how does arctic summer wind modulate sea ice ocean heat balance in the canada basin 1 the impacts of arctic summer wind on ice ocean heat processes in the sea ice reduction zone were investigated using a pan arctic ice ocean model the interannual simulation demonstrated event like sea ice melting because of enhanced solar radiation input and upward ocean heat transfer in the canada basin area both factors were derived from mechanical ice divergence under cyclonic wind stress atmospheric heat penetration into the newly formed open leads was estimated to be a primary contributor to sea ice lateral bottom melting the vertical ocean heat flux associated with wind driven upwelling mixing was the secondary factor on a monthly time scale the modeled interannual variability indicated that the relative role of internal ocean dynamics in sea ice variability was greater under cyclonic wind patterns compared with anticyclonic wind conditions

upper mantle structure of the northern cascadia subduction zone previous knowledge of the structure of the cascadia subduction zone north of the canada united states border has been derived from a variety of geophysical studies that accurately delineated the downgoing juan de fuca plate from the offshore deformation front to depths of 50 60 km beneath south central vancouver island and the georgia strait little is known however of the structure of the cascadia subduction zone farther westward and to greater depths in the upper mantle we have assembled a set of some 1100 teleseismic traveltimes from events recorded on the western canadian telemetered network to augment a previously existing data set recorded on the washington regional seismograph network the composite data set is inverted for upper mantle structure below washington oregon and southwestern british columbia we analyze the new northern portion of the model between 48 5 50 n and 118 127 w which provides the first images of the deep slab structure in this region the model is parameterized using s

springtime north pacific oscillation and summer sea ice in the beaufort sea we investigate the linkage between the spring april june north pacific oscillation npo and the following summer sea ice in the beaufort sea a positive npo is characterized by an intensified and northeastward extended aleutian low a strong beaufort high and enhanced easterly winds in the beaufort sea concomitantly while the deepening east asian trough steers more cyclones from siberia to the sub polar pacific and thus fewer cyclones into the western arctic ocean a weak polar vortex and weak baroclinicity in the beaufort sea does not favor cyclone activity in addition the east asian trough enhances warm air advection from the pacific to the northwestern america the spring npo accounts for 29 of interannual variability of the following september sea ice cover in the beaufort sea and can be used as a potential predictor for ice melting in the beaufort sea during a positive npo the strong easterly winds over the beaufort sea enhance ice advection and reduce ice thickness on the synoptic scale there are more occurrences of the pack ice becoming detached from the coast due to the strong easterly winds moreover increases in shortwave radiation and longwave radiations accelerate the ice melting

downwelling longwave flux over summit greenland 2010 2012 analysis of surface based observations and evaluation of era interim using wavelets this study analyzes the downwelling longwave radiation dlw over the greenland ice sheet gris using surface based observations from summit station 72 n 38 w 3210 m and the european centre for medium range weather forecasts interim reanalysis era interim dlw fields since surface based observations are sparse in the arctic the accuracy of including reanalyses for spatial context is assessed first the dlw at summit is reported including the significant time scales of variability using time frequency decomposition wavelet analysis a new method for evaluating reanalyses is then introduced that also uses wavelet analysis era interim dlw performs reasonably well at summit but because it includes too many thin clouds and too few thick clouds it is biased low overall the correlation between the observations and era interim drops from r2 0 8 to near 0 for time series reconstructed from time scales less than 4 days these low correlations and additional analyses suggest that the spatial resolution of the data sets is a factor in representing variability on short time scales the bias is low across all time scales and is thus likely tied to cloud generation processes in the model rather than the spatial representation of the atmosphere across the gris the exception is autumn when era interim overestimates the influence of clouds at time scales of 1 and 4 weeks the spatial distribution of cloud influence on the dlw across the gris indicates that summit is located in a transition zone with respect to cloud properties the gradient across this transition zone is steepest near summit in autumn so the spatial characteristics of the atmosphere near summit may contribute to the era interim bias during this time

thinning and volume loss of the arctic ocean sea ice cover 2003 2008 1 we present our best estimate of the thickness and volume of the arctic ocean ice cover from 10 ice cloud and land elevation satellite icesat campaigns that span a 5 year period between 2003 and 2008 derived ice drafts are consistently within 0 5 m of those from a submarine cruise in mid november of 2005 and 4 years of ice draft profiles from moorings in the chukchi and beaufort seas along with a more than 42 decrease in multiyear my ice coverage since 2005 there was a remarkable thinning of 0 6 m in my ice thickness over 4 years in contrast the average thickness of the seasonal ice in midwinter 2 m which covered more than two thirds of the arctic ocean in 2007 exhibited a negligible trend average winter sea ice volume over the period weighted by a loss of 3000 km3 between 2007 and 2008 was 14 000 km3 the total my ice volume in the winter has experienced a net loss of 6300 km3 40 in the 4 years since 2005 while the first year ice cover gained volume owing to increased overall area coverage the overall decline in volume and thickness are explained almost entirely by changes in the my ice cover combined with a large decline in my ice coverage over this short record there is a reversal in the volumetric and areal contributions of the two ice types to the total volume and area of the arctic ocean ice cover seasonal ice having surpassed that of my ice in winter area coverage and volume became the dominant ice type it seems that the near zero replenishment of the my ice cover after the summers of 2005 and 2007 an imbalance in the cycle of replenishment and ice export has played a significant role in the loss of arctic sea ice volume over the icesat record

shear wave travel time amplitude and waveform analysis for earthquakes in the kurile slab constraints on deep slab structure and mantle heterogeneity shear wave travel times amplitudes and broadband waveforms from 16 events in the kurile slab are analyzed to place further constraints on the seismic velocity heterogeneity associated with the subducted oceanic lithosphere several previous studies have proposed that the kurile slab penetrates into the lower mantle with little deformation other than a steepening of the dip angle while other studies suggest that strong slab deformations occur with perhaps negligible lower mantle penetration in an effort to detect any deep slab effects we compare anomaly patterns covering a 60 azimuthal range straddling the strike of the slab for events in three different depth intervals 500 km 350 500 km and 100 200 km s and scs pulse broadening is used as a measure of waveform complexity complexity of broadband shear waves does not vary systematically across our azimuthal range as the propagation direction changes with respect to the postulated lower mantle slab model for this region strong residual sphere travel time variations are found for shear waves from events at all depths however these variations do not display the depth dependence apparent in previous p wave analysis that has been interpreted as requiring a change in slab geometry and lower mantle penetration the same s wave residual pattern is apparent in ss depth phases and is well predicted by a recent model of shear velocity structure beneath north america a combination of deep mantle and near receiver aspherical structure can explain most of our observed shear wave travel time patterns with only a minor source depth dependent component remaining after correction individual amplitude and travel time measurements are poorly correlated however scs s amplitude ratios and scs s differential times are significantly correlated consistent with geometric focussing and defocussing effects somewhere along the raypaths the travel time behavior suggests that the responsible velocity heterogeneity is not located near the source the absence of a strong deep slab effect on the shear waves indicates that deep slab heterogeneity is less pronounced for shear waves than expected on the basis of existing p wave models derived from travel time residual sphere analysis various interpretations include estimates of vs t and compositional effects previously used to predict shear velocity structure of the deep kurile slab are in error with actual lower mantle slab anomalies being much weaker than proposed the kurile slab is so distorted in the lower mantle that no source depth dependent trends are produced for our limited focal sphere coverage or the kurile slab does not penetrate into the lower mantle

deconvolution of teleseismic recordings for mantle structure summary delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances this deconvolution problem can be posed in a log spectral domain where the operation of time domain convolution is reduced to an additive form the introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers the system is inherently singular and stabilization is accomplished through the supply of an initial estimate of the source time function although alternative choices exist an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular value decomposition of the suite of aligned seismograms corresponding to a given event the relation of the deconvolution scheme to simultaneous least squares deconvolution is examined application of the methodology to broadband teleseismic p waveforms recorded on the canadian national seismograph network demonstrates the retrieval of effective green s functions including secondary phases associated with upper mantle structure

wind and wave influences on sea ice floe size and leads in the beaufort and chukchi seas during the summer fall transition 2014 sea ice floe size distribution and lead properties in the beaufort and chukchi seas are studied in the summer fall transition 2014 to examine the impact on the sea ice cover from storms and surface waves floe size distributions are analyzed from medea landsat8 and radarsat 2 imagery with a resolution span of 1 100 m landsat8 imagery is also used to identify the orientation and spacing of leads the study period centers around three large wave events during august september 2014 identified by swift buoys and wavewatch iii model data the range of floe sizes from different resolutions provides the overall distribution across a wide range of ice properties and estimated thickness all cumulative floe size distribution curves show a gradual bending toward shallower slopes for smaller floe sizes the overall slopes in the cumulative floe size distribution curves from landsat8 images are lower than while those from radarsat 2 are similar to previously reported results in the same region and seasonal period the medea floe size distributions appeared to be sensitive to the passage of storms lead orientations regardless of length correlate slightly better with the peak wave direction than with the mean wave direction their correlation with the geostrophic wind is stronger than with the surface wind the spacing between shorter leads correlates well with the local incoming surface wavelengths obtained from the model peak wave frequency the information derived shows promise for a coordinated multisensor study of storm effects in the arctic marginal ice zone

the influence of crust and upper mantle heterogeneity on short period waveform distortion abstract short period teleseismic p waves may suffer considerable distortion owing to the effects of upper mantle crustal heterogeneity this sensitivity suggests the use of p waveforms as an aid to constraining upper mantle structure however this requires a means of separating the relative contributions from these two regimes we investigate the nature of short period waveform distortion as recorded on the washington regional seismograph network wrsn a technique for analyzing short period telescismic p waveform distortion is presented to extract the impulse response function of the underlying crust and upper mantle for a given event station pair the technique is shown to yield highly reproducible results and permits direct comparison of response functions for different source locations short period waveform distortion can be strong and is essentially incoherent between stations minimum separation 20 km we subsequently confine our attention to a single station where waveform distortion is evidenced by strong defocusing and examine the behavior of this distortion as a function of source location in addition to crustal defocusing which persists over a broad range of azimuths and epicentral distances we also find evidence for more subtle variations in the initial waveform which may be attributable to upper mantle structure these observations are modeled in two ways we employ a parabolic approximation method to simulate distortion by deep seated large scale structure as exemplified by a best estimate model of the upper mantle below wrsn derived from travel time inversion the resulting synthetics fail to exhibit strong waveform distortion the principal effects are focusing defocusing which accompany geometrical spreading crustal heterogeneity is probably stronger and exists at smaller scales comparable to the wavelengths of the short period waves hence contributions to waveform distortion through scattering processes become important these contributions are investigated qualitatively using an acoustic finite difference code and models of crustal heterogeneity comprising volume heterogeneity and interface topography waveforms vary markedly over station separations as small as 5 10 km but at a given station are relatively insensitive to changes in source location crustal contamination will seriously complicate the retrieval of information on upper mantle structure from short period waveform distortion however these results suggest that the identification of an upper mantle signature can be achieved by comprehensive analysis of the impulse response at a given station as function of epicentral distance and back azimuth

assessment of historical changes 1959 2012 and the causes of recent break ups of the petersen ice shelf nunavut canada aerial photography and satellite imagery of the petersen ice shelf nunavut canada from 1959 to 2012 show that it was stable until june 2005 after which a series of major calving events in the summers of 2005 2008 2011 and 2012 resulted in the loss of 61 of the june 2005 ice shelf area this recent series of calving events was initiated by the loss of extensive regions of 50 year old multi year landfast sea ice from the front of the ice shelf in summer 2005 each subsequent calving event has been preceded by open water conditions and resulting loss of pack ice pressure across the front of the ice shelf and most occurred during record warm summers ground penetrating radar gpr ice thickness measurements and radarsat 2 derived observations of surface motion indicate that tributary glaciers provided total ice input of 1 19 5 65 mt a 1 to the ice shelf from 2011 to 2012 far below the mean surface loss rate of 28 45 mt a 1 with recent losses due to calving and little evidence for current basal freeze on this suggests that the petersen ice shelf will no longer exist by the 2040s or sooner if further major calving events occur

modeling global sea ice with a thickness and enthalpy distribution model in generalized curvilinear coordinates abstract a parallel ocean and ice model poim in generalized orthogonal curvilinear coordinates has been developed for global climate studies the poim couples the parallel ocean program pop with a 12 category thickness and enthalpy distribution ted sea ice model although the poim aims at modeling the global ocean and sea ice system the focus of this study is on the presentation implementation and evaluation of the ted sea ice model in a generalized coordinate system the ted sea ice model is a dynamic thermodynamic model that also explicitly simulates sea ice ridging using a viscous plastic rheology the ted model is formulated such that all the metric terms in generalized curvilinear coordinates are retained following the pop s structure for parallel computation the ted model is designed to be run on a variety of computer architectures parallel serial or vector when run on a computer cluster with 10 parallel processors the parallel performance of the poim is close to that of a correspon

effects of mackenzie river discharge and bathymetry on sea ice in the beaufort sea mackenzie river discharge and bathymetry effects on sea ice in the beaufort sea are examined in 2012 when arctic sea ice extent hit a record low satellite derived sea surface temperature revealed warmer waters closer to river mouths by 5 july 2012 mackenzie warm waters occupied most of an open water area about 316 000 km2 surface temperature in a common open water area increased by 6 5 c between 14 june and 5 july 2012 before and after the river waters broke through a recurrent landfast ice barrier formed over the shallow seafloor offshore the mackenzie delta in 2012 melting by warm river waters was especially effective when the strong beaufort gyre fragmented sea ice into unconsolidated floes the mackenzie and other large rivers can transport an enormous amount of heat across immense continental watersheds into the arctic ocean constituting a stark contrast to the antarctic that has no such rivers to affect sea ice

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

crustal structure of the washington continental margin from refraction data abstract refraction data were collected in an onshore offshore experiment in 1978 near grays harbor washington two dimensional raytracing and synthetic seismograms were used to model the data along the refraction lines the resulting velocity model uses a broad scale subduction zone geometry to explain the major features of the data the three main contributions of the onshore offshore model are 1 evidence for an approximately 9 dip of the subducting oceanic lithosphere 2 a clear indication of the point at which the subducting slab begins to bend beneath the margin and 3 confirmation of the continuity of the slab from the buried trench offshore to beneath puget sound the model also helps explain the distribution and stress orientation of earthquakes within the subduction zone

test of the lower mantle slab penetration hypothesis using broadband s waves bioadband s and scs displacement pulses from deep focus earthquakes in the sea of okhotsk recorded at analog and digital stations in north america and europe show com plexities produced by velocity heterogeneity in the lower mantle the slab beneath the sea of okhotsk strikes n40oe while the stations used in this study range from n40ow to n70oe allowing a test of the hypothesis that multipathing in a lower mantle slab extension gives rise to the waveform complexities comparisons of the s and scs displacement pulses at a given station indicate that in many cases the direct s pulses have a broader more complex waveform than the scs pulses we parameterized the waveform complexity by the energy tempo al centroid difference cs csc s between the s and scs pulses the largest cs csc s anomalies are observed along the strike of the slab there is also a correlation between the cs csc s values and scs s travel time residuals for stations along the strike ofthe slab and no correlation for off strike azimuths however cs csc s anomalies are ob served over the entire azimuth range including azimuths to the southwestern u s for which the raypaths should quickly exit any deep slab structure these data suggest that lateral velocity heterogeneities both near the source possibly due to slab pene tration and deep in the mantle near the s wave turning points contribute to the s and scs complexity

erratum to stratal architecture of western mediterranean margins as the result of the stacking of quaternary lowstand deposits below glacio eustatic fluctuation base level sedimentary geology 112 1997 195 217

seismic tomography constrained by bouguer gravity anomalies applications in western washington tomographic inversions for velocity variations in western washington indicate a high correlation with surface geology and geophysical measurements including gravity observations by assuming a simple linear relationship between density and velocity birch s law it is possible to calculate the gravity field predicted from the velocity perturbations obtained by local tomographic inversion while the predicted gravity matches observations in parts of the model the overall correlation is not satisfactory in this paper we suggest a method of constraining the tomographic inversion to fit the gravity observations simultaneously with the seismic travel time data the method is shown to work well with synthetic data in 3 dimensions where the assumption of birch s law holds strictly if the sources of the gravity anomalies are assumed to be spatially localized integration can be carried out over a relatively small volume below the observation points and sparse matrix techniques can be applied we have applied the constrained inversion method to western washington using 4 387 shallow earthquakes to depths of 40 0 km 36 865 raypaths convering a 150 250 km region and found that the gravitational constraints may be satisfied with minor effect on the degree of misfit to the seismic data

the upper mantle beneath the north east pacific rim a comparison with the gulf of california summary seismograms from 22 earthquakes along the north east pacific rim recorded in southern california form the data set for investigation of the upper mantle beneath the cascade range juan de fuca region a transitional area encompassing both very young ocean floor and a continental margin these data consist of 853 seismograms 6 a 42 which produce 1068 travel times and 40 ray parameter estimates we compare these data directly to another large suite of records representative of structure beneath the gulf of california an active spreading centre we use the spreading centre model gca as a starting point in wkbj synthetic seismogram modelling and perturb gca until the ne pacific data are matched application of wavefield continuation to these two groups of data provides checks on the model s con sistency with the data as well as an estimate of the resolvability of differences between the two areas the resulting ne pacific model cjf is very similar to gca disparities in travel times and waveforms from 16 to 22 however are interpretable in ierms of lateral structural variations between the two regions from 200 to 410 km depth cjf features velocities from 200 to 350 km that are inter mediate between the very low values of gca and the more moderate values of t7 a model for the inland western united states models of continental shield regions have even higher velocities in this depth range but all four model types are similar below 450 km this regular pattern observed in the rate of velocity increase with tectonic regime suggests an inverse relationship between velocity gradient and lithospheric age above 400 km depth

energy budget of first year arctic sea ice in advanced stages of melt 1 during an 8 day drift in july august 2012 in the nansen basin all components of the energy budget of melting first year sea ice were observed absorption of solar radiation by the ice and ponds was the largest source of energy to the ice at almost all times during the drift however oceanic heat flux also provided significant heating and dominated during one wind event longwave fluxes provided a relatively small cooling effect and atmospheric heat fluxes were negligible the aggregate scale albedo of this younger thinner ice was significantly lower than at surface heat budget of the arctic ocean sheba and the transmittance was significantly higher here despite similar pond and open water fractions the oceanic heat flux was only half of the solar flux through the ice to the water producing warm water near the surface that might delay ice growth in autumn an important effect of the transition to thinner first year ice in the high arctic

implications of fractured arctic perennial ice cover on thermodynamic and dynamic sea ice processes decline of the arctic summer minimum sea ice extent is characterized by large expanses of open water in the siberian laptev chukchi and beaufort seas and introduces large fetch distances in the arctic ocean long waves can propagate deep into the pack ice thereby causing flexural swell and failure of the sea ice this process shifts the floe size diameter distribution smaller increases floe surface area and thereby affects sea ice dynamic and thermodynamic processes the results of radarsat 2 imagery analysis show that a flexural fracture event which occurred in the beaufort sea region on 6 september 2009 affected 40 000 km2 open water fractional area in the area affected initially decreased from 3 7 to 2 7 but later increased to 20 following wind forced divergence of the ice pack energy available for lateral melting was assessed by estimating the change in energy entrainment from longwave and shortwave radiation in the mixed layer of the ocean following flexural fracture 11 54 mj m 2 of additional energy for lateral melting of ice floes was identified in affected areas the impact of this process in future arctic sea ice melt seasons was assessed using estimations of earlier occurrences of fracture during the melt season and is discussed in context with ocean heat fluxes atmospheric mixing of the ocean mixed layer and declining sea ice cover we conclude that this process is an important positive feedback to arctic sea ice loss and timing of initiation is critical in how it affects sea ice thermodynamic and dynamic processes

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

sea ice mass balance observations from the north pole environmental observatory in recent years the periphery of the arctic sea ice cover has undergone significant changes with a reduction in summer ice extent a thinning of the ice and a shift from multiyear to first year ice here we examine sea ice conditions during nine summers between 2000 and 2013 in the interior of the ice pack using autonomous measurements of sea ice mass balance deployed near the north pole results exhibit no definitive trends there is large interannual variability with surface melt ranging from 0 02 m to 0 50 m and bottom melt from 0 10 m to 0 57 m the largest amounts of bottom melt have occurred in the past few years for all 9 years the ice at the end of the melt season was at least 1 2 m thick

tomographic imaging of local earthquake delay times for three dimensional velocity variation in western washington tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the puget sound region of western washington the 37 500 square km region is represented by nearly cubic blocks of 5 km per side p wave arrival time observations from 4 387 crustal earthquakes with depths of 0 to 40 km were used as sources producing 36 865 rays covering the target region a conjugate gradient method lsqr is used to invert the large sparse system of equations to diminish the effects of noisy data the laplacian is constrained to be zero within horizontal layers providing smoothing of the model the resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure results of the inversion are correlated with some known geologic features and independent geophysical measurements high p wave velocities along the eastern flank of the olympic peninsula are interpreted to reflect the subsurface extension of crescent terrane low velocities beneath the puget sound further to the east are inferred to reflect thick sediment accumulations the crescent terrane appears to extend beneath puget sound consistent with its interpretation as a major accretionary unit in themore southern puget sound basin high velocity anomalies at depths of 10 20 km are interpreted as crescent terrane and are correlated with a region of low seismicity near mt ranier high velocity anomalies may reflect buried plutons less

late quaternary marine isotope stages 6 1 seismic sequence stratigraphic evolution of the otago continental shelf new zealand abstract a proposed chronostratigraphic model for the late quaternary evolution of the otago continental shelf new zealand includes fluvially incised sequence boundaries eroded during marine isotope stages 2 4 and 6 bounding three sequences subdivided into lowstand transgressive and highstand regressive systems tracts datable material is limited to the uppermost 1 m of the sequence and consequently the model is based on correlation of new high resolution seismic reflection data geopulse to a compilation of independent global sea level curves a second chronostratigraphic model that lacks a stage 4 sequence boundary and distinct stage 3 highstand forced regressive deposit is discounted because its subsidence corrected paleoshoreline depths do not match lowstands in the eustatic curves or temporally constrained sequences at other far field locations in the preferred chronostratigraphic model sea level lowstands during stages 2 4 and 6 reached maximum depths of 126 131 m 101 111 m and 116 135 m respectively producing sequence boundaries landward pinching deltaic wedges on the outer otago shelf and submarine canyon incision lowstand paleoshoreline depths have been corrected for subsidence from thermal cooling sediment compaction and loading but may be erroneously deep owing to erosion of paleoshoreline indicators clastic shallow marine wedges and back barrier valley fill deposits accumulated during the stage 6 5 4 3 and 2 1 marine transgressions contrary to the traditional sequence stratigraphic model deltaic and strandline units deposited during highstand through falling sea level stages 5 4 3 2 1 volumetrically dominate otago late quaternary sequences discrete units and minor marine erosion surfaces within the transgressive and regressive systems tract deposits are interpreted as strong evidence for seventh 20 ka or higher order sea level fluctuations influencing sedimentation on the otago shelf

what drove the dramatic retreat of arctic sea ice during summer 2007 1 a model study has been conducted of the unprecedented retreat of arctic sea ice in the summer of 2007 it is found that preconditioning anomalous winds and ice albedo feedback are mainly responsible for the retreat arctic sea ice in 2007 was preconditioned to radical changes after years of shrinking and thinning in a warm climate during summer 2007 atmospheric changes strengthened the transpolar drift of sea ice causing more ice to move out of the pacific sector and the central arctic ocean where the reduction in ice thickness due to ice advection is up to 1 5 m more than usual some of the ice exited fram strait and some piled up in part of the canada basin and along the coast of northern greenland leaving behind an unusually large area of thin ice and open water thin ice and open water allow more surface solar heating because of a much reduced surface albedo leading to amplified ice melting the arctic ocean lost additional 10 of its total ice mass in which 70 is due directly to the amplified melting and 30 to the unusual ice advection causing the unprecedented ice retreat arctic sea ice has entered a state of being particularly vulnerable to anomalous atmospheric forcing

three dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the kanto tokai district japan we determine the three dimensional p wave velocity structure beneath the kanto tokai district central japan and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the national research center for disaster prevention the variance of the travel time data is reduced by 75 as compared to the variance for the routinely used laterally homogeneous model in general our velocity model is consistent with tectonic models of the kanto tokai district our velocity model is also in good agreement with seismic explosion data and bouguer gravity anomalies the uppermost layer from 0 to 16 km depth is characterized by lateral velocity differences of more than 20 we find low velocity zones beneath tokyo bay the middle of the boso peninsula and omae zaki where there are thick sedimentary layers a marked 30 km thick high velocity zone extends to the northeast and the west of the izu peninsula this zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the philippine sea phs plate which is being subducted at the sagami and the suruga troughs the data unquestionably require a high velocity zone beneath mount tsukuba that cannot be explained in terms of present tectonic models we use our three dimensional velocity model to relocate about 23 000 earthquakes that took place from 1983 through 1985 the accuracy of the hypocentral parameters improved considerably as a result the double seismic zone within the subducting pacific plate is more clearly delineated than in previous studies the inclined seismic zone for the subducting phs plate coincides fairly well with the high velocity zone of our three dimensional velocity model

thin ice and storms sea ice deformation from buoy arrays deployed during n ice2015 arctic sea ice has displayed significant thinning as well as an increase in drift speed in recent years taken together this suggests an associated rise in sea ice deformation rate a winter and spring expedition to the sea ice covered region north of svalbard the norwegian young sea ice2015 expedition n ice2015 gave an opportunity to deploy extensive buoy arrays and to monitor the deformation of the first year and second year ice now common in the majority of the arctic basin during the 5 month long expedition the ice cover underwent several strong deformation events including a powerful storm in early february that damaged the ice cover irreversibly the values of total deformation measured during n ice2015 exceed previously measured values in the arctic basin at similar scales at 100 km scale n ice2015 values averaged above 0 1 d 1 compared to rates of 0 08 d 1 or less for previous buoy arrays the exponent of the power law between the deformation length scale and total deformation developed over the season from 0 37 to 0 54 with an abrupt increase immediately after the early february storm indicating a weakened ice cover with more free drift of the sea ice floes our results point to a general increase in deformation associated with the younger and thinner arctic sea ice and to a potentially destructive role of winter storms

sensitivity of pliocene arctic climate to orbital forcing atmospheric co 2 and sea ice albedo parameterisation abstract general circulation model gcm simulations of the mid pliocene warm period mpwp 3 264 to 3 025 myr ago do not reproduce the magnitude of northern hemisphere high latitude surface air and sea surface temperature sat and sst warming that proxy data indicate there is also large uncertainty regarding the state of sea ice cover in the mpwp evidence for both perennial and seasonal mpwp arctic sea ice is found through analyses of marine sediments whilst in a multi model ensemble of mpwp climate simulations half of the ensemble simulated ice free summer arctic conditions given the strong influence that sea ice exerts on high latitude temperatures an understanding of the nature of mpwp arctic sea ice would be highly beneficial using the hadcm3 gcm this paper explores the impact of various combinations of potential mpwp orbital forcing atmospheric co 2 concentrations and minimum sea ice albedo on sea ice extent and high latitude warming the focus is on the northern hemisphere due to availability of proxy data and the large data model discrepancies in this region changes in orbital forcings are demonstrated to be sufficient to alter the arctic sea ice simulated by hadcm3 from perennial to seasonal however this occurs only when atmospheric co 2 concentrations exceed 300 ppm reduction of the minimum sea ice albedo from 0 5 to 0 2 is also sufficient to simulate seasonal sea ice with any of the combinations of atmospheric co 2 and orbital forcing compared to a mpwp control simulation monthly mean increases north of 60 n of up to 4 2 c sst and 9 8 c sat are simulated with varying co 2 orbit and sea ice albedo values we are able to reproduce proxy temperature records that lean towards modest levels of high latitude warming but other proxy data showing greater warming remain beyond the reach of our model this highlights the importance of additional proxy records at high latitudes and ongoing efforts to compare proxy signals between sites

stratral architectures of late quaternary regressive transgressive cycles in the roussillon shelf sw gulf of lions france abstract a seismic and sequence stratigraphy scheme of the southwestern area of the gulf of lions shelf nw mediterranean basin is presented through the analysis of high resolution seismic profiles obtained with a minisparker system and calibrated with published seismic and core data of the adjacent languedoc area the observed stratigraphic architecture records the repetition of four regressive transgressive cycles which constitute high frequency depositional sequences dss regressive intervals dominate the generation of shelf sedimentary architecture within regressive intervals the identification of progressively shallower clinoforms from proximal to distal in a downdip direction constitutes a strong indicator of the occurrence of forced regressions this stratigraphic pattern documents the preservation of potential sand prone reservoirs encased within widespread regressive wedges and directly connected with organic matter rich muds as distally there is no sharp basal contact between the two the unusual preservation of pre last glacial maximum transgressive deposits is attributed to the combined influence of previous shelf topography and dominant wave regime the most significant transgressions were recorded by shallow water deposits in distal middle and proximal locations distal and proximal deposits develop over relatively steep surfaces which caused slower transgressions and favoured the generation of wave dominated coastal deposits in contrast middle deposits show moderate development over a smooth shelf profile as a consequence of rapid shoreline translation the analysis of spatial changes of regressive transgressive cycles provided important information about the nature of shelf processes and about the relative significance of regressive versus transgressive intervals within each individual ds whose development was led by recent high frequency sea level cycles dominance of 120 ka cycles would imply that transgressive deposits represent a partial record of glacial interglacial transgressions dominance of 20 ka cycles is not favoured by the fact that the preservation of quaternary deposits is apparently limited on the shelf

least squares fitting of marine seismic refraction data summary an iterative procedure is presented for fitting waveform data from a marine seismic refraction experiment the wavefunction from the explosive source is known and the crustal structure is refined using the damped least squares procedure the damping parameter serves the dual purpose of stabilizing an under constrained inversion and improving the linearity by suppressing high frequencies the synthetic seismograms and their model differentials are calculated using the wkbj seismogram method both the synthetic seismograms and the linear algebra are sufficiently straightforward that the computations can be performed on an array processor the inversion procedure is then sufficiently rapid that interactive computations are possible the technique is illustrated using the ff2 refraction data from the fanfare cruise of the scripps institution of oceanography these data had been interpreted previously by trial and error using the reflectivity method starting from two different simple models the inversion procedure obtains essentially one unique model its features are very similar to the previous model

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

the tributary valley systems of the almeria canyon alboran sea sw mediterranean sedimentary architecture three tributary valley systems of the almeria canyon dalias andarax and gata located on the almeria margin alboran sea sw mediterranean were studied using high resolution techniques including swath bathymetry and topas seismic reflection profiles a detailed morphological and seismic stratigraphy study shows that the tributary valley systems tvs are formed by two types of valleys gullies and leveed channels dalias tvs is the best developed system covering an area of more than 300 km 2 from the shelf edge off cabo de dalias to the almeria canyon 100 1500 m depth andarax tvs covers about 65 km 2 from the shelf off the andarax river mouth to the westernmost branch of the almeria canyon 40 300 m depth gata tvs covers more than 100 km 2 from the shelf edge off cabo de gata to the almeria canyon 200 650 m depth tributary valley systems can be divided into two models based on the detailed morpho seismic study the sea level controlled model and the sediment supply controlled model the sea level controlled model includes the dalias and gata tvs this model is mainly governed by sea level fluctuations and the valleys grow as a result of the activity of turbidity currents and hemipelagic sedimentation displaying a retrogressive upslope pattern the valleys development would have been favoured by glacio eustatic oscillations during the late quaternary a tentative chronostratigraphic correlation has been made relating the evolution of dalias and gata tvs with the marine isotopic stages and sea level curves tectonics and margin physiography play a secondary role in the development of this type of valley the sediment supply controlled model is applicable to the andarax tvs the growth of the valleys is controlled by the ephemeral sediment discharges from the andarax river the valleys are small scale gullies located on the shelf and slope which would grow as a result of the turbidity currents which channel the fluvial sediment supplied in form of hyperpycnal currents this model would have been developed during the last highstand of sea level once the coast line reached its present situation d 2005 elsevier b v all rights reserved

effect of frequent winter warming events storms and snow on sea ice growth a case from the atlantic sector of the arctic ocean during the n ice2015 campaign

decline in arctic sea ice thickness from submarine and icesat records 1958 2008 1 the decline of sea ice thickness in the arctic ocean from icesat 2003 2008 is placed in the context of estimates from 42 years of submarine records 1958 2000 described by rothrock et al 1999 2008 while the earlier 1999 work provides a longer historical record of the regional changes the latter offers a more refined analysis over a sizable portion of the arctic ocean supported by a much stronger and richer data set within the data release area dra of declassified submarine sonar measurements covering 38 of the arctic ocean the overall mean winter thickness of 3 64 m in 1980 can be compared to a 1 89 m mean during the last winter of the icesat record an astonishing decrease of 1 75 m in thickness between 1975 and 2000 the steepest rate of decrease is 0 08 m yr in 1990 compared to a slightly higher winter summer rate of 0 10 0 20 m yr in the five year icesat record 2003 2008 prior to 1997 ice extent in the dra was 90 during the summer minimum this can be contrasted to the gradual decrease in the early 2000s followed by an abrupt drop to 55 during the record setting minimum in 2007 this combined analysis shows a long term trend of sea ice thinning over submarine and icesat records that span five decades

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

winter sea ice melt in the canada basin arctic ocean 1 recent warming and freshening of the canada basin has led to the year round storage of solar radiation as the near surface temperature maximum nstm using year round ocean from ice tethered profilers and autonomous ocean flux buoys sea ice from ice mass balance buoys and atmosphere from ncep ncar reanalysis data from 2005 2010 we find that heat from the nstm is entrained into the surface mixed layer sml during winter entrainment can only occur when the base of the sml reaches the top of the nstm if this condition is met the surface forcing and stratification together determine whether the sml deepens into the nstm heat transfer occurs by diffusion or by the erosion of the summer halocline the average temperature of the sml warmed by as much as 0 06 c during storm events solar radiation began warming the sml about 1 month early during the winter of 2007 2008 and this can be explained by thin sea ice

improved green s functions for passive source structural studies over the past two decades teleseismic receiver functions have proved to be a useful tool to investigate crustal structure because they represent a first order approximation to the s wave component of the teleseismic p green s function receiver functions provide valuable information on physical properties related to shear modulus however the implicit use of the p component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus by deconvolving improved estimates of complex source time functions generated by earthquakes one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth s green s function using a new deconvolution method we present estimates of the p component of the teleseismic p green s functions at several stations of the canadian national seismic network cnsn that clearly show the receiver side pure p wave crustal multiple the identificat

shifts in the physical environment in the pacific arctic and implications for ecological timing and conditions abstract the northern bering sea and chukchi sea represent the gateway from the pacific to the arctic this contiguous marine system encompasses one of the largest continental shelves in the world and serves as the sole point of connection between the north pacific and arctic ocean this region has unique attributes and complex dynamics driven by the convergence of distinct water masses dynamic currents advection between pacific and arctic systems and important latitudinal gradients relevant to stratification and water mass structure water temperature and seasonal ice cover many processes and interactions in the region appear to be changing with important implications for both hydrography and ecology our analyses access remote and local data sources in us and russian waters to characterize oceanographic conditions and analyze the implications of dramatic shifts in recent years previously this region appeared resistant to trends apparent elsewhere in the greater arctic now the pacific arctic also appears to be in rapid transition the conditions observed in 2017 2019 are unprecedented we note important shifts in the phenology and magnitude of physical variables including sea ice extent concentration and duration as well as extreme reduction in the extent and intensity of the related bering sea cold pool we also note distinct regional dynamics in sea surface temperature in the bering chukchi system distinguishing western eastern and northern areas of the bering sea specifically our analyses distinguish the northern bering sea as an important transition zone between the pacific and arctic with higher frequency variability in sea surface temperature anomalies our results suggest that the strength and position of the aleutian low may be linked to warm and cold phases in the bering sea and has an important role in large scale circulation while cold winds out of the north are necessary to form ice in the northern bering sea strong winds may be associated with weak sea ice as wind action may break ice and enhance vertical mixing counteracting enhanced sea ice production from the advection of cold air research in this important region is complicated by international borders but may be enhanced through international collaboration this analysis represents an attempt to integrate data across russian and us waters to more fully represent system wide processes to contrast regional trends and to better understand physical interactions

meteorological conditions in a thinner arctic sea ice regime from winter to summer during the norwegian young sea ice expedition n ice2015 atmospheric measurements were made over arctic sea ice north of svalbard from winter to early summer january june 2015 during the norwegian young sea ice n ice2015 expedition these measurements which are available publicly represent a comprehensive meteorological data set covering the seasonal transition in the arctic basin over the new thinner sea ice regime winter was characterized by a succession of storms that produced short lived less than 48 h temperature increases of 20 to 30 k at the surface these storms were driven by the hemispheric scale circulation pattern with a large meridional component of the polar jet stream steering north atlantic storms into the high arctic nonstorm periods during winter were characterized by strong surface temperature inversions due to strong radiative cooling radiatively clear state the strength and depth of these inversions were similar to those during the surface heat budget of the arctic ocean sheba campaign in contrast atmospheric profiles during the opaquely cloudy state were different to those from sheba due to differences in the synoptic conditions and location within the ice pack storm events observed during spring summer were the result of synoptic systems located in the barents sea and the arctic basin rather than passing directly over n ice2015 these synoptic systems were driven by a large scale circulation pattern typical of recent years with an arctic dipole pattern developing during june surface temperatures became near constant 0 c on 1 june marking the beginning of summer atmospheric profiles during the spring and early summer show persistent lifted temperature and moisture inversions that are indicative of clouds and cloud processes

an aeromagnetic study of mount st helens aeromagnetic data from surveys flown by the u s geological survey over mount st helens washington before and after the climactic may 18 1980 eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source we assumed that most of the edifice of preeruption and posteruption mount st helens has a magnetization direction near the present earth s field of 69 inclination and 20 declination and calculated its intensity as 4 1 a m after subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows indicative that their sources were not altered or removed by the may 1980 eruption the residual highs were explained by a 200 m deep source lying mostly within the edifice of mount st helens the source could be terrain that predates mount st helens such as a buried ridge or a cone or a valley filled with lava we calculated the magnetization of the material removed by the may 1980 eruption and found its intensity to be 4 2 a m in a direction near the present earth s field similar to that assumed for the volcano as a whole this similar result confirmed the validity of the magnetization vector assumed for the entire edifice

heat budget analysis of northern hemisphere high latitude spring onset events regional spring onset events are identified within 4 high latitude sectors the primary critical region over north siberia cr greenland north america g na east asia ea and alaska al to identify the primary forcing of the rapid temperature increases observed during spring onset the contributions to the near surface air temperature anomaly tendency are diagnosed within the thermodynamic equation for each of the 4 regional event categories for each region anomalous eddy heat flux convergence is the primary contributor to regional warming prior to and during the early stages of spring onset through day 5 thereafter horizontal advection of the climatological mean temperature by the large scale circulation anomaly field emerges as the leading contributor to regional warming during the later stages of spring onset a parallel diagnostic of storm track strength using the envelope function reveals a systematic weakening of eddy activity within each region leading to a reduction in the northward eddy heat flux out of the domain and an accumulation of heat within the region for cr g na and ea events an east west dipole structure in the sea level pressure anomaly field with lower higher pressure to the west east generates the anomalous southerly flow linked to late period linear warm advection our results indicate that anomalous dynamical processes associated with synoptic eddy activity and stationary wave patterns are the primary contributors to rapid temperature increase during arctic spring onset events with minimal contributions from anomalous diabatic processes

the use of p wave amplitude data in a joint inversion with travel times for upper mantle velocity structure a joint inversion of p wave travel time and amplitude data is performed to test whether the amplitudes can be used to increase the resolution of inversions using travel times alone geometrical spreading amplitudes depend on the curvature of the slowness field and may thus help to resolve sharp gradients travel times and amplitudes of short period p waves observed at the washington regional seismograph network are jointly inverted for upper mantle velocity structure below the array the processing of the observed p wave amplitudes is discussed in detail and it is shown that it is important to correct the raw data for station statics as these dominate the observed amplitude signal a robust averaging procedure is used to identify and remove outliers from the data set which is comprised of 8697 travel time and 4255 amplitude measurements the travel time data alone are used to obtain a reference model for the amplitude data and in this way the nonlinear behavior of the amplitude equations is minimized the results show that the amplitude data induce small short scale slowness perturbations to the starting model producing an amplitude misfit reduction to an arbitrary degree depending on the regularization inversions with synthetic data are performed to explain this result an idealized model of a subducting slab is used to generate synthetic data sets the model resulting from a joint inversion produces an amplitude misfit reduction of 77 while the travel time misfit is unaffected in agreement with the results for the real data sets this is achieved by making small adjustments throughout the model which do not alter the overall slowness variations the amplitude data therefore do not improve the resolution of the sharp gradients present in the synthetic slab model these results can be explained by both the strong sensitivity of geometrical spreading amplitude to slowness perturbations along the ray path and to the distribution of the amplitude data set which is not complete enough to induce more than incoherent changes to the starting model the strong sensitivity is due to the combination of long teleseismic ray paths and small scale about 30 km slowness variations allowed by the model parameterization with the present amplitude data set the curvature is changed on the scale of the node spacing in the model whereas the overall slowness variations on a scale length of several times the node spacing are unaffected the regularization has little effect on these small scale changes as these changes increase the total model roughness by only 1 it is concluded that with the amplitude data set available amplitudes do not improve the resolving power of travel time inversions for upper mantle velocity structure this is due to the number of data required to both decrease the observed variance in the raw amplitudes to the variance in travel time data and to obtain a good coverage of the model the application of p wave amplitude data as a validation tool is suggested for models obtained with for example travel time inversion applications to both synthetic and observed data sets are shown anelastic damping is ignored in the inversions using linear relations between velocity temperature and q it is shown that the effect of anelastic attenuation is of the order of 10 15 of that of geometrical spreading through the upper mantle

modeling teleseismic p wave propagation in the upper mantle using a parabolic approximation teleseismic waves propagating in the upper mantle are subject to considerable distortion due to the effects of laterally heterogeneous structure the magnitude and scale of velocity contrasts representative of features such as subducted slabs may be such that wave diffraction becomes an important process in this case forward modeling methods based on high frequency asymptotic approximations to the wave equation will not accurately describe the wavefield a method is introduced to model the propagation of teleseismic p waves in a laterally heterogeneous upper mantle that accounts for distortion of the initial portion of the wavefield including the effects of multipathing and frequency dependent diffraction the method is based on a parabolic approximation to the wave equation that is solved in the time domain on a finite difference grid which mimics the expected pattern of energy flow in a reference velocity field numerical examples for a simple two dimensional subducting slab model demonstrate the application of the method and illustrate the effects of multipathing and diffraction which dominate waveform distortion at high and low frequencies respectively

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

modeling the formation and fate of the near surface temperature maximum in the canadian basin of the arctic ocean 1 a numerical model is used to investigate the time and space extent of the near surface temperature maximum nstm of the canadian basin of the arctic ocean over the years 2000 2009 the nstm is formed from local summertime absorption of solar radiation which in some circumstances descends through the fall and early winter to form a warm subsurface layer just below the winter mixed layer we find that winter survival of this layer is confined largely to the beaufort gyre of the canadian basin where ekman convergence and downwelling push the summer warm layer down below the winter mixing depth in recent years summer stratification has increased downwelling has accelerated and the nstm has warmed as the sea ice cover in the beaufort gyre has thinned the result is a strengthening nstm which contained enough heat by the end of winter 2007 2008 to melt about 20 cm of sea ice northwest of alaska the model also simulates a second deeper temperature maximum layer that forms from advection of saltier summer pacific water however this layer is difficult to adequately resolve and maintain given the model s resolution

shallow structure effects on broadband teleseismic p waveforms the first 5 sec of source equalized p wave receiver functions from an array of broadband portable instruments located about 50 km from the coast in western washington are strongly influenced by very shallow structure depths less than 3 km these data display behavior associated with a dipping high velocity contrast interface at shallow depths the direct p wave and the p to s converted phase vary strongly with backazimuth producing an apparent delay in first arriving energy at western backazimuths these patterns are attributed to a shallow contact between low velocity sandstones at the surface and an underlying high velocity volcanic unit this shallow structure influences even the original unprocessed teleseismic p waves suggesting that such shallow structure could affect other studies such as seismic source analysis utilizing broadband data

lateral heterogeneity of europe s upper mantle as inferred from modelling of broad band body waves summary the upper mantle velocity structure beneath europe is investigated by body wave modelling of broad band seismograms from earthquakes in the eastern mediterranean recorded by stations in north west europe the gross features of the p and s wave velocity distribution as inferred from the data show a high velocity lid a low velocity zone and two discontinuities at depths of approximately 420 and 650 km the seismograms also present evidence for lateral heterogeneity of at least a few per cent 2 5 per cent for p 4 per cent for s to a depth of approximately 400 km many of the inferred velocity variations can be related to the tectonic setting of the region between source and receiver broad band seismograms are essential for such detailed study of upper mantle structure using body wave modelling

the impact of an intense summer cyclone on 2012 arctic sea ice retreat 1 this model study examines the impact of an intense early august cyclone on the 2012 record low arctic sea ice extent the cyclone passed when arctic sea ice was thin and the simulated arctic ice volume had already declined 40 from the 2007 2011 mean the thin sea ice pack and the presence of ocean heat in the near surface temperature maximum layer created conditions that made the ice particularly vulnerable to storms during the storm ice volume decreased about twice as fast as usual owing largely to a quadrupling in bottom melt caused by increased upward ocean heat transport this increased ocean heat flux was due to enhanced mixing in the oceanic boundary layer driven by strong winds and rapid ice movement a comparison with a sensitivity simulation driven by reduced wind speeds during the cyclone indicates that cyclone enhanced bottom melt strongly reduces ice extent for about 2 weeks with a declining effect afterward the simulated arctic sea ice extent minimum in 2012 is reduced by the cyclone but only by 0 15 106 km2 4 4 thus without the storm 2012 would still have produced a record minimum

small scale deformation of an arctic sea ice floe detected by gps and satellite imagery abstract small scale 100 200 m deformations of an arctic sea ice floe were detected from multiple gps equipped buoys that were deployed on the same ice floe over a nine month period three deformation events were recorded at each case the event was of limited duration each lasting less than a day the events were highly compressive in nature with the area occupied by the buoy array decreasing by over half of the original area the strain rate during the deformation of the order of 10 5 s 1 is about three orders of magnitude larger than previous estimates for brittle fracturing for cracks of about 100 m in length on the 2 day time scale the strain rate became too small and none of the deformation events could be detected this suggests that satellite data with longer time scales may significantly underestimate the amount of intermittent small scale brittle failure of total deformation taken as a whole our results show the influence that large scale wind stress can have on small scale deformation however it is important to note that the impact of large scale wind stress is also dependent on the properties of sea ice as well as on the spatial and temporal evolution of the underlying forces that influence the fracturing process

regional variability in sea ice melt in a changing arctic in recent years the arctic sea ice cover has undergone a precipitous decline in summer extent the sea ice mass balance integrates heat and provides insight on atmospheric and oceanic forcing the amount of surface melt and bottom melt that occurs during the summer melt season was measured at 41 sites over the time period 1957 to 2014 there are large regional and temporal variations in both surface and bottom melting combined surface and bottom melt ranged from 16 to 294 cm with a mean of 101 cm the mean ice equivalent surface melt was 48 cm and the mean bottom melt was 53 cm on average surface melting decreases moving northward from the beaufort sea towards the north pole however interannual differences in atmospheric forcing can overwhelm the influence of latitude substantial increases in bottom melting are a major contributor to ice losses in the beaufort sea due to decreases in ice concentration in the central arctic surface and bottom melting demonstrate interannual variability but show no strong temporal trends from 2000 to 2014 this suggests that under current conditions summer melting in the central arctic is not large enough to completely remove the sea ice cover

sea ice loss and arctic cyclone activity from 1979 to 2014 abstractextensive summer sea ice loss has occurred within the beaufort chukchi east siberian and laptev seas over the last decade associated anomalies in sensible and latent heat fluxes in autumn have increased arctic atmospheric precipitable water and air temperatures with the potential to impact autumn and winter cyclone activity to examine if a connection exists between recent arctic sea ice loss and cyclone activity several cyclone metrics from 60 to 90 n are analyzed results show that following years with less september sea ice there is a subsequent increase in moisture availability regional baroclinicity and changes in vertical stability that favor cyclogenesis however tracking of individual cyclones indicates no coherent increase in cyclone frequency or intensity associated with sea ice loss furthermore no robust northward progression of extreme cyclones is observed

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

regional dependence in the timing of onset of rapid decline in arctic sea ice concentration arctic sea ice concentration from satellite passive microwave measurements is analyzed to assess the form and timing of the onset of decline of recent ice loss and the regional dependence of the response the timing of the onset is estimated using an objective method and suggests differences of up to 20 years between the various subregions a clear distinction can be drawn between the recent onset times of the atlantic sector beginning in 2003 and the much earlier onset times associated with the pacific sector where the earliest transition to rapid loss is found in 1992 rates of decline prior to and following the transition points are calculated and suggest that the postonset rate of loss is greatest in the barents sea and weakest in the pacific sector covariability between the seasons is noted in the sic response both at interannual and longer time scales for two case regions potential mechanisms for the onset time transitions are briefly analyzed in the barents sea the onset time coincides with a redistribution of the pathways of ice circulation in the region while along the alaskan coast the propagation of the regional signal can be traced in the age of the sea ice the results presented here indicate a series of spatially self consistent regional responses and may be useful in understanding the primary drivers of recent sea ice loss

lower pleistocene to present structural deformation and sequence stratigraphy of the continental shelf offshore eel river basin northern california seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore eel river basin of northern california using a closely spaced high resolution multichannel seismic grid geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences producing shifting sedimentation patterns in the offshore basin during the last v1 myr estimated timing of unconformity formation correlates generally with a deep ocean global n 18 o curve both before and after a v500kyr transition from 41 kyr to 100 kyr dominated cycles we suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide however regional tectonic influences on sequence development are also observed folding associated with gorda north america plate convergence ended progressively from south to north between v1 0 ma and v500 ka in response to northward migration of the mendocino triple junction mtj since v500 ka continued encroachment of the mtj produces rotation of preexisting structures uplift of the table bluff anticline tba related periods of channel incision south of the tba and generally reduced sediment preservation across the shelf mtj related uplift may also have induced formation of eel canyon over the past v1 0 myr a dominant northern sediment source becomes progressively less important a southern source probably the paleo eel river has become dominant since v750 ka seismic unconformities fall into two categories 1 irregular surfaces of limited mappable extent interpreted as incision exposure surfaces forming during relative sea level lowstands and 2 smooth laterally extensive regional unconformities interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea level transgressions a sequence stratigraphic model for sediment deposition and preservation on the eel river shelf predicts that preserved sediment on the eel margin is dominated by fluvially derived highstand silts and muds deposited by longshore directed currents and waves lowstand sediments are preserved only in fluvial channels infilled during late lowstand early transgression preserved transgressive sediments are likely limited to a thin veneer of well sorted coarse sediment or shell debris deposited above the transgressive ravinement future deep coring will be required to confirm these predictions 2002 elsevier science b v all rights reserved

how do sea ice concentrations from operational data compare with passive microwave estimates implications for improved model evaluations and forecasting passive microwave sensors have produced a 35 year record of sea ice concentration variability and change operational analyses combine a variety of remote sensing inputs and other sources via manual integration to create high resolution accurate charts of ice conditions in support of navigation and operational forecast models one such product is the daily multisensor analyzed sea ice extent masie the higher spatial resolution along with multiple input data and manual analysis potentially provide more precise mapping of the ice edge than passive microwave estimates however since masie is based on an operational product estimates may be inconsistent over time due to variations in input data quality and availability comparisons indicate that masie shows higher arctic wide extent values throughout most of the year largely because of the limitations of passive microwave sensors in some conditions e g surface melt however during some parts of the year masie tends to indicate less ice than estimated by passive microwave sensors these comparisons yield a better understanding of operational and research sea ice data products this in turn has important implications for their use in climate and weather models

uncertainty in modeled arctic sea ice volume 1 uncertainty in the pan arctic ice ocean modeling and assimilation system piomas arctic sea ice volume record is characterized a range of observations and approaches including in situ ice thickness measurements icesat retrieved ice thickness and model sensitivity studies yields a conservative estimate for october arctic ice volume uncertainty of 1 35 103 km3 and an uncertainty of the ice volume trend over the 1979 2010 period of 1 0 103 km3 decade 1 a conservative estimate of the trend over this period is 2 8 103 km3 decade 1 piomas ice thickness estimates agree well with icesat ice thickness retrievals 0 1 m mean difference for the area for which submarine data are available while difference outside this area are larger piomas spatial thickness patterns agree well with icesat thickness estimates with pattern correlations of above 0 8 piomas appears to overestimate thin ice thickness and underestimate thick ice yielding a smaller downward trend than apparent in reconstructions from observations piomas ice volume uncertainties and trends are examined in the context of climate change attribution and the declaration of record minima the distribution of 32 year trends in a preindustrial coupled model simulation shows no trends comparable to those seen in the piomas retrospective even when the trend uncertainty is accounted for attempts to label september minima as new record lows are sensitive to modeling error however the september 2010 ice volume anomaly did in fact exceed the previous 2007 minimum by a large enough margin to establish a statistically significant new record

the beaufort gyre intensification and stabilization a model observation synthesis a model observation synthesis is conducted to investigate changes in the upper ocean circulation and stratification in the canada basin cb of the arctic ocean results show that the beaufort gyre bg has been generally intensifying during 1992 2015 in conjunction with changes in sea ice and the upper ocean including increasing sea surface height ssh sea ice and ocean speed ekman transport convergence and downwelling and freshwater content decreasing ice thickness and upper ocean salinity shoaling summer halocline and mixed layer and deepening winter halocline and mixed layer increasing ekman transport convergence draws more water from surrounding areas into the cb thus lowering ssh in those areas and raising ssh in the cb the rate of change in the cb began to decrease in 2008 and the bg circulation appears to be stabilizing if not relaxing slightly this is reflected in the general plateauing of ssh the intensity of the sea ice and ocean circulation and various measures of the cb thermohaline stratification the bg intensification and subsequent stabilization appear to have been strongly controlled by atmospheric changes in the cb characterized by generally increasing anticyclonic wind circulation and sea level pressure slp before 2008 and falling wind strength and slp to below average levels in some years after 2008 changes in slp are highly correlated with changes in ocean surface stress curl and downwelling since 2008 the magnitude of the stress curl and downwelling in much of the cb has declined contributing to bg stabilization the general leveling off of sea ice thickness also contributes to the stabilization by limiting melt water input to the cb that increases freshwater content temperatures in the near surface temperature maximum layer trended upward slightly over 1992 2015 which is closely correlated with decreasing sea ice thickness upper ocean heat content increased over the study period mainly due to strong temperature increases in the summer pacific water layer

wind driven mixing at intermediate depths in an ice free arctic ocean recent seasonal arctic ocean sea ice retreat is a major indicator of polar climate change the arctic ocean is generally quiescent with the interior basins characterized by low levels of turbulent mixing at intermediate depths in contrast under conditions of reduced sea ice cover there is evidence of energetic internal waves that have been attributed to increased momentum transfer from the atmosphere to the ocean new measurements made in the canada basin during the unusually ice free and stormy summer of 2012 show previously observed enhancement of internal wave energy associated with ice free conditions however there is no enhancement of mixing at intermediate depths away from significant topography this implies that contrary to expectations of increased wind induced mixing under declining arctic sea ice cover the stratification in the central canada basin continues to suppress turbulent mixing at intermediate depths and to effectively isolate the large atlantic and pacific heat reservoirs from the sea surface

evolution of a canada basin ice ocean boundary layer and mixed layer across a developing thermodynamically forced marginal ice zone a comprehensive set of autonomous ice ocean measurements were collected across the canada basin to study the summer evolution of the ice ocean boundary layer iobl and ocean mixed layer oml evaluation of local heat and freshwater balances and associated turbulent forcing reveals that melt ponds mps strongly influence the summer iobl oml evolution areal expansion of mps in mid june start the upper ocean evolution resulting in significant increases to ocean absorbed radiative flux 19 w m 2 in this study buoyancy provided by mp drainage shoals and freshens the iobl resulting in a 39 mj m 2 increase in heat storage in just 19 days 52 of the summer total following mp drainage a near surface fresh layer deepens through shear forced mixing to form the summer mixed layer sml in late summer basal melt increases due to stronger turbulent mixing in the thin sml and the expansion of open water areas due in part to wind forced divergence of the sea ice thermal heterogeneities in the marginal ice zone miz upper ocean led to large ocean to ice heat fluxes 100 200 w m 2 and enhanced basal ice melt 3 6 cm d 1 well away from the ice edge calculation of the upper ocean heat budget shows that local radiative heat input accounted for at least 89 of the observed latent heat losses and heat storage partitioned 0 77 0 23 these results suggest that the extensive area of deteriorating sea ice observed away from the ice edge during the 2014 season termed the thermodynamically forced miz was driven primarily by local shortwave radiative forcing

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

whither arctic sea ice a clear signal of decline regionally seasonally and extending beyond the satellite record the arctic sea ice has been pointed to as one of the first and clearest indicators of climate change satellite passive microwave observations from 1979 through 2005 now indicate a significant 8 4 1 5 decade 1 trend 99 confidence level in september sea ice extent a larger trend than earlier estimates due to acceleration of the decline over the past 41 years there are differences in regional trends with some regions more stable than others not all regional trends are significant the largest trends tend to occur in months where melt is at or near its peak for a given region a longer time series of september extents since 1953 was adjusted to correct biases and extended through 2005 the trend from the longer time series is 7 7 0 6 decade 1 99 slightly less than from the satellite derived data that begin in 1979 which is expected given the recent acceleration in the decline

recent changes in the dynamic properties of declining arctic sea ice a model study 1 results from a numerical model simulation show significant changes in the dynamic properties of arctic sea ice during 2007 2011 compared to the 1979 2006 mean these changes are linked to a 33 reduction in sea ice volume with decreasing ice concentration mostly in the marginal seas and decreasing ice thickness over the entire arctic particularly in the western arctic the decline in ice volume results in a 37 decrease in ice mechanical strength and 31 in internal ice interaction force which in turn leads to an increase in ice speed 13 and deformation rates 17 the increasing ice speed has the tendency to drive more ice out of the arctic however ice volume export is reduced because the rate of decrease in ice thickness is greater than the rate of increase in ice speed thus retarding the decline of arctic sea ice volume ice deformation increases the most in fall and least in summer thus the effect of changes in ice deformation on the ice cover is likely strong in fall and weak in summer the increase in ice deformation boosts ridged ice production in parts of the central arctic near the canadian archipelago and greenland in winter and early spring but the average ridged ice production is reduced because less ice is available for ridging in most of the marginal seas in fall the overall decrease in ridged ice production contributes to the demise of thicker older ice as the ice cover becomes thinner and weaker ice motion approaches a state of free drift in summer and beyond and is therefore more susceptible to changes in wind forcing this is likely to make seasonal or shorter term forecasts of sea ice edge locations more challenging

arctic sea ice retreat in 2007 follows thinning trend the minimum of arctic sea ice extent in the summer of 2007 was unprecedented in the historical record a coupled ice ocean model is used to determine the state of the ice and ocean over the past 29 yr to investigate the causes of this ice extent minimum within a historical perspective it is found that even though the 2007 ice extent was strongly anomalous the loss in total ice mass was not rather the 2007 ice mass loss is largely consistent with a steady decrease in ice thickness that began in 1987 since then the simulated mean september ice thickness within the arctic ocean has declined from 3 7 to 2 6 m at a rate of 0 57 m decade 1 both the area coverage of thin ice at the beginning of the melt season and the total volume of ice lost in the summer have been steadily increasing the combined impact of these two trends caused a large reduction in the september mean ice concentration in the arctic ocean this created conditions during the summer of 2007 that allowed persistent winds to push the remaining ice from the pacific side to the atlantic side of the basin and more than usual into the greenland sea this exposed large areas of open water resulting in the record ice extent anomaly

mechanisms of summertime upper arctic ocean warming and the effect on sea ice melt 1 in this study we use a numerical sea ice ocean model to examine what causes summertime upper ocean warming and sea ice melt during the 21st century in the arctic ocean our first question is what causes the ocean to warm in the pacific sector during the summer we find that about 80 of total heating over this region comes from ocean surface heat flux with the remaining 20 originating in ocean lateral heat flux convergence the latter occurs mostly within a few hundred kilometers of the northwest alaskan coast in the summer of 2007 the ocean gained just over twice the amount of heat it did over the average of the previous 7 years our second question is what causes sea ice to melt in the pacific sector during summer our analysis shows that top melt dominates total melt early in the summer while bottom melt and in particular bottom melt due to ocean heat transport dominates later in the summer as atmospheric heating declines bottom melt rates in summer 2007 were 34 higher relative to the previous 7 year average the modeled partition of top versus bottom melt closely matches observed melt rates obtained by a drifting buoy bottom melting contributes about 2 3 of total volume melt but is geographically confined to the marginal ice zone while top melting contributes a lesser 1 3 of volume melt but occurs over a much broader area of the ice pack

validating tomographic model with broad band waveform modelling an example from the la ristra transect in the southwestern united states summary traveltime tomographic models of the la ristra transect produce excellent waveform fits if we amplify the damped images we observe systematic waveform distortions across the western edge of the great plains from south american events starting about 300 km east of the centre of the rio grande rift the amplitude decreases by more than 50 per cent within array stations spanning less than 200 km while the pulse width increases by more than a factor of 2 this feature is not observed for the data arriving from the northwest while the s wave tomographic image shows a fast slab like feature dipping to the southeast beneath the western edge of the great plains synthetics generated from this model do not reproduce the waveform characteristics however once we modify the tomographic image by amplifying the velocity contrast between the slab and adjoining mantle by a factor of 2 3 the synthetics produce observed amplitude decay and pulse broadening in addition to the traveltime delay amplitude variation due to wave phenomena such as slab diffraction focusing and defocusing provide much tighter constraints on the geometry of the fast anomaly and its amplitude and sharpness as demonstrated by a forward sensitivity test and snapshots of the seismic wavefield our preferred model locates the slab 200 km east of the rio grande rift dipping 70 75 to the southeast extending to a depth near 600 km with a thickness of 120 km and a velocity of about 4 per cent fast in short adding waveform and amplitude components to regional tomographic studies can help validate and establish structural geometry sharpness and velocity contrast