late cenozoic tectonics of the east ventura basin transverse ranges california the east ventura basin originated in the middle miocene as a rift system bounded on one side by the oak ridge simi hills structural shelf and on the other side by a granitic ridge parallel to the san gabriel fault this fault began accumulating right slip 10 12 m y ago at a rate of 4 5 9 mm yr depending on whether total slip is 45 or 60 km slowing to about 1 mm yr in the quaternary north of the santa clara river rifting ended prior to deposition of the uppermost miocene lower pliocene towsley formation south of the santa clara river the rift axis shifted southwest toward the oak ridge simi hills shelf as the towsley formation accumulated against a normal fault ancestor of the santa susana fault a change to contractile tectonics occurred in the pliocene with depos tion of the fernando formation when the newhall potrero anticline developed as a monocline above a blind reverse fault the pico anticline to the southeast and the temescal and hopper ranch modelo anticlines to the northwest may have a similar origin tectonic inversion and displacement on the southwest verging santa susana fault began about 0 5 ma based on appearance of locally derived clasts in the upper saugus formation and its equivalents and continues today along with the southwest verging san cayetano fault farther west also active are northeast verging backthrusts occurring in the east ventura basin fold belt and a segment of the san gabriel fault which now acts as a northeast dipping oblique slip reverse fault northeast trending discontinuities and structures divide the present deformation zone into four segments in the hopper canyon segment at the west end of the area in the west ventura basin the san cayetano fault places miocene modelo formation over pliocene pleistocene strata more than 5 km thick largely at maximum burial to the southeast the newhall potrero segment is characterized by north vergent backthrusts within the east ventura fold belt and by southward thrusting of the basin sequence tectonic inversion over the structural shelf on the santa susana fault farther southeast the placerita segment is marked by reverse faulting on both the santa susana fault and the san gabriel fault southeast of the basin in the san fernando valley the sylmar segment contains a thick pli cene pleistocene sequence overridden by basement rocks of the san gabriel mountains as well as the south verging mission hills granada hills and northridge hills fault zones

malvinas current volume transport at 41 s a 24 yearlong time series consistent with mooring data from 3 decades and satellite altimetry fil artana camila indira instituto franco argentino sobre estudios del clima y sus impactos argentina centre national de la recherche scientifique francia universite pierre et marie curie francia

on the transport of the antarctic circumpolar current through drake passage and its relation to wind a time series of net transport during 1979 of the antarctic circumpolar current acc through the upper 2500 m of drake passage was obtained as part of the international southern ocean studies program by whit worth 1983 and was presented in a revised form by whitworth and peterson 1985 spectral analyses are performed on records comprising the revised series to determine the major sources of transport variability multiyear records from bottom pressure gauges at each side of the passage are then compared with fields of wind stress over the southern ocean the net transport series is dominated in the subseasonal time scales by the lunar fortnightly and monthly tides and by baroclinic activity in the northern passage processes that are independent of wind for seasonal time scales transport variability appears to be mainly barotropic thus being well represented by bottom pressure records from each side of the passage comparisons of multiyear bottom pressure records with time series of the zonally averaged curl of wind stress in narrow bands along the perimeters of the acc suggest that seasonal fluctuations in the strengths of the subtropical and subpolar atmospheric pressure systems flanking the acc induce changes in the slope of the sea surface across the current the dominant annual cycle in curl associated with the subtropical highs appears to influence sea level across the entire acc whereas the effects of the strong semiannual cycle in curl associated with the subpolar lows are confined to the southern portions of the current

satellite altimetry and current meter velocities in the malvinas current at 41 s comparisons and modes of variations three year long current meter arrays were deployed in the malvinas current at 41 s below a satellite altimeter track at about 10 years intervals surface geostrophic velocities sgv derived from satellite altimetric data are compared with the in situ velocities at the upper current meter 300 m multisatellite gridded sgv compare better with in situ observations than along track sgv in spite of the proximity of the moorings to the complex brazil malvinas confluence bmc region satellite sgv are significantly correlated with the 20 day low passed in situ velocities 0 85 for along isobaths velocities 0 8 for cross isobaths velocities the recent in situ measurement period 2014 2015 stands out in the altimetry record with a long lasting 4 months high level of eddy kinetic energy at the mooring site and a southernmost location of the subantarctic front saf the first two modes of variations of sea level anomaly sla over the bmc remarkably match the first two modes of the low passed in situ velocities the first mode is associated with a latitudinal migration of the saf and the second with a longitudinal displacement of the brazil current overshoot the two modes dominate the 24 year long record of sla in the bmc with energy peaks at the annual and semiannual periods for the first mode and at 3 5 months for the second mode the sla over the southwest atlantic was regressed onto the two confluence modes of sla variations and showed remarkable standing wave train like structures in the argentine basin

two full depth velocity sections in the drake passage in 2006 transport estimates abstract in january february 2006 a hydrographic section under jason track 104 across drake passage dp was performed twice within 3 weeks on board of the r v polarstern two ladcps mounted on the ctd rosette provided the most comprehensive synoptic top to bottom observations of directly measured velocity available in the drake passage to date with an estimated error of less than 3 cm s 1 geostrophic velocities computed from the hydrographic data were referenced to ladcp velocities the velocity structure of the antarctic circumpolar current acc along the track and its evolution within 3 weeks reflect the frontal branches presented by barre et al 2011 and provost et al 2011 the repeated section offers two independent transport estimates computed from the ladcp velocities 145 8 8 and 137 9 10 5 sv respectively and from the adjusted geostrophic velocities 136 6 7 and 129 7 sv respectively error bars of the transport estimates are small contributions of the fronts and eddies to the total transport are modified within 3 weeks but tend to compensate each other the total transport is reduced by about 10 15 17 sv between the southward and northward journey total transport estimations are compared to previous results in particular the isos international southern ocean study data from which the canonical value of 134 sv has been estimated within the uncertainties due to uneven data distribution no trend is discernible neither in the baroclinic nor total transports from 1975 to 2006

circulation in drake passage revisited using new current time series and satellite altimetry 1 the yaghan basin circulation in the yaghan basin

observations of atmosphere ocean coupling in midlatitude western boundary currents oceanographic fields derived from the geosat altimeter and atmospheric fields from the european centre for medium range weather forecasts ecmwf were examined for both the western north pacific ocean and the western north atlantic ocean to determine the nature of the coupling despite the short data records approximately 2 5 years correlation analyses of nonseasonal fluctuations showed similar relationships for both oceans correlations between the height difference across the jets surface transport and the path of the western boundary currents suggested that there is a structural change of the recirculation gyres associated with a path change with timescales of 5 9 months both wind stress and wind stress curl were clearly correlated with the surface transport and path in each ocean although net surface heat flux and wind stress were correlated there was no clear correlation between net surface heat flux from ecmwf and surface transport over the entire study region additional regional analyses of the north atlantic showed a consistent relationship between cooling sea surface temperature sst and surface transport west of 62 w anomalous cooling accompanied by cyclonic wind anomalies corresponds to a weaker sst gradient across the gulf stream and decreasing surface transport because the surface transport fluctuations to the west and east of 62 w in the western north atlantic are not correlated structural changes in the recirculation gyres which have their maximum variations east of 62 w may not be related to surface heat flux variations

deformation rates across the placerita northridge mw 6 7 aftershock zone and hopper canyon segments of the western transverse ranges deformation belt abstract aftershocks of the 1994 northridge m w 6 7 earthquake provide insights into the geometry of the seismic source faults in the san fernando valley and the east ventura basin and allow the calculation of deformation rates for the region the northridge thrust and santa susana faults dip in opposite directions with the northridge thrust entirely beneath the santa susana fault these opposing reverse faults interact resulting in a folded active santa susana fault and uplift in the footwall block of that reverse fault two balanced cross sections suggest thick skinned deformation of the western transverse ranges the western section across the modelo lobe segment of the north dipping san cayetano fault and the easternmost surface trace of the south dipping oak ridge fault is west of any aftershocks of the northridge earthquake and has been termed the hopper canyon segment of the deformation belt structural modeling predicts a dip of 46 s on the oak ridge fault at seismogenic depths horizontal shortening rates are calculated by adding the products of the dip slip displacements and the cosines of the dips of both faults the eastern cross section shows the northridge mainshock with a 45 south dipping nodal plane at a depth of 18 km aftershocks reach a depth of 20 km in a thin skinned paradigm a hinge should occur at the surface near the santa monica mountains due to rocks moving from a decollement at the brittle plastic transition and changing dip as they move up the ramp no hinge of that magnitude occurs there calculation of horizontal short ening rates across this part of the western transverse ranges must take into account the displacement on both the northridge thrust eastern extension of the oak ridge fault and the santa susana fault placerita segment horizontal shortening rates are 8 2 2 4 mm yr across the modelo lobe segment of the san cayetano fault and the oak ridge fault and 5 7 2 5 mm yr across the northridge thrust and the santa susana fault these rates are consistent with those based on tectonic geodesy using gps dip slip displacement rates on the faults are 1 7 mm yr for the northridge thrust since 2 3 ma 4 1 0 4 mm yr for the oak ridge fault since 500 ka 5 9 3 9 3 8 mm yr for the santa susana fault since 600 to 2300 ka and 7 4 3 0 mm yr for the modelo lobe segment of the san cayetano fault since 500 ka this indicates that the slip rates on the north dipping the eastern san cayetano and the santa susana faults are comparable but the slip rate on the south dipping faults decreases eastward the slip rate on the oak ridge fault in the ventura basin is more than double that of the northridge thrust

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

postseismic deformation following the 1994 northridge earthquake identified using the localized hartley transform filter here we present a new mathematical tool the localized hartley hl transform hartley 1942 bracewell 1990 that allows for the filtering of 1 d time series through the identification of the power at various spatial and temporal wavelengths its application to and the associated results are presented from its application to continuous global positioning system gps data from southern california for the time period 1994 through 2006 the hl transform filter removes the high frequency components of the data and effectively isolates the longer period signal this long period signal is modeled as time dependent postseismic deformation using the viscoelastic gravitational model of fernandez and rundle 2004 for six stations selected for their proximity to the northridge earthquake the x y and z components of the postseismic deformation are compared to the filtered data results suggest that this long period deformation is a result of postseismic relaxation and that the hl transform filter provides an important new technique for the filtering of geophysical data consisting of the superposition of the effects of numerous complex sources at a variety of spatial and temporal scales

wind modulation of upwelling at the shelf break front off patagonia observational evidence the south atlantic patagonian shelf is the largest chlorophyll a chl a hot spot in southern ocean color images while a persistent 1500 km long band of high chl a along the shelf break front sbf is indicative of upwelling the mechanisms that drive it are not entirely known along front wind oscillations can enhance upwelling and provide a nutrient pumping mechanism at shelf break fronts of western boundary currents here we assess wind induced upwelling at the sbf off patagonia from daily satellite chl a and winds historical hydrographic observations cross shelf chl a fluorescence transects from two cruises and in situ winds and water column structure from a mooring site satellite chl a composites segregated by along front wind direction indicate that surface chl a is enhanced at the sbf with southerly winds and suppressed with northerly winds northerly winds also result in enhanced chl a further offshore 25 50 km synoptic transects as well as mean hydrographic sections segregated by along front winds show isopycnals tilted upward for southerly winds spring observations from the mooring also suggest that southerly winds destratify the water column and northerly winds restratify in agreement with ekman transport interacting with the front moreover changes in water column temperature lag along front wind forcing by 2 4 days our results suggest that oscillations in along front winds on timescales typical of atmospheric storms 2 10 days can significantly modulate the upwelling and chl a concentrations at the sbf off patagonia revealing the importance of wind induced upwelling for shelf slope exchange at shelf break fronts of western boundary currents

seismogenic deformation field in the mojave block and implications for tectonics of the eastern california shear zone from the aftershocks of the 1992 landers earthquake we infer the orientation of the principal strain rate axes d 1 d 2 d 3 d 1 lengthening their relative magnitude and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic p and t axes the seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central mojave block regionally the aftershock data define two major domains within the central mojave block 1 the western mojave block including the san bernardino mountains and the epicentral area of the big bear earthquake which is characterized by e w d 1 lengthening and n s d 3 shortening and 2 the central mojave block including the landers surface rupture zone which is characterized by nw se d 1 and ne sw d 3 inversion for the principal strain axes of geodetically measured coseismic displacements across the big bear and landers seismogenic zones gives results similar to the aftershock inversions for those areas indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses the background seismicity for 1981 to 1991 shows the same characteristic d 1 and d 3 orientations for the two domains indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation the micropolar inversion also provides values of the relative vorticity parameter w which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault bounded blocks rotating within tabular seismogenic shear zones the observed fault geometry along the kickapoo fault suggests a pinned block model for the local block rotation that is consistent with the values of w obtained from our inversions we interpret the regional nw se orientation of d 1 in the central mojave block to be characteristic of the dextral eastern california shear zone which transfers approximately 22 of the pacific north american plate motion from the san andreas system to the walker lane belt in eastern california our results and geodetic determinations of the secular shear strain in the central mojave block indicate that the locus of nw dextral shear generally lies between the san bernardino mountains and the pisgah fault

