influence of the lower crustal structure on the early coda of regional seismograms we use synthetic seismograms for a flat layered earth model to study the influence of the widely observed strong reflectivity of the lower crust on the propagation of seismic regional phases the lower crustal layering induces a clear increase in the duration of the pg and lg phases on both the vertical and radial components beyond the group velocity of their terminations in a model with a homogeneous lower crust the transverse component is weakly affected the amplitude and the spectral content of the early lg coda are strongly dependent on the distribution of layer thicknesses the predominance of the low frequency components below 2 hz observed in the lg coda of real data is correctly accounted for with 1 km thick layers in the lower crust from particle motions at various depths and the analysis of the dispersion curves of normal modes we hypothesize that the late arrivals result from the interaction of plate type disturbances of the high impedance layers these waves leak energy in the mantle at a low rate this mode of generation of the coda waves explains the puzzling observation that the spreading coefficient of lg seems to be applicable to the early coda of regional seismograms the study of the influence of the lamellae on the average q factor of the continental crust shows that the lower crust layering does not account for the observed low values of the apparent qs another origin is needed for this strong attenuation for example scattering by randomly distributed heterogeneities

magnitude scales for very local earthquakes application for deception island volcano antarctica abstract different magnitude scales are calculated for a set of volcano tectonic earthquakes recorded in deception island volcano antarctica the data set includes earthquakes recorded during an intense seismic series that occurred in january february 1999 with hypocentral distances that range between 0 5 and 15 km this data set is enlarged to include some regional earthquakes with hypocentral distances up to 200 km the local magnitude scale m l fixed at a hypocentral distance of 17 km is used as the reference for the other magnitude scales studied in the present work m l is determined on a standard wood anderson simulated trace assuming a gain of 2080 maximum peak to peak amplitudes are measured on the vertical components of a short period sensor the m w scale is calculated in the vertical component both for p and s waves the attenuation correction of the ground motion displacement spectra is introduced using data from coda waves studied in the area the comparison between m l values and m w estimations indicates severe discrepancies between both values a magnitude duration scale is calibrated from the comparison between coda durations of the recorded events and their assigned local magnitude scales in order to investigate the causes of the discrepancy between the m l and m w values we analyze two possible error sources a wrong coda q value or the effects of the near surface attenuation that initially are not taken into account in the correction of the ground displacement spectra the analysis reveals that the main cause of this discrepancy is the effect of the near surface attenuation the near surface attenuation is also the cause of the determination of an anomalous spectral decay slope after the corner frequency and the determination of this corner frequency value this near surface attenuation represented by is estimated over the data set obtaining an average value of 0 025 with this value the m w scale is recalculated using an automatic algorithm the new m w values are more consistent with the m l values obtaining a relationship of m w 0 78 m l 0 02

the effect of crustal scattering on observed high frequency earthquake seismograms summary a comparison of high frequency 1 100 hz local out to 40 km earthquake seismograms at stations of the southern black forest digital seismic network is made in an attempt to isolate the effects of near receiver structure and of crustal scattering on the waveforms of the direct body waves and on the coda waves angle of incidence and azimuth of approach analyses from lower crustal earthquakes show that near surface weathering layers with low body wave velocities and with laterally varying thicknesses exist under all of the stations even those at hard rock sites of the black forest the similarity of direct p wave pulse shapes at each site for events of different sizes the time domain form of the so called fmax phenomenon is attributed to the response from this weathering zone the amplitude of the p wave coda of an event is found to be independent of the earthquake radiation pattern and to depend only on the absolute size of the event and on epicentral distance to the receiver furthermore the high frequency p wave coda is found to be generated at least in part from random crustal scattering including that within 100 m of the earthquake focus direct s waveforms are shown to be more complicated than their p wave counterparts probably due to the same near receiver and crustal scattering phenomena however in the case of s waves scattered energy can arrive before as well as at and after the direct s arrival making the identification of the direct s difficult especially for nodal arrivals

deep earth structure major developments in earth structure in the last four years have been concentrated in the description of the earth s lateral heterogeneity the regions that are heterogenous and the per cent variation of velocity and density in each region most studies find that lateral variation is concentrated in the upper 400 and lower 200 km of the mantle a radially symmetric earth model has been defined that represents the best average fit to seismic data in a broad frequency band sampling many regions p and s velocity is found to increase in zones of 50 km or less at 400 and 650 to 700 km depth the model is transversely anisotropic in the upper mantle it possesses a vertical axis of symmetry such that the elastic constants are different for vertical propagation than for horizontal and intermediate angles of propagation the real earth generally exhibits azimuthal anisotropy as well but the azimuthal anisotropy cannot be resolved by a global average of data the nature and magnitude of the anisotropy agrees with that found in ultramafic samples of the upper mantle in attenuation models of intrinsic attenuation have included the dispersive properties of intrinsic anelasticity and constructed relaxation models consistent with an observed frequency dependence of q in the body wave band there has been progress in mapping the scattering properties of the lithosphere attenuation due to scattering in the crust and lithosphere has been shown to have strong effects on the amplitudes of seismic waves at local and teleseismic distances

multiple scattering of seismic waves in fractured media crosscorrelation as a probe of fracture intensity this paper presents the theory which describes how the lateral correlation of vertical displacement is influenced by multiple scattering of seismic waves caused by the presence of fluid filled fractures along with an observational methodology to show how to extract fracture related quantities e g fracture porosity degree of anisotropy of the fractures change in orientation of the fractures density of fluid filling the fractures from seismic data using the theoretical development as an underpinning framework a simple physical picture is given of the basic process as well as some rough order of magnitude estimates to show that the parameters quantifying the size of the multiple scattering fall squarely in the seismic regime

travel time and amplitude analysis in seismic tomography in cross hole tomography straight lines connecting the source and receiver are often used to approximate the true ray paths thus linearizing the inverse problem a recent paper by bregman et al 1989a has shown that this approximation may be very poor in the regions of greatest interest where large velocity gradients cause significant ray curvature the interpretation method presented by bregman et al 1989a iteratively ray traces and updates the velocity model with model perturbations obtained by a damped least squares inversion of the travel time residuals this paper presents a modification of the travel time tomography method so that it may be applied to the inversion of first motion amplitudes for inverse quality factor q 1 after determining the velocity structure using travel time tomography the amplitudes are inverted using a similar scheme no additional ray tracing is required for the amplitude inversions the effect of the velocity structure previously determined through travel time tomography is taken into account so that amplitude variations due to the focusing and defocusing effects of the velocity structure are not interpreted as being due to attenuation structure both the travel time and amplitude tomography methods are applied here to field data from a cross hole experiment in crystalline rock wong et al 1983 the frequency range of the seismograms is 1 6 6 khz allowing resolution of velocity structure on a scale of several meters the resulting velocity image shows good agreement with other geological and geophysical data reversing the model by placing the model boreholes on opposite sides relative to the original reconstruction yields almost identical results indicating that the interpolation scheme used in the forward modeling has not biased the final model synthetic maslov seismograms calculated for the derived velocity model agree well with the waveform data further confirming the validity of the model in addition the amplitude tomography results place absorptive regions in the same location as the most pronounced low velocity features which correlate with fractures in the boreholes highly transmissive regions also correlate well with high velocity regions

regional change of coda q in the oceanic lithosphere coda q of 22 local earthquakes recorded on islands located in young and old oceanic lithosphere areas has been measured we found that the q values at periods of from 1 to 2 seconds increase from young to old ocean lithosphere in the same way as for the continent where the intensity of current tectonic activity shows a strong negative correlation with the coda q the q values increase with frequency proportional to fn where n was found to be 0 46 0 61 with the higher values for younger oceanic lithosphere which is again similar to the continents where the active regions have higher n values

coda waves a review the analysis and interpretation of coda waves have received increasing attention since the early seventies in the past few years interest in this subject has spread worldwide and the study of high frequency seismic coda waves has become a very important seismological topic as a conclusion of the studies accomplished in this time coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves p ands waves play a particularly important role in this interaction the process introduces an attenuation which added to the intrinsic absorption gives the observed apparent attenuation therefore coda waves constitute a thumbprint left by the heterogeneities on the seismograms coda waves offer decisive information about the mechanism of how scattering and attenuation take place this review describes coda waves in detail and summarizes the work done in this subject to 1986 the relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed particular attention has been given to the application of coda waves to estimate source and medium parameters the state of the art of the temporal variations of coda wave shape and the possible use of these variations as an earthquake precursor also are presented care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering

comparative analysis of teleseismic p coda and lg waves from underground nuclear explosions in eurasia this study has investigated the characteristics of p coda and lg waves recorded at the norsar and graefenberg arrays from presumed nuclear explosions in western russia lg waves were found to be more energetic relative to the preceding coda in the mid band frequency range 0 2 to 1 0 hz recorded on the broadband graefenberg instruments than in the high frequency band 0 6 to 3 0 hz at either array comparison of the log rms coda envelopes at the two arrays revealed that the early high frequency p coda at norsar is stronger relative to lg than at graefenberg also the amplitudes of p waves and early p coda waves within 55 sec after p recorded at the centers of the norsar subarrays are positively correlated indicating the effect of focusing defocusing on the p and coda waves however for subarrays which cluster spatially there is an inverse correlation which seems to be due to scattering attenuation suggesting that much of the early p coda is generated by local p wave scattering beneath the norsar array also the norsar coda envelopes flatten between about 219 and 352 sec after p whereas the graefenberg coda decays monitonically from the p wave out to the sn and lg arrival times the standard deviation across the norsar array of the log rms amplitudes of lg and s coda phases are higher by about 0 05 to 0 06 magnitude units than those of p coda phases these differences are probably related to differential site and propagation path effects on p s and lg waves in the p coda the flattening of the norsar coda envelopes appears to be a burst of energy not associated with any known regional phases the burst may be caused by forward scattering of lg to p waves along the path between the source and norsar based on a regional travel time curve for western russia the start and end time of the coda flattening is consistent with lg to p scattering in the distance range of 23 to 29 from norsar which brackets the ural mountains the lg propagation path from the source region to graefenberg does not appear to traverse the ural mountains which may explain why the coda flattening is not observed at graefenberg

analysis of near source contributions to early p wave coda for underground explosions iii inversion for isotropic scatterers a systematic array processing analysis is applied to 916 teleseismic shortperiod p wave signals from 25 underground explosions at pahute mesa in order to establish whether coherent arrivals scattered from near source velocity heterogeneity can be identified the signal coherence is measured using semblance which can be statistically evaluated using the noncentral f distribution if the noise properties are suitable the effects of unsuitable correlated noise due to common path and receiver contributions are suppressed by applying station weighting functions obtained by stacking all observations at a given receiver similar corrections for individual events are applied to reduce biases due to variations in surface reflection times and spall effects the characteristics of the spatial and phase velocity resolution of the semblance analysis are established by a series of synthetic calculations including cases with random arrivals in the p wave coda the procedure is applied to the actual data under the assumption that any scattering structures present radiate isotropically although this constraint could be relaxed given independent knowledge of a particular scatterer s nature the pahute mesa data show some evidence for enhanced scattering contributions to the first 15 sec of the p wave signals from the western edge of the silent canyon caldera the overall confidence level in this interpretation is fairly low because simulations with random scattering in the p coda successfully match many features observed in the stacked data

observations of teleseismic p wave coda for underground explosions the earlyp wave coda 5 15 sec after the first arrival of underground explosions at the nevada test site is studied in the time domain using 2082 teleseismic short period recordings with the intent of identifying near source contributions to the signals in the frequency range 0 2 2 0 hz smaller magnitude events tend to have relatively high coda levels in the 0 4 0 8 hz frequency band for both yucca flat and pahute mesa explosions coda complexity in this low frequency passband is negatively correlated with burial depth for pahute mesa events but is only weakly correlated with depth for yucca flat events enhanced excitation of relatively long period scattered waves for smaller less deeply buried events is required to explain this behavior coda complexity in the 0 8 1 1 hz band is positively correlated with magnitude and depth for pahute mesa events but has no such dependence for yucca flat events this may result from systematic variations between the spectra of direct signals and coda arrivals caused bypp interference for the largest events all of which were detonated at pahute mesa another possible explanation is a frequency dependent propagation effect on the direct signals of the larger events most of which were located in the center of the mesa overlying strong lateral velocity gradients in the crust and upper mantle event average complexity varies spatially for both test sites particularly in the 0 8 1 1 hz band providing evidence for frequency dependent focussing or scattering by near source structure both the direct arrivals and the early coda have strong azimuthal amplitude patterns that are produced by defocussing by mantle heterogeneity the direct arrivals have stronger coherent azimuthal patterns than the early coda for pahute mesa events indicating more pronounced deep crustal and shallow mantle defocussing for the direct signals however for yucca flat events the direct arrivals have less coherent azimuthal patterns than the coda suggesting that a highly variable component of near source scattering preferentially affecting the downgoing energy is superimposed on a pattern produced by mantle heterogeneity that affects the entire signal this complicated behavior of the direct arrivals may be the result of triplications and caustics produced by the complex basement structure known to underlie the yucca flat test site the presence of strong azimuthal patterns in the early coda indicates that source strength estimates based on early coda are subject to biases similar to those affecting estimates based on direct arrivals

seismic energy partitioning and scattering in laterally heterogeneous ocean crust we present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust the finite difference formulation is a 2 d solution to the elastic wave equation for heterogeneous media and implicitly calculates p and sv propagation compressional to shear conversion interference effects and interface phenomena random velocity perturbations with gaussian and self similar autocorrelation functions and different correlation lengths a are presented which show different characteristics of secondary scattering heterogeneities scatter primary energy into secondary body waves and secondary stoneley waves along the water solid interface the presence of a water solid interface in the model allows for the existence of secondary stoneley waves which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models

s wave coda q from 3 to 20 hz and p wave q for foreshocks and aftershocks of the petatlan earthquake in order to understand better the variations of mechanical properties close to the source and the failure process in the upper portion of the crust in a geologically very young subduction zone the seismic s wave coda qc from 3 to 20 hz and p wave half pulse width qp from 6 to 12 hz were determined for selected foreshocks and aftershocks of the 1979 petatlan mexico ms 7 6 earthquake to limit the region being sampled the quality factor of s coda qc was obtained from the first 10 s following the s onset qc exhibited a strong spatial temporal and frequency dependence and in the small foreshock portion of the aftershock area qc and to a lesser extent qp were significantly lower before the mainshock than afterward the frequency dependence n in qc qo n was practically constant for aftershocks in four different regions n 1 32 0 03 and for foreshocks n 1 43 0 07 only in one peripheral region station pair did we find a significantly different n n 0 97 0 09 our observations indicate that qc and perhaps the frequency dependence n are sensitive to the physical property changes in the source region and might be useful as predictors of large earthquakes in a shallow subduction zone environment

a field study of seismic attenuation in layered sedimentary rocks i vsp data summary a coherence matching technique was used to estimate q from both real and log derived synthetic vsp data to try to determine the relative contributions of apparent and intrinsic attentuation in layered sedimentary rocks the field experiment was carried out at the imperial college borehole test site in northumberland england the holes have been drilled through a cyclical sequence of the namurian upper limestone group the q analysis indicates that for frequencies from 40 to 200 hz scattering attenuation due to interbed multiples within the horizontally layered sequences is the dominant mechanism and any contribution from intrinsic attenuation is relatively insignificant confirmation of this result is obtained from an analysis of the frequency content of scattered energy in the coda of the downgoing compressional wave although uncertainty bounds on the q values are large these vsp data are not consistent with an intrisic q value as low as 20 which has been estimated from 500 hz to 1 5 khz crosshole data at the same site as described in part ii

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

corner frequency ratios of p and s waves and strain drops of earthquakes recorded by a tight network around the karadere segment of the north anatolian fault zone evidence for non classical source processes

a study of p waves from nevada test site explosions near source information from teleseismic observations spectral analyses of short period p arrivals on teleseismic recordings of nuclear explosions at yucca flats and pahute mesa regions of the nevada test site seem to provide explosion source medium information the spectral ratio p p coda on short period vertical component records at norsar shows significant correlation with explosion medium velocity for a given yield spectrum of the initial p waves from an explosion within a high velocity medium is richer in high frequencies as compared to that for p waves from an explosion within a low velocity medium the spectra of p codas on the other hand are considerably less sensitive to the near source velocity differences from one shot to another so that the spectral ratio p p coda is diagnostic of the shot medium velocity relative insensitivity of the p coda spectra and amplitudes to the overburden velocity is perhaps due to several factors including an effective source function controlled by average properties of rock within a fairly large nonlinear region around the shot point the study provides valuable insight into effects of the shot medium on the spectra and amplitudes of compressional arrivals and offers the exciting possibility of extracting near source information from teleseismic observations

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

coda q estimates in the mount cameroon volcanic region west africa abstract coda q qc has been determined in the mount cameroon volcanic region west africa from the decay of the amplitudes of coda waves on narrow band pass filtered seismograms of local microearthquakes in the frequency range 1 5 to 24 hz qc averages about 121 and 1745 at 1 5 hz and 24 hz respectively and has a frequency dependence of the form qc 65f for each frequency band the coda q values averaged over all the earthquakes show an insensitivity to the compoment of the ground motion considering a model incorporating both scattering and intrinsic attenuation and assuming that the attenuation is entirely due to the scattering loss a frequency independent minimum mean free path of about 38 km was found

inversion of p coda for isotropic scatterers at the yucca flat test site although many studies have addressed the statistical properties of p coda there have been few attempts to analyze coda in a deterministic framework in this analysis 1050 teleseismic p wave seismograms from 32 yucca flat explosions are inverted for the locations of any isotropic point scatterers in the source region that contribute coherent arrivals to the coda a rectangular grid of potential scattering locations is constructed around the source array and the semiblance a measure of waveform ensemble coherence for each point is computed by slant stacking the full set of seismograms at various move out velocities the potential resolution or sensitivity of the method for this data set is tested by constructing a corresponding suite of synthetic seismograms with direct arrivals plus arrivals from various point scatters and calculating the semiblance at each point on the grid the particular source station geometry produces better sensitivity near the array sensitivity is enhanced parallel to the trend of the elongated source array and diminished in the orthogonal direction stacking the data for a velocity of 2 5 km sec yields relatively high semiblance values in the central portion of the test site as well as about 10 km to the ene and moderately high semiblance valuesmore about 10 km to the wnw less

high frequency seismic coda and scattering in the northwest pacific data from a 1500 km linear ocean bottom hydrophone array deployment near wake island by the hawaii institute of geophysics are used to examine seismic coda and scattering in the northwest pacific two earthquakes in the kuril islands that occurred along the trend of the hydrophone array are analyzed in the distance range 2700 to 3700 km the coda of the oceanic s waves 1 5 to 20 hz display 1 a constant decay rate independent of distance and 2 an increasing coda energy falloff rate with increasing frequency both of these observations suggest that substantial scattering is not occurring in the propagation along the array the properties of the oceanic s coda are relatively stable when the propagation paths are nearly the same the observations are consistent with coda generation dominated by back scattering in the source region rather than from forward scattering along the propagation path

shear wave attenuation characteristics over the central india tectonic zone and its surroundings abstract the study area lies between latitude 18 26 n and longitude 73 83 e and mainly covers the central india tectonic zone citz the frequency dependent shear wave quality factor qs has been estimated over the citz and its surroundings using double spectral ratio dsr method we have considered 25 local earthquakes with magnitude ml varies from 3 0 to 4 7 recorded at 11 stations running under national seismic network the fast fourier transformed fft spectra were computed from the recorded waveform having time window from onset of s phase to 1 0 s and for a frequency band of 0 1 10 hz three different shear wave velocities i e 3 87 3 39 and 3 96 km s were obtained over the study area based on a pair of earthquakes recorded at a pair of stations the low qs values of 51 96 at 1 hz i e qs 51f0 49 qs 90f0 488 and qs 96f0 53 were found in the area covering the son narmada tapti sonata lineament citz eastern part of the satpura fold belt vindhyan and gondwana basins godavari and mahanadi grabens and southern part of gangetic plain intermediate qs values of the order of 204 277 i e qs 204f0 56 and qs 277f0 55 were noted in the cartonic areas namely bundelkhand dharwar bhandara and bastar while the higher qs values of 391 628 at 1 hz i e qs 391f0 49 qs 409f0 48 qs 417f0 48 qs 500f0 66 qs 585f0 65 and qs 628f0 69 were found in the eastern part of the sonata citz and the northeastern part of the satpura fold belt the low qs values might be attributing to the more heterogeneous sonata rift system low qs values further may presumably be associated with lower level of seismicity and apparently account for higher tectonic stress accumulation over long duration the long term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the sonata zone surrounding the sonata region the higher qs values possibly accounts for a more homogeneous subsurface structure along the sonata zone

intrinsic and scattering attenuation in the crust of the abu dabbab area in the eastern desert of egypt abstract fifty five microearthquakes recorded by a digital temporary seismic network in the abu dabbab area in the eastern desert of egypt were used to estimate the direct s wave q coda q c intrinsic q i and scattering quality factors q s sato s sato h 1977 single scattering assumption was used to fit the amplitude envelopes of the coda at seven central frequency bands 1 5 3 6 9 12 18 and 24 hz obtaining a q c varying with frequency as generally observed in tectonically active areas lapse time dependence was also studied for the area with the coda waves analyzed at window lengths ranging from 10 to 40 s starting from the onset of the s wave arrival the direct s wave q was estimated using the coda normalization method aki k 1980a the frequency dependence of q was investigated for the direct s waves and coda waves results show a low quality factor and a high frequency parameter indicating that the upper lithosphere of the abu dabbab area is seismically active and heterogeneous using the independent estimates of q c and q the intrinsic quality factor q i was separated from the scattering quality factor the results suggest that intrinsic dissipation plays a predominant role with respect to scattering phenomena in the area the obtained q values seem closer to those reported by analyzing volcano tectonic earthquakes this finding reflects that the cause of abu dabbab earthquake swarms might be igneous activity where the magma is ascending through joints or serpentinized joints that are dewatering

regional wave propagation in southern california and nevada observations from a three component seismic array we present results from a study conducted to characterize regional wave propagation in southern california and nevada the data are from events whose paths sample the southern coast ranges the southern sierra nevada mountains and the southern basin and range these regional earthquake data were recorded by a small aperture three component seismic array located at the pinon flat observatory southern california the data are analyzed using a statistical array processing algorithm that provides estimates of the propagation direction frequency wavenumber and particle motion polarization characteristics of the coherent arrivals observed at the array perhaps the most striking feature shared by these data is the extent to which the p s and lg wave trains are composed of coherent forward scattered multipathed energy the nature of the p s and lg wave trains suggests that spatially anisotropic scattering plays an important role in regional wave propagation in southern california and nevada and that scattering sources are located not on elliptical shells whose foci are the source and the receiver but instead in a limited volume subparallel to the path of the direct arrivals the high amplitude coherent nature of numerous of the pg and lg coda waves suggests that strong scattering in the source zone plays an important role in shaping the pg and lg wave trains while the p s and lg wave trains for these regional events share some general characteristics they also exhibit fairly significant differences these differences illustrate the important role regional and source zone structure play in shaping the wave field and the need to account for both complex scattering and three dimensional propagation effects in regional waveform modeling the coda for these regional data do not begin to exhibit a character consistent with the random scattering model for coda generation until the arrival of the surface waves the fact that much of the surface wave coda cannot be effectively modeled as plane waves suggests that it is not the product of scattering from distant heterogeneities

yield estimation from spectral amplitudes of direct p and p coda recorded by the wake island deep ocean hydrophone array abstract spectral amplitudes of direct p and p coda between 0 5 and 6 hz have been measured for 14 eastern kazakh test explosions using 8 hydrophones of the wake island array and a new model for these data has been developed to estimate relative yields of those explosions each log spectral amplitude is considered to be the sum of four parameters and an error term three parameters are frequency dependent and represent 1 the average eastern kazakh spectrum at the wake island array 2 the spectral shape characteristics unique to each event and 3 the station or hydrophone corrections the fourth parameter represents relative yield a total of 346 parameters including the 14 relative yields were needed to model all 901 spectral amplitudes that were measured and first order linear regression techniques were used to invert the data for these parameters standard deviations of the computed relative yields are very small averaging only 0 015 m b units deviations between the relative yields and their corresponding relative neis body wave magnitudes however are larger averaging 0 14 m b units this difference is interpreted to be an indicator of the level of inhomogeneity in the worldwide pattern of energy radiated from eastern kazakh tests there is evidence that this pattern can be correlated with the precise epicentral location of each explosion within the test site and is due to focusing and defocusing near the source it is proposed that the yield of a large explosion might be disguised by siting the explosion at a location that selectively defocuses energy towards continents where most seismic stations are located the precision with which a relative yield is determined from the data of any single hydrophone averages 0 035 m b units thus the model appears to provide a very stable measure of yield errors associated with frequencies greater than 2 hz are only slightly greater than errors associated with frequencies less than 2 hz verifying the utility of using the higher frequencies observed in direct p and p coda to estimate yield

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

relative role of intrinsic and scattering attenuation beneath the andaman islands india and tectonic implications abstract the attenuation characteristics of seismic waves traversing the andaman nicobar subduction zone ansz are investigated using high quality data from a network of broadband stations operational since 2009 we initially studied the coda wave attenuation q c 1 under the assumption of a single isotropic scattering model subsequently following the multiple isotropic scattering hypothesis we isolated the relative contributions of intrinsic q i 1 and scattering q sc 1 attenuation employing the multiple lapse time window analysis mltwa method within a frequency range 1 5 18 hz results reveal a highly attenuative nature of the crust with the values of q c being frequency dependent the intrinsic absorption is mostly found to be predominant compared to scattering attenuation the dominance of q i 1 in the crust may be attributed to the presence of fluids associated with the subducted slab our results are consistent with the low velocity zone reported for the region a comparison of our results with those from other regions of the globe shows that the ansz falls under the category of high intrinsic attenuation zone interestingly the character of ansz is identical to that of eastern himalaya and southern tibet but entirely different from the garhwal kumaun himalaya and the source zone of chamoli earthquake due to the underlying mechanisms causing high attenuation

seismic q for the lithosphere of the montenegro region yugoslavia frequency depth and time windowing effects abstract the availability of accelerometric data for the montenegro earthquake of 15th april 1979 makes it possible to investigate seismic q of the lithosphere in that region in particular its dependence on frequency on the depth reached by seismic waves and on the length of time windows in which signals are processed two different spectral methods s phase energy ratio and coda envelope decay are applied respectively to direct and scattered shear waves similar results are obtained using different portions of the recordings i e coda waves for the envelope decay fit and the s wave train with a significant duration of 10 s for the energy ratios the same apparent q q 40 f where f is the frequency expressed in hz that is found for other neighbouring central mediterranean regions e g ancona on the central italian adriatic coast valnerina in the central apennines irpinia in the southern apennines is also found for the southern yugoslavian coast in the band 1 25 hz up to a maximum range of 120 km from the focus this strong frequency dependence is probably connected with the type of small scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas in order to compare the apparent q of the whole s wave train 10 s long with the intrinsic apparent q of the single direct s wave usually 1 s or less the maximum entropy method is applied in the energy spectrum computation for shorter wave trains the use of shorter time windows does not reveal any significant variation in the tendency of q to increase linearly with frequency as the length of the time window containing the sample of the s waves decreases this seems to indicate that scattering dependent q is generally inseparable from intrinsic q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used while the type of linear growth with frequency does not seem to undergo any variations it remains of the q qf type the data show there are a considerable decrease in the coefficient of proportionality q with decreasing duration of the window of s waves analysed probably as a result of variations in seismic attenuation with depth

spatial changes of seismic attenuation and multiscale geological heterogeneity in the baikal rift and surroundings from analysis of coda waves abstract the baikal rift system is undergoing an active tectonic deformation expressed by a high level of seismic activity this deformation leads to physical and mechanical changes of crustal properties which can be investigated by the seismic quality factor and its frequency dependence using a single backscattering model a seismic quality factor qc a frequency parameter n and an attenuation coefficient have been estimated by analyzing coda waves of 274 local earthquakes of the baikal rift system for nineteen lapse time windows w from 10 to 100 s every 5 s and for six central frequencies 0 3 0 75 1 5 3 6 and 12 hz the average qc value increases with the frequency and lapse time window from 46 52 at 0 75 hz to 502 109 at 12 hz for w 10 s and from 114 49 at 0 3 hz to 1865 679 at 12 hz for w 100 s the values of qc f and were estimated for the whole baikal rift system and for separate tectonic blocks the stable siberian platform main rift basins spurs and uplifts along the rift system the q0 value qc factor at the frequency f 1 hz varies within 72 109 and the frequency parameter n ranges from 0 87 to 1 22 whereas q0 is 134 and n is 0 48 for the stable siberian platform vertical variations of attenuation reveal that sharp changes of and n are confined to the velocity discontinuities the comparison of lateral variations of seismic wave attenuation and geological and geophysical characteristics of the baikal rift system shows that attenuation is correlated with both seismic activity and heat flow and in a lesser degree with the surface fault density and the age of the crust seismic wave attenuation found across the main shear zones of the south western baikal rift main sayan strike slip fault zone and tunka obruchev and primorsky normal faults is increased by more than 25 60 compared to the neighboring areas

time frequency analysis of seismic excitation and estimates of attenuation parameters for the friuli italy local earthquakes abstract stochastic modelling is applied to the analysis of local earthquake recordings which are usually extremely rich in random incident wave trains that are chaotically superimposed because of scattering effects in the earth s crust the presence in the seismic signal of effects connected with the scale of inhomogeneity in the lithosphere cannot be deterministically described in detail the application of a stochastic second order autoregressive model to accelerometric records for the higher magnitude m l 6 friuli earthquakes and to short period seismometric records for the aftershocks of the strong earthquake of 6 may 1976 has allowed inferences to be drawn about the spectral properties of seismic signals and the propagation mechanisms of seismic waves these inferences are based on an extremely small number of parameters of a mathematical model suitable for simultaneously describing the random sequence of scattered wave trains in the time and frequency domains useful physical information has been obtained about the dynamic characteristic correlation times and the predominant frequency of the seismic signals moreover the strength 2 e t of the innovation of the stochastic process fitting the real digital data set has been estimated from the envelopes of 2 e t the quantity heuristically used in the stochastic approach to describe seismic excitation the mean free path between successive scatterings l or the equivalent diffusivity coefficient d and turbidity g and their dependence on seismic wave frequency have been investigated for friuli using seismometric data at an epicentral distance of 20 km and earthquakes with a magnitude just under 2 mean free path estimates obtained by means of autoregressive parameters vary from 5 km for the strong interaction model to 30 km for the single scattering model furthermore by means of accelerometric records for the strongest earthquakes in friuli during may and september 1976 the dependence for the maximum of the seismic excitation on the epicentral distance r was estimated as 2 e max r with 1 94 0 13 which is in good agreement with results obtained for the same region using standard methods by means of acceleration peaks versus r lastly stochastic modelling provides a method of estimating change versus time for the predominant frequency and characteristic correlation time of narrow band digital recordings these two parameters were computed by means of autoregressive parameters in different physical situations and were found to be functions of the earthquake source the instrumentation frequency response and the earth s filtering effects

attenuation of seismic waves in the trinidad and tobago area abstract the attenuation of seismic waves from earthquakes located within the area bounded by 9 12 n and 60 63 w was estimated from short period seismograms coda q q c determinations were made for each of the six seismograph stations within the area while spectral q values from p phases q were estimated for station trn the s s single scattering model was assumed for coda generation and the 2 source model was assumed for the spectral q determinations the q c values show a strong frequency dependence in the frequency range 1 5 12 hz the value of q at 1 hz q o was found to lie within the range 107 132 while the rate of frequency dependence n extends from 0 80 to 1 06 for shallow events for intermediate depth events q o varies from 101 to 173 and n from 0 80 to 1 02 the q values obtained show a spatial variation within the region the highest attenuation was obtained on land trinidad

multiple scattering of seismic waves in fractured media velocity and effective attenuation of the coherent components of p waves and s waves we show that the multiple scattering by small fractures of seismic waves with wavelengths long compared to the fracture size and fracture spacing is indistinguishable from multiple scattering effects produced by regular porosity except for an orientation factor due to fracture alignment the fractures reduce thep wave ands wave velocities and produce an effective attenuation of the coherent component of the seismic waves the attenuation corresponds to 1000 q of about unity for a gaussian spectrum of fractures and it varies with frequencyf asf3 for a kolmogorov spectrum of fractures of spectral index the attenuation is an order of magnitude or so larger and varies with frequency asf3 v the precise degree of attenuation depends upon the matrix properties the fracture porosity the degree of fracture anisotropy the type of fluid filling the fractures and the incidence angle of the wave

a seismic passive imaging step beyond spac and remi abstractthe basic property of passive imaging is that given any two points one of them can be taken as the source of the waves and the other as the recording station this property can be derived from the statistical self alignment of the rays along the vector joining the two points and applies also to nondiffuse wavefields like seismic tremor it provides a statistical basis for the use of the stationary phase integral allowing passive interferometry under the mild constraint of mechanical homogeneity at a local scale combined with the tremor s large spectral bandwidth it allows one to recover the local green s function from spatial correlation furthermore combining this property also with the azimuthal isotropy of either the wavefield or the array and using the statistical mode as the estimator provides a new technique to measure the local velocity dispersion in the subsoil this technique exploits the potential of spatial autocorrelation spac and refraction microtremor remi allowing one

stress associated intrinsic and scattering attenuation from laboratory ultrasonic measurements on shales seismic attenuation is sensitive to stress induced subtle changes in the physical state of rocks in this study the stress and frequency associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses as an improvement to the single scattering model the elastic monte carlo method is employed to investigate multiple scattering attenuations by incorporating the boundary reflections and wave conversions our results show that as the axial stress increases the intrinsic attenuation decreases in all directions while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions these discrepancies result from different attenuation mechanisms both the intrinsic and scattering attenuation are found to be largest in the direction 45 to the bedding but least in the perpendicular direction the s wave attenuation is larger and more sensitive to stress changes than p wave attenuation due to its shorter wavelength as expected from sandstone examples the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation the frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses but varies in different directions of an individual rock with different heterogeneity and anisotropy scales the peak frequency of s coda is smaller and its peak scattering attenuation is larger than p coda in conclusion the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions indicating significant anisotropy and heterogeneity

imaging of crustal heterogeneous structures using a slowness weighted back projection with effects of scattering modes 2 application to the nagamachi rifu fault japan area 1 this is the second paper in a two part series on a newly developed imaging approach for small scale heterogeneities 1 km in the crust with effects of scattering modes in the present paper we estimated a detail three dimensional spatial distribution of small scale heterogeneities around the nagamachi rifu fault northeastern japan in a frequency range of 2 16 hz using the imaging approach presented in the first paper we used seismograms recorded by dense three component seismic arrays that were deployed in this region for 15 explosion sources as one of our important results there are concentrations for only p s but not p p scatterers near the surface trace of the fault p s scatterers in a frequency range of 8 16 hz are localized near the surface trace of the fault implying the possibility of strong heterogeneities with a size of 0 08 km there in the shirasawa caldera region the characteristics of seismic scatterers seem to convert from large p s to large p p relative scattering coefficients with its transition depth range of 5 8 km this feature implies that the materials composed of seismic scatterers may show a systematic variation with depth finally the strength of scattering coefficients is rather weak in the coseismic area of the 15 september 1998 earthquake with a magnitude of 5 2 the largest recent event in this area this result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system

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

coda q distribution in the iberian peninsula summary several attenuation studies have established a frequency dependence law of the anelastic attenuation factor q in the form q qo f fo v for the approximate 1 10 hz frequency range we propose a method that leads to the determination of q which is a function of the reference frequency fo and the real exponent v with a single station to carry out the problem we determine a set of master curves as a function of y we discuss the method and the different features of the master curves when it is applied to the complicated regions of the iberian peninsula and to several instruments with different responses using this new method and the seismographic stations available in the iberian peninsula we have mapped iso qo lines at a reference frequency of 1 hz applying inversion methods the q values determined for iberia vary between about 100 and about 600 values close to 100 correspond to the southern part of iberia in general q values increase from south to north with values about 600 near the nw part of iberia the pyrenees mountains and adjacent areas present q values between about 200 and about 350 these results suggest a strong qo lateral variation in iberia a considerable frequency dependence of coda q has also been determined the v values vary between 0 3 and 0 8 the q values obtained in the iberian peninsula show very good agreement with several q values obtained in other regions of the world comparison between the iso q lines and other geophysical parameters like regional variations of p velocities heat flow isoseismal intensity distribution and crustal thickness indicates that lower q values are associated with higher isoseismal intensity attenuation higher heat flow lower p velocities and thinner crust

teleseismic coda analysis with simultaneous modeling of single and multiple scattering spectra from teleseismic coda waves are used to estimate earth and source properties through generalized linear inversion gli techniques high frequency teleseismic 66 98 p wave codas recorded by the regional seismic test network rstn are analyzed the spectra of the teleseismic coda were found to be described by the following two models a t2 b exp ct and a t b exp ct a two term model consisting of a time dependent and time independent part were required to fit the single and multiple scattering effects observed in the data two near surface explosions eastern kazakh of magnitude 6 1 and one earthquake northern argentina chile 559 km depth of magnitude 5 8 are analyzed to evaluate near source scattering the five three component north america rstn receiving stations are used to evaluate the effect of near receiver scattering coda q estimates from the inversions increased with frequency in the approximate range from 800 at 1 hz to 1600 at 4 hz the same range of coda q values were obtained for both of the models tested however the a t b exp ct model results had higher variances suggesting poorer fit of this model to the data coda q estimates are higher at stations rscp and rsny compared to the other stations the coda q results from the gli analyses are in agreement with q values determined from the analysis of local coda these results suggest that teleseismic coda q are dominated by near receiver scattering effects the coda analysis method outlined in this paper represents an alternative to normal single scattering analysis methods performed over small time windows early in the coda this method which accounts for multiple scattering effects models the entire coda 500 600 s where signal is above background noise good results were obtained for both earthquake and explosion data while coda analysis on multiple scattered waves at teleseismic distances was the emphasis in this paper these methods should also work at local and regional distances

former extent of glacier like forms on mars abstract mars mid latitude glacier like forms glfs have undergone substantial mass loss and recession since a hypothesised last martian glacial maximum lmgm stand to date there is a lack of knowledge of the nature and timing of the lmgm the subsequent mass loss and whether this mass loss has been spatially variable here we present the results of a population scale inventory of recessional glfs derived from analysis of 1293 glfs 3 identified within context camera ctx imagery to assess the distribution and controls on glf recession a total of 436 glfs were identified showing strong evidence of recession 197 in the northern hemisphere and 239 in the southern hemisphere relative to their parent populations recessional glfs are over represented in the low latitude belts between 25 and 40 and in areas of high relief suggesting that these zones exert some control over glf sensitivity and response to forcing this analysis is complemented by the reconstruction of the maximum extent and morphology of a specific glf for which high resolution imaging science experiment hirise derived digital elevation data are available using nye s nye j f 1951 proc roy soc lond ser a mat phys sci 207 554 572 perfect plastic approximation of ice flow applied to multiple flow lines under an optimum yield strength of 22 kpa we calculate that the reconstructed glf has lost an area of 6 86 km 2 with a corresponding volume loss of 0 31 km 3 since the lmgm assuming the loss reconstructed at this glf occurred at all mid latitude glfs yields a total planetary ice loss from mars glfs of 135 km 3 similar to the current ice volume in the european alps on earth

estimation of coda wave attenuation for the national capital region delhi india using local earthquakes attenuation of seismic waves is very essential for the study of earthquake source parameters and also for ground motion simulations and this is important for the seismic hazard estimation of a region the digital data acquired by 16 short period seismic stations of the delhi telemetric network for 55 earthquakes of magnitude 1 5 to 4 2 which occurred within an epicentral distance of 100 km in an area around delhi have been used to estimate the coda attenuation qc using the single backscattering model the seismograms have been analyzed at 10 central frequencies the frequency dependence average attenuation relationship qc 142f1 04 has been attained four lapse time windows from 20 to 50 seconds duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of qc the qc values show that frequency dependence exponent n remains similar at all the lapse time window lengths while the change in q0 values is significant change in q0 with larger lapsetime reflects the rate of homogeneity at the depth the variation of qc indicates a definitive trend from west to east in accordance with the geology of the region

absence of evidence for a shallow magma chamber beneath long valley caldera california in downhole and surface seismograms a downhole seismometer at 900 m depth and a temporary network of surface stations were deployed to use rays from local microearthquakes to study the upper and middle crust beneath the long valley caldera the downhole seismograms show s waves with high apparent amplitudes from earthquakes located 2 20 km to the south of the downhole seismometer in contrast s waves from earthquakes located in the distance range 20 30 km to the south have low apparent amplitudes if p and s amplitudes are normalized relative to the respective coda amplitudes the s to coda amplitude ratios appear to remain constant but the p to coda amplitude ratios vary significantly with takeoff angle a comparison of the calculated radiation patterns for a double couple in a uniform halfspace and focal mechanisms of the recorded earthquakes suggest that the observed variations in p and s amplitudes are caused by radiation pattern effects reanalysis of possible travel time delays found by elbring and rundle 1986 who used a subset of the borehole data analyzed in this study shows that they underestimated the epicentral distances to three of the earthquakes and hence generated an artificial kink in the reduced travel time versus depth curve synthetic calculations of reduced travel time versus depth suggest that an apparent velocity of 5 7 km s gives less scatter than 6 0 km s used by elbring and rundle 1986 plots of ts tp versus depth show that contrary to the findings of elbring and rundle 1986 the vp vs ratio stays fairly constant with depth and a small 3 km diameter magma chamber cannot easily be resolved furthermore combined analysis of downhole and surface data shows that neither data set requires a low velocity zone or a zone of anomalously high vp vs at depth below the casa diablo area

analysis of near source contributions to early p wave coda for underground explosions i waveform complexity a data set of more than 1600 teleseismic short period p waves from 46 nuclear explosions is analyzed to establish near source contributions to the first 15 sec of the p signals the near source components are isolated by event to event comparisons of first cycle magnitudes rms amplitudes in the first 5 and next 10 sec of the wave forms and energy temporal centroid measurements events from the nevada amchitka and novaya zemlya test sites are analyzed separately to characterize overall source region variations event averaged waveform complexity variations between explosions within a given site are tested for dependence on source strength burial depth location within the site and tectonic release azimuthal patterns of the individual event complexity anomalies are used to discriminate between possible near source influences for events at pahute mesa a strong azimuthal amplitude pattern in both the direct p waves and the p coda for all of the events is produced principally by deep mantle variations defocusing by a high velocity anomaly in the crust and uppermost mantle beneath the site has a stronger effect on the direct p signals than on the early p coda resulting in a systematic spatially varying relative pattern

shallow seismic attenuation and shear wave splitting in the short period range of deception island volcano antarctica abstract the occurrence of a seismic series in deception island volcano antarctica composed of hundreds of local volcano tectonic earthquakes has permitted us to study the seismic attenuation of such a volcanic environment in the short distance and high frequency range this study has been performed using p waves s waves and coda waves and applying different frequency dependent and independent techniques the methods used for this analysis have been spectral and broadening of the pulse for direct p and s waves coda normalization for s waves and single back scattering model for coda waves the results show that in general q values are significantly smaller for the entire frequency range used 6 30 hz than those found in other volcanic and tectonic areas the attenuation for p waves is greater than for s waves in the frequency independent methods with a q q p ratio that ranges between 1 9 and 3 2 comparing the q factor obtained for s waves we have observed clear differences as a function of the method used the coda normalization method has supplied significantly higher q values q d than the other two methods q we have interpreted this discrepancy as an effect of the methods coda normalization and single back scattering methods eliminate the contribution of the near surface attenuation in their q values comparing both q and q d we have estimated the near surface attenuation under the recording site named q on the other hand we have observed that q d has anomalous frequency dependence with a minimum value at 21 hz this pattern is interpreted as an effect of strong scattering of the seismic waves in the source area of the earthquakes q c values depend clearly with frequency and lapse time and the lapse time dependence could be interpreted as a depth dependence of the seismic attenuation in deception island volcano antarctica the obtained q values have allowed us to separate the contribution of intrinsic and scattering attenuation deriving that the scattering attenuation is predominant over the intrinsic effects finally in order to investigate how the heterogeneous medium of the volcanic island could produce other effects we have checked whether it produces polarization of the shear waves the preliminary results of the polarization direction indicate a main e w strain direction all these evidences reveal the strongly heterogeneous structure of deception island volcano

the response of the ubaye valley france for incident sh and sv waves comparison between measurements and modeling l this paper presents a detailed study of the response of an alluvial valley in the alps france the structure ubaye valley was chosen 1 for its moderate dimensions 500 m wide and 65 m thick which allow an accu rate determina tion of the deposit characteristics and 2 for the relatively high seismicity of the region the study included the deployment of five temporary seismological stations a seismic survey of the valley for determining the dynamic proper1ies and the geometry of the soft deposits and 1 d and 2d numerical modeling for incident sh and sv waves comparisons between observed and computed amplifications show a good agreement for frequencies lower than 10hz and the particular direction of arrivai of the input motions corresponding to the 2d case when the ear1h quake sources are not located in the continuation of the line of seismometers the spectral ratios exhibit a great variability between different groups of similar events these observations indicate the azimuthal dependence of the site response and the need for 3d modeling

guided wave tracking in 3 d a tool for interpreting complex regional seismograms abstract the tracking of guided wave trains by raytracing in 3 d structures can provide a means of interpreting complex seismograms at regional distances the method relies on the interpretation of lg as the constructive interference of multiple s reflections within the crust and strong scattering can be simulated by the inclusion of secondary sources this approach has been applied to californian events observed in the southwestern united states whose records at regional distances frequently exhibit an extended and complex lg coda with significant late energy arriving with group velocities of 2 5 km sec or less the general character of this energy precludes an interpretation as the result of stochastic scattering processes or dispersion in low velocity surface sediments therefore some alternative explanation needs to be sought the observed wavetrains of events from two sites in southwestern california coalinga and north baja at stations lac mnv and elk have been compared with the predictions from tracking guided wave patterns multipathing of energy by major changes in the thickness of the crustal wave guide is consistent with the extended and often pulse like nature of the lg coda examples of plausible multipathing mechanisms include reflection from the ocean continent transition scattering associated with topographic features such as the sierra nevada and resonance within the narrow corridor of baja california

spatial and temporal correlation between coda q and seismicity in china a map of coda q at 1 hz q was constructed for china by analyzing more than 1000 seismograms from 80 seismic stations distributed throughout china the resultant contour map of q shows a strong correlation with the epicenters of historic great earthquakes in china the high q regions are devoid of the epicenters and the low q regions are full of them a remarkably linear relation was found between the logarithm of coda q and the maximum earthquake within the area sampled by coda waves radius of about 150 km from each station when we consider earthquakes which occurred from 1700 to present roughly speaking if q is 1000 m max is about 4 and if q is 100 m max is about 8 this relation will be very useful for siting critical facilities such as nuclear power plants and waste disposal sites we also found evidence in support of the migration of low q region coincident with the well known migration of high seismicity region over the distance of several hundred kilometers during the period of several hundred years if the migration is due to a diffusion process the diffusion constant is of the order of 100 km2 yr which agrees very well with those associated with the aftershock migration fluid injection and vp vs anomaly comparison of coda q with q1 estimated from the intensity data shows that they agree very well for q1 150 supporting the validity of the s to s single backscattering model for coda waves for q1 150 however coda q becomes constant independent of q1 this behavior is qualitatively consistent with numerical experiments of frankel and clayton and suggests that coda q may be measuring the intrinsic q of about 150 for these regions where q1 150 this limiting coda q value may depend on regions because coda q lower than 100 has been observed in some active regions in this study and also coda q as low as 30 to 40 has been reported in california mexico and italy

the scattering of seismic waves through a crust and upper mantle with random lateral and vertical inhomogeneities abstract the propagation of seismic waves through earth models with slightly random lateral and vertical inhomogeneities superimposed on one composed of layers with vertical velocity gradients was investigated the maximum deviation of velocity from a mean value at a given depth and a correlation distance derived from a two dimensional smoothing filter were two parameters used to vary the amplitude and size of the velocity anomalies the resulting models show short discontinuous reflectors scattered about at various depths throughout the model and are thus in agreement with many deep seismic reflection experiments on the other hand numerical experiments using ray tracing techniques showed that the effect of the lateral and vertical velocity anomalies is to scatter the energy and break up the continuous travel time lines from vertical gradient models into travel time segments with different slopes similar to those observed in many long range seismic refraction experiments and to those resulting from layering effects in the media many of the numerical experiments which modelled the random crust produced a pg segment and a p segment with an apparent conrad discontinuity at a depth of 10 20 km this apparent depth being related to the correlation distance when a seismic wave propagates through a heterogeneous earth the amount of its energy which is converted into scattered energy will be a function of the inhomogeneous characteristics of the medium through which it has passed if a ray passes through a homogeneous earth the energy arriving at an array station should be relatively coherent whereas if the ray encounters lateral and vertical inhomogeneities its energy will be incoherent and much more complex a series of coherency measurements done on array recordings of earthquakes at various distances showed that large lateral and vertical variations in complexity exist for different ray paths through the earth with the region below the 650 km discontinuity in the mantle tending to be much simpler than the region just below the lithosphere

analysis of near source contributions to early p wave coda for underground explosions ii frequency dependence an analysis of dispersion in more than 1600 teleseismic short period p waves from 46 underground explosions has established that near source effects are responsible for systematic frequency dependent variations observed in the first 15 sec of the p signals explosions from the nevada amchitka and novaya zemlya test sites exhibit a common magnitude dependence of the dispersive behavior with smaller events having relatively enriched low frequency 0 4 to 0 8 hz energy in the coda for the nevada and amchitka sites the larger events have relatively enhanced high frequency 0 8 to 1 1 hz energy in the coda as well which may actually be a consequence of diminished high frequency content of the direct arrivals the dispersive behavior also correlates well with known source depths for the nevada test site and amchitka events and with estimated pp delay times for the novaya zemlya events indicating that burial depth and or explosion size are important factors pahute mesa tests show a secondary dependence on position in the site with centrally located events having stronger dispersion as well as more pronounced slowly varying azimuthal patterns in the frequency dependence stations at azimuths to the north northeast from the mesa have particularly strong dispersion for centrally located events the spatial and azimuthal variations for pahute mesa events do not appear to be the result of aftershock radiation as suggested by douglas 1984 but instead are associated with frequency dependent defocusing and scattering from a high velocity structure beneath the test site some of the dispersion from novaya zemlya events appears to result from deep path properties however the strong magnitude depth dependence for all of the test sites indicates that very near source conditions have the strongest influence on the frequency dependence a likely explanation for the dispersive behavior is a combination of relatively enhanced surface wave excitation and scattering for shallower events and frequencydependent defocusing for sites overlying anomalous high velocity structures as is the case for both pahute mesa and amchitka

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

comparison of amplitude decay rates in reflection refraction and local earthquake records three types of seismic data recorded near coalinga california were analyzed to study the behavior of scattered waves 1 aftershocks of the may 2 1983 earthquake recorded on verticalcomponent seismometers deployed by the usgs 2 regional refraction profiles using large explosive sources recorded on essentially the same arrays above 3 three common midpoint cmp reflection surveys recorded with vibrator sources over the same area records from each data set were bandpassed filtered into 5 hz wide passbands over the range of 1 25 hz corrected for geometric spreading and fit with an exponential model of amplitude decay decay rates were expressed in terms of inverse coda q c 1 q c 1 values for earthquake and refraction data are generally comparable and show a slight decrease with increasing frequency decay rates for different source types recorded on proximate receivers show similar results with one notable exception one set of aftershocks shows an increase of q c 1 with frequency

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

attenuation study in the straits of messina area southern italy abstract the attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering which is closely related to the seismicity and regional tectonic activity of a particular area in this work the seismic attenuation in the straits of messina affected by several and strong historical earthquakes was studied using waveforms recorded by a local seismic network composed of seven stations we measured the coda quality factor q c in the single scattering model hypothesis the direct quality factor q d applying the coda normalization method for s waves and the intrinsic and scattering quality factor q i and q s by the multiple lapse time window analysis mltwa method coda q values were obtained using different lapse times 40 60 and 80 s for the frequency bands centred at 1 5 3 6 and 12 hz our findings indicate that q c increases with increasing lapse time and that q c is frequency dependent this behaviour is usually correlated to the degree of tectonic complexity and to the presence of heterogeneities at several scales in fact by using the coda normalization method we obtained low q d values as expected for a heterogeneous and active zone finally by the mltwa method we observe that the contribution of the scattering attenuation q s 1 prevails on the intrinsic absorption q i 1 until 3 hz conversely q i 1 and q s 1 seem to be of the same order in the higher frequency bands