seismic anisotropy reveals a dynamic link between adjacent magmatic segments prior to dyke intrusion seismic anisotropy has increasingly been proposed as a tool in the monitoring of magmatic systems and potential forecasting of volcanic eruptions we present a detailed study of how seismic anisotropy evolves in an active magmatic rift segment before during and after a dyke intrusion in the afar depression ethiopia results show that seismic anisotropy prior to the dyke intrusion is controlled by a complex and deforming magma plumbing system beneath the adjacent dabbahu and manda hararo magmatic segments approximately eight days prior to the dyke intrusion in the dabbahu segment the pattern of anisotropy coupled with lower crustal seismicity is best explained by the inflation of a lower crustal magma reservoir in the manda hararo segment this is the only clearly observed precursory change in seismic anisotropy during the dyke intrusion the magnitude of seismic anisotropy increases twofold before rapidly returning to predyke values once the intrusion has ended combining our observations with models of magmatically induced crustal stress we propose that when the deep magma reservoir beneath the dabbahu segment becomes overpressured inflation is triggered in the magma reservoir of the neighboring manda hararo segment this provides strong evidence for a hydraulic link between the deep magma systems of the neighboring rift segments and that magma reservoirs beneath the dabbahu segment can be fed by the lateral flow of magma from an adjacent segment our results demonstrate that seismic anisotropy has the potential to be a powerful tool for monitoring deformation in the magma plumbing systems of active volcanoes

shallow seismicity beneath ruapehu crater lake results of a 1994 seismometer deployment virtually all the seismicity within ruapehu volcano recorded during a 2 month deployment in early 1994 with 14 broadband seismographs around the tongariro national park volcanoes in the north island of new zealand was associated with the active vent and occurred within approximately 1 km of ruapehu crater lake high frequency volcano tectonic earthquakes and low frequency events similar to bursts of 2 hz volcanic tremor were both found to have sources in this region the high frequency events which often consisted of a smaller precursor event followed approximately 2 s later by the main event had sharp onsets and were locatable using standard techniques the depth of these events ranged from the surface down to approximately 1500 m below crater lake the low frequency events did not have sharp onsets and were located by phase correlation methods nearly all occurred under a small region on the east side of crater lake at depths from 200 to 1000 m below the surface this low frequency earthquake source region in which no high frequency events occurred may be the steam zone within the actual vent of ruapehu volcano

anisotropic upper crust above the aftershock zone of the 2013 ms 7 0 lushan earthquake from the shear wave splitting analysis we have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with m 2 from the 2013 ms 7 0 lushan earthquake along the longmenshan fault system in southwest china polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction this indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled delay times between fast and slow shear waves do not show a clear trend of increase for deeper events suggesting the anisotropic zone is mostly above the aftershocks which are generally located below 8 km we further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy the three dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it the mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there the main and back reverse fault zones are associated with high anisotropic anomalies above 8 km likely caused by shear fabric or microfractures aligned parallel to the fault zone

reexamination of the near surface airflow over the antarctic continent and implications on atmospheric circulations at high southern latitudes abstract previous work has shown that winds in the lower atmosphere over the antarctic continent are among the most persistent on earth with directions coupled to the underlying ice topography in 1987 parish and bromwich used a diagnostic model to depict details of the antarctic near surface airflow a radially outward drainage pattern off the highest elevations of the ice sheets was displayed with wind speeds that generally increase from the high interior to the coast these winds are often referred to as katabatic with the implication that they are driven by radiational cooling of near surface air over the sloping ice terrain it has been shown that the antarctic orography constrains the low level wind regime through other forcing mechanisms as well dynamics of the lower atmosphere have been investigated increasingly by the use of numerical models since the observational network over the antarctic remains quite sparse real time numerical weather prediction for the u s antarctic program has been

tracking volcanic and geothermal activity in the tongariro volcanic centre new zealand with shear wave splitting tomography abstract we apply a simplified two dimensional tomographic inversion of recorded delay times of shear wave splitting and a spatial averaging of fast direction of anisotropy to data from temporary seismic deployments in the tongariro volcanic centre in order to identify regions of changing seismic anisotropy we observe a region of strong anisotropy 0 025 s km greater than the surrounding area centered on mt ruapehu in 1995 the time of a major magmatic eruption this is interpreted to be due to an increase in fluid filled fractures during the eruption we also observe strong anisotropy 0 018 s km greater than the surrounding area and a change in fast direction 80 at mt tongariro in 2008 and examine the temporal evolution of this anomaly using clusters of earthquakes and permanent seismic stations in operation since 2004 this anomaly is attributed to a change in the geothermal system a pronounced and unchanging feature is observed at waiouru even when the source and receiver locations differ we therefore conclude that the transient features of strong anisotropy associated with volcanic and geothermal activity detected with this method are also robust

monitoring of phreatic eruptions using interferometry on retrieved cross correlation function from ambient seismic noise results from mt ruapehu new zealand abstract since the last major eruption in 1995 96 mt ruapehu has erupted twice on 4 october 2006 and on 25 september 2007 these events occurred without any clear precursors and were mostly phreatic explosions the technique of interferometry on retrieved cross correlation function from ambient seismic noise irccasn is used to monitor subtle temporal changes of mt ruapehu s elastic properties the computation of cross correlation functions of seismic noise recorded at several stations around the volcano allowed us to observe variations during the 2006 eruption period the comparison between a reference cross correlation function and a current cross correlation function allows us to infer relative seismic velocity variations a 0 8 decrease of relative seismic velocity in the edifice starting two days before the 2006 eruption was observed this drop is due to a reversible and ephemeral effect which can be attributed to a pressurization of a magma pocket beneath the east flank of ruapehu due to new magma entering a small reservoir this pressure increase produced an inflation of the east flank of ruapehu and opened fractures in this area leading to a localised drop of seismic velocity we conducted the same analysis for the 2007 eruption but no significant seismic velocity variation was observed this difference is possibly due to the varying time scales of pressurization for the two events and also the irccasn time resolution an analysis of the seismicity before these two eruptions allows us to propose a conceptual model which explains the velocity drop for the 2006 eruption and the lack of velocity variations for the 2007 eruption since there was no surface deformation recorded by the geonet gps network we modelled the maximum radius and pressure change for a simple mogi point source at 5 km depth that produced no deformation we inferred the maximum fresh magma volume of 0 0017 km 3 entering the reservoir as a detectability threshold for gps ground deformation measurements

automatic measurement of shear wave splitting and applications to time varying anisotropy at mount ruapehu volcano new zealand 1 we present an automatic shear wave splitting measurement tool for local earthquakes with the sole manual step of choosing an s arrival time we apply the technique to three data sets recorded on mount ruapehu volcano in new zealand that have previously been determined to have fast polarizations that vary in time and with earthquake depth the technique uses an eigenvalue minimization technique applied over multiple measurement windows the dominant period of each waveform sets minimum and maximum window lengths cluster analysis determines the best solution among all the windows and quality grading criteria assess the results automatically when the same filters are used for events determined to be high quality from manual studies the automatic technique returns virtually identical results scatter increases when the automatic technique chooses the best filter but the average automatic results remain consistent with the manual results when the automatic technique is used on sets that include data previously judged as poor quality some stations yield distributions of fast polarizations that include peaks that were not present in previously published results the difference may stem from two factors automatic grading lets through some measurements that independent analysts consider poor quality but also unconscious bias in the manual selection process may downgrade measurements that do not fit expectations nonetheless the new objective analysis confirms changes in the average fast polarizations between 1994 and 2002 and between shallow and deep events therefore this new technique is valuable for objective assessment of anisotropy and its variation in time

southern hemisphere circulation features associated with el nino southern oscillation events abstract composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the southern hemisphere sh circulation features associated with el nino southern oscillation enso events the period of analyses available 1972 83 has limited the composites to include only three enso events the reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period 1950 79 of sh rawinsonde station data in the sh winter a weak equivalent barotropic wavetrain pattern of anomalies extends over australia and the south pacific ocean to south america this wavetrain pattern is quite variable in amplitude and location between enso events although it is more stable over the subtropical pacific in the sh summer the circulation anomalies are more zonally symmetric with increased height at low and high latitudes and decreased height in middle latitudes the circulation anoma

a case study of antarctic katabatic wind interaction with large scale forcing surface pressure decreases of up to 20 hpa occurred over much of the antarctic continent during a 4 day midwinter period of 1988 the widespread change in the pressure field accompanied intense cyclonic activity to the north of the ice sheet the equatorward mass transport across the antarctic coastline resulted in a redistribution of atmospheric pressure that extended to the subtropics of the southern hemisphere most of the mass flux from antarctica was the result of low level processes and appears tied to the katabatic wind circulation the observed surface pressure decrease over the continent reflects a perturbation of the mean meridional circulation between antarctica and the subpolar latitudes by synoptic scale processes zonally averaged circulations over antarctica were examined using output from the european centre for medium range weather forecasts model results suggest that only a poorly defined return branch of the meridional circulation exists in the middle and upper troposphere this southward directed flow does not compensate for the northward mass transport provided by the katabatic wind outflow in the lower atmosphere isallobaric contours over the antarctic ice sheet roughly match the area of the large scale drainage catchment associated with katabatic wind transport through the ross sea sector an intense extratropical cyclone was present in the circumpolar oceanic belt to the north of the continent the horizontal pressure gradient associated with the cyclone prompted enhanced drainage off the high interior plateau the resulting katabatic flow issued from the continent through a narrow corridor across the ross ice shelf and out over the southern ocean

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

seismic tomography of the tongariro volcanic centre new zealand tomographic inversion of local earthquake p and s wave traveltime data is used to investigate 3 d p wave velocity vp and p wave s wave velocity ratio vp vs variations at the tongariro volcanic centre tgvc new zealand p wave model resolution is generally high throughout the tgvc at depths of around 3 9 km below sea level near surface resolution is lower since rays at shallow depths are predominantly subparallel resolution decreases below 12 km depth due to clustering of seismicity at greater depths the final 3 d vp model shows low velocity volumes from 3 to 7 and 8 km depth respectively beneath the active volcanoes ruapehu and ngauruhoe smaller low velocity volumes are observed beneath the active tongariro volcano and the extinct tama lakes vents synthetic tests indicate that the resolution of the 3 d vp vs model is insufficient to permit any reliable interpretation accurate earthquake depths are determined for the first time within the tgvc seismicity is largely restricted to two clusters west of ruapehu possibly with origins on eastward striking branches of the normal raurimu fault two lineations near waiouru in the south east of the study area and a fault belt between tongariro and lake taupo in the north of the study area earthquake depths indicate possible shallowing of the brittle ductile transition beneath ruapehu and ngauruhoe

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

analysis of phreatic events at ruapehu volcano new zealand using a new som approach abstract we apply self organising maps som to assess the low level seismic activity prior to small scale phreatic events at ruapehu volcano new zealand the som approach allows an automatic pattern recognition virtually independent from a priori knowledge volcanic tremor spectra are randomly presented to the network in a competitive iterative training process followed by a hierarchical clusterization of the som nodes spectra are then projected ordered by time to clusters on the map a coherent time evolution of the data through the clusters can highlight the existence of different regimes and the transitions between them two ruapehu events were examined a phreatic event on 4 october 2006 which displaced the crater lake producing a 4 m high wave on the lake edge and the more energetic 25 september 2007 phreatic eruption the som analysis provides a classification of tremor spectral patterns that clusters into three regimes that we label by colours the pattern for both eruptions is consistent with a pre eruption spectral pattern including enhanced spectral energy in the range of 4 to 6 hz labelled green tremor this gives way to spectra having broader energy between 2 and 6 hz the so called red tremor just prior to the eruption the post eruption pattern includes spectral peaks at generally lower frequencies of 2 to 4 hz the so called blue tremor clusterization into only three groups yields highly non unique solutions which cannot explain the variety of processes operating at ruapehu over long time periods regardless the approach highlights noteworthy similarities that may be explained by a pattern of slow pressurisation under a hydrothermal or magmatic seal green followed by seal failure red and subsequent de pressurisation blue for the two events studied although the application shown here is limited we think it demonstrates the power of this classification approach

impacts of the 1945 and 1995 1996 ruapehu eruptions new zealand an example of increasing societal vulnerability over the past 50 yr the risk to society from volcanic eruptions has increased sharply due to an increased population more developed and diversified economies and a more technologically advanced infrastructure this fact is demonstrated vividly by the impacts from the two largest eruptions of the twentieth century from the cone volcano of ruapehu which suggest that the vulnerability of key sectors in new zealand society has increased by one to two orders of magnitude over this period both the 1945 and 1995 1996 eruptions included explosive phases that dispersed ash over a wide area of the north island for a period of several months individual ash falls were only a few millimeters thick in communities within 150 km of the volcano and only trace amounts were found in communities farther away the 1995 1996 eruption caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts the greatest contribution to the cost of the 1995 1996 eruption estimated in excess of 130 million new zealand dollars was the impact on the alpine tourist industry in the central north island essentially nonexistent in 1945 other significant impacts were felt by the rapidly growing aviation and electricity generation sectors the cost of any future eruption of the same magnitude is likely to grow as the vulnerability of our society increases at a rapid rate

indication of high pore fluid pressures in a seismically active fault zone summary polarisations of seismic shear wave splitting observed above small earthquakes in iceland are typically approximately ne to sw parallel to the direction of maximum horizontal stress in contrast the polarisations of shear waves at three new stations sited over the husav ik flatey fault a major seismically active transform fault in northern iceland are approximately nw to se orthogonal to the stress aligned polarisations elsewhere modelling suggests that these 90 flips in polarisations are caused by propagation through cracks containing fluids at high pore fluid pressures within one or two mpa of the critical stress these observations suggest that high pore fluid pressures which play a key role in earthquake source mechanisms can be monitored by analysing shear wave splitting above seismically active fault planes

stress induced temporal variations in seismic anisotropy observed in microseismic data summary in situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms volcanic eruptions and changes in hydrocarbon reservoirs shear wave splitting induced by stress controlled cracks in the shallow crust provides a way to infer this stress temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations here we present evidence of temporal variations in shear wave splitting from a continuous time series with a high occurrence of microseismic events recorded in a borehole we interpret these observations in terms of stress controlled cracks and are able to infer changes in stress and via modelling suggest the cause of the anisotropy possible origins of the temporal variation in per cent anisotropy are tidal or oil field production processes our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

search for temporal changes in shear wave splitting associated with the 2012 te maari eruptions at mount tongariro new zealand abstract we investigate changes in shear wave splitting and vp vs ratios of local earthquakes from the geonet catalogue during a 16 month period beginning a year before the first te maari eruption at mount tongariro on august 6 2012 focusing on four permanent seismographs located in proximity to the volcano we identify four time periods bounded by sharp transitions that comprise the study period during which moving averages of the shear wave splitting parameters fast direction and t delay time are fairly constant at all stations vp vs is steady throughout most of the study period at 1 75 small variations occur during the earthquake swarm at the volcano which started a month before the first eruption and for some low magnitude events occurring after a change in earthquake location method analysis of data sets in which epicentre location hypocentre depth and event magnitude are restricted illustrates that observed temporal changes in shear wave splitting parameters are likely due to the spatial variation of paths this in turn is governed by the spatial distribution of seismicity and measurement quality we think the short term variation in vp vs ratios is due to event origin time uncertainty of low magnitude earthquakes or incorrect s phase arrival timing for events in the tongariro swarm these results suggest that any volcanic processes able to cause changes in shear wave splitting or vp vs associated with the two eruptions during our study period were too localised to te maari to be observed at the seismographs studied using our methods dominant observed during the study period are oriented approximately tangential to the tongariro ngauruhoe massif at all four stations we suggest that this may result from gravitational loading of tongariro and ngauruhoe mountains inducing fracturing or dilatation of tangentially oriented microcracks there may also be some effect from layered material causing horizontal propagating rays yielding faster speed sh waves than sv at station ngz measurements from shallow earthquakes in immediate proximity to the seismographs indicate the potential presence of a shallow highly anisotropic body in the volcano

extratropical transition of southwest pacific tropical cyclones part i climatology and mean structure changes a database of tropical cyclone best track and intensity information for the southwest pacific ocean basin is used to construct a 28 year climatology for tropical cyclones that move into middle latitudes of the nine or so tropical cyclones that form each year an average of about three can be expected to migrate south of 358s with the greatest fraction in march storms entering the tasman sea west of new zealand nz move almost due south on average and retain greater intensity than those to the east of nz where storms decay quickly while moving rapidly away to the southeast storms east of nz are embedded in a stronger more zonal flow than those to the west which move poleward ahead of a larger amplitude trough during el nino years tropical cyclones that move into middle latitudes exhibit stronger zonal motion and occur over a wider range of longitudes than during la nina years storm intensity is only weakly correlated with concurrent sst anomalies suggesting that atmospheric circulation is the dominant influence on storm properties average structure changes during extratropical transition et are identified using the ncep ncar reanalysis dataset for a subset of 33 transitioning storms during 1980 97 composites are used to construct a three dimensional conceptual model of the transformation from a mature hurricane to an asymmetric baroclinic midlatitude cyclone southwest pacific tropical cyclones encounter the baroclinic westerlies early in their lives accounting for their average eastward and poleward motion at maximum average intensity near 208s baroclinic effects are already important with warm frontogenesis appearing in the southeast quadrant and outflow aloft into a downstream subtropical wind maximum that moves poleward with the storm by 258s the average tc has lost the characteristic symmetric anticyclonic outflow aloft and acquired the characteristics of a baroclinic midlatitude storm including regions of warm and cold frontogenesis a vertical motion dipole and a westward tilt with height from about 308s poleward a second upper tropospheric wind maximum appears west of the storm with strengthening cyclonic vorticity advection aloft below about 400 hpa the storm retains the vertical warm cyclonic core as it migrates poleward

gcm simulations of eastern australian cutoff lows abstract the ability of the csiro 9 general circulation model gcm to capture surface cutoff lows over eastern australia is investigated by comparing composites of ten gcm cases with ten observed lows the lows are also studied individually to compare their development and movement as well as synoptic features which may have been smoothed out in the compositing process finally the incidence of all such lows in the 1 co2 and 2 co2 simulations are examined to determine the possible effects a doubling of co2 will have on their occurrence the gcm surface lows were found to develop from an upper level cutoff low in a manner similar to the observed lows in both sets this development took place between the upper level subtropical and polar jets in all seasons except summer where only one jet was evident latent heat release appeared to play an important role in the intensification of the surface lows the main difference between the two sets of cutoff lows was that the gcm surface lows tended to dev

shear wave velocity structure of the tongariro volcanic centre new zealand fast rayleigh and slow love waves indicate strong shallow anisotropy abstract models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations which can help with monitoring volcanic activity however shear wave velocity of volcanoes is difficult to determine from traditional seismic techniques such as local earthquake tomography let or refraction reflection surveys here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear wave velocity structure of the tongariro volcanic centre tgvc of new zealand focusing on the active ruapehu and tongariro volcanoes we observe both the fundamental and first higher order modes of rayleigh and love waves within our noise dataset made from stacks of 15 min cross correlation functions we manually pick group velocity dispersion curves from over 1900 correlation functions of which we consider 1373 to be high quality we subsequently invert a subset of the fundamental mode rayleigh and love wave dispersion curves both independently and jointly for one dimensional shear wave velocity vs profiles at ruapehu and tongariro volcanoes vs increases very slowly at a rate of approximately 0 2 km s per km depth beneath ruapehu suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases at tongariro we observe larger vs increases with depth which we interpret as different layers within tongariro s volcanic system above altered basement greywacke slow vs on the order of 1 2 km s are compatible with p wave velocities using a vp vs ratio of 1 7 from existing velocity profiles of areas within the tgvc and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross correlation methods most of the measured group velocities of fundamental mode love waves across the tgvc are 0 1 0 4 km s slower than those of fundamental mode rayleigh waves in the frequency range of 0 25 1 hz first higher mode love waves are similarly slower than first higher mode rayleigh waves this is incompatible with synthetic dispersion curves we calculate using isotropic layered velocity models appropriate for ruapehu and tongariro in which love waves travel more quickly than rayleigh waves of the same period the love rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface wave propagation however several measurements at ruapehu have love wave group velocities that are faster than rayleigh wave group velocities the differences between the love and rayleigh wave dispersion curves also vary with the azimuth of the interstation path across ruapehu and tongariro volcanoes significant azimuthal dependence of both love and rayleigh wave velocities are also observed this suggests azimuthal anisotropy within the volcanic structures which coupled with radial anisotropy makes the vs structures of ruapehu and tongariro volcanoes anisotropic with orthorhombic or lower order symmetry we suggest that further work to determine three dimensional volcanic structures should include provisions for such anisotropy

monitoring increases in fracture connectivity during hydraulic stimulations from temporal variations in shear wave splitting polarization summary hydraulic overpressure can induce fractures and increase permeability in a range of geological settings including volcanological glacial and petroleum reservoirs here we consider an example of induced hydraulic fracture stimulation in a tight gas sandstone successful exploitation of tight gas reservoirs requires fracture networks either naturally occurring or generated through hydraulic stimulation the study of seismic anisotropy provides a means to infer properties of fracture networks such as the dominant orientation of fracture sets and fracture compliances shear wave splitting from microseismic data acquired during hydraulic fracture stimulation allows us to not only estimate anisotropy and fracture properties but also to monitor their evolution through time here we analyse shear wave splitting using microseismic events recorded during a multistage hydraulic fracture stimulation in a tight gas sandstone reservoir a substantial rotation in the dominant fast polarization direction i s observed between the events of stage 1 and those from later stages although large changes in have often been linked to stress induced changes in crack orientation here we argue that it can better be explained by a smaller fracture rotation coupled with an increase in the ratio of normal to tangential compliance zn zt from 0 3 to 0 6 zn zt is sensitive to elements of the internal architecture of the fracture as well as fracture connectivity and permeability thus monitoring zn zt with shear wave splitting can potentially allow us to remotely detect changes in permeability caused by hydraulic stimulation in a range of geological settings

ductile precambrian fabric control of seismic anisotropy in the acu dam area northeastern brazil 1 in the joao c mara area ne brazil seismic anisotropy has been interpreted as the result of either precambrian ductile fabric or stress aligned fluid filled grain boundary caused by high pore fluid pressures the first interpretation is based on the fact that the direction of the fastest s wave splitting agrees with the north south trending precambrian fabric the second interpretation related to the extensive dilatancy anisotropy eda is based on the stress field orientation and on the assumption that the pore fluid pressure in the region is high enough to modify microcrack distribution and cause s wave polarization to become parallel to the north south oriented minimum horizontal stress shmin here we address this question by investigating this ambiguity we have analyzed the seismicity in the acu dam area 150 km west of joao c mara over a 3 year period in the acu area the northeast southwest trending ductile precambrian fabric is not parallel or orthogonal to the east west trending maximum horizontal stress shmax we combined the interpretation of fastest s wave polarizations their time delays and fabric orientation measured in the field with side looking airborne radar imagery taken before the lake impoundment thus we discriminate between 1 stress aligned fluid filled cracks 2 high pore pressure fluids modifying the microcrack distribution and therefore causing 90 flips and 3 fabric or paleostrain as causes for the observed s wave anisotropy our main result indicates that the direction of the fastest s wave polarization in the acu dam area agrees with the northeast southwest trending fabric n40 e eda or 90 flips control of seismic anisotropy if occurring in acu have minor effects

atlantic subtropical storms part i diagnostic criteria and composite analysis abstract subtropical cyclones st have only recently gained attention as damaging weather systems a set of criteria for identifying and classifying these systems is introduced here and employed to identify 18 st cases forming in the 1999 2004 hurricane seasons to be classified as an st these systems must have near surface gale force winds and show hybrid structure for more than one diurnal cycle the 18 st cases are partitioned into four classes based upon their genesis environments genesis over waters with sst in excess of 25 c is observed in almost 80 of warm season cases in contrast with only 55 in an st climatology presented in a companion study the low shear magnitude constraint recognized for tropical cyclogenesis is less apparent for st formation with over 50 forming in the two partitions characterized by shear in excess of 10 m s 1 this relatively high shear environment corresponds to equatorward intrusion of upper troughs over the relatively warm sst present in the mid late hurricane s

volcanic earthquakes and their relationship to eruptions at ruapehu and ngauruhoe volcanoes abstract crustal earthquakes near ruapehu and ngauruhoe fall into two classes each of which can be subdivided on the one hand there are high frequency events 3 hz with sharp well defined phases mainly concentrated beneath ruapehu crater lake low frequency events the low frequency events resemble minakami s b type and explosion earthquakes but sometimes occur where no vent exists and rather deeper than his formal definition difficulty in applying minakami s classification rigorously and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path rather than to their suppression or absence at the source has led to reclassification of earthquakes near the volcanoes into two broad groups tectonic and volcanic the former includes all high frequency earthquakes and those discrete events in which dominant low frequencies are due to path effects the latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism dominant low frequencies are ascribed to occurrence in heat weakened material and high frequencies to instantaneous source mechanisms operating in competent rock the term volcano tectonic describes tectonic earthquakes within some arbitrary distance of a volcano at ngauruhoe and ruapehu volcanic earthquakes accompany explosive vent clearing eruptions subsequent open vent degassing and ash emission however although often powerful and prolonged usually occurs without earthquakes such activity is however frequently accompanied by volcanic tremor at ruapehu under closed vent conditions when lake temperature is low low frequency earthquakes up to magnitude ml 3 4 have occurred without any eruption five types of phreatic eruptions are identified at ruapehu each having a distinctive seismic pattern the three most explosive types appear to be generated by a chain reaction process and all involve flashing of water to steam the first by failure of the roof with little precursory seismicity after a closed vent period during which lake temperature decreases the second after prolonged heating of the lake and much preliminary volcanic tremor interpreted as due to rising magma and the third under open vent conditions in the wake of one of the two preceding types a fourth probably occurs in wet sediments near the base of the lake as a result of upward migration of hot gas and a fifth aseismic or accompanied by very weak volcanic tremor is associated with convective overturn within crater lake

calibrating the amplitude source location asl method by using active seismic sources an example from te maari volcano tongariro national park new zealand a frequently applied amplitude source location asl method is here calibrated and optimized by using active seismic sources located at the surface of a dry stream channel the asl produced location discrepancies larger than 1 0 km laterally and 500 m in depth by using independently determined velocity model attenuation and site amplification factors afs sensitivity tests for asl input parameters show that attenuation and velocity have moderate influence on the location but are easy to independently constrain afs are shown to strongly influence the location and their application may introduce substantial location uncertainties model uncertainties are accommodated with either lateral or depth changes depending on the input parameters station corrections and the source station geometry

large scale influences on the evolution of winter subtropical maritime cyclones affecting australia s east coast abstractsubtropical maritime low pressure systems frequently impact australia s eastern seaboard closed circulation lows in the tasman sea region are termed east coast cyclones ecc they can evolve in a range of climatic environments and have proven most destructive during the late autumn winter period using criteria based on pressure gradients inferred wind field and duration an objectively determined database of ecc occurrences is established to explore large scale influences on ecc evolution subclassification based on evolutionary trajectory reveals two dominant storm types during late autumn winter easterly trough lows etl and southern secondary lows ssl synoptic composites are used to investigate the climatological evolution of each storm type etl cyclogenesis occurs along the eastern seaboard at the confluence of warm moist subtropical easterlies and cool air over the continent that is advected from higher latitudes ssl develop when a cold extratropical cyclone moves equatorward and

on the role of the antarctic continent in forcing large scale circulations in the high southern latitudes abstract the antarctic topography and attendant katabatic wind regime appear to play a key role in the climate of the high southern latitudes during the nonsummer months persistent and often times intense katabatic winds occur in the lowest few hundred meters of the antarctic atmosphere these slope flows transport significant amounts of cold air northward and thereby modify the horizontal pressure field over the high southern latitudes three year seasonal cycle numerical simulations using the ncar community climate model version 1 ccm1 with and without representation of the antarctic orography were performed to explore the role of the elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field the katabatic wind regime is an important part of a clearly defined mean meridional circulation in the high southern latitudes the position and intensity of the attendant sea level low pressure belt appears to be tied to the antarctic orography the seasonal movemen

the 1998 valhall microseismic data set an integrated study of relocated sources seismic multiplets and s wave splitting werelocate303microseismiceventsrecordedin1998bysensorsinasingleboreholeinthenorthseavalhalloilfield asemiautomated array analysis method repicks the p and s wave arrival times and p wave polarizations which are needed to locate these events the relocated sources are confined predominantly toa50 m thickzonejustabovethereservoir andlocationuncertaintiesarehalfthoseofpreviousefforts multipletanalysisidentifies 40 multiplet groups which include 208 of the 303 events the largest group contains 24 events and five groups contain 10 or more events within each multiplet group we further improve arrival time picking through crosscorrelation which enhances the relative accuracy of the relocated events and reveals that more than 99 of the seismic activity lies spatially in three distinct clusters the spatial distribution of events and waveform similarities reveal two faultlike structures that match well with north northwest south southeast trending fault planes interpreted from 3d surface seismic data most waveform differencesbetweenmultipletgroupslocatedonthesefaultscanbeattributed to s wave phase content and polarity or p to s amplitude ratio the range in p to s amplitude ratios observed on the faults is explained best in terms of varying source mechanisms wealsofindacorrelationbetweenmultipletgroupsandtemporal variations in seismic anisotropy as revealed by s wave splitting analysis we explain these findings in the context of a cyclic rechargeanddissipationofcap rockstressesinresponsetoproduction driven compaction of the underlying oil reservoir the cyclic nature of this mechanism drives the short term variations in seismic anisotropy and the reactivation of microseismic source mechanismsovertime

shear wave automatic picking and splitting measurements at ruapehu volcano new zealand automatic shear wave picking and shear wave splitting measurement tools multiple filter automatic splitting technique mfast are combined to build a near real time application for monitoring local stress around volcanoes we use an adapted version of diehl et al 2009 on seismograms provided by the new zealand geonet network and having an origin time and location based only on p picks the best automatic picks are processed by mfast which computes the corresponding shear wave fast direction and splitting delay time t interpreted respectively as the principal direction of stress underneath the station and the amount of anisotropy integrated along the wave raypath we applied our system to 9 years of local earthquakes recorded at seven stations around ruapehu volcano new zealand results are compared against mfast measurements from manual s picks when available and show less than 10 difference for 90 of measurements and less than 0 05 s difference for 95 of t measurements shear wave splitting from automatic s arrival times are slightly more consistent than those from manual arrival times at some stations two populations of delay times occur which depend upon computed initial polarization this may be caused in part by cycle skipping an artifact usually associated with monochromatic signals however spatial consistency in the behavior suggests a physical cause as well such as focal mechanisms varying with earthquake source location or a spatially varying near source anisotropic region the numbers of events in each population group vary over time possibly related to activity at ruapehu volcano

the wintertime southern hemisphere split jet structure variability and evolution a persistent feature of the southern hemisphere upper level time mean flow is the presence of a split jet across the south pacific east of australia during the austral winter the split jet is composed of the subtropical jet stj on its equatorward branch and the polar front jet pfj on its poleward branch the ncep ncar reanalysis is used to investigate the structure and evolution of the split jet results show that the presence absence of the pfj determines the degree of split flow given that the stj is a quasi steady feature a splitflow index sfi is developed to quantify the variability of the split jet in which negative values represent strong split flow and positive values nonsplit flow correlations with teleconnection indices are investigated with the sfi positively correlated to the southern oscillation index and negatively correlated to the antarctic oscillation the sfi is used to construct composites of heights temperature and wind for split flow and non split flow days the composites reveal that relatively cold conditions occur in the south pacific in association with nonsplit flow regimes and split flow regimes occur when relatively warm conditions prevail in the latter situation cold air bottled up over antarctica helps to augment the background tropospheric thickness gradient between antarctica and the lower latitudes with a resulting increase in the thermal wind and the pfj it is surmised that frequent cold surges out of antarctica moving into the south pacific are associated with non split flow regimes in this context the variability of the split jet responds to large scale baroclinic processes and is further modulated by synoptic scale disturbances

topographic effects on the mean tropospheric flow patterns around antarctica abstract topographically induced flows around antarctica in a rotating tank experiment with both homogeneous and stratified fluid are analyzed and compared with the mean tropospheric circulation a circular tank of fluid was brought to a state of near rigid clockwise rotation and a topographic model of the antarctic continent was then rotated counterclockwise to simulate a mean westerly zonal wind stratification was chosen to give the same ratios of topographic and dynamical length scales as in the atmosphere as was the rossby number based on the ratio of rotation rates after the onset of the relative rotation of the antarctic model cyclonic eddies evolved in the coastal areas with in the homogeneous case anticyclonic eddies over the antarctic dome after about ten tank rotation periods a dominant wavenumber 3 structure with cyclonic eddies in the ross and weddell seas and prydz bay is observed as an approximately steady state flow over the topography is relatively stagnant with weak anticycloni

definition of antarctic oscillation index following walker s work about his thmous three oscillations published during the 1920 30s many papers were written about atmospheric oscillations a iburth atmospheric oscillation in the middle and high southern latitudes was lbund and nanted the grotarctic oscillation ao a refiers to a large scale alternation of atmospheric mass between the mid latitudes and high latitudes surthce pressure in order to understand the spatial structure of sea level pressure variation in detail empirical orthogonal fitnction analysis is applied an objective index of the antarctic oscillation index aoi is defined as the diffierence of zonal mean sea level pressure between 40os and 65os the aoi has the potential tbr clarifying climate regimes in the southern hemisphere similar to how the na and the npo has been used in the northern hemisphere were there other oscillation s besides nao npo and s especially in the southern hemisphere it had been supposed that there might be other atmospheric oscillation s in the high southern latitudes although as early as in the first quarter of this century walker 1928 had stated that just as in the north atlantic there is a pressure opposition between the azores and iceland there is an opposition between the high pressure belt across chile and the argentine on the one hand and the low pressure area of weddell sea and the bellingshausen sea on the other the scarcity of data in the southern hemisphere hindered the search tbr new oscillation s during the last two decades more comprehensive data over the southern hemisphere became available and more analyses were carried out see kidson 1975 rogers and van loon 1982 mo and l 7 ite 1985 2 4o 1986 connolley 1997 gong and wang 1998 a tburth atmospheric oscillation in the middle and high southern latitudes was tbund and nanted as the antarctic oscillation a wang 1992 gong and wang 1998 the term ao relrs to a large scale alternation of atmospheric mass between the mid latitude sur hce pressure

a climatology of anticyclones and blocking for the southern hemisphere abstract an automated procedure for locating and tracking high pressure centers is applied to 10 years of twice daily ecmwf analyses during 1980 89 to survey the behavior of anticyclones in the southern hemisphere anticyclones are concentrated in the 25 45 s band with preferred regions consistent with previous studies except in the new zealand nz south pacific sector where a double occurrence maximum noted in earlier studies was only weakly evident marked interannual variability was found in this region with the poleward branch of maximum occurrence south of 50 s present in only 3 out of 10 years mean central pressure maximized about 10 south of the time averaged ridge with highest values in winter anticyclones poleward of 50 s involve an anomalous breakdown of the westerlies with mean central pressures there 20 30 hpa higher than climatology highs tend to form and intensify in western regions of ocean basins and weaken and dissipate in the east of these oceans regions southeast of australi

a new source process for evolving repetitious earthquakes at ngauruhoe volcano new zealand abstract since early 2005 ngauruhoe volcano has produced repeating low frequency earthquakes with evolving waveforms and spectral features which become progressively enriched in higher frequency energy during the period 2005 to 2009 with the trend reversing after that time the earthquakes also show a seasonal cycle since january 2006 with peak numbers of events occurring in the spring and summer period and lower numbers of events at other times we explain these patterns by the excitation of a shallow two phase water gas or water steam cavity having temporal variations in volume fraction of bubbles such variations in two phase systems are known to produce a large range of acoustic velocities 2 300 m s and corresponding changes in impedance contrast we suggest that an increasing bubble volume fraction is caused by progressive heating of melt water in the resonant cavity system which in turn promotes the scattering excitation of higher frequencies explaining both spectral shift and seasonal dependence we have conducted a constrained waveform inversion and grid search for moment position and source geometry for the onset of two example earthquakes occurring 17 and 19 january 2008 a time when events showed a frequency enrichment episode occurring over a period of a few days the inversion and associated error analysis in conjunction with an earthquake phase analysis show that the two earthquakes represent an excitation of a single source position and geometry the observed spectral changes from a stationary earthquake source and geometry suggest that an evolution in both near source resonance and scattering is occurring over periods from days to months

statistics and dynamics of persistent anomalies abstract persistent anomalies with recurrent spatial patterns play an important role in the atmosphere s low frequency variability we establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies this is done by computing and analyzing the empirical orthogonal functions eofs in a simple deterministic model on the one hand and in southern hemisphere geopotential heights on the other the dynamical model is governed by the fully nonlinear equivalent barotropic vorticity equation on the sphere with simplified forcing dissipation and topography model solutions exhibit persistent anomalies identifiable with blocked zonal and wave train anomalies in northern hemisphere atmospheric data flow structures similar to the patterns above occur as high variance eofs of this nonlinear model the southern hemisphere data we analyze consist in gridded daily maps of 500 mb heights from june 1972 to july 1983 two types of persistent anomalies appear in t

seismicity associated with the 1995 1996 eruptions of ruapehu volcano new zealand narrative and insights into physical processes abstract seismicity associated with the 1995 1996 eruptions of ruapehu reflects processes associated with the intrusion of magma to shallow depth in the volcanic edifice its subsequent eruption and changes in the volcanic plumbing system resulting from the eruptions the sequence consisted of two distinct periods of eruptive activity the first lasting from september 17 1995 until late october or early november 1995 and the second from june 16 1996 until late july 1996 immediately prior to and during the early stages of the 1995 eruptions seismicity was similar to that recorded at ruapehu during the previous 25 years with the exception of the occurrence between september 17 and 25 1995 of sub 1 hz tremor which is thought to represent magmatic intrusion in early october a new pattern of seismicity was established with the disappearance of the 2 hz tremor resonator source and a change to wideband 2 10 hz tremor and wideband volcanic earthquakes this change coincided with the ejection of the last vestiges of crater lake and the change in eruptive style from phreatomagmatic to magmatic seismicity associated with the 1996 eruptions was very similar to that which accompanied the later part of the 1995 activity implying the same eruption processes and that no significant changes had occurred in the volcanic plumbing system during the intervening period geochemical geodetic and seismic precursors to the 1995 eruptions were both minor and inconsistent highlighting the difficulty in forecasting these eruptions furthermore deep volcano tectonic earthquakes were extremely rare throughout the 1995 and 1996 eruptive sequences suggesting that stresses associated with magmatic intrusion were minor this most likely resulted from the existence of either an open or a ductile pathway from the deep magma source to the surface prior to the eruptions and because the volume of magma intruded and subsequently erupted was relatively small 3

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

analysis and forward modeling of seismic anisotropy during the ongoing eruption of the soufriere hills volcano montserrat 1996 2007 1 volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long term eruption forecasting however because tectonic processes can also produce localized stress field reorientations evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance in this study we show that local stress field reorientations preceding changes in volcanic activity at the soufriere hills volcano montserrat are observable using two independent measures of crustal stress local volcano tectonic earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes we further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at soufriere hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike these observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity

cyclic processes and factors leading to phreatic eruption events insights from the 25 september 2007 eruption through ruapehu crater lake new zealand abstract a dangerous block ash eruption occurred without warning from ruapehu crater lake on 25 september 2007 ending a 6 month period of temperature decline in the lake the northerly directed blast sent ballistics of up to 2 m diameter more than 2 km from the source the ejecta comprised a variety of volcano lacustrine lithologies many showing evidence of intense hydrothermal alteration petrographic studies of the ballistics reveal numerous examples of pores vesicles being filled with assemblages of elemental sulphur anhydrite pyrite and natroalunite and many ballistics exuded liquid sulphur from their pores during cooling at atmospheric pressure up to 5 of the ash was comprised of fine grained juvenile glass material confirming the presence of magma in the shallow vent environment a long recognised characteristic of this volcano is the thermal cycling behaviour of the crater lake behaviour which has remained effectively unchanged through two magmatic eruption events in 1945 and in 1995 96 airborne measurements of co2 and so2 flux from the volcano since 2003 show a strong positive correspondence with temperature cycling in the lake pointing to a definitive link between heat and mass discharge through the system low so2 emission rates and c s mole ratios 30 were typical of the period 2003 to early 2007 and appear to reflect vigorous scrubbing of s gases in the hydrothermal system during this time to better understand the chemical and physical processes operating in this very dynamic volcanic hydrothermal system and its propensity to develop potentially dangerous hydrothermal seals we have developed an integrated finite difference tough2 heat mass transport simulation of the hydrothermal environment the model which is constrained by observed heat and mass flux characteristics of the volcano demonstrates the transient nature of the heating cycles on the vent lake system of particular interest is the role of co2 as a relatively inert pressure transmitting medium in the hydrothermal environment seal formation was simulated using the reactive transport code x1t which showed that condensation of magmatic vapour into a highly porous medium of andesitic composition leads to rapid formation of an elemental sulphur anhydrite natroalunite mineral assemblage and a drastic reduction in permeability we suggest that since the 1995 96 vent clearing eruptions a shroud of elemental sulphur and other alteration phases formed along the margins of the 2 phase liquid vapour region enclosing the conduit this effectively seals the upper portion of the vent from the adjacent hydrostatically controlled environment and allows for development of pressurised vapour static gas columns beneath the northern and central vents

tornillos beneath tongariro volcano new zealand abstract unusual low frequency seismic events have recently begun to occur beneath tongariro volcano new zealand because the seismic waveforms appear similar to waveforms called tornillos which have been recorded at galeras volcano in colombia and have been found to precede small to medium sized ash eruptions there by several weeks to months we are monitoring them closely at tongariro these events are characterised by small amplitude monochromatic or multichromatic waveforms that decay slowly with time features that suggest a source involving magmatic or hydrothermal fluids within a zone surrounded by hard rock the frequency content of monochromatic events is typically between 1 and 3 hz and is often found to be peaked at f 1 2 hz event durations range from about 10 to 80 s the appearance of the shorter duration events is identical to so called lp or long period events the lps and tornillos appear to be co located so that the only difference between lps and tornillos is the extended duration of the latter these events have been identified in the seismograms of the permanent monitoring station kavz since at least january 2001 occurring at a rate of about one per month during june august 2001 the rate of occurrence gradually increased peaking in september 2001 at several events per day we deployed two broadband and several short period seismometers at close distances to the expected source location the resulting seismograms show a complex mixture of seismic waves originating from shallow depth beneath the area of the te maari craters located approximately 0 5 km from the nearest station the close range broadband records reveal some interesting details about the tornillo source the tornillo waveform is found to consist of two parts a low frequency f 1 2 hz part and a higher frequency f 3 hz part the high frequency energy travels at body wave speeds c 3 0 km s while the low frequency energy travels at the speed of shear or surface waves in the shallow crust c 1 km s interestingly for some events the low frequency waves arrive before the high frequency body waves at the closest station this and comparison with several lp events suggests a spatial separation of the high and low frequency sources with the low frequency energy originating at shallow depth and acting as a trigger for the release of tectonic stress at greater depth in the form of high frequency body waves

calculating splitting parameters for plume type anisotropic structures of the upper mantle summary we apply a forward propagator method to calculate shear wave splitting parameters for general 3 d weakly anisotropic upper mantle structures the approach is valid under the assumption that the ray paths of the two quasi shear qs waves can be approximated by a common reference ray within an isotropic background medium along the reference ray the incremental splitting is expressed in terms of qs polarizations and slownesses in the direction of propagation using a ray theory ansatz we show that the resulting shear wave seismogram is equivalent to the coupling ray theory of coates chapman 1990 under the assumption of smoothly varying anisotropy here we use the forward propagator method to calculate apparent shear wave splitting parameters that can be compared directly with observations of teleseismic shear wave splitting a comparison with finite difference calculations is used to assess the validity of the method as an application we consider shear wave splitting due to a plume type anisotropic upper mantle structure the anisotropy is assumed to result from the preferred alignment of orthorhombic olivine we show examples of waveforms and splitting parameters as functions of backazimuth and angle of incidence anisotropic effects for sks are weak at stations near the central upwelling due to the combined effects of initial polarization and olivine a axis orientation here the splitting parameters are irregular and fast axis directions may vary by up to 70 depending on the vertical incidence angle at larger distances splitting parameters slowly converge towards values expected for homogeneous media the results suggest that direct evidence for mantle plumes from shear wave splitting is more likely to come from obs stations at some distance away from the central upwelling at shallower incidence the apparent splitting parameters for s exhibit a 90 periodicity as functions of the initial polarization of the incident shear wave provided that the ray path is kept fixed our results show that this is a general characteristic of shear wave splitting in inhomogeneous anisotropic media the effect can be used to identify complex anisotropic regions within the earth s mantle

a four year study of shear wave splitting in iceland 1 background and preliminary analysis this is part 1 of a report of a four year study of seismic shear wave splitting in iceland designed to seek temporal variations before earthquakes shearwave splitting is observed routinely in iceland whenever shear waves arrive within the shear wave window of seismic stations and whenever adequate data are available temporal and spatial variations in shear wave splitting are observed before both earthquakes and volcanic eruptions shear wave splitting is caused principally by the stress aligned fluid saturated microcracks and pore throats in almost all in situ rocks fluid saturated microcracks are the most compliant elements of the rock mass and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone such changes were found and are reported in paper 2 this paper presents the background preliminary observations and analysis of shear wave splitting in iceland

geodynamics of the middle east domain since the oligocene research summary several analogue models were merged to link the post oligocene closure of the neotethys seaway with the break up of the gulf of aden and the red sea the seaway closure since the early neogene which was a result of northwards subduction of neotethyan lithosphere under south central eurasia generated two styles of kinematic convergence its western branch forms the tethyan subduction under south central eurasia along the hellenic deformation front generating the break up of the aegean sea and the westward migration of anatolia the eastern branch comprises the oblique convergence of arabia with eastern anatolia and iran along the bitlis zagros suture which gradually shifted from subduction to collision because the velocity of plate collision is slower than that of subduction the subducting section of the underthrust slab moves faster than the colliding segment consequently the underthrust arabian slab rotated counterclockwise and developed detachment zones such extension triggered the resumption of the propagation of the carlsberg ridge into the gulf of aden after being stalled for nearly 30 ma it is presumed further that the break up of the red sea in the mid miocene was superimposed on the regional uplift and exhumation of the arabo nubian massif which was caused by a mantle plume in the oligocene a plume that subsequently affected the ethiopian and the east african plateaux in the neogene

transtensional deformation of montserrat revealed by shear wave splitting here we investigate seismic anisotropy of the upper crust in the vicinity of soufriere hills volcano using shear wave splitting sws analysis from volcano tectonic vt events soufriere hills which is located on the island of montserrat in the lesser antilles became active in 1995 and has been erupting ever since with five major phases of extrusive activity we use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases shear wave splitting in the crust is often assumed to be controlled either by structural features or by stress aligned cracks in such a case the polarization of the fast shear wave would align parallel to the strike of the structure or to the maximum compressive stress direction previous studies analyzing sws in the region using regional earthquakes observed temporal variations in which were interpreted as being caused by stress perturbations associated with pressurization of a dyke our analysis which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust shows no clear temporal variation however temporal effects cannot be ruled out as large fluctuations in the rate of vt events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess average delay times of approximately 0 2 s similar in magnitude to those reported for much deeper slab events suggest that the bulk of the anisotropy is in the shallow crust we observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left lateral transtensional array of faults which crosses the volcanic complex

a new look for the southern hemisphere jet stream a new jet stream description defined as the geostrophic streamline of maximum average velocity is proposed an objective algorithm for detecting and tracking the jet has been developed tested and applied to the ncep ncar 200 hpa geopotential height in the southern hemisphere for the period 1958 2002 the results show the variability of the double character of the southern hemisphere jet with a marked seasonality during the warm season a single jet can be found around 40 s while autumn and winter are characterized by a clear double jet structure with a strong and dominant subtropical jet located around 30 s and a polar front jet progressively displaced toward southern latitudes and reaching 60 s by the end of the cold season in general a trend toward slower subtropical jets and stronger polar front jets has been detected during the study period the southern annular mode appears as a main modulator of the latitude and strength of the polar front jet influencing to a minor extent its subtropical counterpart the enso cycle strongly modifies the latitude and specially the strength of the subtropical jet affecting its preferred wavenumber as well nevertheless the effect of this oscillation seems fairly restricted in the pacific thus limiting the ability of this jet to drive the el nino teleconnections along the southern hemisphere the consistency of the results when compared with previous jet climatologies suggests that the new approach is a reliable jet tracking method thus providing a new tool to analyze climatic variability at hemispheric scales

baroclinically induced tropical cyclogenesis abstract the authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 atlantic tropical cyclone seasons in all transitioning cases 10 the 900 200 hpa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis in many of these cases the vertical shear decreased markedly near or prior to the time of tropical cyclone formation in cases that did not become named tropical cyclones either the tropospheric vertical shear remained in excess of 15 20 m s 1 or the underlying sea surface temperature sst dropped below about 26 c prior to or during the weakening of the shear cases in which the shear remained large featured multiple short wave upper tropospheric troughs interacting with the developing lower tropospheric disturbance such that classical occlusion did not occur through detailed analysis and simulation of the development of hurricane michael in 2000

atlantic subtropical storms part ii climatology abstract a 45 yr climatology of subtropical cyclones st for the north atlantic is presented and analyzed the sts pose a warm season forecasting problem for subtropical locations such as bermuda and the southern united states because of the potentially rapid onset of gale force winds close to land criteria for identification of st have been developed based on an accompanying case study analysis these criteria are applied here to the 40 yr european centre for medium range weather forecasts re analysis era 40 to construct a consistent historical database of 197 north atlantic st in 45 yr because st may eventually evolve into tropical cyclones sea surface temperatures sst and vertical wind shear conditions for tropical cyclogenesis are contrasted with the conditions for st genesis identified here around 60 of the 197 st formed over sst in excess of 25 c in a region of weak static stability further the mean environmental vertical wind shear at formation for these storms is 10 7 m s 1 a magnitud

magma system beneath tengchong volcanic zone inferred from local earthquake seismic tomography abstract the tengchong volcanic zone tcvz located in the southeastern part of the india eurasia collision zone is characterized by large scale volcano groups significant thermal activities and active seismicity to reveal the magma system we present new 3 d models of vp vs and vp vs ratio to 10 km depth beneath the tcvz based on the tomographic inversion of p and s wave arrival times from local earthquakes recorded by 10 broadband seismic stations we use arrival times to construct a reference velocity model checkerboard and jackknife tests are carried out to assess the reliability of the inversion results at shallow depths we suggest that the complex distribution of high vp vs ratio is the result of a combination of water and magmatic activity at depth of 10 km only three areas show obvious high vp vs ratio corresponding to areas of high relative geothermal value observed through hot springs at the surface we suggest that the three high vp vs ratio areas are magma chambers the upwelling volatiles heat the water in the shallow layer inducing the corresponding high relative geothermal value at the surface meanwhile the magma chamber beneath rehai spring which corresponds to high he3 he4 ratio may be fed by mantle magma we suggest that the heikongshan volcano and dayingshan volcano are fed by the magma chambers and that maanshan volcano is separated from the magma we also infer that the significant geothermal activity and active seismicity beneath rehai spring are the result of interaction between the magma and hot water reservoir

precipitation over the interior east antarctic ice sheet related to midlatitude blocking high activity abstract intermittent atmospheric blocking high activity in the south tasman sea is shown to play a key role in delivering substantial snowfall as far south as at least 75 s on the central east antarctic ice sheet plateau typically cyclones fail to penetrate this far 1000 km inland and accumulation was thought to be dominated by clear sky precipitation in east antarctica the meridional cloud bands delivering the moisture originate from as far north as 35 40 s and appear to preferentially pass over the east antarctic coast in a corridor from 120 to 160 e comparison of surface observations model and satellite data suggests that a few such episodes contribute a significant proportion of the low mean annual accumulation of the central east antarctic ice sheet e g an estimated 44 at dome c over 18 days in december 2001 january 2002 blocking high related incursions also cause abrupt increases in the surface wind speed snow redistribution and air temperature this has implications for th

a cyclone phase space derived from thermal wind and thermal asymmetry abstract an objectively defined three dimensional cyclone phase space is proposed and explored cyclone phase is described using the parameters of storm motion relative thickness asymmetry symmetric nonfrontal versus asymmetric frontal and vertical derivative of horizontal height gradient cold versus warm core structure via the thermal wind relationship a cyclone s life cycle can be analyzed within this phase space providing substantial insight into the cyclone structural evolution an objective classification of cyclone phase is possible unifying the basic structural description of tropical extratropical and hybrid cyclones into a continuum stereotypical symmetric warm core tropical cyclone and asymmetric cold core extratropical cyclone life cycles are illustrated using 1 navy operational global atmospheric prediction system nogaps operational analyses and 2 5 ncep ncar reanalyses the transitions between cyclone phases are clearly illustrated within the phase space including extratro

the erua earthquake cluster and seismic anisotropy in the ruapehu region new zealand 1 we use seismicity generated from the erua earthquake cluster a consistently active area of seismicity about 20 km to the west of mount ruapehu over the last 12 years to study seismic anisotropy in the ruapehu region in particular we search for changes associated with two minor phreatic eruptions on the 4th of october 2006 and the 25th of september 2007 the seismicity rate magnitude of completeness focal mechanisms and b value of the cluster are also examined to investigate whether the characteristics of the seismicity changed over the duration of the study the hypocenters were relocated which revealed a westward dip in the shallow seismicity shear wave splitting results revealed a decrease in delay time in the 2006 2007 period and a significant variation in the fast shear wave polarization in the same time period the b value also increased significantly from 1 0 0 2 in 2004 to a peak of 1 8 0 2 in 2007 but no other parameters were found to vary significantly over this time period we attribute these changes to an increase in pore fluid pressure in the erua region due to fluid movement and suggest that this fluid movement may be associated with the eruptions in 2006 and 2007

the waiouru new zealand earthquake swarm persistent mid crustal activity near an active volcano 1 a persistent swarm of earthquakes occurs 25 km southeast of the summit of mt ruapehu an active volcano in the central north island of new zealand 319 swarm events recorded during a six month seismograph deployment in 2001 have been relocated using the double difference technique the relocated events define a vertical sheet in the mid crust with most activity 11 21 km deep both focal mechanisms and a stress inversion indicate strike slip motion for the swarm events in contrast to the normal faulting seen on a parallel active fault mapped at the surface various lines of evidence suggest the swarm is of tectonic origin and due to fluid movement it currently reflects the regional stress field rather than local stresses related to volcanic activity at mt ruapehu nevertheless changes in the activity of the swarm appear to correlate with eruptions and the swarm appears to have potential as a stress meter for monitoring nearby volcanic activity

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation

the influence of persistent anomalies on southern hemisphere storm tracks abstract empirical orthogonal function eof analysis has been applied to nine years of daily 500 hpa data for the southern hemisphere from which synoptic scale variations had been removed by application of a 10 day low pass fitter this resulted in 11 principal modes accounting for 66 of the variance which were then regionalized through application of a varimax rotation these rotated patterns included a high latitude mode covering the antarctic eight others defining regional perturbation in the principal storm tracks at high latitudes and two residual patterns not used for this investigation persistent anomalies were defined as spells where the time coefficient of the rotated eof was greater than 1 standard deviation or less than 1 standard deviation for a period of 10 days or more and 220 of these spells were obtained for the nine eofs used in the analysis storm tracks were identified in two ways 1 from high pass filtering of the daily 5oo hpa geopotential height anomalies to include periods

internal structure and volcanic hazard potential of mt tongariro new zealand from 3d gravity and magnetic models abstract a new 3d geophysical model of the mt tongariro volcanic massif tgvm new zealand provides a high resolution view of the volcano s internal structure and hydrothermal system from which we derive implications for volcanic hazards geologically constrained 3d inversions of potential field data provides a greater level of insight into the volcanic structure than is possible from unconstrained models a complex region of gravity highs and lows 6 mgal is set within a broader 20 mgal gravity low a magnetic high 1300 nt is associated with mt ngauruhoe while a substantial thick demagnetised area occurs to the north coincident with a gravity low and interpreted as representing the hydrothermal system the hydrothermal system is constrained to the west by major faults interpreted as an impermeable barrier to fluid migration and extends to basement depth these faults are considered low probability areas for future eruption sites as there is little to indicate they have acted as magmatic pathways where the hydrothermal system coincides with steep topographic slopes an increased likelihood of landslides is present and the newly delineated hydrothermal system maps the area most likely to have phreatic eruptions such eruptions while small on a global scale are important hazards at the tgvm as it is a popular hiking area with hundreds of visitors per day in close proximity to eruption sites the model shows that the volume of volcanic material erupted over the lifespan of the tgvm is five to six times greater than previous estimates suggesting a higher rate of magma supply in line with global rates of andesite production we suggest that our model of physical property distribution can be used to provide constraints for other models of dynamic geophysical processes occurring at the tgvm

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

mesoscale cyclogenesis over the southwestern ross sea linked to strong katabatic winds abstract two years of automatic weather station aws observations and satellite images have been used to study mesoscale cyclogenesis along the transantarctic mountains twice daily regional sea level pressure analyses revealed the frequent formation of mesoscale cyclones adjacent to two regions where the discharge of cold boundary layer air from east antarctica is concentrated near terra nova bay franklin island and byrd glacier between one and two new vortices on average formed each week in the former location with weak frequency maxima in december march and august september there was a large difference between the cyclogenetic activity in the two years the aws array expanded in 1985 and resolved another cyclogenetic area near byrd glacier this feature was half as active as the franklin island area and exhibited many of the same characteristics about half of the byrd glacier cyclones developed simultaneously with vortices near franklin island these developments are the result of a dry baroclinic

eruption source processes derived from seismic and acoustic observations of the 25 september 2007 ruapehu eruption north island new zealand abstract mt ruapehu erupted on 25 september 2007 at 20 26 pm nzdt 8 26 ut generating a steam column to about 15 000 ft 4600 m a directed ballistic and surge deposit of coarse blocks and ash to the north of the crater lake and initiated lahars in the whangaehu catchment and whakapapa ski field the eruption was recorded on three broadband seismometers and two acoustic pressure sensors which indicated broadly coherent waveform characteristics across the network the eruption had a duration of less than 1 min as indicated by strong seismic and acoustic pulses the acoustic wave coincided with observed very long period vlp seismic signal 2 25 s period the acoustic pulse had a positive pressure indicating an explosion and travelled at a velocity of 320 m s the syn eruptive vlp was composed of surface waves having strong radial and transverse components the immediate aftermath of the eruption produced about 4 min of continued high amplitude spasmodic tremor signals that probably resulted from post explosion phase associated vent backfill stabilisation the eruption was preceded by minor volcano tectonic earthquakes and tremor bursts which began 10 min before the main eruption two smaller vlp signals were associated with these tremor pulses at 20 17 and 20 25 these pre eruptive vlp signals had particle motions indicating body and surface waves from a sub surface source region modelling of the vlp sources using a 3d finite difference method shows that that the pre eruption vlp signals are consistent with a volume fluctuation at 3 7 km depth from a radially symmetric source the main eruption vlp was consistent with a south and downward directed single force in response to a northward directed jet and northward directed ballistic deposits

atlantic sea surface temperatures and tropical cyclone formation abstract it has long been accepted that interannual fluctuations in sea surface temperature sst in the atlantic are associated with fluctuations in seasonal atlantic basin tropical cyclone frequency to isolate the physical mechanism responsible for this relationship a singular value decomposition svd is used to establish the dominant covarying modes of tropospheric wind shear and sst as well as horizontal sst gradients the dominant svd mode of covarying vertical shear and sst gradients which comprises equatorially confined near zonal vertical wind shear fluctuations across the atlantic basin is highly correlated with both equatorial eastern pacific sst anomalies associated with el nino and west african sahel rainfall while this mode is strongly related to tropical storm hurricanes and major hurricane frequency in the atlantic it is not associated with any appreciable atlantic sst signal by contrast the second svd mode of covarying vertical shear and horizontal sst gradient variability wh

changes in shear wave splitting before the 2010 eyjafjallajokull eruption in iceland summary we use shear wave splitting sws above microearthquakes to monitor stress variations before the 2010 march and april flank and summit eruptions of eyjafjallaj okull volcano in iceland sws time delays before eyjafjallaj okull show characteristic variations similar to those seen before earthquakes the time delays display a nearly linear increase before the eruption an abrupt change of slope and a rapid nearly linear decrease until the flank eruption begins similar variations before earthquakes are interpreted as stress accumulation increases and stress relaxation decreases as microcracks coalesce onto the eventual fault plane the changes in sws before eyjafjallaj okull are interpreted as a similar stress accumulation increase as magma penetrates the crust and stress relaxation decrease as microcracks coalesce onto the magma conduit prior to magma release we suggest that the remarkable similarity between stress changes before eruption and earthquake is strong evidence for the new geophysics of a critically microcracked crust