global positioning system measurements of strain accumulation across the imperial valley california 1986 1989 global positioning system gps data collected in southern california from 1986 to 1989 indicate considerable strain accumulation across the imperial valley displacements are computed at 29 stations in and near the valley from 1986 to 1988 and at 11 sites from 1988 to 1989 the earlier measurements indicate 5 9 1 0 cm yr right lateral differential velocity across the valley although the data are heavily influenced by the 1987 superstition hills earthquake sequence the 1988 1989 gps displacements are best modeled by 5 0 0 9 cm yr of valley crossing deformation but rates calculated from 15 years of terrestrial geodetic measurements 3 7 0 1 cm yr fit the data nearly as weh1 the relatively high gps rates are suspect because some measurements especially the east trending displacements have large errors there is evidence from gps and very long baseline interferometry observations that the present strain rate along the southern san andreas fault is smaller than the long term geologic estimate suggesting a lower earthquake potential than is currently assumed correspondingly a higher earthquake potential is suggested for the san jacinto fault

regional deformation of the earth model for the los angeles region california abstract a mathematical model for historical horizontal crustal deformation is presented for the los angeles region of california model parameters are estimated from triangulation trilateration and astronomic azimuth data observed during the past 120 years also geologic and seismic data are used indirectly in specifying the model which relates crustal deformation to strain rates over large geographic areas secular slip rates along geologic faults and to episodic movements associated with large m 6 earthquakes the episodic movements are modeled in accordance with the theory of dislocation in an elastic halfspace for the secular motion the region is partitioned into a mosaic of proposed microplates or districts that are allowed to individually translate rotate and deform homogeneously as a linear function of time the model thus accounts for the secular and episodic time variability of the geodetic markers coordinates the results are compared with geodetic geologic and seismic studies by other investigators to evaluate the reliability of the procedure and the estimates in some cases the model provides the first geodetic determinations of strain rates slip rates directions of maximum right lateral shears and values of coseismic displacements experienced within the region the shear strain rates reveal that maximum right lateral shears are oriented parallel to the san andreas fault system and decrease in magnitude with distance from the central transverse ranges the directions of shear strain form a funnel shaped pattern that converges on the point outside the region where the san andreas garlock big pine and san gabriel faults meet the velocity for a point near san pedro relative to a point at the region s northeast corner is 1 8 0 6 cm yr in the direction n 15 w 11 these results are discussed in light of data limitations and modeling inadequacies subsidence due to oil and gas withdrawal and postseismic relaxation following the 1971 san fernando earthquake

application of the global positioning system to crustal deformation measurement 2 the influence of errors in orbit determination networks global positioning system gps measurements of a geodetic network in southern and central california have been used to investigate the errors introduced by adopting different sets of stations as fixed such fixed points called fiducial stations are necessary to eliminate the errors of imprecise satellites orbits which otherwise would dominate the error budget for distances greater than tens of kilometers these fiducial stations also define the reference frame of the crustal deformation network establishing the magnitude of the effect of changing the fiducial network is essential for crustal deformation studies so that these artifacts of the differences between fiducial networks used for the data analyses are not interpreted as geophysical signals solutions for a crustal deformation network spanning distances up to 350 km were computed with a variety of fiducial networks we use fiducial coordinates determined from very long baseline interferometry vlbi we compare these solutions by computing the equivalent uniform strain and rotation that best maps one solution into another if we use a continental scale fiducial network with good geometry the distortions between the solutions are about 10 8 largely independent of the exact choice of stations the one case of a large scale fiducial network where the distortions are larger is when the three fiducial stations chosen all lie close to a great circle use of a fiducial network no larger than the crustal deformation network tan produce apparent strains of up to 10 7 our work suggests that fiducial coordinates determined from gps data analysis may be used although they should be determined using a consistent reference frame such as provided by vlbi and satellite laser ranging

summer enhancement of arctic sea ice volume anomalies in the september ice zone abstractbecause of its persistence on seasonal time scales arctic sea ice thickness sit is a potential source of predictability for summer sea ice extent sie new satellite observations of sit represent an opportunity to harness this potential predictability via improved thickness initialization in seasonal forecast systems in this work the evolution of arctic sea ice volume anomalies is studied using a 700 yr control integration and a suite of initialized ensemble forecasts from a fully coupled global climate model this analysis is focused on the september sea ice zone as this is the region where thickness anomalies have the potential to impact the sie minimum the primary finding of this paper is that in addition to a general decay with time sea ice volume anomalies display a summer enhancement in which anomalies tend to grow between the months of may and july this summer enhancement is relatively symmetric for positive and negative volume anomalies and peaks in july regardless of the initi

sea surface height determination in the arctic ocean from ers altimetry 1 accurate sea surface height measurements have been extracted from ers altimeter data in sea ice covered regions for the first time the data have been used to construct a mean sea surface of the arctic ocean between the latitudes of 60 degreesn and 81 5 degreesn based on 4 years of ers 2 data an rms value for the crossover differences of mean sea surface profiles of 4 2 cm was observed in the ice covered canada basin compared with 3 8 cm in the ice free greenland iceland norwegian seas comparisons are made with an existing global mean sea surface osumss95 highlighting significant differences between the two surfaces in permanently ice covered seas in addition we present the first altimeter derived sea surface height variability map of the arctic ocean comparisons with a high resolution coupled ocean sea ice general circulation model reveal a good qualitative agreement in the spatial distribution of variability quantitatively we found that the observed variability was on average a factor of 3 4 greater than model predictions

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

increased arctic sea ice volume after anomalously low melting in 2013 changes in arctic sea ice volume are difficult to quantify five years of satellite data reveal a reduction in autumn sea ice volume in 2010 2012 but a sharp increase in 2013 and 2014 suggesting that ice volume can recover quickly

strain accumulation in western united states this review is principally concerned with recent geodetic strain measure ments in western united states undertaken by the us geological survey as part of the earthquake studies program and as a consequence is heavily biased toward the author s own publications most of the publications reporting crustal strain measurements in western united states prior to about 1968 have been compiled in one volume national geodetic survey 1973 and more recent work with complete bibliographies is summarized in three successive quadrennial reports to the international union of geodesy and geophysics meade 1971 savage 1975 thatcher 1979b the following conventions are employed in this paper strain a dimensionless quantity is reported in units of 10 6 extension is taken as positive to distinguish between engineering and tensor shear strain we denote the former by y and quote the units as j lrad whereas the latter is denoted by e and given the units of j lstrain uncertainties in all cases are quoted as one standard deviation at the present time geodetic techniques furnish the only reliable measure of strain accumulation in the seismic regions of the world the superiority of geodetic techniques derives principally from the long baseline of the measurements difficulties in coupling fiducial marks to the earth have generally made short baseline measurements of strain accumulation questionable wyatt 1982 however long baselines introduce the dis advantage that the measured strain is not closely localized the current technique of measuring strain accumulation involves the

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

age genesis and paleoclimatic interpretation of the sangamon loveland complex in the lower mississippi valley u s a for more than a century the sangamon paleosol the sangamon has been an integral part of geologic and pedologic investigations in the central united states including the upper mississippi and lower missouri river valleys compositional pedologic micromorphologic stratigraphic and age data indicate that the prominent reddish paleosol developed in silt rich deposits of the lower mississippi valley from southernmost illinois to northwestern mississippi represents multiple periods of soil formation and is wholly or in part time equivalent to the sangamon of the central united states thermoluminescence data for localities where the sangamon developed in loess indicate that the primary period of loess deposition was from 190 to 130 ka oxygen isotope stage ois6 that loess deposition continued intermittently from 130 to 74 ka ois5 and that deposition was wholly or in part coeval with loveland loess deposition in the central united states beryllium 10 chemical and pedologic data indicate that in the lower mississippi valley 1 the sangamon represents a minimum time period of 60 80 k y 2 there were at least two periods of soil formation ca 130 90 ka and 74 58 ka ois4 and 3 rates of weathering and pedogenesis equaled or exceeded the net loess accumulation rate until at least 46 ka ois3 and resulted in development of a paleosol in the overlying basal roxana silt along a n s transect from southern illinois to western mississippi sangamon macroscopic characteristics as well as the micromorphology chemistry and mineralogy suggest a regional paleoclimate during periods of soil formation that 1 was warm to hot with a wider range in temperature precipitation and evapotranspiration than present 2 had seasonal to decadal or longer periods of drought and 3 had down valley southward trends of increasing temperature and precipitation and decreasing seasonality and variation in annual to decadal precipitation

sea ice thickness in the eastern canadian arctic hudson bay complex baffin bay abstract past observations of sea ice thickness in the eastern canadian arctic eca have generally been restricted to drill hole measurements at a few local sites on landfast ice here we use data from the laser altimeter icesat and the radar altimeter cryosat 2 to present a 14 year record 2003 2016 of high resolution and spatially extensive ice thickness observations for the eca and identify 12 sub regions with distinct patterns the mean sea ice growth rate within the seasonally ice covered eca from november to april is 23 cm mo 1 565 km 3 mo 1 with the fastest increase in thickness occurring through strong ice convergence and deformation in eastern hudson bay and foxe basin our results demonstrate characteristically asymmetrical distributions of sea ice thickness in both hudson bay and baffin bay but in opposing directions in hudson bay the spring ice cover is 40 cm thicker in the eastern region compared to the northwestern region whereas in baffin bay the ice is 20 cm thicker in the western half of the bay compared to the eastern half in hudson bay we find that years with strong and positive ice drift vorticity i e cyclonic and convergent conditions correlate with increasingly asymmetrical sea ice covers with the level of west east asymmetry varying from 2 to 11 cm per 100 km however in baffin bay the ice drift vorticity is typically negative i e anticyclonic and divergent with no obvious link to the asymmetry of the spring ice cover finally we estimate that large interannual variations in spring sea ice volume within the eca lead to 15 variations in the volume of freshwater available at the ocean surface during summer

horizontal deformation in the imperial valley california between 1934 and 1980 a triangulation trilateration survey was performed by the national ocean survey national geodetic survey four months prior to the october 15 1979 imperial valley m 6 6 earthquake the survey was repeated in february march 1980 to gauge the coseismic displacement these data combined with triangulation surveys performed in 1934 1935 1939 1941 1954 1955 and 1967 provide a historical record of crustal deformation over a time period that also includes the 1940 el centro m 7 1 earthquake the total data set was analyzed in terms of a dislocation model that relates the observed surface deformation to slip on prescribed geologic faults according to the model the 1941 1980 fault slip at depths 10 km and greater is 1 9 0 3 m this value is approximately twice the 1 0 0 1 m estimated for the 1941 1980 slip on the imperial fault in the 0 to 10 km depth range this discrepancy can be explained by a range of possibilities including the following 1 some of the deep fault slip is due to aftereffects of the 1940 earthquake in which case the seismic cycle over which deep slip and shallow slip are equal is not the interval between large earthquakes but is delayed a few years 2 the deep slip should be equated to the shallow slip over several parallel faults 3 the crust experienced permanent inelastic deformation and 4 deep deformation is accommodated by a mechanism other than slip

on the retrieval of sea ice thickness using smos polarization differences arctic sea ice is going through a dramatic change in its extent and volume at an unprecedented rate sea ice thickness sit is a controlling geophysical variable that needs to be understood with greater accuracy for the first time a sit retrieval method that exclusively uses only airborne sit data for training the empirical algorithm to retrieve sit from soil moisture ocean salinity smos brightness temperature tb at different polarization is presented a large amount of airborne sit data has been used from various field campaigns in the arctic conducted by different countries during 2011 15 the algorithm attempts to circumvent the issue related to discrimination between tb signatures of thin sit versus low sea ice concentration the computed sit has a rms error of 0 10 m which seems reasonably good as compared to the existing algorithms for analysis at the used 25 km grid this new sit retrieval product is designed for direct operational application in ice prediction climate models

spatio temporal variability of antarctic sea ice thickness and volume obtained from icesat data using an innovative algorithm abstract we use total sea ice plus snow freeboard as estimated from ice cloud and land elevation satellite icesat geophysical laser altimeter system glas observations to compute antarctic sea ice thickness and volume in order to overcome assumptions made about the relationship between snow depth and total freeboard or biases in snow depth products from satellite microwave radiometry we implement a new algorithm we treat the sea ice snow system as one layer with reduced density which we approximate by means of a priori information about the snow depth to sea ice thickness ratio we derive this a priori information directly from icesat total freeboard data using empirical equations relating in situ measurements of total freeboard to snow depth or sea ice thickness we apply our new algorithm one layer method or olm which uses the buoyancy equation approach without the need for auxiliary snow depth data to compute sea ice thickness for every icesat glas footprint from a valid total freeboard an improved method for sea ice volume retrieval is also used to derive ice volume at 6 25 km scale spatio temporal variations of sea ice thickness and volume are then analyzed in the circumpolar antarctic as well as its six sea sectors pacific ocean indian ocean weddell east weddell west bell amund sea and ross sea under both interannual and seasonal scales because the olm algorithm relies on only one parameter the total freeboard and is independent of auxiliary snow depth information it is believed to become a viable alternative sea ice thickness retrieval method for satellite altimetry

1991 1992 gps measurements across the nepal himalaya in march 1991 35 control points were measured in the collision zone between the indian and asian plates using the global positioning system gps twenty eight points are located in the nepal himalaya and 7 in s and e tibet in october 1992 6 of the nepal points were remeasured together with 3 bedrock points in bihar state india 5 points in e tibet and one in urumchi the average wrms gps position repeatability between 1991 and 1992 improved from 8 mm to 5 mm in the north 16 mm to 7 mm in the east and 32 mm to 25 mm in the vertical components himalayan convergence rates of 20 mm yr may be resolved to 10 accuracy within 5 years with these uncertainties unless improved vertical measurement techniques are introduced many decades must elapse before regional secular vertical motions may be resolved the 1991 3 1992 8 measurements are consistent with geologically determined himalayan convergence rates of less than 20 mm year

relationship between sea ice freeboard and draft in the arctic basin and implications for ice thickness monitoring we have confirmed our earlier finding that the probability density function pdf of ice freeboard in the arctic ocean can be converted to a pdf of ice draft by applying a simple coordinate transformation based on the measured mean draft and mean elevation this applies in each of six 50 km sections north of greenland of joint airborne laser and submarine sonar profile obtained along nearly coincident tracks from the arctic basin north of greenland and tested for this study detailed differences in the shape of the pdf can be explained on the basis of snow load and can in principle be compensated by the use of a more sophisticated freeboard dependent transformation the measured density ratio r actually mean draft mean elevation ratio for each section was found to be consistent over all sections tested despite differences in the ice regime indicating that a single value of r might be used for measurements done in this season of the year the mean value r from all six sections is 7 89 on the assumption that all six values are drawn from the same population the standard deviation is 0 55 for a single 50 km section and thus 0 22 for 300 km of track in attempting to infer ice draft from laser measured freeboard we would therefore expect an accuracy of about 28 cm in 50 km of track if mean draft is about 4 m and about 11 cm in 300 km of track these accuracies are compatible with the resolution of predictions from numerical models a simple model for the variability of r with season and with mean ice thickness gives results in reasonable agreement with observations they show that although there is a large seasonal variability due to snow load there is a stable period from november to april when the variability is chiefly dependent on the mean ice thickness alone thus in principle r can be mapped over the arctic ocean as a basis for interpreting survey data better field data are needed on the seasonal and spatial variability of three key quantities area averaged snow load mean density of first year and multiyear ice including the effect of ridging with these two ice regimes and density of near surface water

combined airborne laser and radar altimeter measurements over the fram strait in may 2002 knowledge of sea ice thickness is critical for the prediction of future climate and for assessing the significance of changes in thickness sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard however a lack of knowledge of snow depth introduces significant uncertainties into these calculations this paper compares the first coincident airborne laser and radar altimetry data over sea ice collected during the laser radar altimetry lara field campaign lara was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth two new methods were created to analyse these data sets a new radar retracker and a radar power simulator which models radar returns from the laser data we present the first quantitative analysis of data from the lara laser and radar altimeters and demonstrate the potential of combining laser and radar altimetry to estimate snow depth lara elevation estimates compare well with elevations from the radar altimeter onboard ers 2 at the sub meter level and the study provides lessons for future validation of satellite altimetry data over sea ice laser elevations are consistently higher than the radar elevations over snow covered sea ice as lara was a flight of opportunity no coincident in situ measurements were available nevertheless the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons

contemporary deformation of the wasatch fault utah from gps measurements with implications for interseismic fault behavior and earthquake hazard observations and kinematic analysis 1 contemporary strain accumulation rates of the 350 km long wasatch fault utah have been determined by gps measurements and provide key data for evaluating normal fault behavior and related earthquake hazard assessment this paper presents a complete description of the wasatch gps network data archiving and processing and the measured ground deformation the network spans the wasatch fault in an area 300 km long and 200 km wide in north and central utah and consists of 43 gps sites surveyed in 1992 1993 1994 1995 1999 2001 and 2003 plus 8 permanent gps stations operating continuously beginning 1997 observations across a 65 km wide area centered on the wasatch fault indicate the principal horizontal extension rate of 24 6 nstrain yr with the direction nearly perpendicular to the fault this strain rate corresponds to a horizontal displacement rate of 1 6 0 4 mm yr accommodating 50 of the crustal deformation across the 200 km wide eastern basin range analysis of the spatial variation of the strain rate field reveals that the strain accumulation is concentrated near the wasatch fault which suggests an abrupt transition in the horizontal deformation at the fault between the eastern basin range and the rocky mountains we employed a finite strain model that accounts for simple shear deformation of the hanging wall to compare horizontal extension rates measured by gps with vertical fault displacement rates determined by geologic data our results suggest that the average holocene strain release rate is higher than the contemporary strain loading rate for the wasatch fault if the fault dips less than 30 w but these two rates are consistent if the fault dips more steeply

fluvial activity in major river basins of the eastern united states during the holocene in the eastern united states existing paleo reconstructions in fluvial environments consist primarily of site specific investigations of climate and human impacts on riverine processes this paper

age and significance of fossil white spruce picea glauca tunica hills louisiana mississippi abstract radiocarbon dates indicate a farmdalian through woodfordian age for fossil white spruce picea glauca in the tunica hills a greater time span than previously documented these dates furthermore require revision of the ages of fluvial terrace deposits in the region spruce bearing terrace 1 sediments previously regarded as late woodfordian to holocene age are of farmdalian to late holocene age terrace 2 sediments previously assigned a sangamonian or farmdalian age are of probable altonian age plant fossils in terrace 1 sediments represent two climatically and temporally distinct assemblages a cool temperate farmdalian woodfordian assemblage dominated by white spruce but including deciduous hardwoods suggestive of an oak hickory association and a warm temperate holocene assemblage composed of southern hardwoods and nonarboreal species representative of the modern flora of the tunica hills locally fossils of white spruce have been reworked into younger terrace 1 sediments containing the warm temperate plant assemblage the transition from the cool temperate flora to the modern warm temperate flora in the tunica hills probably occurred around 12 000 yr b p

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

influences of relative sea level rise and mississippi river delta plain evolution on the holocene middle amite river southeastern louisiana abstract the holocene geomorphic history of southeastern louisiana s middle amite river is recorded in the stratigraphy of three alloformations identified in decreasing age as the watson wat denham springs ds and magnolia bridge mag the wat meander belt formed by at least 9000 yr b p when sea level was lower and the amite river was tributary to a larger ancestral drainage basin the ds became an active meander belt by at least 3000 yr b p in response to relative sea level rise and eastward progradation of the mississippi river delta plain the mag developed its meander belt in part during the european settlement of the drainage basin and is now attempting to adjust to modern anthropogenic influences geomorphic influences on the middle amite river floodplain have temporal and spatial components that induce regional and local scale effects regional extrinsic influences caused meander belt avulsion that produced alloformations however local influences produced intrinsic geomorphic thresholds that modified channel morphology within a meander belt but did not induce alloformation development base level influences of the relative sea level rise and the mississippi river delta plain were so dominant that the effects of possible climate change were not recognized in the holocene amite river system

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

slow inflation of the furnas volcano sao miguel azores suggested from initial leveling and global positioning system measurements the furnas volcano sao miguel island azores is an active stratovolcano with a 6 km diameter caldera in order to mitigate volcanic risk in the area crustal deformation research based on optical leveling tilt measurements and lake leveling was initiated at the volcano in 1991 progressive tilt as high as 4 rad yr has been observed but the measurements do not all fit a simple model the global positioning system gps was used to measure a network of fourteen stations at the volcano in 1993 and again in 1994 horizontal displacements in the one year period between the surveys are less than 1 cm but significant with 95 confidence stations are being displaced away from the caldera and strain analysis indicates areal dilation of 0 84 0 40 strain yr of the volcanic edifice the areal dilation can be explained by slight inflation of the volcano the location of the center of inflation is rather uncertain but based on both the gps and the leveling data the preferred location is in the northern part of the caldera a central uplift rate of 7 mm year is suggested

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

interdecadal changes in snow depth on arctic sea ice snow plays a key role in the growth and decay of arctic sea ice in winter it insulates sea ice from cold air temperatures slowing sea ice growth from spring to summer the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean impacting ice melt processes knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume and for understanding and modeling sea ice thermodynamics in the changing arctic this study assesses spring snow depth distribution on arctic sea ice using airborne radar observations from operation icebridge for 2009 2013 data were validated using coordinated in situ measurements taken in march 2012 during the bromine ozone and mercury experiment bromex field campaign we find a correlation of 0 59 and root mean square error of 5 8 cm between the airborne and in situ data using this relationship and icebridge snow thickness products we compared the recent results with data from the 1937 1954 1991 soviet drifting ice stations the comparison shows thinning of the snowpack from 35 1 9 4 to 22 2 1 9 cm in the western arctic and from 32 8 9 4 to 14 5 1 9 cm in the beaufort and chukchi seas these changes suggest a snow depth decline of 37 29 in the western arctic and 56 33 in the beaufort and chukchi seas thinning is negatively correlated with the delayed onset of sea ice freezeup during autumn

quaternary paleoecology of the lower mississippi valley copyright c 1996 elsevier science b v all rights reserved in 1938 clair a brown published his classic paleobotanical discoveries from the tunica hills of southeastern louisiana indicating ice age plant migrations of more than 1100km brown collected fossils of both boreal trees such as white spruce picea glaucar and southern coastal plain plants from deposits mapped as the port hickey prairier river terrace by harold n fisk subsequent revisions of terrace mapping radiocarbon dating and paleoecological analysis reconciled browns conceptual and stratigraphic mixingr of these two ecologically incompatible fossil plant groups an older terrace 2 of sangamonian to altonian ager contains the warm temperate assemblage a younger terrace 1 of farmdalian woodfordian and holocene ager includes full glacial and late glacial remains of both boreal and cool temperate plants and a warm temperate suite of plants dates from the holocene interglacial new plant fossil localities with radiocarbon chronologies are now available from within the lower mississippi valley of missouri and arkansas as well as from the adjacent ozark plateaus the interior low plateaus of kentucky and tennessee and the bordering blufflands of tennessee mississippi and louisiana these studies demonstrate that glacial and interglacial patterns of vegetation have been influenced by regional changes in climate glacial runoff and regime of the mississippi river

terrace stratigraphy in the tunica hills of louisiana abstract the age and number of important fossil producing late pleistocene terraces in the tunica hills have recently become quite controversial one hypothesis holds that only a single loess mantled farmdalian terrace flanks the streams in this area the other maintains that there are two terraces the youngest is considered essentially holocene in age while the older is sangamon in age radiometric and stratigraphic evidence collected for this study indicates that there are two terraces the youngest is late woodfordian to holocene in age while the other is farmdalian

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

strain probe best fitting a homogeneous strain to the motions of scattered points abstract in laboratory experiments or e g in studies of glacier motion one sometime wants to estimate a finite strain from images or surveys of scattered marker points taken at successive times similarly crustal geodetic strain can be estimated by measuring velocities of geodetic stations the strain probe presented here is a simple formula that calculates a best fit strain from the motions of n marker points distributed over an area or a volume and monitored in a cartesian reference frame specifically it is the product of two 2 2 or 3 3 in 3 d symmetric matrices whose components are populated from the known positions and the measured displacements of the markers two examples implemented in excel workbooks illustrate how to combine the strain probe with the processing of the data and of its results speck motions in a means urai see through experiment are an example of a finite strain calculation published geodetic site velocities 120 stations in central anatolia are an example of infinitesimal strain

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

evidence for thinning of the arctic ice cover north of greenland in may 1987 a british submarine carried out an ice profiling experiment in the arctic ocean in which the route closely approximated that of an earlier voyage in october 1976 over a zone extending more than 400 km to the north of greenland there is evidence of a significant decrease in mean ice thickness in 1987 relative to that found in 1976 this thinning amounts to a loss of volume of at least 15 over an area of 300 000 km2 the nature of the ice thickness distributions suggests that the thinning is primarily associated with the presence of a larger fraction of young and first year ice a consequence of wind driven divergence in the ice cover during the months preceding the 1987 observations such a large fluctuation in a region previously believed to possess consistently high ice thicknesses illustrates the importance of monitoring arctic ice thickness more systematically to determine whether such fluctuations fall within the limits of normal inter annual variability

fully focused sar altimetry theory and applications in this paper we introduce the concept develop the theory and demonstrate the advantages of fully focused coherent processing of pulse echoes from a nadir looking pulse limited radar altimeter this process similar to synthetic aperture radar sar imaging systems reduces the along track resolution down to the theoretical limit equal to half the antenna length we call this the fully focused sar ff sar altimetry processing the technique is directly applicable to sar altimetry missions such as cryosat 2 sentinel 3 or sentinel 6 jason cs the footprint of an ff sar altimeter measurement is a narrow strip on the surface which is pulse limited across track and sar focused along track despite the asymmetry of the altimeter footprint the fully focused technique may be useful for applications in which one needs to separate specific targets within highly heterogeneous scenes such as in the case of sea ice lead detection hydrology and coastal altimetry applications in addition over rough homogeneous surfaces such as the ocean or ice sheets the improved multilooking capability of ff sar leads to a significant increase in the effective number of looks with respect to the delay doppler processing resulting in better geophysical parameter estimation the proposed processing technique was verified by processing full bit rate cryosat 2 sar mode data over the svalbard transponder hydrology sea ice and open ocean applications are also demonstrated in this paper showing the improvement of this technique with respect to conventional and delay doppler altimetry

late wisconsinan vegetation and environment of the tunica hills region louisiana mississippi abstract pollen plant macrofossil and radiocarbon dating studies of seven exposures of fluvial sediments in the tunica hills region of southeastern louisiana and southwestern mississippi provide new information on late wisconsinan vegetation flora and environment of the region the assemblages date between 25 250 and 17 530 yr b p pollen and macrofossil assemblages are dominated by picea which comprises 40 70 of the pollen assemblages abies and larix pollen and macrofossils are absent in contrast to sites to the north in the central mississippi valley deciduous hardwoods quercus fagus fraxinus carya juglans nigra acer ulmus are minor components of both pollen and macrofossil assemblages radiocarbon dates of picea and quercus wood indicate that these two genera grew contemporaneously in the region regional upland forests were dominated by picea picea cones and cone fragments are not typical of any extant north american species and probably represent either an extinct species or an extinct variety or subspecies of picea glauca late wisconsinan climate of the region was cooler than present but not necessarily as cool as implied by p glauca or other boreal taxa

when will the summer arctic be nearly sea ice free 1 the observed rapid loss of thick multiyear sea ice over the last 7 years and the september 2012 arctic sea ice extent reduction of 49 relative to the 1979 2000 climatology are inconsistent with projections of a nearly sea ice free summer arctic from model estimates of 2070 and beyond made just a few years ago three recent approaches to predictions in the scientific literature are as follows 1 extrapolation of sea ice volume data 2 assuming several more rapid loss events such as 2007 and 2012 and 3 climate model projections time horizons for a nearly sea ice free summer for these three approaches are roughly 2020 or earlier 2030 10 years and 2040 or later loss estimates from models are based on a subset of the most rapid ensemble members it is not possible to clearly choose one approach over another as this depends on the relative weights given to data versus models observations and citations support the conclusion that most global climate model results in the cmip5 archive are too conservative in their sea ice projections recent data and expert opinion should be considered in addition to model results to advance the very likely timing for future sea ice loss to the first half of the 21st century with a possibility of major loss within a decade or two

low rates of deformation of the furnas and fogo volcanoes sao miguel azores observed with the global positioning system 1993 1997 abstract we report results of global positioning system gps measurements at the furnas and fogo agua de pau volcanoes sao miguel island azores gps surveys were carried out in 1993 1994 and 1997 the gps network consists of 16 stations that are mainly distributed in the vicinity of the furnas volcano the measurements indicate slow progressing ground deformation of the furnas volcano in 1993 1997 we propose two alternative hypotheses to explain the observations the first one assumes a source of inflation located northwest of the furnas caldera since there part of the gps network seems to expand the second hypothesis invokes a combination of two processes as being responsible for the observed deformation pattern plate divergence between the eurasian and african plates together with a deflation of the furnas caldera the inferred plate boundary deformation zone is narrow with 75 of the 4 mm yr full plate spreading probably being accommodated over the 10 km width of sao miguel island gps stations at fogo show displacements towards its caldera indicating slight deflation of the volcano possibly due to a pressure decrease in a shallow magma chamber or to extraction of water and steam from the ground by a nearby geothermal power plant further geodetic observations are needed spanning a longer time interval in order to distinguish between different hypotheses

strain accumulation in the shumagin islands results of initial gps measurements deformation in the shumagin seismic gap has been monitored with repeated trilateration edm in the 1980 1987 interval and with the global positioning system gps in the 1987 1991 interval the geodetic network extends for 100 km across the shumagin islands to the alaska peninsula results from the gps surveys are consistent with those previously reported for the edm surveys we failed to detect significant strain accumulation in the n30 w direction of plate convergence using the method of simultaneous reduction for position and strain rates we found the average rate of extension in the direction of plate convergence to be 25 25 nanostrain yr nstrain yr during the 1987 1991 interval of gps surveys compared with 20 15 nstrain yr during the 1981 1987 interval of complete edm surveys we found a marginally significant 26 12 nstrain yr extension rate in the 1981 1991 interval covered by the combined edm and gps surveys strain rates are higher but not significantly so in the part of the network closest to the trench spatial variation in the deformation is observed in the 1980 1991 average station velocities where three of the four stations closest to the trench have an arcward velocity of a few mm yr the observed strain rates are an order of magnitude lower than the 200 nstrain yr rate predicted by dislocation models

the 2017 reversal of the beaufort gyre can dynamic thickening of a seasonal ice cover during a reversal limit summer ice melt in the beaufort sea during winter 2017 the semi permanent beaufort high collapsed and the anticyclonic beaufort gyre reversed the reversal drove eastward ice motion through the western arctic causing sea ice to converge against banks island and halted the circulation of multiyear sea ice via the gyre preventing its replenishment in the beaufort sea prior to the reversal an anomalously thin seasonal ice cover had formed in the beaufort following ice free conditions during september 2016 with the onset of the reversal in january 2017 convergence drove uncharacteristic dynamic thickening during winter by the end of march despite seasonal ice comprising 97 of the ice cover the reversal created the thickest roughest and most voluminous regional ice cover of the cryosat 2 record within the beaufort sea previous work has shown that winter ice export can precondition the region for increased summer ice melt but that a short reversal during april 2013 contributed to a reduction in summer ice loss hence the deformed ice cover at the end of winter 2017 could be expected to limit summer melt in spite of this the beaufort ice cover fell to its fourth lowest september area as the gyre re established during april and divergent ice drift broke up the pack negating the reversal s earlier preconditioning our work highlights that dynamic winter thickening of a regional sea ice cover for instance during a gyre reversal offers the potential to limit summer ice loss but that dynamic forcing during spring dictates whether this conditioning carries through to the melt season

dynap software for estimating crustal deformation from geodetic data abstract the acronym dynap dynamic adjustment program identifies a software package for estimating crustal dynamic parameters from a collection of geodetic data dynap accepts various data types including horizontal directions and angles distances azimuths zenith distances and intersite vectors such as those obtainable with the global positioning system gps or with very long baseline interferometry vlbi dynap incorporates the simultaneous reduction technique whereby crustal dynamic parameters are estimated simultaneously along with station positional coordinates and various nuisance parameters all via the least squares process the estimated crustal dynamics parameters come in groups of six or fewer where each group corresponds to a prespecified geographic region and period of time the six parameters convert mathematically to yield the three components of the horizontal strain rate tensor the horizontal rotation rate and the two components of the vertical tilt rate vector dynap s applicability is illustrated with two examples the first involves over 60 years of triangulation trilateration data in the cascadia subduction zone of northwestern california here a maximum shearing rate was found of 0 161 0 074 radians yr with the corresponding axis of maximum contraction oriented n25 e 14 the second involves a 1980 triangulation trilateration survey and a 1986 gps survey both spanning the imperial fault of southern california here a maximum dextral shearing rate was found of 1 073 0 171 radians yr in the direction of n31 w 5

geodetic measurement of deformation in the ventura basin region southern california we have measured the deformation in the ventura basin region southern california with global positioning system gps measurements carried out over 4 6 years between 1987 and 1992 the deformation within our network is spatically variable on scales of tens of kilometers with strain rates reaching 0 6 1 micro rad yr in the east central basin blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower 0 06 1 micro rad yr to the south we also observed clockwise rotations of 1 deg 7 deg m y shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and gps measurements give consistent though less precise results the geodetic rates of shortening across the basin and western transverse ranges are lower than those estimated from geological observations but the patterns of deformation from the two methods agree qualitatively

deformation in the santa barbara channel from gps measurements 1987 1991 geodesy is one of the few techniques that can be used to estimate strain rates across the submerged structures in the santa barbara channel by resolving relative surface velocities the global positioning system gps can be used to infer crustal deformation rates independent of geologic models gps measurements from the period 1987 1991 have been used to derive velocities for a 8 station network bounding the channel these estimates are based on 10 gps experiments each of several days in length over this 4 5 year period precision of individual determinations of intersite vectors range from 3 to 7 mm for the horizontal components and average 20 mm in the vertical the gps data indicate high rates of shortening in the eastern channel 6 1 mm yr at n16 3e and low rates of deformation in the western channel comparisons with previous geodetic studies suggest that deformation in the region has not been uniform over the last 100 years

contributions of growth and deformation to monthly variability in sea ice thickness north of the coasts of greenland and the canadian arctic archipelago regional variability in monthly cryosat 2 sea ice thickness is partitioned into contributions from dynamics and thermodynamics using ice deformation calculated from large scale ice drift for five winters december to april 2011 2015 over a region of persistent convergence north of the coasts of greenland and the canadian arctic archipelago deformation explains 34 of the overall variance up to 69 in 2014 2015 in monthly thickness changes approximately 42 56 or 0 6 m of the seasonal changes in mean regional ice thickness can be attributed to divergence and shear the estimated area averaged growth of 0 12 0 03 m month compares favorably with measurements from ice mass balance buoys examination of the time variable thickness distributions shows areas covered by ice 3 m increased albeit at fairly coarse resolution this coupled analysis of thickness changes and deformation offered a first look at the character of the regional thickness redistribution process

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

large scale surveys of snow depth on arctic sea ice from operation icebridge 1 we show the first results of a large scale survey of snow depth on arctic sea ice from nasa s operation icebridge snow radar system for the 2009 season and compare the data to climatological snow depth values established over the 1954 1991 time period for multiyear ice the mean radar derived snow depth is 33 1 cm and the corresponding mean climatological snow depth is 33 4 cm the small mean difference suggests consistency between contemporary estimates of snow depth with the historical climatology for the multiyear ice region of the arctic a 16 5 cm mean difference climatology minus radar is observed for first year ice areas suggesting that the increasingly seasonal sea ice cover of the arctic ocean has led to an overall loss of snow as the region has transitioned away from a dominantly multiyear ice cover

circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum september 2007 marked a record minimum in sea ice extent while there have been many studies published recently describing the minimum and its causes little is known about how the ice thickness has changed in the run up to and following the summer of 2007 using satellite radar altimetry data covering the arctic ocean up to 81 5 degrees north we show that the average winter sea ice thickness anomaly after the melt season of 2007 was 0 26 m below the 2002 2003 to 2007 2008 average more strikingly the western arctic anomaly was 0 49 m below the six year mean in the winter of 2007 2008 these results show no evidence of short term preconditioning through ice thinning between 2002 and 2007 but show that after the record minimum ice extent in 2007 the average ice thickness was reduced particularly in the western arctic citation giles k a s w laxon and a l ridout 2008 circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum geophys res lett 35 l22502 doi 10 1029 2008gl035710

field investigations of ku band radar penetration into snow cover on antarctic sea ice monitoring long term large scale changes in the antarctic sea ice thickness is not currently possible due to the sampling constraints of the ship based and airborne observations which comprise most of the available thickness data satellite radar altimetry has been used to measure sea ice thickness variability in the arctic where it is assumed that the highest amplitude radar return originates from the snow ice interface as the arctic snow is cold and dry however this may not be the case in the antarctic due to more complex snow stratigraphy caused by warmer winter temperatures and frequent snow flooding we present the first measurements of radar penetration into snow cover on antarctic sea ice in the ku band at which satellite radar altimeters operate data were taken using a sled borne radar on sea ice off east antarctica during september and october 2007 radar data and field measurements of snow density thickness wetness and layers were taken over a range of locations including snow packs with flooding hard crusts and icy layers detailed snow pit studies showed that the snow ice interface was the dominant scattering surface only for snow without morphological features or flooding analysis of transect data showed that the mean depth of the dominant scattering surface of the radar was only around 50 of the mean measured snow depth indicating that the dominant scattering surface was not always the snow ice interface for the antarctic sea ice surveyed

retrievals of lake ice thickness from great slave lake and great bear lake using cryosat 2 satellite observations have revealed decreases in the duration of the seasonal snow and ice coverage of great slave lake gsl and great bear lake gbl large freshwater lakes in northern canada however limited information is available about ice thickness changes here we present and validate a method to retrieve lake ice thickness using the cryosat 2 cs2 radar altimeter these are the first satellite altimeter retrievals of lake ice thickness under optimal conditions the cs2 signal is scattered from both the snow ice and the ice water interfaces with returns from each interface being of sufficient power to be identified in the radar waveform the distance between the scattering horizons is used to determine the ice thickness similar to traditional ground penetrating radar measurements the seasonal evolution of ice thickness of gbl and gsl is compared with in situ measurements modeled ice thicknesses and previous studies the impact of ice and snow properties on signal penetration and the thickness retrieval are examined with synthetic aperture radar imagery the cs2 ice thickness retrievals are able to observe the seasonal thickening of the lake ice and closely match the in situ measurements over both lakes r 0 65 text rmse m thickness retrievals of thin ice are limited by a minimum waveform peak separation of 2 range bins approximately 0 26 m in ice although not designed for lake ice studies cs2 and future sar satellite altimeter missions offer new possibilities to monitor the ice and water levels of climatically sensitive and influential lakes

discussion of age of tunica hills louisiana mississippi quaternary fossiliferous creek deposits problems of radiocarbon dates and intermediate valley terraces in coastal plains by e g otvos jr

comparison of freeboard retrieval and ice thickness calculation from als asiras and cryosat 2 in the norwegian arctic to field measurements made during the n ice2015 expedition we present freeboard measurements from airborne laser scanner als the airborne synthetic aperture and interferometric radar altimeter system asiras and cryosat 2 siral radar altimeter ice thickness measurements from both helicopter borne and ground based electromagnetic sounding and point measurements of ice properties this case study was carried out in april 2015 during the n ice2015 expedition in the area of the arctic ocean north of svalbard the region is represented by deep snow up to 1 12 m and a widespread presence of negative freeboards the main scattering surfaces from both cryosat 2 and asiras are shown to be closer to the snow freeboard obtained by als than to the ice freeboard measured in situ this case study documents the complexity of freeboard retrievals from radar altimetry we show that even under cold below 15 c conditions the radar freeboard can be close to the snow freeboard on a regional scale of tens of kilometers we derived a modal sea ice thickness for the study region from cryosat 2 of 3 9 m compared to measured total thickness 1 7 m resulting in an overestimation of sea ice thickness on the order of a factor 2 our results also highlight the importance of year to year regional scale information about the depth and density of the snowpack as this influences the sea ice freeboard the radar penetration and is a key component of the hydrostatic balance equations used to convert radar freeboard to sea ice thickness

dynamic preconditioning of the minimum september sea ice extent abstractthere has been an increased interest in seasonal forecasting of the arctic sea ice extent in recent years in particular the minimum sea ice extent here a dynamical mechanism based on winter preconditioning is found to explain a significant fraction of the variance in the anomaly of the september sea ice extent from the long term linear trend to this end a lagrangian trajectory model is used to backtrack the september sea ice edge to any time during the previous winter and quantify the amount of sea ice advection away from the eurasian and alaskan coastlines as well as the fram strait sea ice export the late winter anomalous sea ice drift away from the coastline is highly correlated with the following september sea ice extent minimum it is found that the winter mean fram strait sea ice export anomaly is also correlated with the minimum sea ice extent the following summer to develop a hindcast model of the september sea ice extent which does not depend on a priori knowledge of the minimu

slip in the 1868 hayward earthquake from the analysis of historical triangulation data despite the importance of the hayward fault for earthquake hazards in the san francisco bay area neither the rupture length nor the average coseismic slip in the most recent major earthquake which occurred in 1868 is well known we estimate slip in the 1868 earthquake by analyzing triangulation data collected between 1853 1860 and 1876 1891 we apply a denuisancing procedure that projects out the dependence of that data on station coordinates allowing their displacements to be estimated directly the best fit to the data is obtained with a 52 km long by 10 km deep rupture that extends from warm springs to near berkeley considerably longer than the 30 km of reported surface breakage the estimated slip for an earthquake of this length is 1 9 0 4 m and the seismic moment is m0 3 0 0 7 1019 n m corresponding to mw 7 0 the data are inconsistent with regionally high strain rate prior to the 1906 earthquake as had been previously suggested the triangulation data record northeastward displacement of station loma prieta presumably as a result of the 1865 earthquake the fault normal displacement is not easily explained by slip on either the san andreas fault or the 1989 loma prieta rupture surface this observation is however consistent with an mw 6 3 4 thrust event northeast of the san andreas fault in the southern santa cruz mountains thrust belt

regional deformation of the earth model for the san diego region california a mathematical model for historical horizontal crustal deformation is presented for the southern california region that extends northward from mexico for approximately 100 km to 33 5 n latitude model parameters are estimated from triangulation trilateration and astronomic azimuth data observed during the past century also geologic and seismic data indirectly participate in specifying the model the model relates crustal deformation to the episodic movement experienced during large earthquakes m 6 secular slip rates on geologic faults and secular strain rates over large geographic areas the episodic movement is modeled in accordance with the theory of dislocation in an elastic halfspace for the secular motion the modeled region is partitioned into a mosaic of districts that are allowed to individually translate rotate and homogeneously deform as a linear function of time the derived strain pattern supports the hypothesis that the modeled region spans the pacific north american plate boundary secular shear strain is maximum in the nw direction parallel to the major faults and shear strain rates decrease with distance from a nw se trending strip to the region s north this strip is 40 km wide and is bounded on the west by the san jacinto fault zone and on the east by the southern extension of the san andreas fault to the south this strip converges in width onto the imperial fault the secular velocity for a point near the city of san diego relative to a point at the region s northeast corner is 4 3 3 1 cm yr in the direction n 17 w 5 this rate provides a rough estimate for the relative velocity between the pacific and north american plates

alluvial pedogenesis in pleistocene and holocene mississippi river deposits effects of relative sea level change comparison of pleistocene and holocene soils developed in mississippi river meander belt deposits in louisiana illustrates the influence of relative sea level change on pedogenesis holocene and pleistocene meander belts have similar landscape morphologies lithofacies and soil characteristics and probably formed under similar hydrologic regimes the pleistocene avoyelles prairie meander belt is now terraced but it was originally graded to a lower relative sea level position than the holocene flood plain holocene alluvial soils are generally weakly expressed cumulative profiles with abundant redoximorphic features and formed on an aggrading flood plain with a shallow fluctuating water table the soil morphology varies with respect to landscape position and overbank lithofacies pleistocene alluvial paleosols show evidence for two stages of pedogenesis during the first stage of pedogenesis water table fluctuations and flood plain aggradation produced profiles with color mottles iron and carbonate nodules and weak b and c horizons similar to holocene flood plain soils the second stage of pedogenesis occurred after the avoyelles prairie was terraced and the regional water table fell a minimum of 30 m improved soil drainage created oxidizing and acidic conditions removed free carbonates dehydrated iron oxides and transformed clay minerals this stage produced pleistocene profiles with brighter matrix and mottle hues lower ph values larger and harder iron nodules and greater kaolinite contents than comparable holocene flood plain soils the primary influence on regional water table lowering in the southern lower mississippi valley was relative sea level fall mississippi river flood plain lowering adjacent to the avoyelles prairie most likely occurred between the end of oxygen isotope stage 3 and the maximum period of valley entrenchment during oxygen isotope stage 2 water table fall affected pedogenic processes a minimum of 400 km upvalley of the continental shelf margin pleistocene mississippi river paleosols represent excellent analogs for interpreting ancient alluvial paleosols formed during episodes of base level fall and valley incision as fluvial entrenchment terraced the meandering avoyelles prairie a pedogenic threshold was crossed that altered the pathway of chemical weathering the complexity of a multiple step pedogenic history also highlights the fact that interpretation of relative profile development as a function of time can be problematic

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

horizontal deformation in the cascadia subduction zone as derived from serendipitous geodetic data abstract data from recent global positioning system gps surveys are combined with triangulation trilateration data to estimate horizontal shear strain rates for two regions in the cascadia subduction zone near bellingham washington we estimate that the maximum horizontal shear rate equals 0 116 0 089 rad yr and the direction of maximum horizontal contraction orients n71 e 21 for data spanning the 1905 1985 interval the corresponding estimates for a region near portland oregon are 0 057 0 027 rad yr and n95 e 14 for data spanning the 1881 1988 interval these estimates are consistent with estimates from independent geodetic data in the area moreover the estimates for are consistent with the n68 e direction of ongoing convergence between the juan de fuca plate and the north american plate as predicted by the nuvel 1 plate motion model this consistency between estimates and the direction of plate convergence supports the argument for the possibility of a great subduction earthquake occurring in the cascadia subduction zone the low shear rates however imply that the recurrence interval between such earthquakes would be several centuries long

the arctic ice thickness anomaly of the 1990s a consistent view from observations and models 1 observations of sea ice draft from submarine cruises in much of the arctic ocean show that the ice cover was unusually thin in the mid 1990s here we limit our examination to digitally recorded draft data from eight cruises spanning the years 1987 to 1997 and find a decrease of about 1 m over the 11 year span comparisons of our modeled draft with observed draft show good agreement in the temporal change comparing average draft over entire cruises the rms discrepancy between modeled and observed draft is 0 3 m and the correlation is 0 98 agreement in the spatial patterns of draft is somewhat lower the rms discrepancy of 50 km averages of draft is 0 7 m and the correlation is 0 73 we review reports of interannual variations of ice thickness or volume from other model studies all models agree that thickness decreased by between 0 6 and 0 9 m from 1987 to 1996 our model shows a modest recovery in thickness from 1996 to 1999 for the 1950s 1960s and 1970s models tend to disagree on the size and to a lesser extent the timing or phase of interannual variations

extension across a divergent plate boundary the eastern volcanic rift zone south iceland 1967 1994 observed with gps and electronic distance measurements the average spreading rate in south iceland 19 mm yr is distributed over two parallel rift zones we measured a global positioning system gps network of 42 stations in the eastern zone in 1994 this network consists of stations measured with gps in 1986 and 1992 stations in a 60 km long distance profile measured several times since 1967 and a few new stations the 1994 gps data were processed using the bernese software version 3 5 and the average position uncertainties are about 3 mm in horizontal components and about 8 mm in the vertical component for baselines up to 100 km comparison with results of former gps campaigns gives a uniform extension steady in time perpendicular to the spreading axis a strain rate of 0 12 0 01 strain yr is observed across the 100 km wide network or an extension of about 12 mm yr minor deformation is observed in direction parallel to the spreading axis observations along the distance profile which lies across the rift zone gave a significant contraction during the period 1967 1977 but gave an extension during 1977 1994 this extension is about 138 47 mm and is mainly accommodated in the western part of the profile the observed extension across the rift zone during 1986 1994 can be simulated with a simple model of infinitely long dike intrusions into an elastic layer overlying a viscous layer this model is not able to simulate observed contraction along the distance profile 1967 1977 the observed irregularities along the western part of the distance profile coincide in space and time with volcanic and tectonic unrest near the hekla volcano the disturbances are probably caused by some common underlying process leading to crustal deformation eruptions and earthquakes

sea ice convergence along the arctic coasts of greenland and the canadian arctic archipelago variability and extremes 1992 2014 after the summer of 2013 a convergence induced tail in the thickness distribution of the ice cover is found along the arctic coasts of greenland and canadian arctic archipelago prompted by this a normalized ice convergence index ici is introduced to examine the variability and extremes in convergence in a 23 year record 1992 2014 of monthly ice drift large scale composites of circulation patterns characteristic of regional convergence and divergence are examined indeed the ici shows the june 2013 convergence event to be an extreme i e ici 2 furthermore there is a cluster of 9 months over a 17 month period with positive icis i e 1 following the record summer minimum ice extent smie in 2012 the imprint of ice dynamics from this cluster of positive icis likely contributed to higher smies in 2013 and 2014 the impact of convergence on smie is discussed and the increase in arctic ice volume in 2013 is underscored

crustal velocity field near the big bend of california s san andreas fault we use geodetic data spanning the 1920 1992 interval to estimate the horizontal velocity field near the big bend segment of california s san andreas fault saf more specifically we estimate a horizontal velocity vector for each node of a two dimensional grid that has a 15 min by 15 min mesh and that extends between latitudes 34 0 n and 36 0 n and longitudes 117 5 w and 120 5 w for this estimation process we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes the data include over a half century of triangulation measurements over two decades of repeated electronic distance measurements a decade of repeated very long baseline interferometry measurements and several years of global positioning system measurements magnitudes for our estimated velocity vectors have formal standard errors ranging from 0 7 to 6 8 mm yr our derived velocity field shows that 1 relative motion associated with the saf exceeds 30 mm yr and is distributed on the earth s surface across a band 100 km wide that is roughly centered on this fault 2 when velocities are expressed relative to a fixed north america plate the motion within our primary study region has a mean orientation of n44 w 2 and the surface trace of the saf is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5 and 10 counterclockwise relative to these contours and 3 large strain rates shear rates 150 nrad yr and or areal dilatation rates 150 nstr yr exist near the garlock fault near the white wolf fault and in the ventura basin

geodetic measurement of tectonic deformation in the santa maria fold and thrust belt california we have analyzed geodetic observations to resolve tectonic deformation across the santa maria fold and thrust belt northwest of santa barbara california the geodetic network forms a braced quadrilateral with 40 km sides whose southwest corner is the vandenberg very long baseline interferometry station the data include 1 historical triangulation from the 1880s 1920s and 1950s 2 electronic distance measurement trilateration from 1971 and 1985 and 3 global positioning system observations from 1986 and 1987 we combine the three different types of data to estimate two dimensional station positions and strain rate parameters simultaneously using a model which constrains the relative velocity field to be linear in space and constant in time we find significant strain rates the maximum compressive strain is oriented n17 e 5 e and the compressive strain rate in this direction is 0 13 0 03 strain yr under the assumption that the unresolved rotational component of the velocity field is zero we estimate that the integrated rate of deformation across the basin is 7 1 mm yr oriented at n03 e 13 this vector can be decomposed into 6 2 mm yr of crustal shortening on the general structural trend of n30 e and 3 1 mm yr of right lateral shear across this axis on the basis of these values and earthquake focal mechanisms in the area we infer that the deformation occurs on northwest trending folds and thrusts within the belt these results are consistent with the rate and direction of deformation across the central california coast ranges inferred by balancing pacific north america plate motion against san andreas slip and basin and range extension they imply that the santa maria fold and thrust belt is the primary active element in transforming motion from the coast ranges to the western transverse ranges and the santa barbara channel