stress strain paradox plate coupling and forearc seismicity at the cascadia and nankai subduction zones abstract in the continental forearcs of the cascadia and nankai subduction zones geodetic strain measurements indicate crustal contraction in the direction of plate convergence nearly margin normal but earthquake focal mechanisms and other stress indicators show maximum compression along strike margin parallel it is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles not the absolute stresses the absolute margin normal stress is not only much less than the margin parallel stress but also no greater than the lithostatic value indicating a very weak subduction thrust fault weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime because these small elastic stress perturbations are relatively fast they give large strain rates that are detected by geodetic measurements the present nearly margin normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence with the margin parallel compression being dominant the small increase in margin normal stress may change the forearc seismicity in cascadia from a mixture of thrust and strike slip types into mainly thrust before the next great subduction earthquake in nankai the change should be from strike slip to quiescence as observed in the nankai forearc prior to the 1944 1946 great subduction earthquakes

c band radar polarimetry useful for detection of icebergs in sea ice this paper is focused on investigations of polarimetric c band radar signatures of icebergs in sea ice covered ocean regions the main objective is to assess the potential improvement of iceberg detection when using radar polarimetry the dominant backscattering mechanisms of icebergs are deduced by evaluating different polarimetric parameters magnitudes of the cross polarization ratios the correlation coefficients between hh and vv polarized signals and the entropy alpha parameters indicate a strong contribution of volume scattering in many cases over most icebergs the phase differences between hh and vv polarization are larger than zero spatial patterns of the polarimetric parameters differ from iceberg to iceberg and between different parameters on some bergs they only exhibit slight variations whereas on others they show noiselike textures but also more systematic changes are observed occasionally radar intensities of icebergs are of similar magnitude as those of sea ice only for a number of these cases the combined use of the investigated polarimetric parameters together with intensity improves the discrimination performance between icebergs and sea ice

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

barents sea seasonal ice zone features and processes from ers 1 synthetic aperture radar seasonal ice zone experiment 1992 sea ice features and processes in the barents sea were studied during the seasonal ice zone experiment 1992 sizex92 a dedicated ers 1 satellite synthetic aperture radar sar field campaign carried out in march 1992 sizex92 was based around the research vessels polarsyssel and hakon mosby in situ oceanographic meteorological and ice measurements were made coordinated with low altitude aerial observations and ers 1 sar data acquired in near real time fifty eight low and full resolution sar scenes were obtained during sizex92 which provided a validation data set for ers 1 sar backscatter under winter conditions in the barents sea analysis of sizex92 data has provided geophysical insights including a better understanding of barents sea ice edge freezing processes and ice edge development in response to atmospheric forcings in particular areas of new ice formation were found to be related to bathymetric features through their influence on the circulation of arctic and atlantic water masses ers 1 sar image sequences revealed rapid mesoscale variations in new ice areas and the ice edge in response to wind conditions in addition to geophysical insight sizex92 demonstrated some of the technical capabilities and limitations of ers 1 sar to identify new ice areas the ice edge ice floes and ice types mapping new ice areas using satellite sar data may be considered the most promising new application the limitations of ers 1 sar derived ice classification and ice motion estimates in the marginal ice zone are identified

sea ice monitoring by l band sar an assessment based on literature and comparisons of jers 1 and ers 1 imagery spaceborne single polarization c band synthetic aperture radar sar imagery is widely used to gather information about the state of the sea ice cover in the polar regions c band is regarded as a reasonable choice for all season monitoring capabilities for specific mapping tasks however other frequency bands can be more suitable in the first part of this paper the summary of a literature study dealing with the utilization of l band sar imagery for sea ice monitoring is presented investigations reveal that if deformation features such as ice ridges rubble fields and brash ice are to be mapped l band radar is superior in a number of cases the second part of this paper addresses the comparison of jers 1 and ers 1 sar images that were acquired over sea ice east of svalbard and along the east coast of greenland the effects of the different frequencies polarizations and incidence angles of the two sar systems are discussed it is demonstrated that the images of both sensors complement one another in the analysis of ice conditions resulting in a more detailed view of the sea ice cover state

discrimination of open water from sea ice in the labrador sea using quad polarized synthetic aperture radar abstract a method to differentiate open water from sea ice using quad polarization c band synthetic aperture radar sar imagery is presented based on the roughness properties of the sea surface scattering facets and the polarization ratio pr between vv and hh polarization backscatter from the sar images in quad polarization sar images the normalized radar cross section nrcs which is linked to sea surface roughness can be used to derive the orientation angle shift and its standard deviation as well as to compute the measured polarization ratio pr an x bragg backscatter model is explored to calculate a theoretical look up table for the polarization ratio as a function of incidence angle dielectric constant orientation angle shift and its standard deviation thus ocean surface backscatter can be identified as open water or sea ice by comparing the measured pr with a theoretical model for pr the proposed method is validated by comparison to collocated optical video images and manual visual interpretations estimates of the size shape and locations of open water from our method are shown to be in good agreement with observed data the overall accuracy of our algorithm is about 96 the classification accuracy for sea ice and open water is about 96 and 94 respectively

evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

mechanics of low stress forearcs nankai and cascadia the nankai and cascadia subduction zones have low stress forearcs where the margin normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms in the absence of active back arc spreading the forearc margin normal stress is controlled mainly by the total shear force along the subduction fault or the plate coupling force and the gravitational force the plate coupling force creates compression and the gravitational force in the presence of margin topography creates lateral tension we give a simple expression that relates the forearc stress to topography and average fault strength with the latter represented by an effective coefficient of friction we also use a finite element model of two converging plates in frictional contact to model the forearc stresses the analyses indicate that a low stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth for nankai and cascadia where the coupled fault areas are narrow and shallow because of the young and hot subducting plates is about 0 05 or less and in any case less than 0 09 the low fault strength is probably not a unique property of low stress forearcs since the same low values combined with wider and deeper coupled areas can be shown to create large margin normal compression in addition there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces great earthquakes occur at subduction zones having low stress forearcs such as nankai and cascadia

mapping of different sea ice regimes using images from sentinel 1 and alos synthetic aperture radar airborne c and l band synthetic aperture radar sar images were acquired for three test sites over different sea ice regimes around svalbard in march 2007 complemented by optical imagery environmental data and ice observations one objective was to use the high resolution low noise radar data for investigations on the technical performance of european space agency s sentinel 1 mission for sea ice mapping the other to assess the potential gain of additional use of l band sar data currently available from advanced land observing satellite alos the airborne sar images were employed to simulate data products resembling the interferometric wide swath mode iwsm of sentinel 1 and alos palsar fine resolution mode frm in order to quantify the information loss due to the higher noise level and coarser spatial resolution at the iwsm noise level zones of deformation level ice and new ice can be well separated at like polarization at cross polarization the noise level is too high for a robust automated classification including thin ice but deformed and level ice can be discriminated palsar frm is on average better suited to distinguish deformed and level ice over all ice regimes it was worse for separating new from thicker level ice at two test sites for the third test site with very few patches of new ice the palsar frm showed larger intensity contrasts between new and level ice than sentinel 1 iwsm the spatial resolution provided by the iwsm and frm is sufficient to identify most ice features and types without ambiguities typical characteristics of the imaged ice regimes are described

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

the recent crustal stress field in central europe trajectories and finite element modeling the recent crustal stress field of central europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress shmax the analysis is based on fault plane solutions in situ stress measurements geologic fault slip determinations and repeated precise geodetic triangulations a bending of the direction of shmax from the well known nw se direction in the western part of the study area to directions of ne sw to e w in the eastern part is shown first results on the recent crustal stress field of the study area were presented by grunthal and stromeyer 1986 who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only therefore one aim of this paper was to compile observations on the areas with few data points generally these additional data confirm the previously established pattern in some areas especially in the pannonian basin the stress features became more complicated compared with those solely based on a few data points the second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces simulation of the north atlantic seafloor spreading and the northward directed motion of the african plate by appropriate boundary loads produces a pattern of shmax directions for the western part of the eurasian plate which is compatible with the broad scale observed stress directions subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the western alps or the radial directions around the pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field rigorous introduction of physically constrained model parameters for all these features was not feasible to date therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field they are in a regional scale the north atlantic seafloor spreading the northward motion of the african and the arabian plate and obviously subregionally increased stiffness of the apulian promontary as well as of the bohemian massif

multifrequency polarimetric synthetic aperture radar observations of sea ice the first known fully polarimetric airborne synthetic aperture radar sar data set of sea ice is introduced images were acquired in the beaufort bering and chukchi seas in march 1988 during a campaign for validation of defense meteorological satellite program special sensor microwave imager radiometer ice products statistics of the magnitude phase and polarization of complex backscattered signals recorded by the jet propulsion laboratory three frequency sar are examined in detail for various scenes with different ice characteristics the full stokes matrix information generated from c l and p band data characterize the scattering behavior of different ice types polarization ratios and phase differences between linear copolarized returns are used for discrimination between particular image features and mechanisms are suggested for the observed polarimetric characteristics results indicate that combinations of frequency and polarization enhance current capability to distinguish ice of different properties using single frequency fixed polarization micro wave radar a specific example is the polarimetric identification of new ice formation which may not be easily distinguished in ers 1 5 3ghz vv polarization sar data such findings are consistent with theoretical model simulations of scattering characteristics of sea ice overall these preliminary results demonstrate that radar polarimetry will likely add a new dimension to our current capability for extracting geophysically important ice variables using radar remote sensing methods

classification of sea ice types in envisat synthetic aperture radar images in this paper sea ice in the central arctic has been classified in synthetic aperture radar sar images from envisat using a neural network nn based algorithm and a bayesian algorithm since different sea ice types can have similar backscattering coefficients at c band hh polarization it is necessary to use textural features in addition to the backscattering coefficients the analysis revealed that the most informative texture features for the classification of multiyear ice myi deformed first year ice fyi dfyi and level fyi lfyi and open water nilas are correlation inertia cluster prominence energy homogeneity and entropy as well as third and fourth central statistical moments of image brightness the optimal topology of the nn trained for envisat wide swath sar sea ice classification consists of nine neurons in input layer six neurons in hidden layer and three neurons in output layer the classification results for a series of 20 sar images acquired in the central part of the arctic ocean during winter months were compared to expert analysis of the images and ice charts the results of the nn classification show that the average correspondences with the expert analysis amount to 85 83 and 80 for lfyi dfyi and myi respectively the bayesian pixel based method can provide a higher resolution in the classified image and therefore better capability to identify leads compared to the nn method both methods may be effectively used in the central arctic where myi is predominant

the impact of early summer snow properties on antarctic landfast sea ice x band backscatter up to now snow cover on antarctic sea ice and its impact on radar backscatter particularly after the onset of freeze thaw processes are not well understood here we present a combined analysis of in situ observations of snow properties from the landfast sea ice in atka bay antarctica and high resolution terrasar x backscatter data for the transition from austral spring november 2012 to summer january 2013 the physical changes in the seasonal snow cover during that time are reflected in the evolution of terrasar x backscatter we are able to explain 76 93 of the spatio temporal variability of the terrasar x backscatter signal with up to four snowpack parameters with a root mean squared error of 0 87 1 62 db using a simple multiple linear model over the complete study and especially after the onset of early melt processes and freeze thaw cycles the majority of variability in the backscatter is influenced by changes in snow ice interface temperature snow depth and top layer grain size this suggests it may be possible to retrieve snow physical properties over antarctic sea ice from x band sar backscatter

method for detection of leads from sentinel 1 sar images the university of bremen the german excellence initiative deutsche forschungsgemeinschaft dfg

on the dynamics of the juan de fuca plate the juan de fuca plate is currently fragmenting along the nootka fault zone in the north while the gorda region in the south shows no evidence of fragmentation this difference is surprising as both the northern and southern regions are young relative to the central juan de fuca plate we develop stress models for the juan de fuca plate to understand this pattern of breakup another objective of our study follows from our hypothesis that small plates are partially driven by larger neighbor plates the transform push force has been proposed to be such a dynamic interaction between the small juan de fuca plate and the pacific plate we aim to establish the relative importance of transform push for juan de fuca dynamics balancing torques from plate tectonic forces like slab pull ridge push and various resistive forces we first derive two groups of force models models which require transform push across the mendocino transform fault and models which do not intraplate stress orientations computed on the basis of the force models are all close to identical orientations of predicted stresses are in agreement with observations stress magnitudes are sensitive to the force model we use but as we have no stress magnitude observations we have no means of discriminating between force models model stresses in the north and the south of the juan de fuca plate are similar breakup along the nootka fault zone and not in the gorda region can be explained if the local strength of a lithosphere column depends on lithospheric age stresses are invariant to whether or not transform push is acting on the mendocino transform fault we therefore conclude that the introduction of this extra plate driving force is not required by observations for the juan de fuca plate fl 2001 elsevier science b v all rights reserved

sea ice deformation state from synthetic aperture radar imagery part ii effects of spatial resolution and noise level c and l band airborne synthetic aperture radar sar imagery acquired at like and cross polarizations over sea ice under winter conditions is examined with the objective to study the discrimination between level ice and ice deformation features high resolution low noise data were analyzed in the first paper in this second paper the main topics are the effects of spatial resolution and signal to noise ratio airborne high resolution sar scenes are used to generate a sequence of images with increasingly coarser spatial resolution from 5 to 25 m keeping the number of looks constant the signal to noise ratio is varied between typical noise levels for airborne imagery and satellite data areal fraction of deformed ice and average deformation distance are determined for each image product at l band the retrieved values of the areal fraction get larger as the image resolution is degraded the areal fraction at c band remains constant the retrieved average distance between deformation features increases both at c and l bands as the image resolution gets coarser the influence of noise becomes noticeable if its level is equal or larger than the average intensity backscattered from the level ice the retrieval of deformation parameters using simulated images that resemble ers 2 sar envisat asar and alos palsar data products is discussed basic differences between real and simulated ers 2 sar images are analyzed

strike slip earthquakes in the oceanic lithosphere observations of exceptionally high apparent stress summary the radiated energies e s and seismic moments m0 for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses a e s m0 where is the modulus of rigidity the globally averaged a for shallow earthquakes in all tectonic environments and seismic regions is 0 3 mpa however the subset of 49 earthquakes with the highest apparent stresses a greater than about 5 0 mpa is dominated almost exclusively by strike slip earthquakes that occur in oceanic environments these earthquakes are all located in the depth range 7 29 km in the upper mantle of the young oceanic lithosphere many of these events occur near plate boundary triple junctions where there appear to be high rates of intraplate deformation indeed the small rapidly deforming gorda plate accounts for 10 of the 49 high a events the depth distribution of a which shows peak values somewhat greater than 25 mpa in the depth range 20 25 km suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere a recently proposed envelope for apparent stress derived by taking 6 per cent of the strength inferred from laboratory experiments for young less than 30 ma deforming oceanic lithosphere agrees well with the upper bound envelope of apparent stresses over the depth range 5 30 km the corresponding depth dependent shear strength for young oceanic lithosphere attains a peak value of about 575 mpa at a depth of 21 km and then diminishes rapidly as the depth increases in addition to their high apparent stresses which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10 30 km our set of high a earthquakes show other features that constrain the nature of the forces that cause interplate motion first our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of a for the two types secondly many of the intraplate events have focal mechanisms with the t axes that are normal to the nearest ridge crest or subduction zone and p axes that are normal to the proximate transform fault these observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high a earthquakes inside oceanic plates extant transform boundaries may be misaligned with current plate motion to accommodate current plate motion the pre existing plate boundaries would have to be subjected to large horizontal transform push forces a notable example of this is the triple junction near which the second large aftershock of the 1992 april cape mendocino california sequence occurred alternatively subduction zone resistance may be enhanced by the collision of a buoyant lithosphere a process that also markedly increases the horizontal stress a notable example of this is the aleutian trench near which large events occurred in the gulf of alaska in late 1987 and the 1998 march balleny sea m 8 2 earthquake within the antarctic plate

separability of sea ice types from wide swath c and l band synthetic aperture radar imagery acquired during the melt season abstract differentiating between first year ice fyi and multi year ice myi in c band synthetic aperture radar sar imagery during spring summer melt when wet snow and melt ponds mask the underlying ice is difficult it has been suggested that the use of l band sar may alleviate this concern given increased penetration depths at longer wavelengths however this has not been thoroughly assessed here the separability of fyi and myi is compared using horizontally polarized hh c band radarsat 2 and l band alos palsar scansar images acquired over landfast sea ice in the canadian arctic archipelago in the spring and summer of 2009 l band provided enhanced contrast between fyi and myi during early melt onset and during the drainage phase of advanced melt while c band was found to provide enhanced contrast when the wet snowpack was transitioning from the pendular regime to the funicular regime at the time of the pendular funicular transition the backscatter signatures of fyi and myi reversed at both c and l band this behavior is well established at c band but has not been reported previously at l band the l band imagery also provided improved definition of floe boundaries and ridges throughout the melt season finally the l band data had reduced speckle equivalent number of looks 12 relative to the c band data 9 equivalent looks these results indicate that l band sar data acquired during the melt season could be used to enhance operational and scientific sea ice information products that have traditionally been derived from single frequency c band sar data

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

comparing l and c band synthetic aperture radar estimates of sea ice motion over different ice regimes abstract estimating sea ice motion from synthetic aperture radar sar imagery at c band is the most reliable approach because of its high spatial resolution and ever increasing temporal resolution given the multiple current and upcoming sar platforms however there is still uncertainty in sar derived sea ice motion depending on the ice type and its thermodynamic state there have been suggestions mostly theoretical that use of l band sar and its inherent longer wavelength 15 30 cm and subsequent increased penetration capability could be beneficial for estimating sea ice motion especially during the melt season here we estimate and analyze sea ice motion for 9 pairs of c and l band sar imagery from radarsat 2 palsar 1 and palsar 2 located in the canadian arctic over a variety of sea ice types at different thermodynamic states results show that the increased signal penetration of l band sar into multi year ice myi during the melt season facilitates the detection of more motion vectors with stronger cross correlation coefficients compared to c band sar over newly formed ice and dry first year ice the reduced sensitivity to surface scattering and richer texture from l band sar imagery facilitates the detection of more motion vectors with stronger cross correlation coefficients compared to c band sar over dry myi l band provided stronger cross correlation coefficients but c band detected more motion vectors with a more representative spatial distribution with arctic sea ice continuing shift from a multi year to first year dominated icescape coupled with an increasing melt season length l band sar s ability to provide improved sea ice motion estimates during both the melt and freeze up time periods could prove even more useful in the coming decades

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

microwave and physical properties of sea ice in the winter marginal ice zone surface based active and passive microwave measurements were made in conjunction with ice property measurements for several distinct ice types in the fram strait during march and april 1987 synthetic aperture radar imagery downlinked from an aircraft was used to select study sites the surface based radar scattering cross section and emissivity spectra generally support previously inferred qualitative relationships between ice types exhibiting expected separation between young first year and multiyear ice gradient ratios calculated for both active and passive data appear to allow clear separation of ice types when used jointly surface flooding of multiyear floes resulting from excessive loading and perhaps wave action causes both active and passive signatures to resemble those of first year ice this effect could possibly cause estimates of ice type percentages in the marginal ice zone to be in error when derived from aircraft or satellite borne sensors

stress observations and driving force models for the south american plate stress observations are compared to predictions of driving force models for the south american plate stress observations have the potential to give detailed information about the forces driving the plates in that sets of forces with the same net torque can produce quite different stress patterns for a two dimensional vertical section model of a plate there are three conditions for dynamical equilibrium the net forces and the net torque about any point must all be zero these conditions are used to estimate the magnitudes of forces as a function of age with ridge push and slab pull as active forces and plate drag and slab resistance as passive forces the results are consistent with earlier published models and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces we generalize the model for the possible forces driving the south american plate and compare with observed stress measurements and kinematics the south american plate has a small slab pull component amounting to about one fifth the ridge push force with the other components reasonably isolated geographically thus the plate balance is largely between ridge push and plate drag this gives an opportunity of resolving the effects of trench suction or collision at the subduction zones most remarkable is the nearly uniform e w orientation of the regional stress field shmax directions which is extensional in the high andes a second feature is the n s orientations in the amazon basin the two dimensional horizontal stress patterns are compared for successive models of combinations of driving forces models with ridge push slab pull at the scotia and caribbean arcs and trench suction near the west coast balanced by plate drag produce similar stress patterns to those observed the magnitude of a possible trench suction force is estimated using a model with an eddy between the nazca slab and the overlying south american plate

a left lateral strike slip fault seaward of the oregon convergent margin we have mapped a recently active left lateral strike slip fault the wecoma fault on the floor of cascadia basin west of the oregon convergent margin using seamarc i sidescan sonar seabeam bathymetry and multichannel seismic and magnetic data the fault intersects the base of the continental slope at 45 10 n and extends northwest 293 for at least 18 5 km the fault s western terminus was not identified and the eastern end of the fault splays apart and disrupts the lower continental slope the fault extends to the base of the 3 5 km thick sedimentary section and overlies a basement discontinuity that may be related to movement along the wecoma fault prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left lateral separation allowing the general history of fault motion to be evaluated the fault s average slip rate since 10 24 ka is inferred to be 5 12 mm yr based on the age of an offset submarine channel surficial structural relationships in conjunction with the maximum inferred slip rate indicate that fault movement initiated at least 210 ka and that the fault has been active during the holocene

faulting and hydration of the juan de fuca plate system abstract multichannel seismic observations provide the first direct images of crustal scale normal faults within the juan de fuca plate system and indicate that brittle deformation extends up to 200 km seaward of the cascadia trench within the sedimentary layering steeply dipping faults are identified by stratigraphic offsets with maximum throws of 110 10 m found near the trench fault throws diminish both upsection and seaward from the trench long term throw rates are estimated to be 13 2 mm kyr faulted offsets within the sedimentary layering are typically linked to larger offset scarps in the basement topography suggesting reactivation of the normal fault systems formed at the spreading center imaged reflections within the gabbroic igneous crust indicate swallowing fault dips at depth these reflections require local alteration to produce an impedance contrast indicating that the imaged fault structures provide pathways for fluid transport and hydration as the depth extent of imaged faulting within this young and sediment insulated oceanic plate is primarily limited to approximately moho depths fault controlled hydration appears to be largely restricted to crustal levels if dehydration embrittlement is an important mechanism for triggering intermediate depth earthquakes within the subducting slab then the limited occurrence rate and magnitude of intraslab seismicity at the cascadia margin may in part be explained by the limited amount of water imbedded into the uppermost oceanic mantle prior to subduction the distribution of submarine earthquakes within the juan de fuca plate system indicates that propagator wake areas are likely to be more faulted and therefore more hydrated than other parts of this plate system however being largely restricted to crustal levels this localized increase in hydration generally does not appear to have a measurable effect on the intraslab seismicity along most of the subducted propagator wakes at the cascadia margin

geophysical observations of the northern juan de fuca ridge system lessons in sea floor spreading by virtue of its proximity to the coastline of north america and to numerous oceanographic institutions the juan de fuca ridge has been the focus of a large number of marine geological geochemical and geophysical investigations systematic studies began in the early 1960 s with the geophysical survey of a d raff and r g mason which provided much of the foundation for the development of the extraordinarily successful paradigms of sea floor spreading and plate tectonics subsequent systematic and detailed studies of the plates and plate boundaries of the area by investigators from many academic industrial and government agencies including the geological survey of canada have provided the basis for much of the fundamental understanding we now have of global plate motions and the processes that are involved in the creation of new oceanic crust at sea floor spreading centres much of the success of these studies can be attributed to the geological diversity found along the juan de fuca ridge clear

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

x c and l band sar signatures of newly formed sea ice in arctic leads during winter and spring abstract we examine an extensive synthetic aperture radar sar data set from the arctic ocean spanning a time period from january to june 2015 with the aim of identifying multi polarization parameters that can be used to accurately separate newly formed sea ice from the surroundings newly formed sea ice areas both provide favourable routing for ship traffic and are key to arctic climate science because they enable heat exchange between the ocean and the atmosphere our data set encompasses three different frequencies x c and l band at a range of incidence angles and were acquired under different environmental conditions our results suggest that by combining the scattering entropy and the co polarization ratio we can successfully separate the newly formed sea ice from open water and thicker sea ice within all three frequencies throughout the winter and spring season we observe a high correlation between scattering entropy values calculated using quad polarization c and l band data and scattering entropy values calculated using the same scenes reduced to the co polarization channels hh and vv we therefore conclude that dual polarization hh and vv x band scenes can be directly used to complement quad polarimetric c and l band scenes for studies of newly formed sea ice to confine the quad polarimetric data sets to their co polarization channels one can ensure a higher signal to noise ratio incidence angles below 35 are needed to keep the signal to noise ratios sufficiently high for the scattering entropy and co polarization ratio due to its lack of incidence angle dependency the polarization difference can provide additional support in newly formed sea ice studies the regular coverage of the arctic ocean with c band sar means that such scenes should to be included in any automatic monitoring however x and l band sar can based on their difference in penetration depth provide additional information about newly formed sea ice types and surface structure

structure and seismotectonics of the mendocino triple junction california 1 we analyze and interpret two marine multichannel seismic reflection onshore offshore ocean bottom seismometer obs seismic profiles that cross in the region of the mendocino triple junction over the fault plane of the 1992 petrolia california ms 7 1 earthquake from the offshore reflection data and the wide angle constraints provided by the onshore offshore and obs data we have determined a rough three dimensional view of crustal and uppermost mantle structure beneath the triple junction region this structure is also consistent with gravity data the upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the franciscan terrane a ubiquitous lower crustal layer with velocities of 6 5 7 1 km s 1 is 5 7 km thick throughout most of the region over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick this area is overlain by undisturbed pre 11 ma sediments indicating little deformation since that date the thick lower crust extends beneath the coast suggesting that the deep boundary between pacific and north american terranes lies onshore the northeastern edge of the thick lower crustal layer is marked by concentrated seismicity with the hypocenter of the 1992 petrolia earthquake lying close to the ne corner we speculate that at depth the pacific lower crust acts an asperity acting to concentrate stress from which the petrolia earthquake ruptured

geophysical and atmospheric controls on ku x and c band backscatter evolution from a saline snow cover on first year sea ice from late winter to pre early melt abstract this study investigates changes in ku x and c band microwave backscatter co polarized ratio and dual frequency ratios to changes in in situ measured snow thermophysical properties driven by surface radiation measurements during a 7 day transition from late winter to pre early melt on a highly saline snow cover on smooth first year ice a surface based ku x and c band microwave scatterometer system is used near coincident with in situ thermophysical snow measurements a frequency dependent penetration depth model is utilized to simulate diverse variations in ku x and c band penetration during fluctuating snow thermophysical and atmospheric conditions these results helped improve the interpretation of observed changes in ku x and c band backscatter overall dielectric loss associated with presence of brine throughout the snow volume is observed to be the governing factor affecting microwave penetration our results indicate significant differences in observed microwave backscatter and derived co polarized ratio and dual frequency ratios for all three frequencies during the transition from cold late winter phase to warm pre early melt onset phase c band demonstrates the greatest increase in backscatter and co pol ratios followed by x and ku bands the dual frequency ratio combinations clearly demonstrate their ability to separate sensitivities in frequencies by characterizing changes in backscatter during the 7 day transition our results indicate strong direct influence of downwelling shortwave and longwave radiation on snow brine volume and dielectric properties which drives changes to ku x and c band microwave backscatter and its associated co polarized ratio and dual frequency ratios our results demonstrate the ability of an observational multi frequency active microwave approach to illustrate frequency polarization and incidence angle dependent changes in microwave backscatter from snow covers on first year ice at varying atmospheric and snow thermophysical conditions

diffusion of methane and ethane through the reservoir cap rock implications for the timing and duration of catagenesis to maintain natural gas reserves for even short periods of geologic time there must be an influx of gas to the reservoir to balance any loss because molecular diffusion through the reservoir cap rock is generally regarded as the slowest loss mechanism the minimum influx rate necessary to maintain the reserves is the same as the diffusive loss rate fick s first law of diffusion modified with the stokes einstein relation and the archie equation provides a conservative estimate for the rate of diffusion through a porous medium such as the reservoir cap rock as an example the diffusive flux of methane and ethane through the cap rock of the mcclave field in southeastern colorado was calculated assuming a shale cap rock porosity of only 5 the entire volume of methane would have to be replaced in less than 2 3 m y and 5 3 m y for ethane to maintain the reserves using 10 porosity the replacement time falls to less than 500 000 yr for methane and 1 2 m y for ethane unless natural gas fields are short lived ephemeral phenomena there must be ongoing generation and migration of gas to the reservoir to at least balance the diffusional loss this balance requires that catagenesis be an ongoing and recent phenomenon

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

simplified analysis of horizontal stresses in a buttressed forearc sliver at an oblique subduction zone forearc slivers are commonly present at obliquely convergent margins such slivers are often buttressed at their leading edges but no quantitative model of the buttress effect exists i consider the horizontal stresses in a buttressed sliver using a simple plan view two dimensional elastic model the driving force created by the subducting plate is approximated by a body force in the direction of plate convergence an airy stress function solution relates the stress field to the boundary and body forces that allows the relative importance of different forces driving and resisting the sliver to be evaluated oblique subduction can easily cause arc parallel compression in the buttressed sliver especially near the leading edge because the arc parallel component of the driving force by the subducting plate is integrated over a much larger distance than its arc normal component for the southwest kuril arc the stress field predicted by the simple model agrees with that inferred from geological structures it is possible that the model can also explain the stress field of the cascadia forearc as constrained by earthquake focal mechanisms and in situ stress measurements

revisions to the age of the brunhes matuyama boundary and the pleistocene geomagnetic polarity timescale the currently referenced age of 730 ka for the brunhes matuyama b m geomagnetic polarity boundary mankinen and dalrymple 1979 is based largely on k ar and paleomagnetic data from postcollapse rhyolites in the valles caldera new mexico doell and dalrymple 1966 doell et al 1968 dating of sanidine from valles rhyolites by the 40a 39ar laser fusion technique indicates that samples critical to defining the b m boundary are up to 90 ka older than previously thought the new data combined with earlier results yield an age for the b m boundary of 780 10 ka 40ar 39ar ages from other samples suggest the jaramillo normal polarity subchron extended from 1010 to 915 ka

combined observations of arctic sea ice with near coincident colocated x band c band and l band sar satellite remote sensing and helicopter borne measurements in this study we compare colocated near coincident x c and l band fully polarimetry sar satellite images with helicopter borne ice thickness measurements acquired during the norwegian young sea ice 2015 n ice2015 expedition in the region of the arctic ocean north of svalbard in april 2015 the air borne surveys provide near coincident snow plus ice thickness surface roughness data and photographs this unique data set allows us to investigate how the different frequencies can complement one another for sea ice studies but also to raise awareness of limitations x band and l band satellite scenes were shown to be a useful complement to the standard sar frequency for sea ice monitoring c band for lead ice and newly formed sea ice identification this may be in part be due to the frequency but also the high spatial resolution of these sensors we found a relatively low correlation between snow plus ice thickness and surface roughness therefore in our dataset ice thickness cannot directly be observed by sar which has important implications for operational ice charting based on automatic segmentation

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

incidence angle dependence of hh polarized c and l band wintertime backscatter over arctic sea ice synthetic aperture radar sar incidence angle has a significant effect on the microwave backscatter from sea ice this paper investigates the incidence angle dependence of c and l band hh polarized microwave backscatter coefficient over arctic first year sea ice fyi and multiyear sea ice myi in winter advanced land observation satellite phased array type l band sar l band and radarsat 2 c band images are used to derive ice type specific incidence angle dependencies calculated using linear regression models for l band mean ice type specific incidence angle dependencies for fyi and myi are 0 21 and 0 30 db 1 respectively and for c band they are 0 22 and 0 16 db 1 respectively to validate our results we calculated root mean square deviation rmsd by comparing the ice type specific dependence from 2010 with individual dependencies from 2009 based on ice types and frequencies the rmsd is found to be smaller than the standard deviation of ice type specific dependencies for both frequencies the rmsd values for the l band incidence angle dependencies are 0 03 and 0 04 db 1 for fyi and myi respectively for c band the rmsd values for the fyi and myi dependencies are 0 03 and 0 01 db 1 respectively subsequently we demonstrate that after applying incidence angle normalization the variability of c and l band sar backscatter reduces and separability of ice types increase substantially

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

delineation and interpretation of seismotectonic domains in western north america indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western north america the san andreas east central california ecc mendocino triple junction mtj and pacific northwest pnw domains are found to have similar stress fields although the style and rate of deformation in each is different we propose that all of these domains are generated by shear between the pacific and north american plates in the mtj and pnw domains this shear may be evidence for the initiation of a transform fault connection between the san andreas and the queen charlotte island fault systems the ecc a region including much of the sierra nevada appears to result from the relatively stiff nature of the san andreas fault system the big bend and region of relatively new faulting in northern california act to make the san andreas fault system appear stiff on a regional scale two other domains the northern basin and range nbr and the intermountain seismic belt isb show evidence of being controlled by more local forces the nbr has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the juan de fuca north america plate boundary the isb can be explained by readjustments between the colorado plateau and the basin and range province which may be caused by the lowering of the basin and range due to extension

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

an arctic sea ice multi step classification based on gnss r data from the tds 1 mission abstract this study examines the potential of using bistatic radar reflections from the global navigation satellite system gnss to classify sea ice types in the arctic ocean during the sea ice formation period for this study we used data obtained from the united kingdom uk techdemosat 1 tds 1 satellite mission the main objective of the tds 1 mission is to provide l band radar reflections of the ocean surface in order to infer wind speed through an estimation of the ocean surface roughness given the orbit inclination of tds 1 polar coverage is obtained providing datasets over the arctic sea ice cover recent studies demonstrated the use of tds 1 data for accurately deriving ocean ice detection using the shape of the bistatic radar waveforms derived from delay doppler maps we aim to advance this previous research by demonstrating the sensitivity of gnss bistatic radar signals to sea ice types the originality of the study presented in this manuscript is the classification of sea ice types never implemented before with any gnss r mission for this study we examined the fall period of october 2015 in the beaufort and chukchi seas region when considerable expanses of young ice first year ice fyi and multi year ice myi were present we developed a sea ice multi step classification approach based on bistatic radar reflections to generate not only an ocean ice classification but to classify between the three dominant ice types present the classification was done in multiple steps based on gnss bistatic radar observations only and includes checks for spatio temporal consistency with each dominant class validation results are based on the comparison of the gnss classification against sar derived sea ice type maps produced at the us national ice center first we derived classifications of sea ice open water samples with a success rate of 97 comparable to previous studies next the sea ice type classification results identified fyi myi and young ice with success rates of 70 82 and 81 respectively indicating a strong sensitivity of the l band gnss bistatic radar signals to the different surface scattering properties of these primary ice types with this manuscript we demonstrate the potential of the gnss bistatic radar signals to classify ice types during the sea ice formation period gnss bistatic radar signals provide unique forward scattered measurements at l band potentially on a daily temporal revisit that can be used to produce enhanced information on sea ice characteristics such as those currently generated by other sensors in order to determine the use of gnss r measurements on an operational basis further studies are needed over a year long period of sea ice growth and melt to determine the full benefit of gnss for improving and providing complementary information to that generated by both microwave and optical sensors

a model for large scale plastic yield of the gorda deformation zone the lithosphere east of the gorda ridge often called the gorda plate but referred to here as the gorda deformation zone gorda dz is constricted as it spreads eastward to be subducted beneath the north american plate for the plane strain conditions defined by the seismicity the distortion of the gorda dz is analogous to the ironing of metal cups here the punch boundary is the blanco fracture zone and the die that does the cutting is the mendocino transform fault from the gorda ridge to the mendocino triple junction this analogy is useful in analyzing this problem but is not identical due to differences in boundary constraints the deformation of the gorda dz is determined for an elastoplastic lithosphere with a von mises yield criterion plate motion is constrained to be parallel both to the blanco fracture zone and to the mendocino transform fault west of 1250w the gorda ridge is considered a free boundary unlike the ironing analogy and the lithosphere is pulled through the constriction from the trench any nonlinear feedback between distortion and ridge push forces is not considered traction due to slip along the mendocino transform is varied to fit both seismicity and plate distortion for these conditions the amount of plate movement required before a constant resistance is achieved depends upon the ratio of elastic moduli to plastic yield stress and may vary from hundreds of meters to kilometers the minimum slab pull for steady movement occurs when the transform is weak lubricated and is 60 of the average plate yield stress the yield stress is determined using the temperature structure inferred from plate age and the laboratory constraints on brittle and ductile strength for olivine for an average plate age of 3 m y the yield stress is about 550 mpa so the pull required is 330 mpa when the transform is about as strong as the lithosphere the pull required is twice this value the stress directions in the gorda dz can be inferred from both deformation and seismicity the current seismic data are not precise enough to differentiate between a strong and weak transform but the deformation data are sufficient the constraints placed by distortion of magnetic isochrons on the modeling results indicate a mixed history about 5 ma changes in the position of the pole of rotation of the juan de fuca system began to increase the convergence of the gorda dz with the mendocino transform fault increasing the effective normal stress and brittle shear stress across the fault a strong fault is evident in the deformation data until about 2 ma and this corresponds to the time when convergence stabilized with a stable configuration the analysis indicates that about a kilometer of relative slip reduced the apparent strength of the transform to negligible values compared to the strength of the plate

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

sea ice deformation state from synthetic aperture radar imagery part i comparison of c and l band and different polarization in this paper we present a quantitative comparison of l and c band airborne synthetic aperture radar imagery acquired at like and cross polarizations over deformed sea ice under winter conditions the parameters characterizing the deformation state of the ice are determined at both radar bands and at different polarizations the separation of deformed and level ice is based on a target detection technique the threshold is set such that image pixels with intensities equal to or larger than the highest 2 of the level ice intensity distribution are classified as deformed ice independent of the radar configuration and ice conditions optical imagery of sufficient quality for comparison is available only in a very few cases to characterize the deformation state the areal fraction of deformation features and the average distance between these features are evaluated the values obtained for both parameters are very sensitive to the radar frequency aeral fractions are larger and average distances are smaller at l band than at c band because of the much higher intensity contrast between the deformed and level ice at l band the differences between polarizations at one radar band are smaller but not always negligible in comparison to optical images it was observed that deformed ice areas can be distinguished from level ice over their whole length and extension at l band whereas at c band often only prominent parts are visible

plate deformation at depth under northern california slab gap or stretched slab plate kinematic interpretations for northern california predict a gap in the underlying subducted slab caused by the northward migration of the pacific north america juan de fuca triple junction however large scale decompression melting and asthenospheric upwelling to the base of the overlying plate within the postulated gap are not supported by geophysical and geochemical observations we suggest a model for the interaction between the three plates which is compatible with the observations in this slab stretch model the juan de fuca plate under coastal northern california deforms by stretching and thinning to fill the geometrical gap formed in the wake of the northward migrating mendocino triple junction the stretching is in response to boundary forces acting on the plate the thinning results in an elevated geothermal gradient which may be roughly equivalent to a 4 ma oceanic lithosphere still much cooler than that inferred by the slab gap model we show that reequilibration of this geothermal gradient under 20 30 km thick overlying plate can explain the minor neogene volcanic activity its chemical composition and the heat flow in contrast to northern california geochemical and geophysical consequences of a true slab gap can be observed in the california inner continental borderland offshore los angeles where local asthenospheric upwelling probably took place during the miocene as a result of horizontal extension and rotation of the overlying plate the elevated heat flow in central california can be explained by thermal reequilibration of the stalled monterey microplate under the coast ranges rather than by a slab gap or viscous shear heating in the mantle

dynamics of the terra nova bay polynya the potential of multi sensor satellite observations research on processes leading to formation maintenance and disappearance of polynyas in the polar regions benefits significantly from the use of different types of remote sensing data the sentinels of the european space agency esa together with other satellite missions provide a variety of data from different parts of the electromagnetic spectrum at different spatial scales and with different temporal resolutions in a case study we demonstrate the advantage of merging data from different spaceborne instruments for analysing ice conditions and ice dynamics in and around the frequently occurring terra nova bay polynya tnbp in the ross sea in the antarctic starting with a list of polynya parameters that are typically retrieved from satellite images we assess the usefulness of different sensor types on regional scales several 100 km passive microwave radiometers provide a view on the mutual influence of the three ross sea polynyas on sea ice drift and deformation patterns optical sensors with meter scale resolution on the other hand allow very localized analyses of different polynya zones the combination of different ranges of the electromagnetic spectrum is essential for recognition and classification of ice types and structures radar images together with data from thermal infrared sensors operated at tens to hundreds of meters resolution improve the separation of the outlet zone of the polynya from the adjacent pack ice the direct comparison of radar and passive microwave images reveals the visibility of deformed ice zone in the latter a sequence of radar images was employed to retrieve ice drift around the tnb which allows analysing the temporal changes of the polynya area and the extension and structure of the outlet zone as well as ice movements and deformation that are influenced by the katabatic winds

mapping sea ice types from sentinel 1 considering the surface type dependent effect of incidence angle automated classification of sea ice types in synthetic aperture radar sar imagery is complicated by the class dependent decrease of backscatter intensity with incidence angle ia in the log domain this decrease is approximately linear over the typical range of space borne sar instruments a global correction does not consider that different surface types show different rates of decrease in backscatter intensity here we introduce a supervised classification algorithm that directly incorporates the surface type dependent effect of ia we replace the constant mean vector of a gaussian probability density function in a bayesian classifier with a linearly variable mean during training the classifier first retrieves the slope and intercept of the linear function describing the mean value and then calculates the covariance matrix as the mean squared deviation relative to this function the ia dependence is no longer treated as an image property but as a class property based on training and validation data selected from overlapping sar and optical images we evaluate the proposed method in several case studies and compare to other classification algorithms for which a global ia correction is applied during pre processing our results show that the inclusion of the per class ia sensitivity can significantly improve the performance of the classifier

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

seismic anisotropy beneath the juan de fuca plate system evidence for heterogeneous mantle flow here we use sks shear wave splitting observations from ocean bottom seismometer data to infer patterns of mantle deformation beneath the juan de fuca plate and its adjoining boundaries our results indicate that the asthenosphere beneath the juan de fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone geographically restricted deviations from this pattern indicate the presence of secondary processes at discrete plate boundaries such as the blanco transform fault seismic anisotropy is attributed to relative plate motion within a narrow zone 50 km beneath the deforming southern gorda plate region a diffuse plate boundary splitting observations similarly suggest deformation dominated by relative motion between the rigid juan de fuca and pacific plates but distributed over a broad zone 200 km our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback as suggested by onshore studies instead reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern cascadia

observations of sar polarimetric parameters of lake and fast sea ice during the early growth phase abstract this study examines links between two sets of parameters obtained from radarsat 2 quad pol data and geophysical information of lake ice and landfast sea ice in the resolute bay area nunavut canada the first set includes the three orthogonal backscatter coefficients hh0 vv0 vh0 in addition to the total backscatter power span the second set includes polarimetric decomposition parameters obtained using cloude pottier and yamaguchi decomposition methods data were collected during the early freezing period from 20 september until 28 december 2017 meteorological and ice climatological data from the weather station of resolute bay were used to facilitate this task ice thickness and salinity were obtained from established empirical models results show that single bounce and multiple bounce scattering mechanisms are active in lake ice during the entire freezing period while multiple bounce scattering is active in sea ice only during the initial freezing backscatter parameters can be used to determine the thickness of lake ice up to any depth while both backscatter and the tested decomposition parameters are related to the thickness of young sea ice 0 6 is found to be associated with comparable single and multiple bounce scattering mechanisms hence can be used to identify new ice orthogonal backscatter coefficients and polarimetric decomposition parameters provide different information yet their combination offers clues to better identify ice conditions and investigate questions about scattering mechanisms

stress origins and earthquake potentials in cascadia focal mechanism solutions for shallow earthquakes throughout the cascadia plate system indicate that the primary regional stress is northerly compression even though the juan de fuca plate generally is thought to be subducting n50 e this compressional stress is pervasive throughout the gorda juan de fuca explorer plate system and much of the adjoining section of north american plate modeling using a discrete element code shows that this stress primarily is due to the pacific plate being driven into the gorda block and juan de fuca plate at the mendocino and blanco fracture zones this plate collision causes compression of the offshore plate system northward through the 45 w trending vancouver island and leads to transfer of northerly compression into the overriding plate the northerly compression in the cascadia plate system has caused large crustal earthquakes in the regions of vancouver and northern california and may be capable of causing large earthquakes also in and offshore of washington and oregon several independent lines of evidence confirm that the cascadia subduction interface is coupled strongly and that subduction primarily must be accompanied by significant earthquakes rather than occurring aseismically for numerous earthquakes within the subducted plate focal mechanisms indicate extensional stresses that trend downdip these earthquakes reflect plate extension that is due to sinking of the more deeply subducted plate and to plate motion being resisted at the shallow subduction interface the slab pull force of the sinking plate also can lead to earthquakes at the shallow subduction interface and the potential exists for m 7 5 8 0 subduction earthquakes to occur at segments of the subduction boundary at washington and vancouver and less frequently so at segments of the subduction boundary at oregon the in plate driving forces of the offshore cascadia plate system do not appear to contribute significantly to regional stresses stresses and earthquakes at the cascadia province primarily result from the action of the pacific plate on the gorda juan de fuca explorer plate system a transfer of this action into the overriding north american plate and the independent sinking and slab pull force of the subducted oceanic lithosphere

an explanation for the double seismic layers north of the mendocino triple junction we propose that the gently eastward dipping double planed seismic zone observed at 15 25 km depths in the southern cascadia subduction zone just north of the mendocino triple junction is a direct consequence of the thermally controlled rheology as the oceanic lithosphere subducts to a depth of about 15 km the temperature regime causes a brittle plastic transition to occur within the oceanic crust thus a ductile layer forms in the lower oceanic crust sandwiched between the brittle upper crust and brittle upper mantle the very high strain rates near the triple junction caused by the northward push of the pacific plate on the gorda plate increase the seismicity and thus accentuate the double seismic zone in this region this model explains the focal mechanisms observed in the seismic zone and their spatial change the double seismic layers clearly define the position of the subducting gorda plate previously uncertain in the cape mendocino region

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

direction of maximum horizontal compression in western oregon determined by borehole breakouts elliptical borehole enlargements or breakouts caused by systematic spalling of a borehole wall were identified in 18 wells drilled in the coast range and willamette valley of western oregon the breakouts generally indicate a nnw to nne orientation of maximum horizontal compression hmax that agrees with the predominant direction of hmax determined from earthquake focal mechanisms from post middle miocene structural features and from alignments of holocene volcanic centers in the pacific northwest however this orientation is inconsistent with the n50 e convergence between the juan de fuca and north american plates determined by riddihough 1984 from juan de fuca plate magnetic lineations as young as 730 ka the brunhes matuyama boundary the predominant nnw to nne orientation of hmax may be due to the complex interaction of a northwestward moving pacific plate driving into the gorda and juan de fuca plates and indirectly transmitting n s compression across the strongly coupled cascadia subduction zone into the overriding north american plate spence 1989 alternatively the predominant nnw to nne orientation of hmax may be due to a landward counterclockwise rotation of the direction of hmax from n50 e compression offshore to n s compression in the coast range

coupling of tectonic loading and earthquake fault slips at subduction zones because of the viscoelastic behaviour of the earth accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales if the lithospheric plate is acted upon by distant tectonic forces strain accumulation must occur in a broad region however an earthquake releases strain only in a region comparable to the size of the rupture area a two dimensional finite element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips a fault lock and unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed but determined by the accumulated stress the amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation as well as the rheology of the rock material the larger the region of strain accumulation is compared to the earthquake rupture the smaller is the earthquake fault slip the reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate since rapid permanent plate shortening is not observed at subduction zones there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles the former can be achieved only by significant aseismic fault slip between large subduction earthquakes the most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault which requires significant aseismic fault slip between asperities

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important