electromagnetic induction fields in the deep ocean north east of hawaii implications for mantle conductivity and source fields summary we have analysed a thirty six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north east of hawaii the electromagnetic fields are dominated by tides which have an appreciable oceanic component especially in the east electric and north magnetic components the techniques of data analysis included singular value decomposition svd to remove uncorrelated noise there are three degrees of freedom in the data set for periods longer than five hours indicating a correlation of the vertical magnetic field and the horizontal components suggesting source field inhomogeneity tensor response functions were calculated using spectral band averaging with both svd and least squares techniques and rotated to the principal direction one diagonal component determined mainly by the north electric and east magnetic fields is not interpretable as a one dimensional induction phenomenon the other diagonal term of the response function indicates a rapid rise in conductivity to 0 05 mho m 1 near 160 km no decrease in conductivity below this depth is resolvable polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system

applicability of the site fundamental frequency as a vs30 proxy for central and eastern north america we introduce a new proxy measure for v s 30 time averaged shear wave velocity in the upper 30 m for central and eastern north america cena the new proxy is the site fundamental frequency f peak measured from the horizontal to vertical h v spectral ratios of recorded ground motion or ambient noise in this study h v spectral ratios are obtained from 5 damped pseudospectral acceleration psa from seismograph stations in cena using the next generation attenuation east nga east database we correlate the measured v s 30 values at recording stations with the corresponding f peak values to obtain a predictive relationship for v s 30 the uncertainty of the v s 30 estimate using the f peak based model is small 0 14log10 units in comparison to that for the proxy based methods e g topographic slope and surface geology proxies used in the nga east database 0 25log10 units however values of f peak can be obtained for only about 23 of the nga east recording stations due to data and site type limitations reducing the error in v s 30 estimates can potentially reduce the variability of ground motion prediction equations gmpes to test this hypothesis we use the f peak based v s 30 estimates and select one of the proponent nga east gmpe models for a selected database we are able to reduce the gmpe variability by 3 on average for psa at 1 10 hz just using the f peak based proxy to estimate v s 30 greater variability reductions could be achieved by replacing v s 30 with improved site characterization parameters including f peak in the gmpes

electromagnetic induction fields in the deep ocean off california oceanic and ionospheric sources summary a 56 day long measurement of seafloor electromagnetic field fluctuations is separated into parts coherent and incoherent with the horizontal magnetic field at a terrestrial observatory 1700 km distant but at the same geomagnetic latitude least squares methods show that the m2 and n2 ocean tides are significant in the seafloor data but not on land spectra of the remaining data show high seafloor electric to seafloor magnetic field coherences but markedly lower seafloor to land values especially for the horizontal electric field model studies show that the correlated parts of the seafloor spectra are consistent with induction by broad scale ionospheric sources while the uncorrelated parts are explained by a combination of a barotropic long wave continuum and an internal wave component the oceanic electromagnetic field is discussed in terms of its modal structure and wavelength dependence and implications for seafloor magnetotellurics may be profound

probabilistic assessment of stress normalization for cpt data currently available cone penetration test cpt stress normalization schemes exhibit no consensus on the estimation of the stress normalization component depending on which power law stress normalization exponent is used very different interpretations may result in the analyses where normalized cpt data are used e g cpt based soil classification and seismic soil liquefaction initiation assessment within the confines of this paper it is intended to clarify and resolve some of these differences and to propose improved recommendations for cpt stress normalization for this purpose available stress normalization databases from theoretical numerical and field data analyses approaches were compiled for the soil types and stress conditions where compiled database is not conclusive additional finite element simulations have been performed the resulting relationship not only eliminates several sources of bias intrinsic to previous similar correlations and provides greatly reduced overall uncertainty and variance it also helps to establish a consensus to the stress normalization issue that have long been difficult and controversial key elements in the development of these new correlations are 1 accumulation of a significantly expanded database of analytical numerical cpt simulation results as well as field and chamber test data from homogeneous soil layers 2 use of improved knowledge and understanding of factors affecting cpt and stress normalization and 3 use of high order probabilistic tools bayesian updating

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

observations of the electrical asthenosphere beneath scandinavia the recent recognition that long period i e of the order of hours electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments experiments carried out on the ocean bottom have met with considerable success in delineating the electrical asthenosphere i e a local maximum in electrical conductivity minimum in electrical resistivity in the upper mantle in this paper observations of the time varying magnetic field recorded in three regions of scandinavia northern sweden kiruna kir northern finland northeastern norway kevo kev and southern finland sauvamaki sau are analysed in order to obtain estimates of the inductive response function c for each region the estimated response functions are compared with one from the centre of the east european platform eep and it is shown that the induced eddy currents at periods of the order of 103 104 s in the three regions flow much closer to the surface than under the platform centre specifically at a period of 3000 s these currents are flowing at depths of the order of kev 120 km kir 180 km sau 210 km eep 280 km implying that the transition to a conducting zone of 0 2 s m occurs at around these depths one dimensional inversion of ckev and ckir shows that there must exist a good conducting zone of 0 1 1 0 s m under each of the two regions of 40 km minimum thickness at depths of kev 105 115 km kir 160 185 km this is to be contrasted with eep where the d profile displays a monotonically decreasing resistivity with depth reaching 0 1 s m at 300 km finally a possible temperature range for the asthenosphere consistent with the deduced conducvitity is discussed it is shown that at present there is insufficient knowledge of the conditions water content melt fraction etc likely to prevail in the asthenosphere to narrow down the probable range of 900 1500 c

frequency scaled curvature as a proxy for topographic site effect amplification and ground motion variability we introduce a new methodology to predict the topographic site effect amplification ground motions obtained from a large database of 3d earthquake simulations show that the curvature of the earth s surface defined as the second spatial derivative of the elevation map is correlated with the topographic site amplification the highest correlation between the frequency dependent topographic amplification and the topographic curvature is reached when the curvature is smoothed over a characteristic length equal to the s wavelength divided by two i e frequency scaled curvature fsc this implies the amplification is caused by topographic features for which horizontal dimensions are similar to half of the s wavelength the largest ground motion variabilities are found at sites located on slopes and on the largest summits whereas intermediate variabilities occur over narrow ridges and a stable behavior in the bottom valleys the fsc proxy allows the identification of topographic features with similar characteristic dimensions and probabilistic estimates of amplification values accounting on the variability of ground motions due to source site interactions amplification estimates using the fsc proxy are robust and easily computed from digital elevation maps provided that reasonable values of s wave velocities are available in the area of interest

soil classification analysis based on piezocone penetration test data a case study from a quick clay landslide site in southwestern sweden abstract cone penetration test cpt and piezocone penetration test cptu methods can be effective in site characterization and are important for soil profiling and classification extensive experience exists that relates cpt and cptu results to soil type state and these are often the preferred in situ tools for subsurface investigations soil exploration and the evaluation of different engineering soil properties compared with conventional laboratory testing in this study eight methods consisting of traditional and nontraditional ones for soil classification and prediction of soil type and state using cptu data were examined and tested the cptu data not only provide valuable information on soil types but also can be used for deriving correlations with engineering soil properties for the purposes of hazard analyses and design of foundations we carried out and established a region specific correlation between cptu data and soil properties for three location test points in southwestern sweden at a quick clay landslide site first an analysis of the available cptu data was performed and then classification of the soils was made after determination and identification of the soil profiles the results were further evaluated using the unified soil classification system uscs and converted to cpt spt charts we summarize results from each of these methods including a description of the available charts and their performance for soil classification from cptu data we provide high resolution soil profiles from the three test points our work allowed the detection of potential sensitive fine grained clays which are responsible for landslides in the study area however in most cases occurring above coarse grained materials these materials were also detected in the cptu data and show good correspondence with available reflection seismic profiles from the site

geomagnetic variation sounding of the asthenosphere beneath the juan de fuca ridge two ocean bottom magnetometers have been deployed at sites on either side of the juan de fuca ridge 20 and 40 km from the crest near latitude 47 n twelve days of data were recorded simultaneously at the two sites and these data have been used to determine the electrical conductivity structure beneath the region the vertical field variations at both stations are of very low amplitude in the period range 0 25 to 8 hours with no detectable phase reversal across the ridge electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to victoria and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of oldenburg 1979 resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section the average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of shankland and waff 1976 for a basalt melt fraction within an olivine matrix for the low resistivity zone and depending on the water content of the melt 0 to 10 the estimated temperature range is 1400 c to 1250 c and the corresponding melt fraction 0 05 to 0 09 the data suggest that the melt fraction decreases below 80 km the conductivity profile from magnetotelluric data on 72 ma ocean floor in the north central pacific filloux 1977 has similar characteristics to the 1 ma old conductivity structure near the juan de fuca ridge but with the low resistivity layer at depths of 140 to 220 km in both cases the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere rather than with the base of the lithosphere the increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts

effects of three dimensional structure on magnetotelluric sounding curves significant errors may result when applying one dimensional 1 d or two dimensional 2 d interpretation methods to magnetotelluric mt data collected in three dimensional 3 d environments both depths and resistivities can be grossly incorrect if 1 d or 2 d methods are applied in a 3 d setting we present examples of mt sounding curves generated using a 3 d modeling program madden and park 1982 and illustrate some interpretation pitfalls if 3 d effects are not considered the 3 d effects discussed herein are attributed to a surface layer heterogeneity and can be readily identified in mt data from a well designed mt survey the mt survey must include stations chosen to yield information about regional structure alternatively carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data

magnetotelluric measurements over the alpha ridge abstract magnetic and telluric field variations were recorded on the arctic ocean sea ice over the alpha ridge in april and may 1983 the magnetotelluric measurements were made near the base camp of operation cesar canadian expedition to study the alpha ridge to investigate the deep structure and electrical properties of the lithosphere and upper mantle beneath the ridge thirty days of usable data were obtained within the period range 120 20 000 s during the experiment the base station drifted across a deep 2000 m median valley or graben lying parallel to the ridge axis and then over a steep rise on its northern flank one data set was obtained over the graben and the remainder of the measurements were made as the station travelled back and forth over the rise the directional properties of both the geomagnetic induction arrows and the magnetotelluric impedance tensors were consistent with changes in water depth along the drift path one dimensional inversions of e polarization mt data revealed a resistive lithosphere about 70 85 km thick with a conductive 10 m upper mantle asthenosphere below sea water depths estimated in the inversion process agreed remarkably well with the known bathymetry indicating that the ocean floor contained less than 100 m of highly conducting sediments along most of the drift path two dimensional models required simulation of the topography of the entire section of the alpha ridge between the canada and makarov basins in order that a satisfactory fit be obtained to the h polarization data two dimensional forward modelling indicates that the anisotropic character of the impedance measurements can entirely be attributed to the bathymetry of the ridge the data provided no indication of structural inhomogeneity within the oceanic crust and lithosphere consistent with results from companion seismic refraction and gravity surveys the thickness of the lower conducting region was not resolved because reliable long period data t 20 000 s were not available for analysis impedance estimates at these periods were poorly determined and subject to rapidly rising tensor skew they were also subject to bias errors caused by water movements in particular the m2 tide

assessment of the liquefaction susceptibility of fine grained soils observations from recent earthquakes and the results of cyclic tests indicate that the chinese criteria are not reliable for determining the liquefaction susceptibility of fine grained soils fine grained soils that liquefied during the 1994 northridge 1999 kocaeli and 1999 chi chi earthquakes often did not meet the clay size criterion of the chinese criteria cyclic testing of a wide range of soils found to liquefy in adapazari during the kocaeli earthquake confirmed that these fine grained soils were susceptible to liquefaction it is not the amount of clay size particles in the soil rather it is the amount and type of clay minerals in the soil that best indicate liquefaction susceptibility thus plasticity index pi is a better indicator of liquefaction susceptibility loose soils with pi 0 85 were susceptible to liquefaction and loose soils with 12 0 8 were systematically more resistant to liquefaction soils with pi 18 tested at low effective confining stresses

magnetic and electric fields produced in the sea during geomagnetic disturbances to understand geomagnetic effects on systems with long conductors it is necessary to know the electric field those systems experience for surface conductors such as power systems and pipelines this can easily be calculated from the magnetic field variations at the surface using the surface impedance of the earth however for calculating the electric fields in pipelines and submarine cables at the seafloor it is necessary to take account of the attenuating effect of the conducting seawater assuming that the fields are vertically propagating plane waves we derive the transfer functions between the electric and magnetic fields at the seafloor and the magnetic field variations at the sea surface these transfer functions are then used with surface magnetic field data to determine the power spectra of the seafloor magnetic and electric fields in a shallow sea depth 100 m and in the deep ocean depth 5 km for different values of the kp magnetic activity index for the period range considered 2 min to 3 hrs the spectral characteristics of the seafloor magnetic and electric fields for a 100 m deep sea are very similar to those of the surface fields for the deep ocean the seafloor spectra show a faster decrease in spectral density with increasing frequency compared to the surface fields the results obtained are shown to be consistent with seafloor observations assessment of the seafloor electric fields produced by different levels of geomagnetic activity can be useful in the design of the power feed equipment for submarine cables and cathodic protection for undersea pipelines

attenuation relations of strong ground motion in japan using site classification based on predominant period a spectral acceleration attenuation model for japan is presented in the present study the data set includes a very large number of strong ground motion records up to the 2003 off tokach main and aftershocks site class terms instead of individual site correction terms are used the site classes of recording stations are from a recent study on site classification for strong motion recording stations in japan according to a classification scheme that has been used in japanese engineering design the use of site class terms enables tectonic source type effects to be identified and accounted for in the present model the effects of a faulting mechanism for crustal earthquakes also are accounted for for crustal and interface earthquakes a simple form of an attenuation model with respect to distance is able to capture the main strong motion characteristics and achieves unbiased estimates for subduction slab events a simple distance modification factor is employed to achieve plausible and unbiased predictions the effects of source depth tectonic source type and faulting mechanism of crustal earthquakes are significant the need for magnitude squared terms is evaluated and the use of magnitude squared terms reduces the interevent error further

the tasman project of seafloor magnetotelluric exploration experiment and observations abstract a major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between australia and new zealand simultaneous observations were made by land magnetometers on the australian continent the large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites the parkinson arrows show a strong coast effect near the australian continent and a weaker coast effect over the submerged lord howe rise the occurrence of smoothly varying arrows across the tasman sea suggests a simple large scale pattern for electromagnetic induction in the tasman sea the interpretation of parkinson arrows at seafloor sites is shown to require special care due to the attenuation and phase shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations the magnetotelluric impedances exhibit a strong anisotropy at all sites in the tasman sea the orientation of the impedance axes is constant across the sea and the anisotropy may be attributed to the two dimensional shape of the tasman sea calculations based on the distance scale of the magnetotelluric anisotropy interpreted as an ocean continent boundary effect give a lower bound for the integrated resistivity of the tasman sea oceanic lithosphere of order 107 m2 corresponding to an estimate of 2 102 m for its average resistivity

evaluation of flow liquefaction and liquefied strength using the cone penetration test flow liquefaction is a major design issue for large soil structures such as mine tailings impoundments and earth dams if a soil is strain softening in undrained shear and hence susceptible to flow liquefaction an estimate of the resulting liquefied shear strength is required for stability analyses many procedures have been published for estimating the residual or liquefied shear strength of cohesionless soils this paper presents cone penetration test based relationships to evaluate the susceptibility to strength loss and liquefied shear strength for a wide range of soils case history analyses by a number of investigators are reviewed and used with some additional case histories extrapolations beyond the case history data are guided by laboratory studies and theory

