comment on a paper by sibuet et al 2016 entitled thinned continental crust intruded by volcanics beneath the northern bay of bengal

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

reply to the comment of talwani et al 2017 on the sibuet et al 2016 paper entitled thinned continental crust intruded by volcanics beneath the northern bay of bengal the northern bay of bengal velocity depth profiles do not follow the velocity depth curve for the north atlantic volcanic margins and only partially the kerguelen plume velocity depth curves compared with the south china sea northern margin proxy we still suggest that the bay of bengal crust is thinned continental crust intruded by post rifting volcanics as also shown by the interpretation of the numerous high quality deep multichannel seismic profiles we collected there what was supposed to be underplating might be sills intruded through the lower thinned continental crust

spectral matrix filtering applied to explosion seismology data examples from the western alps abstract a multichannel filtering method is described and its application to explosion seismology data presented the processing method uses the information contained in the eigenelements of the spectral matrix and allows enhancement of the most coherent signal by improvement of the signal to noise ratio the processing of synthetic data shows the performance of the method when the noise level of the section varies and when the reflected signal is differently time shifted the signal to noise ratio always increases allowing better determination of arrival times on real data it is shown that after processing the picking of events is easier and reflectors which were previously invisible may become visible

crustal structure beneath aso caldera southwest japan as derived from receiver function analysis abstract aso volcano experienced a huge pyroclastic eruption 90 thousand years ago and formed a large caldera 18 km 25 km in order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically we investigated seismic velocity discontinuities and velocity structure beneath aso caldera using receiver functions and a genetic algorithm inversion we confirm the existence of the moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath aso caldera from imaging of receiver functions observed only at stations outside the caldera as a result of a more detailed examination with ga inversion a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera s wave velocity of the layer is estimated to be 2 0 2 4 km s we estimate that the low velocity layer contains at most 15 melt or 30 aqueous fluid the layer exists near the conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle eastern part of the caldera fluid contained in the layer might be related with huge pyroclastic eruptions of aso volcano

structure of the northern bay of bengal offshore bangladesh evidences from new multi channel seismic data abstract new multi channel seismic data were acquired in the northern part of the bay of bengal and at the northernmost termination of the 90 e indian ridge offshore bangladesh this survey was coupled with a seismic refraction experiment indicating this offshore basin is here floored by a thinned 15 km thick continental crust injected by mesozoic volcanism this attenuated continental crust is interpreted as formed during gondwana super continent fragmentation during a syn rift period the dominant tectonic pattern is marked by ne sw trending tilted blocks filled by syn rift sediments clearly identified on seismic profiles the uppermost part of this continental crust 3 4 km thick shows a complex assemblage of dipping reflectors and west facing tilted blocks injected by volcanic build ups the lower crustal sequence 11 12 km thick does not reveal significant reflectors this syn rift fabric is attributed to the mesozoic up to the early cretaceous by correlation with published seismic data along the eastern coast of india opposite normal faults vergency on the indian and burma sides indicate an asymmetrical rifting simple shear creating a wide cot on the burma side and a short cot on the opposite indian side a geometry typical of continental crust stretching this crustal fabric is overlain disconformably by a thin reflector attributed to the late cretaceous paleocene pelagic sequence deposited during india bay of bengal drift phase before the cenozoic india asia collision marked by thick clastic sedimentation associated with the ganges delta southward progradation below this delta this mesozoic rift closes axially and is affected by the incipient late miocene shortening of the shillong plateau the ne sw fabric of this attenuated crust might be traced southward to 15 n close to magnetic chron 34 where steady state spreading of the central indian ocean occurred

thinned continental crust intruded by volcanics beneath the northern bay of bengal abstract since the early cretaceous the bay of bengal was formed during rifting between india and antarctica and then by subsequent seafloor spreading the nature of the crust underlying the bay of bengal is oceanic south of 15 n but remains unknown thinned continental crust serpentinized mantle or oceanic crust north of this limit in order to better define the nature of the crust in the northern bay of bengal three wide angle reflection seismic and refraction profiles were acquired during the multichannel seismic reflection bengal cruise nine ocean bottom seismometers were deployed alternatively on three profiles a seismic source consisting of 64 air guns with a volume of 6180 in 3 was used simultaneously with a 10 05 km long streamer to acquire both seismic reflection and refraction data tomographic and forward modelings of the three refraction profiles reveal a 20 km thick crust north of the bengal delta front beneath a minimum of 13 km thick sedimentary cover the crust thins to about 10 km immediately south of the ew trending delta front and the thickness of sediments reaches a minimum of 7 km crustal velocities and velocity gradients are consistent with a continental origin of the crust in this area at the base of the crust high seismic velocities 7 2 km s are interpreted as magmatic underplating wide angle seismic reflection and refraction data cannot resolve the nature of the upper 4 5 km of crust oceanic crust exhumed mantle or thinned continental crust but coincident seismic reflection profiles show the emplacement of a volcanic intrusion sills and some seaward dipping reflectors sdrs located close to the northern prolongation of the ninety east ridge before 70 ma maastrichtian however most of the fan shaped reflectors identified in the northern bay of bengal are synrift features we conclude that the crust in the northern bay of bengal is thinned continental crust intruded by volcanic products with the presence of a minor amount of underplating material at its base such a crustal structure probably extends from the northern bay of bengal 19 n to the shillong plateau 25 n these new findings are critical for the oil and gas exploration presently very active in the northern bay of bengal area

the tectonic origin of the bay of bengal and bangladesh we are able to decipher the tectonic evolution of the bay of bengal a puzzle which has not been satisfactorily solved in the past and we are also able to shed new light on origin of the buried 85 e ridge we do so by incorporating a number of disparate items into a unified solution these items are the marine magnetic anomalies in the western basin of the bay of bengal the rajmahal and sylhet traps and deep seismic sounding lines in india a prominent magnetic anomaly doublet and seismic seaward dipping reflectors in bangladesh and a new precise gravity map of the bay of bengal we identify seafloor spreading magnetic anomalies ranging in age from 132 ma m12n to 120 ma m0 in the western basin these anomalies are one sided the conjugate anomalies lie in the western enderby basin off east antarctica the direction of spreading was approximately nw se and the half spreading rates varied from 2 5 to 4 0 cm yr with the arrival of the kerguelen plume around m0 time seafloor spreading was reorganized and a new spreading axis opened at or close to the line joining the rajmahal and sylhet traps the prominent magnetic anomaly doublet connecting the rajmahal and sylhet traps indicates that these traps are not individual eruptions at about 118 ma but rather together define the new line of opening spreading started at this line and subsequently india changed direction from west to north the new oceanic crust thus generated underlies bangladesh and the eastern basin of the bay of bengal and is younger than 118 ma the western boundary of the new ocean floor is a transform fault which was generated by the spreading axis jump this transform fault appears as the 85 e ridge and further north on land as a negative free air gravity anomaly strip a unique feature of the northern boundary of the new oceanic crust is that due to the later deposition of enormous sediments derived from the himalayan orogeny it lies onshore bangladesh in contrast to most continent ocean boundaries in the world which lie offshore

imaging the moho and subducted oceanic crust at the isthmus of tehuantepec mexico from receiver functions using teleseismic data recorded along a transect which we call veox for veracruz oaxaca seismic line of 46 broadband stations installed across the isthmus of tehuantepec in southern mexico we obtained receiver functions and stacked them to study the moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus we observed a back azimuth dependent moho thickness across the transect particularly beneath the los tuxtlas volcanic field also we observed the cocos plate which subducts with an angle of 26 between 140 and 310 km from the trench comparison with regional seismicity indicates that it occurs below the oceanic crust

shear wave anisotropy of the upper and lower crusts estimated by stripping analysis of ps converted waves abstract the anisotropic structure of the crust is investigated by measuring the shear wave polarization anisotropy of the ps phases converted at the conrad and moho discontinuities the ps converted phases are identified on the p wave receiver functions which are constructed from teleseismic records at the f net and hi net stations in the kinki chugoku shikoku and kyushu districts in southwest japan the ps phase traveling through an anisotropic layer undergoes shear wave splitting which is described by splitting parameters fast polarization direction fpd and split time the upper and lower crustal anisotropies are estimated from the splitting parameters of the conrad and moho ps phases respectively which are determined by a waveform cross correlation method to reliably estimate the lower crustal anisotropy the moho ps phase is corrected for the shear wave splitting effect in the upper crust by a stripping method because the upper crustal anisotropy masks the anisotropic behavior of the ps phase in the lower crust the split times estimated for the upper and lower crusts are less than 0 2 s but their fpd characteristics differ from each other the fpds in the upper crust are predominantly in the e w direction the fpds in the lower crust range from nw se to ne sw in the region with the philippine sea phs slab subduction but from ne sw to se nw in the region without slab subduction thus seismic anisotropy exists in both the lower and upper crusts and slab subduction significantly influences the lower crustal anisotropy the most likely cause for the upper crustal anisotropy is the alignment of vertical cracks induced in the upper crust by the tectonic stress while the lower crustal anisotropy is attributed to the lattice preferred orientation of rock forming minerals along with ductile deformation of the lower crust

receiver functions of seismic waves in layered anisotropic media application to the estimate of seismic anisotropy abstract we investigate the effect of seismic anisotropy on p wave receiver functions calculating synthetic seismograms for p wave incidence on multilayered anisotropic structure with hexagonal symmetry the main characteristics of the receiver functions affected by the anisotropy are summarized as 1 appearance of seismic energy on radial and transverse receiver functions 2 systematic change of p to s ps converted waveforms on receiver functions as ray back azimuth increases and 3 reversal of the ps phase polarity on the radial receiver function in a range of the back azimuth another important influence is shear wave splitting of the ps converted waves and other later phases reverberated as s wave by numerical experiments using synthetic receiver functions we demonstrate that the waveform cross correlation analysis is applicable to splitting ps phases on receiver functions to estimate the seismic anisotropy of layer structure advantages to utilizing the ps phases are 1 they appear more clearly on receiver functions than on seismograms and 2 they inform us about what place along the seismic ray path is anisotropic real analysis of shear wave splitting is executed to the moho generated ps phases that are identified on receiver functions at six seismic stations in the chugoku district southwest japan the time lags between the two arrivals of the split ps phases are estimated at 0 2 0 7 sec and the polarization directions of the fast arrival components are from north south to northeast southwest this result is consistent with recent results of shear wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the chugoku district

regional variations of the shear wave polarization anisotropy in the crust and mantle wedge beneath the tohoku district abstract we investigated the regional variations of the shear wave polarization anisotropy in the upper crust lower crust and mantle wedge beneath the tohoku district in the northeast japan arc the shear wave splitting parameters fast polarization direction fpd and split time for the upper and lower crusts are estimated by shear wave splitting analysis of the ps phases which are generated by the conversion of a p wave into an s wave at the conrad and moho discontinuity interfaces the splitting parameters for the mantle wedge are determined by splitting analysis of the slab ps phase converted at the oceanic crust on the subducting pacific slab and the direct s waves from deep focus earthquakes located just below seismic stations the ps phases are identified on the p wave receiver functions constructed from the teleseismic records to accurately determine the splitting parameters for the lower crust from the moho ps phase the ps phase should be corrected for the shear wave splitting effect in the upper crust similarly to estimate the mantle wedge anisotropy the direct s wave and slab ps phase must be corrected for the splitting effect in the crust the anisotropies of the crust and mantle wedge are estimated from the corrected ps phases and direct s waves the anisotropy of the upper crust exhibits a regional variation where fpd of the split shear wave is predominant in the n s direction in the pacific coast area and in the e w direction in the rest of the tohoku district the split time is less than 0 2 s the upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress in the lower crust fpd is predominant in the e w direction with a split time similar to that in the upper crust and the anisotropy of the lower crust is due to the lattice preferred orientation of rock forming minerals in the mantle wedge fpd is predominant in the n s direction trench parallel in the fore arc side of the volcanic front but in the e w direction trench perpendicular in the back arc side the split time in the mantle wedge is similar to that of the upper crust the most likely cause for the trench perpendicular anisotropy in the back arc mantle wedge is the lattice preferred orientation of a type olivine along flows of the secondary mantle convection for trench parallel anisotropy in the fore arc mantle wedge we considered two possible causes the preferred orientation of b type olivine along the secondary mantle convection and that of dry olivine along the trench parallel flow produced by the along strike dip variation of the pacific slab however we could not determine which one is the origin

