sediment dynamics and resuspension processes in a shallow water posidonia oceanica meadow abstract in the study we describe the sedimentary dynamics in a shallow water seagrass meadow we examined the dynamics of vertical sedimentation flux over a posidonia oceanica meadow during the course of a one year investigation total sedimentary flux was monitored off the northern latium coast italy in the central tyrrhenian sea using sediment traps from october 2015 to december 2016 at three stations established on bare sand in the p oceanica meadow and on a sand patch within the meadow the traps were deployed during 11 surveys planned on the basis of the weather conditions and seagrass growth phase the trap content was analysed for the amount of mass trapped and particle grain size useful for describing sedimentary processes during storms the meteorological data p oceanica leaf biometry and sand patch evolution during the year were analysed to determine their relation to the vertical sedimentation flux the resuspension process dominated the shallow water dynamics but p oceanica buffers the resuspension especially during calm sea conditions the sedimentological tracers showed that when high energy flow regimes were present the vegetation near the bottom enhanced the deposition of particles transported from adjacent unvegetated areas during late summer leaf litter deposited on the unvegetated seafloor buffered the seabed sediment particles preventing resuspension we also detected the active transport of material from the sand patch to the meadow that was dependent on the energy flow regime showing that the vegetation plays two roles preventing resuspension and promoting deposition

boussinesq modeling of wave induced hydrodynamics in coastal wetlands in this paper an improved formulation of the vegetation drag force applicable for the fully non linear boussinesq equations and based on the use of the depth varying higher order expansion of the horizontal velocity in the quadratic vegetation drag law has been presented the model uses the same numerical schemes as funwave tvd but is based on the cactus framework the model is validated for wave height and setup against laboratory experiments with and without vegetation cover the wave attenuation results using the improved formulation were compared with those using the first order reference velocity as well as with analytical solutions using linear wave theory the analytical solution using the depth varying velocity predicted by the linear wave theory was shown to match the model results with the fully expanded velocity approach very well for all wave cases except under near emergent and emergent conditions when the ratio of stem height to water depth is greater than 0 75 and when the ursell ur number is less than 5 simulations during peak storm waves during hurricane isaac showed that vegetation is very effective in reducing setup on platforms and in reducing the wave energy within the first few hundred meters

seagrass on the rocks posidonia oceanica settled on shallow water hard substrata withstands wave stress beyond predictions abstract a multidisciplinary approach was applied to investigate the role of abiotic constraints in the settlement of posidonia oceanica on shallow rocks in two coastal areas of the ligurian sea italy nw mediterranean meadows developed very shallow upper limits at 1 5 m depth in both areas and with a distinctive morphology of stripes growing on rocky outcrops orthogonal to the coastline application of a predictive model to indicate the reference condition zone for the meadow upper limit already validated on meadows developing on soft bottoms was not adequate for these rocky substrata as the meadow upper limits were found shallower than model predictions 5 m depth geological and geomorphological characteristics of the rocky shores were analysed through geomechanic and petrographic analyses i e thin sections sem analyses rock hardness tests whilst the shape and the features of the meadows i e shoot density and maximum leaf length were assessed through scuba diving surveys among the different lithotypes occurring at the sites in the alternating and interbedded outcrops p oceanica was passively selected i e due to the seedlings survival and settlement there on the strongest i e less erodible lithotypes whilst the comparatively weaker and more erodible rocks remained unvegetated and covered by a layer of soft sediments p oceanica settling on specific rocky substrata with favourable lithological and geomechanical characteristics is able to establish outside the theoretical reference zone predicted by the model for soft sediments due to greater attachment strength and possible resistance to hydrodynamic forces combining biological ecological mineralogical geological and geomorphological approaches was effective for explaining the primary role of substratum nature in the spatial variability of seagrass meadows with geomechanical and lithological characteristics of the rocks being equally important abiotic factors than sedimentological features

coastal and near shore changes correlated with die back in eel grass zostera marina l abstract the die back of eel grass zostera marina l is found to have played an important role with regard to near shore sedimentation and coastal changes in a natural harbour at kyholm denmark a hiatus is found between silty sediments that date from about 4000 bc and overlying modern sandy sediments it is suggested that this is a consequence of the die back of eel grass in the nineteen thirties resulting in mobilisation and disturbance of nearshore sediments and the shoreward movement of mobilised sandy material into the harbour the coastal morphology of kyholm was relatively stable from 1802 to 1933 but between 1933 and 1978 there have been two periods of drastic progradation correlated with die back of eel grass

quantitative estimates of interplate coupling inferred from outer rise earthquakes interplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones the spatial and temporal variations of such coupling control the patterns of subduction zone seismicity we calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact to quantitatively estimate the degree of interplate coupling for the tonga new hebrides kurile kamchatka and marianas subduction zones depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models we perform waveform modeling of body waves from the gdsn network to obtain reliable focal depth estimates for 24 outer rise earthquakes a propagator matrix technique is used to calculate outer rise stresses in a bending 2 d elastic plate floating on a weak mantle the modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate we estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location

much damage for little advantage field studies and morphodynamic modelling highlight the environmental impact of an apparently minor coastal mismanagement while coastal management activities have long been known to exert a strong influence on the health of marine ecosystems neither scientists nor administrators have realized that small interventions may lead to disproportionately larger impacts this study investigated the broad and long lasting environmental consequences of the construction of an ill planned although small only 12 m long jetty for pleasure crafts on the hydrodynamic conditions and on the meadow of the seagrass posidonia oceanica of an embayed cove in the ligurian sea nw mediterranean there p oceanica used to develop on a high 1 5 m matte a lignified terrace causing seafloor elevation in which the leaves reach the surface and form a compact natural barrier to waves in front of the beach such a so called fringing reef of p oceanica is today recognized of high ecological value and specific conservation efforts are required the construction of the jetty implied the cutting of the matte which directly destroyed part of the fringing reef in addition meadow mapping and sedimentological analyses coupled with morphodynamic modelling showed that the ecosystem of the whole cove had been greatly altered by the jetty we used the geometric planform approach a proper tool in the study of headland controlled embayment both to characterise the present situation of prelo cove and to simulate the original one before the jetty was built in the long term such a small jetty completely altered the configuration and the hydrodynamic conditions of the whole cove splitting the original pocket beach into two smaller ones and creating strong rip currents flowing seaward along the jetty these rip currents enhanced erosion of residual shallow portions of the meadow and further modified the sedimentary fluxes in shallow waters a century after the construction of the jetty an irreversible environmental damage has occurred as the slow growing rate of p oceanica implies that the high matte terrace and the fringing reef will hardly form again even after the removal of the jetty the lesson learnt from this study is that even such small and therefore reputed intrinsically innocent interventions on the coastal zone require accurate planning based on interdisciplinary studies to understand and respect the delicate interplay among morphological hydrodynamic and ecological components

high sedimentation rates in a karstic lake associated with hydrothermal turbid plumes lake banyoles catalonia ne spain abstract in lake banyoles northeastern spain warm water enters through lake bottom seeps and generates two hydrothermal turbid plumes one of them is perennial in character and the other is active only after fluidisation events of the bottom sediment caused by reactivation of the bottom seeps after rainfall in the aquifer recharge area the vertical development of both hydrothermal plumes is very sensitive to the thermal stratification of the water column when a plume reaches a level of neutral buoyancy a turbidity current spreads laterally and transports sediment particles across the lake silt particles transported by the plume are used in adcp backscatter images to determine its maximum and equilibrium heights field results are compared and found to be in accordance with models for thermal convection from finite isolated sources when the lake is stratified the vertical transport of sediment is confined to the hypolimnion when the lake water column is mixed the plume reaches the surface of the lake the turbidity current is usually confined to the southern sub basin of the lake during stratification resulting in higher sedimentation rates during fluidisation events the sedimentation rates are one order of magnitude greater than in periods without fluidisation for the fluidisation periods turbidity has been estimated to be 10 ftu

effects of short flexible seagrass zostera noltei on flow erosion and deposition processes determined using flume experiments innovative flume experiments were conducted in a recirculating straight flume zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth the aims of this study were i to quantify the combined effects of leaf flexibility and development characteristics of zostera noltei canopies on their interaction with hydrodynamics and ii to quantify the role of zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses velocity within the canopy was significantly damped the attenuation in velocity ranged from 34 to 87 compared with bare sediments and was associated with a density threshold resulting from the flow induced canopy reconfiguration the reduction in flow was higher in dense canopies at higher velocities than in less dense canopies in which the reduction in flow was greater at low velocities these contrasted results can be explained by competition between a rough wall boundary layer caused by the bed and a shear layer caused by the canopy the velocity attenuation was associated with a two to three fold increase in bottom shear stress compared with unvegetated sediment despite the increase in near bed turbulence protection of the sediment against erosion increased under a fully developed meadow while sediment properties were found to be the main factor controlling erosion in a less developed meadow deposition fluxes were higher on the vegetated bed than on bare sediments and these fluxes increased with leaf density fewer freshly deposited sediments were resuspended in vegetated beds resulting in an increase in net sediment deposition with meadow growth however in the case of a very high leaf area index sediment was mostly deposited on leaves which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow

the effect of oceanic crustal structure on phase changes and subduction abstract recent numerical models for the thermal regime of subduction zones are in disagreement with observations of depth to formation of eclogite which require higher temperatures than those found in the models this discrepancy may be reconciled if more accurate geometric depictions of the subducted slab are incorporated in the modelling studies the transition from 15 dip along the thrust plane to 30 along the benioff zone starts at depths of about 40 km dehydration reactions and the gabbro eclogite phase change are presumed to increase the average density of the lithosphere and to cause the steepening of the subducted lithosphere the benioff zone seismicity which starts at about 40 km depth may itself be a result of stresses caused by the phase changes since the gabbro eclogite transformation occurs in gabbroic sections of the crust thickened crustal sections beneath oceanic plateaus will affect subduction first the width of the thrust zone is reduced due to the increased dip angle because of early transformation of lower and warmer crustal sections second incomplete conversion of the entire crustal thickness results in an average density intermediate between unsubducted oceanic lithosphere and fully transformed normal lithosphere the first effect leads to decreased thrust mechanism seismicity because the coupling between the plates is reduced the second effect leads to flat lying subcontinental subduction or arcward advance of the trench in suboceanic subduction

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

wave induced drag force on vegetation under shoaling random waves abstract this paper provides a practical method for estimating the drag force on a vegetation field in shoaling conditions beneath non breaking and breaking random waves this is achieved by using a simple drag formula based on two empirical drag coefficients given by mendez et al 1999 and mendez and losada 2004 respectively in conjunction with a stochastic approach here the waves are assumed to be a stationary narrow band random process and propagating in shallow waters the effects of shoaling and breaking waves are included by adopting the mendez et al 2004 wave height distribution results are presented and discussed for different slopes and an example of calculation is also provided to demonstrate the application of the method

reconstruction of subduction zone paleogeometries and quantification of upper plate material losses caused by tectonic erosion we propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins we first test the method using isostatic and elastic models applying it to 15 different well constrained convergent margins the main conclusion is that most of them are presently close to the isostatic equilibrium consequently we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation then we apply this model to compute the geometry of the palcoslab knowing the palcotopography of the margin the comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge two cases were studied for palcoreconstructions japan and peru the results show that a considerable amount of material was removed during the neogene along both margins a detailed interpretation of the neogene history of the japan margin is proposed on the basis of our results

modified hydrodynamics in canopies with longitudinal gaps exposed to oscillatory flows summary longitudinal gaps are commonly found in aquatic canopies while the ecological significance of gaps may be large we know little about their impact on the hydrodynamics within the canopy we used laboratory experiments to investigate the effect of longitudinal gaps within canopies exposed to a wave field in rigid submerged and emergent vegetation wave velocities were reduced compared to the case without vegetation flexible canopies also attenuated waves but this attenuation was lower than for rigid canopies the presence of the gap modified the mean current associated with the waves in both the gap and the lateral vegetation a gap within a canopy of 5 solid plant fraction did not show differences in the wave attenuation between the gap and the lateral vegetation in contrast gaps within canopies of 10 solid plant fraction resulted in large differences between the gap and the lateral vegetation in all the experiments the effect of a gap within a canopy reduced the wave attenuation within the lateral vegetation adjacent to the gap when compared with a canopy without a gap in canopies with rigid plants the lateral vegetation modified the wave attenuation in the nearby gap in contrast the lateral flexible vegetation did not produce any effect on the wave attenuation of the adjacent gap canopy density plant height and plant flexibility were critical for determining the hydrodynamics throughout the canopy and in the gap

small scale mantle flows and induced lithospheric stress near island arcs summary this paper explores the middle ground between complex thermallycoupled viscous flow models and simple corner flow models of island arc environments the calculation retains the density driven nature of convection and relaxes the geometrical constraints of corner flow yet still provides semianalytical solutions for velocity and stress a novel aspect of the procedure is its allowance for a coupled elastic lithosphere on top of a newtonian viscous mantle initially simple box like density drivers illustrate how vertical and horizontal forces are transmitted through the mantle and how the lithosphere responds by trench formation the flexural strength of the lithosphere spatially broadens the surface topography and gravity anomalies relative to the functional form of the vertical flow stresses applied to the plate base i find that drivers in the form of inclined subducting slabs cannot induce selfdriven parallel flow however the necessary flow can be provided by supplying a basal drag of 1 5mpa to the mantle from the oceanic lithosphere these basal drag forces create regional lithospheric stress and they should be quantifiable through seismic observations of the neutral surface the existence of a shallow elevated phase transition is suggested in two slab models of 300 km length where a maximum excess density of 0 2 g m was needed to generate an acceptable mantle flow a north new hebrides subduction model which satisfies flow requirements and reproduces general features of topography and gravity contains a high shear stress zone 75mpa around the upper slab surface to a depth of 150 km and a deviatoric tensional stress in the back arc to a depth of 70 km the lithospheric stress state of this model suggests that slab detachment is possible through whole plate fracture

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

a triple planed structure of seismicity and earthquake mechanisms at the subduction zone off miyagi prefecture northern honshu japan abstract a detailed cross section of seismicity combined with types of earthquake mechanisms was constructed for the subduction zone off miyagi prefecture northern honshu japan where a large m s 7 5 earthquake occurred on june 12 1978 nodal plane solutions were determined for fourteen earthquakes with body wave magnitudes 5 4 using data recorded at wwssn and japanese stations for smaller magnitude events the types of focal mechanism were analysed using p wave first motion data recorded at regional stations the results obtained are 1 for the area 150 200 km landward from the trench axis the cross section suggests a triple planed seismic zone that is the zone of thrusting overlaps the so called double planed seismic zone the upper and lower planes of the double planed seismic zone are characterized by down dip compression and down dip tension respectively 2 thrusting terminates at a depth of 60 km where the aseismic front in the area is located and it merges into the zone of down dip compression in the deeper part in a down dip direction in contrast the depth of the zone of down dip compression appears to shift by 10 km at the aseismic front 3 the extension of the zone of down dip compression seaward beyond the aseismic front and the shift of the depth of the zone may be caused by loading within the descending slab through a mechanical coupling between the plates at the focal area of the 1978 earthquake prior to its occurrence

volcanic ash deposition eelgrass beds and inshore habitat loss from the 1920s to the 1990s at chignik alaska abstract we quantified the shallowing of the seafloor in five of six bays examined in the chignik region of the alaska peninsula confirming national ocean service observations that 1990s hydrographic surveys were shallower than previous surveys from the 1920s castle bay chignik lagoon hook bay kujulik bay and mud bay lost volume as calculated from mean lower low water chart datum to the deepest depths and four of these sites lost volume from mean high water to the deepest depths calculations relative to each datum were made because tidal datum records exhibited an increase in tidal range in this region from the 1920s to the 1990s our analysis showed that mud bay is quickly disappearing while chignik lagoon is being reduced to narrow channels anchorage bay was the only site that increased in depth over time perhaps due to erosion volcanoes dominate the landscape of the chignik area they have blanketed the region in deep ash deposits before the time frame of this study and some have had smaller ash producing eruptions during the time frame of this study remobilization of land deposited ash and redeposition in marine areas in some locations facilitated by extensive eelgrass zostera marina beds covering 54 of chignik lagoon and 68 of mud bay in 2010 is the most likely cause of shallowing in the marine environment loss of shallow water marine habitat may alter future abundance and distribution of several fish invertebrate and avian species

biological modifiers of marine benthic seascapes their role as ecosystem engineers benthic organisms in marine ecosystems modify the environment on different spatial and temporal scales these modifications many of which are initially at a microscale are likely to have large scale effects on benthic seascapes this is especially so if the species are ecosystem engineers most species of infaunal and epifaunal invertebrates and macrophytes contribute at a geophysical or geochemical level microorganisms also play a key but currently neglected role in the intertidal and immediately sublittoral zone algae and seagrasses and mussels in mussel beds have received considerable attention a substantial fossil record also exists mathematical modelling of these systems is still in its infancy although several sophisticated mathematical tools have been applied the effects of bioturbation and of microorganisms have been less studied and little is known about the activities of benthic organisms in the deep sea this paper addresses all these effects and places them in the context of large scale benthic seascapes and of the extensive literature on species defined as ecosystem engineers in the sea

effects of a turbid hydrothermal plume on the sedimentation rates in a karstic lake 1 the formation of a hydrothermal plume has been recently found in the main basin of a karstic lake the hydrothermal plume carries a concentration of particles in suspension that are transported up to an equilibrium depth at the equilibrium level particles are transported horizontally as a turbidity current the particle volume concentration in the hypolimnion where the turbidity current occurs has been found to be homogeneous for points close to the source of the plume however there is a decrease in the particle concentration points from close to the source of the plume to the points far from the source which indicates that the sedimentation of particles from the turbidity current occurs this sedimentation process has been attributed mainly to double diffusive sedimentation rather than to single particle sedimentation here we demonstrate the effect of this turbidity current on the zonal sedimentation rates at the southern lobe of the lake

characterization of soils beneath a posidonia oceanica meadow the study of a 475 cm core spanning 4316 y bp sampled in a posidonia oceanica meadow portlligat bay nw mediterranean allowed us to make the first detailed description of the soil below this endemic seagrass the sediments under p oceanica often referred to as mat or matte are low density average bulk density of 0 69 g dw cm 3 marine soils mainly composed of siliciclastic 46 and biogenic carbonated 46 fine grained sediments particles 1 mm and finer organic matter 3 of som 1 mm the mat is heterogeneous and has a high total organic matter tom content in the upper layers average of 32 in the top 50 cm after ca 500 years of burial after which the layers become increasingly dominated by inorganic fractions ranging from 20 to 1 of tom in 50 475 cm the tom content in the mat decreases exponentially at an overall rate of 0 0005 y 1 p oceanica sediments have been found to contain the highest areal stocks of tom and organic c 194 kg dw tom m 2 and 79 kg corg m 2 respectively out of all seagrasses the average corg refractory burial rates were estimated to be 21 g corg m 2 y 1 carbon as carbonates accreted at a rate of 54 g ccarb m 2 y 1 for the mat thickness studied the two c fractions yielded a total stock that is among the highest ever recorded in terrestrial and marine soils 282 kg ctot m 2 the mat was also found to trap large amounts of sediment rate 899 g dw m 2 y 1 stock 3342 kg dw m 2 and in particular muddy silt and clay fractions rate 120 g dw m 2 y 1 stock 417 kg dw m 2 the results obtained in this study provide sound additional proof of the valuable role p oceanica plays in stabilizing coastal sediments and sequestering c and also of its potential as a holocene palaeorecord the p oceanica mats located at shallow depths 2 m can be tentatively classified as a limnic subaquatic histosols calcaric eutric world reference base for soil resources 2007

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

sediment retention by a mediterranean posidonia oceanica meadow the balance between deposition and resuspension the role of posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at fanals point nw mediterranean spain by comparing particle deposition within the meadow and adjacent bare sediment small sediment traps were used to measure deposition within and above the meadow and over bare sand a model based on measurements of particle deposition at increasing distances from the bottom was used to partition the total depositional flux between primary sediment particles deposited for the first time at the measuring site and resuspended deposition sediment particles that have been previously deposited at the measuring site measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition and to form a balance of particle transport at the annual time scale significant differences in total deposition were found over time ranging from 1 5 to 500 g dw m 2 d 1 including those between bare and vegetated sediments the effect of p oceanica in increasing primary deposition at an annual scale was modest however p oceanica significantly buffered sediment resuspension which was reduced more than three fold compared to the unvegetated bottom the annual flux of deposition was dominated by settling of resuspended materials which represented 85 of the total flux within the meadow but 95 of the total deposition on bare sand thus seagrass meadows reduce resuspension in the nw mediterranean littoral thereby contributing to increased sediment retention and therefore reducing erosion in the coastal zone

role of shallow phase changes in the subduction of oceanic crust detailed studies of the seismicity of several subduction zones demonstrate that shallow dipping thrust zones turn to steeper angles at depths of about 40 kilometers an increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle in addition the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper benioff zone

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

depositional response to seagrass mortality along a low energy barrier island coast west central florida abstract analysis of aerial photographs and nearshore surficial sediment samples from the northern islands of the west central barrier system of florida indicates that 1 seagrass beds in the nearshore zone have controlled onshore longshore sand transport and 2 resulting sedimentary accumulations within nearshore seagrass beds make differentiation between nearshore and backbarrier facies difficult between 1957 and 1967 an extensive seagrass community occupying the nearshore zone off anclote key disappeared thus allowing the sudden and rapid onshore and longshore transport of sand the 1 000 year old barrier island lengthened 30 percent by recurved spit growth in a 15 year period from 1967 82 although there are no direct observations four possible causes of seagrass mortality have been postulated which are as follows 1 physical destruction by storms 2 infection by pathogens 3 decline of water quality due to human development and 4 overgrazing by sea urchins because of the ability of seagrasses to trap fine grained sediments contribute organic matter and provide for low energy sheltered molluscan biocoenose strong similarities remain between surficial sediments of recently devegetated active nearshore zones and backbarrier lagoonal facies the stable carbon isotopic ratios and the molluscan assemblages within the ubiquitous fine quartz sands in particular are similar within these two normally disparate environments this study indicates that the development and destruction of benthic floral communities should be considered as a process that generates episodic cyclic depositional events in the sedimentary record additionally such changes in seagrass communities should be expected to present a blurred distinction between nearshore and backbarrier sedimentary facies

ecosystem engineering by annual intertidal seagrass beds sediment accretion and modification seagrasses are generally known as ecosystem engineers as they reduce flow velocities in their canopies in perennial subtidal meadows this usually leads to increased net sedimentation rates and reduction of the grain size the present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat within the planting units 79 shoots m 2 4 7 mm of sediment accreted whereas in the most dense parts of these units 199 shoots m 2 accretion amounted to 7 1 mm the silt fraction 63 m increased and the sand fraction 63 500 m decreased in the eelgrass beds which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass presence annual intertidal eelgrass beds significantly contribute to the immobilisation of sediment during the growing season with its magnitude depending on canopy density during winter the accumulated sediments were released again and could even induce additional erosion possible consequences of these sediment dynamics for the larger scale functioning of estuarine ecosystems are discussed

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

environmental evolution and geological significance of the miocene carbonates of the eratosthenes seamount odp leg 160 abstract miocene carbonates of the eratosthenes seamount located offshore from cyprus in the eastern mediterranean odp leg 160 site 966 have been re analysed the use of sedex sequential extraction to evaluate the nutrient concentrations and of ct scan for the identification of large benthic foraminifera has resulted in a more detailed palaeoenvironmental reconstruction and an improved stratigraphic framework three intervals are recognised in the succession and are attributed to the lower middle and upper miocene respectively the lower miocene interval is dominated by large benthic foraminifera and echinoids this association colonized the top of the seamount following its uplift from deep water to the photic zone the middle miocene interval is characterised by rhodoliths and corals at its base and near the top whereas seagrass seaweed assemblages with epiphytic foraminifera and hooked coralline algal crusts dominate its central part the upper miocene is represented by coral reef facies near its base and lagoonal facies toward its top the succession was deposited in an oligotrophic environment and exhibits a general shallowing upward trend that was mainly controlled by tectonic activity related to the northward subduction of the african plate the succession of the eratosthenes seamount is characterised by a diverse large benthic foraminifera assemblage in its lower part and by corals in its uppermost part whereas corallines characterise the central interval this trend is common in miocene mediterranean carbonates and its occurrence on an isolated seamount confirms its connection with the general evolution of the basin the decline of miogypsinids and lepidocyclinids after the early miocene is notable because the eratosthenes seamount was geographically close to the indo pacific ocean where both groups also thrived during the middle miocene this decline was probably caused by the early miocene closure of the deep water connection between the indo pacific and the mediterranean

sediment deposition and production in se asia seagrass meadows abstract seagrass meadows play an important role in the trapping and binding of particles in coastal sediments yet seagrass may also contribute to sediment production directly through the deposition of detritus and also the deposition of the associated mineral particles this study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition to inorganic carbonate and non carbonate and organic sediment production total daily deposition measured with sediment traps varied from 18 8 2 0 g dw m 2 d 1 in silaqui philippines to 681 1 102 g dw m 2 d 1 in bay tien vietnam these measurements correspond to a single sampling event and represent sedimentation conditions during the dry season in se asia coastal areas enhalus acoroides was the most common species in the seagrass meadows visited and together with thalassia hemprichii was present at sites from low to very high deposition halodule uninervis and cymodocea species were present in sites from low to medium deposition the mineral load in seagrass leaves increased with age and was high in e acoroides because it had the largest and long lived leaves up to 417 mg calcium carbonate per leaf and 507 mg non carbonate minerals per leaf and low in h uninervis with short lived leaves 4 mg calcium carbonate per leaf and 2 mg non carbonate minerals per leaf in se asia seagrass meadows non carbonate minerals accumulate at slower rates than the production of calcium carbonate by the epiphytic community consequently the final loads supported by fully grown leaves were as average lower than calcium carbonate loads our results show that organic and inorganic production of the seagrasses in se asia represents a small contribution maximum of 15 of the materials sedimented on a daily base by the water column during the sampling period the contribution of the carbonate fraction can be locally significant i e 34 in silaqui in areas where the depositional flux is low but is minor

effects of natural and artificial thalassia on rates of sedimentation abstract sediment traps were used to measure the rate of deposition in a thalassia seagrass meadow and in an adjacent sandy grass free area the average depositional rate of mud in the thalassia bed was 4 96 g trap 1 week 1 and in the sandy area it was 3 04 g trap 1 week 1 in the summer in winter these rates were 1 60 and 1 50 g mud trap 1 week 1 respectively electromagnetic current meter measurements showed that the mean flow velocity in the grass free area is higher than in the seagrass area therefore the increased depositional rate of mud within the seagrass is due to slowing of water currents by the grass blades confirmation of the blade baffle effect was obtained by placing a 1 m2 plot of artificial thalassia in a grass free area sediment traps in the artificial thalassia contained 5 45 g mud trap 1 week 1 as compared to 3 04 g mud trap 1 week 1 in the adjacent grassfree area in summer in winter these rates were 1 82 and 1 50 g mud trap 1 week 1 respectively epiphytes in the indian river lagoon had no detectable effect on the amount of mud deposited in thalassia beds

geomorphology of the iberian continental margin abstract the submarine features and processes around the iberian peninsula are the result of a complex and diverse geological and oceanographical setting this paper presents an overview of the seafloor geomorphology of the iberian continental margin and the adjacent abyssal plains the study covers an area of approximately 2 3 million km 2 including a 50 to 400 km wide band adjacent to the coastline the main morphological characteristics of the seafloor features on the iberian continental shelf continental slope continental rise and the surrounding abyssal plains are described individual seafloor features existing on the iberian margin have been classified into three main groups according to their origin tectonic and or volcanic depositional and erosional major depositional and erosional features around the iberian margin developed in late pleistocene holocene times and have been controlled by tectonic movements and eustatic fluctuations the distribution of the geomorphological features is discussed in relation to their genetic processes and the evolution of the margin the prevalence of one or several specific processes in certain areas reflects the dominant morphotectonic and oceanographic controlling factors sedimentary processes and the resulting depositional products are dominant on the valencia catalan margin and in the northern part of the balearic promontory strong tectonic control is observed in the geomorphology of the betic and the gulf of cadiz margins the role of bottom currents is especially evident throughout the iberian margin the galicia portuguese and cantabrian margins show a predominance of erosional features and tectonically controlled linear features related to faults

seagrass meadow sedimentary facies in a mixed siliciclastic carbonate temperate system in the tyrrhenian sea pontinian islands western mediterranean abstract the paper presents the characteristics of surface sediments in a seagrass dominated environment of the inner shelf of the pontinian islands tyrrhenian shelf central mediterranean the aims of this work are to characterize the carbonate production and sedimentary processes of a representative high latitude temperate posidonia oceanica dominated area to discuss the role of seagrass in the siliciclastic carbonate mixing processes which provide analogs for interpreting the occurrence and importance of these types of deposits in the rock record the majority of sediments are poorly sorted siliciclastic skeletal sand based on grain type and abundance and carbonate content these deposits include six subfacies the different subfacies reflect environmental changes within the seagrass environment associated with variations in light penetration and hydrodynamic conditions additionally the seagrass facies range from purely carbonate to mixed carbonate siliciclastic depending on the amount of siliciclastic input and on the capacity of seagrasses to trap allochthonous sediments while hosting carbonate producing biota the type of substrate and the erosion and transport processes control the siliciclastic input in these pontinian seagrass meadows sedimentary facies contain abundant siliciclastic grains produced by erosion of the volcanic substrate and trapped by seagrass blades and rhizomes this pontinian example illustrates that in heterozoan dominated seagrass settings mud producing green algae are not present or are not calcifying and consequently seagrass deposits are much more grainy than photozoan counterparts ancient heterozoan tropical seagrass deposits likely will be discriminated from their temperate or subtropical counterparts only by careful taxonomic analysis of skeletal grains

estimation of the bed shear stress in vegetated and bare channels with smooth beds the shear stress at the bed of a channel influences important benthic processes such as sediment transport several methods exist to estimate the bed shear stress in bare channels without vegetation but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production this study proposes a new model to estimate the bed shear stress in both vegetated and bare channels with smooth beds the model which is supported by measurements indicates that for both bare and vegetated channels with smooth beds within a viscous sublayer at the bed the viscous stress decreases linearly with increasing distance from the bed resulting in a parabolic velocity profile at the bed for bare channels the model describes the velocity profile in the overlap region of the law of the wall for emergent canopies of sufficient density frontal area per unit canopy volume a 4 3 m 1 the thickness of the linear stress layer is set by the stem diameter leading to a simple estimate for bed shear stress

spatial distribution of deposition within a patch of vegetation 1 this laboratory study describes the spatial pattern of deposition observed in a patch of vegetation located at the wall of a channel there are two sources of sediment flux to the patch the advection of particles across the upstream leading edge and the lateral dispersion of particles from the adjacent open channel the relative contribution of these two supplies determines the spatial pattern of net deposition in the patch we define the advection length scale within the patch as the longitudinal distance over which advection contributes a significant sediment source at distances from the leading edge that are within the advection length scale net deposition in the patch is laterally uniform reflecting the laterally uniform mean flow delivering the particles at distances farther than the advection length scale from the leading edge the net deposition is highest near the flow parallel edge and decreases into the patch which is the signature of dispersive transport from the patch edge two processes contribute to the lateral dispersion both of which are associated with the shear layer vortices formed at the flow parallel interface between the patch and the channel the vortices generate turbulence and enhance the turbulent diffusion of sediment across the interface in addition the vortices induce a wave oscillation in the flow field within the patch that appears to enhance the lateral transport inside the patch

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

numerical investigation of wave induced flow in mound channel wetland systems abstract coastal wetlands are an important ecosystem in nearshore regions but they are also significant in affecting the flow patterns within these areas wave induced flow in wetlands has complex circulation characteristics because of the interaction between waves and plants especially in discontinuous vegetation here a numerical investigation is performed to analyze the wave averaged flow in vegetated mound channel systems different water levels vegetated conditions and mound configurations are studied with the coulwave cornell university long and intermediate wave boussinesq model model simulations show rip currents in the mound channel systems whose strength varies with different mound separation distances the relative influence of vegetation depends on both mound configuration and water level approximately a 15 change in significant wave height results as waves propagate over the vegetated mounds while up to a 75 decrease in the mean shoreward flow speed through vegetation is observed in addition vegetation influences the spatial distribution of mean water level within the wetlands dimensional analysis shows that rip current strength and primary circulation size depend on mound spacing water depth wave height and vegetation cover

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

sources of organic matter in seagrass colonized sediments a stable isotope study of the silt and clay fraction from posidonia oceanica meadows in the western mediterranean abstract the origin of sedimentary organic matter in 22 sandy beds of posidonia oceanica l delile on the coast of the iberian peninsula and the balearic islands in the northwestern mediterranean sea was investigated using natural abundance stable isotope measurements of carbon and nitrogen 13c and 15n in fine

microseismicity and focal mechanisms of the intermediate depth bucaramanga nest colombia the bucaramanga nest of intermediate depth seismicity in northern colombia was investigated from microearthquakes m sub b less than or equal to4 3 recorded from june 20 to july 6 1979 by a local array of 12 digital three component seismographs of 161 events studied 142 are from the nest and 19 define a thin wadati benioff w b zone mainly from 110 to 190 km depth the w b zone part of the bucaramanga segment strikes n10 sup 0 e with a dip increasing wtih depth from 30 sup 0 e to near vertical beneath the nest the 89 best located events from within the nest define a volume of about 8 km nw by 4 km ne by 4 km depth centered at 161 km depth with relative location accuracies of 0 5 to 1 0 km double couple fault plane solutions of 59 events were determined from an objective procedure using p wave polarities and s p amplitude ratios there is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data the subduction and assimilation of a linear feature such as an island arc undergoing a phase transformation may explain the existence of the bucaramanga nest however the extraordinarilymore high rate of seismicity suggests that the nest may be moving rate several orders of magnitude faster than the rate of relative plate motion alone the intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids perhaps brine or magma that will ultimately result in the initiation of volcanic activity over this recently emplaced w b zone copyright american geophysical union 1987 less

interslab earthquakes and phase changes in subducting lithosphere at 0033 ut on 9 june 1994 a great deep earthquake occurred 635 km beneath the amazonian rain forest of northern bolivia the deep rupture of this record setting magnitude 8 3 shock lasted about a minute harvard university centroid moment tensor catalogue the seismic strain release of this single event as indicated by its seismic moment was greater than the total strain release for all deep earthquakes in the harvard cmt catalogue during the previous 17 years of its coverage of global seismicity incredibly ground motion was felt as far north as seattle washington and toronto canada evidently it occurred within the subducting nazca plate that is descending beneath south america this earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and in particular how such earthquakes may reflect the inner workings of slabs

the 1977 sumba earthquake series evidence for slab pull force acting at a subduction zone the great 1977 sumba earthquake occurred at the eastern sunda trench just west of the collision of australian continental lithosphere with the island arc the length of the aftershock zone of this normal faulting earthquake is about 200 km aftershocks are concentrated 65 115 km east of the main shock epicenter with very few aftershocks in a 50 km long segment that spans the main shock epicenter relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere there is no evidence for thrust faulting of the deeper aftershocks these data imply that the neutral bending surface must be at least 35 40 km deep a second aftershock zone about 180 km northwest of the main shock became active immediately following the main shock but events were concentrated during days 50 52 this zone is a 70 km long lineation that trends toward the main shock epicenter and reflects right lateral strike slip faulting within the subducted oceanic plate seismicity exists to a depth of about 650 km in the very old plate beneath the sunda banda arc and that plate s negative buoyancy causes very large slab pull forces great interface thrust earthquakes are absent at the sumba region and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate this decoupling permits slab pull stresses to be guided updip to the region of the sumba main shock such shallow acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire sumba earthquake series in this model slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake postseismically the released oceanic plate undergoes a pulse of downdip strain returning the plate to a less extended state the moment of this downdip plate motion could exceed the seismic moment of the main shock

evidence for seagrass meadows and their response to paleoenvironmental changes in the early eocene jafnayn formation wadi bani khalid n oman the recognition and understanding of vegetated habitats in the fossil record are of crucial importance in order to investigate paleoecological responses and indirectly infer climate and sea level changes however the low preservation potential of plants and macroalgae hampers a direct identification of these environments in the geological past here we present sedimentological and paleontological evidences as tool to identify the presence of different seagrass vegetated environments in the shallow marine settings of the lower eocene jafnayn platform of oman and their responses to paleoenvironmental changes the studied lower eocene deposits consist of well bedded nodular packstones dominated by encrusting acervulinid and alveolinid foraminifera passing upward to an alternance of packstones with echinoids and quartz grains and grainstones rich in orbitolites smaller miliolid foraminifera and quartz grains the presence of seagrass is inferred by the occurrence of encrusting acervulinids and soritid orbitolites as well as by their test morphologies together with further sedimentological criteria the clear shift observed in the faunal assemblages and sedimentary features may be related to a major reorganization of the carbonate system passing from a carbonate platform to a ramp like platform with increased terrigenous sedimentation heterotroph tubular acervulinids and oligotroph alveolinids of the carbonate platform were replaced upward by more heterotroph organisms such as large discoidal orbitolites and smaller miliolids most likely due to enhanced nutrient levels which would have led to a change of phytal substrate from cylindrical leaf dominated grasses into flat leafed ones

triple seismic zone and the regional variation of seismicity along the northern honshu arc the regional variation of seismicity along the northern honshu arc japan is studied using accurate focal depths and focal mechanism types we use focal depths determined from pp p time intervals reported in the isc bulletins for submarine earthquakes depths are corrected by considering the pp phase reported in the bulletins as the pwp phase the reflection from the ocean surface out of more than 600 well located earthquakes selected from the isc bulletins we determine the types of the focal mechanisms of 184 events using p wave first motion data based on historical seismicity of great and large earthquakes we divide the zone of thrust type earthquakes at the plate interface into two regions the shallow thrust zone 0 40kin where great earthquakes ms 8 0 occur and the deep thrust zone 40 60km where large earthquakes ms 7 4 occur the activity of great or large earthquakes shows a variation along the are in some regions both the shallow and deep thrust zones are active and in other regions only one of the thrust zones is active the seismicity of recent moderate size earthquakes mb 4 combined with the focal mechanism type shows a variation along the are which reflects the variation of the activity of great or large earthquakes where large earthquakes do not occur in the deep thrust zone neither thrust type nor down dip compression tension type events occur in and beneath the deep thrust zone where large earthquakes do occur in the deep thrust zone we find a number of thrust type earthquakes further in the latter case in some regions the down dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple planed structure of seismicity the triple seismic zone this study confirms the hypothesis of previous workers seno and pongsawat 1981 on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone i e the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling respectively here the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone triple seismic zones are found offshore of miyagi prefecture where the deep thrust zone has been broken recently in 1978 and offshore of fukushima prefecture we expect a future large earthquake at the deep thrust zone offshore of fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938

efficient three dimensional reconstruction of aquatic vegetation geometry estimating morphological parameters influencing hydrodynamic drag abstract aquatic vegetation can shelter coastlines from energetic waves and tidal currents sometimes enabling accretion of fine sediments simulation of flow and sediment transport within submerged canopies requires quantification of vegetation geometry however field surveys used to determine vegetation geometry can be limited by the time required to obtain conventional caliper and ruler measurements building on recent progress in photogrammetry and computer vision we present a method for reconstructing three dimensional canopy geometry the method was used to survey a dense canopy of aerial mangrove roots called pneumatophores in vietnam s mekong river delta photogrammetric estimation of geometry required 1 taking numerous photographs at low tide from multiple viewpoints around 1 m2 quadrats 2 computing relative camera locations and orientations by triangulation of key features present in multiple images and reconstructing a dense 3d point cloud and 3 extracting pneumatophore locations and diameters from the point cloud data step 3 was accomplished by a new sector slice algorithm yielding geometric parameters every 5 mm along a vertical profile photogrammetric analysis was compared with manual caliper measurements in all 5 quadrats considered agreement was found between manual and photogrammetric estimates of stem number and of number mean diameter which is a key parameter appearing in hydrodynamic models in two quadrats pneumatophores were encrusted with numerous barnacles generating a complex geometry not resolved by hand measurements in remaining cases moderate agreement between manual and photogrammetric estimates of stem diameter and solid volume fraction was found by substantially reducing measurement time in the field while capturing in greater detail the 3d structure photogrammetry has potential to improve input to hydrodynamic models particularly for simulations of flow through large scale heterogenous canopies

the role of hydrodynamic forcing sediment transport processes and bottom substratum in the shoreward development of posidonia oceanica meadow abstract this paper investigates the combined role of hydrodynamic forcing sediment transport processes and sea bottom features in determining the location of the meadow upper limit of the endemic mediterranean seagrass posidonia oceanica for this purpose an approach including extreme wave analysis and numerical modelling is applied to two sandy beach systems located in southern sardinia italy information about bathymetry p oceanica meadow extension and bottom features has been made available by previous published studies based on the 30 year long noaa hindcast dataset a wave climate analysis is carried out to identify the incoming storm wave conditions 1 year return period to be simulated with the delft3d model package the simulation results extend the current knowledge highlighting the importance of wave induced hydrodynamic parameters as limiting factors for the survival of p oceanica meadows in particular the results suggest that on sandy substrata the meadow upper limit lies well offshore of the surf zone in areas with little morphological activity where the wave orbital velocities associated to storms are on the order of 0 8 m s and the mean current magnitude does not exceed 0 5 m s on rocky bottoms the p oceanica meadow can extend up to the outer surf zone of storms and is thus able to colonize shallow areas subject to stronger hydrodynamic forcings than those observed on sand this difference in the plant tolerance to wave forcing depending on the substratum type shows how both hydrodynamic and geological factors play a key role in defining the environmental conditions for the development of seagrass meadows

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

boundary forces and plate velocities the notion that plate motions result from forces associated with plate boundaries implies that average plate velocities in a mantle hot spot frame of reference are systematically related to the nature and geometry of plate boundaries and to the proportion of continental lithosphere included in the plates several simple linear models relating plate velocity v to net ridge push rp slab pull sp trench suction ts and continental drag cd have been evaluated by multiple linear regression ridge push slab pull and continental drag are found to be vitally important trench suction may be a real force but because effective trench lengths are small and only three of the twelve plates considered are affected it is not a statistically significant contributor to plate motions the preferred relation v cm yr 2 7 4 5 2 1 9 rp 13 2 1 6 sp 4 7 6 cd is a remarkably good predictor of average plate velocities with an rms error of 0 50 cm yr and a correlation coefficient of 0 97 the implied balance between these net forces is generally consistent with the results of force balance models the constant term includes all forces not explicitly considered and suggests that the combined effects of such forces are about equal for all plates finally because the parameters used for any particular plate are independent of all other plates the success of this simple model strongly suggests that the forces which drive plate motions are intimately related to the plates themselves

the influence of waves and seagrass communities on suspended particulates in an estuarine embayment resuspension of bottom sediments by waves corresponding changes in suspended particulate material spm concentrations in the overlying water column and transport pathways of spm were investigated in a shallow estuarine embayment colonized by seagrass communities in chesapeake bay in shallow 2 m unvegetated regions significant resuspension of bottom sediments was evident when northerly winds exceeded 25 km h 1 increasing spm concentrations up to 10 fold high concentrations of spm generated by resuspension dissipated rapidly within 24 h after winds became calm patterns of spm along the embayment s depth gradient suggest that part of this resuspended material was transported offshore into deeper reaches of the estuary in areas of the embayment colonized by seagrasses wave energy was attenuated by the vegetation suppressing resuspension and enhancing deposition as a result spm concentrations were significantly lower inside seagrass beds than in adjacent unvegetated areas during periods of high winds when wave induced resuspension occurred in unvegetated areas spm concentrations remained unchanged inside the bed at normal water levels however when water levels were elevated by spring tides or storm surges plants were less effective at attenuating wave energy and spm concentrations increased inside the seagrass bed due to resuspension and advective processes calculations based on the results of this study indicate that sedimentation rates are substantially higher in seagrass communities than in unvegetated areas

geographic variability in organic carbon stock and accumulation rate in sediments of east and southeast asian seagrass meadows organic carbon oc stored in the sediments of seagrass meadows has been considered a globally significant oc reservoir however the sparsity and regional bias of studies on long term oc accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global oc sink we evaluated the amount and accumulation rate of oc in sediment of seagrass meadows and adjacent areas in east and southeast asia in temperate sites the average oc concentration in the top 30 cm of sediment was higher in seagrass meadows 780 1080 mol g 1 than in sediments without seagrass cover 52 430 mol g 1 the average oc in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 mol g 1 carbon isotope mass balancing suggested that the contribution of seagrass derived carbon to oc stored in sediments was often relatively minor temperate 10 40 subtropical 35 82 tropical 4 34 and correlated to the habitat type being particularly low in estuarine habitats stock of oc in the top meter of sediment of all the studied meadows ranged from 38 to 120 mg ha 1 the sediment accumulation rates were estimated by radiocarbon dating of six selected cores 0 32 1 34 mm yr 1 the long term oc accumulation rates calculated from the sediment accumulation rate and the top 30 cm average oc concentration for the seagrass meadows 24 101 kg ha 1 yr 1 were considerably lower than the oc accumulation rates previously reported for mediterranean posidonia oceanica meadows 580 kg ha 1 yr 1 on average current estimates for the global carbon sink capacity of seagrass meadows which rely largely on mediterranean studies may be considerable overestimations

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes