lithospheric structure in the area of crete constrained by receiver functions and dispersion analysis of rayleigh phase velocities summary we present a case study of lithospheric structure in the forearc of a retreating subduction zone for the hellenic arc lateral structural variations along the arc beneath the island of crete are jointly investigated by receiver functions and rayleigh phase velocities data from temporary short period networks amend previous results from broad band stations by broadening the frequency range available for phase velocity determination and increasing the spatial coverage of receiver function profiles both receiver functions and dispersion analysis reveal distinct structural differences between western and central crete western crete is characterized by nearly constant s velocities of 3 72 3 75 km s 1 from 10 km depth down to a depth of 50 km and no distinct continental moho signal meanwhile central crete shows lower s velocities equal to 3 3 km s 1 in the crust between 10 and 20 km depth which are followed by the aegean moho in about 30 km depth and a mantle wedge with an s velocity of 4 35 km s 1 both methods lead to an average depth of 55 km for the subducted oceanic african moho beneath crete this means that the slab is separated from the aegean crust by a mantle wedge beneath central crete while beneath western crete the corresponding depth region is characterized by crustal velocities this thickened crust in the forearc might be formed by crustal material of the aegean plate dragged down with the subducting african lithosphere furthermore rocks extruded from a melange circulating in a subduction channel might accumulate between a depth of 20 and 50 km and contain low velocity material e g in the form of serpentinized aegean mantle in addition the lateral extent of a prominent negative phase observed around 4 s differential time in receiver functions from western crete is mapped this phase might point to low velocity material around 30 km depth which could be extruded from a subduction channel an important property of the forearc found in this study is its strong lateral heterogeneity

on late miocene to recent vertical motions in the cretan segment of the hellenic arc abstract high resolution foraminiferal and calcareous nannoplankton biostratigraphy and reliable paleobathymetry estimates based on plankton benthos ratios in foraminifera make it possible to reconstruct in detail late neogene vertical motions along the central cretan segment of the hellenic arc these motions are considered to express the surficial effect of the roll back process of the hellenic subduction zone which started about 12 ma ago in contrast to earlier views there is no sustained uplift since the late middle miocene successive paleotopographic profiles for central crete show a predominance of subsidence coupled with an increase of differential reliefs from the latest serravallian until the messinian subsidence was most pronounced between the tortonian messinian boundary interval and the early part of the early pliocene locally up to a magnitude of more than 1000 m a two phased uplift history coupled with tilting to the north or northeast can be inferred from the late early pliocene recent record separated by a short early late pliocene episode of subsidence rates of uplift were highest in the early pliocene up to about 125 cm ka the central cretan late neogene to recent paleotopographic profiles are compared with those reconstructed from seismic interpretations and drilling data from the cretan sea the combined results show that paleotopographic configurations between the cyclades and crete were fairly similar to those on central crete until messinian time foundering of the cretan basin started in the course of the early pliocene but subsidence rates were less than the contemporaneous uplift rates of crete the timing and magnitude of the vertical motions along the cretan sea central cretan transect put detailed geological constraints on tectonophysical modelling of the surficial effects of the roll back process these vertical motions are discussed in view of the model of southward translation of a large supracrustal slab resulting in the origin of the cretan sea and in thrusting and uplift in the frontal parts of the slab where it thrusted over the northern limb of the subducted ionian plate

depth extent of the fault zone seismic waveguide effects of increasing velocity with depth summary the damage zone of a major fault can act as a low velocity seismic waveguide the fault zone guided waves provide a potential method to constrain the in situ physical properties of the fault zone fz at depth recently there has been debate over the depth extent of observed fault waveguides and whether fault properties at seismogenic depth can be constrained by guided waves gws to address these questions elastic finite difference synthetic seismograms were generated for fault zone models that include an increase in seismic velocity with depth both inside and outside the fz previous synthetic studies for a homogeneous fault showed that earthquakes off of the fault do not generate gws unless the waveguide is restricted to a few kilometres depth in contrast earthquakes both inside and outside of a depth varying fault waveguide generate strong gws within the near surface portion of the fz this is because the frequency dependent trapping efficiency of the waveguide changes with depth the nearsurface fault structure efficiently guides waves at lower frequencies than the deeper fz the low frequency waves that are guided at the surface are not efficiently guided at greater depth and therefore travel as body waves fault structure at seismogenic depth requires the analysis of data at higher frequencies than the gws that dominate at the surface and have been the subject of most previous investigations

imaging crustal discontinuities and the downgoing slab beneath western crete the migration of teleseismic receiver functions yields high resolution images of the crustal structure of western crete data were collected during two field campaigns in 1996 and 1997 by networks of six and 47 short period three component seismic stations respectively a total of 1288 seismograms from 97 teleseismic events were restituted to true ground displacement within a period range from 0 5 to 7 s the application of a noise adaptive deconvolution filter and a new polarization analysis technique helped to overcome problems with local coda and noise conditions the computation and migration of receiver functions results in images of local crustal structures with unprecedented spatial resolution for this region the crust under crete consists of a continental top layer of 15 20 km thickness above a 20 30 km thick subducted fossil accretionary wedge with a characteristic en echelon fault sequence the downgoing oceanic moho lies at a depth of 40 60 km and shows a topography or undulation with an amplitude of several kilometres as a consequence of slab depth and distribution of local seismicity the mediterranean ridge is interpreted as the recent accretionary wedge

anomalous high frequency wave propagation from the tonga kermadec seismic zone to new zealand summary bulletins of the international seismological centre isc show very large residuals up to 15 s early for arrivals from events in the tongakermadec subduction zone to the new zealand network of seismometers the very early arrivals are confined to events south of about 22 s and shallower than about 350 km the waveforms show two distinct phases an early emergent first phase with energy in the high frequency band 2 10 hz and a distinct second phase containing lower frequency energy arriving at about the time predicted by jb tables the residuals are attributed to propagation through the cold subducted lithosphere which has a seismic velocity 5 per cent faster on average than normal ray tracing shows that the ray paths lie very close to the slab for events south of 22 s but pass well beneath the slab for events further north corresponding to the change in residual pattern this characteristic of the ray paths is due to the curved shape of the seismic zone and in particular to the bend in the zone where the louisville ridge intersects the trench at 25 s the residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36 s the very high frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high q and to transmit 2 10 hz energy with little attenuation the absence of low frequency energy in the first phase is due to the narrowness of the high velocity slab which transmits only short wavelength waves the second phase which contains low frequencies is identified as a p wave travelling beneath the subducted slab in normal mantle there is no need to invoke any special structures such as low velocity waveguides or reflectors to explain any of the observations the s wave arrivals show similar effects

partitioning of deformation within a subduction channel during exhumation of high pressure rocks a case study from the western alps the metamorphic belt of the western alps was subjected to widespread extensional tectonism at the end of the eocene ca 45 35 ma extension was accommodated by hinterland directed movements along gently inclined extensional shear zones which facilitated rapid exhumation of high pressure and ultra high pressure rocks this deformation resulted in a normal metamorphic sequence extension in the inner parts of the western alps was coeval with shortening at the front of the belt foreland directed thrusts which took place during decompression and emplaced higher grade metamorphic units over lower grade metamorphic rocks thus forming an inverse metamorphic sequence two mechanisms for this extensional episode are discussed 1 collapse of an overthickened lithosphere and 2 internal readjustments within the orogenic wedge due to subduction channel dynamics we favour the latter mechanism because it can account for the development of the observed inverse and normal metamorphic sequences along foreland directed thrusts and hinterland directed detachments respectively this hypothesis is supported by published structural metamorphic and geochronological data from four geological transects through the western alps this study also emphasizes the importance of post shearing deformation e g horizontal buckling versus vertical flattening which can modify the distribution of hinterland and foreland directed shear zones in orogenic belts

high frequency precursors to p wave arrivals in new zealand implications for slab structure this report revisits the very early high frequency slab phases from earthquakes in the kermadec slab between 25 s and 37 s that arrive as a precursor to the p wave onset at stations in new zealand the analysis of short period digital records for station snzo south karori new zealand for the time period between 1980 and 1986 involved 4 times as many data as in previous studies we confirm most of the earlier observations in particular the relationship between source region and observed waveform numerical waveform modeling using two dimensional slab models confirm that the high frequency precursor can be explained by p wave propagation in a thin 8 10 km high velocity layer hvl the sensitivity of the high frequency phase to source position in the slab is investigated and we demonstrate that the slab phase does not survive interruptions or strong lateral variations of the hvl assuming the shape of the slab from tomographic images and continuity of the hvl we infer that the large negative travel time residuals can be explained if p wave propagation is 4 to 5 faster than in the ambient mantle for depths to 300 km and about 2 faster for depths between 300 and 600 km rather than resorting to regional differences in structure and composition of the hvl we argue that specific source receiver configurations and the shape of the slab explain the observed regional variation in the character of the high frequency slab phases

sea level reconstructions from the peri vocontian zone south east france point to valanginian glacio eustasy the valanginian is a period of global environmental change as illustrated by sedimentary palaeontological geochemical and climatic perturbations a production crisis in most of the carbonate platforms suggests important changes in palaeoenvironmental conditions during the same time interval a major positive excursion in 13c the weissert event suggests perturbations of the carbon cycle from the latest early valanginian to the early hauterivian in order to better understand the link between these changes sea level fluctuations have been reconstructed in detail from the middle berriasian to the earliest hauterivian sections from the peri vocontian zone south east france have been investigated because of the good quality of outcrops on the carbonate platforms their margins and in the vocontian basin sections ranging from the most proximal zone swiss jura to the basin were interpreted in terms of sequence stratigraphy and cyclostratigraphy and correlated at high resolutions using the identified small medium and large scale sequences as well as depositional geometries sea level fluctuations were reconstructed two main trends are evidenced during the studied interval i the peak amplitude magnitude of the sea level fluctuations increased gradually from the middle berriasian to the early valanginian and reached a maximum more than 50 m from the middle early valanginian to the valanginian hauterivian boundary and ii sea level variations were quite symmetrical during the late berriasian slightly asymmetrical during the early valanginian and strongly asymmetrical fast sea level rise slow fall from the latest early valanginian to the earliest hauterivian moreover three orders of sea level fluctuations were recognized in the sedimentary rocks of the peri vocontian zone platform basin correlations and cyclostratigraphic interpretations of the basinal sections evidence an astronomical control on the sea level variations mainly by the two eccentricity cycles of 100 and 400 kyr the increase in the amplitude of the sea level fluctuations and their change from symmetrical to asymmetrical can be related to the onset of a major cooling event in the early valanginian fast transgressions followed by slower regressions would correspond to waxing and waning of high latitudinal ice during most of the valanginian especially from the latest early valanginian to the latest late valanginian glacio eustatic sea level fluctuations in tune with the 100 and 400 kyr eccentricity cycles are in agreement with glaciations during the valanginian

modeling of seismic guided waves at the dead sea transform 1 on several recordings of linear seismometer arrays crossing the arava fault af in the middle east we see prominent wave trains emerging from in fault explosions which we interpret as waves being guided by a fault zone related low velocity layer the af is located in the arava valley and is considered the principal active fault of the mainly n s striking dead sea transform system in this section observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations they exhibit large amplitudes and are almost monochromatic we model them by a two dimensional 2 d analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces a hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space resolution is investigated by synthetic tests the observations are adequately fit by models with a narrow only 3 12 m wide waveguide with s wave velocity reduced by 10 60 of the surrounding rock we relate this vertical low velocity layer with the damage zone of the af since the location of receivers observing and of shots generating the guided waves respectively match with the surface trace of the fault the thickness of the damage zone of the af at least at shallow depths seems to be much smaller than in other major fault zones this could be due to less total slip on this fault

seismic wavefield calculation for laterally heterogeneous whole earth models using the pseudospectral method summary a method for simulating seismic wave propagation in a laterally heterogeneous whole earth model is presented by solving the elastodynamic equations in 2 d cylindrical coordinates using the pseudospectral method psm the psm is an attractive timedomain technique that uses the fast fourier transform for an accurate diierentiation of eld variables in the equations since no dispersion error arises in fourier diierentiation even when using a large grid spacing computer memoryand time are reduced by several orders of magnitude compared to traditional nite diierence methods in order to examine body wave phases with current computing resources a slice through the sphere is approximated with a 2 d cylindrical model an irregular grid spacing is used in the vertical coordinate to improve the treatment of the various structural boundaries appearing in the earth model by matching the heterogeneity in the model synthetic seismograms obtained by the psm calculation are compared with those calculated from an exact simulation method for a spherically homogeneous 1 d earth model and achieve good agreement the psm method is illustrated by constructing the seismic p sv wave eld for strongly heterogeneous earth models including a shield structure near the free surface and velocity perturbations just above the core mantle boundary the visualization of the evolution of seismic p and s waves in time and space is tracked using a sequence of snapshots and synthetic seismograms these displays allow direct insight into the nature of the complex seismic wave behaviour in the earth s interior

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

high resolution imaging of the bear valley section of the san andreas fault at seismogenic depths with fault zone head waves and relocated seismicity summary detailed imaging of fault zone fz material properties at seismogenic depths is a difficult seismological problem owing to the short length scales of the structural features seismic energy trapped within a low velocity damage zone has been utilized to image the fault core at shallow depths but these phases appear to lack sensitivity to structure in the depth range where earthquakes nucleate major faults that juxtapose rocks of significantly different elastic properties generate a related phase termed a fault zone head wave fzhw that spends the majority of its path refracting along the fault we utilize data from a dense temporary array of seismometers in the bear valley region of the san andreas fault to demonstrate that head waves have sensitivity to fz structure throughout the seismogenic zone measured differential arrival times between the head waves and direct p arrivals and waveform modelling of these phases provide high resolution information on the velocity contrast across the fault the obtained values document along strike fault normal and downdip variations in the strength of the velocity contrast ranging from 20 to 50 per cent depending on the regions being averaged by the ray paths the complexity of the fz waveforms increases dramatically in a region of the fault that has two active strands producing two separate bands of seismicity synthetic waveform calculations indicate that geological observations of the thickness and rock type of the layer between the two strands are valid also for the subsurface structure of the fault the results show that joint analysis of fzhws and direct p arrivals can resolve important smallscale elements of the fz structure at seismogenic depths detailed characterization of material contrasts across faults and their relation to earthquake ruptures is necessary for evaluating theoretical predictions of the effects that these structures have on rupture propagation

new evidence for a low velocity layer on the subducting philippine sea plate in southwest japan abstract a distinct precursory phase is sometimes found prior to s arrival when deep focus and intermediate depth earthquakes are observed at seismic stations in the chugoku district southwest japan the phase from intermediate depth earthquakes has apparent velocity close to that of s wave in the philippine sea phs plate but at epicentral distance of 100 250 km it arrives earlier by about 2 5 s than the s wave propagated as a head wave along the upper boundary of the hps plate thus we interpreted the phase as sp wave into which the s wave refracted upward after traveling in the high velocity phs plate was converted at a boundary while propagating on a ray path between the plate and seismic station the precursory phase to s wave from deep focus earthquakes was also identified as sp wave that was caused by conversion of the direct s wave into p wave because the s sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter the sp conversion interface was located at the upper boundary of the phs plate further it was demonstrated that a thin low velocity layer should overly just above the phs plate in order to explain both the observed difference in polarity of the s and sp waves and the amplitude ratio of the sp wave to the direct s wave