from mountain summits to roots crustal structure of the eastern alps and bohemian massif along longitude 13 3 e abstract the crustal structure of the eastern alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth of particular interest are the nature of a previously proposed moho gap south and east of the tauern window the plate tectonic affinity of the steeply dipping eastern alpine slab and the relationship of the alps to the neogene sedimentary basins and the bohemian massif to address these questions we use various seismological approaches based on converted waves from the temporary passive experiment easi eastern alpine seismic investigation a complementary experiment of the alparray project the easi is a densely spaced 540 km long seismic network along 13 3 e we operated for more than a year the uppermost crustal structures in and near the alps exhibit dipping layers and or tilted anisotropy that correlate well with surface geology observations the moho despite its variable appearance is clearly identified along most of the swath the variscan lithospheric blocks beneath the bohemian massif are imaged with sub vertical boundaries beneath the eastern alps the shape of the moho is consistent with bi vergent orogenic thickening with a steeper and deeper reaching adriatic plate plunging northwards beneath the european plate in the north at the junction of these plates at depth around the previously proposed moho gap the root of the eastern alps is a broad trough characterized by a zone of low velocity gradient that is up to 20 km thick transitioning between crust and mantle our receiver function results corroborate earlier lithosphere upper mantle seismic tomography images and highlight the adriatic affinity of the eastern alpine slab the zigzag deployment pattern of stations in the easi experiment also allows distinction of short wavelength variations perpendicular to the profile both within the shallow and the deep crust this underlines the importance of applying 3d imaging in complex geodynamic systems

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

regional variation in shear wave polarization anisotropy of the crust in southwest japan as estimated by splitting analysis of ps converted waves on receiver functions abstract regional variation in shear wave polarization anisotropy of the crust is investigated in the kyushu chugoku shikoku and kinki districts southwest japan using ps converted phases generated at the moho discontinuity the ps phases on radial and transverse receiver functions are analyzed to determine shear wave splitting parameters of fast polarization direction and time lag by waveform cross correction analysis as a result the fast polarization directions are shown to be obviously different between the chugoku district and the kyushu and shikoku districts which are separated from each other by the seto inland sea in the kyushu shikoku and south kinki districts the ps phases exhibit fast polarization directions ranging from e w to nw se the polarization directions are not only consistent with those of s waves from shallow crustal earthquakes but also parallel to the direction of maximum horizontal compression acting on southwest japan and the strike directions of metamorphic belts accretionary zones and rift zones thus the polarization anisotropy is understood to reflect mainly the upper crustal anisotropy that is attributable to stress induced vertical cracks and geological lineament structure on the other hand in the chugoku and north kinki districts fast polarization directions are in a range from n s to ne sw they are not consistent with the tectonic stress field and the s wave splitting observed for shallow crustal earthquakes the disagreement between the fast polarization directions estimated from the s waves and the ps converted phases is interpreted as arising because the lower crustal anisotropy attributed to mineral preferred orientation has a significant influence on the ps phase splitting the regional variation in ps phase polarization anisotropy of the crust in southwest japan is dependent on which anisotropy of the upper and lower crust is prominently reflected in the moho ps phase splitting

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

crustal architecture and moho topography beneath the eastern indian and bangladesh margins new insights on rift evolution and the continent ocean boundary the thick sedimentary cover 18 km along the eastern continental margin of india ecmi and over the bay of bengal has hindered understanding of the mechanisms of rifting that operated on the eastern indian and bangladesh margins analysis of multichannel seismic reflection data together with 3d gravity inversion and 2d gravity forward modelling illuminates basement configuration crustal thickness and moho topography thereby revealing the modes of rift evolution and the location of the continent ocean boundary cob along these margins the basement particularly in the shelf and slope regions of eastern peninsular india is traversed by nearly coast perpendicular graben and their seaward continuity is delineated up to 125 km from the coastline three different types of continental margin are present along the ecmi 1 sheared rift on the southern segment of the ecmi up to 14 n latitude offshore the southern granulite terrain 2 hyper extended rift in the central segment between 14 n and 17 n offshore the dharwar craton and 3 hypo extended rift on the northern segment between 17 n and 20 n offshore the eastern ghats mobile belt the graben terminations crustal thickness and moho topography of both the ecmi and offshore bengal basin clearly suggest that the cob runs nearly parallel to the coastline of peninsular india but takes an orthogonal turn offshore in the bengal basin and connects the palaeo continental shelf and the rajmahal sylhet line onshore in the bengal basin crustal thickness 8 10 km and depth to the moho discontinuity 28 km beneath the bangladesh margin are unusual because the crustal thickness increased through interaction of the kerguelen plume with the existing oceanic lithosphere the moho deepened largely due to the load of the bengal fan sediments the absence of rifted crustal blocks on the bangladesh margin and the continuity of the cob into the onshore bengal basin together imply that the present bangladesh region was under marine conditions at least until the beginning of bengal fan sedimentation i e no less than 23 ma

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

anisotropy of the mantle inferred from observations of p to s converted waves summary seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km in this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range the idea of the method is to detect and analyse the sh component of the waves converted from p to s in the mantle the procedure of record processing includes frequency filtering axis rotation transformation of the record to a standard form stacking the standardized sh component records of many seismic events and the harmonic analysis of amplitude as a function of the direction of wave propagation when applied to the long period records of norsar the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizontal axis of symmetry our preferred model postulates anisotropy of 1 per cent in a layer 50 km thick at the base of the upper mantle

azimuthal shear wave velocity anisotropy in the basin and range province using moho ps converted phases an eight station large aperture seismometer array was deployed in the basin and range province of west central nevada for the 1988 1989 program for array seismic studies of the continental lithosphere passcal passive source seismic experiment during the 10 months of data collection a total of 100 teleseismic events were recorded 30 source equalized p wave receiver functions show waveform variations in timing amplitude and polarity in the moho ps phase that is diagnostic of shear wave velocity anisotropy information regarding the crustal anisotropic component can be recovered through the ps phase since it is generated at the moho and is sensitive to shear wave splitting in the crust only we utilize single event stacked and array beam formed receiver functions to minimize the influence of structural variations and scattering on the observed converted phases analysis of shear wave splitting of the ps conversion from the crust mantle boundary indicates a fast azimuth of anisotropy oriented approximately nw se with observed time delays of 0 20 s this strongly suggests coherent crustal anisotropy with a fast direction perpendicular to the strike of fault block mountain ranges this fabric is most likely due to the preferred crystallographic orientation of seismically anisotropic minerals in the middle to lower crust rather than the distribution of basin and range upper crustal fracture systems

on the detection and identification of converted and reflected phases from receiver functions summary we develop a simple methodology to extract information related to crustal structure such as the mean vp vs ratio the magnitude of lateral variations and the presence of discontinuities from a selection of receiver functions we show that crustal phases can be detected and exploited on individual receiver functions two filtering techniques based on singular value decomposition and spectral matrix filtering are proposed to enhance the coherent phases in receiver functions additionally the relative traveltimes and slownesses between the different phases are utilized for a more reliable phase identification we apply these methods to data of station pru czech republic and compare the results with a classical time domain waveform inversion we find a good qualitative agreement between the simple crustal models inferred from the two approaches waveform modelling suggests that the identification of converted phases can still be ambiguous and other geophysical or geological constraints are needed

the so2 camera a simple fast and cheap method for ground based imaging of so2 in volcanic plumes 1 so2 flux is widely monitored on active volcanoes as it gives a window into the hidden subsurface magma dynamics we present here a new approach to so2 flux monitoring using ultraviolet imaging of the volcanic plume through carefully chosen filters to produce images of so2 column amount the so2 camera heralds a breakthrough in both our ability to measure so2 flux at unprecedented frequencies 2 hz and at unprecedented accuracy thanks to the application of correlation techniques to determine wind speed directly from the images and the ability to measure the whole profile simultaneously in this paper we detail the commercially available pieces required to construct the so2 camera introduce a retrieval scheme to determine so2 amounts from the images and present results from a field campaign in november 2005 on sakurajima volcano japan

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

the relationship between degassing and ground deformation at soufriere hills volcano montserrat we examine the correlations between so2 emission rate seismicity and ground deformation in the month prior to the 25 june 1997 dome collapse of the soufriere hills volcano montserrat during this period the volcano exhibited a pattern of cyclic inflation and deflation with an 8 14 h period we find that so 2 emission rates measured by cospec correlate with the amplitude of these tilt cycles and that higher rates of so 2 emission were associated with stronger ground deformation and enhanced hybrid seismicity within tilt cycles degassing peaks coincide with maximum deformation gradients increases in the amount of gas in the magma conduit feeding the dome probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude hybrid seismicity and so2 emission rate q 2000 elsevier science b v all rights reserved

anisotropic upper mantle stratigraphy and architecture of the slave craton the earth s physical properties show a dominantly radial structure which is the result of compositional differentiation isochemical phase changes1 and rheological layering2 rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property elastic anisotropy may closely mimic the effects of isotropic heterogeneity3 nonetheless an improved characterization of earth s rheological structure promises important insights into such processes as plate dynamics and continental evolution here i present a methodology for effectively characterizing sharp transitions in anisotropic upper mantle structure using the coda of teleseismic p waves application to seismograms from the slave craton reveals a well developed stratigraphy at least in part anisotropic with major boundaries occurring at nominal depths of 75 135 and 195 km the geometry and sharpness of these discontinuities suggest a structural origin perhaps involving shallow subduction

the poisson ratio of the australian crust geological and geophysical implications the poisson ratio which depends on the vp vs ratio provides much tighter constraints on the crustal composition than either the compressional or the shear velocity alone the crustal poisson ratio can be determined from the joint analysis of the travel times of waves converted at the moho and of crustal multiples reflected at the top of the moho we have analyzed the records of the permanent stations installed on the australian continent complemented by the data of the skippy experiment the results reveal substantial variations in the poisson ratio in the different tectonic units for the proterozoic crust an increase of the poisson ratio with increasing crustal thickness is systematically observed while for the phanerozoic crust the poisson ratio tends to decrease for increasing crustal thicknesses these observations are in remarkable agreement with the results of the deep seismic soundings that were performed in the former soviet union the variations observed in the proterozoic provinces can perhaps be explained by underplating of mafic materials at the base of the crust fl 2000 elsevier science b v all rights reserved

examining the interior of llaima volcano with receiver functions abstract llaima volcano in chile is one of the largest and most active volcanoes in the southern andes with over 50 eruptions since the 1600s after years of persistent degassing llaima most recently erupted in a series of violent strombolian eruptions in 2007 2009 this period had few precursory signals which highlights the need to obtain accurate magma storage information while petrologic advancements have been made in understanding magma degassing and crystallization trends a comprehensive seismic study has yet to be completed here we present results of a receiver function survey utilizing a dense seismic array surrounding llaima volcano application of h stacking and common conversion point stacking techniques reveals a new moho estimate and two structural anomalies beneath llaima volcano we interpret a low velocity zone between 8 and 13 km depth as a newly imaged magma body

shallow structure effects on broadband teleseismic p waveforms the first 5 sec of source equalized p wave receiver functions from an array of broadband portable instruments located about 50 km from the coast in western washington are strongly influenced by very shallow structure depths less than 3 km these data display behavior associated with a dipping high velocity contrast interface at shallow depths the direct p wave and the p to s converted phase vary strongly with backazimuth producing an apparent delay in first arriving energy at western backazimuths these patterns are attributed to a shallow contact between low velocity sandstones at the surface and an underlying high velocity volcanic unit this shallow structure influences even the original unprocessed teleseismic p waves suggesting that such shallow structure could affect other studies such as seismic source analysis utilizing broadband data

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

configuration of the continental moho and philippine sea slab in southwest japan derived from receiver function analysis relation to subcrustal earthquakes abstract we carried out receiver function imaging to estimate detailed configurations of the continental moho discontinuity and the philippine sea phs slab beneath southwest japan we calculated the receiver functions by an extended time multiple taper method a clear discontinuity at about 30 km depth was found beneath northern chugoku and kinki and we interpreted this as the continental moho we also found a northward dipping discontinuity at about 20 40 km depth beneath shikoku part of chugoku and the southern kii peninsula we present a new interpretation that this clear boundary is the upper part of the phs slab furthermore a less prominent northward dipping discontinuity is thought to be the aseismic phs slab at the depth of about 60 km beneath northern chugoku and kinki most subcrustal earthquakes occur at the plate boundary and or within the oceanic crust and their p axes tend to be oriented at high angles to the strike of the phs slab in northern chugoku existence of the aseismic phs slab is consistent with the adakitic rocks which are generated by the melting of younger slabs

protocols for uv camera volcanic so2 measurements abstract ultraviolet camera technology offers considerable promise for enabling 1 hz timescale acquisitions of volcanic degassing phenomena providing two orders of magnitude improvements on sampling frequencies from conventionally applied scanning spectrometer systems this could for instance enable unprecedented insights into rapid processes such as strombolian explosions and non aliased corroboration with volcano geophysical data the uptake of this technology has involved disparate methodological approaches hitherto as a means of expediting the further proliferation of such systems we here study these diverse protocols with the aim of suggesting those we consider optimal in particular we cover choice and set up of hardware calibration for vignetting and for absolute concentrations using quartz so 2 cells the retrieval algorithm and whether one or two filters or indeed cameras are necessary this work also involves direct intercomparisons with narrowband observations obtained with a scanning spectrometer system employing a differential optical absorption spectroscopic evaluation routine as a means of methodological validation

a hybrid method to compute short period synthetic seismograms of teleseismic body waves in a 3 d regional model we present a hybrid method to simulate the propagation of short period teleseismic body waves through 3 d regional models the incident wavefield is computed in an spherically symmetric reference earth model based on the direct solution method the global and regional wavefields are matched at the boundaries of the regional mesh in the regional domain we implement a spectral element method with absorbing boundaries to cancel the outgoing scattered wavefield the hybrid method is successfully benchmarked against the direct solution method in the reference earth model iasp91 the potential of the method is illustrated by computing short period p wave synthetic seismograms in a 3 d model with a 20 km moho offset and or topography on the free surface focusing on the actual example of the pyrenees

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

the lithosphere in central europe seismological and petrological aspects abstract the lithosphere thickness in the variscan belt of central europe varies between about 60 and 150 km with typical values of 100 120 km our estimates derived from directionally independent representative p wave residuals are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle the large scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative p residuals vary between 6 5 and 15 2 for p velocities the s wave anisotropies determined from sks polarizations vary between 2 2 and 6 7 these values are in reasonable agreement with the anisotropies of peridotites determined in laboratory systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere the residual patterns at most stations in the saxothuringicum rhenohercynicum and in the massif central imply northwesterly orientated dips of the anisotropic structures while stations in the moldanubicum the alpine foredeep and most of the alps north of the insubric line suggest southeasterly orientated dips in our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere the variscides of central europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the moldanubicum and the saxothuringicum

receiver functions for the tien shan analog broadband network contrasts in the evolution of structures across the talasso fergana fault we determined the structure of the crust beneath the central and western tien shan by analyzing broadband 1 20 s analog seismograms of converted p sv phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations the one dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the talasso fergana fault specifically the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault also velocities at depths between 10 and 35 km are about 10 lower in the east although the depth range of these lower velocities is not well resolved the talasso fergana fault is an important boundary for other observables in addition to the structural discontinuities observed at the surface the areas east of the talasso fergana fault are associated with abnormally low mantle velocities outcrops of basalt low q and short wavelength variations in anisotropy integrating these observations we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust likewise the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids the gross contrast in structure east and west of the talasso fergana fault could reflect a contrast in the dynamics of mountain building in these two regions we postulate that while crustal shortening is the dominant mechanism controlling topography west of the talasso fergana fault vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault