along strike variations of earthquake apparent stress at the nicoya peninsula costa rica subduction zone oceanic plates vary in temperature topography and sediment load as they enter subduction zones these variations persist along the subduction interface causing perturbations in coupling and earthquake rupture processes we explore the effects of variable subducting plate structure on microseismicity rupture characteristics along the nicoya peninsula costa rica the subducting cocos plate has low relief along the northern and central portion of the peninsula with seamounts present at the southern tip of the peninsula we compute apparent stresses for 94 ml 2 5 4 2 earthquakes along the plate interface using waveform coda and find along strike variations that mimic bathymetric variability median stress values are higher 3 2 mpa in the smooth northern region with lower values in the central 2 1 mpa and southern 0 7 mpa segments higher apparent stresses along a zone of little suspected subduction topography imply increased coupling or higher friction along the interface these results agree with geodetic and other seismic studies that suggest variable plate coupling along the nicoya peninsula

quaternary deformation and uplift of coral reef terraces produced by oblique subduction and underthrusting of the bahama platform below the northern hispaniola forearc abstract active tectonics linked to the rise of the hispaniola forearc explain the morphological characteristics spatial distribution and u th geochronological ages of the uplifted coral reef terraces of the la isabela formation in both the eastern cordillera septentrional and the samana peninsula of northeastern dominican republic depositional coral reef terraces of mis 5e tii 7e tiii and 9 tiv stages occur respectively at maximum heights of about 25 m 80 m and 120 m above the current sea level the development of heterogeneous brittle deformation structures in the coral limestone at all scales document active tectonics and include small faults shear fractures extensional joints and calcite veins their geometric and kinematic characteristics as well as the stress tensors calculated from fault slip and fracture orientation data are all compatible with ene to ne trending regional shortening and indicate that a left lateral transpressive regime is deforming the forearc during the quaternary along the northern hispaniola coastline the tectonic uplift mimics the distribution of the bathymetric features on the subducting bahamas platform outboard the hispaniola puerto rico trench the highest calculated uplift rates in the eastern cordillera septentrional and samana peninsula correspond to the collision zone of the silver spur and navidad bank 0 4 m ka respectively uplift rates decrease towards the west 0 2 m ka where the oblique collision of carbonate ridges and related forearc deformation would not have been recorded since at least the early pleistocene oblique subduction and underthrusting of the bahamas platform followed by basal erosion at the subduction channel explain the observed time transgressive pattern of uplift and subsidence in the northern hispaniola forearc and the change from accretionary to erosive processes in the convergent margin

observations of beach cusp evolution at melbourne beach florida usa abstract morphological observations terrestrial laser scanning and sediment analysis are used to study beach cusp morphodynamics at melbourne beach florida usa a moderately sloped beach characterized by fine sand the study couples rapid high resolution surveying with surficial sediment sampling over multiple tidal cycles surveys were run 500 m alongshore and sediment samples were collected intertidally over a period of 5 days beach cusp location within larger scale beach morphology is shown to directly influence cusp growth as either gross erosional or gross accretional sediment characteristics within the beach cusp morphology are reported coincident with cusp evolution variations in particle size distribution kurtosis are exhibited as the cusps evolve however no significant correlation is seen between grain size and position between horn and embayment during the end of the study a storm resulted in beach cusp destruction and increased sediment sorting

a numerical model for shore normal sediment size variation on a macrotidal beach this paper describes a computer simulation model which is designed to predict the selective shore normal sorting of grain sizes in the nearshore environment the model simulates wave shoaling wave height attenuation due to frictional losses and breaking using linear theory up to the break point and a breaker decay model in the surf zone peak horizontal orbital velocities at the bed are calculated from stokes second order wave theory the peak onshore and offshore velocities are used with the threshold expression of komar and miller 1975 to generate a spatial pattern of size variation of threshold grain diameter along a profile normal to the shore from deep water to the swash zone the predicted grain size is used in an hydraulic interpretation of grain size distribution on the intertidal profile based on the hydrodynamic variations over a tidal cycle on a macrotidal beach the model is successful in predicting the broad pattern of increasing grain size in the onshore direction which has been observed in nature comparisons between measured and predicted grain size distributions indicate that the predictions of the model are better than those of previous models but the model is more successful at predicting sediment size distributions than at predicting mean sizes on a beach profile

accreted seamounts in north tianshan nw china implications for the evolution of the central asian orogenic belt the carboniferous bayingou ophiolitic melange is exposed in north tianshan accretionary complex in the southwestern part of the central asian orogenic belt caob the melange is mainly composed of serpentinised ultramafic rocks including harzburgite lherzolite pyroxenite dunite and peridotite pillowed and massive basalts layered gabbros radiolarian cherts pelagic limestones breccias and tuffs and displays block in matrix structures the blocks of ultramafic rocks gabbros basalts cherts and limestones are set in a matrix of serpentinised ultramafic rocks massive basalts and tuffs the basaltic rocks in the melange show significant geochemical heterogeneity and two compositional groups one ocean island basalt like and the other mid ocean ridge like can be distinguished on the basis of their isotopic compositions and immobile trace element contents such as light rare earth element enrichment in the former but depletion in the latter the more enriched basaltic rocks are interpreted as remnants fragments of seamounts derived from a deep mantle reservoir with low degrees 2 3 of garnet lherzolite mantle melting the depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with 15 partial melting it is probable that both groups owe their origin to melting of a mixture between plume and depleted morb mantle the results from this study when integrated with previous work indicate that the junggar ocean crust comprising a significant number of seamounts was likely to have been subducted southward beneath the yili central tianshan block in late devonian early carboniferous the seamounts were scraped off and accreted along with the oceanic crust in an accretionary wedge to form the bayingou ophiolitic melange we present a model for the tectonomagmatic evolution of this portion of the caob involving prolonged intra oceanic subduction with seamount accretion

control of seafloor roughness on earthquake rupture behavior earthquake rupture complexity is described for three recent large underthrusting earthquakes along the costa rican subduction zone the 1983 osa 1990 nicoya gulf and 1999 quepos events these earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate the 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities in contrast the 1983 event occurred along the subducting cocos ridge and had a complex rupture history comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

an overview of oceanic island basalts in accretionary complexes and seamounts accretion in the western central asian orogenic belt abstract the central asian orogenic belt caob is one of the largest and long lived accretionary collages worldwide and provides an ideal opportunity to address the fundamental geological processes and continental growth of accretionary orogens however the tectonic history of the caob is still controversial previous studies mainly focused on the process and model of accretion and subduction but did not pay much attention to oceanic intraplate volcanoes in this paper we provide an overview of the salient features of twelve oib hosting accretionary complexes with ages from late neoproterozoic to early carboniferous in western the caob and distinguish rock association of typical seamounts from accretionary complexes and shear zones including pillow basalt volcanic breccia limestone olistostromes and terrestrial clastic rocks combined with unique structural features of seamount the stratigraphic framework of seamount was preliminarily reconstructed in paleo asian ocean but the distribution and scale of seamounts is still not clear geochemically volcanic rocks from seamounts are mainly alkaline basalts with litte tholeiitic basalts the alkaline basalts are characterized by lree enrichment and hree depletion and no obvious nb ta and ti negative anomalies suggesting typical oib affinity the nd sr and pb isotopic features indicate that magma sources have enriched components and show fluid metasomatism in short this is consistent with geochemical features of louisville ontong java and hawaiian seamounts which are related to mantle plumes but do not contain high mg picrite and komatitic basalt based on our observations and in combination with previous work we present an innovative model in which the caob was involved multiple intra oceanic arc with seamounts subduction and accretion which is similar to the present day pacific and can better explain the tectonic evolution of the caob however further studies are needed to document the distribution scale and mantle dynamics of seamounts in the caob

late paleozoic mesozoic tectonic evolution of sw japan a review reappraisal of the accretionary orogeny and revalidation of the collisional model abstract this paper makes a review of the interpretations of the tectonic evolution of sw japan during the last three decades in the late 1970s the dominant model was the so called pacific type orogeny emphasizing the purported absence of nappes and the contrast with the alpine chains and interpreting the evolution as due to a steady oceanic subduction since the paleozoic time in the 80s the discovery of the actual structure made of a pile of large thrust sheets led authors to propose collisional models involving the intermittent underthrusting of buoyant blocks like micro continents at the same time the use of high resolution biostratigraphy allowed several authors to recognize ancient accretionary wedges with a reconstructed ocean plate stratigraphy of individual accreted units especially in the tanba and shimanto zones also precise radiometric dating permitted the distinction of metamorphosed units especially in sanbagawa and shimanto belts as a result of these new data since the 1990s the plate tectonic interpretation of the history of the japanese islands was revised by japanese scientists and presented again in terms of accretionary processes linked to a steadily oceanic subduction with an episodic ridge subduction the so called miyashiro type orogeny the review of different data leads to the following conclusions the structure of sw japan is made of a pile of sub horizontal nappes polydeformed with a geometry similar to the one encountered in collisional orogens the geodynamic mechanisms advocated for the tectonic building within the accretionary orogeny concept miyashiro type orogeny are inappropriate a permanent oceanic subduction with the intermittent collision actually subduction of an active ridge or seamount chain is unable to build such structures as this process induces in fact an acceleration of the tectonic erosion and collapse of the upper plate the underthrusting of a micro continent or mature arc is likely needed the exhumation story of sanbagawa hp schists suggests the setting of a continental subduction the petrological and new geochemical data from the literature strongly support the existence beneath the nappes of accretionary complexes of continental bodies showing affinities with south china from which they were once separated the episodic collision underthrusting of such blocks was responsible for the tectonic piling tectonic erosion plaid likely a major role in removing material during the intervening subduction stages a revised geodynamic model implying the collision of the honshu south kitakami kurosegawa and shimanto blocks is proposed for explaining the three orogenic crises which took place respectively at around 240 130 and 80 60 ma ago in sw japan the paleogeographic position and affinity of the hida block with surrounding units in the hinterland are still unclear more work is needed to solve this question

seismogenic zone structure of the southern middle america trench costa rica 1 the shallow seismogenic portion of subduction zones generates damaging large and great earthquakes this study provides structural constraints on the seismogenic zone of the middle america trench offshore central costa rica and insights into the physical and mechanical characteristics controlling seismogenesis we have located 300 events that occurred following the mw 6 9 20 august 1999 quepos costa rica underthrusting earthquake using a three dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers we use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region these events define a plane dipping at 19 that marks the interface between the cocos plate and the panama block the majority of aftershocks occur below 10 km and above 30 km depth below sea level corresponding to 30 35 km and 95 km from the trench axis respectively relative event relocation produces a seismicity pattern similar to that obtained using absolute locations increasing confidence in the geometry of the seismogenic zone the aftershock locations spatially correlate with the downdip extension of the oceanic quepos plateau and reflect the structure of the main shock rupture asperity this strengthens an earlier argument that the 1999 quepos earthquake ruptured specific bathymetric highs on the downgoing plate we believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level

effect of subducting sea floor roughness on fore arc kinematics pacific coast costa rica fault kinematics and uplift in the costa rican fore arc of the middle america convergent margin are controlled to a large extent by roughness on the subducting cocos plate along the northwest flank of the incoming cocos ridge seafloor is characterized by short wavelength roughness related to northeast trending seamount chains onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore arc thrust belt and separate blocks with contrasting uplift rates trunk segments of pacific slope fluvial systems typically follow these margin perpendicular faults regionally developed marine and fluvial terraces are correlated between drainages and across faults along the costa rican pacific coast terrace separations across block bounding faults reveal a pattern of fore arc uplift that coincides roughly with the distribution of incoming seamounts magnitude and distribution of quaternary uplift along the costa rican pacific coast suggests that despite a thin incoming sediment pile the inner fore arc shows an accumulation of mass a characteristic that may be due to underplating of seamounts beneath the fore arc high

subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of central ecuador the uplift of the coastal cordillera of central ecuador is a likely consequence of the subduction of the carnegie ridge a 400 km long and 2 km high topographic asperity this study aims at analyzing the impact of the subduction of the carnegie ridge on the continental shelf sedimentation during the quaternary we interpret high resolution 50 450 hz frequency multi channel 72 channels and single channel sparker seismic data piston cores sediment profiles 3 5 khz and high resolution multibeam bathymetry acquired during the atacames cruise with the rv l atalante in 2012 in the la plata island cabo san lorenzo peninsula region the results show a full and detailed record of the last ten quaternary sedimentary sequences deposited in intra shelf basins at and just seaward of the shelf break these 100 ka scale sequences are tied to piston core data to well dated quaternary onshore exposures and to a flight of coastal marine terraces on cabo san lorenzo peninsula and la plata island these sequences correlate with the global ice volume and deep marine temperature changes d18o for the last 0 7 ma which points to a strict climatic control on depositional sequence development and preservation however the subsidence of the continental shelf acoustic basement estimated by the stepwise backstripping of the sedimentary record exhibits a complex deformation pattern with uplifting and subsiding regions deep marine seismic data currently under processing show evidence for a subducted seamount beneath la plata island and gps data indicate an important interplate coupling in the same area with the potential to generate a mw 7 7 5 earthquake the pattern of the continental shelf deformation is consistent with the shape of this seamount we tested the hypothesis of a link between the margin deformation and the subduction of the seamount by comparing a stepwise subduction of the seamount to the palinspastic restoration of the deformation of the continental shelf for the last 1 ma this comparison shows that the collision probably started c 500 ka ago together with the syntectonic sedimentation and drastically slowed down by c 50 ka with the sealing of most of the deformation on the shelf

tomographic evidence for a subducted seamount beneath the gulf of nicoya costa rica the cause of the 1990 mw 7 0 gulf of nicoya earthquake 1 tomographic images constrain the existence of a subducted seamount beneath the gulf of nicoya costa rica the subducted seamount is found at a depth of 30 km within the rupture area of the march 25 1990 mw 7 0 gulf of nicoya earthquake the gulf of nicoya earthquake was a typical thrust type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting cocos plate and the overriding plate precise relocation of the mainshock and its aftershocks in a 3 d p wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate we interpret the subducted seamount as an asperity whose rupture caused the 1990 gulf of nicoya earthquake

wave refraction patterns and sediment transport in monifieth bay tay estuary scotland abstract monifieth bay is floored by unconsolidated sediments dominated by medium to fine sand which rest on andesitic or sandstone bedrocks wave activity dominates the shoreline processes and refraction diagrams constructed for the prevailing southwesterly winds show waves sweeping towards the northwest between monifieth and broughty ferry only short period waves from the southwest cross aberty sands and they refract to give northwestern motion paths in this area injection and recovery of fluorescent tracers at many sites under a range of tidal and wind conditions suggested that the volume of sand transported in the intertidal zone ranges from 4 15 cm 3 s to 129 76 cm 3 s along the beach the complex bottom geomorphology of monifieth bay leads to differential potential wave erosion depending on the level of the tide and wave direction especially at low water 5 to 2 m waves from the southwest are refracted and create divergent paths of sediment movement defining a zone of potential erosion waves from the southeast however transport the sediment in a uniformly westerly direction along the coast

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

evolution of tectono sedimentary systems in the kumano basin nankai trough forearc abstract sedimentary deposits in the distal kumano forearc basin of the nankai accretionary margin off kii peninsula japan have been imaged using three dimensional 3d seismic data the seismic data along with logging and core data from the integrated ocean drilling program iodp show that the unconformity between the accretionary prism and overlying forearc sediments is time transgressive the unconformity at site c0002 separates 5 ma prism rocks from 3 65 ma basin deposits at site c0009 it separates 5 6 ma prism from 3 8 ma basin sediments acoustic reflections in the basal deposits are sub parallel to the underlying accretionary prism the acoustic facies varies in thickness from 50 to 750 m the mudstone deposits and laterally equivalent turbidites are interpreted as lower trench slope deposits the condensed slope sediment ss section decreases in age from 3 5 to 1 5 ma at site c0002 to 1 5 0 9 ma at c0009 acoustic sequences within the lower forearc basin lfb contain higher proportions of silt and sand turbidites and progressively onlap the ss unit along a low angle discontinuity kl in a landward direction because of the landward onlap of the lfb unit the oldest lfb strata at c0002 are older than 1 67 ma whereas those at c0009 are younger than 0 9 ma thus the amount of time missing or characterized by condensed sedimentation across the kl unconformity decreases in duration in the landward direction the landward onlapping sequences tilt progressively landward in response to regional uplift along an out of sequence thrust oost mega splay fault regional tilting shifted the basin s depocenter progressively landward expanding that part of the basin from 10 km in width to 30 km the onset of sand silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at 1 9 ma conversely turbidites of the upper forearc basin ufb progressively onlap lfb in a seaward direction furthermore the respective thicknesses of the lfb and ufb units switch from the seaward side of the basin c0002 farther landward c0009 the lfb unit is 800 m thick in the seaward region whereas it is only 200 300 m thick in the landward region the ufb unit is

the seismogenic zone in the central costa rican pacific margin high quality hypocentres from an amphibious network transition from subduction of normal to thickened oceanic crust occurs in the central portion of the costa rican margin where large interplate earthquakes m 7 and abundant interseismic seismicity have been associated with subduction of bathymetric highs we relocated 1 300 earthquakes recorded for 6 months by a combined on and offshore seismological network using probabilistic earthquake relocation in a 3d p wave velocity model most of the seismicity originated at the seismogenic zone of the plate boundary appearing as an 18 dipping planar cluster from 15 to 25 30 km depth beneath the continental shelf several reverse focal mechanisms were resolved within the cluster the upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying plate thickness and coherency which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than 100 to 120 c along the plate boundary the downdip limit of the stick slip behaviour collocates with relative low temperatures of 150 to 200 c suggesting that it is controlled by serpentinization of the mantle wedge the distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths unlike in northern costa rica rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity

aseismic zone and earthquake segmentation associated with a deep subducted seamount in sumatra the subduction of topographic features has been linked both to earthquake generation and the segmentation of faults seismic imaging reveals a seamount subducted to 40 km depth below sumatra that is associated with an aseismic zone suggesting that at this location the seamount reduces coupling of the slab and overriding plate

wave and tidal current sorting of sand on a wide surf zone beach abstract experiments with fluorescent tracers have shown that on a wide surf zone beach selective transport of different grain size material may play an important role in the sorting of beach sediments by waves and tidal currents in the alongshore direction the results have shown that at the mid tide level small particles in general move more rapidly than coarse material although there is evidence to suggest that with increasing particle size the net rate of movement passes through a minimum such behaviour would be consistent with komar s 1977 observations and suggestion of a minimum corresponding to the transition from bedload to suspended load transport of sediment across the beach fluorescent tracer samples have shown onshore offshore sorting about the mean position of the breaker line with coarse material moving offshore and fine material moving onshore

analysis of normal fault populations in the kumano forearc basin nankai trough japan 1 multiple orientations and generations of faults from 3 d coherency mapping 1 analyses of normal faults in the kumano forearc basin of the nankai trough reveal multiple normal fault populations in a region generally thought to be under compression most faults have offsets of less than 20 m and dips of 60 70 and show no growth structures indicating that the faults were active for short periods of time the oldest generation of faults is older than 0 9 ma and strikes 50 60 the next oldest faults strike 160 170 are older than 0 44 ma and are related to local uplift along the western edge of the region the youngest faults cut the seafloor shallow faults near the se margin of the basin curve from 100 in the middle of the survey area to 145 at the se corner of the area the pattern of the two youngest fault populations is consistent with the regional stress pattern maximum horizontal stress subparallel to the trench orientations of older fault populations are caused by uplift of the underlying accretionary prism implying that the forearc basin region is not as stable as previously thought reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2 concentrated near the seaward edge of the basin the active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge

the use of the textural parameters of sand in studying the characteristics and depositional processes of coastal sediments south of jeddah saudi arabia abstract the subtidal sediments of four coastal environments beach swash zone lagoon and nearshore bar south of jeddah have been studied in terms of their textural characteristics and depositional process the skeletal grains are composed mainly of fragments of corals benthic foraminifera and different types of shells together with sand sized rock fragments and quartz and feldspar grains the skeletal material of the sediments has been derived from the mechanical breakdown of the adjacent reefs while the small clastic portion was provided from the hinterland by rivers bimodal and polymodal grain size distributions are common and can be attributed to heterogeneity in the types of skeletal debris the size distribution of such sediments suggests that the various processes that are responsible for their transportation and deposition are wave current and wind action the different energy conditions and the morphology of each coastal environment seem to be reflected in the characteristics of the sediments in each environment

field test comparison of an autocorrelation technique for determining grain size using a digital beachball camera versus traditional methods abstract this extensive field test of an autocorrelation technique for determining grain size from digital images was conducted using a digital bed sediment camera or beachball camera using 205 sediment samples and 1200 images from a variety of beaches on the west coast of the us grain size ranging from sand to granules was measured from field samples using both the autocorrelation technique developed by rubin rubin d m 2004 a simple autocorrelation algorithm for determining grain size from digital images of sediment journal of sedimentary research 74 1 160 165 and traditional methods i e settling tube analysis sieving and point counts to test the accuracy of the digital image grain size algorithm we compared results with manual point counts of an extensive image data set in the santa barbara littoral cell grain sizes calculated using the autocorrelation algorithm were highly correlated with the point counts of the same images r 2 0 93 n 79 and had an error of only 1 comparisons of calculated grain sizes and grain sizes measured from grab samples demonstrated that the autocorrelation technique works well on high energy dissipative beaches with well sorted sediment such as in the pacific northwest r 2 0 92 n 115 on less dissipative more poorly sorted beaches such as ocean beach in san francisco results were not as good r 2 0 70 n 67 within 3 accuracy because the algorithm works well compared with point counts of the same image the poorer correlation with grab samples must be a result of actual spatial and vertical variability of sediment in the field closer agreement between grain size in the images and grain size of grab samples can be achieved by increasing the sampling volume of the images taking more images distributed over a volume comparable to that of a grab sample in all field tests the autocorrelation method was able to predict the mean and median grain size with 96 accuracy which is more than adequate for the majority of sedimentological applications especially considering that the autocorrelation technique is estimated to be at least 100 times faster than traditional methods

neogene overflow of northern component water at the greenland scotland ridge in the north atlantic ocean flow of north atlantic deep water nadw and of its ancient counterpart northern component water ncw across the greenland scotland ridge gsr is thought to have played an important role in ocean circulation over the last 60 ma the iceland plume has dynamically supported an area which encompasses the gsr consequently bathymetry of the gsr has varied with time due to a combination of lithospheric plate cooling and fluctuations in the temperature and buoyancy within the underlying convecting mantle here we reassess the importance of plate cooling and convective control on this northern gateway for ncw flow during the neogene period following wright and miller 1996 to tackle the problem benthic foraminiferal isotope data sets have been assembled to examine 13c gradients between the three major deep water masses i e northern component water southern ocean water and pacific ocean water composite records are reported on an astronomical timescale and a nonparametric curve fitting technique is used to produce regional estimates of 13c for each water mass confidence bands were calculated and error propagation techniques used to estimate ncw and its uncertainty despite obvious reservations about using long term variations of 13c from disparate analyses and settings and despite considerable uncertainties in our understanding of ancient oceanic transport pathways the variation of ncw through time is consistent with independent estimates of the temporal variation of dynamical support associated with the iceland plume prior to 12 ma 13c patterns overlap and ncw cannot be isolated significant long period variations are evident which are consistent with previously published work from 12 ma when lithospheric cooling probably caused the gsr to submerge completely long period 13c patterns diverge significantly and allow reasonable ncw estimates to be made our most robust result is a dramatic increase in ncw overflow between 6 and 2 ma when dynamical support generated by the iceland plume was weakest between 6 and 12 ma a series of variations in ncw overflow have been resolved

sedimentology and morphodynamics of a macrotidal beach pendine sands sw wales abstract the foreshore of pendine sands forms the seaward part of an extensive sandy coastal barrier in a shallow carmarthen bay sw wales the sedimentological features of the macrotidal foreshore reflect a tide induced modification of nearshore wave characteristics as the tide ebbs the breaker height may decrease the surf zone widens and becomes increasingly dissipative and swash backwash velocities diminish a concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient a zero net transport model demonstrates that the beach profile is self stabilising in the short term and periodic levelling has shown that the beach is in long term equilibrium with prevailing conditions though this does not preclude a significant dynamic response to changing tides and waves the flow regimes of wave generated currents decline as the tide ebbs and normal beach processes do not usually affect the lower foreshore accordingly there is an overall seaward fining of the primary framework component of the sands in more detail this framework component displays a slight seaward coarsening across an upper foreshore dominated by high water swash and surf a rapid seaward fining across the mid foreshore in response to the ebb attenuating swash zone flow velocities and a slight seaward fining across the lower foreshore under the action of nearshore shoaling waves bedforms vary from a swash backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore the surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions partly to storms the upper foreshore is characterised by swash backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities the mid foreshore is composed of coarser grained shelly traction clogs arranged as landward and seaward dipping large scale cross bedding and or plane bedding these are probably storm breaker surf deposits the lower foreshore though partially and sometimes totally bioturbated shows landward dipping small scale cross bedding in very fine sands sorted by nearshore shoaling waves tide and storm induced modification of the nearshore flow regimes therefore produces a distinctive shore normal array of sedimentary facies each facies is characterised by diagnostic textural and structural signatures a prograding sequence of such macrotidal deposits would be similar to but more extensive than a comparable microtidal sequence

supraslab earthquake clusters above the subduction plate boundary offshore sanriku northeastern japan seismogenesis in a graveyard of detached seamounts 1 thousands of offshore repeating earthquakes with low angle thrust focal mechanisms occur along the subduction plate boundary of ne japan double difference relocation methods using p and s wave arrivals reveal clusters of events above these repeating events to assure good depth control we restrict our study to events that are close to seismic stations these supraslab earthquake clusters are regional features at depths of 25 to 50 km and most of these clusters are below the depth of the forearc moho which we determined from converted waves seismicity over this depth range does not occur under the inland area of ne japan except just below the vicinity of the arc volcanoes re entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the pacific plate during slab descent driven by the resistance of seamounts to subduction detachment during slab descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth high fluid pressure produced during dehydration of clay minerals and other low temperature hydrous minerals could enable detachment at depths seamount crust is thus accreted to forearcs possibly leading to a long term component of near coastal uplift supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system

potential of 3 d vertical seismic profiles to characterize seismogenic fault zones the potential of a 3 d vertical seismic profile vsp to improve resolution of seismogenic plate interfaces was explored with synthetic modeling the 3 d vsp modeled is at a proposed site for a 1 to 1 5 km deep open hole that provides background for riser drilling three dimensional vsp images could resolve 30 60 m spaced reflective horizons in a costa rican subduction zone it can record a great amount of high fidelity s wave data to invert for physical properties directions of strain and pore pressure above and below the plate interface fault a 6 km 12 km grid of shots with a surface ship will illuminate a 4 km 7 km area of the plate interface fault zone with a high data density acquisition adds 5 to 9 days to drill ship time on site and a shooting ship seismic image resolution falls between that of borehole information and 3 d surface ship seismic images a multiple kilometer 3 d volume of high fidelity s wave data is an exceptional addition not available with other techniques

the java convergent margin structure seismogenesis and subduction processes the java margin is characterized by a distinct variation in lower to upper plate material transfer and recurring catastrophic tsunamogenic earthquakes both processes are closely linked to the subduction of oceanic basement relief resulting in varying degrees of fore arc deformation tomographic models of refraction seismic profiles and reflection seismic lines in combination with high resolution multibeam bathymetric data reveal the variability in the deep structure and deformation of the java fore arc shallow subduction processes are governed by the sediment supply in the trench as well as by the nature and fabric of the oceanic lithosphere the deep structure of the fore arc reveals a shallow upper plate crust mantle transition present along the entire java margin section the serpentinized fore arc mantle wedge governs the depth extent of the seismogenic zone here which is narrower compared to its sumatran analogue in addition offshore central java high relief oceanic basement features potentially act as asperities as well as barriers to seismic rupture limiting the possible magnitude of subduction thrust earthquakes however the potential for geohazards in particular tsunamis is high along the entire margin this results from tsunamogenic earthquakes ubiquitous splay faults and potentially tsunamogenic landslides which further increase the risk of future tsunamis

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio