the role of the critical angle in brightness reversals on sunglint images of the sea surface 1 a wide variety of oceanic and atmospheric phenomena are often observed in and around the sunglint region on optical images of the sea surface the appearance of these phenomena depends strongly on the viewing geometry with areas on the sea surface that are rougher or smoother than the background appearing as either brighter or darker than the background depending on their position relative to the specular point to understand these sea surface signature variations this paper introduces the concept of a critical sensor viewing angle defined as the sensor zenith angle at which different sea surface roughness variances produce identical sunglint radiance it is when the imaging geometry transitions through the critical angle that a surface feature goes through a brightness reversal knowledge of where this transition takes place is important for properly interpreting the characteristics of the sea surface signature of these phenomena the theory behind the concept of the critical angle is presented and then applied to sunglint imagery acquired over the ocean from space by the moderate resolution imaging spectroradiometer onboard nasa s aqua and terra satellites

drifter observations of submesoscale flow kinematics in the coastal ocean fronts and eddies identified with aerial guidance are seeded with drifters to quantify submesoscale flow kinematics the lagrangian observations show mean divergence and vorticity values that can exceed five times the coriolis frequency values are the largest observed in the field to date and represent an extreme departure from geostrophic dynamics the study also quantifies errors and biases associated with lagrangian observations of the underlying velocity strain tensor the greatest error comes from under sampling even with a large number of drifters a significant bias comes from inhomogeneous sampling of convergent regions that accumulate drifters within a few hours of deployment the study demonstrates a lagrangian sampling paradigm for targeted submesoscale structures over a broad range of scales and presents flow kinematic values associated with vertical velocities o 10 m h 1 that can have profound implications on ocean biogeochemistry

joint sun glitter and radar imagery of surface slicks a method is proposed to retrieve and interpret fine spatial variations of the sea surface roughness in sun glitter imagery observed sun glitter brightness anomalies are converted using a transfer function determined from the smoothed shape of sun glitter brightness the method is applied to modis and meris sun glitter imagery of natural oil seeps and the catastrophic deepwater horizon oil spill in the gulf of mexico the short scale roughness variations in the presence of mineral oils slicks are consistently extracted and compared to variations associated with the biogenic slicks in doing so the wind speed dependency on the roughness anomalies is also considered a comparison to normalized radar cross section nrcs anomalies taken from the corresponding high resolution asar images is performed and similarities as well as differences are investigated the results document significant benefit from the synergetic use of sun glitter and radar imagery for detection and monitoring of surface slicks

seismicity and structure in central mexico evidence for a possible slab tear in the south cocos plate the morphology of the transition from flat to normal subduction in eastern central mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays observations of a sharp transition in slab dip near the abrupt end of the trans mexican volcanic belt tmvb suggest a possible slab tear located within the subducted south cocos plate the eastern lateral extent of a thin ultra slow velocity layer usl imaged atop the cocos slab in recent studies along the meso america subduction experiment array is examined here using additional data we find an end to this usl which is coincident with the western boundary of a zone of decreased seismicity and the end of the tmvb near the sharp transition in slab dip waveform modeling of the 2 d structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the location of the usl analysis of intraslab seismicity patterns reveals clustering sudden increase in depth variable focal mechanism orientations and faulting types and alignment of source mechanisms along the sharp transition in slab dip the seismicity and structural evidence suggests a possible tear in the south cocos slab this potential tear together with the tear along the orozco fracture zone to the northwest indicates a slab rollback mechanism in which separate slab segments move independently allowing for mantle flow between the segments

sunglint images of current gradients at high resolution critical angle and directional observing strategy abstract high resolution satellite images of the ocean surface in and around the sunglint often provide unique observations of sub mesoscale upper ocean surface processes local anomalies of wind waves currents or surfactants appear on the images as local anomalies of brightness a quantitative interpretation of those brightness anomalies must relate them to slope properties of the wave field which are to the lowest order described by the mean square slope mss the prevailing paradigm for such interpretation is that of the critical zenith angle it states that for sub critical zenith view angle brightness and mss anomalies have opposite signs and this defines the so called inversion region this prevailing paradigm implicitly builds on the assumption that the mss decomposition between upwind and crosswind components is conserved the mss anomalies are then isotropic and can be reduced to a scalar i e one dimensional quantity in such a case one single sunglint image would be sufficient to retrieve the mss anomaly this isotropic case likely applies for surface wave changes induced by varying wind speed or by surfactants yet satellite and airborne observations at multiple view angles recently revealed anisotropic mss anomalies e g with mss increase in the upwind direction and decrease in the crosswind direction this anisotropic behavior likely characterizes wave modulations by anisotropic surface current gradients this paper details the expected properties of such anisotropic mss modulations it is shown that 1 the classical concept of critical angle does not systematically hold neither for frontal current shear nor for internal wave divergence 2 at least two sunglint images at different zenith and azimuth angles are needed to retrieve the mss anomalies and a single observation is not sufficient 3 a satellite with radiometers looking at multiple zenith angles is capable of providing a geometry favorable to retrieve mss anomalies an illustration is discussed with internal waves observed by the multi angle imaging spectroradiometer misr where the upwind and crosswind components of the retrieved mss anomalies are anisotropic those results provide guidelines to interpret available observations and to help refine strategy for future satellite missions

characteristics of a surface buoyant jet tidal outflow from leschenault estuary in winter forms an unsteady surface buoyant jet in koombana bay on each outgoing tide as the water moves away from the exit channel and spreads radially over the bay it assumes a plumelike nature the mean features of this buoyant outflow such as the plume shape frontal celerity internal circulation and the bulk entrainment were measured in an intensive field program these features are shown to compare favorably with results from previous laboratory investigations and predictions from a simple inertia buoyancy force balance which includes entrainment at the front the stability of the plume frontal region was examined and it is shown that subfronts were present but their behavior cannot be explained by reference to previous work

stereo observation of lakes and coastal zones using aster imagery abstract one of the unique features of the visible and near infrared vnir subsystem of the advanced spaceborne thermal emission and reflection radiometer aster is the near infrared back looking view which is obtained 55 s later than the corresponding nadir view although the stereoscopic capability derived from a combination of these two views was originally designed for use in land applications serendipitous observation has shown that it also represents a promising source of data over water bodies in fact when coupled with surface temperature measurements derived from the sensor s thermal infrared tir component stereo viewing forms a powerful observational capability that can be exploited in the monitoring of lake and marine environments one important aspect introduced by the stereo system is the capability to measure the short term displacements associated with surface currents and waves by comparing the nadir and back looking views in initial results presented here surface motion was deduced from the displacement of a ship wake appearing in the sun glitter regime of an aster image of the izu shoto islands which lie to the southwest of tokyo bay japan within the natural limitations imposed by spatial and temporal resolutions this measurement was found to be consistent with simultaneous surface current values derived from a nearby shore based high frequency hf radar system the success of this basic comparison introduces the possibility that stereo imaging from space if well tuned to follow the motion of surface slicks and other surface features within the sun glitter regime could provide a future means of observing surface current on a global scale data from the vnir stereo system also highlights the influence of observational geometry in the case of a surface with sun glitter this often manifests as a brightness reversal between the nadir and back looking views in which relatively bright dark features at nadir appear dark bright in the back looking view such a region of brightness reversal robr can be indicative of a locally rough or smooth surface texture as may be found for example in the vicinity of internal waves rough or ship wakes smooth simple analysis based on a back to nadir ratio of relative radiances shows that the signal strengths measured by aster within a robr can yield a crude estimate of the mean square surface slope which is an important parameter in theories of sea surface roughness imaging by devices such as synthetic aperture radar sar several other potential applications of vnir stereo data in coastal zone and lake monitoring are considered here including the study of internal waves in the strait of gibraltar derivation of wind velocity at cloud height swell wave propagation suspended sediment transport marine traffic monitoring and ship wake patterns as the reflectivity of water in the near infrared reverses from low to high in the presence of sun glitter most of the applications discussed here involve glitter and hence some discussion of the influence of sensor view geometry is presented the general conclusion of the paper is that future missions such as the advanced land observing satellite alos which aim to deploy high spatial resolution stereo viewing systems primarily for land applications should also return data of considerable value in oceanography and limnology particularly if their observational geometry can be adjusted to permit the imaging of sun glitter from water surfaces

theory of radar imaging of internal waves radar images taken over ocean areas in particular those obtained by the synthetic aperture radar sar onboard the american seasat satellite in 19781 2 sometimes show features that seem to be surface manifestations of oceanic internal waves3 11 here i present a theory explaining the large radar signatures of internal waves in which the imaging is attributed to variations in the short scale surface roughness induced by current variations associated with internal waves

source characteristics of the 12 november 1996 m w 7 7 peru subduction zone earthquake the 12 november 1996 m w 7 7 peru subduction zone earthquake occurred off the coast of southern peru near the intersection of the south american trench and the highest topographical point of the subducting nazca ridge we model the broadband teleseismic p waveforms from stations in the global seismic network to constrain the source characteristics of this subduction zone earthquake we have analyzed the vertical component p waves for this earthquake to constrain the depth source complexity seismic moment and rupture characteristics the seismic moment determined from the nondiffracted p waves is 3 5 1020n m corresponding to a moment magnitude m w of 7 6 7 7 the source time function for the 1996 peru event has three pulses of seismic moment release with a total duration of approximately 45 50 seconds the largest moment release occurs at approximately 35 40 seconds and is located 90km southeast of the rupture initiation approximately 70 of the seismic moment was released in the third pulse

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

accretion and exhumation structures formed by deeply subducted seamounts in the kamuikotan high pressure temperature zone hokkaido japan 1 the lower cretaceous iwashimizu complex in the southern kamuikotan zone japan was formed by accretion of seamounts which subducted to the lawsonite albite to blueschist facies the complex is a pile of four nappe units each unit comprises a duplex with horses of seamount volcanics sedimentary rocks and ramp shear zones suggesting a thin skinned peeling of subducted seamounts each unit bounded fault is regarded as the floor thrust of an overlying duplex and the roof thrust of an underlying duplex reflecting a downward progression of underplating accretion the structurally upper pirashuke unit was metamorphosed at higher pressure than the structurally lower units this inverted arrangement of metamorphic grade can be explained by shallowing of progressively younging subducted slab structurally upper units were exhumed synchronously with growth of the structurally lower units this synaccretionary exhumation can be explained by repetition of undercritical duplex forming compression and overcritical extensional unroofing during seamounts subduction

on radar imaging of current features 2 mesoscale eddy and current front detection 1 the surface signatures of meandering fronts and eddies have been regularly observed and documented in synthetic aperture radar sar images wave current interactions the suppression of short wind waves by natural film and the varying wind field resulting from atmospheric boundary layer changes across an oceanic temperature front all contribute to the radar image manifestation of such mesoscale features the corresponding imaging mechanisms are quantitatively explored using a new radar imaging model kudryavtsev et al 2005 that solves the energy balance equation where wind forcing viscous and wave breaking dissipation wave wave interactions and generation of short waves by breaking waves are taken into account high quality and synoptic in situ observations of the surface conditions should ideally be used in this model however such data are rarely available instead the fields of temperature and ocean current are herein derived from two distinct numerical ocean models sar image expressions of current fronts and eddies are then simulated based on these fields the comparison of simulated images with european remote sensing ers sar and envisat advanced sar asar images is favorable we consequently believe that the new radar imaging model provides promising capabilities for advancing the quantitative interpretation of current features manifested in sar images

seismic coupling and structure of the hellenic subduction zone in the ionian islands region abstract the western hellenic arc has been commonly considered as a largely aseismic subduction zone from the comparison of a small rate of shortening derived from the seismic moment release with a large rate of convergence inferred from geology complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces because of the trenchward velocity of the hellenic aegean upper plate now confirmed with gps measurements in the region of the ionian islands a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate from reflection seismic profiling and local earthquake tomography in the appropriate model for such an earthquake source region which considers a single interplate fault and takes into account these features the moderate seismic moment release is found consistent with complete seismic coupling of this subduction the shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened hellenides

fault kinematics in northern central america and coupling along the subduction interface of the cocos plate from gps data in chiapas mexico guatemala and el salvador new gps measurements in chiapas mexico guatemala and el salvador are used to constrain the fault kinematics in the north america na caribbean ca and cocos co plates triple junction area the regional gps velocity field is first analysed in terms of strain partitioning across the major volcano tectonic structures using elastic half space modelling then inverted through a block model we show the dominant role of the motagua fault with respect to the polochic fault in the accommodation of the present day deformation associated with the na and ca relative motion the na ca motion decreases from 18 22 mm yr 1 in eastern guatemala to 14 20 mm yr 1 in central guatemala assuming a uniform locking depth of 14 28 km down to a few millimetres per year in western guatemala as a consequence the western tip of the ca plate deforms internally with 9 mm yr 1 of east west extension 5 mm yr 1 across the guatemala city graben alone up to 15 mm yr 1 of dextral motion can be accommodated across the volcanic arc in el salvador and southeastern guatemala the arc seems to mark the northern boundary of an independent forearc sliver ar pinned to the na plate the inversion of the velocity field shows that a four block na ca co and ar model that combines relative block rotations with elastic deformation at the block boundaries can account for most of the gps observations and constrain the overall kinematics of the active structures this regional modelling also evidences lateral variations of coupling at the co subduction interface with a fairly high coupling 0 6 offshore chiapas and low coupling 0 25 offshore guatemala and el salvador

synergistic surface current mapping by spaceborne stereo imaging and coastal hf radar 1 well validated optical and radar methods of surface current measurement at high spatial resolution nominally 100 m from space can greatly advance our ability to monitor earth s oceans coastal zones lakes and rivers with interest growing in optical along track stereo techniques for surface current and wave motion determinations questions of how to interpret such data and how to relate them to measurements made by better validated techniques arise here we make the first systematic appraisal of surface currents derived from along track stereo sun glitter atssg imagery through comparisons with simultaneous synoptic flows observed by coastal hf radars working at frequencies of 13 9 and 24 5 mhz which return averaged currents within surface layers of roughly 1 m and 2 m depth respectively at our tsushima strait japan test site we found that these two techniques provided largely compatible surface current patterns with the main difference apparent in current strength within the northwest southern comparison region the magnitudes of the atssg current vectors derived for 13 august 2006 were on average 22 40 higher than the corresponding vectors for the 1 m 2 m depth radar these results reflect near surface vertical current structure differences in the flow components sensed by the two techniques and disparities in instrumental performance the vertical profile constructed here from atssg hf radar and adcp data is the first to resolve downwind drift in the upper 2 m of the open ocean the profile e folding depth suggests stokes drift from waves of 10 m wavelength visible in the images

pattern formation at the ocean surface sargassum distribution and the role of the eddy field positively buoyant organisms such as the macroalga sargassum and the cyanobacterium trichodesmium often form surface accumulations visible in satellite imagery that have lateral scale separation of tens of kilometers and cannot be explained by langmuir circulation here we discuss the accumulation of floating materials in the ocean in presence of meso and submesoscale activity using high resolution simulations of the ocean mesoscale in both idealized a 3 dimensional box where coherent eddies are forced by small scale winds and realistic domains western gulf of mexico where extensive concentrations of floating sargassum have been recorded in satellite images we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few tens of meters below it such distribution does not resemble that observed for passive tracers in quasi geostrophic turbulence the strong divergence and convergence zones generated at the surface by ageostrophic processes in the submesoscale range are responsible for the creation of areas where the floating material accumulates floating particles are expelled from the core of mesoscale eddies and concentrate in convergence regions in patterns comparable to the ones observed through the satellite images in light of those results sargassum and or trichodesmium may provide a useful proxy to track convergence divergence processes resulting from ageostrophic processes at the ocean surface

surface roughness changes by finescale current gradients properties at multiple azimuth view angles abstractat times high resolution images of sea surface roughness can provide stunning details of submesoscale upper ocean dynamics as interpreted transformations of short scale wind waves by horizontal current gradients are responsible for those spectacular observations those observations could prove particularly useful to validate numerical ocean models that reach increasingly high resolutions focusing on surface roughness at optical wavelengths two steps have recently been performed in that direction first it was shown in a previous paper by rascle et al that surface roughness variations not only trace surface current divergence but also other characteristics of the current gradient tensor mainly the strain in the wind direction the wind direction with respect to the current gradient thus stands out as an important interpretative parameter the second step is the purpose of the present paper where the effect of the viewing direction is investigated to this end the authors discuss pairs of

causes of earthquake spatial distribution beneath the izu bonin mariana arc abstract statistics about the occurrence frequency of earthquakes 1973 2015 at shallow intermediate and great depths along the izu bonin mariana ibm arc is presented and a percent perturbation relative to p wave mean value llnl g3dv3 is adopted to show the deep structure the correlation coefficient between the subduction rate and the frequency of shallow seismic events along the ibm is 0 605 proving that the subduction rate is an important factor for shallow seismic events the relationship between relief amplitudes of the seafloor and earthquake occurrences implies that some seamount chains riding on the pacific seafloor may have an effect on intermediate depth seismic events along the ibm a probable hypothesis is proposed that the seamounts or surrounding seafloor with high degree of fracture may bring numerous hydrous minerals into the deep and may result in a different thermal structure compared to the seafloor where no seamounts are subducted fluids from the seamounts or surrounding seafloor are released to trigger earthquakes at intermediate depth deep events in the northern and southern mariana arc are likely affected by a horizontal propagating tear parallel to the trench

ocean convergence and the dispersion of flotsam floating oil plastics and marine organisms are continually redistributed by ocean surface currents prediction of their resulting distribution on the surface is a fundamental long standing and practically important problem the dominant paradigm is dispersion within the dynamical context of a nondivergent flow objects initially close together will on average spread apart but the area of surface patches of material does not change although this paradigm is likely valid at mesoscales larger than 100 km in horizontal scale recent theoretical studies of submesoscales less than 10 km predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices here we show that such structures can dramatically concentrate floating material more than half of an array of 200 surface drifters covering 20 20 km2 converged into a 60 60 m region within a week a factor of more than 105 decrease in area before slowly dispersing as predicted the convergence occurred at density fronts and with cyclonic vorticity a zipperlike structure may play an important role cyclonic vorticity and vertical velocity reached 0 001 s 1 and 0 01 ms 1 respectively which is much larger than usually inferred this suggests a paradigm in which nearby objects form submesoscale clusters and these clusters then spread apart together these effects set both the overall extent and the finescale texture of a patch of floating material material concentrated at submesoscale convergences can create unique communities of organisms amplify impacts of toxic material and create opportunities to more efficiently recover such material

surface roughness imaging of currents shows divergence and strain in the wind direction images of sea surface roughness for example obtained by synthetic aperture radars sar or by radiometers viewing areas in and around the sun glitter at times provide clear observations of meso and submesoscale oceanic features interacting with the surface wind waves particular deformation properties of surface currents are responsible for those manifestations ignoring other sources of surface roughness variations the authors limit their discussion to the mean square slope mss variability this study confirms that vortical currents and currents with shear in the wind direction shall not be expressed in surface roughness images only divergent currents or currents with no divergence but strained in the wind direction can exhibit surfaceroughnesssignatures morespecifically nondivergentcurrentsmightbetracedwitha458sensitivityto the wind direction a simple method is proposed in order to interpret high resolution roughness images where roughness variations are proportional to u x 1 a y y a linear combination of the along wind and crosswind current gradients the polarization parameter a depends upon the sensor look direction and the directional properties of the surface waves selected by the sensor the use of multiple look directions or possible acquisitions with different wind directions shall thus help to retrieve surface currents from surface roughness observations

the shear convergence and thermohaline structure of a front abstract with the objective of measuring convergence directly to confirm previous observations of frontal subduction a seaward upwelling jet off central california was studied using satellite infrared images hydrographic sections ship drift and clusters of surface drifters the cyclonic front of the jet was sharper than 1 km resulting in a shear several times larger than f a cross frontal convergence of 7 cm s 1 over the width of the front equivalent to 0 8f was visible as a 20 m wide accumulation of debris the sharpness of the front lasted at least for a day away from the cyclonic front the divergence of the flow was small and the shear was less than 0 6f thermohaline layers originating at the front were interleaving along isopycnals suggesting water subduction it is proposed that the asymetry between anticyclonic and cyclonic sides of the jet and the strong convergence at the cyclonicfront resulted from a frictionally driven ageostrophic secondary circulation superimposed on the geostr

roughness characteristics of oceanic seafloor prior to subduction in relation to the seismogenic potential of subduction zones we have developed a new approach to characterize the seafloor roughness seaward of the trenches as a proxy for estimating the roughness of the subduction interface we consider that abrupt elevation changes over given wavelengths play a larger role in the seismogenic behavior of the subduction interface than the amplitude of bathymetric variations alone the new database subrough provides roughness parameters at selected spatial wavelengths here we mainly discuss the spatial distribution of short 12 20 km and long wavelength 80 100 km roughness rsw and rlw respectively along 250 km wide strips of seafloor seaward of the trenches compared with global trend seamounts show distinct roughness signature of much larger amplitudes at both wavelengths whereas aseismic ridges only differ from the global trend at long wavelengths fracture zones cannot be distinguished from the global trend which suggests that their potential effect on rupture dynamics is not the consequence of their roughness at least not at these wavelengths based on rlw amplitude segments along subduction zones can be defined from rough to smooth subduction zones like the solomons or the ryukyus appear dominantly rough whereas others like the andes or cascadia are dominantly smooth the relative contribution of smooth versus rough areas in terms of respective lateral extents probably plays a role in multipatch rupture and thus in the final earthquake magnitude we observe a clear correlation between high seismic coupling and relatively low roughness and conversely between low seismic coupling and relatively high seafloor roughness

high resolution measurements made across a tidal intrusion front high spatial resolution current and hydrographic measurements have been made across a tidal intrusion front in the james river estuary virginia during the first few hours of flood the current measurements were made with a surface towed acoustic doppler current profiler which was used in both profile and side scan configurations the profile data show a 30 50 cm s 1 inflow of water from the chesapeake bay a time evolving across front density gradient and downward velocity beneath the front of about 15 cm s 1 the side scan observations directly measure the across front change in current and provide a view of the three dimensional character of the frontal interface the near surface horizontal strain rate a critical parameter for understanding the modulation of surface waves across the front is estimated to be about 0 04 s 1 using phillips 1984 theory as a guide fronts with such large strain rates as these should be discernible in imagery from the ers 1 c band synthetic aperture radar for wind speeds of 4 m s 1

convergence and downwelling at a river plume front abstract the small scale structure of the circulation and hydrography at the frontal boundary of the connecticut river plume in long island sound has been resolved using a novel combination of instruments a towed acoustic doppler current profiler adcp and a rigid array of current meters and conductivity temperature sensors observations were made during the latter half of the eastward ebb tide when the river plume was well established and the front was moving to the west at approximately 0 3 m s 1 two across front transects revealed a horizontal convergence rate in the across front velocity components at 0 6 m of 0 05 0 1 s 1 this was associated with a salt induced horizontal density gradient of 10 2 kg m4 observations obtained during a period in which the towed adcp was caught in the zone of maximum surface convergence showed significant downwelling with a near surface maximum of 0 2 m s 1 vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0 6

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 mw 9 0 tohoku oki earthquake 1 imaging the extent to which the rupture areas of great earthquakes coincide with regions of pre seismic interplate coupling is central to understanding patterns of strain accumulation and release through the earthquake cycle both geodetic and seismic estimates of the coseismic rupture extent for the march 11 2011 mw 8 9 9 0 earthquake tohoku oki earthquake may be spatially correlated 0 26 0 05 to 0 82 0 05 with a region estimated to be partially to fully coupled in the interseismic period preceding the earthquake though there is substantial variation in the estimated distribution and magnitude of coseismic slip the 400 km long region estimated to have slipped 4 m corresponds to an area of the subduction zone interface that was coupled at 30 of long term plate convergence rate with peak slip near a region coupled 80 the northern termination of rupture is collocated with a region of relatively low 20 interseismic coupling near the epicenter of the 1994 mw 7 6 sanriku oki earthquake and near a region of potential long term low coupling or ongoing slow slip slip on the subduction interface beneath the coastline 40 50 km depth is best constrained by the land based gps data and least constrained on the shallowest portion of the plate interface due to the 230 km distance between geodetic observations and the japan trench

body wave and surface wave analysis of large and great earthquakes along the eastern aleutian arc 1923 1993 implications for future events the 500 km long alaska peninsula and shumagin islands segments of the aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades to understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface we determine focal mechanisms depths and source time functions from seismic records for the largest events since 1917 the great earthquake of november 10 1938 and seven events with surface wave magnitude ms of 6 9 to 7 5 teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of november 10 1938 along the alaska peninsula body wave inversion of five p and four sh waves gives a duration of about 110 s in which moment was released in two episodes each of about 50 s duration with the second being larger than the first the first source was located in the general vicinity of the epicenter of the mainshock and the second which occurred about 60 s later was centered about 180 km to the northeast the body wave derived seismic moment is 3 7 1021 n m mw 8 3 we corroborate the body wave results by calculating surface waves by normal mode summation and comparing them with data at periods greater than 50 s an adequate fit to observed seismograms is obtained for either a single point source or two point sources with one located about 180 km northeast of the mainshock epicenter rupture in 1938 appears to have been confined to the alaska peninsula segment uniform rupture into the shumagin region is not supported by the data the ms 6 9 earthquake of may 13 1993 ruptured a small portion of the shumagin gap the earthquake of may 14 1948 ms 7 5 occurred on a shallow dipping thrust fault with a depth of about 31 km not 60 km as originally suggested five ms 7 events in the 1938 rupture zone have locations depths and mechanisms that define a shallow dipping 16 19 plate interface which shallows to 8 under kodiak island subduction of seamounts of the gulf of alaska seamount province may explain the location moment focal mechanism and depth of the five ms 7 events the plate interface dips nearly uniformly between the alaska peninsula and shumagin segments indicating that segmentation if any is not controlled by the orientation of the plate interface a slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the kodiak and alaska peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary

intense deformation field at oceanic front inferred from directional sea surface roughness observations fine scale current gradients at the ocean surface can be observed by sea surface roughness more specifically directional surface roughness anomalies are related to the different horizontal current gradient components this paper reports results from a dedicated experiment during the laser lagrangian submesoscale experiment drifter deployment a very sharp front 50 m wide is detected simultaneously in drifter trajectories sea surface temperature and sea surface roughness a new observational method is applied using sun glitter reflections during multiple airplane passes to reconstruct the multi angle roughness anomaly this multi angle anomaly is consistent with wave current interactions over a front including both cross front convergence and along front shear with cyclonic vorticity qualitatively results agree with drifters and x band radar observations quantitatively the sharpness of roughness anomaly suggests intense current gradients 0 3 m s 1 over the 50 m wide front this work opens new perspectives for monitoring intense oceanic fronts using drones or satellite constellations

analytical solution for two dimensional groundwater flow in presence of two isopotential lines 1 the groundwater flow pattern is examined in an idealized situation which is an approximation for situations as found at the confluence of rivers or where streams flow into or out of a lake or at peninsulas or bays two isopotential lines meet at an angle it is assumed that the aquifer is well connected to all surface water bodies moreover groundwater flow is determined by a base flow component and recharge or pumping wells the flow field is constructed using superposition of analytical solutions and conformal mapping classical results for concerning the onset of bank filtration and the length of the infiltration zone are extended for the general case finally an outlook is given on general problem setups for which the proposed methodology can be applied too

shocks in a box an analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution 1 we introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones the quasi two dimensional analogue model features rate and state dependent elastic frictional plastic and viscoelastic material properties and is scaled for gravity inertia elasticity friction and viscosity the experiments are monitored with a high resolution strain analysis tool based on digital image correlation particle imaging velocimetry piv providing deformation time series comparable to seismologic geodetic and geologic observations in order to separate elastic and nonelastic effects inherent the experimental deformation patterns we integrate elastic dislocation modeling edm into a hybrid approach we use the analogue earthquake slip and interseismic locking distribution as edm dislocation input and forward model the coseismic and interseismic elastic response the residual which remains when the edm prediction is subtracted from the experimental deformation pattern highlights the accumulation of permanent deformation in the model the setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations by applying the model to an accretionary type plate margin we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc including a tectonically stable shelf and coastal high 20 plate convergence accommodated by internal shortening overlying the area of large megathrust earthquake slip fifty to 75 of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench the inner forearc region remains undeformed and represents a basin

relation between the subducting plate and seismicity associated with the great 1964 alaska earthquake tectonic studies of the great 1964 alaska earthquake have underappreciated the nature of the subducted plate in influencing seismicity we compare seismological observations in the prince william and kodiak areas that ruptured during this earthquake with the corresponding morphology and structure of the subducting plate the upper plate geology prince william terrane and velocity structure are the same in both areas in the prince william area where the yakutat terrane subducted the energy released and coupling were stronger than above the kodiak subduction zone where thick trench sediment subducts the conjecture that lower plate character or the amount of subducted sediment affects coupling helps explain variability in seismology geodetic inversions and the horizontal velocity of gps stations

simulated radar imagery of an ocean spiral eddy ocean submesoscale features appear to be widespread in the surface mixed layer and thus may be an important link in the energy pathway from large to small scales an example is the spiral eddy for which several theories have been proposed high resolution radar imagery should be useful in testing these theories but there have as yet been no simulations of radar imagery from first principles as a step in this direction we developed a capability to simulate imagery using a full spectral calculation that includes the effects of both wave current interaction and wave damping due to a surface film a particular model of a spiral eddy is used to specify the surface velocity field and film distribution imagery is then simulated for a range of radar frequencies wind speeds initial film pressures and relative radar view directions for winds of 3 8 m s and an initial film pressure of 0 5 mn m imagery for shorter radar wavelengths x and c band is dominated by the effects of film damping for longer wavelengths l and p band wave current interactions and film damping are of comparable magnitude but for higher initial film pressures the l and p band images also become dominated by film damping l band imagery in particular is highly sensitive to the initial value of film pressure and such a result may have implications for determining properties of seawater films overall the radar simulations produce surface patterns having characteristics that resemble radar imagery of real ocean spiral eddies

comparison of airborne synthetic aperture radar imagery with in situ surface slope measurements across gulf stream slicks and a convergent front synthetic aperture radar sar imagery showing two types of features near the inshore edge of the gulf stream are compared with nearly simultaneous estimates of the sea surface slope field derived from optical shipboard measurements one class of feature consists of a set of narrow dark lines having radar signal modulations of about 10 db at l band these modulations are comparable to those observed in the in situ wave slope data over wavenumbers corresponding to the radar resonant bragg waves these modulations are also predicted by a simplified equilibrium wave spectrum model using a surface elasticity of about 25 mn m as determined from in situ measurements of surface tension and a wind friction velocity of about 10 cm s from buoy and shipboard measurements these results support lyzenga and marmorino s 1998 inference that the gulf stream dark lines represent relatively passive biogenically derived surfactant slicks that are advected and strained by the large scale surface flow field the second class of feature consists of a pair of bright dark signatures over the shallow continental shelf region near cape hatteras north carolina these features are characterized by signal modulations of about 4 and 4 db in both the l band sar and in situ data these modulations appear to be due to the interaction of surface waves with the current convergence and divergence regions associated with a rotary circulation in a shallow convergent front model calculations of this interaction by lyzenga 1998 indicate that the positive perturbations associated with current convergence should be relatively independent of the wavenumber which is in approximate agreement with the in situ measurements of the present paper the model also indicates that the negative perturbations associated with the region of current divergence should decrease for wavenumbers higher than l band that trend is observed in the sar data but not in the in situ data which show significant perturbations at higher wavenumbers it is conjectured that this is due to the effects of surfactants and either along front or time variability in the frontal dynamics which are not included in the model additional comparisons of this kind should be useful to further test and improve physical models of near surface phenomena

sun glitter imagery of surface waves part 2 waves transformation on ocean currents under favorable imaging conditions the sentinel 2 multi spectral instrument msi can provide spectacular and novel quantitative ocean surface wave directional measurements in satellite sun glitter imagery ssgi owing to a relatively large swath with high spatial resolution 10 m ocean surface roughness mapping capabilities changes in ocean wave energy and propagation direction can be precisely quantified at very high resolution across spatial distances of 10 km and more this provides unique opportunities to study ocean wave refraction induced by spatial varying surface currents as expected and demonstrated over the grand agulhas current area the mesoscale variability of near surface currents documented and reconstructed from satellite altimetry can significantly deflect in coming south western swell systems based on ray tracing calculations and unambiguously revealed from the analysis of sentinel 2 msi ssgi measurements the variability of the near surface current explains significant wave current refraction leading to wavetrapping phenomenon and strong local enhancement of the total wave energy in addition to its importance for wave modeling and hazard prediction these results open new possibilities to combine different satellite measurements and greatly improve the determination of the upper ocean mesoscale vorticity motions

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

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

dynamics and early post tsunami evolution of floating marine debris near fukushima daiichi the devastating tsunami triggered by the t hoku oki earthquake of 11 march 2011 caused a crisis at the fukushima daiichi nuclear power station where it overtopped the seawall defences on retreating the tsunami carried loose debris and wreckage seaward and marshalled buoyant material into extensive plumes widespread concern over the fate of these and numerous other t hoku tsunami depositions prompted attempts to simulate debris dispersion throughout the wider pacific however the effects of locally perturbed wind and wave fields active langmuir circulation and current induced attrition determine a complex and poorly understood morphology for large floating agglomerations here we show that the early post tsunami evolution of marine debris plumes near fukushima daiichi was also shaped by near surface wind modifications that took place above relatively calm lower surface roughness waters covered by surface films derived from oil and other contaminants high spatial resolution satellite tracking reveals faster than expected floating debris motions and invigorated plume evolution within these regions while numerical modelling of turbulent air flow over the low drag film covered surface predicts typically metre per second wind strengthening at centimetric heights sufficient to explain the observed debris speed increases wind restructuring probably stimulates the dispersion of flotsam from both biological and anthropogenic sources throughout a global ocean of highly variable surface roughness the tsunami triggered by the 2011 t hoku oki earthquake carried debris seaward satellite data and numerical simulations suggest that small scale wind modifications over a smooth film covered sea surface affected debris motions

imaging mesoscale upper ocean dynamics using synthetic aperture radar and optical data a synergetic approach for quantitative analysis of high resolution ocean synthetic aperture radar sar and imaging spectrometer data including the infrared ir channels is suggested this approach first clearly demonstrates that sea surface roughness anomalies derived from sun glitter imagery compare very well to sar roughness anomalies as further revealed using these fine resolution similar to 1 km observations the derived roughness anomaly fields are spatially correlated with sharp gradients of the sea surface temperature sst field to quantitatively interpret sar and optical in visible and ir ranges images equations are derived to relate the surface roughness signatures to the upper ocean flow characteristics as developed a direct link between surface observations and divergence of the sea surface current field is anticipated from these satellite observations intense cross frontal dynamics and vertical motions are then found to occur near sharp horizontal gradients of the sst field as a plausible mechanism it is suggested that interactions of the wind driven upper layer with the quasi geostrophic current field via ekman advective and mixing mechanisms result in the generation of secondary ageostrophic circulation producing convergence and divergence of the surface currents the proposed synergetic approach combining sst sun glitter brightness and radar backscatter anomalies possibly augmented by other satellite data e g altimetry scatterometry ocean color can thus provide consistent and quantitative determination of the location and intensity of the surface current convergence divergence upwelling downwelling this in turn establishes an important step toward advances in the quantitative interpretation of the upper ocean dynamics from their two dimensional satellite surface expressions