sediment dynamics over sand ridges on a tideless mid outer continental shelf abstract a sand ridge field with crests oriented ne sw 52 located between 55 and 85 m water depth on the valencia continental shelf spain was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling boundary layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three year recurrence period from the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field the near bottom shear stress generated during storm conditions reached around 1 n m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h the total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160 it is argued that these sand ridges were developed in a shallow environment and there is no evidence of present day ridge migration or morphological degradation it is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology

deep structure of the central lesser antilles island arc relevance for the formation of continental crust oceanic island arcs are sites of high magma production and contribute to the formation of continental crust geophysical studies may provide information on the configuration and composition of island arc crust however to date only few seismic profiles exist across active island arcs limiting our knowledge on the deep structure and processes related to the production of arc crust we acquired active source wide angle seismic data crossing the central lesser antilles island arc north of dominica where the oceanic tiburon ridge subducts obliquely beneath the forearc a combined analysis of wide angle seismics and pre stack depth migrated reflection data images the complex structure of the backstop and its segmentation into two individual ridges suggesting an intricate relation between subducted basement relief and forearc deformation tomographic imaging reveals three distinct layers composing the island arc crust a three kilometer thick upper crust of volcanogenic sedimentary rocks and volcaniclastics is underlain by intermediate to felsic middle crust and plutonic lower crust the island arc crust may comprise inherited elements of oceanic plateau material contributing to the observed crustal thickness a high density ultramafic cumulates layer is not detected which is an important observation for models of continental crust formation the upper plate moho is found at a depth of 24 km below the sea floor upper mantle velocities are close to the global average our study provides important information on the composition of the island arc crust and its deep structure ranging from intermediate to felsic and mafic conditions in this study we model the deep structure of the lesser antilles island arc we use a hybrid analysis of refraction and reflection seismic data we image the complex structure of two ridges forming the backstop island arc crust composition ranges from intermediate to felsic to mafic conditions we discuss the formation of island arc and continental crust

surface rupture of the 2005 kashmir pakistan earthquake and its active tectonic implications to provide a detailed record of a relatively rare thrust surface rupture and examine its active tectonic implications we have conducted field mapping of the surface rupture associated with the 2005 m w 7 6 kashmir earthquake despite the difficulty arising from massive earthquake induced landslides along the surface rupture we found that typical pressure ridges and warps extend northwestward for a distance of 70 km with a northeast side up vertical separation of up to 7 m neither the main frontal thrust nor the main boundary thrust is responsible for the earthquake but three active faults or fault segments within the sub himalaya collectively called the balakot bagh fault compose the causative fault although the fault exhibits substantial geomorphic expression of repeated similar surface ruptures only a part of it had been mapped as active before the earthquake the location of the hypocenter suggests that the rupture was initiated at a deep portion of the northern central segment boundary and propagated bilaterally to eventually break all three segments our obtained surface rupture traces and the along strike slip distribution are both in good agreement with results of prompt analyses of satellite images indicating that space geodesy can greatly aid in time consuming field mapping of surface ruptures assuming that the extensive fill terrace in the meizoseismal area was abandoned during 10 30 ka we tentatively estimate the earthquake recurrence interval and shortening rate on the balakot bagh fault to be 1000 3300 yr and 1 4 4 1 mm yr respectively these estimates indicate that the balakot bagh fault is not a main player of himalayan contraction accommodation graphic 1 selected field photographs and arcgis files of the mapped surface rupture traces and measured vertical separations are available in the electronic supplement to this article 1 embed inline graphic 1 gif

suspended sediment transport in the surf zone response to cross shore infragravity motion a field experiment to investigate the magnitude and temporal and spatial scales of suspended sediment transport in the inner surf zone was conducted at san marine oregon on october 7 13 1984 instrumentation consisted of five electromagnetic current meters three strain gauge type pressure transducers and 25 optical backscatter sensors to measure the concentrations of suspended sediment the instruments were deployed approximately 70 m x seaward of the mean high water line mean depth h 1 1 1 3 m in a closely spaced cross shaped pattern data were collected for 68 min periods spanning high tide during the 7 day experiment during the experiment waves varied in angle of approach from shore normal to southwesterly offshore significant swell heights were 3 5 m with periods of 12 14 s local sea conditions with wind waves of approximately 8 s also occurred a visual estimation of the width of the surf zone indicated it to be approximately 500 m and the breaker pattern suggested that there were two longshore bars at approximately 200 and 500 m offshore the data resulting from the experiment encompass a broad range of wave and current conditions which included periods of extreme infragravity motion 30 t 300 s and weak longshore currents and periods of low infragravity motion and strong longshore currents periods of incident wave dominance also occurred the field study section of this paper qualitatively describes the sediment response to strong 240 cm s 1 low frequency 0 01 hz cross shore fluid motions which dominated the inner surf zone during two of the data runs the infragravity band accounted for 85 of the cross shore velocity variance the peak 0 01 hz of which was associated with shore normal standing waves concentration gradients of suspended sediment appear to be associated with diffusive processes and develop and decay on time scales of the infragravity motions rather than those of incident waves suspension events during times of infragravity dominance often persisted for periods of 30 45 s and reached peak concentrations of 20 40 g l 1 at elevations in excess of 26 cm mean suspended load was 3 4 times larger than that associated with incident wave motions the modeling section of the paper attempts to quantitatively describe the observed time dependent vertical suspended sediment distribution with a one dimensional turbulent diffusion model formulations of eddy diffusion and erosion rate coefficients associated with peak cross shore flows are found to agree with or fall within the range of values from previous laboratory and field studies and are used in the model the model is evaluated by its ability to reproduce the time dependent vertical distribution of suspended sediment and to estimate the observed mean suspended load and the correct direction and magnitude of the mean longshore and cross shore flux level above the bed z 31 cm of suspended sediment

deformation of the oceanic crust between the north american and south american plates fracture zone trends and magnetic anomalies in the atlantic ocean indicate that the north american plate must have moved with respect to the south american plate during the opening of the atlantic a comparison of plate tectonic flow lines with fracture zones identified from geosat and seasat altimeter data suggests that the north american south american plate boundary migrated northward from die guinea demarara shear margin to the vema fracture zone before chron 34 84 ma to north of the doldrums fracture zone before chron 22 51 9 ma and to north of the mercurius fracture zone between chron 32 72 5 ma and chron 13 35 5 ma the paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area indicate that the triple junction may have been located between the mercurius and the fifteen twenty fracture zones after 67 ma chron 30 plate reconstructions indicate a late cretaceous phase of transtension followed by transpression in the tertiary for the tiburon barracuda ridge area south of the fifteen twenty fracture zone the ocean floor in this area is characterized by a series of ridges and troughs with large bouguer gravity anomalies up to 135 mgal we use smoothing spline estimation to invert bouguer anomalies for crustal layer structure our model results suggest that the moho is uplifted 2 4 km over short wavelengths 70 km at the barracuda and tiburon ridges and imply large anelastic strains the severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted we propose that the north south american plate boundary migrated to the latitude of the tiburon ridge bounded by the vema and marathon fracture zones before chron 34 84 ma post chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the vema marathon mercurius and fifteen twenty fracture zone troughs but reaching the fifteen twenty fracture zone and future barracuda ridge area only after chron 32 72 5 ma this interpretation concurs with our crustal structural model which shows stronger crustal thinning underneath the tiburon ridge than at the barracuda ridge subsequent transpression may have continued along the existing zones of weakness in the tertiary creating the presently observed crustal deformation and uplift of the moho accompanied by anelastic failure of the crust middle eocene upper oligocene turbidites on the slope of the tiburon ridge now located 800 m above the abyssal plain suggest that most of its uplift occurred at post oligocene times the unusually shallow moho underneath the tiburon and barracuda ridges represents an unstable density distribution which may indicate that compressive stresses are still present to maintain these anomalies and that the north american south american plate boundary may still be located in this area

along arc segmentation and interaction of subducting ridges with the lesser antilles subduction forearc crust revealed by mcs imaging highlights we image the deep structure of the lesser antilles subduction zone by mcs profiles the complex deformation of the outer forearc crust is induced by subducting ridges we discuss also the effect of the subducting compressive nam sam plate boundary along strike variations of the seaward edge of the outer forearc crust are discovered the updip limit proxy of the seismogenic part reaches 20 km trenchwards than believed abstract we present the results from a new grid of deep penetration multichannel seismic mcs profiles over the 280 km long north central segment of the lesser antilles subduction zone the 14 dip lines and 7 strike lines image the topographical variations of i the subduction interplate decollement ii the top of the arcward subducting atlantic oceanic crust toc under the huge accretionary wedge up to 7 km thick and iii the trenchward dipping basement of the deeply buried forearc backstop of the caribbean upper plate the four northernmost long dip lines of this new mcs grid reveal several kilometre high topographic variations of the toc beneath the accretionary wedge offshore guadeloupe and antigua islands they are located in the prolongation of those mapped on the atlantic seafloor entering subduction such as the barracuda ridge this mcs grid also provides evidences on unexpected huge along strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments their mapping clearly indicates two principal areas of active deformation in the prolongation of the major barracuda and tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the atlantic seafloor although different in detail the two main deforming forearc domains share similarities in style the imaged deformation of the sedimentary stratification reveals a time and space dependent faulting by successive warping and unwarping which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely oriented elongated and localized topographical ridges the induced faulting producing vertical scarps in this transport does not require a regional arc parallel extensional regime as proposed for the inner forearc domain and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver a contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone its mapping documents along arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench with the exception of the southernmost part the newly mapped updip limit reaches 25 km closer to the trench thus indicating a possible wider seismogenic zone over almost the whole length of the study area

structure of the lesser antilles subduction forearc and backstop from 3d seismic refraction tomography in 2007 the sismantilles ii experiment was conducted to constrain structure and seismicity in the central lesser antilles subduction zone the seismic refraction data recorded by a network of 27 obss over an area of 65 km 95 km provide new insights on the crustal structure of the forearc offshore martinique and dominica islands the tomographic inversion of first arrival travel times provides a 3d p wave velocity model down to 15 km basement velocity gradients depict that the forearc is made up of two distinct units a high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south short wavelength deformations of the outer forearc basement are observed beneath a 3 to 6 km thick sedimentary pile in relation with the subduction of the tiburon ridge and associated seafloor reliefs north offshore dominica island the outer forearc is 70 km wide it extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed its width decreases strongly to the south to terminate offshore martinique where the inner forearc acts as the backstop the inner forearc is likely the extension at depth of themesozoicmagmatic crust outcropping to the north in la desirade island and along the scarp of the karukera spur the outer forearc could be either the eastern prolongation of the inner forearc but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes or an oceanic terrane more recently accreted to the island arc

searching for unconventional seismic signals on a subduction zone with a submerged forearc obs offshore the lesser antilles abstract detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer obs records requires the analysis and identification of noise due to instrumental problems deployment sites or sea state conditions the temporary obs deployment at the lesser antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor nvt long period lp or ultra long period ulp events analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data in order to reliably detect nvt 1 10 hz originating from inside the solid earth one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea bottom as well as on land the semidiurnal modulation of noise amplitude can be shown here not to be due to that of the nvt from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs here the semidiurnal modulation is rather due to the effect of the tides themselves such as tidal currents since they do not affect all types and all components of the unique multi station array of obs that could be deployed on this submerged forearc the short period cut off of the strong noise due to ocean surface infragravity waves increases to longer periods with obs depth thereby increasing the observational window with low noise to lower frequencies and deep obs sites may be advantageous for detecting lp events

field measurements of the suspended sand concentration profiles and fluxes and of the resuspension coefficient γ0 over a rippled bed measurements of the acoustic backscatter from suspended sand over a rippled bed and of currents were made near the crest of a ridge and runnel beach on the macrotidal north norfolk coast of the united kingdom two 12 min records were taken close to high water in a depth of 1 7 0 2 m visual inspection of the data shows that sand was intermittently suspended to heights of 20 30 cm but that these events were not always correlated with strong wave induced currents the structure of some of the suspension events suggest that they are due to vortex shedding from a rippled bed the observed time averaged concentration profiles were compared with predictions generated using glenn and grant s 1987 combined flow model the nondimensionalised concentrations showed good correspondence in the first 8 to 10 cm above the wave boundary layer supporting the use of a value of 0 74 drawn from atmospheric data for the constant of proportionality between sediment and momentum diffusivity above this height observed values of the proportionality constant tended to approach 1 0 which may have been due to a decrease in sediment settling velocity with height which is consistent with finer sand being suspended higher in the flow or alternatively to nonuniform turbulence in the vicinity of the wave ripples absolute concentrations measured 2 cm above the bed were consistent with values of o 10 4 for the empirical constant 0 in smith and mclean s 1977 pickup function concentrations within the wave boundary layer were observed to decrease much more rapidly than predicted by the model sand transport fluxes were computed for both the cross shore and alongshore components of the flow using a current profile based on the combined flow model of glenn and grant 1987 for the two records vertically integrated fluxes of 1 5 mg s cm width and 0 89 mg s cm width directed towards the shore were calculated for the wave oscillatory component of the transport the mean flow produced shoreward vertically integrated fluxes of 5 1 and 10 5 mg s cm width the uncertainty associated with these estimates is considerable 50 the vertical profile of the total flux wave oscillatory and mean for both records showed a strong shoreward transport close to the bed an offshore transport between 5 and 15 cm above the bed and a further weak onshore flux above this demonstrating the necessity for measurements of the whole concentration profile rather than a few point samples for the calculation of sand transport over a rippled bed in a wave dominated environment

slow subduction of old lithosphere in the lesser antilles abstract the lesser antilles subduction zone is an extreme case of the subduction of old 90 m y lithosphere at a slow 2 cm y convergence rate focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data during the time period 1950 1978 studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation all three large magnitude seven earthquakes were from intraplate normal faults no large thrust faulting earthquakes and few small ones occurred these observations suggest that the plate boundary is largely decoupled that subduction is at least partially aseismic and that the downgoing slab is in a state of extension

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

velocity measurements close to rippled beds in oscillatory flow the results are reported of velocity measurements in oscillatory flow over rippled beds velocities were measured with a laser doppler anemometer in both an oscillating tray rig and an oscillatory flow water channel both self formed and artificial ripples were examined in addition some measurements were made with an apparently plane bed with intense sediment motion the experimental results were compared with the predictions of kalkanis 1964 kajiura 1968 bakker 1973 1974 and sleath 1974 the closest agreement between theory and experiment was obtained with the method of sleath measurements of the eulerian drift velocities showed drift towards the adjacent ripple crest in the immediate vicinity of the bed and away from the crest further out

vertical fluxes of sediment in oscillatory sheet flow time series of vertical sediment fluxes are derived from concentration time series in sheet flow under waves while the concentrations c z t vary very little with time for z 10d50 the measured vertical sediment fluxes qzs z t vary strongly with time in this vertical band and their time variation follows to some extent the variation of the grain roughness shields parameter 2 5 t thus sediment distribution models based on the pickup function boundary condition are in some qualitative agreement with the measurements however the pickup function models are only able to model the upward bursts of sediment during the accelerating phases of the flow they are so far unable to model the following strong downward sediment fluxes which are observed during the periods of flow deceleration classical pickup functions which essentially depend on the shields parameter are also incapable of modelling the secondary entrainment fluxes which sometimes occur at free stream velocity reversal the measured vertical fluxes indicate that the effective sediment settling velocity in the high 0 3 c z t 0 4 concentration area is typically only a few percent of the clear water settling velocity while the measurements of richardson and jeronimo chem eng sci 34 1979 1419 from a different physical setting lead to estimates of the order 20 the data does not support gradient diffusion as a model for sediment entrainment from the bed that is detailed modelling of the observed near bed fluxes would require diffusivities that go negative during periods of flow deceleration an observed general trend for concentration variability to increase with elevation close to the bed is also irreconcilable with diffusion models driven by a bottom boundary condition

historical seismicity of the southeastern caribbean and tectonic implications we have relocated the twenty eight largest magnitude 4 3 ms 7 3 historical 1922 1963 earthquakes of the southeastern caribbean we also present new focal mechanisms for seven of these events the relocations are based on reported issp ands arrival times that we analyzed using generalized linear inversion techniques the new focal mechanisms were constrained by first motionp polarities as reported by the iss and as picked by us where records were available and by the polarities and ratios ofsh andssh andsv andssv arrivals that we determined from seismograms the results of the relocations are commensurate with the distribution of seismicity observed in the recent era hypocenters are shallow and intermediate in depth 0 200 km and the events occur almost exclusively in areas known to be currently seismic the frequent seismic activity in the vicinity of the paria peninsula venezuela is clearly a persistent feature of the regional earthquake pattern intermediate depth earthquakes indicative of subduction beneath the caribbean plate occur here and along the lesser antilles arc the grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era trinidad and the eastern gulf of paria were also largely quiescent

long term seismic behavior of a fault involved in a multiple fault rupture insights from tectonic geomorphology along the neodani fault central japan abstract multiple faults with different paleoseismic histories sometimes rupture simultaneously resulting in a larger earthquake than expected to test behavioral models for a fault involved in a multiple fault rupture we performed detailed tectonic geomorphological mapping along the neodani fault central japan one of several faults that ruptured simultaneously during the devastating 1891 m w 7 5 nobi earthquake our mapping showed that the along strike distributions of the long term rates of left lateral and vertical slips generally mimic in shape those of the 1891 slip implying that the slip patch behavior in which smaller more frequent earthquake ruptures fill in low slip portions of the 1891 rupture does not apply to the neodani fault if the earthquake recurrence interval estimated from a previous paleoseismic trench is correct however the neodani fault should not be characterized by repetition of same slip function earthquake ruptures or the characteristic slip function behavior the observations can instead be explained by the semicharacteristic slip function behavior in which a similarly shaped slip distribution is repeated independently of the rupture of adjacent faults but the amount of slip varies nonetheless characteristic slip function behavior may still be a valid interpretation if some of the paleoseismic events identified in the trench are minor sympathetic slip events although we found that the shape of the slip distribution has been quite stable through successive earthquakes including a multiple fault rupture earthquake further paleoseismic efforts are warranted to examine the stability in the amount of slip

measuring suspended sediment concentrations using acoustic backscatter devices abstract in recent years the requirement to develop a technique for measuring suspended sediment concentration profiles with similar temporal and spatial resolution to that of electromagnetic current meter measurements of the flow has led to the development of an acoustic approach acoustic backscatter devices have been constructed and deployed in marine environments and measurements of backscattered signal profiles obtained however the translation from acoustic returns to suspended load has been an empirical procedure resulting in some uncertainty regarding the accuracy and reliability of the conversion process to address this problem a series of laboratory measurements of acoustic backscattering from marine suspensions were conducted using two acoustic backscatter instruments operating at 3 0 and 5 65 mhz a simple semi empirical backscattering theory has been developed to predict the form of the scattered signal and comparisons have been made of the concentrations predicted from the theory against concentration measurements made in laboratory tanks the results show that the main features of the backscattering signal can be explained and accurate concentration estimates can be made from the acoustic returns it is concluded that acoustic backscatter techniques are potentially powerful tools for examining at high resolution the structure of suspended sediment concentration profiles near the seabed

a suspended sediment stratification correction for combined wave and current flows a simple model for the near bottom combined wave and current flow over a moveable sediment bed is presented the model is an extension of the grant and madsen 1979 combined wave and current model with moveable bed effects on the physical bottom roughness included as by grant and madsen 1982 the unsteady conservation of fluid momentum and sediment mass equations which are coupled through an eddy diffusivity closure scheme are solved for the wave and current velocity profiles along with the suspended sediment concentration and transport profiles sample runs are presented to illustrate the effect of varying wave conditions on the current the mean sediment concentration and the mean sediment transport results show that for fine to medium sands self stratification of the flow can be important during storms owing to the large amount of sediment suspended by the waves and the enhanced turbulent mixing associated with the wave current interaction the theory easily can be generalized for inclusion in a full ekman layer model

s wave velocities of the lithosphere asthenosphere system in the lesser antilles from the joint inversion of surface wave dispersion and receiver function analysis abstract we present an overview of s wave velocities vs within the crust and upper mantle of the lesser antilles as determined with 19 seismic broadband stations receiver functions rf have been computed from teleseismic recordings of earthquakes and rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the caribbean area local smoothness optimization lso procedure has been applied combined with an h k stacking method the spatial distribution of hypocenters of local earthquakes and of the energy they released in order to identify an optimum 1d model of vs below each station several features of the caribbean plate and its interaction with the atlantic subducting slab are visible in the resulting models a relatively thick oceanic crust below these stations ranges from 21 km to 33 km being slight thinner in the middle of the island arc b crustal low velocity zones are present below stations saba seus ski smrt cbe dsd gcmp and tdba c lithospheric thickness range from 40 km to 105 km but lithosphere asthenosphere boundary was not straightforward to correlate between stations d the aseismic mantle wedge between the caribbean seismic lithosphere and the subducted slab varies in thickness as well as vs values which are in general lower below the west of martinique than below the west of guadeloupe e the depth of the subducted slab beneath the volcanic arc appears to be greater to the north and relatively shallower below some stations e g dlpl sam bim and fdf than was estimated in previous studies based on the depth distribution of seismicity f the wbz is 10 15 km deeper than the top of the slab below the central lesser antilles martinique and dominica where the presence of partial melt in the mantle wedge seems also to be more evident

sorted bed forms as self organized patterns 1 model development 1 here we present a modified version of the exploratory numerical model originally presented by murray and thieler 2004 and use it to further investigate the development of sorted bed forms on the inner continental shelf the new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model the shallow water wave equations are replaced by the general equations and changes in water depth related to morphological development are accounted for this model reproduces the main results of the previous one supporting the hypothesis that sorted bed forms are a self organized pattern and develop as a result of a coupling between bed composition and sediment flux the sensitivity of the model predictions evaluated in terms of the geometry of the sorted bed forms and their growth rate with respect to external forcing variables and internal model parameterizations shows increasing decreasing sorted bed form spacing height with increasing wave height and period the opposite occurs for increasing water depths more complicated behavior is observed when the ratio fine to coarse sediment diameter or the percentage of fine and coarse material is varied the shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave generated ripples can significantly affect or even inhibit growth rates of sorted bed forms results from a sensitivity analysis show that there are deficiencies in the description of small scale processes that limit the ability to quantitatively predict sorted bed form evolution highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled

examination of reference concentration under waves and currents on the inner shelf 1 we examine reference concentration using three different data sets of near bed suspended sediment concentration observed under combined waves and currents the data include observations made at 15 and 20 m depth off dounreay scotland uk and observations obtained at 13 m depth off duck north carolina usa these data accommodate different dynamic conditions from wave dominated conditions at dounreay to wind driven current dominated conditions at duck and sediment properties median size of bed sediment ranging from 120 to 350 m near bed concentration profiles to elevations of about 80 cm were obtained using acoustic backscatter sensors with 1 cm resolution the reference concentrations cr at 1 cm were then evaluated by regressing the observed suspended sediment concentrations against a rouse type model bed shear stresses associated with each estimate of cr were estimated using the wave current interaction model of grant and madsen existing equations for reference concentration based on shear stress alone fail to accommodate all cr estimates from different environments we introduce a new empirical relationship between cr and the product of shields and inverse rouse numbers these dimensionless parameters represent the ratio of bed shear stress and submerged particle weight and the ratio of shear velocity and particle settling velocity respectively the new formula adjusts the amount of mobile sediment at the bed related to the shields number to that available for suspension at the reference height related to the inverse rouse number the new formula for reference concentration accommodates observations from different environments suggesting that it may have wide applicability on sandy inner shelves

a cause of rupture segmentation and synchronization in the nankai trough revealed by seismic imaging and numerical simulation 1 a giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes however it is still unknown if a giant earthquake coincidentally occurs or if there is a causal factor to control its generation in this study we show a cause and a growth process of a giant earthquake which may occur along southwestern japan on the basis of seismic images obtained from wide angle seismic data and a numerical simulation incorporating the structural images the wide angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 tonankai moment magnitude mw 8 1 and the 1946 nankai mw 8 4 earthquakes the seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary the numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire nankai trough a distance of over 600 km long mw 8 7 the growth process revealed from the simulated slip history in and around the strongly coupled patch is 1 prior to the giant earthquake a small slow event or earthquake occurs near the segmentation boundary 2 this accelerates a very slow slip slower than the plate convergent rate at the strong patch which reduces a degree of coupling and 3 then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary consequently growing into a giant earthquake

seismic activity offshore martinique and dominica islands central lesser antilles subduction zone from temporary onshore and offshore seismic networks this work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the central lesser antilles island arc from martinique to guadeloupe islands during the whole recording period extending from january to the end of august 2007 more than 1300 local seismic events were detected in this area a subset of 769 earthquakes was located precisely by using hypoellipse we also computed focal mechanisms using p wave polarities of the best azimuthally constrained earthquakes we detected earthquakes beneath the caribbean forearc and in the atlantic oceanic plate as well at depth seismicity delineates the wadati benioff zone down to 170 km depth the main seismic activity is concentrated in the lower crust and in the mantle wedge close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed we propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense thick and rigid crustal block and the lighter and more flexible outer forearc seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate

fault slip and earthquake recurrence along strike slip faults contributions of high resolution geomorphic data article i nfo understanding earthquake eq recurrence relies on information about the timing and size of past eq ruptures along a given fault knowledge of a fault s rupture history provides valuable information on its potential future behavior enabling seismic hazard estimates and loss mitigation stratigraphic and geomorphic evidence of faulting is used to constrain the recurrence of surface rupturing eqs analysis of the latter data sets culminated during the mid 1980s in the formulation of now classical eq recurrence models now routinely used to assess seismic hazard within the last decade light detection and ranging lidar surveying technology and other high resolution data sets became increasingly available to tectono geomorphic studies promising to contribute to better informed models of eq recurrence and slip accumulation patterns after reviewing motivation and background we outline requirements to successfully reconstruct a fault s offset accumulation pattern from geomorphic evidence we address sources of uncertainty affecting offset measure ment and advocate approaches to minimize them a number of recent studies focus on single eq slip distribu tions and along fault slip accumulation patterns we put them in context with paleoseismic studies along the respective faults by comparing coefficients of variation cv for eq inter event time and slip per event and find that a single event offsets vary over a wide range of length scales and the sources for offset variability differ with length scale b at fault segment length scales single event offsets are essentially constant c along fault offset accumulation as resolved in the geomorphic record is dominated by essentially same size large offset in crements and d there is generally no one to one correlation between the offset accumulation pattern constrained in the geomorphic record and eq occurrence as identified in the stratigraphic record revealing the higher resolution and preservation potential of the latter while slip accumulation along a fault segment may be dominated by repetition of large nearly constant offset increments timing of surface rupture is less regular

discovery controls and hazards of widespread deep seated gravitational slope deformation in the etsumi mountains central japan deep seated gravitational slope deformation dsgsd is a largely unnoticed but important long term mass wasting process that may result in catastrophic failure of mountain slopes manifested by small topographic irregularities such as ridge parallel scarps and linear depressions it has been predominantly reported in alpine landscapes above timber lines on the basis of area wide high resolution topographic data acquired by light detection and ranging lidar surveys we here show that 96 of existing gravitational scarps have been hidden under forest canopies in the etsumi mountains central japan the scarps are surprisingly widespread over the mountains with a mean line density of as large as 0 87 km km2 our analyses of the scarp distribution suggest that uphill facing scarps are primary geomorphic signals of dsgsd with a destabilized rock mass larger than 105 m2 whereas downhill facing scarps principally occur in response to more localized slope deformation in terms of controls topography is by far the most influential factor in triggering and promoting dsgsd despite the m 7 5 earthquake in 1891 impact of large local earthquakes proves to be not very strong comparison with preexisting landslide maps further suggests that dsgsd and large scale landslide are not different slope processes but represent different stages of the same process our results highlight limitation of aerial photograph interpretation in forest covered mountains and the need for lidar assisted mapping for deeper understanding of this long term process and interactions between surface and tectonic processes

developing sub 5 m lidar dems for forested sections of the alpine and hope faults south island new zealand implications for structural interpretations abstract kilometre wide airborne light detection and ranging lidar surveys were collected along portions of the alpine and hope faults in new zealand to assess the potential for generating sub 5 m bare earth digital elevation models dems from ground return data in areas of dense rainforest bush cover as an aid to mapping these faults the 34 km long franz whataroa lidar survey was flown along the densely vegetated central most portion of the transpressive alpine fault six closely spaced flight lines 200 m apart yielded survey coverage with double overlap of swath collection which was considered necessary due to the low density of ground returns 0 16 m 2 or a point every 6 m2 under mature west coast podocarp broadleaf rainforest this average point spacing 2 5 m allowed for the generation of a robust high quality 3 m bare earth dem the dem confirmed the zigzagged form of the surface trace of the alpine fault in this area originally recognised by norris and cooper 1995 norris and cooper 1997 and highlights that the surface strike variations are more variant than previously mapped the 29 km long hurunui hope lidar survey was flown east of the main divide of the southern alps along the dextral slip hope fault where the terrain is characterised by lower rainfall and more open beech forest flight line spacings of 275 m were used to generate a dem from the ground return data the average ground return values under beech forest were 0 27 m 2 and yielded an estimated cell size suitable for a 2 m dem in both cases the lidar revealed unprecedented views of the surface geomorphology of these active faults lessons learned from our survey methodologies can be employed to plan cost effective high gain airborne surveys to yield bare earth dems underneath vegetated terrain and multi storeyed canopies from densely forested environments across new zealand and worldwide

satellite imaging of the 2015 m7 2 earthquake in the central pamir tajikistan elucidates a sequence of shallow strike slip ruptures of the sarez karakul fault

sediment transport near the break point associated with cross shore gradients in vertical eddy diffusivity sediment clouds entrained and mixed vertically into the water column at the breakpoint were observed to advect offshore with the return current following wave breaking at a natural beach an advection diffusion model is applied to this phenomenon providing for cross shore gradients in vertical eddy diffusivity within and beyond the surf zone which result in differential entrainment and settlement near the break point results indicate that the horizontal gradient in turbulent vertical sediment mixing can provide a significant mechanism for bar generation even if the surface gravity and infra gravity waves remain uncoupled quantitative estimates of the rate of bar accretion are obtained after calibrating the sediment transport model using field measurements of currents and suspended sediment loads in the absence of other processes accretion rates of order 20 mm hour associated with the horizontal gradient in vertical diffusivity are predicted on the offshore bar

laboratory flume studies on monochromatic wave fine sandy bed interactions part 2 sediment suspensions abstract as reported in preceding paper part 1 soil fluidization the observed phenomena of sediment suspensions above a fluidized sandy bed of sand ii d50 0 092 mm under monochromatic wave actions are quantitatively investigated the suspended sediment concentration ssc at a single point within 5 cm above the bed was synchronously measured with water waves and bed soil s pore pressures with an intrusive optical sediment concentration probe the measurements show that ssc initiates several wave cycles after initiation of bed soil s fluidized response and grows to a peak value mainly in the post fluidization phase under similar wave loadings in the same test series ssc is usually higher over a resonantly fluidized rf bed than over a non resonantly fluidized nrf bed on the contrary only relatively low scc can be identified above an unfluidized bed the analyses illustrate that to certain extent peak values of ssc are directly proportional to the thickness of fluidized soil layer df values of df usually decrease with repeated fluidized response longer consolidation periods and in deeper water depths once the fluidized responses initiate pore pressures are generally much significantly amplified in both shallow fluidized soil layers and near below the fluidized layer especially during the resonance event the resulting depth gradients of dynamic pore pressure amplitudes in shallow layers are likely to have caused higher initial rises of ssc in a rf bed than in the subsequent nrf bed those in deeper layer should have contributed to sustain the fluidization state for further ssc increments immediately after termination of wave loading re deposited suspended sediments always result in a typical flat bed form for a pre fluidized bed wave induced drastic sediment suspensions are still obtainable very near above the bed with even a rather thin fluidized surface soil layer

suspended sediment load during an asymmetric wave cycle over a plane bed abstract suspended sediment measurements made by ribberink and al salem 1992 between 5 mm and 46 mm from a plane bed exhibited apparent changes to the boundary layer thickness and eddy diffusivity during the wave cycle bed ripples were absent but the data still showed entrainment near the time of flow reversal these laboratory measurements were examined using a numerical model to study sediment suspension at short time scales to assess relevance the data were considered in conjunction with concentration time series of similar character from a natural beach each of three distinct concentration peaks during one wave cycle evident in the laboratory measurements had to be treated differently however all peaks exhibited a high correlation to steady state logarithmic concentration profiles with uniform but unequal vertical eddy diffusivities the first peak under the wave crest could be predicted with standard empirical relationships derived from long term averaged data the remaining peaks could be simulated only when unexpected entrainment was permitted for a very short time 0 1 s near flow reversal and separately during the accelerating phase of the wave trough the paper provides the basis for further work but at present the results cannot be used for predictive purposes

microseismicity and stresses in the lesser antilles dipping seismic zone abstract microearthquake data from a telemetered array have been used to study the central antilles arc system the zone lying between lat 14 17 n and long 59 5 62 w has been investigated during the period 1979 1980 no shock with a magnitude larger than 4 5 was recorded results are well correlated with previous conclusions derived from teleseismic records the dip of the subducting plate is 60 everywhere in this area no composite focal mechanism can be produced for the region north of dominique for the southern region composite focal mechanisms reveal two zones for depths between 45 and 80 km the stress field seems mainly related to the interplate action beneath 80 km the slab dominantly neither in compression nor in tension earthquakes are more likely related to local stress fields a double layered structure has not been found in the earthquake distribution in the subducted plate nor in the stress field deduced from focal mechanisms

spatial distribution of microearthquakes beneath the japan trench from ocean bottom seismographic observations summary the area of the japan trench about 40 n off the t6hoku district of north eastern honshu japan was for the first time extensively covered by an ocean bottom seismograph obs array seven obss were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 july and august during this period each obs registered between 300 and 600 earthquakes about 100 events were simultaneously recorded by at least three obss hypocentres were calculated by using a generalized inversion technique sixty five events mostly with magnitudes m from 1 5 to 3 0 which were not located by a land network were determined to be within or near the area covered by the obs array the obs array had nearly uniform detectability for earthquakes with m over 2 0 within the latitude range of 39 s0 41 5 n and the longitude range of 143 146 5 e within this area there is a highly active region of microseismicity beneath the seaward wall of the japan trench the seaward activity is concentrated within 100 km of the trench axis and is low beneath the northwest pacific basin a seismicity gap was found just westward of the trench axis beneath the landward wall of the trench at a water depth of between 6000 and 4000m seismic activity continues landward from the region west of the gap the obs array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible the results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere on the seaward side most events occur at a depth of less than 30 km beneath the trench wall while the activity on the landward side starts to deepen westward exactly at the trench axis this paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and or subducting of the oceanic plate just beneath the japan trench axis

stereopaired morphometric protection index red relief image maps stereo mpi rrims effective visualization of high resolution digital elevation models for interpreting and mapping small tectonic geomorphic features

dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves abstract bedform dimensions bed position changes near bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of england the study was conducted in 0 5 2 25 m water depth under both swell and wind generated storm waves with both weak and strong currents present bed positions and suspended sand concentrations were measured using the 3 transducers of a multi frequency acoustic backscatter sensor with 5 mm vertical resolution two distinct bed types based on wavelength were observed each with two or more subtypes possible i small scale bedforms 20 cm which include two dimensional 5 forms and three dimensional vortex 6 forms small scale forms were dominant under non breaking conditions while the large scale forms occurred under both non breaking and breaking waves the large scale forms dominate under breaking conditions both types but particularly low steepness forms were highly mobile with maximum horizontal migration rates of 5 cm min 1 large upto 15 cm and rapid upto 3 0 cm min 1 changes in vertical bed elevation were also observed in association with the development and migration of large scale forms large scale bedforms were also highly variable spatially often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes observations suggest these forms are present even under high energy surf zone conditions wave shields 1 models for predicting ripple dimensions did not perform well in this environment this lack of agreement reflects the complex hydrodynamic regimes associated with random grouped shoaling and breaking waves together with the presence of currents often at large angles to the waves as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration estimates of transport rates are subject to potentially large errors upto 30 without compensation for bed elevation changes relative to sensor position

relative motion between the caribbean and north american plates and related boundary zone deformation from a decade of gps observations global positioning system gps measurements in 1986 1994 and 1995 at sites in dominican republic puerto rico cuba and grand turk define the velocity of the caribbean plate relative to north america the data show eastward motion of the caribbean plate at a rate of 21 1 mm yr 1 standard error in the vicinity of southern dominican republic a factor of 2 higher than the nuvel 1a plate motion model prediction of 11 3 mm yr independent measurements on san andres island and an euler vector derived from these data also suggest a rate that is much higher than the nuvel 1a model available data combined with simple elastic strain models give the following slip rate estimates for major left lateral faults in hispaniola 1 the north hispaniola fault offshore the north coast of hispaniola 4 3 mm yr 2 the septentrional fault in northern dominican republic 8 3 mm yr and 3 the enriquillo fault in southern dominican republic and haiti 8 4 mm yr the relatively high plate motion rate and fault slip rates suggested by our study combined with evidence for strain accumulation and historical seismicity imply that seismic risk in the region may be higher than previous estimates based on low plate rate low fault slip rate models and the relatively low rate of seismicity over the last century

segmentation of the lesser antilles subduction zone abstract precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the benioff zone beneath the lesser antilles island arc there are two distinct segments to the zone one to the north of martinique which trends nnw and one to the south of st lucia which trends nne the northern zone dips at 60 50 the southern zone has a dip of 50 45 in the north but is vertical in the south pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the benioff zone mid miocene swarms of dykes in martinique and st lucia have nw and ne azimuthal modes respectively we interpret this to mean that this kink in the subduction zone existed as long ago as the miocene the kink may represent the deformation of a single subducting plate or two separate north and south american plates subducting beneath the caribbean plate with a triple junction in the region of martinique st lucia

seismicity and seismic gap in the lesser antilles arc and earthquake hazard in guadeloupe summary a seismic study of the lesser antilles arc has been carried out first for the period 1950 1978 for which we can use local seismic networks to draw maps of instrumental seismicity then for the period 1530 1950 for which we have catalogues of felt earthquakes the striking feature of the spatial distribution of foci is the cluster of epicentres in the northern half of the arc all large earthquakes m 7 5 are located north of 14 latitude seismicity cross sections through the arc show a variable dipping subduction zone along the arc the deep seismic zone is steeper in the centre of the arc than on the extremity the time space diagram for historical seismicity and the evidence of a seismic gap at the east of guadeloupe lead us to consider the northern half arc as a likely site for a large earthquake in the near future the seismic slip rate calculated from all major earthquakes since 1530 is of much greater value than that obtained from recent plate tectonic models suggesting that the recurrence rate of earthquakes is more than many hundreds of years with a possible aseismic creep

control of forearc density structure on megathrust shear strength along the chilean subduction zone abstract unveiling how the interaction of friction pore pressure p and normal stress n controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains an existing density model of the chilean margin 18 45 s allows the computation of the so called vertical stress anomaly vsa that accounts for the component of normal stress due to the weight of the overlying crustal column a spatial correlation between vsa surface geology and several earthquake parameters suggests a significant control exerted by the geologically dominated density structure of the crust on megathrust shear strength the very seismogenic forearc in front of the high central andes 18 34 s has positive vsa values up to 60 mpa and a mafic dominated crust which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column in addition to a likely dry subduction channel characterized by high and low p in contrast the southern andean forearc 34 45 s shows neutral to negative vsa down to 30 mpa correlated with a felsic dominated crust which added to a presumably wet subduction channel low and high p configures a rather weak megathrust below a light forearc earthquakes here are concentrated around the arauco peninsula a first order morpho structural anomaly characterized by comparatively higher values of vsa than the rest of this segment which is the nest of the giant mw 9 5 valdivia 1960 earthquake and the southern rupture termination of the recent mw 8 8 maule earthquake the ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative vsa values the correlation between vsa and the along strike segmentation of the andean orogen indicates that the geologically inherited density structure of the forearc also rules long term mechanical coupling between converging plates and prescribes the possible structure of the orogen because central and southern andes represent two extremes in terms of orogenic structure and seismogenic behavior results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins

swell dominated sediment re suspension in a silty coastal seabed abstract waves are fundamentally important for sediment re suspension in estuary and coastal areas especially for silty sediments which can be easily suspended by waves but the differential effects of swell and wind waves are still unclear integrated field observations were made from november 2012 to march 2013 including waves currents and suspending sediments on the offshore seabed of the huanghe delta to explore the mechanism of sediment re suspension in silty coastal zones during the five months of observation there were more than 30 winter wind events that affected the study area and induced sediment re suspension with varying suspended sediment concentration the observed wave composition was separated into swell and wind waves using a bandpass filter results show that large swell with significant height 1 0 m coming from the offshore direction ne in our study area dominated sediment resuspension in the coastal seabed due to the fact that this wind direction had the longest average fetch winds from the onshore direction usually had smaller swell due to their short fetches and caused limited sediment re suspension the residual currents caused by ne winds also transport larger sediment an individual ne wind event could transport sediment 8 13 6 t m2 and 5 1 8 2 t m2 in directions parallel and perpendicular respectively to the isobaths which is much higher than the sediment transportation during an individual nw wind event which could transport 1 5 4 t m2 and 0 6 5 t m2 parallel and perpendicular respectively to the isobaths our research shows that large swell and the accompanying residual currents caused by ne winds from offshore direction are a vital driving force for sediment resuspension and transportation in the offshore zone

experimental study of site effects in the fort de france area martinique island abstract we used weak motion recordings of regional events to quantify the site effects in the fort de france martinique area in the frequency band 1 to 25 hz the studied area extends over three distinct geological units the morne pitault volcanic massif 15 to 11 m a the carbet volcanic massif 4 to 2 m a and the lamentin alluvial plain quaternary age the topography being complex 31 sites were selected in order to get a representative sampling of the various topographical and geological configurations the large number of studied sites also allowed us to define a reference site whose selection was not obvious large broadband amplifications are observed in the lamentin plain instead of the expected peak resonances more surprising such resonance is observed around 2 to 3 hz on several sites of the volcanic vauclain pitault chain though no sedimentary layer is present on the other hand in the inner fort de france where significant effects were expected because of the very complex topography no large amplication is observed except for at one site clear three dimensional 3d effects are also observed at some stations though not very well documented the low frequency behavior deduced from teleseismic observations seems rather different illustrating the difficulty in finding a convenient reference site in such a volcanic area

predicting suspended sediment concentration on continental shelves field observations of the near bottom suspended sediment concentration current and wave at a depth of 90 m on the washington shelf and at a depth of 1 5 m on the coastal beaufort sea show that wave induced oscillatory flow plays a dominant role in resuspending bottom sediment the suspended sediment concentration can be reasonably predicted from the measured flow data by applying a diffusion model good agreement between the predicted and observed concentrations is obtained by using a reference concentration proportional to the square root of the excess boundary shear stress for a sediment bed consisting of multiple grain sizes the differential rate at which material is eroded leads to a significant change in the surfacial grain size composition the fraction of the fine grain sediment in the surface layer decreases sharply with increasing boundary shear stress a procedure is presented to model this change of the grain size distribution in the surface layer the analysis indicates that neglecting the change of the surfacial grain size distribution with flow may overestimate the erosion depth by a factor of 10 at bottom orbital velocities exceeding 70 cm s 1

comparison of time averaged acoustic backscatter concentration profile measurements with existing predictive models time averaged concentration profiles over bedforms measured using acoustic backscatter abs instrumentation on a macrotidal beach at whitsand bay cornwall uk were dominated by vortex type suspension profiles wave heights were typically in the range 0 25 1 0 m with wave periods of between 5 and 8 s mean currents measured at 0 2 m above the bed were generally weak 0 1 m s 1 near bed concentrations were compared with two reference concentration cref models smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 the resuspension coefficient 0 showed a strong dependence on the maximum skin friction shields parameter the smith and mclean smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 model implies that cref z0 increases with excess skin friction at the bed excess however field data showed no such correlation and the inclusion of a variable 0 gave poor model predictions that tended to have cref z0 decreasing with increasing excess reference concentrations at the bed c0 were determined from the time averaged concentration profiles using regression analysis the nielsen nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 model predicts that c0 increases rapidly with the modified skin friction shields parameter r however extrapolated values of c0 showed no dependence on r in addition model predictions were orders of magnitude higher than extrapolated values of c0 the influence of the bedforms on the magnitude of near bed concentration measurements has been examined we conclude that the observed variations in cref over different bedforms vortex ripples lunate ripples lunate mega ripples will need to be taken into account if model predictions of cref as a function of the skin friction shields parameter are to improve the mixing height l predictions of the nielsen nielsen p 1992 combined convection diffusion modelling of sediment entrainment proc 3rd int conf on coastal engineering venice 4 9 oct 1992 pp 3202 3215 and the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 models have been compared with measured data although the predictions of both models were typically within a factor of two of l measurements the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 model is believed to be more appropriate for modelling l over bedforms as field data showed a strong dependence of l on and model predictions do not rely on bedform data

slip in the 1857 and earlier large earthquakes along the carrizo plain san andreas fault the moment magnitude mw 7 9 fort tejon earthquake of 1857 with a 350 kilometer long surface rupture was the most recent major earthquake along the south central san andreas fault california based on previous measurements of its surface slip distribution rupture along the 60 kilometer long carrizo segment was thought to control the recurrence of 1857 like earthquakes new high resolution topographic data show that the average slip along the carrizo segment during the 1857 event was 5 3 1 4 meters eliminating the core assumption for a linkage between carrizo segment rupture and recurrence of major earthquakes along the south central san andreas fault earthquake slip along the carrizo segment may recur in earthquake clusters with cumulative slip of 5 meters

deep structure of an island arc backstop lesser antilles subduction zone 1 we present the results from a coincident seismic reflection refraction grid conducted at the lesser antilles subduction zone near 16 n this paper focuses on the seismic refraction data and constraints these data place on the three dimensional structure of the island arc backstop we find that the backstop in this region contains considerable topography in both the strike and dip directions two ridges each 25 35 km in length and 10 km in width rise 1 6 km above the adjacent basement the eastern edge of one of the ridges deepens by 4 6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop in contrast to the complex nature of the backstop the adjacent accretionary wedge displays little lateral variability at large scales this may be a consequence of the spatial scales involved the backstop topography is 10 35 km in width while the accretionary wedge extends 125 km from the deformation front to the backstop the top of the subducting oceanic crust as identified by an increase in velocities to 6 6 5 km s intersects the backstop at a depth of 14 15 km the updip limit of plate boundary seismicity is located 75 100 km west and downdip of the backstop however two earthquake clusters are observed at the intersection of the subducted barracuda ridge and tiburon ridge with the backstop suggesting active deformation associated with the backstop edge at these locations

structure of the oceanic lithosphere and upper mantle north of the gloria fault in the eastern mid atlantic by receiver function analysis receiver functions rf have been used for several decades to study structures beneath seismic stations although most available stations are deployed on shore the number of ocean bottom station obs experiments has increased in recent years almost all obss have to deal with higher noise levels and a limited deployment time 1 year resulting in a small number of usable records of teleseismic earthquakes here we use obss deployed as mid aperture array in the deep ocean 4 5 5 5 km water depth of the eastern mid atlantic we use evaluation criteria for obs data and beam forming to enhance the quality of the rfs although some stations show reverberations caused by sedimentary cover we are able to identify the moho signal indicating a normal thickness 5 8 km of oceanic crust observations at single stations with thin sediments 300 400 m indicate that a probable sharp lithosphere asthenosphere boundary lab might exist at a depth of 70 80 km which is in line with lab depth estimates for similar lithospheric ages in the pacific the mantle discontinuities at 410 km and 660 km are clearly identifiable their delay times are in agreement with prem overall the usage of beam formed earthquake recordings for obs rf analysis is an excellent way to increase the signal quality and the number of usable events

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

subduction seismicity and tectonics in the lesser antilles arc we have studied the mechanisms of 17 earthquakes along the lesser antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate no large thrust earthquakes occurred during the 1950 1978 study period the three large magnitude seven events are all normal faults one is a normal faulting event seaward of the trench its aftershock sequence includes strike slip events on differently oriented faults probably due to lateral block motion in response to the main shock a second indicates extension within the slab at depth these observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative the third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench this unusual geometry may represent a flexural response to the subduction of the barracuda ridge a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some mid atlantic ridge fracture zones thus the barracuda ridge is not buoyant and does not affect benioff zone dip strike slip faulting occurs at depth in the subduction zone along a concentration normal to the arc and maymore indicate a fossil fracture zone there is no direct evidence in the shallow seismicity for the hypothetical north america south america caribbean triple junction through some of the oceanic intraplate seismicity is consistent with such a boundary less

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

suspended sediment concentrations under waves abstract a predictive formula is derived for calculation of the magnitude of time averaged suspended sediment concentrations under waves by taking into account the effect of flow contraction near ripple crests the formula applies equally well for rippled beds and sheet flow conditions the formula is in qualitative agreement with the bed load flux formula of madsen and grant 1976

two hundred thirty years of relative sea level changes due to climate and megathrust tectonics recorded in coral microatolls of martinique french west indies we sampled six coral microatolls that recorded the relative sea level changes over the last 230 years east of martinique on fringing reefs in protected bays the microatolls are cup shaped which is characteristic of corals that have been experiencing submergence x ray analysis of coral slices and reconstructions of the highest level of survival hls curves show that they have submerged at rates of a few millimeters per year their morphology reveals changes in submergence rate around 1829 11 1895 and 1950 tide gauges available in the region indicate a regional sea level rise at a constant mean rate of 1 1 0 8 mm yr which contrasts with our coral record implying additional tectonic subsidence comparing our coral morphology with that of synthetic corals generated with matlab by using the key west tide gauge record florida we show that their growth was controlled by tectonics and that a sudden relative sea level increase drowned them around 1950 simple elastic models show that this sudden submergence probably occurred during the 21 may 1946 earthquake which ruptured the plate interface in front of martinique in the mantle wedge in an area of sustained seismic activity the 1839 m8 earthquake probably occurred in the same area long term subsidence of microatolls indicates that this deep portion of the megathrust is probably locked down to 60 km depth during the interseismic period our oldest coral recorded a long lasting period 50 years of stable relative sea level after the 1839 earthquake indicating that transient interseismic strain rate variations may occur in the lesser antilles

spatial and temporal structures of suspension and transport over megaripples on the shore face megaripples of 0 10 0 15 m height and 0 75 0 9 m wavelength occurred under swell wave activity on a macrotidal beach in southern cornwall england megaripples are relatively common features on the shore face in wave dominated environments although their occurrence is not well predicted by bed form models of the four bed form models examined clifton 1976 mogridge et al 1994 nielsen 1981 wiberg and harris 1994 only that of clifton 1976 predicted the formation of megaripples the average suspended sand concentration profiles measured using acoustic backscatter differed considerably in absolute magnitude and vertical gradient depending on whether the measurements were made over the crest or trough of the bed form the temporal structure of the suspension also varied with position relative to the megaripple utilizing the motion of the bed form beneath the instruments patterns of suspension over the megaripple were calculated as a function of the phase of the wave sand transport was computed from the cospectra of the suspended concentration with an electromagnetic current meter at 10 cm from the seabed transport was dominantly at the infragravity frequencies and was consistently directed offshore while the transport associated with the swell waves was smaller and varied with position over the megaripple the motion of the megaripple itself was toward the shore

lidar and field observations of slip distribution for the most recent surface ruptures along the central san jacinto fault we measured offsets on tectonically displaced geomorphic features along 80 km of the clark strand of the san jacinto fault sjf to estimate slip per event for the past several surface ruptures we identify 168 offset features from which we make over 490 measurements using b4 light detection and ranging lidar imagery and field observations our results suggest that lidar technology is an exemplary supplement to traditional field methods in slip per event studies displacement estimates indicate that the most recent surface rupturing event mre produced an average of 2 5 2 9 m of right lateral slip with maximum slip of nearly 4 m at anza a m w 7 2 7 5 earthquake average multiple event offsets for the same 80 kms are 5 5 m with maximum values of 3 m at anza for the penultimate event cumulative displacements of 9 10 m through anza suggest the third event was also similar in size paleoseismic work at hog lake dates the most recent surface rupture event at ca 1790 a poorly located large earthquake occurred in southern california on 22 november 1800 we relocate this event to the clark fault based on the mre at hog lake we also recognize the occurrence of a younger rupture along 15 20 km of the fault in blackburn canyon with 1 25 m of average displacement we attribute these offsets to the 21 april 1918 m w 6 9 event these data argue that much or all of the clark fault and possibly also the casa loma fault fail together in large earthquakes but that shorter sections may fail in smaller events online material topographic contour maps and hillshades generated from b4 lidar data corresponding field photographs and data tables comparing lidar based and field based slip measurements for individual geomorphic features along the clark fault

the 2004 2005 les saintes french west indies seismic aftershock sequence observed with ocean bottom seismometers on november 21 2004 an mw6 3 intraplate earthquake occurred at sea in the french caribbean the aftershock sequence continues to this day and is the most extensive sequence in a french territory in more than a century we recorded aftershocks from day 25 to day 66 of this sequence using a rapidly deployed temporary array of ocean bottom seismometers obs we invert p and s wave arrivals for a tomographic velocity model and improve aftershock locations the velocity model shows anomalies related to tectonic and geologic structures beneath the les saintes graben 3d relocated aftershocks outline faults whose scarps were identified as active in recent high resolution marine data the aftershocks distribution suggests that both the main november 21 event and its principal aftershock on february 14 2005 ruptured roseau fault which is the largest of the graben extending from dominica island to the les saintes archipelago aftershocks cluster in the lower part of the roseau fault plane between 8 and 12 6 km depth that did not rupture during the main event shallower aftershocks occur in the roseau fault footwall probably along smaller antithetic faults we calculate a strong negative vp anomaly between 4 and 8 km depth within the graben along the roseau fault plane this low vp anomaly is associated with a high vp vs ratio and may reflect a strongly fracturated body filled with fluids we infer from several types of observation that fault lubrication is the driving mechanism for this long lasting aftershock sequence

airborne lidar analysis and geochronology of faulted glacial moraines in the tahoe sierra frontal fault zone reveal substantial seismic hazards in the lake tahoe region california nevada usa we integrated high resolution bare earth airborne light detection and ranging lidar imagery with field observations and modern geochronology to characterize the tahoe sierra frontal fault zone which forms the neotectonic boundary between the sierra nevada and the basin and range province west of lake tahoe the lidar imagery clearly delineates active normal faults that have displaced late pleistocene glacial moraines and holocene alluvium along 30 km of linear right stepping range front of the tahoe sierra frontal fault zone herein we illustrate and describe the tectonic geomorphology of faulted lateral moraines we have developed new three dimensional modeling techniques that utilize the high resolution lidar data to determine tectonic displacements of moraine crests and alluvium the statistically robust displacement models combined with new ages of the displaced tioga 20 8 1 4 ka and tahoe 69 2 4 8 ka 73 2 8 7 ka moraines are used to estimate the minimum vertical separation rate at 17 sites along the tahoe sierra frontal fault zone near the northern end of the study area the minimum vertical separation rate is 1 5 0 4 mm yr which represents a two to threefold increase in estimates of seismic moment for the lake tahoe basin from this study we conclude that potential earthquake moment magnitudes m w range from 6 3 0 25 to 6 9 0 25 a close spatial association of landslides and active faults suggests that landslides have been seismically triggered our study underscores that the tahoe sierra frontal fault zone poses substantial seismic and landslide hazards

moment tensor solutions along the central lesser antilles using regional broadband stations abstract using waveform data gathered from the seismological networks of the lesser antilles we calculate 38 moment tensors for earthquakes with m 3 from 2013 to middle of 2015 by full waveform inversion nine of these moment tensor solutions are in good agreement with those previously reported by other institutions it provides some guarantee for 29 new moment tensors for the central region of the lesser antilles for earthquakes within the upper caribbean lithosphere our results evidence that extensional and strike slip focal mechanisms are predominant resulting from the intra plate deformation produced by the subduction of the north america and south american plates under the caribbean plate whereas very few thrust events are observed for deeper earthquakes 90 km our results compare well with older focal mechanisms from previous studies showing normal oblique or strike slip faulting within the subducted slab however the inversion for most of the deeper events is less reliable as documented for example by their larger condition numbers we use the newly obtained moment magnitudes to estimate the scaling relationship with the local magnitude mlv computed by the regional seismic network the two magnitudes are consistent for earthquakes with magnitudes m 4 with a slope close to unity further work is needed to precise the scaling relation for m

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

micro seismicity in the gulf of cadiz is there a link between micro seismicity high magnitude earthquakes and active faults abstract the gulf of cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes occasionally punctuated by high magnitude events such as the m 8 7 1755 great lisbon earthquake and the m 7 9 event of february 28th 1969 micro seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers obss in an area of high seismic activity encompassing the potential source areas of the mentioned large magnitude earthquakes we combined micro seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the gulf of cadiz area three controlling mechanisms are explored i active tectonic structures ii transitions between different lithospheric domains and inherited mesozoic structures and iii fault weakening mechanisms our results show that micro seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric rheological domains even though the crustal structure is well imaged in the seismic profiles and in the bathymetry crustal faults show low to negligible seismic activity a possible explanation for this is that the crustal thrusts are thin skinned structures rooting in relatively shallow sub horizontal decollements associated with aseismic serpentinization levels at the top of the lithospheric mantle therefore co seismic slip along crustal thrusts may only occur during large magnitude events while for most of the inter seismic cycle these thrusts remain locked or slip aseismically we further speculate that high magnitude earthquake s ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers

synchronized gravitational slope deformation and active faulting a case study on and around the neodani fault central japan abstract earthquakes are considered to be important triggers of deep seated gravitational slope deformation dgsd in seismically active regions and if that is the case dgsd features could be candidates for off fault paleoseismology investigations on the basis of pit excavations and sediment cores at an off fault dgsd site and a trench excavation across the active neodani fault at a nearby site we examined the records of dgsd and surface rupturing paleoearthquakes of the neodani fault the four most recent dgsd events were dated at after 240 cal bp 1710 340 cal bp 4730 3970 cal bp and 5570 5340 cal bp the four most recent surface rupturing earthquakes were dated at 1891 ce the nobi earthquake 2010 1220 cal bp 7180 2110 cal bp and before 9540 cal bp we conclude that the ages of the four dgsd events are consistent with surface rupturing earthquakes on the adjacent neodani fault we infer that static crustal strain from repeated seismogenic faulting plays an important role in the occurrence of dgsd events at least in the immediate vicinity of active faults although coseismic severe shaking would have at least some effect on them our case study suggests that off fault dgsds can be used to reconstruct or refine the paleoseismic history of a nearby active fault we propose that an ideal dgsd for that purpose would be located in an area of concentrated strain near a termination bend or stepover of the target fault

decreasing uplift rates and pleistocene marine terraces settlement in the central lesser antilles fore arc la desirade island 16 n this study investigates the lesser antilles fore arc at the latitude of guadeloupe archipelago and evidences that la desirade island the eastermost island of the forearc displays a staircase coastal sequence including four uplifted marine terraces and an upper reefal platform with mean shoreline angle elevations ranging from 10 to 210 m above sea level asl the platform paleobathymetry is constraint by a detailed analysis of the sediments we propose a revised morphostratigraphy for this coastal sequence including 5 paleo shorelines based on six u th dating from aragonitic corals from the three lowest terraces combined with paleobathymetric analysis of the fossil corals present in the upper platform terrace and upper platform carving of construction periods occurred during marine isotopic stages mis 5e mis 9 and during the intervals mis 15 17 mis 19 25 and mis 31 49 upper coral reef platform our results evidence a bulk decreasing uplift rate since early calabrian to present day clearly documented since 310 ka mis 9 from 0 14 to 0 19 to ca 0 mm y our data are consistent with first the transient influence of the subducting oceanic tiburon ridge during calabrian then with other parametres of the subduction zone since late calabrian to present day dip of the slab basal erosion of the upper plate inherited structures

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface seawards of the breaking zone during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break off bed shear stress is exceeded no direct measurements of bedforms were available and the combined wave current interaction model of grant and madsen j geophys res vols 84 1979 and 87 1982 with extensions to movable bed roughness ripple dimensions from carstens et al s 1969 laboratory data u s army corps eng cerc tech mem 28 and to sediment induced self stratification glenn and grant j geophys res vol 92 1983 was used to predict the current and suspended sand concentration profile matching the measured and modeled concentrations at 2 cm above the bed the values of the resuspension coefficient 0 were found to decrease as excess skin friction increased in a manner similar to that suggested by drake and cacchione cont shelf res vol 9 1989 but were an order of magnitude larger using the much smaller ripple dimensions of nielsen j geophys res vol 86 1981 changed the model results very little significant differences were found between the time averaged suspended sand profiles and those predicted by the model sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents the current being the sum of steady and wave induced components the wave induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5 10 cm above the bed and offshore transport in the few centimetres closest to the bed little net transport occurred above 15 cm these fluxes show a weak dependence on the wave energy becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased again consistent with the influence of the ripple steepness

sediment concentration and vertical mixing under breaking waves abstract field measurements of near bed sediment concentrations and sediment diffusivity in the breaker zone and in the inner surf zone of natural beaches were obtained using stacks of optical and fiber optical backscatter sensors under conditions with strongly plunging breakers as well as with spilling surf bores the field data suggest that significant differences exist between these wave types in terms of near bed sediment concentration levels and vertical mixing of sediment from the bed upward into the water column when plotting mean sediment concentration profiles in log c z space profiles are upward concave for spilling surf bores and sediment diffusivity increases approximately linearly upward from the bed for plunging waves however mean sediment concentration profiles are linear in a log c z plot indicating vertically constant sediment diffusivity this suggests that under surf bores sediment suspension from the bed occurs through diffusion processes while vertical mixing is mainly a convective process under plunging waves under surf bores sediment is largely confined to the lower layers of the water column while it is lifted to significantly higher elevations under plunging breakers

subduction and seismic hazard in the northern lesser antilles revision of the historical seismicity an important set of french and british archives on the historical seismicity of the northern lesser antilles is analyzed we present the most significant data which help to constrain the 8 february 1843 the 16 may 1851 and the 29 april 1897 earthquakes the 1843 event is the largest reported historical earthquake of the lesser antilles subduction arc and produced intensity ix on a segment about 100 km long between antigua and grande terre guadeloupe which contradicts the 300 km long proposed by other authors who mainly rely on robson s catalog 1964 this suggests a magnitude 7 5 to 8 0 the absence of tsunami and noticeable vertical deformation of the coasts in the source area makes very unlikely a normal fault mechanism but does not rule out the possibility of a subduction event because of the low dip angle of the interplate plane the dominance of moderate instrumental thrust earthquakes in the source area suggests a subduction mechanism for the 1843 event the northern and southern limits of the 1843 source area roughly coincide with the barracuda ridge and the tiburon ridge respectively which may have acted as barriers during the rupture process the northwestern portion of the tiburon ridge is associated with a lack of instrumental seismicity near grande terre which could indicate a possible magnitude 7 5 to 8 0 event in the next future moreover its mechanism could be normal which would provide the additional risk of a damaging tsunami the evaluation of seismic hazard must also take into account possible shallow moderate earthquakes as the 1851 and the 1897 events which produced intensities vii to viii in guadeloupe and may be related to observed major plioquaternary faults in this island

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern

detection of subtle tectonic geomorphic features in densely forested mountains by very high resolution airborne lidar survey abstract despite successes elsewhere in the world 2 m resolution dems from a standard airborne lidar survey failed to detect small tectonic geomorphic features in densely forested high relief mountains of central japan our new 0 5 m dems from an unprecedentedly high resolution lidar survey along the neodani fault now reveal a number of previously unknown fault scarps as well as other hidden geomorphic features the survey achieves a ground return density of 6 2 m 2 out of a total shot density of as much as 12 7 m 2 the main factor to gain sufficient ground returns in unfavorable conditions is a large side lap of 70 between flight swaths which means that any specific area in the target zone is scanned three or more times from different angles evaluation of dems with resolution from 0 25 to 10 m assures that a 0 5m resolution lidar dem is necessary for the detection of subtle tectonic breaks another key factor for complete detection of small tectonic geomorphic features is the application of a recently developed dem visualization red relief image map rrim which allows mapping of all the small features with various sizes orientations and morphology overcoming major drawbacks of classic dem visualizations a very high resolution lidar survey aided with rrim visualization as used in this study provides a more reliable approach for the detailed mapping of slight active fault traces hidden under dense vegetation

spatial variability of the wave bottom boundary layer over movable rippled beds observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected the dynamics between the ripple elements is dominated by coherent vortices characterized by the swirling strength and evidenced in the temporal and spectral characterization within the boundary layer spectral energy in the second harmonic 3f0 is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients first moment and second moment velocity statistics were used to address the spatial variability of the intra ripple hydrodynamics estimates of displacement and momentum thicknesses and mom are smaller than suggested by the higher harmonics but consistently highlight areas of adverse and favorable pressure gradients shear stress and roughness estimates were inferred by fitting a logarithmic model to first moment and second moment statistics of the velocity field the maximum shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half wave period onshore first moment roughness estimates are similar in magnitude to bed load parameterizations provided by li et al 1997 and about a factor of 3 larger than second moment estimates assessment of the vertical transfer of horizontal momentum derived using a reynolds decomposition suggests that stresses inferred from the logarithmic law using first moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half wave period

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates