climatic variability in upper ocean ventilation rates diagnosed using chlorofluorocarbons the chlorofluorocarbon cfc 12 ccl2f2 distributions from two occupations of a meridional hydrographic section in the eastern north atlantic are used to describe the oceanic penetration of cfcs and change in the integrated ventilation patterns over the five years from 1988 to 1993 the cfc 12 water column inventories increased by 30 40 despite a slowing atmospheric growth rate 14 because of continuing uptake by undersaturated subsurface water masses whose response is lagged by the ventilation time scales after removing the long term cfc temporal trend using a tracer age based normalization technique we observe a distinct dipole pattern in upper ocean ventilation with reduced convection in the subpolar gyre and enhanced production of saline subtropical underwater in 1993 these differences are discussed in relation to interannual variability in atmospheric surface forcing upper ocean anomalies and convection patterns associated with the north atlantic oscillation

surface climate variations over the north atlantic ocean during winter 1900 1989 abstract the low frequency variability of the surface climate over the north atlantic during winter is described using 90 years of weather observations from the comprehensive ocean atmosphere data set results are based on empirical orthogonal function analysis of four components of the climate system sea surface temperature sst air temperature wind and sea level pressure an important mode of variability of the wintertime surface climate over the north atlantic during this century is characterized by a dipole pattern in ssts and surface air temperatures with anomalies of one sign cast of newfoundland and anomalies of the opposite polarity off the southeast coast of the united states wind fluctuations occur locally over the regions of large surface temperature anomalies with stronger than normal winds overlying cooler than normal ssts this mode exhibits variability on quasi decadal and biennial time scales the decadal fluctuations are irregular in length averaging 9 years before 1945 and 1

a new mixed mode failure criterion for weak snowpack layers the failure of a weak snow layer is the first in a series of processes involved in dry snow slab avalanche release the nature of the initial failure within the weak layer is not yet fully understood but widely debated the knowledge of the failure criterion is essential for developing avalanche release models and hence for avalanche hazard assessment yet different release models assume contradictory criteria as input parameters we analyzed loading experiments on snow failure performed in a cold laboratory with samples containing a persistent weak snow layer of either faceted crystal depth hoar or buried surface hoar the failure behavior of these layers can be described well with a modified mohr coulomb model accounting for the possible compressive failure of snow we consequently propose a new mixed mode shear compression failure criterion that can be used in avalanche release models

post seismic relaxation following the 2009 april 6 l aquila italy earthquake revealed by the mass position of a broad band seismometer summary post seismic relaxation is known to occur after large or moderate earthquakes on time scales ranging from days to years or even decades in general long term deformation following seismic events has been detected by means of standard geodetic measurements although seismic instruments are only used to estimate short timescale transient processes albeit inertial seismic sensors are also sensitive to rotation around their sensitive axes the recording of very slow inclination of the ground surface at their standard output channels is practically impossible because of their design characteristics however modern force balance broad band seismometers provide the possibility to detect and measure slow surface inclination through the analysis of the mass position signal this output channel represents the integral of the broad band velocity and is generally considered only for state of health diagnostics in fact the analysis of mass position data recorded at the time of the 2009 april 6 l aquila mw 6 3 earthquake by a closely located sts 2 seismometer evidenced the occurrence of a very low frequency signal starting right at the time of the seismic event this waveform is only visible on the horizontal components and is not related to the usual drift coupled with the temperature changes this analysis suggests that the observed signal is to be ascribed to slowly developing ground inclination at the station site caused by post seismic relaxation following the main shock the observed tilt reached 1 7 10 5 rad in about 2 months this estimate is in very good agreement with the geodetic observations giving comparable tilt magnitude and direction at the same site this study represents the first seismic analysis ever for the mass position signal suggesting useful applications for usually neglected data

eastward flow through the mid atlantic ridge at 11 n and its influence on the abyss of the eastern basin abstract the dilute antarctic bottom water of the north atlantic eastern trough is supplied from the western trough through fractures in the mid atlantic ridge in particular the influence on eastern trough property distributions of flow through the romanche and vema fracture zones near the equator and 11 n respectively has been noted previously here new observations are reported that document the abyssal circulation of the northeastern atlantic basins gambia abyssal plain south canary basin and north canary basin in particular the dominance of vema influence the absence of romanche influence and the existence of a system of deep western boundary currents and estimated transport deep isopycnals slope steeply across the vema s eastern end near 39 w corresponding to a geostrophic transport through the vema of 2 1 to 2 3 106 m3s 1 colder than 2 0 c this is half or more of the estimated bottom water that flows north across the equator into the subtropical western north atlantic this transp

hidden dykes detected on ultra long period seismic signals at piton de la fournaise volcano broadband seismic data enable us to test whether ultra long period ulp signals can be used to investigate magma chamber pressure state and to monitor volcanic eruptions by systematically investigating seismic signals at geoscope station rer we find ulp signals associated with volcanic events during the last 2 decades we tentatively propose to interpret these signals as events highlighting the activity of the upper magma feeding system of the piton de la fournaise volcano and as being related to the deep magma chamber some of these unusual signals were previously detected and usually interpreted in term of tilt battaglia j aki k montagner j j p tilt signals derived from a geoscope vbb station on the piton de la fournaise volcano geophys res lett 27 5 2000 605 608 they are detected using sts1 seismometers sensitive in the bandwidth 10 3 to 10 2 hz this alternative model enables to quantify each event in terms of a pressure drop in the upper magma reservoir of the volcano two kinds of seismic waveforms can be associated with two types of fracturing events the first one is related to the fast rise of a magma batch coming from the deep magma chamber and the second is the result of a slow rise in pressure inside a shallow reservoir in both cases we are able to detect these events on very long period seismic data the interpretation of these signals supports the idea of a single magma source at the sea level thus we propose to classify all the eruptions occurring during the last two decades into two groups corresponding to the two primary pressure states of the magma chamber 2007 elsevier b v all rights reserved

broadband sensor nonlinearity during moderate shaking abstract we observe that broadband seismometers may produce artifact long period signals that resemble impulse responses similar to a step in acceleration in the presence of shaking as moderate as 0 2 g this observation accords with recent observations in europe and elsewhere with similar instruments e g zahradnik and plesinger 2005 we present two case studies for both the 8 october 2006 m 4 5 earthquake near mt rainier in the pacific northwest and an m 5 0 event on 29 september 2004 in southern california artifact signals possible step tilts and apparent sensor problems are observed as far as about 200 km from the epicenters such long period artifacts if not recognized complicate and degrade estimation of source parameters of moderate and larger earthquakes on regional networks the exact cause of the artifacts currently remains obscure but may require alterations to instrument installation and or design strategies

ghost tilt signal during transient ground surface deformation events insights from the september 3 2010 mw7 1 darfield earthquake new zealand 1 this work assesses whether tiltmeters can reliably measure ground tilt in the 2 20 s band similar to that of very long period volcanic earthquakes seismic waves recorded with co located high rate gps borehole seismometer and bubble tiltmeter during the 2010 mw7 1 darfield earthquake in new zealand are used to determine the response of the tiltmeter the joint analysis of this dataset shows that at least for periods within the aforementioned band the observed tilt signal was in fact not related to ground tilt but instead to translational ground acceleration caused by passing s and surface waves if no independent control on translational motion is provided for recorded very long period volcanic earthquakes within the 2 20 s band the signal recorded on bubble tiltmeters may therefore be misinterpreted the use of co located high rate gps data provides such control and is essential to discriminate translational motion from ground tilt on the recorded seismic and tiltmeter signals

inferring the subduction rate and period over the north atlantic abstract the annual rate at which mixed layer fluid is transferred into the permanent thermocline that is the annual subduction rate sann and the effective subduction period x1d4af eff is inferred from climatological data in the north atlantic from its kinematic definition sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer geostrophic velocity fields computed from the levitus climatology assuming a level of no motion at 2 5 km are used the vertical velocity at the base of the mixed layer is deduced from observed surface ekman pumping velocities and linear vorticity balance a plausible pattern of sann is obtained with subduction rates over the subtropical gyre approaching 100 m yr twice the maximum rate of ekman pumping the subduction period x1d4af eff is found by viewing subduction as a transformation process converting mixed layer fluid into stratified thermocline fluid the effective

evaluation of slope stability with respect to snowpack spatial variability the evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas the spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation since it strongly influences failure initiation and crack propagation in weak snow layers hence the determination of the link between these spatial variations and slope stability is very important in particular for avalanche public forecasting in this study a statistical mechanical model of the slab weak layer wl system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches this model accounts in particular for the spatial variations of wl shear strength and stress redistribution by elasticity of the slab we show how avalanche release probability can be computed from release depth distributions which allows us to study the influence of wl spatial variations and slab properties on slope stability the importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so called knock down effect on slope stability is revisited using this model finally critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests

a comment on hidden dykes detected on ultra long period seismic signals at piton de la fournaise volcano by n houlie and j p montagner epsl 261 2007 abstract based on horizontal displacements extracted from a very broadband seismic station located near the piton de la fournaise volcano houlie and montagner houlie n montagner j p 2007 hidden dykes detected on ultra long period seismic signals at piton de la fournaise volcano earth planet sci lett 261 doi 10 1016 j epsl 2007 04 018 conclude that there have been intrusions of large volume hidden dykes into the volcano and suggest the presence of two distinct types of eruptions related to the pressure states of a magma chamber in this comment we point out that the recordings of horizontal components may be dominated by tilt and that obtaining displacements with proper amplitudes is prone to errors we also show that the use of an oversimplified model to explain a single datum leads the authors to conclusions that are inconsistent with other data collected on the volcano as well as with previous interpretations

dynamic dyke propagation deduced from tilt variations preceding the march 9 1998 eruption of the piton de la fournaise volcano during the hour preceding the march 9 1998 eruption of the piton de la fournaise volcano the deformation network of the observatory recorded some large deformations in the summit area a broadband seismic station of the geoscope global network rer is located about 8 km away from the summit of the volcano signals from that station may be interpreted as tilt changes the combination of the above two kinds of signals allows by using a tensile fault model to constrain the geometry as well as some characteristics volume propagation velocity of the dyke that intruded the summit area during the hour preceding the beginning of eruptive activity

recent variability of the north atlantic thermohaline circulation inferred from surface heat and freshwater fluxes abstract an annual mean surface forced component of the north atlantic thermohaline circulation thc in density space surf is diagnosed from observed surface heat and freshwater fluxes the climatological mean of surf over 1980 97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing 8 sv of greenland sea deep water gsdw 4 sv of labrador sea water lsw and 4 sv of 18 water stmw obtained for each year over 1980 97 surf reveals interannual to decadal variability in the renewal and overturning of these water masses the total surface forced overturning rate varies in the range 10 20 sv reaching peak values in 1989 90 and generally decreasing over the period 1990 97 more dramatic changes are apparent in the overturning of different water masses the overturning of stmw exhibits large amplitude interannual variability a background positive trend over 1980 97 is dominated by variance of 13 4 sv by contrast the overtur

early detection of large eruptions at piton de la fournaise volcano la reunion island contribution of a distant tiltmeter station we report a compilation of data recorded at a distant tiltmeter station rer during recent episodes of dyke emplacement and eruption 2003 2007 at piton de la fournaise volcano la reunion island this sensitive station provides useful information for evaluating the extent of deformation distinct responses of this station were recorded based on the eruption type dykes feeding summit eruptions did not significantly influence the rer tiltmeter signals whereas dykes feeding large distal eruptions with vents located more than 4 km from the summit generated up to 1 4 rad of tilt an amplitude 2 to 4 times greater than for proximal eruptions 0 3 0 7 rad on the flanks of the summit cone the distinct tilt amplitude is directly linked to the location depth and volume of the dyke comparison with summit tiltmeters reveals that up to one third to half of the rer tilt signal associated to dyke propagation is recorded when the dyke is still below the summit crater thus before large distal eruptions more than 0 5 rad of tilt is recorded in less than 20 min when the dyke is below the summit crater i e a few minutes hours before the beginning of the eruption we can thus propose for the rer station a threshold value of 0 5 rad which when reached as a dyke rises beneath the summit crater suggests a high likelihood of a large distal eruption the distant rer tiltmeter station thus appears to be a powerful tool for forecasting the type of eruption that is likely to occur and can contribute to the early detection of large distal eruptions at piton de la fournaise which are the most dangerous to inhabitants for volcano monitoring installation of high precision distant tiltmeters along the lower slopes of a volcano may provide warnings of large eruptions with enough lead time to allow for short term hazards mitigation efforts

highlights from a seismic broadband array on stromboli volcano an array of nine three component broadband seismometers was deployed in two different configurations on stromboli volcano the analysis of the seismic wavefield related to volcanic explosions revealed some observations which offer a completely new insight into the internal dynamics of a volcano these new observations are restricted to the low frequency range below 1 hz and underline therefore the superiority of broadband recordings over conventional short period observations surprisingly simple wavelets indicate an initially contracting source mechanism gas jets that could not be seen in a short period seismic record at all generate a clear dilatational wavelet in a broadband recording suggesting the same contracting source mechanism the analysis of particle motion and seismic array techniques permits a location of the seismic source we find low frequency signals of 3s and 6s period that are not related to eruptions and do not share a common source with the eruption related events a video recording of visible volcanic activity at the crater region allows one to correlate precisely eruptive features with seismic signals

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

performance of wielandt streckeisen sts 1 seismometers in the tidal domain preliminary results abstract the aim of this work is to show that sts 1 seismometers are able to provide a signal with a very long period vertical component which can be used for earth tide studies provided a number of recording conditions are improved we use data from the geoscope network for bng bangui central african republic mbo m bour senegal crzf crozet taaf terres australes et antarctiques frandc aises indian ocean ech echery france and paf port aux frandc ais kerguelen taaf indian ocean stations operated by the ecole et observatoire de physique du globe de strasbourg eopgs and institut frandc ais de recherche scientifique pour le de veloppement en coope ration orstom these data are analysed with a least squares adjustment method to fit the earth tidal waves and the results for these stations are compared special attention is paid to the comparison of results from ech with those from the superconducting gravimeter of strasbourg j 9 for several stations used in this study crzf mbo and ech the results are promising but they are disappointing for others paf and bng we did not find any clear reason for such a difference in quality we propose some improvements that will enable researchers to achieve a better recording of this long period signal and obtain higher quality data allowing gravimetric studies from global seismological networks

measurement of the deformation field associated with fracture propagation in weak snowpack layers 1 dry snow slab avalanches release as a result of the failure of a weakly bonded layer located below a slab like layer of cohesive snow traditionally the failure of the weak layer was attributed to the formation and propagation of volume conserving simple shear cracks over the past decade however evidence for slope normal subsidence associated with fracture propagation in snow has accumulated pointing toward a new understanding of fracture propagation in snow the typically very high porosity of weak layers implies a loss of volume during fracture as the aggregate of ice grains composing the weak layer collapses and rearranges in a tighter packing order therefore in the new models failure is attributed to the formation and propagation of mixed mode anticracks in order to experimentally investigate the nature of the deformation field associated with fracture propagation through weak snowpack layers we monitored propagating fractures in field tests on natural snowpacks with a high level of detail using high speed photography and analyzed the data with particle tracking velocimetry the collapse of the weak layer was always observed and ranged between 0 3 cm and 4 cm in all experiments the collapse was found to coincide with the fracture front a shear crack preceding the collapse front was not detected at any stage of the process it is concluded that the volumetric collapse of the weak layer which is characteristic of anticracking is the rule rather than the exception for the fracture process in stratified snow and therefore for slab avalanche release

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

the early twentieth century arctic high fact or fiction reconstructions of monthly mean sea level grid point pressure from station pressure data over the arctic region using an objective technique have revealed two serious errors in the gridded data first all gridded data from the u s historical weather map series for the arctic ocean area away from the north atlantic sector appears to be between 4 and 6 mb too high before about 1931 the semi permanent arctic high produced in the early historical map series was the result of a lack of basic station data and the belief amongst many north american meteorologists of the 1920s and 1930s of the existence of a polar or glacial anticyclone the second error affects only north western north america for the years 1899 1909 here pressure is again too high compared with reconstructed data the cause of this error is not immediately apparent but is probably the result of an erroneous correction to sea level for stations in the region both errors seriously affect the homogeneity of the gridded arctic series the objective scheme used to reveal the errors can also be used to derive correction factors for the gridded data prior to 1931

decadal changes in the thermohaline circulation of the north atlantic abstract a set of repeated full depth and mostly ocean wide hydrographic sections is used to describe the temporal and spatial changes of temperature and salinity at the intermediate layers of the north atlantic over the past 40 years they cover three distinct periods namely the late 1950s the early 1980s and the early 1990s but do not constitute a time series most parts of the recently observed changes in the subpolar and subtropical gyres are attributed to the changes of the labrador sea water lsw properties the latter is formed in the labrador sea and spreads into the north atlantic the spreading time of the lsw from the source region is evaluated it takes less than two years for the lsw to arrive in the newfoundland basin and 18 20 yr to reach the deep western basin interior at 24 5 n changing characteristics of the lsw are related to the varying intensity of the meridional overturning circulation moc and the meridional heat transport mht the intensification of the moc and mht around 1981 coincides with the rapid decrease of the lsw production the increase of the lsw production that began just after 1987 and resulted in a dramatic cooling and densification of this water mass was accompanied by a reduction of both moc and mht two scenarios of the moc interaction with the full depth circulation of the north atlantic are suggested these scenarios indicate that the development of the moc is governed by the changing transports of the deep waters of arctic origin the direct contribution of the lsw to the intensity of the moc is negligible the lsw is seen as an important part of the feedback loop in the north atlantic moc system

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

how does the deep western boundary current cross the gulf stream abstract the manner in which the deep western boundary current dwbc crosses the gulf stream is investigated using data from a hydrographic survey conducted in 1990 absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level three density layers are considered in detail two mid depth layers which together make up the shallowest water mass component of the dwbc 500 1200 m and a deep layer consisting of the norwegian greenland overflow water 2500 3500 m the shallowest layer does not make it through the crossover and is completely entrained by the gulf stream however the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover in the intermediate layer which is denser than the gulf stream coming off the shelf part of the dwbc recirculates to the northeast while the onshoremost portion continues equatorward in the deep layer only a small amount of recirculation occurs the lateral

modelling snow failure with a fibre bundle model dry snow slab avalanches initiate from a failure in a weak snow layer below a cohesive slab snow is considered as a porous ice structure and the strength distribution of the single elements of this structure i e grains and bonds between grains shows a high degree of disorder on the bond or microstructural level the failure process is believed to start if the fracturing of bonds between snow grains is not balanced by the formation of new bonds we use a statistical fracture model a fibre bundle model to study the failure process in a weak snow layer the model consists of fibres of various strengths representing single snow grains between two rigid plates which represent the slab above and the substratum below the weak layer the fibres deform in a linear elastic manner and break instantly at their rupture strength broken fibres may sinter re bond and regain strength after a finite sintering time we show that the different characteristic times for breaking and sintering lead to the rate dependence of snow strength this is to our knowledge the first statistical model to reproduce the ductile to brittle transition which snow exhibits with increasing strain rate when the model is applied to simulate experimental stress strain curves for different strain rates the model and experimental results are in fair agreement

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

variability of the simulated meridional heat transport in the north atlantic for the period 1951 1993 a 43 year ocean model simulation for the period 1951 1993 is analyzed with a focus on the meridional heat transport mht as a proxy for the strength of meridional overturning cell moc at 25 n the surface heat flux forcing associated with the north atlantic oscillation nao pattern is related to the variability in mht both on interannual and longer timescales the manifestation of the interannual and decadal oceanic response to nao is nonlocal as evidenced by the concentrated heat content and sea level variability at the gulf stream and north atlantic current regions and shown by the comparison of the model sea level variability to the altimeter data between low mht years in mid 1980s and high mht years in early 1990s the same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level mht and time series of the leading sea level mode are highly correlated which reflects the importance of the moc in determining the variability of the heat content of the whole basin also the model suggests that the mht moc has entered a very strong phase since the mid 1980s and this trend has continued up to the end of 1993 this behavior follows the general trend in the nao index

oceanic deep water formation as a sink of persistent organic pollutants 1 the formation of deep oceanic waters occurs as part of the global thermohaline circulation due to gradients in salinity and temperature and moves surface waters including persistent organic pollutants pops directly to the deep ocean for the four main deep water formation regions removal fluxes of polychlorinated biphenyl pcb congeners were calculated based on their surface water concentrations and deep water formation rates pcb fluxes were higher in the norwegian sea ca 420 kg yr than in the labrador ross and weddell seas ca 140 160 kg yr each in the four regions considered more pcbs were removed due to deep water formation ca 870 kg yr than by the settling of pcbs associated to organic carbon ca 320 kg yr whereas the settling flux dominates on a basin scale several pops could serve as tracers for oceanic deep water plumes as they have only been produced for a few decades

three dimensional microstructure and numerical calculation of elastic properties of alpine snow with a focus on weak layers abstract the microstructure and stratigraphy of a snowpack determine its physical behaviour weaklayers or weak interfaces buried under a slab are prerequisites for the formation of dry snow slabavalanches and a precise characterization of weak layers or interfaces is essential to assess stability yettheir exact geometry and micromechanical properties are poorly known we cast weak layers and theiradjacent layers in the field during two winters and reconstructed their three dimensional microstructureusing x ray microcomputer tomography the high resolution of 10 20 m m allowed us to study snowstratigraphy at the microstructural scale we quantified the microstructural variability for 32centimetre sized layered samples and we calculated young s modulus and poisson s ratio bytomography based finite element simulations layers in a sample could therefore be differentiatednot only by a change in morphology or microstructure but also by a change in mechanical properties we found a logarithmic correlation of young s modulus with density for two different density ranges consistent with previous studies by calculating the relative microstructural changes within our samples we showed that a large change could indicate a potential weak layer but only when the weak layer andboth adjacent layers i e the sandwich were considered keywords

tilt signals derived from a geoscope vbb station on the piton de la fournaise volcano we report some very long period transients observed on a very broadband seismic station of the geoscope network which is situated 8 km away from the summit of the piton de la fournaise volcano we transform the ground velocity measured by the seismic station into tilt and test the result by comparing with data from blum type classical pendulum tiltmeters located in the same vault the comparison validates the use of the horizontal seismometers for measuring tilt for periods shorter than 25 hours the method is applied to the transients observed prior to several eruptions and magmatic intrusions since 1991 because of the moderate distance between geoscope station and the summit of the volcano the obtained tilt measurements provide information about eruptions and intrusions which complement the information on the geometry and location of the magmatic source provided by the summit tilt network

analysis of critical length measurements for dry snow slab weak layer shear fracture snow slab avalanches primarily release by propagation of shear fractures within thin weak layers under much thicker slabs in some cases the weak layer is on the order of 1 mm thick and such fractures may be considered to be a mode ii shear fracture at initiation in the cases analysed in this paper the weak layer has finite thickness and slope normal effects may be present field data from 500 snow shear fracture tests are analysed and applied to the problem of weak layer fracture the paper contains a detailed analysis using a simple analytical model to estimate the critical length prior to an unstable shear fracture the model contains the assumption of a finite fracture process zone which may be a significant fraction of the slab depth d or the critical length l for weak layer shear fracture the field results show that the l d ratio varies from 0 1 to just over 2 and the model results are close to the same range the analysis also shows that both the field and model results for l d follow a gumbel probability density function since the experimental field data contain rate dependent viscoelastic and slope normal effects it is imperative to account for these in the model and for snow slab instability evaluation when using test data detailed evaluations considering both these effects are given the applicability of the test data to avalanche release is discussed

estimating the effective elastic modulus and specific fracture energy of snowpack layers from field experiments measurements of the mechanical properties of snow are essential for improving our understanding and the prediction of snow failure and hence avalanche release we performed fracture mechanical experiments in which a crack was initiated by a saw in a weak snow layer underlying cohesive snow slab layers using particle tracking velocimetry ptv the displacement field of the slab was determined and used to derive the mechanical energy of the system as a function of crack length by fitting the estimates of mechanical energy to an analytical expression we determined the slab effective elastic modulus and weak layer specific fracture energy for 80 different snowpack combinations including persistent and nonpersistent weak snow layers the effective elastic modulus of the slab ranged from 0 08 to 34 mpa and increased with mean slab density following a power law relationship the weak layer specific fracture energy ranged from 0 08 to 2 7 j m 2 and increased with overburden while the values obtained for the effective elastic modulus of the slab agree with previously published low frequency laboratory measurements over the entire density range the values of the weak layer specific fracture energy are in some cases unrealistically high as they exceeded those of ice we attribute this discrepancy to the fact that our linear elastic approach does not account for energy dissipation due to non linear parts of the deformation in the slab and or weak layer which would undoubtedly decrease the amount of strain energy available for crack propagation

relative influence of mechanical and meteorological factors on avalanche release depth distributions an application to french alps the evaluation of avalanche release depth distributions represents a challenging issue for the mapping zoning and long term hazard management in mountainous regions to that aim both the distribution of snowfalls and the occurrence probability of an avalanche release for a given snow height need to be assessed in this study a rigorous formalism allowing coupling of these two ingredients into a mechanical statistical model is presented the stability criterion of a layered snowpack is investigated using a finite element analysis accounting for the spatial heterogeneity of weak layer mechanical properties while the available snow depth is evaluated by studying the distribution of 3 day extreme snowfalls the release depth distributions predicted by this coupled model are then compared to a well documented database encompassing 369 natural slab avalanches recorded in la plagne france it appears that with only one adjustable parameter an excellent agreement can be obtained both for the power law tail of the distribution corresponding to large slab depths and for its core corresponding to shallow slab depths two important conclusions can be drawn 1 small to medium sized avalanches are controlled mainly by mechanics whereas large avalanches are influenced by a strong mechanical meteorological coupling 2 the release depth distributions including the value of the power law exponent obtained for large slab depths are highly variable in space and cannot be regarded as universal finally the model is extended using a robust interpolation procedure in order to produce maps of expected release depths for different return periods

probabilistic size effect law for mode ii fracture from critical lengths in snow slab avalanche weak layers from field observations dry snow slab avalanche initiation is associated with fracture within relatively thin weak layers under stronger cohesive slabs for risk based avalanche prediction it is important to understand the fracture properties of alpine snow alpine snow is a quasi brittle material with a fracture mechanical size effect on nominal shear strength meaning that strength decreases with increasing specimen size a related size effect is the critical length required for rapid propagation of a shear fracture in that case the probability of fracture increases with increasing crack length in this paper 45 sets of field measured critical lengths are presented based on 591 individual tests from analysis a probabilistic size effect law based on critical lengths is derived analogous to the deterministic size effect law for nominal shear strength related to fracture mechanics the new size effect law may be useful in applications particularly since the critical length is easily measured in the field and it is a principal component of weak layer fracture toughness

influence of weak layer heterogeneity on snow slab avalanche release application to the evaluation of avalanche release depths slab avalanches are generally caused by the collapse of a weak layer underlying a cohesive slab the two key ingredients for the mechanical description of such slab avalanche releases are the heterogeneity of the weak layer and the redistribution of stresses via the elasticity of the slab the finite element code cast3m is used to build a complete full scale mechanical model of the slab weak layer system including inertial effects the weak layer is modeled as a cohesive mohr coulomb interface with cohesion softening which accounts for shear induced collapse the overlying slab is represented by an elastic layer the system is loaded by increasing the slope angle until rupture we first study the influence of a single weak spot in the weak layer in order to validate the model against analytical solutions the interaction between two weak spots is also analyzed the case of heterogeneous weak layers represented through gaussian stochastic distributions of the cohesion with a spherical spatial covariance is then studied several simulations for different realizations of the heterogeneity of the weak layer are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is examined in particular a heterogeneity smoothing effect caused by slab elasticity and characterized by a typical length scale of the system is evidenced finally the model is coupled with extreme snowfall distributions belonging to the gev class which allows to recover with very good accuracy release depth distributions obtained from field data

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

the effects of seismic rotations on inertial sensors summary with the important increase in the number of instruments especially in the near field of quite significant earthquakes unsaturated traces have led seismologists to question what they are really measuring we have performed a review of previous studies related to the effects of rotations on both horizontal and vertical components of various sensors illustrations of near field records show that the recovering static displacement requires an accurate rotation estimation that present day seismometers cannot achieve estimations of coseismic tilts rotation around an horizontal axis using seismometers cannot be achieved independently of translational motion therefore for specific configurations such as near field or long period far field reconstruction of the ground motion requires specific rotation measurements effects of the chi chi earthquake 1999 on accelerograms make the static displacement estimation unreliable for long period background noise far field horizontal seismic signals present unexpected n45 polarization which is explained by the similar influence of the rotation around the vertical axis on the two horizontal components for the geoscope network this feature has been seen for stations with a streckeisen sts 1 sensor while those with a streckeisen sts 2 sensor do not show this polarization feature this study suggests that sensor installation should follow a protocol that will better guarantee the verticality of the sensors moreover rotational recordings with ad hoc sensors are necessary and by adequately correcting the traces they will enable us to reconstruct the translational motion from recorded seismic signals

sea ice transport a highly variable link between arctic and north atlantic the variability of the ice volume flux into the northeast atlantic is investigated with an optimized dynamic thermodynamic sea ice model using 40 years 1958 1997 of atmospheric forcing fields derived from the ncep ncar reanalysis project the simulated sea ice export from the arctic exhibits considerable interannual to decadal variability and is primarily a linear response to sea level pressure anomalies over greenland and over the barents and kara seas our model results suggest that ice export anomalies such as in 1968 which supposedly caused the so called great salinity anomaly in the northern north atlantic are not unique but rather frequent events as part of the variability of the arctic climate system

anticrack nucleation as triggering mechanism for snow slab avalanches snow slab avalanches are believed to begin by the gravity driven shear failure of weak layers in stratified snow the critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases however recent experiments show that the critical length of artificially introduced cracks remains constant or if anything slightly decreases with decreasing slope this surprising observation can be understood in terms of volumetric collapse of the weak layer during failure resulting in the formation and propagation of mixed mode anticracks which are driven simultaneously by slope parallel and slope normal components of gravity such fractures may propagate even if crack face friction impedes downhill sliding of the snowpack indicating a scenario in which two separate conditions have to be met for slab avalanche release

the critical size of macroscopic imperfections in dry snow slab avalanche initiation 1 dry snow slab avalanches initiate after mode ii fracture propagation within a thin weak layer under a planar slab dry alpine snow in which avalanches form is a porous material typically with volume fraction filled by solids of between 10 and 50 if snow slab avalanches were caused by small scale flaws there would be continuous avalanches and alpine snow would not survive on steep slopes instead snow slab avalanches initiate from macroscopic imperfections or weak zones within the weak layer these are called sweet spots by practitioners and deficit zones in the pioneering work by conway and abrahamson in the 1980s in this paper the results of 750 in situ shear fracture tests from 68 slab weak layer combinations are summarized in relation to the critical sweet spot length and weak layer crystal form the results show that the critical sweet spot lengths follow a gamma probability density function with a range from 0 14 to 1 3 m and most probable value 0 5 m the results also imply that critical sweet spot length depends on weak layer grain type with significantly larger values for persistent forms surface hoar and facets than for nonpersistent forms decomposing and fragmented grains

measurement of crack face friction in collapsed weak snow layers 1 in this letter we analyze the frictional contact forces during and immediately after the collapse of a weak snowpack layer when the sliding plane consists of the freshly collapsed and crushed but not yet eroded granular debris of the weak layer the results from thirty four field experiments show that frictional contact forces per unit area are on the order of 0 6 times the normal stress equivalent to a friction angle close to 30 degrees the measurements show that there is a transient sharp drop in the coefficient of friction during the collapse of the weak layer and relatively constant values afterwards one implication of our findings is that the minimum angle for avalanche release does not depend on shear strength as commonly thought but results from crack face friction which comes into play only as the fracture through the weak layer is already propagating

seismic precursors of a basaltic paroxysmal explosion track deep gas accumulation and slug upraise 1 basaltic volcanoes though fed with low viscosity magma occasionally produce sudden paroxysmal explosions that are unforecasted and whose triggering mechanism remains poorly elucidated here we report on the first detection of seismic signals precursory to such an explosion on 5 april 2003 at stromboli volcano italy this strongest event in the past 73 years was preceded by 25 h of seismic tremor variation broadly coincident with strong geochemical anomalies in crater plume emissions followed by 15 h of tilt related long period inflation pulses of increasing amplitude these precursory signals are best explained by accelerating growth and leakage of a bubble melt foam layer formed at 10 km depth whose collapse triggered the fast ascent of co2 rich gas slugs and then the explosion our results open new perspectives for the forecasting of such paroxysmal explosions and associated hazards

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

the 2007 eruptions and caldera collapse of the piton de la fournaise volcano la reunion island from tilt analysis at a single very broadband seismic station seismic records from la reunion island very broadband geoscope station are investigated to constrain the link between the 2007 eruptive sequence and the related caldera collapse of the piton de la fournaise volcano tilt estimated from seismic records reveals that the three 2007 eruptions belong to a single inflation deflation cycle tilt trend indicates that the small volume summit eruption of 18 february occurred during a phase of continuous inflation that started in january 2007 inflation decelerated 24 days before a second short lived small volume eruption on 30 march almost simultaneous with a sudden large scale deflation of the volcano deflation rate which had stabilized at relatively low level increased anew on 1 april while no magma was erupted followed on 2 april by a major distal eruption and on 5 april by a summit caldera collapse long term tilt variation suggests that the 2007 eruptive succession was triggered by a deep magma input

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

texture and strength changes of buried surface hoar layers with implications for dry snow slab avalanche release buried layers of surface hoar are the failure plane for many slab avalanches including fatal human triggered avalanches in various mountain regions these layers may persist as weak layers in the snow cover for weeks or months it is therefore essential for operational avalanche forecasters to monitor the evolution of persistent weak layers such as buried surface hoar traditional grain shape observations of isolated grains with a magnifier and crystal screen do not show bonding that is decisive for strength in this study we used in situ microphotography and observations of texture to complement strength measurements from shear frame tests buried layers of surface hoar consist of crystals most of which extend from the layer below to the layer above and may exhibit a columnar or truss like structure observations and measurements show that texture and crystal size change little over periods of up to several months during which the snowpack remains dry under these conditions layer thickness decreases while density and strength increase based on field measurements we argue that the increase in strength is primarily due to penetration of the surface hoar crystals into the adjacent layers especially at the bottom of the buried surface hoar layer where bonding is critical the weak bonding at the bottom implies that shear failure occurs at the lower interface rather than within the weak layer on slopes we find that surface hoar crystals that were initially surface normal are tilted downslope faster than predicted by published shear strain rates for settled snow indicating that shear strain is concentrated in these layers the characteristic texture of buried surface hoar columnar or truss like permits collapsing at the time of fracture the gravitational energy released by the displacement of the slab may contribute to the extensive fracture propagation associated with buried surface hoar layers

avalanche crown depth distributions 1 the literature disagrees about the statistical distribution of snow avalanche crown depths large datasets from mammoth mountain california and the westwide avalanche network show that the three parameter generalized extreme value distribution provides the most robust fit followed by a two parameter variation the frechet distribution the most parsimonious explanation is neither self organized criticality nor other complex cascades but the maximum domain of attraction implying that distributions of individual avalanche crown depths are scaling we also show that crown depths do not have a universal tail index rather they range from 2 8 to 4 6 over different avalanche paths consistent with other geophysical phenomena such as wildfires which show similar variability

strainmeters and tiltmeters despite steady effort over the last century continuously recording tiltmeters and strainmeters have not yet been successful except for earth tide measurements this article reviews the techniques and instrument designs that have been developed in this field the continuous measurement of true tectonic motions which have rates of 10 14 s 1 requires extremely high instrument stability but more rapid motions tides and seismic waves apart are even smaller and not much more easily detectable much past effort has been spent improving the transducers used in these instruments which usually measure small displacements the commonest choices are optical the most accurate capacitive the most sensitive and inductive the most rugged nowadays building a transducer is the easiest part a much harder task equally important to a good design is attaching the instrument to the ground so that it can detect the signal wanted this part of the design includes choosing a location a large underground opening is thermally stable but distorting a surface installation is easy to build but noisy because of soil weathering and a borehole is inaccessible to weather but also to the operator all such installations and indeed the whole field of strain and tilt measurement of course rely on the assumption that the deformations of interest can be measured over baselengths of a few hundred meters or less this is true for tides and seismic waves but unproven for tectonic motions appropriate attachment techniques differ for each location and many mundane details such as cementing techniques for borehole instruments remain to be worked out the goal of measuring tectonic and tidal tilt has created many tiltmeter designs most are relatively small instruments that use some form of pendulum or bubble level a few use a liquid surface to measure tilts over a long baseline repeated experience has shown the latter type when well built to be superior to the former proving end monument motions to be a dominant source of noise the same appears to hold true for strainmeters the best results coming from long base laser instruments though strainmeters using rigid or flexible material length standards are useful for special purposes and borehole instruments show promise because achieving a good design has been so difficult it is important to select the goals an instrument must meet and be prepared to compromise on others the difficulty of validating results from this type of instrument makes comparison tests essential such tests as have been made so far show that good results cannot be gotten at low cost or without careful attention to details

ultra long period seismic signals and cyclic deflation coincident with eruptions at santiaguito volcano guatemala abstract broadband seismic observations made near the active vent at santiaguito provide constraints on changing pressures inside the edifice during january 2009 we recorded episodic ultra long period signals ulps 30 s that were coincident with eruptive activity centered on the active el caliente vent these signals lasted 2 5 min and corresponded to depressurization of a volume beneath the dome coincident shorter period seismic signals 0 5 hz appeared to be associated with shallow level pyroclastic emissions and dome surface motions these shorter period signals varied significantly from event to event unlike the ulp waveforms which were self similar as opposed to the short period signals which were recorded across the network the ulps were only recorded on the horizontal channels of 3 stations that were within 1 1 km of the vent this and other factors suggest that they were a result of long period rotational motion rather than horizontal translation ulp source locations derived from horizontal particle motions point to the northwestern part of el caliente 150 m from the vent axis the general locus of activity from 1967 until flank collapses in 1989 1990 we modeled the ulp signal as a mogi source and solved for three dimensional location and equivalent source strength through analysis of 78 events we identify a mean mogi source that is 200 m west of and 250 m beneath the centre of the vent with the peak volume changes accompanying eruptions ranging from 125 to 1825 m 3 these inferred volume losses coincide with the onset of gas venting and or explosive eruptions at the surface of caliente based upon these observations we develop a model to describe the pre eruptive eruptive and inter eruptive physical processes occurring at the surface and inside santiaguito

field measurements of sintering after fracture of snowpack weak layers 1 this research documents two cases where field workers unintentionally fractured a snowpack weak layer but no avalanche released measurements from before and after the fractures provide unique data sets on the temporal change of snow stability shear strength decreased immediately after fracture on both slopes subsequent strengthening occurred in both cases though the rates differed presumably due to the characteristics of the weak layers our results have two important implications first they suggest the sub critical weak layer fractures assumed as a prerequisite in some snow slab avalanche release models are transient features and future modeling efforts must take this into account second they provide insights into interpreting snow stability tests and assessing the stability of slopes with fractured weak layers

10s period volcanic tremors observed over a wide area in southwestern japan unusual seismic oscillations with a period of 10 seconds were observed over a wide area of southwestern japan several lines of evidence indicate that the origin of these 10s waves must be around the aso volcano located in the central part of kyushu island japan although a 10s period is unusually long for a volcanic tremor and no corresponding surface activity at the crater was reported by the local observatory the volcanic origin of the tremor is indisputable a further astonishing aspects of the observation is that there exists a strong correlation between the onset of 10s waves observed remotely and cessation of high frequency 1hz volcanic tremors at the aso volcano the aso volcano appears to be trembling with a 10s period without this having been noticed since seismic monitoring of volcanic activity is limited to high frequencies above 1 hz at the most of the volcanoes in the world long period tremors of the type observed here might be a widespread phenomenon that has been overlooked

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

tilt recorded by a portable broadband seismograph the 2003 eruption of anatahan volcano mariana islands 1 the horizontal components of broadband seismographs are highly sensitive to tilt suggesting that commonly deployed portable broadband seismic sensors may record important tilt information associated with volcanic eruptions we report on a tilt episode that coincides with the first historical eruption of anatahan volcano on may 10 2003 the tilt was recorded by a strekheisen sts 2 seismograph deployed in an underground insulated chamber 7 km west of the active vent an ultra long period signal with a dominant period of several hours was recorded on the e w component beginning at 06 20 gmt on may 10 which coincides with the onset of continuous volcano tectonic vt seismicity and is one hour prior to the eruption time estimated by the volcanic ash advisory center the signal is much smaller on the n s component and absent on the vertical component suggesting it results from tilt that is approximately radial with respect to the active vent an estimate of tilt as a function of time is recovered by deconvolving the record to acceleration and dividing by the acceleration of gravity the record indicates an initial episode of tilt downward away from the volcanic center from 06 20 09 30 gmt which we interpret as inflation of the shallow volcanic source the tilt reverses recording deflation from 09 30 until 17 50 after which the tilt signal becomes insignificant the inflation corresponds to a period of numerous vt events whereas fewer events were recorded during the deflation episode and the vt events subsequently resumed after the end of the deflationary tilt the maximum tilt of 2 microradians can be used to estimate the volume of the source inflation 2 million m3 assuming a simple mogi source model these calculations are consistent with other estimates of source volume if reasonable source depths are assumed examination of broadband records of other eruptions may disclose further previously unrecognized tilt signals

very long period pulses at asama volcano central japan inferred from dense seismic observations summary we observed seismic events consisting of several one sided velocity pulses with durations of 5 30 s hereafter called very long period or vlp pulses using up to 14 broad band seismometers installed near the summit area of asama volcano japan particle motions do not point to a single source location despite their rectilinearities suggesting a non isotropic source mechanism together with path effect we conducted moment tensor inversion analyses for these events using the green s functions of not only translational motions but also of tilt motions the best fitting source locations are located north of the crater centre at depths of 100 150 m below the crater bottom the principal value ratios of moment tensor solutions can be approximated as 5 3 2 which is indicative of a combination of a tensile crack and a cylinder with the strike and dip angle of the crack both being nearly 70 and symmetry axis of the cylinder being inclined by nearly 30 from the vertical the time histories of moment tensors are indicative of an initial sudden pressurization and subsequent gradual depressurization at the source region we sought to model vlp pulses at asama volcano by considering the inflow and outflow of gas at the source cavity compensated by boiling in another deeper reservoir

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

monitoring gulf stream transport by radar altimeter and inverted echo sounders abstract profiles of the gulf stream sea surface height ssh from the geosat altimeter were compared with thermocline depth measurements from inverted echo sounders ies for a period of about 19 months the correlation between ssh anomalies and thermocline depth anomalies at about 68 w was 0 92 to distinguish between agreement in the location of the gulf stream jet and agreement in the strength of the jet separate comparisons were made for jet location and the ssh difference across the gulf stream based on an error function fit to each geosat and ies profile the correlation between the jet centers from the two types of measurements was 0 97 and the correlation between the amplitudes of the error functions was 0 76 although the altimeter measures surface geostrophic velocity the high degree of vertical correlation in the gulf stream velocity profiles allows the altimeter to monitor transport fluctuations as well as changes in jet position based on this assessment of the accuracy of the altimeter

decadal variability in the outflow from the nordic seas to the deep atlantic ocean the global thermohaline circulation is the oceanic overturning mode which is manifested in the north atlantic ocean as northward flowing surface waters which sink in the nordic greenland iceland and norwegian seas and return southwards after overflowing the greenland scotland ridge as deep water this process has been termed the conveyor belt and is believed to keep europe 5 8 c warmer than it would be if the conveyor were to shut down the variability of today s conveyor belt is therefore an important component of climate regulation the nordic seas are the only northern hemisphere source of deep water and a previous study has revealed no long term variability in the outflow of deep water from the nordic seas to the atlantic ocean here i use flows derived from hydrographic data to show that this outflow has approximately doubled and then returned to previous values over the past four decades i present evidence which suggests that this variability is forced by variability in polar air temperature which in turn may be connected to the recently reported arctic warming

persistent organic pollutants in ocean sediments from the north pacific to the arctic ocean concentrations of polychlorinated biphenyls pcbs organochlorine pesticides oc pesticides and polybrominated diphenyl ethers pbdes are reported in surficial sediments sampled along cruise transects from the bering sea to the central arctic ocean ocs and pcbs all had significantly higher concentrations in the relatively shallow water 500 m of the bering sea and arctic ocean e g canada basin pcb 149 102 pg g 1 dw concentrations were similar to or slightly lower than studies from the 1990s indicating a lack of a declining trend pbdes excluding bde 209 displayed very low concentrations e g range of median values 3 5 6 6 pg g dw in the shelf areas the sediments comprised similar proportions of silt and clay whereas the deep basin sediments were dominated by clay with a lower total organic carbon toc content while significant positive correlations were observed between persistent organic pollutant pop concentrations and toc pearson correlation r 0 66 0 75 p 0 05 the lack of strong correlations combined with differing chemical profiles between the sediments and technical formulations and or marine surface waters indicate substantial chemical processing during transfer to the benthic environment marked differences in sedimentation rates between the shallow and deeper water regions are apparent the 5 cm depth grab samples collected here representing 100 years of accumulation for the shelf sediments and 1000 years for the deeper ocean regions which may bias any comparisons nonetheless the sediments of the shallower coastal arctic seas appear to serve as significant repositories for pops deposited from surface waters

observed decadal midlatitude and tropical atlantic climate variability two common indicators of atlantic climate variability viz the north atlantic oscillation nao and the cross intertropical convergence zone itcz sea surface temperature sst gradient are examined for their frequency characteristics and midlatitude tropical links sst anomalies north and south of the itcz are found to be uncorrelated on all time scales while the sea level pressure slp fluctuations associated with the nao display a coherent seesaw between iceland and the azores this out of phase relationship spans a broad range of time scales but is particularly strong in the 5 10 year period band strong broadband coherence between the nao and the tropical atlantic cross itcz sst difference is found in the 8 20 year period band suggesting a significant midlatitude tropical interaction moreover tropical atlantic ssts on both sides of the itcz separately exhibit significant coherence with the nao index and slp variability over iceland and the azores based on these findings we hypothesize that the tropical atlantic ta ocean atmosphere interaction is affecting north atlantic climate variability