propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

coda of long range arrivals from nuclear explosions short period three component recordings from peaceful nuclear explosions pne of the profile quartz located in russia are used to constrain the nature of the coda of pne arrivals in particular we examine the unusually strong and extensive coda of the long range p n interpreted as a whispering gallery wg phase propagating to beyond 3000 km energy balance considerations in three dimensions show that such an extensive coda is inherent not only to wg but to all other p wave phases and can be explained by crustal scattering the long coda is a result of excitation of short period scattered waves pg sg lg rg within the crust by the waves incident from the mantle or conversely by generation of mantle phases from crustal guided waves within the source region the resulting estimates of coda q range between q 380 near 2 hz and q 430 near 5 hz and can be associated with crustal attenuation including the sediments our coda model also explains quantitatively the observed build up of the arrival amplitude with time and the apparent lack of a pronounced coda of the body wave arrivals from the mantle transition zone these effects result from adding up the energy of the later arrivals arriving during the codas of earlier arrivals we deconvolve the overlapping coda patterns and show that the true relative energies of the arrivals are significantly lower than the apparent energies measured from the raw records a whispering gallery interpretation of the long range p n and crustal scattering accounts for the entire range of observations of kinematic spectral and amplitude pattern of the pne wavefield and allows the derivation of constraints on attenuation

guided wave propagation in laterally varying media ii lg waves in north western europe summary lg wave observations at norsar from a sequence of explosions straddling the central graben in the north sea show that the phase is almost extinguished by passing through this structure for models of the graben structure based on seismic refraction experiments with crustal thinning beneath a zone of thickened sediments numerical modelling of lg propagation shows very poor transmission the inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity the theoretical effect of a crustal pinch is to redistribute s energy over a wide range of groups velocities a pattern that fits well with the observations three components recordings at norsar of explosive charges fired in water covering a wide range of azimuths show significant transverse components to the seismograms the transverse energy builds up with travel time and for lg is of the same order as the vertical component these results are consistent with the theoretical prediction of progressive net transfer of energy from rayleigh to love modes as the wavetrain propagates through a three dimensionally heterogeneous medium

theoretical study of the excitation spectral characteristics and geometrical attenuation of regional seismic phases abstract we present a theoretical study of the generation and geometrical attenuation of regional crustal phases we do this through the computation of seismograms in the epicentral distance range from 60 to 500 km the geometrical attenuation of lg waves with epicentral distance is of the form r 0 83 pg wave amplitudes display a much stronger decay of the form r 1 5 the spectral density of the crustal transfer function for pg waves is relatively flat for frequencies between 0 1 and 5 hz while lg wave spectra strongly fall off beyond 2 to 3 hz the excitation of pg wave is insensitive to the depth of the source within the crust while the lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust the amplitudes of these two phases drastically decrease when the source is below the moho these results illustrate the important role of wave guide played by the crust for the propagation of lg and pg we find that the geometrical attenuation of pg and lg waves is independent of source mechanisms in the case of an explosion the excitation of pg is insensitive to the source depth the lg wave amplitude is small in comparison to pg and rayleigh waves and depends on the closeness of the source to an interface or to the free surface

mapping of crustal heterogeneity in the north sea basin via the propagation of lg waves summary the propagation of lg waves within the continental area in and around the north sea basin shows strong dependence on the path between source and receiver paths lying within the british isles and norway show very clear lg phases but paths which cross the graben zone lying in the middle of the north sea basin have very weak lg over 150 paths have now been studied across the region and the character of the lg wave has been described by comparison with the size of the sn phase for shallow events this gives a stable measure of the efficiency of lg propagation the regions which appear to block the transmission of lg are quite localized in the middle of the north sea in the region with graben structures and extend on into the netherlands a weak zone of poor transmission appears to be associated with the oslo graben with the relatively dense areal coverage provided by paths crossing the basin with a wide range of azimuths it is possible to attempt to invert for the pattern of crustal heterogeneity which gives rise to the observed character of the lg propagation an interative scheme has been devised to find the propagation properties in cells of a 1 x 1 grid and a well defined map of the heterogeneity is obtained the strongest heterogeneities correlate very well with the major tectonic features of the viking and central grabens running north south through the basin where the path coverage is greatest the position of the heterogeneity can be tightly constrained and lies within a rather narrow zone around 100 km wide despite the number of paths employed it is not possible to obtain this level of resolution over the whole region seen in the light of theoretical studies of lg wave transmission the heterogeneity map gives an indication of the strength of horizontal gradients in seismic properties in the crust these are most likely to be associated with crustal thinning beneath the graben structures

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

frequency dependent attenuation of shear waves in the crust of the southern kanto area japan the attenuation characteristics of shear waves in the crust of the southern kanto area central japan were estimated using strong motion data including acceleration data recorded in the pre tertiary basement rocks by means of downhole observation the quality factor qs f was determined for a range of discrete frequencies from 0 5 to 16 hz from the analysis of data from 13 local earthquakes with focal depths of less than about 50 km that occurred in the philippine sea plate and in the boundary zone between the lower part of the eurasian plate and the upper part of the philippine sea plate the estimated 1 qs f shows a peaked structure in this frequency range on the assumption that the geometrical spreading exponent is 1 body waves the estimated peak 1 qs f is of the order of 10 2 at 0 8 hz

geometrical spreading and attenuation of lg waves a comparison between western anatolia turkey and southern spain abstract we compared the attenuation and geometrical spreading coefficient of lg waves for crustal paths in western anatolia and southern spain at source receiver distances between 80 and 400 km we used the coda normalization method which is based on the elimination of the source and site effects from observed spectra of lg waves q for lg waves in the crust was determined as a function of frequency in the range 1 5 10 0 hz for western anatolia and in the range 1 5 12 hz for southern spain we found that qlg shows a strong frequency dependence in both regions in western anatolia the frequency dependence of qlg is qlg f 82 f1 0 and the geometrical spreading coefficient ranges from 0 6 to 0 9 we found qlg 83 f0 88 and a geometrical spreading coefficient close to 0 5 in southern spain comparing our results with coda q values obtained in both regions qlg qc are similar qlg is close to qi in all frequency bands we interpret the geometrical spreading coefficient higher than 0 5 for western anatolia as due to a strong layered and heterogeneous crustal structure

synthetic sh seismograms in a laterally varying medium by the discrete wavenumber method summary we present a new method to calculate the sh wavefield produced by a seismic source in a half space with an irregular buried interface the diffractiog interface is represented by a distribution of body forces the green s functions needed to solve the boundary conditions are evaluated using the discrete wavenumber method our approach relies on the introduction of a periodicity in the source medium configuration and on the discretization of the interface at regular spacing the technique developed is applicable to boundaries of arbitrary shapes and is valid at all frequencies some examples of calculation in simple configurations are presented showing the capabilities of the method

nonlinear frequency dependence of the effective seabottom acoustic attenuation from low frequency field measurements in shallow water hamilton s seabed geo acoustic model which is widely accepted predicts that the attenuation of sound in marine sediments increases linearly with frequency over the full frequency range of interest in ocean acoustics a few hertz to megahertz however biot stoll s physics based seabed geo acoustic model predicts that the bottom attenuation should exhibit non linear frequency dependence particularly in sandy bottoms since the publication of previous papers zhou j acoust soc am 78 1003 1009 1985 kibblewhite ibid 86 716 738 1989 more low frequency field data collected from different coastal zones around the world have shown inconsistencies with the often used linear frequency dependence this paper attempts to support the non linear frequency dependence of sea bottom attenuation through a review of shallow water acoustic field measurements with a special emphasis on the 50 1000 hz frequency range the relevant measurements include bottom reflection loss sound transmission loss dispersion analysis vertical coherence of both propagation and reverberation normal mode spatial filtering and optimum frequency and transition range of sound propagation a non linear frequency dependence of equivalent bottom acoustic attenuation derived from these measurements will be introduced work supported by onr and nnsf of china

lg wave propagation in a laterally varying crust and the distribution of the apparent quality factor in central france the aim of this study is to evaluate the sensitix ity of lg waves to lateral changes of the earth s structure considering a simple model of uplift of the moho numerical simulations show that the geometrical attenuation of lg is not much affected by a smooth anomaly of the moho depth on the other hand the passing of the lg wave through the region of the moho uplift results in a clear deterioration of the wave shapes which confirms the occurrence of mode conversions the presence of an overlying sedimentary basin causes a local amplification of lg above the basin itself and the appearance behind the basin of a secondary surface wave guided in the sediments the effect of the basin on the lg wave is found to be reasonably taken into account by applying a local amplification function to the data we use a data set consisting of records of lg phases in france to test the conclusions of our numerical study the mapping of the apparent quality factor computed from lg at different frequencies confirms the interpretation of the strong attenuation of s waves around i hz in terms of scattering and shows the weak sensitivity of the amplitude of lg to smooth changes in the depth of the moho

studies of coda using array and three component processing the application of standard array processing techniques to the study of coda presents difficulties due to the design criteria of these techniques typically the techniques are designed to analyze isolated short arrivals with definite phase velocity and azimuth and have been useful in the frequency range around 1 hz coda is long in time and may contain waves of different types phase velocities and azimuths nonetheless it has proved possible to use or adapt array methods to answer two questions what types of waves are present in coda and where are they scattered most work has been carried out on teleseismicp coda work on local coda has lagged due to lack of suitable data and the difficulties of dealing with high frequencies the time domain methods of beamforming and vespagram analysis have shown that there is coherent energy with a high phase velocity comparable top orpp in teleseismicp coda these methods can detect this coherent coda because it has a fairly definite phase velocity and the same or close to azimuth as firstp orpp this component must consist ofp waves and is either scattered near the source or reflected in the mantle path as apdpp or precursorpp reflection the fourier transform method of the frequency wavenumber spectrum has been adapted by integrating around circles of constant phase velocity constant total wavenumber to produce the wavenumber spectrum which shows power as a function of wavenumber or phase velocity for teleseismicp coda wavenumber spectra demonstrate that there is a diffuse coda of shear lg or surface waves scattered from teleseismicp near the receiver wavenumber spectra also suggest that the coherent coda is produced by near source scattering in the crust not mantle reflection since it is absent or weak for deep focus events crustal earthquakes have a very strong coherent component of teleseismic coda suggesting scattering from shear to teleseismicp near the source three component analysis of single station data has shown the presence of off azimuth arrivals and may lead to the identification of waves scattered from a single scatterer

frequency dependent lg q within the continental united states frequency dependent crustal attenuation 1 q is determined for seven distinct physiographic tectonic regions of the continental united states using high quality lg waveforms recorded on broadband stations in the frequency band 0 5 to 16 hz lg attenuation is determined from time domain amplitude measurements in one octave frequency bands centered on the frequencies 0 75 1 0 3 0 6 0 and 12 0 hz modeling errors are determined using a delete j jackknife resampling technique the frequency dependent quality factor is modeled in the form of q q 0 f regions were initially selected based on tectonic provinces but were eventually limited and adjusted to maximize ray path coverage in each area earthquake data was recorded on several different networks and constrained to events occurring within the crust m b 3 5 in size a singular value decomposition inversion technique was applied to the data to simultaneously solve for source and receiver terms along with q for each region at specific frequencies the lowest crustal q was observed in northern and southern california where q is described by the functions q 152 37 f 0 72 0 16 and q 105 26 f 0 67 0 16 respectively the basin and range province pacific northwest and rocky mountain states also display lower q and a strong frequency dependence characterized by the functions q 200 40 f 0 68 0 12 q 152 49 f 0 76 0 18 and q 166 37 f 0 61 0 14 respectively in contrast in the central and northeast united states q functions are q 640 225 f 0 344 0 22 and q 650 143 f 0 36 0 14 respectively show a high crustal q and a weaker frequency dependence these results improve upon previous lg modeling by subdividing the united states into smaller distinct tectonic regions and using significantly more data that provide improved constraints on frequency dependent attenuation and errors a detailed attenuation map of the continental united states can provide significant input into hazard map mitigation both scattering and intrinsic attenuation mechanisms are likely to play a comparable role in the frequency range considered in the study

sequential inversion of modal data for sound attenuation in sediment at the new jersey shelf this paper presents a method for estimating bottom geoacoustic properties especially the sediment attenuation from information contained in normal modes of a broadband signal propagating modes are resolved using the time warping technique applied to signals from light bulb sound sources deployed at ranges of 5 and 7 km in the shallow water 06 experiment a sequential inversion approach is designed that uses specific features of the acoustic data that are highly sensitive to specific geoacoustic model parameters the first feature is the modal group speed which is inverted for seabed sound speed density and sediment thickness the second feature is the modal depth function for inverting receiver depths the third feature is related to the modal coefficient spectra and this is inverted for source depth and sediment attenuation in each subsequent stage estimates from the previous stage s are used as known values the sequential inversion is stable and generates estimates for the geoacoustic model parameters that agree very well with results from other experiments carried out in the same region notably the inversion obtains an estimated attenuation of 0 078 db in the band 120 180 hz for the de watered marine sediment characteristic of the continental shelf at the site

frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations campillo m and plantet j l 1991 frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations phys earth planet inter 67 48 64 we processed digital records from 431 earthquakes at 25 stations of the laboratoire de detection geophysique network in france we computed spectral amplitudes of pg and lg in different group velocity windows this large data set aliowed us to compute a map of mean crustal attenuation through simultaneous consideration of source amplitude site response and apparent q our inversion procedure combines an iterative reconstruction technique with an adjustment of source amplitudes and site responses at each step the stability of the result is tested with different subsets of data and the resolution is evaluated maps of apparent attenuation at frequencies between 1 5 and 10 hz were computed from pg lg and early coda of lg these results are in a good agreement with the predictions of the singie scattering model both for frequency dependence and for qs qp ratios the comparison between the attenuation anomaly found in the yariscan belt and results from a deep reflection seismic profile confirms the prominent part played by scattering in the apparent attenuation of seismic waves in the crust

frequency dependence of coda q part i numerical modeling and examples from peaceful nuclear explosions frequency dependent coda attenuation values are often reported however such measurements usually depend on the types of the q f models employed we use numerical modeling of peaceful nuclear explosion pne coda at far regional to teleseismic distances to compare two such models namely the conventional frequency dependent quality factor q coda f q f and frequency independent coda attenuation q c with geometrical attenuation the results favor strongly the q c model and illustrate the mechanisms leading to apparent q coda f dependencies tests for variations of the crustal velocity structures show that the values of are stable and related to lithospheric structural types and the inverted q c values can be systematically mapped into the true s wave attenuation factors within the crust modeling also shows that could increase in areas where relatively thin attenuating layers are present within the crust such areas could likely be related to younger and active tectonics by contrast when interpreted by using the traditional q approach the synthetic coda shows a strong and spurious frequency dependence with 0 5 which is also similar to the observations observed lg coda from two pnes located in different areas in russia show similar values of 0 75 10 2 sec 1 which are also remarkably close to the independent numerical predictions at the same time coda q c values vary strongly from 850 in the east european platform to 2500 within the siberian craton this suggests that parameters and q c could provide stable and transportable discriminants for differentiating between the lithospheric tectonic types and ages and also for seismic coda regionalization in nuclear test monitoring research

an investigation of the link between lead induced thermohaline convection and arctic eddies a recent laboratory study indicates that a turbulent buoyant line plume discharging into a rotating stratified fluid provides a natural mechanism for generating a series of anticyclonic geostrophic vortices bush and woods 1999 here we examine the implications of these experimental results in the context of the thermohaline convection which develops when saline water is released as leads freeze over in the polar oceans using the experimental results in conjunction with a simple numerical model of plume dynamics in a non uniformly stratified environment we develop a model which characterizes the geometry of the eddies which would develop owing to lead induced convection the model predicts that in the absence of strong currents lead induced thermohaline convection may generate anticyclonic geostrophic vortices of characteristic radius 2 10 km at the base of the mixed layer however this mechanism cannot account for the arctic eddies observed at substantial depths beneath the mixed layer cyclonic or strongly ageostrophic eddies

a frequency dependent relation of coda q c for koyna warna region india attenuation properties of the lithosphere around the koyna warna seismic zone is studied by estimating the coda q c from 30 local earthquakes of magnitude varying from 1 5 to 3 8 an average lapse time of 65 sec used in the single scattering model sampled a circular area with an average radius of 114 km the estimated q c values show a frequency dependent relation q c 169 f0 77 and range from 169 at i hz to 1565 at 18 hz a comparison of worldwide q studies reveals that for a large frequency range the q for active regions is low as compared to that for stable regions however south carolina and norway are exceptions in that their q is low in the low frequency range while new england and north iberia are exceptions as they have a q value similar to that for active regions like spain turkey italy and garhwal himalaya stih in the higher frequency range in contrast to this the q for the koyna warna area which belongs to a stable region is low in the entire frequency range as compared to the stable regions and similar to the active stih regions