crustal investigation of the hellenic subduction zone using wide aperture seismic data abstract we present the results of a wide aperture seismic onshore offshore study crete seismic experiment in the cretan region as part of the hellenic arc compressional system three seismic lines were carried out on and around the island of crete in order to investigate the crustal structure of the region up to 119 three component recording stations were deployed on each profile that observed seismic energy generated by a 48 l airgun array and eight 20 kg landshots a total of 6208 shots were fired upon completing the fieldwork the vertical components of all stations were evaluated 300 common receiver gather crg sections of the ocean bottom seismographs obs and land stations as well as 100 common source gather csg sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2d p wave velocity depth model for each profile the accuracy of the model depends on the depth and position along the profiles and does not exceed 5 for both depth and p wave velocity we identified strong lateral variations in crustal and sedimentary thickness mainly in a north south direction but also along strike east west the crust is continental and has a maximum thickness of 32 5 km below northern central crete its subdivision in an upper v p 5 8 6 3 km s locally up to 6 5 km s and a lower v p 6 4 6 9 km s part is justified by a first order discontinuity with v p velocity a contrast of up to 0 6 km s the eastern part of crete shows a significantly thinner crust of 24 to 26 km to the north the crustal thickness decreases to 15 km below the central cretan sea the prominent decrease of the moho depth north of central crete is interpreted to represent the northern end of a microcontinent that was subducted in oligocene times and later surfaced by buoyant escape stockhert et al 1999 thomson et al 1999 the p t t d history of the high pressure rocks of crete greece denudation by buoyant escape in exhumation processes normal faulting ductile flow and erosion ring u lister g willet s brandon m eds spec publ of the geol soc of london p 154 to the south and southwest of the island the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of crete it is in contact with oceanic crust below the mediterranean ridge upper mantle velocities were determined to be 7 7 km s below the cretan sea and 8 0 km s south of crete below the continental cretan crust a 6 to 7 km thick layer with v p velocities between 6 6 and 7 1 km s was identified on each line and could be followed by reflections to a depth of 42 km it is decoupled from the overlying continental crust at central crete and is interpreted as oceanic crust presently under subduction towards the nne below the aegean sea

numerical simulation of fault zone guided waves accuracy and 3 d effects fault zones are thought to consist of regions with reduced seismic velocity when sources are located in or close to these low velocity zones guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth observations above several fault zones suggest that they are common features of near fault radiation yet their interpretation may be highly ambiguous analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities these solutions can be used for accurate modeling of wave propagation in plane parallel layered fault zone structures however at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion as more complicated models cannot be solved by analytical means numerical methods must be employed in this paper we discuss 3 d finite difference calculations of waves in modestly irregular fault zone structures we investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3 d structures we also show that simple mathematical operations on 2 d solutions generated with line sources allow accurate modeling of 3 d wave propagation produced by point sources the discussed simulations indicate that structural discontinuities of the fault zone e g fault offsets larger than the fault zone width affect significantly the trapping efficiency while vertical properly gradients fault zone narrowing with depth small scale structures and moderate geometrical variations do not the results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer

love type seam waves in washout models of coal seams the propagation of love seam waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite difference method seam interruption seam end and seam thinning models ere investigated the horizontal offset the dip of the discontinuities and the degree of erosion served as variable parameters maximum displacement amplitudes relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted love seam waves the degree of amplitude reduction depends on the horizontal offset and the degree of erosion it is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness in a seam cut vertically the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected love seam waves they diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27 sup 0 and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22

visualization of 3d wave propagation from the 2000 tottori ken seibu japan earthquake observation and numerical simulation the dense networks of strong ground motion instruments in japan k net and kik net provide a means of direct visualization of regional wave propa gation during large earthquakes for the 2000 tottori ken seibu earthquake mw 6 6 in western japan snapshots of ground motion derived directly from interpolation of a large number of array observations demonstrate clearly the nature of the source radiation and the character of the seismic wave field propagating to regional dis tances in western japan the wave field from the earthquake is characterized in most parts by the dominance of high frequency 0 2 5 hz lg waves on three component acceleration records and longer period t 10 sec fundamental mode love waves in tangential displacement the presence of strong lateral variations in the crust and upper mantle structures such as the low velocity superficial layer and the high velocity philippine sea plate with shallow subduction into the mantle impose sig nificant modifications on the regional wave field further insight into the nature of the seismic wave field is gained by comparison of the observations with numerical simulation for a 3d model including sedimentary basins by an embedded submesh a multigrid parallel computation using a hybrid pseudospectral finite difference method allows the inclusion of a realistic model of the source process for the 2000 tottori ken seibu earthquake there is good agreement in the dominant features of the regional wave field propagating through the complex structure of western japan however the differences between the observations and the computer simulations indicates the need for further refinement of the source and structural models the inclusion of amplification effects for the population centers in sedimentary basins means that the modeling procedure is already suitable for estimating the main pattern of ground motion for future earthquake scenarios

modelling of rayleigh type seam waves in disturbed coal seams and around a coal mine roadway summary wave propagation in coal seams is numerically modelled in order to identify approaches towards the reconnaissance beyond the heading face of an advancing coal mine roadway complete synthetic wavefields including p sv body waves and rayleigh type seam waves are calculated using a green s function approach for simple laterally homogeneous models and a parallel elastic 2 d 3 d finite difference modelling code for more realistic geometries for a simple three layer model the wavefield within the seam is dominated by a fundamental rayleigh seam mode symmetrical with respect to the centre of the seam on the vertical component and antisymmetrical on the horizontal component if the seam contains an interleaved dirt band with higher velocities and density higher modes dominate the wave propagation depending on the thickness of the dirt band wave propagation in laterally inhomogeneous coal seam models with disturbances like seam ends faults thinning washouts and seam splitting is strongly influenced by the type of disturbance amplitudes of seam waves reflected from these disturbances strongly depend on the fault throw and the degree of thinning or washout in some cases conversion to higher modes can occur in all investigated models those rayleigh seam wave phases are preferably reflected which have frequencies above the fundamental mode airy phase lower frequency phases are preferably transmitted however seam waves are not reflected from a seam splitting disturbance thus a detection of seam splitting with reflected seam waves appears to be impossible fd computations for 3 d models containing an ending tunnel parallel to the seam and a source beyond the heading face of the tunnel show that seam waves are converted into rayleigh waves at the tunnel face they propagate along the surface of the tunnel and interfere with the seam waves propagating beside the tunnel this effect has to be taken into account for subsequent treatment of experimental data where the locations of sources and receivers are restricted to a small seismic layout in the vicinity of the tunnel as tunnel surface waves have slightly lower frequencies than seam waves it may be feasible to separate tunnel waves from the seam wave reflections particularly because higher frequency phases of the seam wave are preferably reflected at seam disturbances polarization analysis showed that the elliptically polarized rayleigh type seam waves in the vertical radial plane can be distinguished from rayleigh tunnel waves propagating on the sidewall of the tunnel adjoining the coal layer with elliptical polarization in the radial transversal plane

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

propagation of high frequency seismic waves inside the subducted lithosphere from intermediate depth earthquakes recorded in the vanuatu arc three seismic phases associated with the slab of subducted lithosphere are identified on seismograms of intermediate depth earthquakes depths 70 km recorded in the distance range up to 350 km by the vanuatu formerly new hebrides seismograph network they are interpreted as 1 a p wave refracting into the inner portion of the slab 2 a p wave traveling mainly through the outer upper portion of the slab and 3 a p to s converted wave in which the conversion occurs across the upper surface of the slab the existence of the two p arrivals from intermediate depth earthquakes suggests that these earthquakes are not located in the strong and cold inner core of the slab but within an outer zone where the velocity is lower than that within the inner portion of the slab the low frequency character of the shear wave converted from the p wave across the outer boundary of the slab indicates that either the boundary is a transition layer with a finite thickness or the material above the slab has high attenuation for high frequency shear waves large spatial variations in the transmission of high frequency shear waves also indicate that there is a localized zone of anomalous shear wave attenuation in the mantle wedge lying just above the subducted slab and beneath the active volcanos

receiver function study of the hellenic subduction zone imaging crustal thickness variations and the oceanic moho of the descending african lithosphere summary we use data from recently installed broad band seismographs on the islands of crete gavdos santorini naxos and samos in the hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of crete into the aegean sea the stations are equipped with sts 2 seismometers and they are operated by gfz potsdam university of chania and eth zurich teleseismic earthquakes recorded by these stations at epicentral distances between 35 and 95 have been used to calculate receiver functions the receiver function method is a routinely used tool to detect crustal and upper mantle discontinuities beneath a seismic station by isolating the p s converted waves from the coda of the p wave converted p s energy from the oceanic moho of the subducted african plate is clearly observed beneath gavdos and crete at a depth ranging from 44 to 69 km this boundary continues to the north to nearly 100 km depth beneath santorini island because of a lack of data the correlation of this phase is uncertain north of santorini beneath the aegean sea moho depths were calculated from primary converted waves and multiply reflected waves between the moho and the earth s surface beneath southern and eastern crete the moho lies between 31 and 34 km depth beneath western and northern crete the moho is located at 32 and 39 km depth respectively and behaves as a reversed crust mantle velocity contrast possibly caused by hydration and serpentinization of the forearc mantle peridotite the moho beneath gavdos island located south of crete in the libyan sea is at 26 km depth indicating that the crust south of the crete microcontinent is also thinning towards the mediterranean ridge this makes it unlikely that part of the crust in crete consists of accreted sediments transported there during the present day subduction process which began approximately 15 ma because the backstop i e the boundary between the current accretionary wedge of the mediterranean ridge and the crete microcontinent is located approximately 100 km south of gavdos a seismic boundary at 32 km depth beneath santorini island probably marks the crustal base of the crete microcontinent a shallower seismic interface beneath santorini at 20 25 km depth may mark the depth of the detachment between the crete microcontinent and the overlying aegean subplate the moho in the central and northern aegean at naxos and samos is observed at 25 and 28 km depth respectively assuming a stretching factor of 1 2 1 3 crustal thickness in the aegean was 30 35 km at the inception of the extensional regime in the middle miocene

microseismic activity and seismotectonics of heraklion area central crete island greece abstract a seismological network of 10 portable analogue stations was installed in the area of heraklion central crete from september to december 1995 during this period more than 1000 events were recorded by at least 4 stations with magnitudes ranging from 0 5 to 4 6 and depths up to 70 km analysis of 336 well located events revealed high seismic activity in the onshore area seismicity is shallow

seismic radiation from an sh line source in a laterally heterogeneous planar fault zone a solution to the class of problems in which an sh line source is operating in a medium consisting of two quarter spaces with an arbitrary number of semi infinite vertical layers in between is given in a frequency wavenumber domain the general result is applied to a model with a single vertical layer and synthetic seismograms are calculated for a source specified as an sh line dislocation at the interface between the vertical layer and the faster quarter space the numerical examples demonstrate that moderate fault normal material heterogeneity can have significant effects on the seismic response of a medium the overall distribution of travel times amplitudes and motion polarities in the presence of fault zone material heterogeneity is different from a corresponding distribution in a homogeneous half space the effects are most prominent near the fault zone where head waves surface waves and trapped modes result in waveform complexities large amplifications over an order of magnitude in the fault zone itself and first motion polarities which are reversed from homogeneous medium predictions the results imply that material heterogeneity should be included in fault zone models which are used for the interpretation of observed seismic data and theoretical calculations of near fault responses such models can be utilized to obtain important fault zone and earthquake source parameters

waveform modeling of the slab beneath japan 1 the tomographic p wave model for the japan subduction zone derived by zhao et al 1994 has two very striking features a slab about 90 km thick with p wave velocities 3 6 higher than the surrounding mantle and a mantle wedge with 6 low velocity anomalies we study three component seismograms from more than 600 hi net stations produced by two earthquakes which occurred in the downgoing pacific plate at depths greater than 400 km we simulate body wave propagation in the three dimensional 3 d p wave model using 2 d finite difference fdm and 3 d spectral element sem methods as measured by cross correlation between synthetics and data the p wave model typically explains about half of the traveltime anomaly and some of the waveform complexity but fails to predict the extended sh wave train in this study we take advantage of the densely distributed hi net stations and use 2 d fdm modeling to simulate the p sv and sh waveforms our 2 d model suggests that a thin elongated low velocity zone exists atop the slab extending down to a depth of 300 km with an s wave velocity reduction of 14 if a thickness of 20 km is assumed further 3 d sem simulations confirm that this model explains a strong secondary arrival which cannot easily be imaged with standard tomographic techniques the low velocity layer could explain the relatively weak coupling associated with most subduction zones at shallow depths 50 km generally involving abundant volcanic activity and silent earthquakes and it may also help to further our understanding of the water related phase transition of ultramafic rocks and the nature of seismicity at intermediate depths 70 300 km

imaging the deep structure of the san andreas fault south of hollister with joint analysis of fault zone head and direct p arrivals summary we perform a joint inversion of arrival time data generated by direct p and fault zone fz head waves in the san andreas fault south of hollister ca to obtain a high resolution local velocity structure the incorporation of head waves allows us to obtain a sharp image of the overall velocity contrast across the fault as a function of depth while the use of near fault data allows us to resolve internal variations in the fz structure the data consist of over 9800 direct p and over 2700 head wave arrival times from 450 events at up to 54 stations of a dense temporary seismic array and the permanent northern california seismic network in the area one set of inversions is performed upon the whole data set and five inversion sets are performed on various data subsets in an effort to resolve details of the fz structure the results imply a strong contrast of p wave velocities across the fault of 50 per cent in the shallow section and lower contrasts of 10 20 per cent below 3 km with the southwest being the side with faster velocities the presence of a shallow low velocity zone around the fault which could corresponds to the damage structures imaged in trapped wave studies is detected by inversions using subsets of the data made up of only stations close to the fault the faster southwest side of the fault shows the development of a shallow low velocity fz layer in inversions using instruments closer and closer to the fault 5 and 2 km such a feature is not present in results of inversions using only stations at greater distances from the fault on the slower northeast side of the fault the presence of a low velocity shallow layer is only detected in the inversions using the stations within 2 km of the fault we interpret this asymmetry across the fault as a possible indication of a preferred propagation direction of earthquake ruptures in the region using events from different portions of the fault the head wave inversions also resolve small scale features of the fault visible in the surface geology and relocated seismicity

fast p wave propagation in subducted pacific lithosphere refraction from the plate p waves traveling from events in the tonga kermadec seismic zone to stations in new zealand are very fast with highly emergent dispersed waveforms ray tracing has shown the waves to travel close to the subducted pacific plate throughout their length and synthetic seismogram calculations have shown the dispersion requires a very thin 8 12 km fast layer previous work has been based on data from analogue records and one digital single component short period instrument no polarization analysis was possible and measurements of dispersion were limited by the bandwidth from january 1991 to august 1992 we deployed nine broad band three component seismometers in good sites for observing these arrivals the data are augmented by three component short period digital records from new stations of the new zealand national network in this study we analyze 1191 broad band and 2076 short period seismograms from 71 events for polarization of the initial p wave the polarization directions are found to be up to 30 off the great circle path and consistently steep 20 from vertical they are too steep to be explained by standard ray paths or refraction from a fast horizontal layer we invert the polarization directions for a tilted interface beneath the array and use arrival times to control the depth to the interface which is found to lie close to the top of the subducted plate inferred from the seismicity we conclude that these precursive emergent p waves have traveled through a fast layer close to the top of the subducted plate and refract upward to the station a second arrival with lower dominant frequency near 1 hz and normal travel time is occasionally seen on both broad band and short period stations its polarization direction is similarly steep but difficult to measure the evidence suggests that it also travels within the plate with similar ray path to the precursor

seismic modelling of seam waves excited by energy transmission into a seam1 the traditional method of exciting channel waves in coal deposits underground consists of firing explosive sources in a mid seam position generating seam waves of the rayleigh and love type we investigate various source positions and excitation mechanisms within the bedrock structure surrounding the seam to evaluate the effects of source positions adjacent to the seam the investigation is based on analogue and numerical modelling of half and full space cases for which the excitation and the nature of rayleigh channel waves are examined in the analogue modelling sources located from mid seam out into the bedrock along the edge of a 2d plate model excited channel waves through a conversion of the free surface rayleigh wave at the edge of the plate the excited channel wave belongs to the normal mode range frequency wavenumber analysis shows that the symmetric 2nd mode of the channel wave is excited with frequencies comparable to the forcing frequency of the source signal the polarization changes from retrograde to prograde as the wave develops from the front to the rear of the seam respectively the amplitude depth distribution resembles that of an ordinarily excited seam wave for the symmetric component however the antisymmetric component does not show the characteristic change of sign in amplitudes across the mid seam axis numerical modelling with sources located in the bedrock full space case shows that relocating the source away from the seam lowers the frequency content of the excited channel wave based on these investigations the influence of a lower frequency source signal on the excitation of the channel wave is examined in an analogue experiment sources are sited in the bedrock adjacent to the seam at three locations a lower frequency wavelet is calculated for each source location from the results obtained in the numerical analysis for comparison a higher frequency wavelet is also used which is known to be optimal for this model geometry when excited by a mid seam source location it is found that in two cases the use of the lower frequency wavelet improves the channel wave excitation while no amplification is achieved in one case

quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 landers california earthquake evidence for a shallow trapping structure summary we analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 landers earthquake a grid search method with station delay corrections is used to locate events that do not have catalogue locations the quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct s waves at stations close to and off the fault zone approximately 70 per cent of the events with s p times of less than 2 s including many clearly off the fault produce considerable trapped wave energy this distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the landers rupture zone the time difference between the s arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth the dispersion measured from the trapped waves is weak these results imply that the seismic trapping structure at the landers rupture zone is shallow and does not extend continuously along strike by more than a few kilometres synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2 4 km a width of approximately 200 m an s wave velocity reduction relative to the host rock of approximately 30 40 per cent and an s wave attenuation coefficient of approximately 20 30 the fault zone waveguide north of the array appears to be shallower and weaker than that south of the array the waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break

guided waves at subduction zones dependencies on slab geometry receiver locations and earthquake sources summary we investigate the geometry of deep subduction zone waveguides depth 100 km the wavefield characteristics for up dip profiles are described and compared with data recorded at the chile peru subduction zone observed distorted p onsets at stations in northern chile near 21 s can be matched by 2 d finite difference simulations of a thin low velocity layer lvl atop the slab in an iasp91 velocity model the replacement of the lvl by simple random velocity undulations in the slab in the same model cannot explain the observations varying slab geometries are investigated and the distribution of guided wave onsets originating in deep waveguides is predicted relative to the slab surface further double couple source position and orientation is explored and found to be closely limited by the guided wave observations sources situated above the layer and at distances more than 2 layer widths below the subducted moho are not suitable for the remaining favourable source locations a strong link between pulse shapes and fault plane dip angle is evident we conclude that up dip guided wave observations at subduction zones follow a simple pattern given by slab geometry and modified by source position the resulting onsets are shaped by layer thickness and velocity contrast and further influenced by the shape of the slab surface

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

dispersion of regional body waves at 100 150 km depth beneath alaska in situ constraints on metamorphism of subducted crust phase delays at high frequencies are observed in body waves that travel in the alaska slab along its strike at 100 150 km depth the delays between 2 6 hz energy and the direct 0 5 1 hz arrival are 0 5 1 5 s for p waves and 1 5 4 s for s waves such dispersion suggests a waveguide structure that parallels the slab perhaps near its top a channel that is 2 6 km thick and 2 5 5 slower than surrounding mantle can explain the observations the thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner the layer may be crust that is slow at these depths the required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer it may indicate a mixture of the two or persistence of hydrated mineral assemblages to depth

seismicity of the hellenic subduction zone in the area of western and central crete observed by temporary local seismic networks temporary local seismic networks were installed in western crete in central crete and on the island gavdos south of western crete respectively in order to image shallow seismically active zones of the hellenic subduction zone more than 4000 events in the magnitude range between 0 5 and 4 8 were detected and localized the resulting three dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central crete from the mediterranean ridge to the cretan sea furthermore a three dimensional structural model of the studied region was compiled based on results of wide angle seismics surface wave analysis and receiver function studies the comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished high interplate seismicity along the interface between the subducting african lithosphere and the aegean lithosphere was found south of western crete where the interface is located at about 20 to 40 km depth an offset between the southern border of the aegean lithosphere and the southern border of active interplate seismicity is observed in the area of crete the offset varies laterally along the hellenic arc between about 50 and 70 km a southwards dipping zone of high seismicity within the aegean lithosphere is found south of central crete in the region of the ptolemy trench it reaches from the interface between the plates at about 30 km depth towards the surface in comparison the aegean lithosphere south of western crete is seismically much less active including the region of the ionian trench intraplate seismicity within the aegean plate beneath crete and north of crete is confined to the upper about 20 km between 20 and 40 km depth beneath crete the aegean lithosphere appears to be seismically inactive in western crete the southern and western borders of this aseismic zone correlate strongly with the coastline of crete

wave propagation in 3d spherical sections effects of subduction zones abstract in order to understand details in the seismic wave field observed on regional and global scales on the earth s surface accurate modeling of 3d wave propagation is necessary while numerical techniques are now routinely applied to local seismic wave propagation only recently has the possibility of simulating wave propagation on larger scales in spherical geometry been investigated we apply a high order staggered grid finite difference scheme to the elastic wave equations in spherical coordinates r using regular grid spacing in a single domain the physical space is limited to spherical sections which do not include the axis 0 while the staggering of the space dependent fields improves the overall accuracy of the scheme some of the tensor elements have to be interpolated to the required grid locations by comparing with quasi analytical solutions for layered earth models we demonstrate the accuracy of the algorithm finally the technique is used to study the effects of a source located in a simplified slab structure the 3d technique will allow us to study the wave field due to laterally heterogeneous structures such as subduction zones plumes or oceanic ridges

seismic waves guided by untransformed oceanic crust subducting into the mantle the case of the kanto district central japan a model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the kanto district central japan a distinct pair of later p and s phases xp and xs commonly observed at specific stations for these events in a depth range of 40 60 km and is considered to take place in association with the subducting motion of the philippine sea plate the xp phase is found from its particle motions to be a longitudinal seismic wave a principal component analysis of wavelets including the initial p and the xp phases shows that the latter arrives at the earth s surface with a slightly shallower angle relative to the former close agreement between the relative arrival time ratio of xs s xp p and the vp vs ratio of crustal or mantle rocks suggests that both xp and xs travel along the same path as longitudinal and tangential waves respectively one can clearly observe these later phases at stations located where the upper surface of the philippine sea plate is apparently in direct contact with the bottom of the crust of the honshu island apparent velocities of the xp and the xs phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous travel time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region the hypocentral distribution of the events related to the subducting motion of the philippine sea plate does not form such a thin seismic zone as observed in southwest japan but extends over the entire thickness of the philippine sea plate the earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone the deeper portion does not generate such events these characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the philippine sea plate taking the result of our earlier work into account we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the philippine sea plate

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

travel times and attenuation relations for regional phases in the barents sea region a database containing 45 events in the barents sea region has been compiled and analyzed with the aim of evaluating crustal models travel times and attenuation relations in the context of performing regional detection threshold monitoring of this region the 45 events are mostly located around the circumference of the study area due to the virtually aseismic nature of the barents sea itself regional p n and s n phases were observable for most events in the database while p g and l g phases were only observable for events with raypaths that do not cross the tectonic structures in the barents sea this corroborates a number of previous observations of l g wave blockage within the barents sea three existing velocity models were evaluated with a model having slightly lower s velocities than earlier assumed in the upper mantle giving the overall best fit to the observed arrivals in order to estimate magnitudes short term average sta and spectral amplitude values were calculated in several frequency bands for all phase arrivals in the database there were no significant differences between spectral and sta amplitudes so the latter were used as this parameter is more efficient to calculate in real time processing an inversion was performed in order to determine an attenuation relation specific for this region the resulting magnitudes based on p n p g s n and l g phases gave an internally consistent reasonably stable set of values which can be calibrated towards any existing global or regional scale

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

guided waves propagating in subducted oceanic crust 1 we use guided waves traveling updip along the surface of the nazca slab to image subducted oceanic crust at the chile peru subduction zone observed p onsets of intermediate depth events near 21 s in northern chile reveal waveguide behavior with growing focal depth low frequency energy 2 hz becomes more and more dominant and higher frequencies arrive delayed sometimes resembling two distinct phases to explain the observations we employ two dimensional finite difference fd simulations of complete p sv wave propagation along an updip profile of the subduction zone the fd calculations shed some light on several basic issues regarding crustal waveguides the development of guided waves dependent on event focal depth is simulated further we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle simulations also yield constraints on source locations relative to the low velocity structure finally the frequency content of p onsets is used to constrain the thickness of the waveguide the results indicate that a structure of 4 5 km average width and 7 low velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km the layer is interpreted as the unaltered lower part of the subducted oceanic crust suggesting that complete eclogite transformation in the chile peru subduction zone is unlikely to take place until beyond the volcanic front

numerical modelling of seismic wave propagation along the plate contact of the hellenic subduction zone the influence of a deep subduction channel summary we model seismic wave propagation from intermediate depth earthquakes in a subduction zone using a 2 d chebyshev pseudospectral method particular attention is directed to the influence of a deep low viscosity subduction channel on top of the plate contact where metamorphic rocks may be exhumed by forced return flow the study is motivated by observations of complicated dispersive and high amplitude p and s wave trains in the forearc of the hellenic subduction zone the basic model is a subducted slab with a thin oceanic crust forming a lowvelocity layer our model setup closely follows recent results on the structure of the hellenic subduction zone obtained from receiver functions and surface wave studies they exhibit an abrupt change of the dip of the downgoing slab at about 70 km depth the subduction channel is modelled as a thin wedge shaped layer of intermediate seismic velocity above a slower oceanic crust and below a faster overlying mantle wedge we also look into the effects of a continuous phase transition from basalt to eclogite in the subducted oceanic crust and near surface crustal structures in all models wave propagation is characterized by dispersive guided channel waves trapped in the low velocity subducted crust they produce high amplitude arrivals in the forearc a fast guided wave train gp originates from the direct p wave and a slower one gs from the direct s wave guided waves are radiated into the overlying mantle where the dip of the slab is abruptly changing seismogram sections for models without a subduction channel typically show two spatially separated guided wave trains one following the oceanic crust and one travelling more steeply towards the forearc high a subduction channel above the plate contact enhances the radiation effect of gp waves at the slab bend due to the weaker velocity contrast and inhibits the separation of the wave trains in models with additional near surface crustal structures the wave field is dominated by reverberations however guided waves remain discernible in seismogram sections because of their high amplitudes when a basalt to eclogite phase transition is considered guided waves are preferentially generated in the presence of a subduction channel a pronounced gp wave train develops particularly for sources inside this channel on the contrary guided waves are hardly distinct for sources below the subducted crust

dispersion calculations for sh and p sv waves in multilayered coal seams recent work by rader schott dresen and ruter has provided a means of calculating dispersion and amplitude distribution curves for sh channel waves in multilayered media an alternative calculation procedure is presented and the p sv wave type is also included numerical results are given for a coal seam containing a band of dirt of either higher or lower seismic velocities than the coal itself

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

san andreas fault zone head waves near parkfield california microearthquake seismograms from the borehole seismic network on the san andreas fault near parkfield california provide three lines of evidence that first p arrivals are head waves refracted along the cross fault material contrast first the travel time difference between these arrivals and secondary phases identified as direct p waves scales linearly with the source receiver distance second these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct p arrivals third the first motion polarities of the emergent arrivals are reversed from those of the direct p waves as predicted by the theory of fault zone head waves for slip on the san andreas fault the presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms the fault zone head waves indicate that the velocity contrast across the san andreas fault near parkfield is approximately 4 percent further studies of these waves may provide a way of assessing changes in the physical state of the fault system

slab low velocity layer in the eastern aleutian subduction zone summary local earthquakes in the vicinity of the alaskan peninsula s shumagin islands often produce arrivals between the main p and s arrivals not predicted by standard traveltime tables based on traveltime and polarization these anomalous arrivals appear to be from p to s conversions at the surface of the subducted pacific plate beneath the recording stations the p to s conversion occurs at the top of a low velocity layer which extends to at least 150 km depth and is 8 f 2 per cent slower than the overlying mantle the slab is 7 per cent faster than the mantle the low velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines ps ray paths to lie within it these observations indicate that layered structures persist to positions well past the surface location of the volcanic front reactions forming high pressure minerals do not yield slab like velocities until beyond the point that subduction zone magma genesis occurs if the subducted oceanic crust forms the layer it is subducted essentially intact

spatial transition from compression to extension in the western mediterranean ridge accretionary complex the mediterranean ridge has been described since 1982 as an accretionary prism related to the hellenic subduction zone however little attention has been paid to the actual mechanical boundary between the sedimentary wedge and the crustal backstop of the outer hellenic arc using seismic velocity structures from recent esp s pasiphae cruise we propose a new interpretation of existing geological and geophysical data around the western hellenic trenches the velocity structure of the inner unit of the mediterranean ridge suggests that the late neogene to quaternary sequences were deposited above a high velocity rigid body we propose to correlate the boundary of this rigid body backstop of the accretionary wedge with a series of oblique reflectors dipping to the southwest near dsdp sites 126 and 377 the backstop extends further south than previously thought and decreases by one third the total width of the compressional wedge it has thus major implications on the volume of accreted material the short wavelength folding of plio quaternary sediments observed on the outer wall of the south matapan trench is probably a consequence of gravity gliding layers above the high velocity body gently dipping 2 to the northeast using the messinian evaporite as a decollement on the other hand the hellenic trenches appear to be narrow half grabens affected by the widespread extensional deformation active on the outer hellenic arc they are situated on the landward boundary of a wider and poorly sedimented fore arc basin we conclude that extensional deformation extends at least over the width of the hellenic trenches on the continental side of the backstop whereas compressional deformation stops near the edge of the backstop itself 100 km to the southwest

exhumation rates of high pressure metamorphic rocks in subduction channels the effect of rheology 1 exhumation of high pressure metamorphic rocks can take place with typical plate velocities of cm year this is consistent with a model of forced flow in a subduction channel the micro structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep according to a power law rheology instead deformation appears to be localized in low strength shear zones and is dominated by dissolution precipitation creep or fluid assisted granular flow implying a newtonian rheology 1d modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation when the subduction flux either equals or exceeds the return flux the maximum exhumation rate for newtonian behavior of the material is at least twice as high 1 3 of the subduction burial rate compared to that for power law creep 1 6 of the subduction burial rate

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

guided waves from sources outside faults an indication for shallow fault zone structure using 3 d numerical modeling of seismic wave propagation we investigate the possibility of generating fault zone fz trapped wave energy from sources well outside a fault the fz is represented by a o 200 m wide vertical low velocity layer in a half space we find that fz trapped waves can be excited from sources well outside the fault if 1 the low velocity structure extemds only to shallow depth and the source is located at greater depth or 2 the structure of the low velocity zone is such that only the shallow part of the fz traps energy fz trapped waves are not excited from sources well outside a fz continuous with depth the results support in conjunction with recent observational evidence a model for natural faults with shallow trapping structures rather than ones that span the entire seismogenic zone this may have implications for fault mechanics as well as for aspects of shaking hazard near faults

late pleistocene and holocene sea level change in greece and south western turkey a separation of eustatic isostatic and tectonic contributions summary sea level change during late pleistocene and holocene times is a combination of eustatic isostatic and tectonic contributions in the central mediterranean sea region of greece and western turkey the isostatic components are important due to the changing gravitational potential of the earth the readjustment of the crust upon the removal of the large ice sheets and the addition of the meltwater into the oceans including the mediterranean changes in relative sea level due to these glacio hydro isostatic adjustments have reached amplitudes of 1 mm yr 1 during the last few thousand years a model for the isostatic contribution to sea level change including the movement of shorelines due to the combined eustatic isostatic changes is developed based on earth model and ice sheet parameters estimated from sea level observations from other areas comparisons of this model with observations of sea level change permit rates of vertical movement to be estimated for late holocene time allowance for the isostatic factors makes a significant difference to these rates the plains of argolis lakonia messinia and navarine in the southern peloponnese for example appear to be tectonically stable on time scales of a few thousand years and longer consistent with the position of the last interglacial shoreline close to the present day sea level the observations here of a rising sea level are largely the consequence of the glacio hydro isostasy and not of a long term tectonic process crete as a whole is subject to tectonic uplift but at variable rates and forms part of an arcuate zone of uplift extending from rhodes and karpathos in the east to kithera in the west at rates that locally exceed 4 mm yr 1 the southern shore of the gulf of corinthos is also subject to uplift at rates approaching 1 5 mm yr 1 these isostatically corrected estimates for the past few thousand years are in agreement with longer term estimates based on the position of the last interglacial shoreline only on the perachora peninsula do the tectonic rates over these two time intervals appear to be in disagreement with the late holocene rates being much higher than the long term rates

one dimensional models of shear wave velocity for the eastern mediterranean obtained from the inversion of rayleigh wave phase velocities and tectonic implications summary on a sw ne profile from the libyan coast towards central turkey phase velocity curves of the fundamental rayleigh mode were measured using a two station method the inversion of phase velocity curves yields 1 d models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile strong lateral variations in the shear wave velocity structure are imaged along the profile the subducted oceanic african mantle lithosphere is indicated in 1 d models for the region around crete by significantly enlarged shear wave velocities it is also imaged by an average model of the structure between stations on crete and santorini on a path crossing the libyan sea south of crete the resulting model is slower than a model expected for 110 myr old oceanic lithosphere the passive african margin is thus assumed to extend northwards beneath the libyan sea anomalous low shear wave velocities are found for the uppermost mantle beneath central turkey down to a depth of approximately 130 km using two stations on crete the average depth of the oceanic moho within the subducting slab is estimated to be at approximately 50 km beneath crete for this arc parallel path an enlarged standard deviation of the measured phase velocities of approximately 0 2 km s 1 between 10 and 30 mhz is observed that is probably caused by strong lateral heterogeneity related to the subducting slab in addition in this frequency range an anomalous propagation of the fundamental rayleigh mode is detected that is indicated by measured phase velocities that are approximately one standard deviation faster than phase velocities expected from a great circle approximation an average shear wave velocity of approximately 3 5 km s 1 is observed above the oceanic moho in order to explain the recent lithospheric structure of the hellenic subduction zone a tectonic model is assumed for the ne sw striking profile considered it is based on the calculated 1 d models tectonic reconstructions and on a model derived from the metamorphic history of rocks exposed on crete the suggested model summarizes the tectonic development at a lithospheric scale starting in the late cretaceous accretion of crustal material of two microcontinents to eurasia is assumed while continuous subduction of the oceanic lithosphere of different ocean basins and possibly of the mantle lithosphere of the microcontinents resulted in a single slab the length of the oceanic lithosphere that was subducted south of crete is estimated to be not greater than approximately 550 km

microearthquake seismicity and fault plane solutions in the southern aegean and its geodynamic implications summary seismicity mi 3 5 in the southern aegean located using data collected during seven weeks of recording by a temporary network of seismological stations largely follows the hellenic arc the sea of crete is nearly aseismic and only little activity is located south of the hellenic trench within the african plate focal mechanisms exhibit reverse faulting in the external part of the arc and normal faulting inside it this normal faulting indicates n s extension in the northern aegean the gulf of corinth the cyclades and dodecanese islands but nw se extension in southern peloponnese and western crete and e w extension in eastern crete this non uniform strain pattern suggests that the aegean region not only extends in a n s sense with the hellenic arc moving south westward relative to the eurasian plate but also by e w extension of its southern margin so that there is a net divergence of material

the geometry of the wadati benioff zone and lithospheric kinematics in the hellenic arc abstract accurate locations of 961 shallow and intermediate depth earthquakes which occurred between 1956 and 1995 in the hellenic arc 34 39 n 19 28 e are used to define the plate boundaries in this area reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc an ocean continent type of interaction occurs on a curved surface which is defined by the shallow branch 20 100 km of the wadati benioff zone the intersection of this zone with the earth s surface is a curve which follows the convex outer side of the sedimentary arc western peloponnese west of cythera south coast of crete east coast of rhodes and dips at low angle 30 to the aegean sea coupling between the subducted oceanic crust and the overriding of the aegean lithospheric plate takes place on this surface the deep branch 100 180 km of the wadati benioff zone is dipping freely without coupling at a high angle 45 beneath the south aegean trough and the volcanic arc the high shallow seismicity h 20 km which is observed in the southwestern convex outer side of the arc ionian sea is attributed to the fast southwestward motion of the aegean plate location of strong deep earthquakes h 100 km in the fore arc area of the southwestern part of the hellenic arc west of cythera indicates that oceanic crust is destroyed in this part of the hellenic trench due to roll back of the descending lithospheric slab

regional wave propagation from mexican subduction zone earthquakes the attenuation functions for interplate and inslab events the seismic waves from subduction zone earthquakes are significantly affected by the presence of 3d variation in crust and upper mantle structure around the source area these heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances this is illustrated by studying shallow interplate earthquakes along the mexican subduction zone and deeper inslab normal faulting earthquakes in the subducted cocos plate beneath mexican mainland the strong motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and mexico city we compare the wavefield from two large earthquakes of different source type during the shallow h 17 km interplate 1995 guerrero earthquake m w 7 3 the lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source the presence of a lateral velocity gradient in the crust caused by the subduction of the cocos plate enhances the lg wave amplitude which is then amplified further in the mexican volcanic belt by amplification in the low velocity volcanic rocks both effects lead to very large ground motions along the path from the coast to the mexican inland in the frequency band from 0 2 to 4 hz however for the deeper h 40 km inslab normal faulting 1999 oaxaca earthquake m w 7 5 the amplitude of the lg phase is too small to produce the abnormal wave propagation and the direct s wave and its multiple sms reflections between the free surface and moho show a simple attenuation with increasing distance we compare these observations with numerical simulations of seismic wave propagation using the fourier spectral method the results provide a key to the understanding of seismic wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3d heterogeneous structure

in seam seismic love wave scattering modeled by the finite element method the dynamic finite element method allows frequency dependent reflection and transmission coefficients to be computed for love waves scattered by two dimensional inhomogeneities in coal seams clean faults of zero hade angle show a throw dependent cross over frequency in reflection spectra and throw dependent conversion from fundamental to first higher mode energy in transmission spectra oblique faults show mode conversion in reflection spectra which is relatable to the fault hade angle by simple huygens theory models thin fracture zones or dykes normal to the seam show a reflection maximum when thickness of the zone is of order one quarter of the seam wave wavelength published field data from two known faults and a dyke are compared with the modeling results and support the belief that broad band seam wave data are capable of characterizing a seam discontinuity throw hade angle dyke thickness as well as locating it development of such procedures will require extensions to existing field practice and processing 19 references

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

three dimensional calculations of fault zone guided waves in various irregular structures summary the detailed structure of fault zones fzs plays an important role in problems related to fault mechanics earthquake rupture wave propagation and seismic hazard fzs are thought to consist of an o 10 100 m wide region of decreased seismic velocity but structural details such as their depth extent lateral and vertical variations etc are elusive the small spatial scales involved make such structures difficult to image with ray theoretical methods such as tomography however seismic energy trapped inside fz layers can provide dispersive wave trains that carry information on the fz structure these waves can travel many kilometres inside the fz before reaching the surface and are therefore strongly altered by its properties candidate trapped waves have been observed above several active faults inversion algorithms exist that can model these observations in terms of planar fault zone structures however at present it is not clear how reliable these estimates are as the effects of even small 3 d variations on trapped waves are not well understood the goal of this study is to distinguish 3 d structures that do and do not significantly affect fz waves to achieve this we perform numerical calculations of wave propagation in various fz geometries and analyse the waveforms spectra and envelopes of the synthetic seismograms the main results are that 1 moderate changes of the shape of fz or 2 small scale heterogeneities or 3 depth dependent properties do not strongly affect the observed fz waves in contrast strong effects are to be expected from 4 breaks in the continuity of fz structure e g offsets which may at some point allow imaging of such features at depth

p sv wave scattering by coal seam inhomogeneities the dynamic finite element method is used to calculate the normal incidence p sv guided wave scattering spectra of 124 models of coal seam obstructions commonly encountered in in seam seismology the models are faults of varying throw and dip thick faults with an associated region of fracturing dikes of varying width hardness and dip sills of varying hardness stone rolls of varying height and seam washouts reflection and transmission spectra including mode conversion are studied for incident fundamental symmetric p sv seam modes the dynamic finite element method is adapted to the calculation of scattering parameters through the consistent boundary condition of lysmer and waas the results complement an earlier study of sh seam wave scattering they show that faults reflect a much higher percentage of sh wave energy than p sv but that p sv waves undergo substantially greater mode conversion on scattering from faults dikes exhibit thin film interferometric effects for both sh and p sv waves sil

subduction zone guided waves and the heterogeneity structure of the subducted plate intensity anomalies in northern japan 1 the subducting pacific plate acts an efficient waveguide for high frequency signals and often produces anomalously large intensity on the eastern seaboard of northern japan during deep earthquakes the waveform records in the region of high intensity show a low frequency f 2 hz later arrivals with a long coda this behavior is not explained by a simple subduction zone model comprising a high velocity plate with low attenuation from the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the pacific subduction zone we demonstrate that the high frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0 5 km in thickness the standard deviation of wave speed fluctuations from the averaged background model is about 2 this new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness but low frequency waves with long wavelengths can easy tunnel through such lamina structure the result is frequency selective propagation characteristics with a faster low frequency phase followed by large and high frequency signals with very long coda a low wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high frequency coda three dimensional simulations using the earth simulator supercomputer for modeling of high frequency seismic wave propagation in the pacific subduction zone including plate heterogeneity clearly demonstrate the scattering waveguide effects for high frequency seismic waves traveling in the plate the region of large intensity for the heterogeneous model migrates away from the hypocenter into northern japan with an elongated zone along the pacific coast almost comparable to the observations from deep events in the pacific plate

modeling the influence of moho topography on receiver functions a case study from the central hellenic subduction zone 1 pseudospectral 2d modeling of wave propagation provides a fresh look at receiver function data from the hellenic subduction zone it is shown that distinct moho topography which is suggested for this complex subduction environment from various geophysical data sets can provide a conclusive explanation for previously observed inverted moho phases in the forearc as well as for a complex sequence of apparently dipping conversions in the volcanic arc prominent effects of strongly dipping discontinuities include the change of sign in ps phases refracted past the vertical and multiple p reflections that cannot be separated from ps conversions during receiver function processing unambiguous identification of these phenomena can be hampered by poor azimuthal coverage of the data still in regions where discontinuity topography can reasonably be expected its effects should be considered when interpreting receiver functions as results might be spurious otherwise

calculation of dispersion curves and amplitude depth distributions of love channel waves in horizontally layered media we present dispersion curves and amplitude depth distributions of the fundamental and first higher mode of love seam waves for two characteristic seam models the first model consists of four layers representing a coal seam underlain by a root clay of variable thickness the second model consists of five layers representing coal seams containing a dirt band with variable position and thickness the simple three layer model is used for reference it is shown that at higher frequencies depending on the thickness of the root clay and the dirt band the coal layers alone act as a wave guide whereas at low frequencies all layers act together as a channel depending on the thickness and position of the dirt band and the root clay in the dispersion curves of the group velocity secondary minima grow in addition to the absolute minima furthermore the dispersion curves of the group velocity of the two modes can overlap in all these cases wave groups in addition to the airy phase of the fundamental mode propagating with minimum group velocity occur on the seismograms recorded in in seam seismic surveys thus impeding their interpretation hence we suggest the estimation of the dispersion characteristics of love seam waves in coal seams under investigation preceding actual field surveys all numerical calculations were performed using a fast and stable phase recursion algorithm

forearc decoupling of guided waves in the chile peru subduction zone 1 the structure and alterations of subducted oceanic lithosphere e g thickness and seismic velocity of oceanic crust can be obtained by analyzing guided seismic waves generated by earthquakes within the slab wadati benioff zone in northern chile prominent secondary phases from intermediate depth seismicity observed in the forearc region can be interpreted as guided waves for the observation of guided waves it is usually required to have stations close to the wave guide a fact which is not directly given for forearc stations in subduction zone environments with the help of finite difference simulations we model the decoupling mechanism of guided waves at the contact between the descending oceanic plate and the upper plate crust where the wave guide is opened due to the equalization of seismic velocities provided that suited stations are available this mechanism allows for the use of intermediate depth seismicity to study the shallow subduction zone structure 100 km depth

geodetic determination of the kinematics of central greece with respect to europe implications for eastern mediterranean tectonics we use a new satellite laser ranging global positioning system slr gps solution at seven sites in anatolia and aegea to obtain a better definition of the extrusion motion of the anatolian aegean block with respect to europe we show that this motion can be described in a first approximation by a counterclockwise rotation which transfers most of the motion of arabia to anatolia we combine 78 displacement vectors obtained at common points of two triangulation nets measured in central greece in 1895 and 1975 with the slr gps measurements to compute the velocity field over greece with respect to europe these measurements indicate that central greece is a zone of extension between the anatolian aegean counterclockwise rotation to the south and the northern greece clockwise rotation to the north this extension is principally localized within the gulf of corinth to the east but is distributed to the west we then extrapolate this velocity field to the whole aegea and western anatolia using recently published gps results as well as the slr results the narrow dextral north anatolian fault which limits the velocity field to the north progressively gives way to a much wider boundary zone where extension becomes dominant we show that the collision between the mediterranean ridge and africa began 3 6 ma and we describe the modifications that this collision has produced on the kinematic pattern both in aegea and on the mediterranean ridge