what is a reference site many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a reference motion the critical assumption in these methods is that the surface rock site record reference is equivalent to the input motion at the base of the soil layers data collected in this study show that surface rock sites can have a site response of their own which could lead to an underestimation of the seismic hazard when these sites are used as reference sites data were collected from local and regional earthquakes on digital recorders both at the surface and in boreholes at two rock sites and one basin site in the san jacinto mountains southern california the two rock sites keenwild and pinon flat are located on granitic bedrock of the southern california peninsular ranges batholith the basin site garner valley is an ancestral lake bed with watersaturated sediments on top of a section of decomposed granite which overlies the competent bedrock ground motion is recorded simultaneously at the surface and in the bedrock at all three sites when the surface rock sites are used as the reference site i e the surface rock motion is used as the input to the basin the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 hz this underestimation by a factor of 2 to 4 depending on frequency and site results from the rock sites having a site response of their own above the 2 to 5 hz frequencies the near surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest even at sites that appear to be located on competent crystalline rock the bedrock borehole ground motion can be used as the reference motion but the effect of the downgoing wave field and the resulting destructive interference must be considered this destructive interference may produce pseudo resonances in the spectral amplification estimates if one is careful the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

numerical modeling of site effects at san giuliano di puglia southern italy during the 2002 molise seismic sequence the seismic sequence that occurred in october and november 2002 in the molise region southern italy was characterized by two mw 5 7 earthquakes within 24 h followed by one month long aftershocks series the mainshocks caused substantial structural damage in the village of san giuliano di puglia the damage distribution was highly non uniform heavy and widespread damage was observed to all buildings constructed in the recently developed part of the village where subsoil conditions are characterized by a bowl shaped basin filled with stiff clays whereas in the historical center built on an adjacent rock outcrop many buildings showed no or light damage several accelerograms were recorded during the aftershocks sequence by a temporary network installed on two sites in the san giuliano village located on rock and soil respectively the geological seismological geotechnical and structural relevant information of the earthquakes are presented in the first part of the paper the second part of the p

site amplification in the san fernando valley california variability of site effect estimation using the s wave coda and h v methods during the months that followed the 17 january 1994 m 6 7 northridge california earthquake portable digital seismic stations were deployed in the san fernando basin to record aftershock data and estimate site amplification factors this study analyzes data recorded on 31 three component stations from 38 aftershocks ranging from m 3 0 to m 5 1 and depths from 0 2 to 19 km site responses from the 31 stations are estimated from coda waves s waves and ratios of horizontal to vertical h v recordings for the coda and the s waves site response is estimated using both direct spectral ratios and a generalized inversion scheme results from the inversions indicate that the effect of qs can be significant especially at high frequencies site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other depending on the choice of the reference site the difference is reduced when an average of six rock sites is used as a reference site in addition when using this multi reference site the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the s waves however for nonrock sites the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the s waves the h v method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods the amplifications determined from the hiv method are however different from the amplifications determined from the other methods finally the stations were grouped into classes based on two different classifications general geology and a more detailed classification using a quaternary geology map for the los angeles and san fernando areas average site response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification

analytical solution to the seismic response of visco elastic soil layers the response of a multi layered soil deposit subjected to a horizontal earthquake ground motion is obtained in a form which is exact in the sense that both the starting transients and the steady state parts are included in the solution and can be evaluated to any degree of accuracy the solution is compared with that obtained as a steady state one for harmonic excitations it is shown that the transient part of the response which is required for the solution to satisfy initial conditions and which does not appear in the steady state may be important particularly for long period layers subjected to short period acceleration trains for small values of the radiation coefficients radiation damping may be introduced in the solution in the form of equivalent viscous damping la reponse d un sol multicouches soumis a des mouvements sismiques horizontaux est presentee sous une forme exacte puisque l etat transitoire du depart et l etat permanent sont tous deux pris en compte dans la solution et peuvent etre

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

surface and near surface effects on seismic waves theory and borehole seismometer results a simple plane wave model is adequate to explain many surface versus borehole seismometer data sets using such a model we present a series of examples which demonstrate the effects of the free surface near surface velocity gradients and low impedance surface layers on the amplitudes of upcoming body waves in some cases these amplitudes are predictable from simple free surface and impedance contrast expressions however in other cases these expressions are an unreliable guide to the complete response and the full plane wave calculation must be performed large surface amplifications are possible even without focusing due to lateral heterogeneities or nonlinear effects both surface and borehole seismometer site responses are almost always frequency dependent ocean bottom versus borehole seismic data from the 1983 ngendei seismic experiment in the southwest pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation the borehole seismic response to upcoming p waves is reduced at high frequencies because of interference between the upgoing p wave and downgoing p and sv waves reflected from the sediment basement interface however because of much lower borehole noise levels the borehole seismometer enjoys a p wave signal to noise advantage of 3 to 7 db over nearby ocean bottom instruments

on the electrical crust mantle structure in fennoscandia no moho and the asthenosphere revealed summary an interpretation of the geomagnetic inductive response function c 0 observed at kiruna in northern sweden is herein undertaken the bounds of acceptable solutions are initially discovered by a monte carlo random search procedure and the best fitting solutions are examined by the application of linear theory to the problem the data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum a further monte carlo inversion of the variance reduced data set gives solutions with well defined model parameters the two major features of the models are 1 a small or non existent electrical conductivity variation across the seismic moho boundary and 2 the unequivocal existence of an electrical asthenosphere under the fenno scandian shield beginning at a depth of between 155 185 km and of 60km minimum thickness both of these observations have seismic counterparts finally possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed

statistical evaluation of mt and amt methods applied to a basalt covered area in southeastern anatolia turkey ilkisik o m and jones a g 1984 statistical evaluation of mt and amt methods applied to a basalt covered area in southeastern anatolia turkey geophysical prospecting 32 706 724 the efficacy of the magnetotelluric and audiomagnetotelluric mt amt methods for detailing the structure of a hypothetical geological section is investigated by using the singu lar value decomposition svd technique the section is representative of southeastern turkey which is mostly covered by basalt and is a prime area for oil exploration one of the geological units the germav shale at a depth of 600 m is a problem layer for electromagnetic surveys because of its very low resistivity on average 3 om and highly variable thickness across the area 200 900 m in the mt frequency range 0 0004 40 hz its total conductance or since its resistivity is known from resistivity log information its thickness is the best resolved model parameter the total depth to the germav shale and the resistivity of the cambrian precambrian basement are the marginally resolved parameters in the amt frequency range 4 10000 hz the resistivity of the surface basalt layer strongly affects the resolution of the other less important model parameters which are the total depth to the germav shale and the total conductance of the germav shale the errors in the measure ments determine the number of model parameters resolvable and are also important for interpretation of the geological model parameters to within a desired accuracy it is shown that statistical evaluation of the mt and or amt interpretations by using an svd factorization of the sensitivity matrix can be helpful to define the importance of some particular stage of the interpretation and also provides a priori knowledge to plan a pro posed survey arrangements of mt and amt observations together with some schlumber ger resistivity soundings on a large grid will certainly provide three dimensional detailed information of the deep geoelectric structure of the area

site effects in a volcanic environment a comparison between hvsr and array techniques at colima mexico colima city is the capital of the mexican federal state of the same name it is located close to the pacific coast and is subjected to a large seismic risk we present a microzonation study in this city based on microtremors using single station and array measurements we applied horizontal to vertical spectral ratios hvsr analysis to single station measurements at 310 sites within the city concentrating measurements in zones that were damaged by the january 2003 m 7 4 earthquake the results show that a seismic zonation based exclusively on single station microtremor measurements is not a reliable alternative when the local geology is complex and site effects are not the result of a single impedance contrast for this reason we applied two independent analysis techniques to array measurements of microtremors the spatial autocorrelation spac method and the refraction microtremor remi method we used linear arrays to record 25 sec microtremor windows at eight sites within the city which were analyzed with those two techniques the result of both techniques of analysis is a phase velocity dispersion curve which can be inverted to obtain a shallow s wave velocity profile two of the sites were the location of shallow 50 m boreholes where p and s wave velocity profiles were measured using a p s suspension log the phase velocity dispersion curves obtained from the remi and spac analyses of the microtremor records showed very good agreement the velocity profiles inverted from the phase velocity dispersion curves showed good agreement with the suspension logging measurements at one of the two sites where they were available and poor agreement at the other site the transfer functions computed from the inverted soil profiles are in good agreement with previous estimates of local amplification from spectral ratios analysis of earthquake records our results are compatible with previous indications of site effects and explain the failure of single station microtremor measurements when the concept of dominant frequency loses its meaning finally we propose an estimate of local site amplification at the city of colima which will be useful for future predictions of ground motion at this city