low frequency sound speed and attenuation in sandy seabottom from long range broadband acoustic measurements a joint china u s underwater acoustics experiment was conducted in the yellow sea with a very flat bottom and a strong and sharp thermocline broadband explosive sources were deployed both above and below the thermocline along two radial lines up to 57 2 km and a quarter circle with a radius of 34 km two inversion schemes are used to obtain the seabottom sound speed one is based on extracting normal mode depth functions from the cross spectral density matrix the other is based on the best match between the calculated and measured modal arrival times for different frequencies the inverted seabottom sound speed is used as a constraint condition to extract the seabottom sound attenuation by three methods the first method involves measuring the attenuation coefficients of normal modes in the second method the seabottom sound attenuation is estimated by minimizing the difference between the theoretical and measured modal amplitude ratios the third method is based on finding the best match between the

simulation of topographic effects on seismic waves from shallow explosions near the north korean nuclear test site with emphasis on shear wave generation 1 we performed high resolution 8 hz three dimensional simulations of ground motions from shallow explosions in the presence of rough surface topography near the north korean nuclear test site to study elastic propagation effects with emphasis on theoretical aspects of shear wave generation interaction with rough topography causes significant p to rg scattering along the surface with amplification of high frequency 2 8 hz shear waves relative to the flat earth case shear waves of different polarizations are coupled by topographic scattering rg precursors composed of p to rg conversions traveling as surface waves have the spectral amplitudes comparable to the p wave while the rg phase has the low frequency 0 5 3 hz spectral shape of the rg from the flat case plus the high frequency 3 8 hz p wave spectra motions at near vertical takeoff angles corresponding to teleseismic propagation are increased or decreased indicating that waves are focused or defocused by topographic features above the source topographic roughness has a dramatic effect as short wavelength features 2 5 km are included higher frequencies are amplified by topography including frequencies corresponding to wavelengths shorter than the shortest topographic scale length overall topography enhances energy propagating along the surface near the source amplifies surface waves and tends to balance sv and sh polarized motions all of which impact shear wave observations used for nuclear explosion monitoring further simulation studies could elucidate how the wavefield emerging from a topographically rough area ultimately propagates to regional and or teleseismic distances

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

attenuation of regional phases in western europe abstract the purpose of this study is to evaluate the attenuation of local seismic phases in france we use the 25 short period vertical stations of the french seismic network and select a set of six earthquakes which occurred in france or in its vicinity and within a range of epicentral distances from 1 to 10 we have determined the attenuation versus distance for pn sn pg and lg phases and analyzed the influence of frequency attenuation coefficients for pn and sn waves are of the order of 2 they are slightly higher for pg 2 3 and lg 2 5 waves but the amplitude of these waves is largely dependent on the azimuth of propagation lg pg ratios present a large scatter between 1 and 10 with a mean trend around 3 to 4 we found that the lg anelastic attenuation coefficient is of the order of 0 2 deg 1 at 1 hz a result to be compared with the values of 0 07 deg 1 eastern united states 0 15 deg 1 northern ussr and 0 35 deg 1 south of the caspian sea obtained by nuttli 1973 1981 the quality factor computed for each one of these local phases shows a clear dependence on frequency increasing from around 100 at 1 hz to around 1000 at 16 hz

2 d image of seismic attenuation beneath the deep seismic sounding profile quartz russia we present a 2 d image of the upper mantle attenuation using nuclear explosion data from the ultra long refraction reflection profile quartz our analysis is based on a modified common spectrum technique followed by least squares inversion for q and iterative ray tracing in the velocity structure obtained earlier the resulting attenuation structure corroborates the earlier model for northern eurasia as well as our recent estimate based on the analysis of the long range p n phase and provides significantly more detail than the existing models the resulting upper mantle attenuation structure is characterised by q values ranging from 400 to 1800 down to the depths of 150 190 and probably 400 km the attenuation increases horizontally in se direction away from the baltic shield our model exhibits strong 2 d vertical and horizontal attenuation contrasts a high attenuation layer in the depth range of 120 150 to 160 180 km can apparently be associated with the presence of a partial melts within the base of the lithosphere

mesoscale eddies of the arctic ocean subsurface baroclinic eddies in the arctic ocean have a diameter of 10 20 km and are confined in depth between 50 and 300m during the arctic ice dynamics joint experiment aidjex in the central beaufort sea between march 1975 and may 1976 extensive information on eddy characteristics statistics and behavior was collected from four manned drifting ice camps during this 14 month period 146 eddies were encountered of which 19 were found to be repeated crossings making a total of 127 separate eddies observed during this period on the basis of the 1975 1976 aidjex data set it is concluded that eddies are 1 prevalent in the amerasia basin of the arctic ocean and in particular the beaufort sea where they may occupy up to a quarter of the available surface area 2 located in the depth range of 50 300 m although a few deeper eddies are also present 3 the second largest producers 32 of kinetic energy found within the upper 200 m of the beaufort sea and 4 predominantly anticyclonic in rotational tendency these eddies apparently originate north of point barrow alaska as a result of instability in the eastward flowing alaskan coastal current and transfer seasonally varying water properties from the chukchi sea and alaskan shelf into the arctic ocean they appear to translate in response to barotropic forcing over short time scales and over longer time periods move with the mean geostrophic field the season of formation for an eddy may be identified by the anomalous thermal properties still residing in its core

temporal variations of coda q an attenuation coefficient view abstract spatial and temporal variations of coda wave attenuation were identified in many studies and particularly in relation to major earthquakes and volcanic eruptions both the coda quality factor qc and its frequency dependence often change following such events which is often attributed to variations in the properties of large volumes of the subsurface however qc is also strongly sensitive to the assumed theoretical models which are usually insufficiently accurate for constraining the actual relationships between the geometrical spreading anelastic dissipation and scattering this inaccuracy often leads to significant exaggeration of attenuation effects and complicates the interpretation of temporal variations to resolve this problem this study uses a phenomenological approach based on the temporal attenuation coefficient instead of qc the attenuation coefficient often linearly depends on frequency f with intercept 0 related to the geometrical spreading and slope giving the effective quality factor qe as d df q e 1 two published examples of temporal variations of local earthquake coda are revisited a non volcanic stone canyon in central california and volcanic area mount st helens washington in both cases linear f patterns are found with the effects of on coda decay rates being significantly stronger than those of q e 1 at stone canyon ranged from 0 035 to 0 06 s 1 and qe varied from 3000 to 10 000 with increasing and qe decreasing during the winter season at mount st helens remained constant at 0 18 s 1 and qe changed from 400 before the eruption to 750 after it the observed temporal variations are explained by the near surface changes caused by seasonal variations in the non volcanic case and gas magma and geothermal system related in the volcanic case scattering attenuation does not appear to be a significant factor in these areas or otherwise it may be indistinguishable due to its fundamental trade off with the background structure and anelastic attenuation in the data

frequency dependence of q in the mantle underlying the shield areas of eurasia part iii the q model summary a large set of teleseismic body wave data covering a broad range in frequency was analysed to determine the frequency arid depth dependence of q for p and s waves under the northern shield areas of eurasia based on numerous f estimates for p and s waves covering the seismic band between 0 02 and 8 hz a q model of the eurasian shield was constructed the data require a model in which q increases with frequency and which is characterized by q values in the upper mantle that are generally higher than those of global average models the model with the best fit includes a minimum in q between about 100 and 200 km depth and high q values on the order of thousands throughout most of the deeper mantle these results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth preliminary results suggest that t under tectonic regions is higher than t under shield regions for all frequencies over the 0 02 to 8 hz frequency range suggesting that q varies regionally as well as with frequency and depth

on the nature of the teleseismic pn phase observed on the ultralong range profile quartz russia the most prominent secondary phase observed in the records from the ultralong profile quartz crossing northern eurasia is the high frequency long range teleseismic pn this phase propagating with the group velocity of about 8 1 to 8 2 km sec to over 3000 km distances dominates the records within the frequency range above about 5 hz within this teleseismic pn we distinguish onsets of several branches having higher apparent velocities between 8 5 and 8 6 km sec using travel time modeling techniques we associate the teleseismic pn with whispering gallery modes traveling within the top 160 km of the mantle the long incoherent coda of this phase results from scattering and from reverberations of seismic waves within the crust the contrast in frequency contents between the teleseismic pn and deeper refracted and reflected phases is explained by the increase of inelastic attenuation within the prominent low velocity zone lvz below the depth of about 150 km

attenuation of seismic shear wave energy in switzerland summary modelling the attenuation of shear wave energy is an important component of seismic hazard analysis previous studies have shown how attenuation particularly in the uppermost layers of the crust is regionally dependent the impact of this is that the decay of energy radiating from an earthquake will vary from place to place to quantify the regional attenuation in switzerland we model the fourier spectral amplitude of small to moderate earthquakes recorded on the local seismic networks high frequency decay is parametrized by q and while apparent geometrical spreading models account for the frequency independent decay of energy we analyse ground motion encompassing the significant duration of shaking to provide models that are useful for the purpose of seismic hazard analysis two methods are used to estimate the whole path attenuation parameter t first a simultaneous fit of the source model and attenuation effects across the entire spectral bandwidth for earthquakes with m 2 and secondly a linear fit of an attenuation model to the high frequency part of the spectrum for earthquakes with m 3 5 the t parameter is found to vary with hypocentral distance consistent with a weakly attenuating crust and strongly attenuating uppermost layer 1 d tomographic inversions indicate a profile of increasing q with depth down to the moho frequency independent decay is parametrized using a three part model which allows for the inclusion of moho reflection phases in the spectrum in the range of 20 140 km in the swiss foreland and from 70 to 140 km in the swiss alps

estimating marine sediment attenuation at low frequency with a vertical line array this paper presents a method of determining the compressional wave attenuation in marine sediment from a short range measurement the data were collected on a vertical line array at a range of 230 m during the shallow water 2006 experiments the sediment attenuation is extracted from the signal strength ratio of the sea bottom reflection to a sub bottom reflection at different frequencies from 1 75 to 3 15 khz linear frequency dependence of the attenuation is found from the estimation the sediment attenuation estimate is lower than the values estimated from the inversions of acoustic field data previously done in the vicinity

scattering and attenuation abstract an extensive existence of strong small scale heterogeneity in the earth is suggested by the great variability of direct waves and remarkable stability of scattered waves both in the case of teleseismic p coda and in the case of local earthquake s coda quantitative studies of small scoale heterogeneities have been made by the following three methods a lower bound of heterogeneity in the lithosphere has been estimated by the deterministic or stochastic modeling of travel time and amplitude fluctuation of teleseismic p waves across a large aperture seismic array an upper bound of heterogeneity has been estimated by modeling the attenuation of s waves as a scattering loss due to heterogeneity these two approaches suggest the existence of heterogeneity with rms velocity fluctuations from 3 to 10 per cent the third method is based on the estimation of heterogeneity from the amplitude of coda waves we found a significant discrepancy between the result obtained by the third method and those from the first two and attribute the discrepancy to the assumption of a single scattering model adopted in the coda generation theory the need for including multiple scattering as well as for considering vector elastic waves rather than scalar waves is emphasized

lead convection measured with an autonomous underwater vehicle the winter lead experiment leadex was conducted during 1992 in the beaufort sea there the autonomous conductivity temperature vehicle actv was operated under and around leads these are the first measurements of this kind measurements of temperature salinity and turbulence parameters were also made at lead edges with sensors fixed to rigid masts by combining information from the fixed measurements and the actv an inertial dissipation method is used to derive horizontal profiles of salt flux from actv salinity measurements comparisons also show that turbulent vertical velocity perturbations can be estimated from the vertical motion of the actv with these velocity estimates horizontal profiles of the turbulent fluxes of heat w t and salt w s are also computed directly data from a wide 1000 m wide lead with rapid ice motion indicate stronger turbulence under the lead than downstream but the character of the flow is that of a convective boundary layer modified by a change in surface buoyancy flux the convection is characterized by multiple intermittent plumes that scale in size with the mixed layer depth at a medium sized 100 m wide lead with slower ice motion the convective circulation set up by the lead is more apparent convective plumes are stronger and more prevalent at the lead edges particularly the downstream edge the comparison between the horizontal profiles from the actv and time series from fixed sensors suggests that lead convection is a combination of a near surface convective boundary layer and a deeper quasi stationary convection pattern

elastic character of seismic coda envelopes within east indian shield seismic codas are usually characterized by the values of coda q qc however interpretation of this quantity is often tricky because of its frequency dependence and acute sensitivity to subjective theoretical assumptions and processing parameters here a simpler and physically more consistent parameterization of coda envelopes is proposed by noting that their temporal decay rates are often nearly frequency independent this weak frequency dependence shows that codas mostly consist of elastic reverberations and scattering on larger scale structures and the subwavelength scale scattering and q type wave attenuation are weak a recent study of the eastern indian shield by singh et al in this journal gives an illustration of such elastic coda from that study the inferred qc steeply increases with frequency lapse times window lengths and distances from the seismic station however we show that all of these dependencies of qc represent a common artifact of the acquisition geometry and inversion procedure in an alternate interpretation we explain the same coda envelopes by two frequency independent properties of the earth s subsurface geometrical attenuation denoted c earth and effective q denoted qc earth based on these parameters the model becomes independent of theoretical assumptions and comparable to other areas and the acquisition inversion artifact is reduced the estimated c earth is above 0 01 s 1 which is also found in other areas of active tectonics the effective attenuation is weak qc earth 5700 likely below the measurable level which is typical for stable tectonic areas the data indicate near surface resonances beneath the recording station effects of these resonances on coda envelopes also exceed those of q type attenuation thus in the eastern indian shield and likely many other areas coda envelopes are principally controlled by elastic structures such as crustal and near surface layering and not necessarily by the s wave q and uniformly distributed random small scale scattering as commonly thought

characterizing the eddy field in the arctic ocean halocline ice tethered profilers itp deployed in the arctic ocean between 2004 and 2013 have provided detailed temperature and salinity measurements of an assortment of halocline eddies a total of 127 mesoscale eddies have been detected 95 of which were anticyclones the majority of which had anomalously cold cores these cold core anticyclonic eddies were observed in the beaufort gyre region canadian water eddies and the vicinity of the transpolar drift stream eurasian water eddies an arctic wide calculation of the first baroclinic rossby deformation radius rd has been made using itp data coupled with climatology rd 13 km in the canadian water and 8 km in the eurasian water the observed eddies are found to have scales comparable to rd halocline eddies are in cyclogeostrophic balance and can be described by a rankine vortex with maximum azimuthal speeds between 0 05 and 0 4 m s the relationship between radius and thickness for the eddies is consistent with adjustment to the ambient stratification eddies may be divided into four groups each characterized by distinct core depths and core temperature and salinity properties suggesting multiple source regions and enabling speculation of varying formation mechanisms

attenuation of lg waves in the new madrid seismic zone of the central united states using the coda normalization method abstract unique properties of coda waves are employed to evaluate the frequency dependent quality factor of lg waves using the coda normalization method in the new madrid seismic zone of the central united states instrument and site responses are eliminated and source functions are isolated to construct the inversion problem for this purpose we used 121 seismograms from 37 events with moment magnitudes m ranging from 2 5 to 5 2 and hypocentral distances from 120 to 440 km recorded by 11 broadband stations a singular value decomposition svd algorithm is used to extract q values from the data while the geometric spreading exponent is assumed to be a constant inversion results are then fitted with a power law equation from 3 to 12 hz to derive the frequency dependent quality factor function the final results of the analysis are q v lg f 410 38 f 0 49 0 05 for the vertical component and q h lg f 390 26 f 0 56 0 04 for the horizontal component where the term after sign represents one standard error for stations within the mississippi embayment with an average sediment depth of 1 km around the memphis metropolitan area estimation of quality factor using the coda normalization method is not well constrained at low frequencies f

yield estimates of nevada test site explosions obtained from seismic lg waves a methodology is presented for determining the yield of underground nuclear explosions from lg wave amplitudes the methodology is applied to nevada test site nts explosions for which the data from short period vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water saturated rock source conditions the latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the united states and in the french sahara suggesting that they may be applicable to all continental areas if so they also can provide an estimate of the bias of mb p magnitudes between different continental sites for example the lg data from nts explosions indicate a 0 31 0 02 magnitude unit bias between nts and eastern north america similar to the approximately 0 33 unit bias found between western and eastern north america previously by use of earthquake data

modeling some empirical vertical component lg relations abstract higher mode surface wave theory is used to model the vertical component lg wave observed in eastern north america at regional distances tests of the model are made to determine whether it is capable of describing empirical spectral scaling laws spatial attenuation and peak time domain lg amplitudes it is found that a simple crustal model and a rough estimate of crustal qb are all that are required to accomplish this good results are obtained if the average crustal qp is equated to the coda q of the same frequency introduction the lg wave is a wave observed on short period seismograms of local and regional seismic events which travels with a group velocity of approximately 3 5 km sec in the central united states it is usually the largest arrival on the seismogram which makes it suitable for defining a magnitude scale nuttli 1973 herrmann and nuttli 1982 as part of developing the magnitude scale there have been many studies on the spatial attenuation of the lg wave nuttli 1973 street 1976 bollinger 1979 singh and herrmann 1982 the lg wave is usually thought of as a superposition of higher mode surface waves propagating within the crustal wave guide herrmann and nuttli 1975a b attempted to deterministically model observed lg amplitudes in terms of focal mechanism focal depth and seismic moment using surface wave techniques they were limited by their inability to compute surface wave eigenfunctions at frequencies greater than 0 5 hz however their results were encouraging harkrider 1979 drawing upon the work of apsel 1979 discovered a way to compute surface wave energy integrals of the higher modes very precisely bache

p wave mantle velocity structure beneath northern eurasia from long range recordings along the profile quartz high resolution seismic sounding of the mantle using two peaceful nuclear explosions pnes along the 3950 km long quartz profile in northern eurasia reveal major unexpected vertical and lateral mantle inhomogeneities in contrast to standard seismological earth models such as prem and iasp91 three rather than two mantle discontinuities are found between 400 and 700 km depth they are located at about 420 550 and 670 km depth lateral variations in depths amount to 10 20 km and associated p wave velocities vary by 0 1 km s 1 in the upper mantle the uppermost layer through which pn propagates extends down to about 100 km depth the mantle below this depth but above the 410 km discontinuity is characterized by alternating high and low p wave velocities in the depth range between 100 and 200 km the velocity fluctuates between 8 3 and 8 7 km s 1 and shows variations from shotpoint to shotpoint average velocities are clearly higher than standard seismological reference models this part of the mantle overlies a low velocity zone between about 200 and 300 km depth this channel also displays substantial lateral variation in depth which could be interpreted as a laterally variable asthenosphere beneath the west siberian basin the asthenosphere seems to be well established whereas it is only poorly defined under the uralian belt and the tectonic units to the northwest with the addition of more data from this and other profiles future aims include two dimensional and three dimensional tomographic modelling as well as modelling of petrological composition of the mantle down to 850 km depth beneath northern eurasia

seismic attenuation above 10 hz in southern california from coda waves recorded in the cajon pass borehole we estimate the attenuation around cajon pass southern california for frequencies above 10 hz and we find total q to exhibit only weak frequency dependence ranging from 800 at 10 hz to 1500 at 100 hz the intrinsic attenuation is approximately twice the level of the scattering attenuation measurements are made using earthquake seismograms recorded at 0 3 km depth using the multiple lapse time window method the results are not dependent on receiver depth and are consistent with previous estimates of q made from direct waves recorded at 2 5 km depth our q values are therefore thought to be representative of the seismogenic crust and the technique used is uncontaminated by the highly attenuating near surface at least above 10 hz we also calculate q from surface data between 1 and 10 hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at 10 hz q exhibits strong frequency dependence below 10 hz 1 8 consistent with previous studies in active tectonic regions and only weak frequency dependence at higher frequencies 0 34 this change in behavior renders it unwise to extrapolate q measurements outside the frequency range from which they were derived for example in earthquake source studies possible factors responsible for the apparent change in frequency dependence of q are considered further work is required however to resolve the causes of this change

on the attenuation of lg waves in western and central asia and their use as a discriminant between earthquakes and explosions data from earthquakes and explosions in the western and central parts of the soviet union are used to determine the values of the coefficient of anelastic attenuation of 1 sec period lg waves from the russian uplands region and for the region from the caspian sea to the southern perimeter of the soviet union for the former region the value is approximately 0 15 deg 1 and for the latter 0 35 deg 1 using the empirical veith and clawson 1972 curve for the amplitude of p waves at regional distances and theoretical curves for the attenuation of higher mode surface waves for lg curves are constructed which show the variation of the amplitude ratio of lg to p with epicentral distance with the coefficient of anelastic attenuation as a parameter when data from explosions and earthquakes in the soviet union are compared with the calculated curves the amplitude ratios of lg and p on the average are larger for earthquakes than explosions however there is much overlap so that this ratio by itself does not appear to be a useful discriminant between earthquakes and explosions

the oceanography of winter leads leads in pack ice have long been considered important to the thermodynamics of the polar regions a winter lead affects the ocean around it because it is a density source as the surface freezes salt is rejected and forms more dense water which sinks under the lead this sets up a circulation with freshwater flowing in from the sides near the surface and dense water flowing away from the lead at the base of the mixed layer if the mixed layer is fully turbulent this pattern may not occur rather the salt rejected at the surface may simply mix into the surface boundary layer in either event the instability produced at the surface of leads is the primary source of unstable buoyancy flux and as such exerts a strong influence on the mixed layer here as many as possible of the disparate and almost anecdotal observations of lead oceanography are assembled and combined with theoretical arguments to predict the form and scale of oceanographic disturbances caused by winter leads the experimental data suggest the velocity disturbances associated with lead convection are about 1 5 cm s 1 these appear as jets near the surface and the base of the mixed layer when ice velocities across the lead are less than about 5 cm s 1 the salinity disturbances are about 0 01 to 0 05 psu scaling arguments suggest that the geostrophic currents set up by the lead density disturbances are also of the order of 1 5 cm s 1 the disturbances are most obvious when freezing is rapid and ice velocity is low because the salinity and velocity disturbances in the upper ocean are not smeared out by turbulence in this vein lead convection may be characterized at one extreme as free convection in which the density disturbance forces the circulation at the other extreme lead convection may be characterized as forced convection in which the density disturbance is mixed rapidly by boundary layer turbulence the lead number lo which is the ratio of the pressure term to the turbulence term in the momentum equation and the turbulent lead number lot which is the ratio of buoyant production to shear production in the turbulent kinetic energy equation define the boundary between the free and forced regimes for lo and lot less than one both the large scale circulation and the turbulence are forced by surface stress for lo and lot greater than one both the large scale circulation and the turbulence are forced by the buoyancy flux the magnitudes of velocity and salinity disturbances from a model developed elsewhere suitable to free convection agree with what few observations we have the results of a forced convection model developed here suggest salinity disturbances of the order of 0 01 0 02 practical salinity units with the maximum occurring at the surface of the lead and decreasing substantially below 5 10 m this unstable gradient is a unique characteristic of lead convection though the salinity disturbances may be small when ice velocities are large the buoyancy flux in leads has a major effect on the boundary layer turbulence

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

bayesian geoacoustic inversion in a dynamic shallow water environment this paper presents results for matched field bayesian geoacoustic inversion of multitonal continuous wave data collected on the new jersey continental shelf to account for effects of significant spatial and temporal variation of the water column sound speed the sound speed profile was represented by empirical orthogonal functions data error information for the inversion was estimated from multiple time windows of the data inversion results for the sediment sound speeds at three ranges are in excellent agreement with the ground truth

lg coda q and its relation to the structure and evolution of continents a global perspective tomographic maps of lg coda q q lg c variation are now available for nearly the entire african eurasian south american and australian continents as well as for the united states q lg c at 1 hz q 0 varies from less than 200 to more than 1000 and q lg c frequency dependence varies between 0 0 and nearly 1 0 q 0 appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region a plot of q 0 versus time since that activity indicates that a single q 0 time relation approximates most mean q 0 values those that deviate most from the trend lay in australia the arabian peninsula and the east african rift the increase in q 0 with time may be due to a continual increase in crustal shear wave q q caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity extrapolated values of q lg c at 5 hz using q 0 and values measured at 1 hz and assuming that is constant in all regions between 1 and 5 hz show a similar percentage wise increase with times that has elapsed since the most recent activity other factors that can reduce q 0 in continental regions include thick accumulations of sediment especially sandstone and shale of mesozoic age and younger severe velocity gradients at the crust mantle transition and possibly lateral variations in the depth thickness and severity of those gradients severe and large increases of q in the mid crust of some regions can cause relatively large values of even if the frequency dependence of q is small

the effect of crustal structure on the regional phases pg and lg at the nevada test site the effect of local structure on the characteristics of the regional phases pg and lg was investigated by analyzing events at epicentral distances less than 10 from the nevada test site the stations used are on markedly different geological structures three on granitic intrusions two on pahute mesa a feature overlaying a buried volcanic caldera and finally four on yucca flats a deep alluvial valley the crustal effect on amplitudes is quite significant for both lg and pg yucca flats gives amplitudes as much as a full order of magnitude higher than the stations on granite while pahute mesa stations are 0 7 0 8 magnitude units higher in amplitude than the granite sites besides gross amplitude effects the local crust also induces prolonged ringing at yucca flats resulting in the lengthening of the signal wave trains the observed site amplification effects show no clear dependence on the azimuths of arrivals indicating that the phenomenon is due to local structures and is not caused by lateral refraction of surface wave modes the observed amplification is similar in magnitude for the vertical component of both pg and lg thus not affecting the pg lg amplitude ratio a possible discriminant between earthquakes and explosions at regional distances finite difference theoretical simulations using two dimensional models of the yucca basin reproduce in a qualitative sense the ringing effect observed at the yucca flats any attempt at matching characteristics of pg and lg with synthetic seismograms computed for laterally homogeneous layered structures is apt to lead to false conclusions about the source and path properties unless the recording sites used are carefully chosen for small crustal effects

frequency dependent characteristics of coda wave quality factor in central and southcentral alaska coda wave quality factor q c was investigated by using digital data 100 sample sec 1 recorded by a vertical component short period station installed for this study the station was located in the greater fairbanks area in central alaska from several hundred earthquakes recorded by this station in about a year 27 earthquakes were selected for the above study 7 of these selected earthquakes were located along the alaska wadati benioffzone pacific plate the other 20 earthquakes were located in the area of intraplate seismicity north american plate the data was filtered using 9 pass bands with center frequency varying from 1 5 hz to 16 hz with octave bandwidth the values of q c obtained from the coda amplitude decay rates measured on the filtered data after corrections due to the recording instrument and source receiver separation show appreciable frequency dependence the value of q c varies in the range of 253 and 1190 corresponding to the frequency interval from 1 5 hz to 16 hz for the study area this variation is close to that reported by others for the kanto region of japan moreover the characteristics of q c obtained in the present case seem to be independent of epicentral distance and hypocentral depth

high frequency wave propagation in the uppermost mantle short period three component recordings of the seismic wave field of peaceful nuclear explosions recorded on deep seismic sounding profiles quartz and ruby i collected in 1984 and 1988 respectively in northern russia are used to constrain the nature of the high frequency teleseismic p n phase which can be observed for receiver distances of 3000 km we suggest that this phase is caused by velocity fluctuations in the upper mantle acting as scatterers to test this hypothesis and to determine the properties of the upper mantle scatterers the elastic reflectivity method one dimensional isotropic models was used to model the coda of the high frequency teleseismic p n phase both the observed data and the synthetic record sections were analyzed by examining the coda decay rates of the teleseismic p n phase at 5 hz synthetic seismograms are computed for different models based on the global model iasp91 with an added zone of randomly distributed velocity fluctuations lamellae just below the crust mantle boundary these models may be characterized by the thickness of the scattering layer l the vertical heterogeneity correlation length a and the heterogeneity standard deviation the numerical simulation of the wave propagation in these models generated a high frequency teleseismic p n phase with a well pronounced long coda a comparison of the modeling results with the observations shows clearly that a velocity model containing fluctuations in the upper mantle can easily explain the presence of a high frequency teleseismic p n comparing the coda decay rates of synthetic seismograms with the observation we tried to constrain the properties of the velocity fluctuations in the upper mantle l a and in the distance range of the uninterfered high frequency teleseismic p n phase beyond 1300 km the coda decay rates for our best fitting model are similar to the observed ones this model has a 75 km thick zone of scatterers below the moho containing lamellae with an average thickness of 2 km and a rms velocity perturbation of 5 the modeling results show also that two other possible models the whispering gallery phase along the crust mantle boundary and scattering in the lower crust taken alone are not able to explain the coda properties of the high frequency teleseismic p n phase

crustal reflections and the nature of regional p coda abstract f k spectra for regional p coda are presented for events recorded at the scandinavian noress finesa and arcess arrays and the new england nynex array the spectra are dominated by on azimuth energy with apparent velocities between pn or faster and lg following this analysis reflection coefficients calculated with an efficient boundary integral scheme are used to study the role irregular interfaces play in the creation of regional p coda we find that observed crustal scattering in these regions is strikingly consistent with p p and p sv scattering from the two dimensional 2d irregular moho and even more consistent with scattering from a 2d irregular near surface interface

propagation and attenuation characteristics of the crustal phase lg thelg wave consists of the superposition ofs waves supercritically reflected and thus trapped in the crust this mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed the numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models in regions with strong lateral variations the influence of large scale heterogeneities on thelg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure

observation of high frequency teleseismic p n on the long range quartz profile across northern eurasia short period three component recordings of the seismic wave field of peaceful nuclear explosions pnes on long range profiles are used to determine the fine structure of the mantle lithosphere by analyzing the frequency content of the recorded phases and applying different band pass filters to the data it is possible to divide the wave field into two distinctly different constituents low frequency body waves traveling along fermat paths first arrivals and the high frequency teleseismic or long range pn phase traveling with a group velocity of 8 1 km s and accompanied by a long incoherent coda this high frequency teleseismic pn phase is observable from about 750 km where it separates from the faster first arrival to the maximum recording distance of 3145 km it is recorded from shots at different locations and appears to be almost unaffected by the major tectonic feature along the profile the urals the frequency spectrum of this pn phase contains more high frequency energy up to 12 hz than first arrivals that penetrate deeper into the upper mantle the teleseismic high frequency pn arrival has a remarkable coda which is incoherent between closely spaced stations the coda duration is dependent on the component of motion being shortest on the vertical and longest on the transverse component we propose a velocity model that is characterized by a zone extending from the crust mantle boundary to a depth of about 100 km this zone has randomly distributed spatially anisotropic velocity fluctuations we propose that these velocity heterogeneities are stretched in the horizontal direction this zone forms a scattering waveguide that confines the high frequency teleseismic pn there are indications that below this pn waveguide either the scale of the velocity fluctuations or the q factor changes this is expressed in a separation of the teleseismic pn phase from the phase diving deeper into the mantle lithosphere

brine driven eddies under sea ice leads and their impact on the arctic ocean mixed layer abstract eddy generation induced by a line shaped salt flux under a sea ice lead and associated salt transport are investigated using a three dimensional numerical model the model is designed to represent a typical condition for the wintertime arctic ocean mixed layer where new ice formation within leads is known to be one of the primary sources of dense water the result shows that along lead baroclinic jets generate anticyclonic eddies at the base of the mixed layer and almost all the lead originated salt is contained inside these eddies these eddies survive for over a month after closing of the lead and transport the lead originated salt laterally consequently the lead origin salt settles only on the top of the halocline and is not used for increasing salinity of the mixed layer sensitivity experiments suggest that the horizontal scale of generated eddies depends only on the surface forcing and is proportional to the cube root of the total amount of salt input this scaling of eddy size is consi

ground variation of radon 222 for location of hidden structural features example of the south of france alpes maritimes we report two examples from the south of the french alps showing that radon emanation monitored by alpha sensitive film may be used to locate certain discrete structural features revealed in data collected by remote sensing from a satellite the variations observed in our data over a period of several months are in accordance with atmospheric changes and might correlate with local seismic activity when the detectors are located directly above structural fractures and the magnitude of the seismic event is greater than 2

strong crustal heterogeneity in the bolivian altiplano as suggested by attenuation of lg waves using the seismological data of four temporary networks installed across the andes of bolivia and chile between 1990 and 1995 we study the amplitudes of the lg phase for 45 regional events located at crustal depths this analysis reveals that lg is strongly attenuated for most of the ray paths crossing the altiplano independent of the azimuth of propagation to map the anomalies and evaluate their frequency dependence we conduct a regionalization of the crustal average qs factor using a damped least squares inversion technique in the frequency range 0 6 5 hz the resulting maps delineate regions with contrasting values of apparent attenuation the altiplano is characterized by strong attenuation with an average qs of 100 at 1 hz whereas the cordilleras are regions of weaker attenuation the frequency dependence of qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption within the hypothesis of a weak frequency dependence of the intrinsic q we show that at 1 hz scattering plays a major part in the attenuation of lg waves whereas at 4 hz both mechanisms are of equal importance these results indicate that the attenuation of lg waves in the altiplano is due more to scattering by small scale heterogeneities than to a high percentage 1 of partial melt involving a large part of the crust

a summary of attenuation measurements from borehole recordings of earthquakes the 10 hz transition problem earthquake seismograms recorded by instruments in deep boreholes have low levels of background noise and wide signal bandwidth they have been used to extend our knowledge of crustal attenuation both in the near surface and at seismogenic depths site effects are of major importance to seismic hazard estimation and the comparison of surface shallow and deep recordings allows direct determination of the attenuation in the near surface all studies to date have found that q is very low in the near surface 10 in the upper 100 m and increases rapidly with depth unlike site amplification attenuation at shallow depths exhibits little dependence on rock type these observations are consistent with the opening of fractures under decreasing lithostatic pressure being the principal cause of the severe near surface attenuation seismograms recorded in deep boreholes are relatively unaffected by near surface effects and thus can be used to measure crustal attenuation to higher frequencies 100 hz than surface recordings studies using both direct and coda waves recorded at over 2 km depth find q to be high 1000 at seismogenic depths in california increasing only weakly with frequency between 10 and 100 hz intrinsic attenuation appears to be the dominant mechanism these observations contrast with those of the rapidly increasing q with frequency determined from surface studies in the frequency range 1 to 10 hz further work is necessary to constrain the factors responsible for this apparent change in the frequency dependence of q but it is clearly unwise to extrapolate q estimates made below about 10 hz to higher frequencies

attenuation of high frequency shear waves in the crust measurements from new york state south africa and southern california we compare the attenuation of high frequency 3 30 hz shear waves for crustal paths in new york state south africa and southern california over source receiver distances of about 10 400 km the data consist of digital recordings of s waves 5 100 km and lg waves 100 400 km produced by earthquakes we use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the s and lg waves over the entire distance range studied 10 400 km the amplitude decay of 3 hz shear wave energy is considerably less for the tectonicaily stable areas of new york and south africa than for the tectonicaily active region of southern california high frequency 30 hz s wave attenuation is significantly less for new york and south africa than for southern california for distances between 15 and 90 km we parameterize the decay with distance r of coda normalized shear wave amplitudes with a frequency independent q and geometrical spreading exponent where geometrical spreading is proportional to r for new york state the s wave amplitude decay 3 30 hz is well described by a frequency independent q of 2100 330 490 and of 1 3 0 1 the decay of lg wave amplitudes from 3 to 15 hz in the new york state region is fit with a frequency independent q of 1600 280 330 of 0 70 0 2 the s wave amplitudes 3 30hz in south africa yield a q of 1500 190 380 and of 1 3 0 1 fixing the geometrical spreading at r 0 5 produces an lg wave q estimate at 3 hz in south africa of 360 50 80 this lg wave q is low considering that south africa is a cratonic tectonicaily stable area the s wave amplitudes from southern california are described with a frequency independent q of 800 150 240 and a large geometrical spreading exponent of of 1 9 0 2 we find an lg wave q at 3 hz of 260 30 for southern california after constraining the geometrical spreading at r 0 5

quantifying the uncertainty of geoacoustic parameter estimates for the new jersey shelf by inverting air gun data this paper describes geoacoustic inversion of low frequency air gun data acquired during an experiment on the new jersey shelf hybrid optimization and bayesian inversion techniques based on matched field processing were applied to multiple shots from three air gun data sets recorded by a vertical line array in a long range shallow water geometry for the bayesian inversions full data error covariance matrix was estimated from a set of consecutive shots that had high temporal coherence and small spatial variation in source position the effect of different data error information on the geoacoustic parameter uncertainty estimates was investigated by using the full data error covariance matrix a diagonalized version of the full error covariance and a diagonal matrix with identical variances the comparison demonstrated that inversion using the full data error information provided the most reliable parameter uncertainty estimates the inversions were highly sensitive to the near sea floor geoacoustic parameters including sediment attenuation of a simple single layer geoacoustic model the estimated parameter values of the model were consistent with depth averaged values over wavelength scales of a high resolution geoacoustic model developed from extensive ground truth information the interpretation of the frequency dependence of the estimated attenuation is also discussed

heterogeneity of the uppermost mantle beneath russian eurasia from the ultra long range profile quartz the 3850 km long deep seismic sounding profile quartz crosses six major geologic provinces in eurasia and is sourced by 3 nuclear and 48 chemical explosions we present the first interpretation of the entire data set using two dimensional 2 d ray tracing and inversion resolution analysis and 1 d amplitude modeling our interpretation shows a 42 km thick high velocity crust under the baltic shield a 29 km thick crust and high velocity upper mantle under the mezenskaya depression 52 km thick crust with high velocity lower crust and uppermost mantle under the urals and 40 km thick crust under the west siberian basin deepening to 45 km under the altay sayan fold belt high velocity 8 4 km s uppermost mantle is found under the mezenskaya depression and under the east flank of the urals one almost continuous upper mantle boundary occurs at 65 to 80 km depth and another with an approximately 40 km thick lvz occurs at 120 to 140 km depth the shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity resolution analysis based on direct multivariate model perturbations artificial neural network and principal component analysis indicate the depth uncertainty of the 410 km discontinuity within 6 km and also its trade off with dip and velocities above and below the discontinuity decreased near critical amplitudes of reflections from the 410 km and 660 km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15 20 km thick lithosphere thins asthenospheric velocity decreases and the 410 km discontinuity dips to the se approaching the himalayan orogenic belt

seabed acoustics of a sand ridge on the new jersey continental shelf acoustic measurements were made on a sand ridge on the new jersey continental shelf data collected on two l arrays separated by 20km from a single multi frequency tow suggest small horizontal environmental variability values for the sound speed structure of the seabed are extracted by first applying a geo acoustic inversion method to broadband and narrowband acoustic data from short range sources then a parabolic equation algorithm is used to properly include the bathymetry and sub bottom layering finally the frequency dependence of the seabed attenuation is inferred by optimizing the model fit to long range transmission loss data in the 50 3000hz band

multi mode analysis of rayleigh type lg part 2 application to southern california and the northwestern sierra nevada the uc diagram technique described in the companion paper part 1 is applied to nine sets of lg phases recorded through the cedar system in southern california and two sets of lg phases recorded along the northwestern margin of the sierra nevada a clear image of the signal is obtained in time frequency wavenumber space and we discuss in particular observations at 2 5 sec period for events 200 to 300 km outside the profiles from the gross features of uc diagrams we conclude that a representation of lg as a single coherent multi mode wave train is oversimplified in the case of southern california but is more appropriate for the sierra block in southern california peaks observed at group velocities smaller than 3 2 km sec are not predicted by realistic crustal models of the area and are probably due to lateral heterogeneities effects such as mode conversion and multipathing on the other hand for group velocities between 3 2 and 3 6 km sec peaks observed in either area can generally be interpreted in terms of overtones excited at the source and propagating through spatially averaged structures although care must be taken to monitor the stability of the algorithm on actual short period records

measurement of frequency dependent lg attenuation in the great basin measurement of the spatial decay of spectral amplitudes of the higher mode seismic surface wave train lg indicates that in the great basin the apparent seismic quality factor q is a function of frequency observations along a 300 km profile recording of a nuclear explosion yield the function q 206 0 68 over the band 0 3 10 0 hz similar analysis using numerous recordings of earthquakes along paths which provide a good average sampling of the great basin crust gives q 214 15 0 54 0 09 for 0 3 5 0 hz if the crustal sampling by lg energy from nuclear explosions surface sources is primarily in the shallow crust as has been suggested then these results indicate a greater frequency dependence of apparent q there than at depth

moment energy stress drop and source spectra of western united states earthquakes from regional coda envelopes we present a new method to estimate stable seismic source parameters such as energy moment and orowan stress drop using regional coda envelopes from as few as one broadband station we use the method to compute path and site corrected seismic moment rate spectra for 117 recent western united states earthquakes empirical green s function corrections were applied to our surface and body wave coda envelope measurements to generate s wave source spectra these source spectra provide stable single station estimates of radiated seismic energy es and seismic moment mo that for common events are in excellent agreement with network averaged estimates obtained using local and regional data teleseismic moment estimates are compatible with our regional results but teleseismic energy estimates appear to be nearly an order of magnitude low we estimated the seismic moment of events ranging between mw 2 2 and 7 3 and energy estimates for which we had measured at least 70 of the total energy generally events above mw 3 3 we use these estimates to examine the behavior of derived parameters such as the orowan stress drop 2 es mo while the earthquakes we studied have a small range in orowan stress drop generally between 0 1 and 20 mpa they show a strong tendency for orowan stress drop to increase with moment approximately as mo0 25 we believe this is a source effect and is not due to inadequate bandwidth or attenuation correction and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern california many of the large high stress drop earthquakes show complexity in their moment rate spectra near the corner frequency and cannot be fit by a simple square model instead above the first corner frequency the spectral decay ranges between f 1 0 and f 1 5 this leads to larger estimates of radiated energy than predicted with a simple square model and has implications for seismic hazard estimation coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy 1 coda amplitudes vary little with geology and source radiation anisotropy and allow accurate single station applications 2 path corrected coda amplitude measurements can be applied to very large regions allowing a comparison of source parameters throughout the western united states using a common methodology and stations 3 because long period coda can last for hours for large local and regional events it allows the analysis of seismograms with clipped early arrivals

regional phases in continental and oceanic environments summary the very different crustal thicknesses between continental and oceanic crust lead to the establishment of distinct styles of regional seismic wavefield observations and numerical simulations of events near subduction zones in japan allow a direct comparison of seismic wave propagation in oceanic and continental structures the thick zone of lower seismic velocities in the continental crust acts to trap s wave energy in particular and generates crustally guided phases such as pg lg in the oceanic environment there is no efficient s wave trapping and crustal energy progressively leaks into the mantle reverberations in the crust and the water above help to sustain the amplitude of the mantle arrivals pn sn to large distances

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

modeling short period crustal phases p lg for long range refraction profiles abstract the short period seismic phases known as p and lg are often recorded at distances of 200 1000 km on long range refraction profiles and are usually the largest amplitude features on record sections for this distance range p and lg propagate as multiply reflected compressional and shear waves in a crustal waveguide whose principal boundaries are the moho and the free surface equivalently they can be interpreted as the interference pattern produced by a superposition of higher mode p sv and sh waves propagating in a leaky waveguide for compressional waves the waveguide efficiency is a strong function of frequency and depends on the presence or absence of low velocity layers within a few kilometers of the surface such as deep sedimentary sections commonly found in active tectonic areas such low velocity surface layers create constructive interference effects for upcoming p waves incident at near grazing angles at the free surface and lead to efficient p propagation several good examples of strong p phases can be found on long range refraction profiles for the tectonically active western united states the 550 km profile eastward from shoal to delta ut is analyzed here we have used a modified reflectivity method computer program to model crustal phases for the shoal delta profile the reflectivity technique accounts for all body and surface waves contributing to the short period seismograms it is found that the synthetic waveforms realistically model the observed p characteristics in this case the decay of p amplitudes with distance appears to be dominated by surface reflection leakage from the waveguide rather than by anelastic attenuation due to q of crustal rocks

frequency dependent attenuation in the crust beneath central france from lg waves data analysis and numerical modeling abstract the quality factor of shear waves in the crust beneath central france is evaluated by the combined use of data analysis and theoretical modeling the measurement is made from lg wave records of 18 earthquakes at 22 short period vertical seismic stations of the french seismic network the geometrical attenuation of lg waves has been calculated through a numerical simulation in a perfectly elastic model the station responses including site effects have been evaluated from lg waves and from records of the seismic noise q has been calculated for a set of 11 frequencies between 0 5 and 10 hz and has been found to be in the form q 290 f 0 52 where f is the frequency the numerical simulation leads to the interpretation of q of lg waves as the mean value of q within the crust theoretical and experimental decay of energy with distance are in very good agreement the lg waves present a different sensitivity to a low q zone whether this zone is associated with the sediments or with scattering in the lower crust the lower crust beneath central france appears from our experimental results to be associated with high values of q

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

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

statistical analysis of sound transmission results obtained on the new jersey continental shelf experiments have been conducted near the site of amcor borehole 6010 on the new jersey shelf to evaluate propagation predictability in sandy shallow water environments the influence of a nonlinear frequency dependence of the sediment volume attenuation in the uppermost sediment layer at this location is examined previously it was determined that a frequency power law exponent of 1 5 was required for the best modeling of experimental results over the band 50 1000hz the approach here references the attenuation to an accepted value at 1khz and makes extensive comparisons between measurements and calculations to determine a power law exponent of 1 85 0 15

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

scales of structure in the lithosphere images of processes advances in deep seismic sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust mantle transition in this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near vertical reflection and wide angle refraction experiments the abrupt termination of near vertical reflections at the moho and the coincident presence of a strong coda accompanying the supercritical pmp reflection phase in record sections of continental seismic wide angle refraction experiments reveal a new property of the crust mantle boundary the moho forms a sandwiched mix of crust mantle material between lower crust and topmost mantle in combination with a step in mean velocity a significant change in scale of the structural dimensions and of velocity variance this view is strongly supported by the recognition of the subcrustal lithosphere as a high frequency 5 10 hz scattering wave guide for long distance propagation of the pn phase over more than 3000 km detected during seismic experiments in russia using peaceful nuclear explosions evidence is brought forward that this high frequency wave guide is likely to be present also in the phanerozoic upper mantle of western europe it is furthermore hypothesized that the global propagation of teleseismic sn or corresponding pn waves observed on continents and oceans combined with the recording of strong pmp codas for wave paths on continents points to a world wide change of scale of structure and velocities near the moho a change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the moho the global presence of the scattering pn wave guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle locations of present day tectonic activity where horizontal decoupling between crust and mantle are likely to occur e g subduction at continent continent collision zones change of tectonic regimes and continental transform faults are discussed

modelling short period crustal phases at regional distances for the seismic source parameter inversion abstract waveforms of short period crustal phases observed at regional distances are modelled in an attempt to retrieve the source parameters of small earthquakes for a simple shield type crustal structure the synthetic seismograms calculated for elementary source mechanisms show that the waveforms of crustal phases are quite distinct for different source mechanisms and suggest that these features can be used to constrain the source mechanisms waveforms of the crustal phases are also strongly affected even by small variations in the crustal thickness as well as in the focal depth a generalized linearized waveform inversion technique is used to invert the regional crustal phases for a deviatoric moment tensor source the inversion technique is applied to regional short period records from a small earthquake in central sweden for determining its source parameters the waveform inversion which mainly used lg waves produced a satisfactory fit between the observed and synthetic records suggesting that the source mechanism and focal depth estimates were reasonable the focal depth of 40 km obtained for the event studied suggests the occurrence of a deep crustal earthquake and demonstrates the usefulness of crustal phases in studying small intraplate earthquakes

lateral variations of high frequency seismic wave propagation at regional distances across the turkish and iranian plateaus in this study we investigate crustal and uppermost mantle physical properties that characterize some of the continental plateaus of the middle east this is done as part of a larger effort to map and compare high frequency wave propagation at regional distances across the earth s continental plateaus thousands of short period wwssn seismograms recorded at stations located in the middle east and produced by earthquakes with epicentral distances less than about 20 were examined visually in an effort to study lateral variations of high frequency 0 5 2 hz seismic wave propagation across this area particularly to the north of the zone of continental collision between the africa arabian and eurasian plates variations of frequencies and amplitudes of sn and lg relative to p are examined and mapped throughout the region and this work is supplemented by a study of velocities of pn sn and lg sn amplitude variations are very striking in this area an important observation of this study is that sn propagates efficiently beneath a major part of the turkish and iranian plateaus sn is strongly attenuated however in the northernmost portion of the plateaus south of the black and caspian seas and in an area between the two seas these regions are characterized in general by active tectonism including volcanism faulting and folding however this active tectonism is not restricted to the areas of high sn attenuation but appears to extend beneath other parts of the iranian and turkish plateaus patterns of lateral variations in the propagation of lg are not as consistent as those for sn lg propagates efficiently across turkey iran and adjacent regions but the lg waves that cross the turkish and iranian plateaus are weak and have relatively long predominant periods of about 2 5 s the lg phase is not observed when the path of propagation crosses the southern caspian and the black seas consistent with the evidence of oceanic type crustal structure beneath these seas lg is also not observed from subcrustal events located in the hindu kush region the velocity of pn beneath most of the plateau areas and particularly in the regions of sn attenuation ranges between 8 0 and 8 2 km s velocities for sn and lg throughout the middle east are about 4 5 and 3 4 km s respectively it is possible to interpret the efficient sn propagation and the relatively normal uppermost mantle p and s velocities beneath a major part of the turkish iranian plateau as indicating a partial underthrusting of the arabian continental plate beneath the iranian and turkish continental blocks alternatively the mapped regions of efficient sn propagation and high sn attenuation may result from a differential cooling of a past thermal anomaly in this case the isostatic compensation of the plateaus could be due to an anomalously hot uppermost mantle that developed behind the mid to upper tertiary subduction zone s in this region and to a subsequent crustal shortening after the collision of the arabian and eurasian plates our results cannot uniquely confirm the validity of the above proposed models

effects of rotation on convective plumes from line segment sources abstract effects of rotation on finite length line plumes are studied with a three dimensional nonhydrostatic numerical model geophysical convection with this source geometry occurs for example as the result of fissure releases of hot hydrothermal fluids at the seafloor from terrestrial release of hot gases and ash during volcanic activity along fissures and in the descent from the sea surface of brines formed during freezing of ice leads at high latitudes here the model treats the case of a starting plume of dense fluid descending into a rotating environment results are compared with laboratory experiments so that the validity of the model particularly the nonlinear subgrid scale mixing formulation might first be established differences in plumes caused by varying rotation rate ohm and buoyancy flux b0 are the primary focus with experiments in fluid of depth h spanning a convective rossby number b01 3 2 h of 0 01 1 0 rotation initiates spiraling of the descending plumes but it has litt

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p