance of an m4 current input to the radiation condition and the difficulty of using an elevation specified open boundary condition to reproduce the m4 tide in this region are considered in some detail the nonlinear advective terms are shown to have a limited local influence namely around headlands in generating the m4 tide the influence of wind effects upon m4 tidal currents particularly through the nonlinear term associated with a time dependent eddy viscosity is examined and shown to produce a significant increase in the m4 near surface tidal current and a modification of the current profile in essence the change of the m4 tidal current by meteorological effects will give rise to noise in any observed m4 current due to the random nature of the wind fields

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

wind influence on water exchange between the ria of ferrol nw spain and the shelf abstract the wind induced water exchange between the ria of ferrol nw spain and the adjacent shelf is analyzed by means of a three dimensional hydrodynamic model mohid2000 since thermohaline measurements indicate that the estuary can be considered vertically homogeneous a barotropic version of the model has been used real in situ wind measurements have been used for model calibration and further analysis the approach to study the wind effect on water exchange through the ria strait consisted in subtracting the signal calculated with only tidal forcing t s from the one with wind and tidal forcing wt s this shows the existence of a residual circulation with water flowing in wind direction along surfaces layers and a counter current flowing in the opposite direction at bottom layers the flux across the strait of ferrol has shown to increase linearly on wind strength with a correlation larger than 96 and reaching values close to 20 m 3 s 1 under real wind forcing

water input into the mariana subduction zone estimated from ocean bottom seismic data the water cycle at subduction zones remains poorly understood although subduction is the only mechanism for water transport deep into earth previous estimates of water flux1 3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres the main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle previous active source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate bending region near the oceanic trench4 7 however these studies do not constrain the depth extent of hydration and most investigate young incoming plates leaving subduction zone water budgets for old subducting plates uncertain here we present seismic images of the crust and uppermost mantle around the central mariana trench derived from rayleigh wave analysis of broadband ocean bottom seismic data these images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the moho discontinuity combined with estimates of subducting crustal water these results indicate that at least 4 3 times more water subducts than previously calculated for this region3 if other old cold subducting slabs contain correspondingly thick layers of hydrous mantle as suggested by the similarity of incoming plate faulting across old cold subducting slabs then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates3 because a long term net influx of water to the deep interior of earth is inconsistent with the geological record8 estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards9 seismic images of earth s crust and uppermost mantle around the mariana trench show widespread serpentinization suggesting that much more water is subducted than previously thought

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

gold deposits and occurrences of the greater caucasus georgia republic their genesis and prospecting criteria abstract the south central part of the greater caucasus region georgia republic represents an extremely prospective region for significant orogenic gold deposits gold bearing quartz veins are concentrated in two extensive wnw trending belts the mestia racha and svaneti districts within the northern margin of the southern slope zone of the great caucasus orogen this metalliferous region is dominated by early to middle jurassic slates which are part of a terrane that likely accreted to the continental margin from late paleozoic to jurassic the slates were subsequently intruded by both middle to late jurassic and neogene granitoids quartz veins in the more carbonaceous slate units are most consistently enriched in as au hg sb and w and show mineralization styles most consistent with typical orogenic gold deposits quartz veins in the mestia racha district were mined in soviet times for as sb and w but many of these are now being recognized as gold resource targets the veins occur in the footwall of a thrust fault between the southern slope zone and an earlier accreted terrane the main zone to the north many veins in the district continue along strike for 1 km and some cut neogene intrusions constraining ore formation to the most recent 4 to 5 million years gold deposition thus correlates with final collision of the arabian plate to the south and uplift of the ore hosting greater caucasus the zopkhito deposit previously mined for antimony contains an estimated 55 t au at a cutoff grade of 0 5 g t the veins are localized in an area where smaller order structures show a major change in strike from n s to more e w trends a pyrite arsenopyrite ore stage includes gold concentrated in both sulfide phases it is overprinted by a later stibnite dominant stage fluid inclusion studies of ore samples from the zopkhito deposit indicate minimum trapping temperatures of 300 to 350 c and 200 to 300 c for the two stages respectively and minimum trapping pressures of 0 2 to 0 5 kbar ore forming fluids with approximately 5 to 20 mol non aqueous gas evolved from n2 dominant to co2 dominant during evolution of the hydrothermal system 34s values of 1 to 4 for ore related sulfides at zopkhito are consistent with a sedimentary rock source for the sulfur and 18o quartz measurements of 16 to 21 are consistent with either a magmatic or metamorphic fluid more than 60 gold bearing lodes and placers in the svaneti district occur along the thrust between the southern slope and main zones lode gold potential was first recognized in the historic placer district in the 1980s with many auriferous quartz veins cutting middle jurassic igneous rocks brecciated veins in the 18 t au lukhra deposit cut a small granodioritic to dioritic stock the latter intrudes devonian schist immediately north of the thrust presently there are three recognized ore zones in the deposit with the most significant occurring over an area 140 m in length and 12 m wide with typical grades of 7 to 9 g t au reconnaissance fluid inclusion studies of ore samples from the lukhra deposit indicate minimum trapping temperatures of 220 c measurements of 18oquartz of about 10 suggest buffering of isotopic composition by the igneous host rocks

