stability of the viscous plastic sea ice rheology abstract the large scale two dimensional rheology of a sea ice pack arises from the local contact forces between adjacent does in convergence it is conventionally modeled by a viscous plastic constitutive relation to reflect the low or zero stress in a divergent flow field and the rate independent ridging process in convergence the authors demonstrate how for a simple one dimensional configuration the elliptical yield curve results in tensile in plane stresses in divergence that cause a short wave length linear instability as the ice pack diverges the ice strength which is the root cause of the instability is rapidly reduced and so although the model is linearly unstable it is nonlinearly stable the practical significance is that the model does not blow up but systematic grid size dependent errors associated with the linear instability are introduced throughout the pack artificial diffusion terms can be used to prevent the growth of the linear instability numerical models must almost always

rayleigh waves guided by sea trench topography a later phase of large amplitude is observed at a station in hokkaido from several events of the december 1991 off urup earthquake swarm in the kurile islands region the phase is characterized by a narrow frequency band with a dominant period of about 15 s and retrograde particle motion in a vertical plane with a strike very different from the great circle direction between the station and the epicenter the phase is observed from only the earthquakes that are located at shallow depths beneath the inner wall of the kurile trench where sea depth is greater than 4 km these characteristics indicate that the phase is a packet of rayleigh waves that arrive at the station from an azimuth considerably different from the great circle direction rayleigh waves with periods around 15 20 s are well excited by the shallow earthquakes beneath the inner wall of the trench this is supported by the dominance of a spectral peak at 15 20 s in the spectra of the direct rayleigh waves and the later phase rayleigh waves that propagate in an oceanic region are strongly influenced by the thickness of the water layer at periods shorter than about 25 s the group velocity decreases by 80 at 15 s with the increase of the sea depth from 4 km to 6 km the sea trench topography therefore forms a strong low velocity zone the observed later phase can be interpreted as a rayleigh wave packet guided by this velocity complexity

development of long period ground motions from the nankai trough japan earthquakes observations and computer simulation of the 1944 tonankai mw 8 1 and the 2004 se off kii peninsula mw 7 4 earthquakes strong ground motions recorded in central tokyo during the 1944 tonankai mw8 1 earthquake occurring in the nankai trough demonstrate significant developments of very large 10 cm and prolonged 10 min shaking of long period t 10 12 s ground motions in the basin of tokyo located over 400 km from the epicenter in order to understand the process by which such long period ground motions developed in central tokyo and to mitigate possible future disasters arising from large earthquakes in the nankai trough we analyzed waveform data from a dense nation wide strong motion network k net and kik net deployed across japan for the recent se off kii peninsula mw 7 4 earthquake of 5 september 2004 that occurred in the nankai trough the observational data and a corresponding computer simulation for the earthquake clearly demonstrate that such long period ground motion is primarily developed as the wave propagating along the nankai trough due to the amplification and directional guidance of long period surface waves within a thick sedimentary layer overlaid upon the shallowly descending philippine sea plate below the japanese island then the significant resonance of the seismic waves within the thick cover of sedimentary rocks of the kanto basin developed large and prolonged long period motions in the center of tokyo the simulation results and observed seismograms are in good agreement in terms of the main features of the long period ground motions accordingly we consider that the simulation model is capable of predicting the long period ground motions that are expected to occur during future nankai trough m 8 earthquakes

imaging lateral heterogeneity in the northern apennines from time reversal of reflected surface waves summary prominent arrivals in the coda of seismograms from the wider alpine area can be associated with lateral reflections of love waves at the northern apennines mountain chain italy where structural heterogeneity causes an abrupt contrast in phase velocity we discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate period threecomponent coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time we numerically compute three dimensional sensitivity kernels for the dependence of coda waveforms on p velocity s velocity and density based upon correlations between the adjoint and the regular forward wavefields we consider synthetic coda waveforms for a simplified model of the northern apennines as well as real coda observations from five moderate magnitude earthquakes m w 4 6 5 6 in the southern alps wave propagation is simulated using the spectral element method for which a 3 d regional earth model is used in the case of real data single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern apennines in kernels for density and s wave speed the kernels show that surface wave reflections occur near the axial zone of the mountain chain apart from the apennines the approach is able to image other smaller reflectivity patches from the coda waveforms like the ivrea zone in the southern alps our coda misfit kernels can be integrated in a gradient based waveform tomography where they could enhance the sharpness of the model at lateral discontinuities

constituents of vertical component coda waves at long periods aki 1969 first modeled coda waves of a local earthquake as a superposition of scattered surface waves this paper attempts to clarify the constituents of surface wave coda at long periods at very long lapse times for a large earthquake of magnitude 7 or larger vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake origin time although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered rayleigh waves by heterogeneities distributed all over the earth the later potion of the observed coda envelope roughly later than 35 000 s has systematically larger amplitude than theoretical prediction to clarify the cause of this discrepancy we studied the constituents of vertical component seismograms of three large earthquakes recorded by the f net in japan using the f k power spectral analysis we found that the direct and scattered fundamental mode rayleigh waves of velocity about 3 7 km s are dominant in the earlier part of each envelope it justifies the use of a scattering model of the fundamental rayleigh waves for synthesizing the envelope at lapse times later than 20 000 s 35 000 s higher modes with phase velocities around 20 km s become dominant the transition time to the dominance of higher modes is found to become earlier for a deeper focus earthquake the small coda attenuation factor from 1 90 0 23 10 3 to 2 38 0 32 10 3 estimated from later coda envelopes recorded at iris stations distributed worldwide also agrees with the attenuation factor of spheroidal modes according to prem we may interpret that higher mode waves are uniformly distributed at large lapse time due to large velocity dispersion and or scattering and they dominate over the fundamental mode waves because of smaller attenuation in the lower mantle the coda attenuation measurement proposed by aki is found to be useful even for long periods and at very large lapse times

images of the iberian lithosphere from one local earthquake abstract on 12 august 2007 a magnitude m w 4 7 intraplate earthquake occurred near the center of the iberian peninsula an area characterized by comparably simple crustal structure within the complex iberia maghreb plate boundary zone and characterized by complete azimuthal coverage with seismic broadband stations we analyze regional intermediate period 20 to 50 s coda waveforms for this earthquake they contain energy representing late arriving surface waves that have been reflected laterally at lithospheric heterogeneities in or around iberia but complexity of the coda waveforms hampers a direct interpretation we use coda recordings as the source for a back propagating adjoint wave field and compute 3d born sensitivity kernels for the dependence of least squares waveform misfits of coda waves on wave speed variations we hereby image the origin of single scattering recorded in the coda misfit kernels for p and s wave velocity show azimuth dependent intensity variation as a result of source radiation and an appropriate compensation significantly improves imaging quality thereby revealing several clear lineaments these are interpreted as surface wave reflectors due to deep rooted heterogeneity such as terrain boundaries or moho topography demonstrating the ability of the approach to unravel complex waveforms and providing a new point of view on regional lithospheric structure

progressive evolution of a fault related fold pair from growth strata geometries sant llorenc de morunys se pyrenees abstract new structural stratigraphical mapping constrains the three dimensional kinematics and mechanisms of eocene oligocene growth folding at sant llorenc de morunys ne ebro basin spain a 1 km wide sub vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold pair to shallowlydipping strata the axial surface of the anticline comprises either continuous or en echelon segments while that of the syncline is concave and usually continuous while converging upwards the axial surfaces do not define growth triangles principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb dips within the common limb decrease up stratigraphy and up dip mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels and concentration of cleavage in the syncline indicates that this hinge was essentially fixed sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration particle movement vectors generated by section restoration are arcuate about a hinterland pinpoint a new trishear model of fault propagation folding involving non rigid limb rotation reproduces the rounded hinge forms thickening geometries and limb dip variations observed simple kink band migration models fixed axis and constant thickness theories do not replicate these features

off great circle propagation of intermediate period surface waves observed on a dense array in the french alps array analysis is performed on surface waves recorded in the french alps using a small aperture 25 km temporary array of six broad band stations the analysis shows that both rayleigh and love waves deviate relative to the great circle path the deviations are particularly strong up to 30 between 20 and 40 s period to interpret these observations we first study the effect of large scale structures using ray tracing in a smooth laterally heterogeneous model of the earth second we evaluate the local effect by considering a model for the french alps including strong lateral heterogeneities around the array that were not taken into account in the ray tracing by combining these two possible causes of the observed deviations we propose an explanation for the general trend in the observed deviations finally we show that by taking into account azimuthal deviations phase velocities measured at a regional scale can be significantly improved

shallow brine driven free convection in polar oceans nonhydrostatic numerical process studies a three dimensional nonhydrostatic convection model which accounts for small scale ice ocean interactions is used to study convection in shallow sea coastal ice formation regions which contribute significantly to water mass formation in both the arctic and antarctic ocean for certain conditions the results presented in this paper are also transferable to shallow open ocean convection the model is applied to an initial well mixed ocean at rest with a temperature close to the freezing point the ocean initially free of ice is exposed to cold and dry polar air we consider situations in which the mean wind stress is negligible but wind fluctuations result in small sensible and latent heat fluxes corresponding to a wind speed of 2 m s 1 cellular convection patterns develop in the ocean finally occupying a mean aspect ratio of 2 convection is driven by salt release during frazil ice formation due to supercooling newly forming sea ice is collected along convergent downwelling regions at the surface thus showing also cellular structures because the area of insulating sea ice remains small new ice can be formed continuously and the surface buoyancy forcing remains large this collection of ice in small fractions of the sea surface results in a latent heat polynya type which is very effective in terms of dense water mass formation a comparison of the three dimensional model and a two dimensional slice model shows that key results can be reproduced with the slice model in summary the results of the process studies indicate that cellular features in the sea ice cover which may be detectable by remote sensing techniques are closely related to active brine driven convection

scattering of teleseismic p waves by the japan trench a significant effect of reverberation in the seawater column abstract we detected a scattered wave train in data from the high sensitivity seismograph network in japan hi net following the arrival of the near vertically incident p wave generated by the 2009 earthquake mw 7 8 off the south island of new zealand the scattered wave train represented predominantly vertical ground motion at a period of 20 to 50 s and with an apparent velocity of 3 5 km s it propagated cylindrically westward through the kanto area of central japan array analysis showed that the scattered wave train developed beneath the pacific ocean near the boso triple junction southeast of the kanto area a 3d finite difference simulation of seismic wave propagation using a high resolution model incorporating subsurface structure topography and bathymetry revealed that the strong scattered waves that were generated along the japan trench and propagated normal to the trench axis represented multiple reverberations of seismic waves between the seafloor and the pacific plate boundary in addition strong reverberation of acoustic waves in the seawater column above the boso triple junction causes elongated scattered waves which reasonably explains our observations

wave guide effects in subduction zones evidence from three dimensional modeling we have simulated seismic wave propagation in models of the subduction zone off the pacific coast of mexico using a three dimensional finite difference method the results show that a significant part of the energy generated by earthquakes in the subduction zone can be trapped inside a wave guide generated by the low velocity material of the accretionary prism the trapped waves in the simulations appear as long duration wave trains that follow the onset of the surface waves similar to the signature of broadband seismograms recorded by stations along the pacific coast of mexico for subduction earthquakes

on the influence of coastline orientation on the steady state width of a latent heat polynya this paper investigates under what conditions the steady state shape of a coastal latent heat polynya is influenced by 1 the direction of flow of free drifting frazil ice and the consolidated offshore ice sheet and 2 coastline orientation analytical solutions are obtained for coastal polynyas when the coastline is straight and when a coastline has a single step from the solutions it is shown that the asymptotic polynya width is given by the generalized pease 1987 width l cos where l is the classical pease width and is the angle between the consolidated ice sheet velocity and the normal to the coastline an expression is also obtained for the alongshore length scale l c over which the polynya adjusts toward its asymptotic width and it is shown that l c depends upon the directions of travel of the frazil ice and the consolidated ice sheet when the frazil ice and the consolidated ice sheet both drift offshore normal to the coast l c 0 in agreement with the classical pease theory variations in the coastline which occur on a length scale much smaller than l c are not reproduced in the polynya shape two numerical simulations are presented for terra nova bay antarctica and the st lawrence island polynya in the bering sea both examples employ realistic coastline geometry and information about the direction of drift of the consolidated ice sheet in each example a realistic polynya shape is obtained the examples highlight the versatility of the model because in the former case the ice sheet drifts almost parallel to the coast while in the latter it drifts offshore

interference of long period seismic wavefield observed by the dense hi net array in japan 1 in order to recover small signals of long period ground motions recorded by a short period seismometer we designed a time domain recursive filter that simulates seismic waves recorded with broadband seismometers we applied the time domain filter to seismograms recorded by the dense hi net array which consists of 800 stations at intervals of approximately 20 km the effectiveness of the time domain filter is demonstrated through the comparison of broadband waveforms obtained by the filter and sts 2 true broadband seismograms from the f net at the same site good agreement is observed between filtered and true broadband waveforms with high correlation coefficients for periods of up to 100 s for waveforms from the 2007 sumatra earthquake m7 9 which occurred approximately 5000 5800 km away from japan the applicability of the time domain filter to the recovery of long period signals from the record of a short period seismometer depends on the signal to noise ratio as a result a limited bandwidth of broadband signal is recovered for smaller earthquake from the spatiotemporal variation of the long period t 20 100 s wavefield obtained by the time domain filter we newly found an anomalous phase pattern of splitting of polarity along the wavefront particularly for the relatively short period band of from 20 50 s array analysis of the dense broadband waveforms revealed multipathing of the rayleigh waves which propagated toward central japan from two directions with slightly different slowness vectors the interference of two rayleigh waves caused such splitting of wavefield

air ice drag coefficients for first year sea ice derived from aircraft measurements turbulent flux measurements were made at four levels 42 90 195 and 340 m by the noaa p 3 aircraft over first year sea ice in the northern bering sea during february 1982 three profiles of momentum flux and mean wind were used to calculate an air ice drag coefficient cd of 3 0 6 10 3 referenced to a 10 m anemometer height the boundary layer was slightly unstable zi l 1 2 where zi was the inversion height of 660 m and l the monin obukhov length the mean wind speed at the 42 m height was 17 m s and the air temperature was 20 c from turbulent heat flux measurements the value of the bulk heat transfer coefficient ch was 0 73 1 6 10 3 giving a ch cd of 0 24 the bowen ratio was greater than 2 8 comparison of the present turbulent flux and variance profiles with those collected over the ocean shows agreement which increases confidence in the calculations the geostrophic drag coefficient u g where u is the friction velocity and g is the geostrophic wind was 0 047 the turning angle of the surface wind measured by an anemometer at a height of 3 m on the ice was 32 and the ratio of surface wind speed to the geostrophic wind speed was 0 76

development of the colombian foreland basin system as a consequence of diachronous exhumation of the northern andes this study addresses multiple controls on foreland basin accommodation and contributes to enhanced understanding of the evolution of the northern andes the middle magdalena valley basin mmvb eastern cordillera and llanos basin are part of a late cretaceous cenozoic foreland basin system east of the colombian central cordillera mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses facies and unconformities was lithospheric flexure in response to crustal loads from the central and eastern cordilleras shorter wavelength folding and paleoaltitude determined the local character of strata our mechanical modeling consists of the application of orogenic and sedimentary loads extracted from geologic data on a continuous elastic lithosphere the results validate two major basin configurations the first configuration was a maastrichtian early eocene foreland basin coupled with central cordillera uplift growth strata record continuous sedimentation in the eastern cordillera whereas regional unconformities in the llanos basin distal foreland basin reflect isostatic adjustments of the basin s amplitude and wavelength to central cordillera episodic uplift and tectonic quiescence the second major basin configuration was characterized by central cordillera erosion since middle eocene times recorded by a regional pediment surface in the absence of central cordillera effective loading loads from onlapping sediments and eastern cordillera piggyback sub basins provoked post middle eocene accommodation in the mmvb and llanos basin intensified eastern cordillera uplift during the neogene produced basinal tilting recorded by unconformities in the mmvb this study highlights the importance of assessing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins

rayleigh wave multipathing along the west coast of north america we have created a movie of surface ground motion for the 3 november 2002 denali fault earthquake based on spectral element simulations using crustal model crust2 0 mantle model s20rts topography and bathymetry model etopo5 and a finite fault slip model the movie features two anomalous wave packets that travel along the west coast of the north american plate following off great circle paths these wave packets are rayleigh waves with dominant periods around 20 sec which are also found in seismograms recorded by the southern cali fornia seismic network one of these packets is the direct surface wave whose group arrival time changes laterally as dictated by the shape of the oregon coast the other packet is a surface wave reflected by a lateral interface underneath the rocky moun tains a linear reflector parallel to the canadian coast offsetting it by a few hundred kilometers can explain its arrival time but the offsetting distance derived from the synthetic seismograms puts the reflector 350 km northeast of the result obtained from the data indicating a need to update the crustal and mantle models in this area online material animation of normalized simulated vertical displacement for the 2002 denali earthquake

reflection of seismic surface waves at the northern apennines abstract seismic surface waves with periods of 15 to 20 s are reflected laterally at the northern apennines for earthquakes originated and recorded in the wider alpine area a few hundred kilometres north of the apennines we can directly observe late arrivals of reflected regional surface waves these have a characteristic polarization of particle motion and often dominate the intermediate period surface wave coda love waves are the most prominent coda arrivals while reflected rayleigh waves show smaller amplitudes and become clear only after rotation of the recordings to the reflection incidence direction we can track the development of the love wave reflection along a temporary broad band transect across the northern apennines indicating the location of the reflector near to the highest topography from spectral element simulations of three dimensional wave propagation we attribute the reflection to a continuous 270 km long offset in the crust mantle boundary under the mountain belt with vertical throw of 20 km thus supporting a deep crustal root under the outer side of the apennines fold and thrust belt and significant crustal thinning on the inner side amplitude reflection coefficients for near normal incidence can be estimated as 7 for both love and rayleigh waves

anomalous surface waves associated with deep earthquakes generated at an ocean ridge summary broad band seismograms observed at regional distances from deep earthquakes display large amplitude rayleigh and love waves with a predominant period of about 20 s although deep earthquakes in laterally homogeneous structures do not generate such surface waves since the propagation direction of the surface waves inferred from particle motion is approximately radial and the group velocity of the surface waves observed at each station is similar it can be assumed that the surface waves have travelled close to the great circle paths back projection allows us to determine the region in which these surface waves are generated manyof the zones from which the surface waves appear to originate are located close to the norfolk ridge where crustal structure has strong lateral heterogeneity this suggests that the surface waves are generated by the interaction of body waves from the deep earthquakes with this strongly heterogeneous crustal structure in order to investigate the eiect of such heterogeneity in crustal structure on seismic wave elds we have modelled the p sv wave eld using a pseudospectral method the results show that surface waves such as those observed can be generated by ridge structures and indicate that sedimentary layer thickness plays a very important role we suggest that such unexpected surface waves from deep earthquakes may prove useful for delineating heterogeneity in shallow structure

syntectonic cenozoic sedimentation in the northern middle magdalena valley basin of colombia and implications for exhumation of the northern andes development of the colombian middle magdalena valley basin mmvb was determined by late cretaceous early eocene uplift of the central cordillera to the west and subsequent transferal of deformation to the eastern cordillera to the east these phases are separated in the tectono stratigraphic record by a major unconformity the middle magdalena valley unconformity mmvu paleocene coastal to alluvial facies underneath the mmvu were deposited in a foreland basin coupled to central cordillera kilometer scale uplift the middle eocene to neogene continental strata that onlap the mmvu document transformation of the mmvb into an interior basin due to eastern cordillera deformation recorded by growth strata in seismic lines and changing provenance and paleoflow patterns exhumation histories of the mmvb bounding ranges are further constrained by apatit fission track and vitrinite reflectance thermochronology and volcanic ash chronology this study indicates that the mmvb is fundamentally related to evolution of the entire andean margin of south america

reverse propagation of surface waves reflected from seamounts in the northwestern pacific abstract the reverse propagation of a surface wave traveling through the japanese islands from southwest to northeast was detected for major shallow earthquakes that occurred in northeastern japan the wave is predominant in the transverse component having a period of 20 sec and lasting longer than 200 sec the propagation velocity is around 3 8 km sec which is characteristic of a typical surface wave semblance analysis using a dense seismograph network shows that the wave was generated near the kyushu palau ridge southeast of kyushu island numerical simulation with a 3d finite element method can explain the surface wave reflection predominant in the transverse component as reflection from seamounts with a crustal root 10 km deeper than the moho discontinuity at the surrounding ocean floor the observed long duration of the reverse propagating surface wave might represent the superposition of a reflected wave from other seamounts and or the thick sedimentary layer along the nankai trough

drag measurements for first year sea ice over a shallow sea for a free drift case a method is developed for simultaneously determining air ca and water cw drag coefficients by solving the vertically integrated stress balance equation for sea ice the method allows the algebraic determination of ca and cw from measurements of relative wind and relative current at single reference levels from estimates of the densities of all three fluids and ice thickness and from estimates of the accelerations caused by the nonsurface stress terms derived from position measurements the method treats the ice as a natural drag plate and therefore includes contributions from both skin and form drag one shortcoming of the technique is that if the relative wind and current are colinear then only the ratio of ca and cw can be determined another is that the accelerations caused by other forces need to be independently determined an experiment was conducted on a single floe in the eastern bering sea over the continental shelf during march 1981 the site was about 80 km from the ice edge and was occupied for approximately 3 days following the passage of a storm which had broken the pack into 10 20 m diameter floes both profile and slab inversion estimates of drag were made from the current meter and anemometer measurements at the site ca at 3 m was 4 55 10 3 by the profile method compared to 3 63 4 39 10 3 for slab depending on assumptions cw at 1 1 m by profile was 24 2 10 3 compared to 18 3 22 0 10 3 for slab the two methods gave results which were not statistically separable and which were among the highest drags observed for sea ice the instantaneous stress balance on the floe included contributions from material acceleration coriolis force the two surface stresses and sea surface tilt relative to mean current and tide

a numerical study of dense water formation and transport on a shallow sloping continental shelf the circulation and transport of dense water generated by an idealized coastal polynya is studied using a three dimensional primitive equation model starting with a homogeneous quiescent ocean a constant negative buoyancy flux is imposed at the surface over a half elliptical region adjacent to the coastal boundary on a gently sloping continental shelf the flow response can be divided into the following three phases geostrophic adjustment instability and offshore eddy transport during geostrophic adjustment the fluid within the forcing region becomes denser and the flow at the edge of the forcing region accelerates in response to the strong density gradient there eventually the flow at the leading edge of the forcing region relative to kelvin wave propagation becomes unstable and a train of counterrotating eddies develops these eddies then form a complex three dimensional flow field and rapidly transport dense water offshore across isobaths the density within the forcing region reaches a maximum which remains fairly constant after the eddies begin to transport the dense fluid offshore the results are qualitatively insensitive to weakening of the negative buoyancy forcing and to changing the bottom slope eddy scales and velocities are consistent with observations in the arctic the results suggest that instability processes and eddy fluxes are important in transporting dense water off continental shelves and into marginal seas

a new look at sedimentation rates and the completeness of the stratigraphic record several recent papers have suggested that completeness of stratigraphic sections can be estimated by using the ratio of long term sedimentation rates to short term rates we show that the median short term sedimentation rates used in these papers are erroneous and are mostly an artifact arising from improper data manipulation to demonstrate the artificial character of short term median rates we have generated a series of plots assuming a constant sedimentation rate with fixed variance and defined limits of measurement precision these plots demonstrate that increasing variance in the time required to deposit sediments will result in spuriously high short term median sedimentation rates a new method for calculating median sedimentation rates which eliminates the problem of spurious median rates is then applied to make an improved estimate of expected completeness recalculated median values show that sedimentation rates for pelagic sediments are close to identical for both short term and long term tim

estimating surface wind direction over drifting open pack ice surface wind direction data vital for the study of drifting pack ice are not routinely available for polar oceans these data may be estimated from the difference between the direction of motion of detached ice floes as determined from sequential satellite images and the angle of sea ice deflection which varies with the geostrophic wind speed

ocean feedback to wind driven coastal polynyas for lack of observations a coupled numerical model of an ice layer and a coastal ocean is used to conduct thought experiments on the evolution of wind driven coastal polynyas attention is focused on the possible ocean feedback to polynya developments before and after wind relaxation the coupled system is initially motionless having an ice layer of uniform concentration and thickness in the ocean the salinity is initially uniform and the temperature is at the freezing point subsequent cold air outbreaks of several days in duration push the ice layer offshore frazil is produced and collected in the polynya owing to continuous heat loss to the atmosphere ocean feedback is particularly effective after wind relaxation under large scale seaward winds the displaced ice edge does not return landward after wind abatement if the wind event is strong and long over a sizable coastal bottom obstacle however anticyclonic circulation develops and its landward arm may force a landward return of the ice edge after wind relaxation further dipole vortices in the ocean may develop under a mesoscale seaward wind event landward ocean currents around the rim of a dipole may also force a postwind return of the ice edge the ocean feedback is generally stronger and faster in shallower basins this can be demonstrated by a one dimensional stress driven coastal ocean model

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

seismic channel waves in the accretionary prism of the middle america trench a visual examination of seismograms of earthquakes located near the middle america trench and recorded on vbb seismographs along the pacific coast of mexico when bandpass filtered between 6 and 30 s shows the presence of two types of waves the first is a short pulse and the second is a dispersed wavetrain the first type of wave is quickly attenuated while the second type propagates without significant attenuation we applied a frequency mime analysis to identify the nature of these waves a comparison of period group velocity diagrams for signals recorded on different stations shows that the first arrival is a rayleigh wave propagating in the continental plate while the second one propagates along the trench in the thickened sediment wedge of the accretionary prism we have also measured group velocities of rayleigh wave propagating in the cocos plate far from the coast at periods between 15 and 20 s the group velocities of the wave propagating along the trench are lower than the velocity of rayleigh waves propagating in adjacent continental and oceanic plates we conclude that the dispersed wavetrain observed near the coast is a wave trapped in a channel formed by low velocity sediments of the accretionary prism

the interaction of an ocean eddy with an ice edge ocean jet in a marginal ice zone remotely sensed images in the east greenland current marginal ice zone region indicate a variety of mesoscale motions these range from isolated monopoles and dipoles to multiple eddy fields recent observations indicate that many of the eddies may be advected into the region from the west spitsbergen current region they then interact with the east greenland current marginal ice zone and its associated along ice edge jet in this paper we illustrate how the ice floe fields can evolve during the interaction of an ocean eddy with an along ice edge ocean jet the effects of eddy sense of rotation flow strength and bottom topography are considered in barotropic and baroclinic two layer numerical experiments initially barotropic experiments involve the combined effects of flat bottom eddy jet interactions and eddy topography interactions over a flat bottom anticyclones interact with the cyclonically sheared side of the jet and move seaward exporting ice away from the ice edge in a dipolar distribution cyclones move toward the jet and are advected downstream a second dipole formation mechanism is associated with eddy topography interaction and is a consequence of the shallow pycnocline depth of the region rapid topographic rossby wave radiation in the lower layer spreads energy along slope and upslope downslope for cyclones anticyclones the upper ocean response to this process is a dipole which propagates parallel to the ice edge with shallow water to the left topographic eastward jet induced and topography induced eddy propagation tendencies combine to enhance cyclone jet interaction and limit anticyclone jet interactions the dipole formation associated with eddy topography interaction is weakened by the presence of the jet but not eliminated eddy induced distortions in the ice edge are suppressed by the stabilizing effect of topography on the jet sensitivity of the initially barotropic results to variation in the rossby number and degree of topographic slope illustrates that eddies are more efficient in producing ice edge distortions for stronger flows and weaker topographic slopes for initially baroclinic simulations over topography results are comparable to flat bottom cases strong upper layer eddy jet interactions can occur independent of topographic effects which are constrained to the lower layer