dynamics of remotely forced intraseasonal oscillations off the western coast of south america abstract six years of sea level observations at six locations along the south american coast from peru to chile were analyzed at intraseasonal frequencies periodicity 1 2 months a high resolution low frequency numerical coastal model having realistic shelf and slope topography and bottom friction and forced by an equatorial signal was used to analyze the intraseasonal coastal dynamics at these latitudes the following results were obtained lagged correlations between sea level stations imply a sea level poleward propagation of 250 km day 1 the frictionless vertical wall theory poleward phase speed was considerably less at 216 km day 1 model runs with realistic bottom friction and shelf and slope bottom topography gave poleward phase speeds much closer to that observed for typical parameters the phase speed was 253 km day 1 both bottom friction and topography significantly affect the propagation speed past work has shown that the sea level amplitude should grow alongshore like f f coriolis

the dynamics of large scale wind driven variations in the antarctic circumpolar current a nonlinear model is devloped and analytical results obtained to discuss the response of the antarctic circumpolar current to wind forcing over a wide range of frequencies the main results are as follows i the nonlinear equations of motion can be conveniently separated into one baroclinic and one barotropic mode ii for forcing with period t equal to less than a few years wind driven fluctuations in the antarctic circumpolar current are barotropic and governed by the linearized laplace tidal equations theory suggests that fluctuations in the transport should lag and be most strongly correlated with the circumpolar averaged wind stress these theoretical results are consistent with recent measurements made in drake passage an interested untested theoretical prediction is that the sea level fluctuations measured at the southern side of drake passage with t between one month and a few years should be coherent at zero lag with sea level fluctuations at the same latitude around the earth iii for longer period forcing baroclinic fluctuations are important the baroclinic pressure current and associated density variations all decrease exponentially with depth exponential depth decay of these baroclinic fields is in fact observed the decay scale being about 1 km iv the theory indicates thatmore significant large scale wind driven fluctuations in the strength of the baroclinic antarctic circumpolar current can only occur at frequencies with periodicity or approx 70 years climatic changes associated with such variability must therefore consist of oscillations of similar or longer period this is consistent with limited observations which suggest that wind and sea surface temperature in the region of the antarctic circumpolar current have fluctuated through one cycle over the last 100 years less

remote and local forcing in the brazil malvinas region origins of the seasonal variability observed in current meter data from the malvinas falkland current are sought in the wind field on both a regional and circumpolar scale a singular value decomposition of the covariance of the fields makes it possible to distinguish between a local and a remote source of variability the local mode is the result of changes in the wind stress curl around 408s causing an annual modulation transverse to the mean path of the current and thus contributes little to the variability in transport it seems likely that the wind stress curl drives the annual excursions of the brazil malvinas front and forces the retroflection of the malvinas current the remote mode features increased transport trailing negative wind stress curl anomalies at drake passage with a lag of 20 30 days repeating this analysis with the winds on a circumpolar domain suggests that this might be the regional manifestation of a more global feature associating globally negative wind stress curl anomalies north of 608s with increased transport this analysis also shows that the malvinas current transport is anticorrelated with the zonal wind stress hence with the antarctic circumpolar current although initially counterintuitive this is confirmed through coherence analyses with bottom pressure measurements at the south of drake passage these results are further assessed using a 5 yr series of transport in the malvinas current derived from topex poseidon altimeter data which is compared with the circumpolar averages of both zonal wind stress and wind stress curl coherences are largest with the wind stress curl at latitudes north of drake passage up to the subtropics and also indicate that it is most influential in the pacific sector it is suggested that the antarctic circumpolar current and the malvinas current respond differently to the wind forcing and that these two modes cohabit at drake passage while the barotropic variations of the former have been shown to respond to the zonally averaged wind stress the fluctuations of the latter at timescales of 100 200 days are more sverdrupian that is sensitive to the wind stress curl lag 20 days explaining why they fluctuate with opposite phase superimposed on this intraseasonal variability at shorter periods 70 days results from baroclinic shelf waves trapped along the edge of the patagonian plateau these propagate from drake passage and may originate from equatorial kelvin waves in the pacific

an equivalent barotropic mode in the fine resolution antarctic model abstract both the 6 year time mean flow and the eddy kinetic energy in the fine resolution antarctic model are found to be approximately self similar in the vertical with the velocity showing a rapid decay with depth a simple argument is given as to why this would be expected to occur in this and other numerical models and an analysis is given as to the degree of steering of the mean flow by topography

sea level changes at port stanley falkland islands 1 time series of sea level change at port stanley present a rich spectrum of variability from seiches tides and storm surges through to the seasonal interannual and secular changes of interest to ocean circulation and climate change studies this paper emphasizes the strategic importance of the port stanley sea level station within a global sea level network through analysis of the decade length tide gauge records from port stanley itself together with sets of data from a series of temporary tide gauge deployments at several other locations in east falkland the characteristics of seiches and of tidal storm surge and mean sea level variability are described and the frequency of extreme sea level events is determined port stanley sea level variability is shown to be inverse barometer like over a wide range of frequencies although with a significant dependence on the local wind field and an intriguing association with the distant tropical pacific wind field altimeter and tide gauge data in combination demonstrate that the sea level variability is representative of much of the patagonian shelf and neighboring deep ocean consequently port stanley is an ideal site to include within a global network for the purpose of altimeter calibration the utility of port stanley data for monitoring variability in the falkland malvinas current and antarctic circumpolar current is explored port stanley possesses one of the longest records from a southern hemisphere island which indicates an upward sea level secular trend adjusted for station air pressure of 0 70 0 18 statistical 0 25 systematic mm yr since 1964

antarctic circumpolar current frontal system in the south atlantic monitoring using merged argo and animal borne sensor data 1 we describe large scale features of the antarctic circumpolar current acc in the atlantic part of the southern ocean by merging argo data and data obtained by novel animal borne ctd sensors twenty one of these ctd satellite relay data loggers ctd srdls were attached to southern elephant seals mirounga leonina on south georgia the merged data yield unified gridded hydrogaphic fields with high temporal and spatial resolution enabling the determination of features absent in each of the data sets separately the structure and variability of the frontal field revealed by this data set were compared with those in daily quarter degree optimally interpolated sea surface temperature fields and fields of weekly gridded sea level anomaly in general the frontal positions derived using our data set are in agreement with previous work especially where the pathways are constrained by topography e g at the north scotia ridge and the south scotia ridge however with the improved temporal and spacial resolution provided by the ctd srdls we were able to observe some novel features all frontal positions are more variable than previously indicated across the scotia sea and west of the mid atlantic ridge on seasonal time scales the merged data set shows the temporal variability of the southern acc front saccf north of south georgia and in its position east of the island where the saccf lies further north than has been suggested in previous work in addition the subantarctic front crosses the mid atlantic ridge about 400 km further north when compared to previous work

causes of large scale sea level variations in the southern ocean analyses of sea level and a barotropic model 1 we analyze a decade of sea surface height ssh measurements in the southern ocean from the topex poseidon and ers altimeters with a focus on the variability at timescales 2 years among the different processes contributing to large scale ssh variations the barotropic response to the winds dominates poleward of 50 s while thermosteric processes dominate equatorward except for resonant basins for the barotropic modes and regions of intense eddy activity a finite element barotropic model has been developed to analyze the vorticity budget the ssh from the model agrees well with observations the leading barotropic mode which is annular and is confined near antarctica is responsible for most of the barotropic circumpolar transport it is coherent with the zonally integrated eastward wind stress consistent with a free mode response although previously evidenced in bottom pressure data this mode is only partially seen in altimeter data because of ice coverage it nevertheless distinctly appears above the pacific ridges where it expands meridionally up to midlatitudes in the rest of the domain several regions coherent with the local wind stress curl are found these are regions isolated by f h contours mostly deep basins an analysis of the vorticity budget shows that generally topographic sverdrup balance is the leading process for periods 50 days but in some regions resonant basins diffusive and nonstationary terms are important a model experiment shows that transients redistribute energy along f h waveguides contributing to drain resonant regions as was hypothesized in previous works

evidence for interannual and interdecadal climate variability in the south atlantic a singular value decomposition svd analysis is used to determine the coupled modes of variability of monthly sea surface temperature sst and sea level pressure slp data from the south atlantic region for the period 1953 1992 we find that the three leading svd modes respectively account for 63 20 and 6 of the total square covariance the first mode represents an approximately 15 year period oscillation in the strength of the subtropical anticyclone accompanied by fluctuations of a north south dipole structure in the sst it appears to be linked to the global scale interdecadal 15 year joint mode in sst and slp recently studied by mann and park the second mode is characterized by east west displacements of the anticyclone center in association with strong 6 to 7 year period fluctuations of sst off the coast of africa the third mode is characterized by north south displacements of the anticyclone and 4 year period fluctuations in the sst in a broad band across the central south atlantic this mode is strongly correlated with enso

circulation in drake passage revisited using new current time series and satellite altimetry 1 the yaghan basin the complex bathymetry of the drake passage and the meridional extent of the shackleton fracture zone in particular force the subantarctic front saf and the polar front pf to veer to the north and the flow of the antarctic circumpolar current concentrates in the yaghan basin we have studied the circulation in the yaghan basin using 3 years of velocity data january 2006 march 2009 at five mooring sites and 18 years of satellite altimetry data mean velocities at our mooring sites show a dominant eastward component which decreases with depth as expected with a notable exception in the center of the yaghan basin where mean velocities reveal a dominant westward component increasing with depth the mooring data suggest the existence of a permanent strong deep cyclonic circulation over the yaghan seafloor depression in the northeastern part of the yaghan basin the in situ data provide the first opportunity to compare altimetry derived velocities with high temporal resolution near surface current meter velocities in a large eddy kinetic energy environment at high latitudes globally altimetry derived velocities compare rather well with the in situ velocities at 500 m depth both in strength and direction correlations are high between the in situ velocities and the surface velocities derived from satellite altimetric data mean sea level estimates lead to reasonable mean surface velocities with however a slight underestimation of the mean velocity at the mean location of the saf on the continental slope and a more important underestimation of the westward current in the center of the yaghan basin a dominant mode of velocity variations 23 of the variance is observed both in the in situ and satellite data corresponding to a strong southward meander of the saf upstream of the mooring line and a northward meander of the pf downstream of the latter the 18 yearlong altimetry time series shows that the mode is robust and has a strong semiannual component

interannual to decadal changes in the western south atlantic s surface circulation 1 a combination of surface drifters and altimetry is used to analyze the seasonal to interannual variability of the surface velocity field in the brazil malvinas confluence of the western south atlantic ocean longer term changes are inferred from wind and sea surface temperature fields during the period october 1992 to december 2007 a southward shift of 0 6 to 0 9 decade 1 is found in the confluence latitude of the brazil and malvinas currents a comparable trend is found in the latitude of the maximum wind stress curl averaged across the south atlantic basin allowing a proxy for the confluence latitude to be calculated for the prealtimeter time period this longer time series suggests that the recent trend may be part of a longer term oscillation which has returned to values last sustained in the early 1980s this variation does not appear to be related to the multidecadal trend in the southern annular mode but instead is inversely related to long term variations in the sea surface temperature anomaly in the agulhas benguela pathway of the eastern south atlantic subtropical basin