guided waves propagating in subducted oceanic crust 1 we use guided waves traveling updip along the surface of the nazca slab to image subducted oceanic crust at the chile peru subduction zone observed p onsets of intermediate depth events near 21 s in northern chile reveal waveguide behavior with growing focal depth low frequency energy 2 hz becomes more and more dominant and higher frequencies arrive delayed sometimes resembling two distinct phases to explain the observations we employ two dimensional finite difference fd simulations of complete p sv wave propagation along an updip profile of the subduction zone the fd calculations shed some light on several basic issues regarding crustal waveguides the development of guided waves dependent on event focal depth is simulated further we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle simulations also yield constraints on source locations relative to the low velocity structure finally the frequency content of p onsets is used to constrain the thickness of the waveguide the results indicate that a structure of 4 5 km average width and 7 low velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km the layer is interpreted as the unaltered lower part of the subducted oceanic crust suggesting that complete eclogite transformation in the chile peru subduction zone is unlikely to take place until beyond the volcanic front

a detailed comparison of a range of three dimensional models of the m2 tide in the faeroe shetland channel and northern north sea abstract a three dimensional hydrodynamic model of the faeroe shetland channel and northern north sea is used to investigate the spatial variability of m2 tidal elevations and currents in the region this area is chosen because it covers a range of water depths also there is a significant database of tidal elevations namely 41 gauges and current meters namely 89 observations with which comparisons can be made with the exception of a couple of measurements made at the shelf edge which may be influenced by the internal tide namely a 180 phase shift across the thermocline due to a first mode internal tide the observations correspond to those of a barotropic tide two different approaches are used to represent the profile of tidal currents in the vertical in the first a spectral functional method is used while in the second a finite difference grid is applied a range of parameterizations of vertical eddy viscosity suitable for deep water regions are used from ones in which viscosity is relate

imaging subducted slab structure beneath the sea of okhotsk with teleseismic waveforms the structure of subducted slabs is not well imaged in most global tomographic models with anomalies typically less than 1 synthetic waveforms for such models are not noticeably different from their 1d reference models in contrast we find that observed teleseismic waveforms sampling slabs in the down dip direction display multipathing features indicative of substantial structure such waveform complexity patterns appear distinctly different for various source locations with outer rise events generally displaying relatively simple waveforms here we model these waveform patterns for a portion of the kuril subduction zone the best fitting slab models have relatively fast cores 5 with smooth edges compatible with thermal modeling in shape complete seismograms for these 2d models indicate that multibounce p and s waves are slab sensitive and can be used in refining models

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

effect of subseabed salt domes on tidal residual currents in the persian gulf geological studies in the persian gulf pg have revealed the existence of subseabed salt domes with suitable filtering of a high resolution pg seabed topography it is seen that the domes leave their signature in the seabed i e numerous hills and valleys with amplitudes of several tens of meters and radii from a few up to tens of kilometers it was suspected that the shark skin of the pg seabed may affect the tidal residual flow the interaction of tidal dynamics and these obstacles was investigated in a nonlinear hydrodynamic numerical tidal model of the pg the model was first used to characterize flow patterns of residual currents generated by a tidal wave passing over symmetric elongated and tilted obstacles thereafter it was applied to the entire pg the model was forced at its open boundary by the four dominant tidal constituents residing in the pg each tidal constituent was simulated separately results i e tidal residual currents in the pg as depicted by lagrangian trajectories reveal a stationary flow that is very rich in eddies each eddy can be identified with a topographic obstacle this confirms that the tidal residual flow field is strongly influenced by the nonlinear interaction of the tidal wave with the bottom relief which in turn is deformed by salt domes beneath the seabed different areas of maximum residual current velocities are identified for major tidal constituents the pattern of trajectories indicates the presence of two main cyclonic gyres and several adjacent gyres rotating in opposite directions and a strong coastal current in the northern pg

constraints on subducting plate strength within the kermadec trench four specially designed surveys parallel to the kermadec trench allow localized estimates of plate strength within the subducting pacific plate to be made the transfer function between topography and gravity is estimated for five trench parallel ship tracks at distances of 25 110 km from the trench axis we find a clear reduction in the magnitude and peak wavelength of the transfer function from the outer rise to the trench axis the change in the transfer function indicates a decrease in plate strength and is consistent with a reduction in flexural rigidity by 3 5 orders of magnitude or a decrease in effective elastic thickness by more than 15 km such a large magnitude decrease in the effective elastic strength suggests that the plate has little or no elastic strength within the trench and that viscous stresses play an important role in transferring slab pull forces to the subducting plate and regulating plate speeds in subduction zones

elasticity of superhydrous phase b at the mantle temperatures and pressures implications for 800 km discontinuity and water flow into the lower mantle the thermodynamic properties and elasticity of superhydrous phase b shyb at high pressure and temperature are calculated using first principles calculations based on density functional theory the velocities and densities of shyb are significantly lower than those of bridgmanite and periclase the major minerals in the lower mantle the anisotropy of shyb is unremarkable at relevant mantle conditions the decomposition of shyb into bridgmanite periclase and water which can occur at a depth around 800 km at a cold slab will cause an increase of 7 5 15 0 and 12 on shear velocity compressional velocity and density respectively thus the decomposition of a small amount of shyb can produce a local discontinuity at the depth of 800 km the water released from the decomposition of shyb may rise upward and promote the partial melt which can further explain the low velocity zones just above 800 km discontinuity in western pacific subduction zones

the influence of mixing length formulation and stratification upon tidal currents in shallow seas abstract the mathematical formulation of a three dimensional shallow sea hydro dynamic model using a slip condition at the sea bed with sigma coordinates in the vertical and a range of turbulence energy closure schemes is briefly presented in this paper the equations are solved using finite difference methods in both the horizontal and the vertical and a time splitting approach is used to reduce the computational time the influence of changes in bed friction coefficient and formulation of the turbulence energy scheme upon tidal elevations currents and mixing intensities in a homogeneous sea is examined in detail comparisons are made with observations and with previous published current profiles in the irish sea using models of identical resolution and similar closure schemes but a no slip condition at the sea bed also comparisons are made with models using simple closure schemes in terms of a flow dependent eddy viscosity and a spectral approach in the vertical the influence of an imposed and maintained idealized vertical stratification upon tidal elevations current profiles turbulence profiles and mixing lengths is also considered the resulting changes in vertical structure of tidal current profiles both in terms of amplitude and phase are explained using previously published results from an idealized point model

modelling the response of the north channel of the irish sea to idealized time varying winds and boundary forcing abstract a three dimensional high resolution horizontal grid of order 1 km model of the north channel of the irish sea is used to examine the response of the region to wind and open boundary forcing at a number of periods calculations show that with wind forcing at the inertial period currents are produced through the water column although with longer and shorter period forcing the depth of penetration of the wind s momentum is confined to a near surface layer in the case of short period forcing the wind stress and elevation gradient terms are balanced by the acceleration term whereas at longer periods the acceleration term is small and the balance is between rotational and elevation gradient terms and the wind stress the direction of the wind stress which produces a maximum out flowing current in the centre of the north channel depends upon the period of the forcing and the depth below the surface of the out flowing current although calculations show that to first order the motion in the centre of the channel can be described by linear dynamics coupling between the time varying eddy viscosity and wind shear in the surface layer modifies the m 2 tidal current in the surface region in the case of flow produced by open boundary forcing the current is in essence aligned with the orientation of the channel and above the near bed region is dominated by currents at the period of the boundary forcing close to the bed higher frequency harmonics are generated although there is no evidence at any depth of the m 2 harmonic found in the wind driven flow case

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

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

single crystal equation of state of phase d to lower mantle pressures and the effect of hydration on the buoyancy of deep subducted slabs 1 an understanding of the physical properties of the hydrous magnesium silicate phase d is important for the interpretation of the seismic anomalies observed in subducted slabs and to evaluate the effect of hydration on slab dynamics here we report the equation of state of phase d mg1 1si1 8h2 5o6 up to 65 gpa obtained from high precision single crystal x ray diffraction a single crystal of phase d was loaded in a diamond anvil cell using helium as pressure transmitting medium to ensure quasi hydrostatic conditions during the entire data collection the volume of phase d decreases smoothly over the entire pressure range without the anomalies in the compressibility reported at 40 gpa in previous powder diffraction studies if existing in phase d a hydrogen bond symmetrization transition as predicted by first principles calculation is therefore not associated with anomalies in the volume compression behavior the isothermal bulk modulus kt and its pressure derivative k t obtained from the fitting of the unit cell volumes using a third order birch murnaghan equation of state are kt 151 4 1 2 gpa and k t 4 89 0 08 respectively this bulk modulus is in good agreement with a recent single crystal brillouin scattering experiment the presence of 16 vol of phase d in hydrous peridotite lithologies reduces the density by up to 2 6 at upper most lower mantle pressure temperature conditions 1273 k and 30 gpa phase d is thus a potential candidate to influence the buoyancy of hydrated stagnant slabs below the transition zone

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

elasticity of superhydrous phase b seismic anomalies in cold slabs and implications for deep water transport abstract seismic anomalies in the vicinity of cold slabs including low velocity zones and enhanced seismic shear wave splitting have been commonly attributed to the presence of hydrated slab material the dense hydrous magnesium silicate dhms superhydrous phase b shyb mg 10 si 3 h 4 o 18 is considered an important water carrier to transition zone depth due to its large pressure stability and abundance up to 20 vol in hydrous peridotites to interpret the observed seismic anomalies in terms of hydration and to assess the role of shyb in deep water recycling we have investigated the sound velocities and single crystal elasticity of shyb to pressures of 17 5 gpa at ambient temperature by brillouin scattering spectroscopy in diamond anvil cells the voigt reuss hill averages for the adiabatic bulk moduli k s shear moduli and their pressure derivatives yield k s 150 2 gpa 99 1 gpa k s p 4 7 2 and p 1 44 5 the aggregate compressional and shear wave velocities of shyb at transition zone pressures are comparable to those of hydrous iron bearing ringwoodite but are significantly lower than anhydrous phases in peridotites the calculated velocity contrast between dry and shyb bearing hydrous peridotite containing 1 2 wt of water indicate that 17 vol of shyb decreases both compressional v p and shear v s wave velocities by at most 2 5 at transition zone depths our results combined with data for the deformation mechanisms of shyb indicate that shyb aggregates develop strong textures under down dip compression regime and yield a maximum shear wave splitting of 1 1 in the plane perpendicular to the compression axis at 17 5 gpa although shyb in hydrous subducted peridotite would reduce its seismic velocities the splitting geometry v sv v sh generated by shyb fabrics in peridotite is incompatible with the pattern observed in tonga and sangihe and would require the contribution of other phases to be explained textured phase d is a plausible candidate to explain the shear wave splitting in the lower transition zone while at intermediate transition zone depths the coexistence of textured phase d and shyb in hydrous deformed peridotite would generate the observed array polarization however the later scenario will require locally very high water concentrations at depth

application of an unstructured mesh model to the determination of the baroclinic circulation of the irish sea 1 a three dimensional finite volume model with horizontally varying but fixed in time mesh is used to compute the baroclinic circulation of the irish sea during 1995 tidal forcing was applied along the model s open boundary with meteorological forcing taken from observations initial calculations were performed with a mesh that had high resolution in the well mixed near coastal region a necessary requirement in order to reproduce tides in the region although offshore in the stratified area the mesh was slightly coarser than that used in earlier finite difference models subsequent calculations were performed using an enhanced resolution which is significantly finer than earlier finite difference models in the offshore region which is thermally stratified in summer due to solar heating and low tidal mixing this produces a cold water bottom dome separated from the well mixed shallow water regions by strong tidal fronts calculations show that both model meshes can reproduce the observed major features of the baroclinic circulation of the western irish sea with the coarse mesh model giving comparable results to earlier finite difference models in the case of the finer mesh model there are sharper horizontal density gradients in the region of the fronts which show the presence of baroclinic instability and associated small scale variability as observed in satellite images but not found in the coarser mesh model due to lack of resolution results from the fine mesh model show significantly more spatial variability comparable to that found in the measurements

on the modification of tides in shallow water regions by wind effects 1 the influence of non linear effects upon tides in shallow coastal regions due to the presence of a significant storm surge is examined using a two dimensional model of the west coast of britain the model has an unstructured grid designed to have a high resolution mesh in the near coastal region of the eastern irish sea the area chosen as the focus of this study the influence of tide surge interaction upon the m2 m4 and m6 components of the tide due to surges produced by steady uniform wind stresses is examined in detail calculations show that in deep regions the tide is unaffected by the surge however in shallow coastal regions there is significant modification of tidal elevations and currents this arises because of changes in bottom stress and the non linear interaction term in the hydrodynamic equations in addition the locations of regions that wet and dry are changed during the tidal cycle due to the influence of the surge this gives rise to significant spatial variations and changes in magnitude of the tide and its higher harmonics depending upon wind stress direction and water depth these results explain why tidal energy remains in the surge residual in shallow water when it is computed by de tiding the total signal using a tide only calculation an effect often found in observed surge residuals

review of recent developments in tidal hydrodynamic modeling i spectral models in this paper we present a brief overview with a comprehensive set of references to recently published papers for detail of a range of techniques used in tidal hydrodynamic modeling and recent progress in the field although the paper aims to introduce the topic to someone new to the area the main emphasis is on new developments in near coastal shallow sea tidal modeling and consequently we concentrate on three dimensional modeling which has progressed significantly in recent years the paper gives a brief account of methods used to discretize the hydrodynamic equations in the horizontal namely finite difference finite element and boundary fitted coordinates and through time the representation of tidal current structure in a three dimensional model using a spectral method in the vertical is also described in this method the subgrid scale vertical diffusion of momentum which has a major influence on tidal current profiles is parameterized using a flow dependent eddy viscosity recent work on

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

three dimensional modal model of wind induced flow in a sea region abstract the solution of the linear three dimensional hydrodynamic equations describing wind induced flow in a sea region is developed using the galerkin method through the vertical a basis set of b splines is shown to have some computational advantages over a set of eigenfunctions vertical modes however a basis set of modes leads to a system of essentially uncoupled equations and current profiles can be interpreted in terms of vertical modes the influence of wind induced surface turbulence and turbulence at depth due to tidal motion upon current profiles in both deep 260 m and shallow 35 m sea regions is examined the variation in the angle between surface current and surface wind for different viscosity profiles and the effect of bottom friction upon it is considered the magnitude and direction of the surface current is significantly influenced by surface eddy viscosity however viscosity at depth due to tidal motion also has an important effect upon the surface current the time evolution of current structure following the sudden onset of a wind is examined using the modal model calculations show that the rate of damping of the internal modes is inversely proportional to the square of the depth consequently wind induced current structure takes longer to reach a steady state in a deep sea region than a shallow area the influence of sea surface elevation gradients in determining the direction of surface current is also considered

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

hydrodynamic model study of the ria de pontevedra under estuarine conditions the ria de pontevedra is one of the galician rias baixas partially mixed estuaries on the north west coast of the iberian peninsula the hydrodynamics of the rias is far from being fully understood in this application the hydrodynamics of the ria de pontevedra is studied by means of a 3 d baroclinic model our aim was to establish the circulation pattern driven by tide and the horizontal density gradient between river and shelf waters under estuarine conditions the spatial variability of tidal velocities and salinities is studied the residual component of the flow related to transport is described and discussed as expected in a partially mixed estuary a double layered residual pattern is observed with water flowing seaward in a surface layer and upstream in a bottom layer a cross channel residual flow not described up to date is presented

subduction zone guided waves and the heterogeneity structure of the subducted plate intensity anomalies in northern japan 1 the subducting pacific plate acts an efficient waveguide for high frequency signals and often produces anomalously large intensity on the eastern seaboard of northern japan during deep earthquakes the waveform records in the region of high intensity show a low frequency f 2 hz later arrivals with a long coda this behavior is not explained by a simple subduction zone model comprising a high velocity plate with low attenuation from the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the pacific subduction zone we demonstrate that the high frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0 5 km in thickness the standard deviation of wave speed fluctuations from the averaged background model is about 2 this new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness but low frequency waves with long wavelengths can easy tunnel through such lamina structure the result is frequency selective propagation characteristics with a faster low frequency phase followed by large and high frequency signals with very long coda a low wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high frequency coda three dimensional simulations using the earth simulator supercomputer for modeling of high frequency seismic wave propagation in the pacific subduction zone including plate heterogeneity clearly demonstrate the scattering waveguide effects for high frequency seismic waves traveling in the plate the region of large intensity for the heterogeneous model migrates away from the hypocenter into northern japan with an elongated zone along the pacific coast almost comparable to the observations from deep events in the pacific plate

late cretaceous early eocene tectonic development of the tethyan suture zone in the erzincan area eastern pontides turkey six individual tectonostratigraphic units are identified within the izmir ankara erzincan suture zone in the critical erzincan area of the eastern pontides the ayikayasi formation of campanian maastrichtian age is composed of bedded pelagic limestones intercalated with polymict massive conglomerates the ayikayasi formation conformably overlies the tauride passive margin sequence in the munzur mountains to the south and is interpreted as an underfilled foredeep basin the refahiye complex of possible late cretaceous age is a partial ophiolite composed of 75 by volume serpentinized peridotite mainly harzburgite 20 diabase and minor amounts of gabbro and plagiogranite the complex is interpreted as oceanic lithosphere that formed by spreading above a subduction zone unusual screens of metamorphic rocks e g marble and schist locally occur between sheeted diabase dykes the upper cretaceous karayaprak melange exhibits two lithological associations 1 the basalt radiolarite serpentinite association including depleted arc type basalts 2 the massive neritic limestone lava volcaniclastic association that includes fractionated intermediate composition lavas and is interpreted as accreted neotethyan seamount s the several kilometre thick karadag formation of campanian maastrichtian age is composed of greenschist facies volcanogenic rocks of mainly basaltic to andesitic composition and is interpreted as an emplaced upper cretaceous volcanic arc the campanian early eocene sutpinar formation 1500 m thick is a coarsening upward succession of turbiditic calcarenite sandstone laminated mudrock volcaniclastic sedimentary rocks that includes rare andesitic lava and is interpreted as a regressive forearc basin the late paleocene eocene sipikor formation is a laterally varied succession of shallow marine carbonate and siliciclastic lithofacies that overlies deformed upper cretaceous units with an angular unconformity structural study indicates that the assembled accretionary prism supra subduction zone type oceanic lithosphere and volcanic arc units were emplaced northwards onto the eurasian margin and also southwards onto the tauride gondwana related margin during campanian maastrichtian time further mainly southward thrusting took place during the eocene in this area related to final closure of tethys our preferred tectonic model involves northward subduction supra subduction zone ophiolite genesis and arc magmatism near the northerly eurasian margin of the mesozoic tethys

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

the response of the irish sea to boundary and wind forcing results from a three dimensional hydrodynamic model a three dimensional nonlinear hydrodynamic model of the irish sea is used to examine the response of the region to forcing along its open boundary and wind stress over the area frictional dissipation at the bed is parameterized with a quadratic law of bottom friction which can take account of enhanced frictional effects due to wind wave current interaction a standard finite difference grid is used in the horizontal with an expansion of functions through the vertical giving a continuous current profile from sea surface to seabed the model has a grid resolution of the order of 3 km and hence can resolve some features of the nearshore region the model is forced with m2 tidal input along its open boundary and time invariant wind fields corresponding initially to spatially uniform northerly and westerly wind stresses of 1 n m2 and subsequently to a west east gradient wind changing from 1 n m2 to 1 n m2 going from west to east and a north south gradient wind changing from 1 n m2 to 1 n m2 going from north to south the response to open boundary forcing through both elevations and currents is also examined and the influence of the effects of enhanced bottom friction due to wind waves is also briefly considered