can mesoscale models reproduce meandering motions the influence of meandering flow on dispersion of pollutants is frequently underrepresented in dispersion models in terms of measurements meandering is primarily associated with time scales between the turbulence and the applied averaging time which is usually 1 h the related spatial scales thus range roughly from 10 2 to 10 4 m referred to here as submesoscales as the state of the art mesoscale models should be capable of reproducing flow features on scales larger than the turbulence and as the meandering generating mechanisms are not fully understood yet it is useful to examine if the mesoscale models can reproduce meandering for that purpose the wrf chem model at 1 3 km horizontal resolution is used to simulate a weak wind night during the cases99 experiment the measurements are used for detailed model verification the model with its typical set up fails to reproduce the variability at submesoscales and the locus of the under representation is traced to too strong horizontal diffusion reducing or removing the model diffusion allows the appearance of the submeso variability whose spectral properties and the resulting plume behaviour agree well with the measurements the linear correlation between the simulations with reproduced variability and the measurements is low as is the case between two simulations with only slightly different set up the conclusion is that mesoscale models are able to reproduce the strength of variability and the effects of meandering but only with reduced or removed horizontal diffusion the question arises whether it is possible to obtain a linear correlation i e to correctly reproduce individual modes at these scales at all copyright c 2010 royal meteorological society

seismicity and attenuation in the central vanuatu new hebrides islands a new interpretation of the effect of subduction of the d entrecasteaux fracture zone the spatial distribution of the intermediate depth earthquakes in the central vanuatu new hebrides islands includes a remark able gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km no well located earth quakes determined from teleseismic d ata during a 16 year period nor any microearthquakes located by a local seismograph network during a 5 year period have occurred in the gap a zone of attenuation of high frequency shear waves over laps the seismicity gap no contortion of the benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent however the gap is close to the extrapolated location of the subducted part of the d entrecasteaux fracture zone dfz the dfz is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island a rc a pogitive thermal anomaly associated with the dfz may cause the gap of intermediate depth seismic ity and produce anomalous absorption of high frequency shear waves alternatively a zone of strong scattering associated with the subducted part of the dfz may also account for the anoma lous attenuation

anomalously deep earthquakes beneath the new hebrides trench abstract the central new hebrides island arc is a morphologically complex area where the island blocks of santo and malekula replace the trench and override the d9entrecasteaux fracture zone dfz a major aseismic ridge on the subducting plate earthquakes recorded by a local network have been located beneath the trench southwest of malekula at depths of 60 to 90 km because these locations are unusual and because tests with hypothetical data showed that two minima one for shallow depths and one for deep depths exist on the travel time surface for events in this area we investigated these hypocenters in more detail location quality was judged primarily by the fit to p and s wave arrival times calculated in a variety of velocity models we found that while the standard deviations of the travel time residuals from some of the events were nearly the same for shallow and deep locations they were significantly lower for several events at the deeper minimum moreover the marginal a posteriori density functions for depth indicate that the deeper depths are significantly more probable activity in this area appears to be sporadic most of the events were aftershocks of a large earthquake m w 7 1 in 1981 and a cluster of events was reported to have occurred in the same area over a period of one month in 1986 the existence of deep earthquakes in this area may be due to interactions of the dfz with santo and malekula because their locations follow a trend in the seismicity that parallels the western coast of the island blocks they also occur directly north of a sharp bend in the trench and near a major seismic discontinuity and may be a manifestation of recent en echelon subduction if these earthquakes are not occurring along the boundary of a new subduction zone the inferred existence of brittle failure at depths usually associated with a ductile rheology brings into question our understanding of the mechanics of the oceanic lithosphere

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 2 structure from multichannel seismic data the d entrecasteaux zone dez collides with the central new hebrides island arc and consists of two subparallel ridges that strike east west stand 1 2 km above the surrounding oceanic plate and subduct obliquely 15 northward beneath the arc rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin crystalline volcanic rocks are common along the lower flank of the ridge but sedimentary probably volcaniclastic rock caps the ridge seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge these deposits appear to form discrete bodies suggesting that mass wasting occurred episodically large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope apparently slope rocks have low strength so that mass wasting deposits formed instead of large relief structures mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge the toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement footwall lithologies change from ocean basin to volcaniclastic ridge material and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition thrust faults deform the accretionary wedge whereas south of the transition steep reverse faults crosscut the wedge and pierce the north flank of the ridge this piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the new hebrides arc may be incorporated into the accretionary wedge

formation of forearc basins by collision between seamounts and accretionary wedges an example from the new hebrides subduction zone seabeam data reveal two deep subcircular reentrants in the lower are slope of the new hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin the malekula reentrant lies just south of the partly subducted bougainville seamount this proximity as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off japan suggests that the malekula reentrant formed by the collision of a seamount with the arc an arcuate fold thrust belt has formed across the mouth of the reentrant forming the toe of a new accretionary wedge the efate reentrant may show the next stage in basin development this reentrant lies landward of a lower slope ridge that may have begun to form as an arcuate fold thrust belt across the mouth of a reentrant this belt may have grown by continued accretion at the toe of the wedge by underplating beneath the reentrant and by trapping of sediment shed from the island arc these processes could result in a roughly circular forearc basin basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the aleutian trenches

studies of crustal converted waves using short period seismograms recorded in the vanuatu island arc abstract on three component short period digital array recordings s to p converted phases from three major crustal discontinuities beneath efate island are identified between the primary p and s arrivals the time differences between the main s arrival and these converted phases sp1 sp2 and sp3 provide constraints on the crustal structure beneath efate island a crustal model with three layers over a half space is determined which is consistent with the results of a published refraction experiment in the southern vanuatu formerly new hebrides island arc

horizontal meandering as a distinctive feature of the stable boundary layer abstractoscillations in the horizontal components of the wind velocity associated with oscillations in air temperature during low wind speed episodes are ubiquitous in the stable boundary layer and

use of two inverse techniques application to a local structure in the new hebrides island arc summary two inversion techniques stochastic inverse and linear programming are used in the determination of hypocentre and velocity model parameters simultaneously by using p wave arrivals from local earthquakes at a local seismograph array these procedures are applied to p wave data obtained from a combined land and ocean bottom seismographs network in the new hebrides island arc two hundred and fifty two residuals data are inverted simultaneously with 46 earthquake parameters the displacements of each initial hypocentre location are practically the same with the two methods and are small 2 4km the main result for the velocity model parameters is the existence at a low velocity zone between depths of 10 and 19km and may be deeper an estimate of the degree of confidence of the solution is determined and can confirm that the extremal values of the 7 3 km s 1 are 7 2 and 7 5 km s 1 these results may be related to an intrusion of ultrabasic magma from the mantle beneath malekula and efate islands

leveling arrays as multicomponent tiltmeters slow deformation in the new hebrides island arc two benchmark arrays located on the islands of santo and efate in the new hebrides island arc have been releveled 2 to 4 times per year during the past five years 1975 1980 each array is thereby used as a multicomponent tiltmeter the arrays are located within 35 50 km of the interplate thrust zone of this convergent plate boundary each array has an aperture of about 1 km and in addition contains small subarrays with apertures of about 70 m this configuration leads to a resolving power for tilt of about one microradian the most important result of this study is that at the devils point array on efate island there has been a consistent tilting down of about five microradians in a direction subparallel to the strike of the arc this tilting is clearly resolved by both the large array and by the small aperture sub arrays the observations are accounted for by a combination of planar tilting and random error consistent with the observed misclosures although tilting is clearly established at the devils point array it is not possible at this stage to establish a unique cause for the observed tilt hypotheses consistent with the tectonic setting include interseismic strain accumulation associated with the earthquake cycle of loading and interplate boundary rupture deformations near a transition between a creeping and a locked segment of the interplate boundary and near surface deformations associated with magmatic activity and or block movements also reflected in the pattern of faulting of the uplifted holocene coral terraces at the second array at ratard santo island no tilt trend clearly emerges from the noise level however two marginally significant tile excursions precede in each case large earthquakes located several hundred km north of the array at the ratard array the ground noise small wavelength elevation disturbances and benchmark instability contributes to residuals almost as much as the measurement error thereby degrading the threshold level for detection of tilt signals the difference between the two arrays is not clearly established but seems to reflect an inherently larger level of short wavelength ground noise associated with geological properties of the ratard site

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

analysis of gravity waves generated at the top of a drainage flow abstract drainage or katabatic flows are common mesoscale circulations established as a result of differential radiative cooling of near surface air masses in sloping terrain the initial irruption of these flows with sudden shifts in wind speed and direction may result in vertical displacements of air parcels from their equilibrium position which prove to be a common source of internal gravity waves this paper illustrates this mechanism and describes the main features of the oscillations following the study of observational data gathered throughout one night during the stable atmospheric boundary layer experiment in spain 2006 sables2006 field campaign pressure differences measured by microbarometers set at different levels of a tower help to interpret the evolution of other atmospheric variables provide a detailed picture of the irruption of a drainage current and reveal the formation of gravity waves at its top the main parameters of the waves are derived from wavelet cross correlation of p

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

wavelet analysis for geophysical applications wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals since then significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields in geophysics the power of wavelets for analysis of nonstationary processes that contain multiscale features detection of singularities analysis of transient phenom ena fractal and multifractal processes and signal com pression is now being exploited for the study of several processes including space time precipitation remotely sensed hydrologic fluxes atmospheric turbulence can opy cover land surface topography seafloor bathymetry and ocean wind waves it is anticipated that in the near future significant further advances in understanding and modeling geophysical processes will result from the use of wavelet analysis in this paper we review the basic properties of wavelets that make them such an attractive and powerful tool for geophysical applications we dis cuss continuous discrete orthogonal wavelets and wavelet packets and present applications to geophysical processes

velocity structure in northern chile evidence of subducted oceanic crust in the nazca plate summary 2 d p wave velocity models were determined for the subduction zones near iquique and antofagasta in northern chile simultaneously inverting p and s wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations a 2 d parametrization was used because of the paucity of data but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3 d inversions the crust and upper mantle are parameterized by constant velocity regions of irregular shape with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve p wave velocities beneath the antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting nazca plate this crust has a p wave velocity of 7 3 f 0 1 km s and is observed down to a depth of 60 f 10 km this depth also corresponds to the maximum depth of seismogenic coupling in the chilean subduction zone the subducted oceanic crust overlies an oceanic upper mantle with a p wave velocity of 8 0 f 0 1 km s apparently oceanic crust is being subducted beneath iquique as well however this feature is less constrained by the data available from this region

p wave velocity of the uppermost mantle and crustal thickness in the central vanuatu islands new hebrides island arc the p wave velocity of the uppermost mantle and crustal thickness were determined for the central vanuatu formerly mew hebrides islands using data from a local seismograph network two different methods and data sets were lased in this investigation the first method used travel time plots for earthquakes located well outside of the seismograph network the second method used timeterm analysis on sets of earthquakes both methods yielded a mantle velocity of 7 6 90 1 km sec and a crustal thickness of 27 92 km for the region located just trenchward of the volcanic arc published refraction studies have demonstrated that the mantle velocity near the trench has a more typical value of 8 0 kmlsec this contrast in mantle velocities between the trench and the volcanic arc has also been reported for the japanese and kurile kamckatka island arcs and therefore may be a general feature of subduction zones

patterns of seismicity associated with asperities in the central new hebrides island arc in the central part of the new hebrides island arc efate malekula region 16 s 18 6 s a sequence of moderate sized earthquakes their aftershock sequences and other clusters of small earthquakes together form an intricate but coherent time space pattern that probably reveals a major asperity complex along the interplate boundary of the subduction zone this pattern is determined by study of data from a local network the sequences including one event with magnitude mw 7 1 eight events with magnitudes mw between 5 8 and 6 3 and nearly 13 000 smaller events occurred during the six year period 1978 1984 the seismicity is very unevenly distributed in space a sharply defined east west line near 17 2 s separates the very active efate region to the south where nearly 10 500 earthquakes occurred from the less active malekula region to the north where less than 2 500 earthquakes occurred during the six year period in the efate region several spatial patterns are highlighted first the seismic regimes of the updip and the downdip part of the interplate boundary are different the updip part is characterized by a low background activity and main shocks with large aftershock zones while the downdip part is characterized by a very high level of background activity and main shocks with much smaller aftershock zones or no aftershocks this difference in seismic regimes suggests that asperities are located in the updip part of the interplate boundary while the downdip part may slip predominantly by creep a creep episode may have been responsible for the tilt signal observed by periodic relevelings of a l km aperture network of benchmarks on efate island second specific locations along the interplate boubdary are identified either as sites of very intense and repeatedly activated concentrations of hypocenters or as sharp boundaries limiting the spatial development of aftershock zones two areas of concentrated activity particularly stand out one located in the updip part of the interplate boundary and the other in the downdip part most of the clustered activity that occurred during the six years of observation is concentrated in these two areas four boundaries limiting the spatial development of aftershock zones are located in the updip part of the interplate boundary one of these boundaries coincides with one of the areas of intense activity epiceniers of moderately large events that occurred in the region since 1960 are also concentrated along the boundaries the zones of concentrated activity and the boundaries limiting the development of aftershock zones can be interpreted either as locations of penties or as edges of asperities an alternative interpretation is that a giant asperity is located in the ylekula region with the different spatial patterns observed in the efate region being interpreted as the southern edge of that asperity a transition zone between a locked part to the north and normal i subduction to the south study of the temporal patterns of the seismicity shows that the main shocks in tbe downdip part of the interplate boundary are not preceded by obvious long term months to year nor short term days to week precursory activity in the updip part of the interplate boundary short term fipreshocks preceded each main shock but only one of these foreshock sequences is outstanding although no clear cyclic pattern related to the main shocks is evident clustered activity that occurred in the two zones of concentrated activity and two other zones located in the back arc region are correlated ih time with the occurrence of the major sequences located in the updip part of the interplate boundary activation of these four zones always follows the sequences in the updip part of the interplate boundary qith varying time intervals days to months these four zones are also sometimes activated before sequences in the updip part of the interplate boundary in particular one of the zones of concentrated a ctivity shows possibly repeated long term few months precursory activity this phenomenon could b e an indicator of increasing probability of occurrence of main events in the updip part of the interplate b oundary i tuk ukiis lmcui l ilu tl6aj2 qfiztqf

a shallow double seismic zone beneath the central new hebrides vanuatu evidence for fragmentation and accretion of the descending plate a shallow double seismic zone sdsz has been found in the descending australian plate beneath the central part of the new hebrides island arc directly above a large gap in intermediate depth seismicity and between two seismic boundaries ambient seismicity occurs mostly in the upper part of the sdsz while earthquakes in the lower part occur in clusters swarms or aftershocks of large earthquakes the distance between the upper and lower levels of the sdsz is 50 70 km and they are joined at 80 km depth by a near horizontal band of seismicity thrust faulting mechanisms predominate for earthquakes in the upper level of the sdsz those in the lower level however appear to be normal faulting despite their being aftershocks of large thrust events we suggest that with the absence of a pull from the detached lithosphere the upper part of the australian plate in the region of the sdsz is resistant to subduction and thus the downward displacements caused by large earthquakes in the adjoining regions result in a localized rebound the location of the aftershocks within the plate suggests that a new plate boundary is forming which will eventually replace that outlined by the residual seismicity in the upper level thus the leading edge is decoupling and the boundary will eventually shift back to the lower level of the sdsz

the december 1994 seismic swarm near aoba ambae volcano vanuatu and its relationship with the volcanic processes abstract a seismic swarm which occurred in december 1994 near the aoba volcano vanuatu is investigated in detail by analyzing local and regional broad band seismic records the seismic area is situated in a poorly instrumented region although major seismic and volcanic events are possible the data collected by four permanent short period stations the nearest one being installed in the espiritu santo island located 80 km from the volcano allowed us to locate epicenters with a 2 3 km precision and to document the present local seismicity the quality of available regional broadband data allows us to determine reliable focal parameters though the magnitude of the strongest event is 4 8 data have been selected from the permanent geoscope and iris stations located in the south western pacific region and from two temporary networks one installed in new caledonia and vanuatu by i r d and the e o s t and another one installed in fiji and tonga islands by the washington university st louis usa results obtained either with local epicentral distance d km body waves or regional 500 km surface waves lead us to conclude that the swarm is associated with the volcanic edifice but is not of volcanic type according to the definitions given by minakami phys volcanol 1974 1 and schick bull volcanol 44 1981 491 the observed mechanisms evidence an important local influence of the volcano itself on the stress field and the observed near horizontal p axis argues against a strong increase of phreatic activity in addition seismic and volcanic observations are analyzed in the framework of the interpretation of earthquake swarms occurring in the vicinity of active volcanoes

seismicity and shallow slab geometry in the central vanuatu subduction zone the vanuatu arc in the southwest pacific ocean is one of the world s most seismically active regions with almost 39 magnitude 7 earthquakes in the past 43 years convergence rates are around 90 120 mm yr along most of the arc but drop to 25 43 mm yr in the central section probably due to the subduction of the d entrecasteaux ridge we characterize the slab geometry and tectonic state in this central section by analyzing data from a 10 month deployment of 30 seismometers over this section we located more than 30 000 events all less than magnitude 5 5 constructed an improved 1 d velocity model calculated focal mechanisms and cluster geometries and determined the 3 d geometry of the interplate seismogenic zone the seismogenic zone has a shallow bulge in front of the d entrecasteaux ridge which could be explained by the ridge s buoyancy contributing to the uplift of the fore arc islands the seismogenic zone extends to 45 km depth significantly below the 26 27 km depth of the fore arc moho indicating that the upper mantle wedge is not significantly serpentinized which is consistent with the relatively high thermal parameter of the subducting plate the maximum width of the seismogenic zone is 80 km indicating an upper earthquake magnitude limit of mw 7 85 0 4 assuming standard rupture zone aspect ratios the data also reveal a double seismic zone 20 to 30 km below the seismogenic zone which is presumably caused by flexure of the downgoing plate

three dimensional numerical modeling of detachment of subducted lithosphere recent seismological studies suggest that slab detachment has occurred in the mediterranean and the new hebrides subduction zones subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km presumably caused by the lateral migration of the tear along the strike of the slab to investigate the physical mechanism of the slab detachment and in particular its migration we constructed a viscoelastic three dimensional finite element model and introduced a small initial tear from one side of the slab we investigated spatio temporal variations in the state of stress within the slab as a function of tear length rheology and a variety of force distribution our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab which is probably sufficient to cause further lateral migration of the tear the stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension from our modeling we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate forming in plate tensional stress at intermediate depths such a condition is indeed observed in the dinarides hellenic and the new hebrides subduction zone

probabilistic seismic hazard model for vanuatu abstract this study develops a preliminary probabilistic seismic hazard model for vanuatu the area of investigation formerly referred to as the new hebrides lies in the center of a chain of partly vulcanologically active islands that mark the present day boundary between the australia india plate and the microplate of the north fiji basin the seismicity of the vanuatu arc is dominated by an east dipping subduction zone which shows striking structural anomalies in the central part between 14 and 18 s our historical catalog contains 7519 events within the vanuatu region for the period from 1964 to 2003 drawn from the global teleseismic catalogs by the united states geological survey national earthquake information center usgs neic see data and resources section and engdahl et al 1998 as a measure of seismic hazard we use horizontal peak ground acceleration pga and horizontal spectral ground acceleration sga at a period of 1 sec the hazard estimates are based on a logic tree approach to account for the epistemic uncertainties associated with our analysis our results suggest that the entire island arc experiences a high and uniform seismic hazard typical values for pgas range from 0 65 g to 0 77 g with a 10 probability of exceedence in 50 yr for port vila the capital and largest city in vanuatu we additionally present a pga hazard curve and a uniform hazard spectrum over the period range 0 1 2 sec

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

unfolding the subducting plate in the central new hebrides island arc geometrical argument for detachment of part of the downgoing slab in the central part of the new hebrides island arc a sizable gap exists in the seismic activity seismic observations in the zone of the gap have been previously interpreted as either related to a detachment of part of the downgoing slab or associated with a continuous slab we give a geometrical argument which added the seismic observations from these previous studies adds one step in favor of the detachment hypothesis after unfolding the downgoing plate and restoring it to its initial state using a 3d balance technique we show that the gap can be filled with three pieces two of these pieces are seismic zones located beneath the seismic gap and the third one is a deep 550 650 km seismic zone located about 200 km north east of the arc these three pieces could be detached parts of the subducting plate

seismicity in subduction zones from local and regional network observations the data provided by local and regional seismograph networks are essential for the solution of many problems of subduction zone seismology the capabilities of such networks are limited by the instrumentation currently in common use and the unfavorable source station geometry often imposed by the regional geography nevertheless important contributions have come from the data gathered in many of the earth s subduction zones the accuracy of hypocenter locations based on regional data is affected by the complex velocity structures characteristic of subduction zones but the problems are now well understood examples of numerous studies of the spatial configurations of the seismicity in subduction zones and consequent interpretations of seismogenesis and subduction processes are reviewed studies of the distributions of earthquakes in time and with magnitude for events down to the microearthquake level have the potential for clarifying the earthquake generating processes and possibly a basis for earthquake prediction other uses of local and regional network data have been for investigations of coda s and the identification of asperities in subduction zones

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 1 sea beam morphology and shallow structure sea beam bathymetric data closely spaced single channel seismic reflection sections and geopotential field data were collected aboard the r v j charcot over the collision zone between the north d entrecasteaux ridge ndr and the new hebrides island arc the ndr trends east at a small angle 14 to the plate convergence direction so that the ridge creeps northward along the trench at about 2 5 cm yr requiring a continual structural adjustment of the accretionary complex in this report we study the shallow structure tectonic erosion and consequent mass wasting of the accretionary complex all of which were produced by the collision of the ndr in a complex deformational environment the accretionary complex in this collision zone can be divided into northern central and southern parts the northern part of the collision zone which lies north of the leading flank of the ridge shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and n60 w trending strike slip faults a middle accretionary complex that bulges seaward and appears to be rotated 45 e from the regional arc trend and an upper accretionary complex that is shaped by slumps and a canyon network the central part of the collision zone which directly overlies the crest of the ridge forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps rocks forming this protrusion have been uplifted possibly by as much as 1500 2500 m and tilted to the north causing northward block sliding along an extensional detachment surface the southern part of the collision zone which lies in the wake of the ridge is deformed by large slumps and normal faults that trend parallel to the ridge axis rocks forming this southern part collapse causing widespread mass wasting these geophysical data indicate that normal trench convergence has apparently produced only a few trench parallel structures confined to the toe of the accretionary complex whereas the along trench motion of the ndr has resulted in well developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern the area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the ndr and the time required to heal the disturbance may be about 0 8 m y

a wavelet analysis of low wind speed submeso motions in a nocturnal boundary layer in low wind regimes wind speed less than 1 5 m s 1 the nocturnal boundary layer is still inadequately understood in such conditions turbulence is weak and often intermittent whereas dynamics and total fluxes are often driven by submeso motions as a consequence the momentum mass and energy transfers are poorly represented by dispersion models in low wind conditions an important fraction of submeso motions is represented by meandering modes which can be detected through the eulerian autocorrelation functions of horizontal wind components and temperature such an approach has been proven to be reliable however a deeper insight could be useful for understanding the phenomenon especially when complex or multiple mesoscale motions simultaneously develop in this work eulerian autocorrelation functions and the morlet continuous wavelet transform were used to investigate an 8 h nocturnal period characterized by the coexistence of horizontal meandering and vertical oscillations rarely observed elsewhere this nice episode represents a good case study to compare the two used methodologies and to investigate the phenomenology of the two submeso phenomena simultaneously occurring in a low wind speed regime the two methodologies identified the same time scale for the detected meandering structures over the whole period moreover the wavelet analysis i was able to discriminate horizontal meandering and gravity waves which simultaneously developed in the second part of the analysed period ii showed that in the investigated case the horizontal meandering was not triggered by gravity waves which appeared later in the night iii highlighted how both gravity waves and meandering can contribute to an increase of the vertical turbulent energy and fluxes confirming the crucial role of submeso structures in the turbulence production during low wind regimes in stable conditions

regional circulation and its impact on upper ocean variability south of tasmania ocean colour images of the subantarctic zone saz south of tasmania show a higher biomass in the east than in the west to identify the main features of the regional circulation and the physical drivers of the east west contrast we used world ocean circulation experiment hydrographic sections sr3 and p11s west and east of tasmania respectively argo float profiles and trajectories and high resolution climatology the east australian current and the tasman outflow are the mechanisms driving the variability in the eastern subantarctic zone this region has a weak flow and an enhanced input of subtropical waters through eddies interleaving and a subsurface salinity maximum intruding from the north to south in the western subantarctic zone the regional circulation is dominated by a northwestward circulation and a deep reaching anticyclonic recirculation the south tasman rise acts as a barrier inhibiting exchange between waters southeast and southwest of tasmania the regional circulation and mixing processes result in the strong contrast in water properties between the eastern and western subantarctic zone cooler and fresher in the west and warmer and saltier in the east the subantarctic mode water samw pycnostad is more prominent in the west with a local variety of samw associated with the anticyclonic recirculation west of the south tasman rise antarctic intermediate water aaiw formed in the southeastern pacific and southwestern atlantic oceans meet in the saz south of tasmania cool fresh and well ventilated aaiw is found in the west and southeast saz relatively warm salty and low oxygen aaiw enters the saz from the tasman sea after having traversed the pacific ocean subtropical gyre enhanced input of subtropical water high in micronutrients such as iron in the east likely supports the higher surface biomass observed there the physical processes responsible for maintaining the east west contrast south of tasmania e g regional circulation eddies and intrusions are likely to drive variability in physical and biogeochemical properties of samw aaiw and the subantarctic zone elsewhere in the southern ocean

spatial distribution and mechanisms of earthquakes in the southern new hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations in 1977 a wide aperture seismic network of land and ocean bottom stations obs was operated for 6 weeks in the southern new hebrides island arc data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size new hebrides earthquakes of the past 17 years to study the structure and deformational processes in the new hebrides subduction zone surprisingly small differences less than about 10 km were found between the locations of shallow and intermediate depth earthquakes as located by the temporary local network and by the international seismological centre using worldwide stations analysis of the p wave travel time data from the obs station closest to the trench shows limited evidence for a high velocity zone about 5 higher than the surrounding mantle associated with the descending plate the thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks but it is not very well defined by the spatial distribution of small size events located in this study shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of erromango and tanna is divided into a series of blocks that are differentially uplifted along mainly nw se striking faults during the obs experiment several shallow events were well located beneath the coriolis trough a riftlike feature located to the east of the volcanic arc well determined depths of these events are between 11 and 22 km the well located intermediate depth events define a 20 km thick benioff zone that has a dip of about 70 the spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short term sample of small events determined by the local network and the long term sample of locations based on worldwide data several focal mechanisms of moderate to large size intermediate depth events show components of lateral extension along the strike of the arc and some show components of lateral compression both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern new hebrides arc where the trench begins to curve around to the east

detachment of part of the downgoing slab and uplift of the new hebrides vanuatu islands several seisniological observations suggest that there is a gap within the downgoing slab of australian lithosphere plunging beneath the new hebrides and ages of elevated coral tenaces on the new hebrides islands suggest that the islands arc rising rapidly we suggest that the creation of the gap in the slab within the last 1 m y occurred by part of the slab detaching from the rest and sinking rapidly into the underlying asthenosphere this lowered the downward force on the slab at shallower depths and therefore on the overriding plate and the continued decrease of this force as the deeper slab sinks freely has allowed the islands above it to rise although this suggestion cannot account for all aspects of the uplift of the islands it provides a simple mechanism for explaining the fairly young uplift and its distribution along the new hebrides arc

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

a climatological study of internal gravity waves in the atmospheric boundary layer overlying the brunt ice shelf antarctica internal gravity waves are frequently observed in stably stratified regions of the atmospheric boundary layer in order to determine the statistical influence of such waves on the dynamics of the boundary layer it is necessary to compile information concerning properties of the waves such as frequency of occurrence propagation and spectral characteristics gravity wave climatologies have been compiled from relatively few locations in this paper a climatological study of gravity waves in the period range 1 20 min propagating in the stably stratified atmospheric boundary layer overlying an antarctic ice shelf is presented an extensive set of boundary layer measurements were compiled throughout 1991 surface pressure fluctuations were recorded from a spatial array of six sensitive microbarographs wind and temperature records from an instrumented mast were also available a beam steering technique has been used to determine wave parameters from the surface pressure data the microbarographs detected the presence of internal gravity waves throughout the observational campaign rootmean square pressure values were typically in the region 16 40 mb but a significant number of isolated events with amplitudes of up to 180 mb were also found wave properties have been studied in conjunction with the mean wind and temperature profiles in the boundary layer it was found that most of the wave activity did not originate locally but from shear layers aloft or more commonly from the katabatic flow regime where the ice shelf joins the antarctic continent

mapping of low p wave velocity structures in the subducting plate of the central new hebrides southwest pacific arrival times of compressional p and shear s waves generated by earthquakes located in the new hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large scale one and three dimensional elastic wave velocity structures of the subduction zone between 15 and 20 s and from the surface to about 250 km depth the results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures the results for one dimensional structure indicate a gradual increase of velocity with depth until a 9 reversal appears between 60 and 100 km depth the three dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending benioff zone taken with other observations such as the attenuation of high frequency shear waves travelling across this gap and the locations of active volcanoes at the surface we infer that the low velocity region represents a thermal anomaly of about 750 c which alters the physical properties of the descending plate in this region at shallower depths there is evidence of low velocity structures included in the descending plate one beneath north malekula island and another between malekula and efate islands corresponding to the large embayment of the leading edge of the upper plate the locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small scale features such as seamounts and accretionary wedges