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

on the temporal variability of the transport through drake passage a recently obtained series of bottom pressure measurements from various positions in the drake passage dp has been used to estimate the transport variability of the antarctic circumpolar current acc under the assumption that the variability is predominantly barotropic and thus proportional to the north minus south pressure difference the standard deviations in derived transport range from 5 3 sv in 1993 to 8 9 sv in 1990 10 day filter 1 sv 106 m3 s 1 all values are less than the 10 sv 10 day filter obtained during the international southern ocean studies isos program at dp between the mid 1970s and early 1980s whitworth and peterson 1985 although some of this discrepancy is due to differences in procedure in dealing with gaps in the data our data suggest that the acc transport can be persistently less variable over several years than was previously thought in particular the isos data set contains two large amplitude changes in pressure difference over timescales less than 2 weeks while no similar events were observed in the more recent data although the more recent pressure gauges were deployed at different positions to those used in isos their reliability as indicators of acc transport changes has been established by examination of simultaneous pressure measurements within dp

the malvinas current at the confluence with the brazil current inferences from 25 years of mercator ocean reanalysis twenty five years of high resolution 1 12 ocean reanalysis are used to examine the confluence of the malvinas current mc with the brazil current bc from synoptic to interannual time scales the model transports of the mc 38 0 sv 7 4 sv 57 at 41 s and the bc 23 0 sv 11 sv at 36 s agree with observations the model shows the branching of the mc near the confluence with an offshore branch returning south and an inner branch sinking below the bc and managing to continue northward along the continental slope northward velocities associated with the subsurface inner branch peak at 40 cm s at 36 s at 700 m depth the model documents the migrations of the subantarctic saf and subtropical fronts stf at the confluence the saf and stf positions vary over a large range at synoptic 800 km and interannual scale 300 and 200 km respectively compared to the rather small seasonal migrations of the stf 150 km and saf 50 km while trends in the mc are small over the 25 years of the reanalysis the bc becomes more intense 12 5 cm s saltier 0 37 psu and warmer 2 5 c in the upper 1 000 m these trends are accompanied with a southward displacement of the stf and the saf of 150 and 50 km

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

co seismic displacements of the 1994 northridge california earthquake the 17 january 1994 northridge california earthquake significantly deformed the earth s crust in the epicentral region displacements of 66 survey stations determined from global positioning system gps observations collected before and after the earthquake show that individual stations were uplifted by up to 417 5 mm and displaced horizontally by up to 216 3 mm using these displacements we estimate parameters of a uniform slip model fault geometry and slip are estimated independent of seismological information using monte carlo optimization techniques that minimize the model residuals the plane that best fits the geodetic data lies 1 to 2 km above the plane indicated by aftershock seismicity modeling for distributed slip on a coplanar yet larger model fault indicates that a high slip patch occurred up dip and northwest of the mainshock hypocenter and that less than 1 m of slip occurred in the uppermost 5 km of the crust this finding is consistent with the lack of clear surface rupture and with the notion that the intersection with the fault that ruptured in 1971 formed the up dip terminus of slip in the northridge earthquake displacements predicted by either of these simple models explain most of the variance in the data within 50 km of the epicenter on average however the scatter of the residuals is twice the data uncertainties and in some areas there is significant systematic misfit to either model the co seismic contributions of aftershocks are insufficient to explain this mismatch indicating that the source geometry is more complicated than a single rectangular plane

the coupled patterns between sea level pressure and sea surface temperature in the midlatitude north atlantic abstract monthly variability of atmosphere ocean interactions in the midlatitude north atlantic during the winter months november april is examined composite and singular value decomposition svd analyses are applied to the observed sea level pressure slp and sea surface temperature sst anomalies of each winter month for the period 1950 1987 the slp anomaly composites i e slp differences between selected warm and cold sst months are constructed based on the averaged sst anomalies over the rm region 60 40 w and 50 40 n these composites shift from a positive monopole pattern in early winter to a dipole pattern in midwinter and then back to a monopole pattern in late winter a complementary svd analysis reveals that the first svd mode is dipole structured and especially dominant in midwinter the second svd mode is monopole featured and more dominant in early and late winter than in midwinter by examining the spatial distributions of the svd modes and especially their similarities to the p

the oak ridge fault system and the 1994 northridge earthquake the 17 january 1994 northridge earthquake moment magnitude m w 6 7 in the san fernando valley in southern california illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1 this fault lies beneath the active north dipping santa susana fault system and uplifted both the footwall and the hanging wall of the santa susana fault during the earthquake1 here we argue that footwall uplift on the santa susana fault before the earthquake could have been used to identify the northridge blind thrust as an active fault moreover we propose that the northridge earthquake occurred on a continuation of the oak ridge fault system which reaches the surface in the ventura basin to the west slip rates on the western part of this fault system 2 3 are nearly three times larger than on the northridge blind thrust 4 increasing the probability of a n or t bridge sized earthquake in the heavily populated ventura basin

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

malvinas current variability from argo floats and satellite altimetry the malvinas current mc is an offshoot of the antarctic circumpolar current acc downstream of drake passage the northern fronts of the acc veer northward cross over the north scotia ridge nsr and the malvinas plateau and enter the argentine basin we investigate the variations of the mc circulation between the nsr and 41 s and their possible relations with the acc circulation using data from argo floats and satellite altimetry the data depict meandering and eddy shedding of the northern acc jets as they cross the nsr the altimetry fields show that these eddies are trapped break down and dissipate over the malvinas plateau suggesting that this region is a hot spot for dissipation of mesoscale variability variations of sea level anomalies sla across the nsr do not impact the mc further north except for intra seasonal variability associated with coastal trapped waves altimetry and float trajectories show events during which a large fraction of the mc is cut off from the acc blocking events at around 48 5 s are a recurrent feature of the mc circulation over the 23 year altimetry record we detected 26 events during which the mc surface transport at 48 5 s was reduced to less than half its long term mean blocking events last from 10 to 35 days and do not present any significant trend these events were tracked back to positive sla that built up over the argentine abyssal plain future work is needed to understand the processes responsible for these blocking events

dynamics of sea surface temperature anomalies in the southern ocean diagnosed from a 2d mixed layer model we analyze the processes responsible for the generation and evolution of sea surface temperature anomalies observed in the southern ocean during a decade based on a 2d diagnostic mixed layer model in which geostrophic advection is prescribed from altimetry anomalous air sea heat flux is the dominant term of the heat budget over most of the domain while anomalous ekman heat fluxes account for 20 40 of the variance in the latitude band 40 60 s in the acc pathway lateral fluxes of heat associated with anomalous geostrophic currents are a major contributor dominating downstream of several topographic features reflecting the influence of eddies and frontal migrations a significant fraction of the variability of large scale sst anomalies is correlated with either enso or the sam each mode contributing roughly equally the relation between the heat budget terms and these climate modes is investigated showing in particular that anomalous ekman and air sea heat fluxes have a co operating effect with regional exceptions hence the large sst response associated with each mode it is further shown that enso or sam locked anomalous geostrophic currents generate substantial heat fluxes in all three basins with magnitude comparable with that of atmospheric forcings for enso and smaller for the sam except for limited areas enso locked forcings generate sst anomalies along the acc pathway and advection by mean flows is found to be a non negligible contribution to the heat budget exhibiting a wavenumber two zonal structure characteristic of the antarctic circumpolar wave by contrast sam related forcings are predominantly zonally uniform along the acc hence smaller zonal sst gradients and a lesser role of mean advection except in the southwest atlantic while modeled sst anomalies are significantly correlated with observations over most of the southern ocean the analysis of the data model discrepancies suggests that vertical ocean physics may play a significant role in the nonseasonal heat budget especially in some key regions for mode water formation

on the origins of the variability of the malvinas current in a global eddy permitting numerical simulation 1 this article investigates the origins of the variability of the malvinas current mc transport using the results of an eddy permitting ocean general circulation model first the dynamical links between the variabilities of the mc and the antarctic circumpolar current acc transports at the drake passage are established time series analyses indicate that the connection between the transport variations of the acc and the mc is masked by high frequency oscillations although a substantial portion of this highfrequency variability is produced and dissipated locally there are also anomalies that propagate between those regions the most conspicuous of these signals has an average speed of 6 7 cm s 1 and a period of 150 days second the contribution of the wind forcing to the variability of the acc and the mc is determined from principal estimator patterns analysis the analysis indicates that the variance of the transport of both currents is more directly related to the variability of the wind stress than to the wind stress curl the transport variability is particularly sensitive to zonal wind stress anomalies in the latitudinal range of the drake passage that extends farther north over the south indian ocean a rather counterintuitive result of our analysis is that the wind stress forcing over the south atlantic ocean is unimportant to the variability of the mc transport

late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures paleoseismic investigation of the northridge hills fault in the northern san fernando valley california helps assess the timing and style of near surface late quaternary deformation in the epicentral area of the 1994 northridge earthquake the northridge hills fault a 15 km long north dipping reverse fault exhibits geomorphic evidence of late quaternary surface deformation including topographic scarps across late quaternary fluvial terraces and aligned alluvial fan apices on the footwall block we excavated one 40 m long trench and six test pits and drilled nine boreholes across a 2 m high scarp developed on a probable holocene fluvial terrace adjacent to aliso canyon wash a continuous clayey gravel identified in the trench test pits and boreholes defines a south facing monocline with 6 1 m of vertical separation across the northridge hills fault based on pedochronology the clayey gravel ranges in age from 6 to 30 ka the borehole data also suggest that an unconformity developed on the plio pleistocene saugus formation is warped into a monocline that has 13 2 m of vertical separation across the fault these preliminary data yield a dip slip rate of 1 0 0 7 mm yr for the northridge hills fault the absence of distinct scarp derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small incremental uplifts rather than large uplifts that generate distinct scarp relief we postulate that such uplift could be produced via moderate magnitude earthquakes m w 6 on the northridge hills fault or secondary deformation induced by earthquakes on other faults for instance evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the northridge hills fault produced by movement on the underlying northridge blind reverse fault or other nearby large structures based on our geologic investigations the distribution of aftershocks following the 1994 earthquake and pre and post 1994 leveling and geodetic surveys we interpret that the northridge hills fault underwent triggered slip during the 1994 earthquake

anatomy of subinertial waves along the patagonian shelf break in a 1 12 global operational model the patagonian slope hosts a variety of waves we used a state of the art ocean reanalysis to examine waves at the shelf break and in the core of the malvinas current mc at periods larger than 10 days statistics over 25 years indicated three types of signals in phase signals at specific locations of the shelf break to the south of 47 s fast propagating signals all along the shelf break phase speed from 140 cm s to 300 cm s at periods between 5 and 110 days and slower signals in the core of the mc phase speeds from 10 cm s to 30 cm s at 20 day 60 day and 100 day periods the large zonal wind stress variations south of 47 s forced in phase along slope velocity variations and triggered fast propagating waves at distinct sites corresponding to abrupt changes in the shelf break orientation the shelf break waves modulated the intensity of the inshore jet which varied from 0 to 30 cm s at 100 m depth and had spatial and temporal structures and scales consistent with those of observed upwelling events slow propagating waves in the core of the mc had along slope wavelengths between 450 and 1200 km and were not forced by the local winds they were tracked back to the drake passage and the malvinas escarpment

combined gps and insar models of postseismic deformation from the northridge earthquake models of combined global positioning system gps and interferometric synthetic aperture radar insar data collected in the region of the northridge earthquake indicate that significant afterslip on the main fault occurred following the earthquake additional shallow deformation occurred to the west of the main rupture plane both data sets are consistent with logarithmic time dependent behavior following the earthquake indicative of afterslip rather than postseismic relaxation aftershocks account for only about 10 of the postseismic motion the two data sets are complimentary in determining the postseismic processes fault afterslip and shallow deformation dominate the deformation field in the two years following the earthquake lower crustal deformation may play an important role later in the earthquake cycle

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

a test of the ability of topex poseidon to monitor flows through the drake passage topex poseidon t p altimeter data from the drake passage dp area of the southern ocean near to the highest latitudes accessible by the satellite are found to agree as well in general to data from an extensive set of in situ instrumentation as has been observed for other parts of the ocean root mean square rms differences between subsurface pressures ssp measured by altimetry and bottom pressures bp measured by in situ devices are approximately 2 mbar in the northern dp and 4 mbar in the south after application of a 20 day low pass filter two mbar accuracy would suggest that altimetry could be capable of providing useful additional information on antarctic circumpolar current acc transport fluctuations through the dp on seasonal and interannual timescales the larger rms differences on what is considered to be the more important southern side are explainable in terms of baroclinic variability at the southern end of the world ocean circulation experiment woce sr1 section the first woce conductivity temperature depth data sets from sr1 have confirmed the existence of a major baroclinic eddy feature in the south which could account to a great extent for the larger rms values better southern dp locations could possibly have been chosen for woce however further recent in situ measurements from other locations near to the dp have shown that the southern dp acc related signals in ssp or bp are only slightly larger than t p accuracy which suggests that as yet they cannot be adequately monitored by means of altimetry alone whichever location is chosen even if the altimetric and in situ observations were perfect would they provide information directly relatable to transport variability through the passage this question has been studied by means of fine resolution antarctic model fram numerical modeling investigations which have demonstrated the complexity of the correlation between the variability in dp transport and ssp or sea level within fram northern ssp or bp variations are virtually uncorrelated with transport fluctuations and the modelling has confirmed the importance of southern measurements as inferred from in situ measurements over a decade ago it is evident that for the foreseeable future an extensive set of in situ instrumentation will be required in addition to the altimetry if further insight into circulation in this part of the southern ocean is to be obtained

kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 northridge california earthquake geodetic observations of surface deformation associated with the 1994 northridge southern california earthquake generally are reproduced by simple models of a large scale elastic dislocation on a blind or buried thrust fault the smaller scale aftershocks of the northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock these aftershocks averaged over volumes that are large relative to their rupture radii reflect a distributed permanent deformation that is accommodated by local brittle fracture we use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain aftershock deformation that occurred at depths below about 6 km and which is associated with the primary rupture zone is consistent with slow continuation of the southwest side up reverse slip on the blind northridge thrust fault in contrast aftershock deformation from the upper 5 7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal ne sw shortening and horizontal nw se i e fault parallel extension this pattern of deformation is similar to that associated with the mainshock as observed geodetically and as calculated from the elastic dislocation model we interpret that the aftershock activity in the hanging wall represents the quasi ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block the aftershocks along the mainshock rupture zone are interpreted as resulting from either 1 the time dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or 2 the continued slip in the weakened fault zone driven by the deformation of a ductile elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust in either case the aftershock activity suggests that the crust undergoes quasi ductile flow as a brittle elastic material and is not a strictly elastic material

the slip history of the 1994 northridge california earthquake determined from strong motion teleseismic gps and leveling data we present a rupture model of the northridge earthquake determined from the joint inversion of near source strong ground motion recordings p and sh teleseismic body waves global positioning system gps displacement vectors and permanent uplift measured along leveling lines the fault is defined to strike 122 and dip 40 to the south southwest the average rake vector is determined to be 101 and average slip is 1 3 m the peak slip reaches about 3 m our estimate of the seismic moment is 1 3 0 2 10 26 dyne cm potency of 0 4 km3 the rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike nearly 20 km in the dip direction and there is no indication of slip shallower than about 5 to 6 km the up dip strong motion velocity waveforms are dominated by large s wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals a few seconds apart stations at southern azimuths indicate two main s wave arrivals separated longer in time about 4 to 5 sec these observations are best modeled with a complex distribution of subevents the initial s wave arrival comes from an asperity that begins at the hypocenter and extends up dip and to the north where a second larger subevent is centered about 12 km away the secondary s arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km 8 km west of the hypocenter the resolving power of the individual data sets is examined by predicting the geodetic gps and leveling displacements with the dislocation model determined from the waveform data and vice versa and also by analyzing how well the teleseismic solution predicts the recorded strong motions the general features of the geodetic displacements are not well predicted from the model determined independently from the strong motion data likewise the slip model determined from geodetic data does not adequately reproduce the strong motion characteristics whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data the strong motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong motion velocity recordings it does a poor job of predicting the geodetic data consequently a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets

short term velocity and water pressure variations down glacier from a riegel storglaciaren sweden during the 1991 94 summer field seasons time correlated measurements of water pressure and surface speed were made over and down glacier from a major riegel on storglaciaren sweden measurements were made at sub hourly time scales in order to discern details in the diurnal cycle large water input events typically associated with rain storms produced coherent lagged surface velocity responses that could be understood in terms of till deformation or decoupling and these have been discussed elsewhere the consequences of smaller diurnal water pressure events were more enigmatic in that acceleration of ice flow generally preceded the onset of the local water pressure rise from consideration of these data and other work done on the hydrology of storglaciaren we infer that the ice in this area is generally pushed from behind via a relaxation in extensional strain across the riegel hence accelerations occur in response to increases in water pressure that occur up glacier and that precede local water pressure rises in addition following a period of large storm events surface speeds became more spatially coherent and were in phase with the diurnal ater pressure cycle this suggests that the large water pressure events lead to a spatially more homogeneous subglacial drainage system sliding laws need to take into accoun such temporal changes in spatial coherence of the subglacial drainage system

the barotropic response of the subpolar north pacific to stochastic wind forcing in a recent observational study koblinsky et al 1989 attempted to demonstrate that the topographic sverdrup balance can account for mesoscale current fluctuations with periods between 10 and 100 days in the abyssal north pacific however the topographic sverdrup balance proved difficult to verify systematically over the north pacific in this paper the conditions required to obtain the topographic sverdrup balance are investigated through numerical experiments with a stochastically forced barotropic quasi geostrophic numerical model of the subpolar north pacific in the numerical experiments the topographic sverdrup balance emerges from area averages of the terms in the vorticity equation but only for periods longer than about 40 days when the averages are taken over 4 4 region this period increases when averages are taken over progressively smaller regions an examination of vorticity balances at individual points along sections through the model domain indicates that the topographic sverdrup balance is not obtained at individual points even for very long period motions the possibility of verifying topographic sverdrup dynamics with oceanic data is considered it is argued that topographically generated small scales in the velocity field reduce the likelihood that the topographic sverdrup balance can be systematically observed in velocity measurements from individual current meter moorings the results suggest that the vorticity balance is complex and that topographic sverdrup dynamics cannot be expected to provide an adequate explanation for the seasonal modulation of the abyssal eddy kinetic energy that is observed with moored instrumentation in the north pacific

variations in the malvinas current volume transport since october 1992 1 we present a set of new current meter measurements collected to monitor the malvinas current mc near its merger with the brazil current at 40 41 s from december 2001 to february 2003 below a jason 1 altimeter track these measurements are compared to former measurements obtained 8 years earlier at the same location they also provide new information on the core of the mc on the continental slope above the 1000 m isobath where a mooring had been previously lost there most of the velocity variation is along isobath 80 of the variance and shows a significant annual cycle the two data sets provide coherent means and statistical parameters on the vertical structure of the flow a 14 year long time series of mc volume transport is derived using satellite altimetry the good correlation between the altimetry derived transport and the transport estimated from the current meter data persists in time over 0 7 for each measurement period a spectacular shift in the spectral composition of transport variations was observed from 1992 until the end of 1997 transport variations occurred at rather short periods 50 90 days and to some degree around 180 days whereas after year 2000 longer periods including a seasonal cycle predominated altimetry derived anomalies of surface geostrophic velocities along the core of the mc show a similar shift in spectral composition suggesting a remote forcing origin

northridge earthquake rupture models based on the global positioning system measurements abstract we use global positioning system gps data to study the rupture mechanism of the 1994 northridge earthquake in southern california we include data from 62 observation sites of which two palos verdes and jet propulsion lab are permanent gps geodetic array pgga sites we use a grid search scheme to study the range of single and dual plane uniform and varied slip models consistent with the data we find that in order to fit the geodetic data with a fault model whose primary fault patch is confined to a plane through the aftershocks a secondary fault plane is required above the primary fault plane the moment release of the secondary fault can be as large as 1 9 10 18 n m 14 of the moment release of the primary fault this result implies significant deformation in the shallow crust associated with the mainshock our preferred model has a 14 14 array of dislocation patches on a plane through the main aftershock cluster and a 5 6 array of patches in the hanging wall west of the epicenter we estimate the displacements on the patches by linear inversion with a first order smoothness constraint the estimated displacements on the main fault for this model are confined to a simple region between depths of 5 and 18 km in the interior of the modeled fault surface the mainshock lies at the bottom of the aftershock zone near which about 1 m slip is shown on our modeled fault surface the maximum slip on the fault surface is about 2 2 m located at 34 28 n 118 55 w and 12 4 km at depth the seismic moment release estimate of 1 34 0 15 10 19 n m on the main fault at the 95 confidence is consistent with the estimate from strong motion studies

complex faulting associated with the 22 december 2003 mw 6 5 san simeon california earthquake aftershocks and postseismic surface deformation we use data from two seismic networks and satellite interferometric synthetic aperture radar insar imagery to characterize the 22 december 2003 m w 6 5 san simeon earthquake sequence absolute locations for the mainshock and nearly 10 000 aftershocks were determined using a new three dimensional 3d seismic velocity model relative locations were obtained using double difference the mainshock location found using the 3d velocity model is 35 704 n 121 096 w at a depth of 9 7 0 7 km the aftershocks concentrate at the northwest and southeast parts of the aftershock zone between the mapped traces of the oceanic and nacimiento fault zones the northwest end of the mainshock rupture as defined by the aftershocks projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the oceanic fault near the santa lucia range front and the 5 mm postseismic insar imagery contour the oceanic fault in this area as mapped by hall 1991 is therefore probably a second order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth the southeast end of the rupture projects closer to the mapped oceanic fault trace suggesting much of the slip was along this fault or at a minimum is accommodating much of the postseismic deformation insar imagery shows 72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture and 48 and 45 mm at the northwest and southeast end of the aftershock zone respectively from these observations we model a 30 km long northwest trending northeast dipping mainshock rupture surface called the mainthrust which is likely the oceanic fault at depth a 10 km long southwest dipping backthrust parallel to the mainthrust near the hypocenter several smaller southwest dipping structures in the southeast and perhaps additional northeast dipping or subvertical structures southeast of the mainshock plane discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic nacimiento fault zone at depth the insar data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone the well defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the santa lucia range as a popup block aftershocks in the southeast part of the zone also indicate a popup block but it is less well defined the absence of backthrust features in the central part of the zone suggests range front uplift by fault propagation folding or backthrusts in the central part were not activated during the mainshock

multiple oscillatory modes of the argentine basin part i statistical analysis observations of the sea surface height in the argentine basin indicate that strong variability occurs on a time scale of 2030 days the aim of this study is to determine the physical processes responsible for this variability first results are presented from two statistical techniques applied to a decade of altimetric data a complex empirical orthogonal function ceof analysis identifies the recently discovered dipole mode as the dominant mode of variability a principal oscillation pattern pop analysis confirms the existence of this mode which has a period of 25 days the second ceof displays a propagating pattern in the northern argentine basin plus a rotating dipole in the southwest corner the pop analysis identifies both patterns as individual modes with periods of 30 and 20 days respectively second the barotropic normal modes of the argentine basin are studied using a shallow water model capturing the full bathymetry of the basin coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability this analysis implies that the 20 day mode detected by recent bottom pressure measurements is a true barotropic mode however the 25 day variability as found in altimeter data cannot be directly attributed to the excitation of a free rossby basin mode this study indicates that the results of several apparently conflicting observations of the flow variability in the argentine basin can be reconciled by assuming that multiple basin modes are involved

theory of slickenline patterns based on the velocity gradient tensor and microrotation abstract brittle fault zones commonly are characterized by a multitude of lineated shear planes having a wide distribution of orientations in our model we assume the faulted rock comprises an aggregate of rigid blocks whose surfaces are the shear planes we define two independent scales of motion to distinguish the local rigid rotation of the blocks with their shear planes the micromotion from the average deformation of the material on a macroscopic scale the macromotion the macromotion is defined by the macrovelocity gradient tensor which is separated into symmetric and antisymmetric parts the deformation rate and the macrospin tensors respectively the micromotion is defined by a microspin tensor on any shear plane slickenlines form parallel to the direction of the maximum rate of shear which we assume is determined by the direction of the maximum component of the macrovelocity gradient tensor tangent to the shear plane and by a component of the microspin in the shear plane for a restricted kinematic model patterns of slickenline orientations are defined on a uniform distribution of shear plane orientations by the values of d and w d is a ratio of differences in the principal values of the deformation rate tensor and w is a ratio of the net spin to the maximum rate of macroshear we distinguish between instantaneous slickenline patterns and finite deformation patterns the two are different if the shear planes rotate relative to the principal deformation rate axes in which case finite deformation slickenlines form curved or crossing lineations if w 0 the instantaneous slickenline patterns have orthorhombic symmetry 0 d d 1 or 0 if w 0 the instantaneous slickenline patterns have monoclinic symmetry for pure shear in an isotropic body instantaneous slickenline patterns have orthorhombic symmetry w 0 d 0 5 whereas for pure shear accommodated by shear on a planar anisotropy w 1 d 0 5 the pattern is monoclinic for simple shear with the rigid rotation of local shear planes defined by the shear induced spin w 0 d 0 5 the instantaneous slickenline pattern is orthorhombic but the finite deformation pattern is monoclinic for simple shear with no shear plane rotation w 1 d 0 5 the instantaneous pattern is monoclinic special cases of our hypothesis are equivalent to the hypothesis that slickenlines are parallel to the direction of maximum resolved shear stress e g angelier 1979 1984 the slickenline patterns predicted for this case are always of orthorhombic or higher symmetry and are equivalent to our instantaneous patterns for which w 0 and to our finite deformation patterns for which in addition there is no microspin