polynya dynamics drive primary production in the larsen a and b embayments following ice shelf collapse 1 the climate driven collapses of the larsen a and b ice shelves have opened up new regions of the coastal antarctic to the influence of sea ice resulting in increases in seasonal primary production in this study passive microwave remote sensing of sea ice concentration and satellite imagery of ocean color are employed to quantify the magnitude of and variability in open water area and net primary productivity npp in the larsen embayments between 1997 and 2011 numerical model output provides context to analyze atmospheric forcing on the coastal ocean following ice shelf disintegration the embayments function as coastal sensible heat polynyas the larsen a and b are as productive as other antarctic shelf regions with seasonally averaged daily npp rates reaching 1232 and 1127 mg c m 2 d 1 and annual rates reaching 200 and 184 g c m 2 yr 1 respectively a persistent cross shelf gradient in npp is present with higher productivity rates offshore contrasting with patterns observed along the west antarctic peninsula embayment productivity is intimately tied to sea ice dynamics with large interannual variability in npp rates driven by open water area and the timing of embayment opening opening of the embayment is linked to periods of positive southern annular mode and stronger westerlies which lead to the vertical deflection of warm maritime air over the peninsula and down the leeward side causing increases in surface air temperature and wind velocity high productivity in these new polynyas is likely to have ramifications for organic matter export and marine ecosystem evolution

laboratory earthquakes along inhomogeneous faults directionality and supershear we report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch rupture is always found to be asymmetric bilateral in one direction rupture always propagates at the generalized rayleigh wave speed whereas in the opposite direction it is subshear or it transitions to supershear the lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in parkfield california and in north anatolia

dynamic modeling of the 2004 mw 6 0 parkfield california earthquake 1 we present two spontaneous rupture models of the 2004 mw 6 0 parkfield earthquake constrained by near source ground motions we start with a stress drop distribution calculated from a kinematic slip distribution using a linear slip weakening friction law we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records the material contrast across the san andreas fault is incorporated using different one dimensional velocity structures on each side of the fault an approximately constant s parameter of 0 3 and a uniform slip weakening distance of 0 15 m are used in the dynamic models in our preferred dynamic model consistent with the ground motion and gps the slip is bounded by seismicity streaks at 5 and 10 km depths confirming a locked zone at depth the stress drop is approximately 10 mpa in the hypocentral region and about 2 mpa elsewhere the material contrast across the fault causes significant normal stress variations 1 mpa leading to a larger strength drop to the southeast than to the northwest the main rupture front propagates at nearly a constant subshear rupture velocity 3 km s in both directions the total radiated energy determined from the preferred dynamic model is 1 1 1013 j seismic moment is 1 0 1018 nm and fracture energy is 3 0 1013 j the limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

breakup and conditions for stability of the northern larsen ice shelf antarctica the breakup of ice shelves has been widely regarded as an indicator of climate change1 with observations around the antarctic peninsula having shown a pattern of gradual retreat associated with regional atmospheric warming and increased summer melt and fracturing processes2 3 4 5 6 7 8 9 the rapid collapse of the northernmost section of the larsen ice shelf larsen a over a few days in january 1995 indicated that after retreat beyond a critical limit ice shelves may disintegrate rapidly here we use a finite element numerical model that treats ice as a continuum without fracture10 to examine the breakup history2 between 1986 and 1997 of the two northern sections of larsen ice shelf larsen a and larsen b from which we establish stability criteria for ice shelves analysis of various ice shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf on larsen a only the initial and final ice front configurations show a stable pattern larsen b at present exhibits a stable pattern but if the ice front were to retreat by a further few kilometres it too is likely to enter an irreversible retreat phase

rupture directivity of small earthquakes at parkfield 1 theoretical modeling of strike slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction we test this model with 450 small earthquakes 2 3 exhibiting southeast rupture characteristics some spatial and temporal variability in rupture directivity is also apparent we observe a higher proportion of northwest directivity ruptures following the 2004 m 6 parkfield earthquake which ruptured toward the northwest our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors

a seasonal cycle in the export of bottom water from the weddell sea dense water formed over the antarctic continental shelf rapidly descends into the deep ocean where it spreads throughout the global ocean as antarctic bottom water1 2 the coldest and most voluminous component of this water mass is weddell sea bottom water1 3 4 5 6 7 here we present observations over eight years of the temperature and salinity stratification in the lowermost ocean southeast of the south orkney islands marking the export of weddell sea bottom water we observe a pronounced seasonal cycle in bottom temperatures with a cold pulse in may june and a warm one in october november but the timing of these phases varies each year we detect the coldest bottom water in 1999 and 2002 whereas there was no cold phase in 2000 on the basis of current velocities and water mass characteristics we infer that the pulses originate from the southwest weddell sea we propose that the seasonal fluctuations of weddell sea bottom water properties are governed by the seasonal cycle of the winds over the western margin of the weddell sea interannual fluctuations are linked to the variability of the wind driven weddell sea gyre and hence to large scale climate phenomena such as the southern annular mode and el nino southern oscillation

the 2004 mw6 0 parkfield california earthquake inversion of near source ground motion using multiple data sets on 28 september 2004 the m w 6 0 parkfield earthquake became the most densely recorded earthquake for near source ground motion to infer the kinematic nature of this event we invert the strong motion data the non linear global inversion yields slip amplitude slip rake average rupture velocity and rise time over the fault by using subsets of the data we study the dependence of the kinematic solutions on data input the inversions reveal that the slip amplitude was less than 0 65 m and outline two major areas of slip one that laterally surrounds the hypocenter preferentially extending to its se the other 10 to 20 km nw of the hypocenter at a depth between 2 and 8 km the slip amplitude we obtain for each point on the fault varies less than 0 15 m depending on data set used the rake angle variability is less than 40 degrees

late pleistocene holocene marine conditions in the ross sea antarctica evidence from the diatom record statistical analyses of diatom assemblages from radiocarbon dated sediment cores were used to reconstruct changing palaeoceanographic conditions in the western and west central ross sea antarctica from c 12 14c kyr bp to the present data from three kasten cores support a north to south time transgressive glacial interglacial transition assemblages at the base of each core suggest that glacial processes affecting frustule preservation were significant during the late pleistocene increasing biogenic silic and decreasing per centages of reworked species suggest that upper water column productivity instead of preservational processes increasingly dominates diatom assemblages during grounding line recession c 12to c 6 14c kyr bp a warm ing during the middle to late holocene c 6to c 3 14c kyr bp may have resulted in a spring sea ice melting prior to the annual inception of the ross sea polynya in the west central ross sea and b increased plateletice delivery due to increased melting beneath t

surface circulation at the tip of the antarctic peninsula from drifters an array of 40 surface drifters drogued at 15 m depth was deployed in february 2007 to the east of the tip of the antarctic peninsula as part of the antarctic drifter experiment links to isobaths and ecosystems adelie project data obtained from these drifters and from a select number of local historical drifters provide the most detailed observations to date of the surface circulation in the northwestern weddell sea the antarctic slope front asf characterized by a 20 cm s 1 current following the 1000 m isobath is the dominant feature east of the peninsula the slope front bifurcates when it encounters the south scotia ridge with the drifters following one of three paths drifters i are carried westward into bransfield strait ii follow the 1000 m isobath to the east along the southern edge of the south scotia ridge or iii become entrained in a large standing eddy over the south scotia ridge drifters are strongly steered by contours of f h coriolis frequency depth as shown by calculations of the first two moments of displacement in both geographic coordinates and coordinates locally aligned with contours of f h an eddy mean decomposition of the drifter velocities indicates that shear in the mean flow makes the dominant contribution to dispersion in the along f h direction but eddy processes are more important in dispersing particles across contours of f h the results of the adelie study suggest that the circulation near the tip of the antarctic peninsula may influence ecosystem dynamics in the southern ocean through antarctic krill transport and the export of nutrients

ice retreat in the eastern antarctic peninsula region the application of diatoms for understanding climate change abstract climate driven retreat of ice shelves on the eastern side of the antarctic peninsula has opened up new coastal marine environments enabling the development of primary productivity diatom assemblages in surface sediment samples from the eastern side of the antarctic peninsula were analyzed to investigate the use of diatoms as paleoenvironmental indicators of changes in glacial and sea ice extent here we present surface sediment diatom assemblage data from samples taken on 5 cruises between 2000 2012 from the northeastern most tip of the antarctic peninsula prince gustav channel larsen a and larsen b embayments and a single sample from just east of larsen c ice shelf absolute diatom abundance increases to the northeast reflecting greater contribution of diatom valves originating from surface waters that are ice free for a longer period of time this trend is mirrored by the relative abundance of chaetoceros subg hyalochaete a spring bloom indicator group the genus fragilariopsis is dominated by sea ice associated species including fragilariopsis curta fragilariopsis cylindrus fragilariopsis obliquecostata fragilariopsis sublinearis and fragilariopsis vanheurckii fragilariopsis spp relative abundance varies inversely with chaetoceros subg hyalochaete distributions of thalassiosira antarctica t1 and thalassiosira antarctica t2 interpreted as a cooler and warmer forms respectively while generally inverse to one another with the t antarctica t2 more common to the northeast are not straightforward with a pocket of high relative abundance in larsen b embayment these diatom data and their comparison to modern oceanography of the eastern antarctic peninsula provide a basis for further paleoceanographic work in the region

climatically induced retreat and collapse of northern larsen ice shelf antarctic peninsula the areal changes of the northern larsen ite shelf lis antarctic peninsula between march 1986 and march 1997 have been analyzed based on synthetic aperture radar images of the european remote sensing satellites ers 1 and ers 2 and on landsat images this analysis is complemented by data on ice motion and surface mass balance which have been obtained during several field campaigns since the early 1980s after a period of retreat coinciding with atmospheric warming and with decreasing net accumulation at the surface due to melt losses the two northernmost sections of lis disintegrated almost completely within a few days in january 1995 recent observations of the ice shelf section north of jason peninsula which is presently the northernmost section of lis show increased summer melt and intensification of the rifting processes probably causing accelerated retreat of this section in the near future the retreat and the disintegration event of lis indicate high sensitivity of ice shelves to prolonged perturbations of the mass balance

aftershock asymmetry on a bimaterial interface to better understand the asymmetric distribution of microearthquake aftershocks along the central san andreas fault we study dynamic models of slip weakening ruptures on an interface separating differing elastic half spaces subshear ruptures grow as slightly asymmetric bilateral cracks with larger propagation velocities slip velocities and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed the southeast front in the context of the san andreas when the se front encounters a stress barrier the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse this slip pulse smooths the stress field and reduces the static stress change beyond the se front furthermore because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon the se rupture front can be left far below the failure threshold while the nw front remains quite close to failure both mechanisms could contribute to the observed aftershock asymmetry formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip weakening law to achieve this while minimizing off fault damage requires either substantial velocity contrasts or small reductions in friction the simulations also show a pronounced asymmetry in the timescales over which barriers to the se and nw experience increasing stresses a result that has implications for the asymmetric distribution of subevents in compound earthquakes

on eddy scales in the eastern and northern north atlantic ocean as a function of latitude infrared satellite images and two ensembles of satellite tracked buoys have been used to analyze eddy scales in the central and eastern north atlantic between 35 n and 60 n both data sets yield eddy scales which decrease northward from the subtropical atlantic and can be closely related to the rossby radius of the first baroclinic mode

late quaternary sea ice history in the indian sector of the southern ocean as recorded by diatom assemblages abstract a modern analog technique mat5201 31 has been applied to fossil diatom assemblages to provide down core estimates of february sea surface temperatures ssts and of sea ice duration over the past 220 000 years at 56 40 s 160 14 e at the core location sea ice progression lagged the sst drop by 1 ka at interglacial glacial transitions and sea ice retreat was almost synchronous to the sst increase at glacial interglacial terminations sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times although ssts were almost constant during glacials this indicates that ssts are the major parameter determining the advance and retreat of sea ice at transitions but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo air temperature and meridional wind stress the strong correlation r 0 75 between sea ice duration at the core location and the vostok co2 record argues for a control of antarctic sea ice extent on atmospheric co2 concentration via the modification of the ocean to atmosphere gas balance

kinematic inversion of the 2004 m 6 0 parkfield earthquake including an approximation to site effects the 2004 m 6 0 parkfield earthquake yielded one of the largest amounts of near source strong ground motion seismic data ever we invert strong motion seismograms to obtain a model for the space time distribution of coseismic slip on the fault to reduce noise in the inversion we take into account local amplifications that affect each station by using records of the 1983 m 6 5 coalinga earthquake site amplification correlates well with large peak ground velocities registered during the 2004 parkfield mainshock the inversion for a kinematic rupture model yields a nonunique solution we therefore analyze various rupture models that explain the data equally well our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter where the maximum slip is 57 cm a secondary slip area over which contours are not well resolved is located northwest of the hypo center the rupture speed is highly heterogeneous we infer an average rupture ve locity of 2 8 km sec close to the hypocenter and of 3 3 km sec in the secondary region of large slip to the northwest of the hypocenter by correlation of our rupture model with both microseismicity and velocity structure we identify six patches on the fault plane that behave in seismically distinct ways online material kinematic rupture model parameters

introduction to the special issue on the 2004 parkfield earthquake and the parkfield earthquake prediction experiment the 28 september 2004 m 6 0 parkfield earthquake a long anticipated event on the san andreas fault is the world s best recorded earthquake to date with state of the art data obtained from geologic geodetic seismic magnetic and electrical field networks this has allowed the preearthquake and postearthquake states of the san andreas fault in this region to be analyzed in detail analyses of these data provide views into the san andreas fault that show a complex geologic history fault geometry rheology and response of the nearby region to the earthquake induced ground movement although aspects of san andreas fault zone behavior in the parkfield region can be modeled simply over geological time frames the parkfield earthquake prediction experiment and the 2004 parkfield earthquake indicate that predicting the fine details of future earthquakes is still a challenge instead of a deterministic approach forecasting future damaging behavior such as that caused by strong ground motions will likely continue to require probabilistic methods however the parkfield earthquake prediction experiment and the 2004 parkfield earthquake have provided ample data to understand most of what did occur in 2004 culminating in significant scientific advances

evidence of deep and bottom water formation in the western weddell sea abstract during ice station polarstern ispol r v polarstern cruise ant xxii 2 november 2004 january 2005 hydrographic and tracer observations were obtained in the western weddell sea while drifting closely in front of the larsen ice shelf these observations indicate recently formed weddell sea bottom water which contains significant contributions of glacial melt water in its upper part and high salinity shelf water in its lower layer the formation of this bottom water cannot be related to the known sources in the south the filchner ronne ice shelf we show that this bottom water is formed in the western weddell sea most likely in interaction with the larsen c ice shelf by applying an optimum multiparameter analysis omp using temperature salinity and noble gas observations helium isotopes and neon we obtained mean glacial melt water fractions of about 0 1 in the bottom water on sections across the weddell gyre farther north melt water fractions are still on the order of 0 04 using chlorofluorocarbons cfcs as age tracers we deduced a mean transit time between the western source and the bottom water found on the slope toward the north 9 3 years this transit time is larger and the inferred transport rate is small in comparison to previous findings but accounting for a loss of the initially formed bottom water volume due to mixing and renewal of weddell sea deep water a formation rate of 1 1 0 5 sv in the western weddell sea is plausible this implies a basal melt rate of 35 19 gt year or 0 35 0 19 m year at the larsen ice shelf this bottom water is shallow enough that it could leave the weddell basin through the gaps in the south scotia ridge to supply antarctic bottom water these findings emphasize the role of the western weddell sea in deep and bottom water formation particularly in view of changing environmental conditions due to climate variability which might induce enhanced melting or even decay of ice shelves

marine record of holocene climate ocean and cryosphere interactions herbert sound james ross island antarctica abstract the sediment record offshore james ross island northeast antarctic peninsula presents an unparalleled opportunity to directly compare marine and terrestrial climate records spanning the holocene in maritime antarctica an 11 m drill core was collected between herbert sound and croft bay as part of the shaldril nbp 0502 initiative and produced the southernmost sediment record from the eastern side of the ap thirty eight radiocarbon ages are used to construct an age model of centennial scale resolution multi proxy records including magnetic susceptibility pebble content particle size total organic carbon and diatom assemblages were interrogated in the context of nearby holocene age ice core lake and drift records from james ross island differences in the timing and expression of holocene events reflect marine controls on tidewater glaciers such as water mass configurations and sea ice glacial behavior mimics ice core paleotemperatures during the holocene with the exception of distinct ocean warming events herbert sound was fully occupied by grounded ice during the last glacial maximum and experienced rapid lift off followed by a floating ice phase the canopy of floating ice receded by 10 2 4 cal kyr bp presumably in response to early holocene warming herbert sound and croft bay fully deglaciated by 7 2 cal kyr bp when the mid holocene hypsithermal commenced and the sound became open and productive an extreme peak in productivity 6 1 cal kyr bp indicates an oceanic warming event that is not reflected in atmospheric temperature or lacustrine sediment records increase in sea ice cover and ice rafting mark the onset of the neoglacial 2 5 cal kyr bp when pronounced atmospheric cooling is documented in the james ross island ice core our comparison facilitates more holistic understanding of atmosphere ocean cryosphere interactions that may aid predictions of glacial response to future warming and sea level scenarios

synchronous intensification and warming of antarctic bottom water outflow from the weddell gyre 1 antarctic bottom water aabw the densest water in the global overturning circulation has warmed in recent decades most notably in the atlantic time series recorded within the boundary currents immediately upstream and downstream of the most significant outflow of aabw from the weddell sea indicate that raised outflow temperatures are synchronous with stronger boundary current flows these changes occur rapidly in response to changes in wind forcing suggesting that barotropic dynamics and the response of the bottom ekman layer are significant the observed synchronicity indicates that the previously detected weakening of the export of the colder forms of aabw from the weddell sea need not be associated with a reduction in the total flux of aabw exported via this route these points need careful consideration when attributing the observed aabw warming in the atlantic and when determining its contribution to global heat budgets and sea level rise citation meredith m p a l gordon a c naveira garabato e p abrahamsen b a huber l jullion and h j venables 2011 synchronous intensification and warming of antarctic bottom water outflow from the weddell gyre geophys res lett 38 l03603 doi 10 1029 2010gl046265

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 1 total diatom abundance 1 late pleistocene to holocene climate variability in the antarctic peninsula is described from ocean drilling program odp leg 178 site 1098 palmer deep using total diatom abundance valves per gram dry sediment as a paleoproductivity proxy diatom frustules are absent in the diamicton at the base of the core which was deposited prior to age calibrated circa 13 2 ka immediately above the diamicton frustules are in low abundance as rare fragments and become increasingly more abundant and well preserved up core a rapid and major increase in productivity after circa 12 8 ka is evident from distinctly laminated sediments that overlie the diamicton the strongly laminated section was deposited during a deglaciation phase that followed the retreat of grounded ice after the last glacial maximum diatom productivity decreased during a climatic cooling and or change in meltwater supply until circa 9 0 ka between 9 0 and 4 4 ka productivity increased steadily again with diatom abundance maxima reached during a mid holocene climatic optimum the optimum is interrupted by a brief cold event of 1000 years duration 6 0 5 0 ka after 4 4 ka productivity generally decreased and the last 500 years have experienced the lowest diatom productivity of the postglacial period the palmer deep sediment is compared to ice core and lake sediment records from other regions around the antarctic peninsula these diverse data sets confirm that the antarctic peninsula has been a region of rapid and variable climate fluctuation since the last glacial maximum

san andreas fault geometry in the parkfield california region in map view aftershocks of the 2004 parkfield earthquake lie along a line that forms a straighter connection between san andreas fault segments north and south of the parkfield reach than does the mapped trace of the fault itself a straightedge laid on a geologic map of central california reveals a 50 km long asymmetric northeastward warp in the parkfield reach of the fault the warp tapers gradually as it joins the straight creeping segment of the san andreas to the northwest but bends abruptly across cholame valley at its southeast end to join the straight locked segment that last ruptured in 1857 we speculate that the san andreas fault surface near parkfield has been deflected in its upper 6 km by nonelastic behavior of upper crustal rock units these units and the fault surface itself are warped during periods between large 1857 type earthquakes by the presence of the 1857 locked segment to the south which buttresses intermittent coseismic and continuous aseismic slip on the parkfield reach because of nonelastic behavior the warping is not completely undone when an 1857 type event occurs and the upper portion of the three dimensional fault surface is slowly ratcheted into an increasingly prominent bulge ultimately the fault surface probably becomes too deformed for strike slip motion and a new more vertical connection to the earth s surface takes over perhaps along the southwest fracture zone when this happens a wedge of material currently west of the main trace will be stranded on the east side of the new main trace

numerical study of splay faults in subduction zones the effects of bimaterial interface and free surface 1 splay faults are a characteristic branching fault system in some subduction zones we model a megathrust earthquake rupture with a branching fault inhomogeneous media and a free surface using a spontaneously propagating mode ii crack on a bimaterial interface for this purpose we develop an explicit finite element code to solve the elastodynamic equations and a slip weakening friction law on the fault plane in a homogeneous prestress condition the rupture on the branching fault is enhanced when the upper medium is more compliant even when the upper medium is less compliant a fast rupture propagating at a velocity close to the generalized rayleigh velocity produces the strong dynamic stress around the rupture front and activates the branching fault when prestress linearly increases with depth from zero at the free surface only a slight change of normal stress that is due to seismic waves causes an initiation of rupture on the branching fault at the free surface and this rupture propagates downward on the branching fault the arrival of this descending rupture at the branching point greatly changes the stress field in the surrounding area it affects the slip distribution on the main fault and even terminates the main rupture propagation if it occurs before the arrival of the main rupture on the branching point

rupture directivity of microearthquakes on the san andreas fault from spectral ratio inversion summary earthquakes often occur on faults separating materials with different elastic properties on theoretical grounds it is expected that earthquakes on such bimaterial interfaces might have a preferred rupture propagation direction that being the direction of motion of the more compliant material the goal of this paper is to determine whether a large sample of natural earthquakes on a bimaterial interface exhibits this tendency since the creeping section of san andreas fault has a large across fault velocity contrast and has produced thousands of microearthquakes over the last few decades the rupture directions of 3100 magnitude 0 5 3 0 earthquakes were studied using spectral ratio analysis the spectral ratios of all earthquake pairs in spatially defined clusters were fitted with synthetic spectral ratios at qualified stations the synthetics were computed from a simple moving point source model in which each modelled earthquake has four parameters two rupture lengths one to the se and one to the nw and their propagation velocities the resolution of rupture directivity increases with event size such that nearly 900 events mostly those larger than 70 m appear reasonably well resolved the inversion results suggest that 40 per cent of the well resolved events are roughly bilateral although more than 80 per cent of the 144 events classified as strongly unilateral rupture to the se consistent with the theoretical prediction for those rupture halves that were large enough for the propagation speed to be somewhat resolved that speed was greater by roughly 10 per cent for those halves propagating to the se qualitatively consistent with numerical and laboratory experiments we also found that events with nearby foreshocks within several hours tend to rupture away from those foreshocks whether to the nw or to the se indicating that asymmetry of prior stressing history can exert a stronger influence on rupture directivity than the material contrast

summertime water masses off the northern larsen c ice shelf antarctica 1 we report on oceanographic observations made at the northern end of larsen c ice shelf in the western weddell sea it appears that the larsen c continental shelf is flushed not by high salinity shelf water from the southern continental shelf but by modified weddell deep water mwdw flowing across the shelf break mwdw is observed at the ice front having tracked west along the northward facing slopes of depressions that reach to the shelf break ice shelf water observed near the ice front is not however derived from mwdw directly but from mwdw pre conditioned by winter cooling and by salinification from sea ice production if the ice shelf base generally is being melted only by pre conditioned mwdw then contrary to recent suggestions changes in the temperature of the deep weddell sea are unlikely to have a major impact on melt rates at the base of larsen c ice shelf

oceanographic variability on the west antarctic peninsula during the holocene and the influence of upper circumpolar deep water recent intensification of wind driven upwelling of warm upper circumpolar deep water ucdw has been linked to accelerated melting of west antarctic ice shelves and glaciers to better assess the long term relationship between ucdw upwelling and the stability of the west antarctic ice sheet we present a multi proxy reconstruction of surface and bottom water conditions in marguerite bay west antarctic peninsula wap through the holocene a combination of sedimentological diatom and foraminiferal records are for the first time presented together to infer a decline in ucdw influence within marguerite bay through the early to mid holocene and the dominance of cyclic forcing in the late holocene extensive glacial melt limited sea ice and enhanced primary productivity between 9 7 and 7 0 ka bp is considered to be most consistent with persistent incursions of ucdw through marguerite trough from 7 0 ka bp sea ice seasons increased and productivity decreased suggesting that ucdw influence within marguerite bay waned coincident with the equatorward migration of the southern hemisphere westerly winds sww ucdw influence continued through the mid holocene and by 4 2 ka bp lengthy sea ice seasons persisted within marguerite bay intermittent melting and reforming of this sea ice within the late holocene may be indicative of episodic incursions of ucdw into marguerite bay during this period the cyclical changes in the oceanography within marguerite bay during the late holocene is consistent with enhanced sensitively to enso forcing as opposed to the sww forcing that appears to have dominated the early to mid holocene current measurements of the oceanography of the wap continental shelf suggest that the system has now returned to the early holocene like oceanographic configuration reported here which in both cases has been associated with rapid deglaciation

sedimentary record of disintegrating ice shelves in a warming climate antarctic peninsula 1 seafloor sediments from beneath the former larsen a and prince gustav ice shelves document the recent breakup of the shelves and provide evidence for interpretation of previous events at three of five sites sediment texture coarsens upward with up to 40 sand at the surface radiometric 210pb dating shows this to have occurred between 1985 and 1993 several years before breakup of the shelf and that rates of accumulation of sediment on the seafloor doubled to quadrupled during this period these events are related to the release of eolian sediment in periodic rapid draining of small lakes and crevasses on the ice shelf before breakup x ray radiographs of sediment cores also document the recent influx of coarse particles gravel related to ice rafting during ice shelf disintegration because sediment is released irregularly in time and space from well separated point sources on the ice shelf during at least several years before final disintegration only a portion of the seafloor is affected

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 2 diatom assemblages 1 the distribution of late pleistocene to holocene diatom assemblages from ocean drilling program odp leg 178 site 1098 palmer deep is described and compared using multivariate statistical methods cluster analysis and analysis of variance five assemblages are identified from site 1098 which are correlated with the climatic zones identified previously in the palmer deep domack et al 2001 a thalassiosira diatom assemblage characterizes the deglaciation phase 13 2 11 0 ka and was deposited during glacial retreat at the end of the regional last glacial maximum a well preserved rhizosolenia assemblage was deposited during a climatic reversal 11 0 9 0 ka and indicates periods of seasonally open water with diatom blooms an open water fragilariopsis kerguelensis assemblage characterizes the mid holocene climatic optimum 9 0 3 7 ka in the neoglacial late holocene 3 7 ka climatic cooling increased sea ice cover and increased wind are inferred from a cocconeis assemblage during the climatic optimum neoglacial transition 4 4 1 8 ka laminae characterized by corethron criophilum were preserved it is uncertain whether the laminae represent short lived interannual bloom events with high settling rates or have been deposited over longer time periods

bimaterial effects in an earthquake cycle model using rate and state friction we have developed a computational framework to study earthquake cycles in 2 d plane strain and apply it to faults separating dissimilar material we consider a planar strike slip fault governed by rate and state friction where quasi dynamic events nucleate spontaneously due to remote tectonic loading we investigate the influence of material contrast over the course of many hundreds of years for the parameters we consider we find that the presence of bimaterial properties influences the earthquake nucleation site such that rupture in the preferred direction that is in the direction of particle motion of the side of the fault with lower shear wave velocity is favorable for large values of the critical slip distance dc events propagating in the preferred rupture direction occur for a wide range of material contrasts for smaller values of dc small events emerge even in the monomaterial case with material mismatch present some of these small events propagate in the nonpreferred direction made possible by a favorable stress distribution left on the fault from previous ruptures these results may shed light on our understanding of rupture directivity on large strike slip faults like the san andreas fault in california which occasionally host events rupturing in the nonpreferred direction

boundary condition of grounding lines prior to collapse larsen b ice shelf antarctica grounding zones where ice sheets transition between resting on bedrock to full floatation help regulate ice flow exposure of the sea floor by the 2002 larsen b ice shelf collapse allowed detailed morphologic mapping and sampling of the embayment sea floor marine geophysical data collected in 2006 reveal a large arcuate complex grounding zone sediment system at the front of crane fjord radiocarbon constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12 000 years ago previous studies and morphologic mapping of the fjord suggest that the crane glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse this implies that the 2002 larsen b ice shelf collapse likely was a response to surface warming rather than to grounding zone instability strengthening the idea that surface processes controlled the disintegration of the larsen ice shelf

material contrast does not predict earthquake rupture propagation direction 1 earthquakes often occur on faults that juxtapose different rocks the result is rupture behavior that differs from that of an earthquake occurring on a fault in a homogeneous material previous 2d numerical simulations have studied simple cases of earthquake rupture propagation where there is a material contrast across a fault and have come to two different conclusions 1 earthquake rupture propagation direction can be predicted from the material contrast and 2 earthquake rupture propagation direction cannot be predicted from the material contrast in this paper we provide observational evidence from 70 years of earthquakes at parkfield ca and new 3d numerical simulations both the observations and the numerical simulations demonstrate that earthquake rupture propagation direction is unlikely to be predictable on the basis of a material contrast

effects of a low velocity zone on a dynamic rupture dynamic crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous tomographic and near fault seismic studies indicate that the crust near faults is instead comprised of rocks of varying material velocities we have tested the effects of adding material velocity variation to simulations of spontaneously propagating earthquakes we used two dimensional plane strain conditions coupled with a slip weakening fracture criterion and examined earthquakes on faults that bisect finite width low velocity zones embedded in country rock and earthquakes on faults that bound two different velocity materials when a fault bisects a low velocity zone the normal stress remains unchanged but both the rupture velocity and slip velocity pulse shape are perturbed the presence of the low velocity zone induces high frequency oscillations in the slip function near the rupture front when the fault is on the edge of the low velocity zone the oscillations are more pronounced and repeated sticking and slipping can occur near the rupture front for the slip weakening velocity independent friction model however the temporary sticking does not lead to permanent arrest of slip and slip duration is still controlled by the overall rupture dimension when an earthquake ruptures a fault juxtaposing a lower velocity material against a higher velocity material the normal stress across the fault near the crack tip is perturbed the sign of the normal stress perturbation depends on the direction of rupture leading in some cases to a directional dependence of rupture velocity when slip is accompanied by stress reduction a positive feedback develops between the normal and shear stress changes as previously noted by andrews and ben zion 1997 resulting in an apparently unavoidable grid size dependence in computation of stress change near the rupture front numerical experiments indicate however that the rupture velocity is insensitive to this zone size dependence which is highly localized immediately behind the crack tip the factors controlling the rupture velocity in the simulations including directional dependence are further elucidated by a new analytical solution for rupture of an asperity on a frictionless interface

extensive phytoplankton blooms in the atlantic sector of the glacial southern ocean 1 the sources and sinks of atmospheric carbon dioxide over glacial interglacial cycles are under debate variation in productivity of the antarctic circumpolar current acc could potentially play a significant role but current interpretations of sedimentary geochemical proxies suggest that glacial productivity was not higher than today we present areal and down core distribution patterns of previously overlooked diatom resting spores that indicate the occurrence of extensive phytoplankton blooms across the entire atlantic sector of the acc particularly in the seasonal ice zone siz linked to higher iron input during the last glacial sea ice acts as an effective transporter of iron and enhances its bioavailability the dominance of the deep living radiolarian cyladophora davisiana in glacial siz sediments indicates that organic carbon export to mesopelagic depths was at least tenfold higher than today

biogenic silica and diatom thanatocoenosis in surface sediments below the peru chile current controlling mechanisms and relationship with productivity of surface waters abstract based on 76 surface sediment samples collected between ca 22 and 44 s along the chilean coast in the southeast pacific ocean we report on the north south distribution of opal content and diatom concentration and the quantitative and qualitative composition of the preserved diatom community latitudinal differences in the siliceous signal reflect the influence of two main mechanisms of surface water production coastal upwelling off northern and central chile till ca 38 s and nutrient enrichment by the input of cold southern non upwelling associated waters south of ca 38 s the poleward increase in biogenic opal and diatom values parallels well the north south increase of pigment concentration in surface waters as recorded by satellites between 22 and 33 s where low pigment concentrations are measured opal content and diatom concentration are mostly low except at 22 23 s in contrast both higher opal and diatom values at 34 38 s and 41 42 s correspond well with higher pigment estimations preserved diatom populations document the present day dominant hydrographical conditions nutrient enrichment of surface waters due to coastal upwelling results in intensive production of diatoms off northern and central chile reflected by the predominance of the richly composed association of chaetoceros spores north of ca 38 s the diatom association shows that the area between 35 and 38 s is transitional the abrupt decrease of chaetoceros spores south of 38 s off chile coupled with the enhanced contribution of thalassiosira spp representatives of high productivity low temperature waters points to the presumed high nutrient supply by the iron limited nutrient rich antarctic circumpolar water the contribution of benthic diatoms reflects some lateral transport from the near shore area into deeper positions south of 38 s

rupture dynamics on a bimaterial interface for dipping faults dip slip faulting may juxtapose different geologic materials with differ ent properties such that a strong material contrast will naturally tend to form both the material contrast across a fault and dip slip motion on a nonvertical fault lead to normal stress variations during earthquake rupture this normal stress variation will significantly affect dynamic rupture propagation to demonstrate this we model dynamic rupture propagation on two dimensional 2d reverse and normal faults 30 45 and 60 dipping with 20 material contrasts for predominantly up dip rupture propagation we find that normal stress variations due to the free surface and material contrast can either reinforce or counteract each other depending on the configuration for reverse faults we find a larger strength drop for a more com pliant hanging wall and a lower strength drop for a more compliant footwall for normal faults we find a larger strength drop for a more compliant footwall and a lower strength drop for a more compliant hanging wall for both reverse and normal faults ground motion will be more symmetric between the hanging wall and footwall with compliant material on the footwall and more asymmetric if more compliant materials are on the hanging wall our results have important implications for the dynamics of crustal and perhaps subduction zone earthquake faulting where strong bimaterial contrasts across dipping faults are possible in continental settings reverse faulting will tend to advect rigid materials from greater depth onto the hanging wall such that the effects of fault dip and material contrast will counteract one another in subduction zones the hanging wall is likely to be more compliant and hence the material and geometric effects may reinforce one another

aftershock asymmetry rupture directivity among central san andreas fault microearthquakes using a waveform cross correlation technique we have obtained precise relative locations for nearly 75 of the northern california seismic network catalog 4300 earthquakes occurring between 1984 and 1997 along 50 km of the san andreas fault errors in relative location are meters to tens of meters for events separated by tens to hundreds of meters we find that consecutive earthquakes in the relocated catalog occur no closer than a distance approximately equal to the radius of the first rupture as estimated from the moment magnitude relationship of abercrombie 1996 assuming a 10 mpa stress drop when the relative position vectors between consecutive events are normalized by this distance and projected onto the fault surface they define a hole whose shape suggests that typical microearthquakes are elongate in the mode ii slip parallel direction by several tens of percent moreover of the 100 immediate aftershocks occurring closest to the mode ii edges of the prior rupture more than twice as many occur to the northwest than to the southeast we interpret this asymmetry as resulting from the large contrast in material properties across the fault models of dynamic rupture between dissimilar media predict that ruptures in this region may run preferentially to the southeast in the direction of motion of the lower velocity material if so then the barriers that stop rupture fronts moving to the southeast should initially be farther from failure on average than the barriers that stop rupture fronts moving to the northwest once the rupture stops the induced stress change is more symmetric but the fault remains farther from failure on average to the southeast this interpretation receives some support from pulse width measurements on a localized set of 72 magnitude 0 6 to 3 6 earthquakes

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

comment on material contrast does not predict earthquake rupture propagation direction by r a harris and s m day 1 harris and day 2005 hereinafter referred to as hd concluded from some numerical simulations and low resolution analysis of 8 m4 m6 earthquakes near parkfield ca that material contrast in fault zone structures is not important for earthquake propagation direction the limited theoretical and observational results of hd do not justify such a general conclusion considerations of a larger body of results suggest that material contrast may produce a statistically preferred direction of earthquake ruptures the inferred diversity of the 8 discussed events is consistent with their small number and relatively small size in addition the behavior has a natural explanation in a structure with two material interfaces the san andreas fault saf and the southwest fracture zone sfz on the opposite sides of a deformation damage region large earthquakes propagating predominantly as mode ii ruptures and affected primarily by properties of the bounding crustal blocks rather than the local structure are likely to exhibit a smaller amount of diversity

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

deep and bottom water of the bransfield strait eastern and central basins temperature salinity oxygen and chlorofluorocarbon cfc data obtained in september 1997 define the water types ventilating the deep eastern and central basins of the bransfield strait these water types are observed adjacent to joinville island and are clearly derived from the weddell sea the eastern basin bottom water characteristics closely match that of the bottom water at 310 dbar depth 50 km east of joinville island the eastern basin bottom water is a simple mixture of low salinity freezing point shelf water 65 and relatively warm saline weddell deep water 35 with a cfc 113 cfc il ratio age of 8 5 years the eastern basin bottom water shares a common origin with a weak salinity minimum observed at 1000 dbar within the central basin though overflow from the central basin to the eastern basin floor may also occur the bottom water within the central basin is colder and saltier with higher concentrations of oxygen and cfc il than that of the eastern basin the central basin bottom water with a cfc ratio age of 7 5 years is composed of same weddell water types that form the eastern basin bottom water plus a large contribution of high salinity freezing point shelf water of the characteristics of water observed 10 km northeast of joinville island and a small amount of pacific pycnocline water the components and their percentage are weddell deep water 11 low salinity shelf 24 high salinity shelf 60 to pacific pycnocline 5

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis