borehole response studies at the garner valley downhole array southern california the garner valley downhole array gvda consists of a set of seven downhole strong motion instruments ranging from 0 to 500 m depth one of the objectives of this experiment is to estimate site response and study wave propagation as the energy travels from the bedrock underneath the site up through the soil column the gvda velocity structure is studied by computing synthetic accelerograms for a small event located at an epicentral distance of 10 km these synthetics simulate well the data recorded at the borehole stations in addition theoretical transfer functions are calculated using the obtained velocity model and compare well with the empirical transfer functions from 54 recorded events it is also observed that the downgoing wave effect is predominant in the first 87 m and is strongly reduced at depth using the velocity structure at gvda and the transfer function results it has also been possible to develop a simple method to compute the incident wave field which is needed in nonlinear site response for instance recently there have been many comparative studies between horizontal to vertical h v spectral ratios and traditional spectral ratios although many of these studies show that h v spectral ratios can reproduce the shape of the site response curve most show differences in the amplitude level in the case of garner valley where we have both surface and multiple borehole instruments we find that this discrepancy in amplitude of the site response estimates is because the vertical component has significant site response associated with it due to s to p conversions that begin in the weathered granite boundary at 87 m depth manuscript received 27 august 2001

clinker deposits powder river basin wyoming and montana a new source of high fidelity paleomagnetic data for the quaternary clinker rock baked and fused by in situ burning of underlying coal posesses a thermoremanent and or thermochemical magnetization that resides in magnetite hematite and goethite data from individual sites imply that clinker may provide a high resolution recording of the geomagnetic field e g within site virtual geomagnetic pole angular standard deviations are usually less than 10 but this is dependent on its thermal history and the time over which magnetic phases formed data from 17 sites in the powder river basin 13 normal polarity 4 reverse yield a mean of decl 348 incl 62 95 5 k 42 and a corresponding pole position of 81 lat 158 long with a virtual geomagnetic pole angular standard deviation of 17 3 the mechanism of clinker formation the wide geographic distribution and the range of available fission track ages suggest that clinker may contain a nearly complete geomagnetic field record for much of the quaternary

a comparison and test of various site response estimation techniques including three that are not reference site dependent abstract we compare various site response estimation techniques using after shock data of the 1989 loma prieta earthquake collected in oakland california because of recent interest in comparing results from weak and strong motion to infer any nonlinearity and between direct s and coda waves we pay particular attention to the uncertainties first sediment to bedrock spectral ratio estimates between pairs of sites are compared with those obtained from various generalized inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously we find that the site amplification factors are very similar among these approaches but that the uncertainties can be significantly different depending on how the data are weighted we also examine and test three site response estimation techniques that do not rely on a reference site to estimate source and path effects the first involves a parameterized source and path effects inversion even when the bedrock data are excluded from consideration this approach is found to reveal the frequency dependence of site response at each of the sediment sites the second technique involves taking horizontal to vertical component spectral ratios receiver function type estimates of shear wave aftershock data these are also found to reveal the frequency dependence of site response at the sediment sites and the results for the bedrock site are relatively flat and near unity the third estimate is formed by taking horizontal to vertical component ratios of ambient seismic noise and these are shown to reveal the fundamental resonant frequency of the sediment sites unfortunately discrepancies exist among all of the site response estimates and with one dimensional predictions with respect to a frequency independent scaling factor nevertheless the highly frequency dependent character of site response is well constrained and the fact that non reference site dependent methods are capable of revealing this is promising for site specific hazard assessments in regions that lack adequate reference sites

a note on the secular variation of the geomagnetic field abstract geomagnetic variations with apparent periods from 13 to 30 y have been reported by alldredge 1977 who has argued that the origin of these signals must be found in the core of the earth rather than outside the earth it is shown in this paper that a portion of these geomagnetic variations perhaps most of the variations might well be due to geomagnetic signals of much longer period originating in the core that appear to have periods of 13 30 y because of an artifact of the data processing much of the remainder of these variations could well be of external origin a method for processing these data is suggested that is superior to alldredge s method in minimizing the generation of spurious waves by long period secular variation

geomagnetic field intensity and reversal asymmetry in late precambrian keweenawan rocks summary fifty four palaeointensity values measured by the thellier technique are used to study the cause of reversal asymmetry in late precambrian keweenawan rocks these data obtained mainly from diabase dykes carrying primary trm and their baked host rocks show that reversed palaeointensities are on average 40 per cent higher than their normal counterparts this result cannot be explained by any overall difference in remanence characteristics grain size or cooling rate dyke width between the rocks comprising the two polarity groups a significant difference in the local intensity of the geomagnetic field between times of normal and reversed polarity is thus indicated this conclusion is reinforced by positive baked contact tests for both normal and reversed thellier palaeointensities secondary component and single dipole offset models as explanations for keweenawan reversal asymmetry are in conflict with the palaeointensity results the difference in palaeointensity between normal and reversed rocks disappears when the data are reduced to the palaeoequator in accordance with a geocentric axial dipole field the palaeointensity results are therefore consistent with the idea that reversal asymmetry is caused by the north american plate occupying a lower palaeolatitude when the field had normal polarity acceptance of a geocentric axial dipole would imply that the keweenawan geomagnetic field intensity was about 45 per cent higher than at present however evidence at one locality of possibly three asymmetric reversals poses problems for a plate movement interpretation and forces consideration of a model whereby the asymmetry is caused by persistent non dipole field components rock magnetic studies show that reliable palaeointensity estimates can be obtained for rocks which have stable nrm with single domain or pseudo single domain grain sizes and with a high koenigsberger ratio baked contact rocks are the most reliable whereas hydrothermally altered lavas are unsuitable for thellier intensity determinations it is suggested that af demagnetization studies on companion specimens should be performed before thellier experiments in order to select specimens which have high values of median destructive field and which lack viscous or other overprints

nonlinearity liquefaction and velocity variation of soft soil layers in port island kobe during the hyogo ken nanbu earthquake clear nonlinear behavior is analyzed from the acceleration records of the 1995 hyogo ken nanbu earthquake at port island kobe from four triaxial in struments placed at four different depths the surficial effects during strong ground motions were compared with those during weak motions before and after the main shock we used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data from the spectral analysis we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state the spectral ratios after the mainshock i e after the liquefied state are different from those before the mainshock the peak frequencies in the spectral ratios after the mainshock are shifted to lower fre quencies with respect to those in the spectral ratios before the mainshock we in verted the s wave velocities using a genetic algorithm technique to determine the velocity structure before during and after the mainshock the s wave velocity struc ture before and after the mainshock was found to be different specifically the s wave velocity of the second layer 5 m to 16 m depth after the mainshock was 20 lower than before our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr the rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve the strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions so that the observed horizontal peak acceleration was only about 25 of the peak acceler ation expected from the linear theory however the nonlinear effects in the vertical peak acceleration were not significant

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

evaluation of local site effects and their removal from borehole records observed in the sendai region japan abstract we present results from a borehole observation network composed of 12 borehole sites deployed in a 20 20 km area in the sendai region japan the subsurface quaternary layers vary with each site but a pliocene layer is commonly underlying at a depth of 0 to 80 m throughout the sendai region we define this pliocene layer whose s wave velocity is larger than 500 m sec and whose n value of the standard penetration test is greater than 50 as an engineering bedrock in the region once we characterize ground motions at the engineering bedrock we can either use these motions directly in the response analysis of whole soil building systems or use them in the nonlinear analysis of soft soil layers as an input the purpose of this study is to evaluate the local site effects due to surface layers overlying the engineering bedrock and to remove them by using one dimensional 1d soil models whose properties are determined by weak motion records of 18 events first we identify s wave velocities and frequency dependent quality factors q from amplification factors between surface records and borehole records observed at 10 sites at depths of several tens of meters the identified s wave velocities are very close to s wave logging values the identified q values show strong frequency dependence proportional to f 0 46 to 1 16 where f is frequency we confirm that the observed amplification factors in the frequency range from about 0 1 to 20 hz can be explained well by the theoretical ones based on the 1d wave propagation theory with the identified s wave velocities and q then we estimate the so called engineering bedrock waves which are supposed to be observed on the outcrop of the engineering bedrock from borehole records by using the 1d models with these identified soil constants we confirm that local site effects due to surface layers overlying the engineering bedrock are properly removed so that we have similar characteristics in the resultant engineering bedrock waves from records at different sites

geoid anomalies over cooling lithosphere source for a third kernel of upper mantle thermal parameters and thus an inversion summary the geoid anomaly over an isostatic mass redistribution on an infinite flat earth is simply 27 c g times the differential mass moment on a spheroidal earth the limiting value can never quite be reached because the external effect of a spherical shell of mass dipole is identically zero so distant portions of the spherical surface subtract from the effect of the region under the point of observation the width of the integrating kernel varies with the depth of the mass distribution but the observable fraction of the limiting geoid slope can be estimated at least for ocean basins of moderate extent where it should reach about 90 per cent for very small 3 cm yr 1 ocean basins the observable geoid slope is smaller a consideration of published geoid data together with gravity anomalies in the same area compared to the theoretical free geoid gravity anomalies gravity observed on the geoid leads with great caution to a crude estimate of the thermally produced slope of 0 14 0 02 m myr 1 given a value of the thermal geoid signal it is possible to conduct an inversion for the upper mantle thermal parameters if two the density and specific heat are assumed to be known the parameter kernels for the heat flow topographic decay and geoid give three equations to solve for expansion coefficient diffusivity and initial temperature ti the results are different from common modelling assumptions 3 1 10 5 c 1 17 per cent 0 0065 cm2 s l 45 per cent and 1660 c 29 per cent manipulation of the equations in the case of general temperature variable parameters leads to partial explanations appears multiplied by k0 7 k0 conductivity at 0 7 of t1 at surface temperature while k is weighted toward the lower values in the low to medium temperature range an estimate of t1 using the shape of the temperature variations of k and k in the most reliable available data for olivine remains at about 1660 c 30 per cent in spite of the large error band the higher initial temperature is plausible because decompression of upwelling material produces enough partial melting to generate the oceanic crust the latent heat of melting must be supplied from somewhere and the only other possible source is a large amount of mechanical work applied to the system in the divergent flow

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

estimation of q for long period 2 sec waves in the los angeles basin we simulate 0 to 0 5 hz 3d wave propagation through the southern california earthquake center seismic velocity reference model version 2 for the 1994 northridge earthquake in order to examine the effects of anelastic attenuation and amplification within the near surface sediments we use a fourth order finite difference staggered grid method with the coarse grained frequency independent anelastic scheme of day and bradley 2001 and a variable slip distribution from kinematic inversion for the northridge earthquake we find that the near surface material with s wave velocity v s as low as 500 m sec significantly affects the long period peak ground velocities compared with simulations in which the s wave velocity is constrained to 1 km sec and greater anelastic attenuation also has a strong effect on ground motion amplitudes reducing the predicted peak velocity by a factor of up to 2 5 relative to lossless simulations our preferred q model is q s v s 0 02 v s in meters per second for v s less than 1 2 km sec and much larger q s v s 0 1 v s in meters per second for layers with higher velocities the simple model reduces the standard deviation of the residuals between synthetic and observed natural log of peak velocity from 1 13 to 0 26 relative to simulations for the lossless case the anelastic losses have their largest effect on short period surface waves propagating in the los angeles basin which are principally sensitive to q s in the low velocity near surface sediments of the basin the low frequency ground motion simulated here is relatively insensitive to q p as well as to the values of q s at depths greater than roughly that of the 2 km sec s wave velocity isosurface

temporal changes in site response associated with the strong ground motion of the 2004 mw 6 6 mid niigata earthquake sequences in japan we analyzed temporal changes in site response associated with the strong ground motion of the 2004 mw 6 6 mid niigata earthquake sequence in japan the seismic data were recorded at a site with accelerometers at the surface and a 100 m deep borehole we computed the empirical surface to borehole spectral ratios and used them to track temporal changes in the top 100 m of the crust we observed that the peak spectral ratio decreases by 40 60 and the peak frequency drops by 30 70 immediately after large earthquakes the coseismic changes are followed by apparent recoveries with the time scale ranging from several tens to more than 100 sec the coseismic peak frequency drop peak spectral ratio drop and the post seismic recovery time roughly scale with the input ground motions when the peak ground velocity is larger than 5 cm sec or the peak ground acceleration is larger than 100 gal our results suggest that at a given site the input ground motion plays an important role in controlling both the coseismic change and the postseismic recov ery in site response

secular variation as recorded in lake sediments a discussion of north american and european results type curves depicting secular variations in declination and inclination through holocene time have been constructed by stacking data from individual cores of lake sediment from east central north america and the u k there is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 14 c years b p but from 4500 to 8500 14 c years b p the correlation is strongly negative which we interpret in terms of fluctuations in intensity of standing geomagnetic sources a source located under the atlantic ocean appears to have been dominant before ca 4500 14c years b p and then another similar source located either to the east or to the west of both observation areas gained dominance however the inclination type curves show optimum positive correlation for a phase shift of ca 400 years suggesting that the geomagnetic sources that caused them were drifting westward at ca 0 2 degrees a year there is no way of adjusting the timescales so as to obviate this apparent paradox secular variations of the total geomagnetic vector have been investigated by noting the sense clockwise anticlockwise of looping of the v g p paths though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core as demonstrated by specific radial dipole models

the late pleistocene geomagnetic field as recorded by sediments from fargher lake washington u s a measurement of the remanent magnetization of a 6 88 m oriented core of soft sediments and tephras from fargher lake near mount st helens in southwestern washington state shows that no significant geomagnetic reversals were recorded in the sediments of the lake radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17 000 34 000 years b p although geochemical correlation of a prominent tephra layer in the core with tephra set c of mount st helens could mean that the maximum age of the sediments may be at least 36 000 years b p the core was divided into specimens 0 02 m long each representing approximately 55 years of deposition assuming a constant rate of sedimentation pilot alternating field demagnetization studies of every tenth specimen indicated a strong stable remanence with median destructive field of 15 mt and the remaining specimens were subsequently demagnetized in fields of this strength the mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56 1 which is 8 shallower than the present axial dipole field at this site perhaps because of inclination error in the detrital remanent magnetization of the sediments although because of the variability in the data this departure from the axial dipole field may not be significant the ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes although three isolated specimens have remanence with negative inclination these anomalous directions are due to sampling and depositional effects measurement of a second core of 6 86 m length also revealed only normal magnetic polarity but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and x ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin the magnetite occurs in a wide range of grain sizes with much of it of sub multidomain size 15 m as a whole this study provides substantial evidence against the existence of large scale worldwide geomagnetic reversals during the time interval of fargher lake sedimentation a segment of geological time for which many excursions and reversals have been reported elsewhere

a natural seismic isolating system the buried mangrove effects the belleplaine test site located in the island of guadeloupe french lesser antilles includes a three accelerometer vertical array designed for liquefac tion studies the seismic response of the soil column at the test site is computed using three methods the spectral ratio method using the vertical array data a numerical method using the geotechnical properties of the soil column and an operative fre quency domain decomposition fdd modal analysis method the belleplaine test site is characterized by a mangrove layer overlaid by a stiff sandy deposit this con figuration is widely found at the border coast of the caribbean region which is exposed to high seismic hazard we show that the buried mangrove layer plays the role of an isolation system equivalent to those usually employed in earthquake engineering aimed at reducing the seismic shear forces by reducing the internal stress within the structure in our case the flexibility of the mangrove layer reduces the distortion and the stress in the sandy upper layer and consequently reduces the potential of liquefaction of the site

the paleomagnetism and u th dating of three canadian speleothems evidence for the westward drift 5 4 2 1 ka bp three speleothems from western canada have been analyzed for their paleomagnetism and u th activity ratios these cave deposits do not appear to show the kind of depositional effects on the recorded magnetic signal as is often the case with sediments in addition the signal may be referred directly to geographic coordinates so that virtual geomagnetic pole plots may be constructed this in turn has allowed assessment of any possible bias of the paleofield the u th activity ratios of two flowstones from the crowsnest pass area of british columbia indicated ages greater than the 350 ka dating limit of the dating method one of the samples was reversely magnetized besides their use in the study of the ancient field these samples have useful geomorphic applications a stalagmite from vancouver island was shown to have recorded clockwise loops of the field vector in the period 5 4 2 1 ka bp this implies a mainly westward drift of the paleofield for this period the corresponding vgp s were mostly far sided

influence of a sedimentary basin infilling description on the 2 d p sv wave propagation using linear and non linear constitutive models numerical modelling is a useful tool to understand the role of different parameters site geometry impedance contrast material properties constitutive model input motion governing site effects we focus here on the 2 d p sv seismic wave propagation in a simple shaped asymmetric model we consider two ways of describing the sedimentary infilling properties the basin is either composed of distinct homogeneous geological layers or is described through properties progressively changing as a function of depth smooth model in order to study the wave propagation in the two basins with similar soil properties p and s velocities of the layered basin are deduced from the smooth basin ones making both models compatible in a kinematic point of view p and s velocities globally increase with depth following epri non linear properties are considered as constant in layers for the two basins we assess the influence of the model choice on the wave propagation while taking into account linear and non linear constitutive models we test the influence of the input motion by propagating two different input motions the first being a simple impulsive one and the second a real complex accelerogram we find that transfer functions computed in each basin are quite similar irrespective of the soil constitutive behaviour and the input motion moreover we show that the maximum shear strain spatial distribution depends on the input motion frequency content and maximum values depend on the input motion strength and complexity maximum shear strains are generally higher for superficial layers than for deeper ones we show that the highest shear strain values are located above discontinuities where velocities are lower and the soil is more non linear than in the underlying layer regardless of the input motion indeed in a layered basin the shear strain is higher above interfaces for all the soil constitutive models that were used in the smooth visco elastoplastic model a kind of layering appears absent in a viscoelastic model due to the different non linear properties in each layer moreover no matter what the model structure and the input motion used maximum shear strain levels are higher in the visco elastoplastic soil than in the viscoelastic one such a numerical study helps to understand the non linearity location and therefore the consequences of a model choice when performing a site specific study

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

inversion of strain dependent nonlinear characteristics of soils using weak and strong motions observed by borehole sites in japan during the 1995 hyogo ken nanbu earthquake strong motions with horizontal peak ground accelerations of 507 cm sec 2 or more 187 cm sec 2 and 126 cm sec 2 were observed at surface stations of borehole sites at amagasaki takasago and nanko in japan respectively we showed qualitative evidence of nonlinear soil response during strong motions at amgasaki and takasago but not at nanko using cross correlation and spectral ratio analyses of weak and strong motions we evaluated soil nonlinearity by inversion for s wave velocities and frequency dependent damping factors the strain dependence of shear modulus ratios inverted for gravel is larger than that for clay which is consistent with laboratory test results in the shear strain range from about 10 4 to 10 3 the strain dependence of shear modulus ratios inverted for clay and gravel at amagasaki and takasago is consistent with empirical relations based on laboratory test results for clay and gravel respectively these results support using laboratory estimates of shear modulus ratios to simulate strong motions at clay and gravel sites the damping factors for gravel inverted from the mainshock at amagasaki are the largest among the others but the strain dependence of damping factors are not as significant as that of shear modulus ratios the reason why the strain dependence of damping factors is not so significant may be because scattering attenuation is relatively dominated in the low strain range

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift although conductive intracrustal layers appear to be characteristic of continental rift zones they also have been detected in stable crustal environments the low resistivity layers are often explained by free water or crustal magma however alternative explanations are possible purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings certain minerals such as graphite and hydrated minerals e g serpentine may be important thermally activated processes associated with ductile flow mechanisms may also enhance conductivity two dimensional analysis of a portion of 25 new magnetotelluric stations in the central rio grande rift supports the presence of a conductive zone at 10 km depth or less however very significant differences are apparent in the crustal sections at two close localities mid crustal magma lenses well defined by seismic data cannot explain these differences in fact the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels only the possible effects of regional three dimensional current channeling could conceivably alter this conclusion to explain the unexpected result it is suggested that a conductive horizon occurs where an impermeable ductile cap traps pore fluids beneath the cap may be nearly an order of magnitude more conductive 100 s m than the dry brittle crust above the zone of trapped pore fluids much more conductive by more than an order of magnitude 10 m where active magma injection destroys the integrity of the ductile cap trapped fluids may escape and produce an overall decrease in conductivity the final electrical signature with this dynamic concept depends on the thermal gradient the relative impermeability of the cap the extent of pore fluids beneath and the amount and frequency of magma intrusion the temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection

in situ assessment of the g γ curve for characterizing the nonlinear response of soil application to the garner valley downhole array and the wildlife liquefaction array abstract we analyze the nonlinear and near surface geological effects of two network for earthquake engineering simulation at university of california santa barbara nees ucsb instrumented sites the garner valley downhole array gvda and the wildlife liquefaction array wla the seismic interferometry by deconvolution method is applied to earthquake data recorded by the multisensor vertical array between january 2005 and september 2013 along the cross section local shear wave velocity is extracted by estimating travel time between sensors the s wave velocity profiles are constructed and compared with classical in situ geophysical surveys we show that velocity values change according to the amplitude of the ground motion and we find anisotropy between east west and north south directions at the gvda site the ratio between average peak particle velocity v and local s wave velocity between two boreholes is tested as a deformation proxy using average peak particle acceleration a the a versus curve is used to represent the stress strain curve for observing the site s nonlinear responses under different levels of excitation nonlinearity is observed from quite low shear strain levels 1 10 5 and a classic hyperbolic model is derived proves to be a good deformation proxy finally the shear modulus degradation curves are constructed for each depth and test site and they are similar to previous laboratory measurements or in situ geophysical surveys a simple comparison regarding nonlinear behavior between gvda and wla is performed

geomagnetic reversal frequency since the late cretaceous models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the heirtzler et al 1 magnetic polarity time scale which has subsequently been superseded examination of newer time scales shows that the mean reversal frequency is dominated in the cenozoic and late cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed subdivision into two periods of stationary behavior is no longer warranted the distribution of polarity intervals is visibly not poissonian but lacks short intervals the labrecque et al 2 polarity time scale shows the positions of 57 small wavelength marine magnetic anomalies which may represent short polarity chrons after adding these short events the distribution of all polarity intervals in the age range 0 40 myr is stationary and does not differ significantly from a poisson distribution a strong asymmetry develops in which normal polarity chrons are poisson distributed but reversed polarity chrons are gamma distributed with indexk 2 this asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the late cenozoic if short wavelength anomalies arise from polarity chrons the geomagnetic field may be more stable in one polarity than the other alternative explanations of the origin of short wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons however the observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long term memory

a geomagnetic secular variation study 31000 19 500 bp in western canada summary palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern british columbia are reported radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp no evidence for any large geomagnetic excursions such as the so called mono lake excursion is found but a distinctive pattern of normal secular variation is observed with declination and inclination swings of 45 and 25 peak to peak amplitude respectively for the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude but three relatively large perturbations occur at approximately 4000 yr intervals these perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non isotropic secular variation model proposed by cox the bias involved is about 6 in declination and 3 in inclination overall mean d 5 8 e i 64 2 95 0 9 n 212 horizons which leads to a pole which is both far sided and right handed if the horizons involved in the three major perturbations are eliminated the mean direction d 1 2 e i 67 2 95 0 8 n 125 horizons does not differ from that of a geocentric axial dipole despite the small cone of confidence

experimental study of site effects in the fort de france area martinique island abstract we used weak motion recordings of regional events to quantify the site effects in the fort de france martinique area in the frequency band 1 to 25 hz the studied area extends over three distinct geological units the morne pitault volcanic massif 15 to 11 m a the carbet volcanic massif 4 to 2 m a and the lamentin alluvial plain quaternary age the topography being complex 31 sites were selected in order to get a representative sampling of the various topographical and geological configurations the large number of studied sites also allowed us to define a reference site whose selection was not obvious large broadband amplifications are observed in the lamentin plain instead of the expected peak resonances more surprising such resonance is observed around 2 to 3 hz on several sites of the volcanic vauclain pitault chain though no sedimentary layer is present on the other hand in the inner fort de france where significant effects were expected because of the very complex topography no large amplication is observed except for at one site clear three dimensional 3d effects are also observed at some stations though not very well documented the low frequency behavior deduced from teleseismic observations seems rather different illustrating the difficulty in finding a convenient reference site in such a volcanic area

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

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

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

a comparison of geomagnetic secular variation as recorded by historical archaeomagnetic and palaeomagnetic measurements palaeomagnetic methods can extend the documentary record of changes in the earth s magnetic field far into the past tolerable agreement is found between various methods demonstrating the geophysical value of palaeomagnetic experiments combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour lake sediments have recently been found to hold remarkably detailed signatures of past field changes a mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka

electrical conductivity of dry lower crustal rocks this study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings we present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 c to 1000 c we observe statistically significant differences between rock types mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group aphanitic fine grained rocks have higher conductivity than phaneritic coarse grained ones best fitting curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity because the laboratory rocks are dry their conductivities are lower at a given temperature than they would be if fluids or volatiles were present hence any temperatures inferred from

a paleomagnetic record from tangle lakes alaska large amplitude secular variation at high latitudes a paleomagnetic study of a core from the tangle lakes area of alaska has provided information about the nature of secular variation between 2 700 and 4 600 years b p the data indicate that the amplitudes of fluctuations in inclination and declination were substantially larger than those which would be expected from the present configuration of the geomagnetic field from this single site it is not possible to determine whether the observed secular variation arises mainly from the dipole or the non dipole component of the field if the dipole component is responsible then the data indicate that about 3 350 years b p the geomagnetic pole shifted by about 20 in a period of less than 150 years this shift is comparable to one proposed to have occurred about 500 years ago

numerical modeling of seismic site effects in a shallow estuarine bay suai bay shantou china abstract this paper presents the results of numerical modeling of seismic site effects in a shallow estuarine bay suai bay shantou china which is meaning for seismic hazard assessment and mitigation of estuarine and coastal structures such as immersed tunnels pipelines caissons and breakwaters the subsurface stratum and soil properties in the study area are obtained from geological and geotechnical surveys large scale 2d finite element refined nonlinear modeling is used to simulate ground motion for near field and far filed input motions with different design earthquake levels the results show that for the study area 1 the spatial distribution of pga amplification factor presents obvious vertical and horizontal variations and is strongly affected by the overall geometry of interfaces of different materials 2 the attenuation in soft sediments have significant influences on surface ground motion which leads to a deamplification of the acceleration amplitude a prolongation of the significant duration and a filtration of the high frequency spectral content 3 the pga alone cannot be regarded as an adequate parameter to quantify nonlinear site effects while the energy content expressed by arias intensity is more closely related to transfer functions patterns 4 the seismic intensity at surface is greater than the intensity of input motion during weak motions while the seismic intensity at surface is smaller than the intensity of input motion during strong motions suggesting that isolation effects may be expected in the areas having thick soft soils with a low plasticity index during strong motions

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

late quaternary secular variation of geomagnetic declination in western new york a secular variation record of the geomagnetic declination has been obtained from glaciolacustrine varved clays and postglacial sediments sampled both from outcrop and from piston cores from lake ontario it appears to be uninterrupted and covers the interval 14 000 years b p 14c to the present the record for the interval 14 000 12 300 years b p consists of true declination and was obtained from samples collected from outcrop the record for the interval 12 500 years b p to the present consists of relative declination and was obtained from piston cores the secular variation record is internally consistent and is compatible with the chronology of events known to have occurred during and following the deglaciation of western new york use of the record for magnetic correlation within the study area is demonstrated

site effects and microzonation in the city of thessaloniki greece comparison of different approaches ten seismological stations were installed for 3 months in the city of thessaloniki greece to record earthquakes and ambient noise over areas of different geotechnical characteristics about 40 local earthquakes were recorded with a good signal to noise ratio in more than two stations we analyze the records with three different techniques a the classical spectral ratios to a reference station b the receiver functions and c the horizontal to vertical h v spectral ratios on noise recordings the results are compared in terms of predominant frequencies and amplification levels the three methods are found equally able to reveal the fundamental frequency regarding local site amplification they provide significantly different results with a general trend of the two h v ratio methods b and c to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique a the results are then used in relation with the microzonation of the city of thessaloniki a good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock finally we compare our observations with the damage distribution in the city which was hit by a magnitude 6 5 earthquake in 1978 the amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity

late quaternary paleomagnetic record of the goderich basin lake huron paleomagnetic analysis of two cores taken of the late quaternary sequence of the goderich basin lake huron resulted in high resolution declination inclination intensity of magnetization j magnetic susceptibility k and q ratio j k curves there was a consistent relative relationship of the features along the curves of all five parameters for both cores studied which confirms the reproducibility of the method and suggests that paleomagnetic curves can be used for time parallel correlation of the late quaternary sequence of the goderich basin

long period geomagnetic secular variations since 12 000 yr bp a comparison of secular variation records of the geomagnetic field inclination obtained from palaeomagnetic studies of european and north american post glacial and late glacial sediments suggests that geomagnetic westward drift has persisted through the past 12 000 yr between 12 000 and 2 500 yr bp the overall average drift rate was between 0 14 and 0 16 deg yr 1 at 2 500 yr bp it is tentatively suggested that the drift rate increased and rose to a maximum of nearly 0 5 deg yr 1 before decreasing to historically observed values of 0 25 deg yr 1

numerical correlation of recent paleomagnetic records in two lake tahoe cores abstract recent paleomagnetic fluctuations are smooth and well correlated in two short cores collected 28 km apart in lake tahoe cross correlation analysis of the unit vector time series data shows that the characteristic angular difference between the two records is only 11 for the past 3000 years this compares favorably with the characteristic angular differences of 7 8 between adjacent samples within each core taken separately the correspondingly high values of pairwise between cores and serial within core correlation are significant at 99 99 confidence these data collected and previously studied qualitatively by a s goldstein show that the sediment in lake tahoe can provide reliable recent paleosecular variation records the paucity of fine scale paleomagnetic correlation between other cores from that basin is due likely to stratigraphic rather than rock magnetic difficulties

archaeo and paleosecular variation and long term asymmetries of the geomagnetic field this report summarizes progress made during the last four years in i obtaining records of the geomagnetic secular variation using paleomagnetic techniques ii analysis and modelling of the secular variation and iii the study of asymmetries in the time averaged field seen both as departures from the geocentric axial dipole model and as statistical differences between polarity states

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

contribution of dense array analysis to the identification and quantification of basin edge induced waves part ii application to grenoble basin french alps settled on a deep sediment filled valley the city of grenoble french alps faces important site effects large amplification and significant duration in crease of ground motion even for moderate size events in order to study multidi mensional site effects a very dense array composed of 29 three component seis mometers over a 1 km aperture was operated during spring 1999 in the center of the city a total of 18 events 6 local 4 regional and 8 teleseismic with an acceptable signal to noise ratio could be recorded over a 4 month period the complexity of the wave field and in situ seismic noise constraints led us to develop a procedure based on time frequency coherence and the multiple signal classification algorithm to iden tify and characterize wave arrivals cornou et al 2003 applying the procedure to the 18 records it is clearly indicated that ground motion inside the valley is domi nated by basin edge induced waves that carry 4 times more energy than the direct wave field regardless of the type of event considered in addition the basin induced wave field is composed of 60 rayleigh waves and 40 love waves when consid ering energy carried by the three components if one considers only the energy of horizontal components this proportion is 50 rayleigh waves and 50 love waves the diffraction phenomena are mostly constrained by the 3d structure of the basin regardless of the azimuth of the event a study of the relative contribution of 1d and 2d 3d effects on recorded ground motion suggests at least at frequencies below 1 hz that the difference between the standard spectral ratio and 1d transfer function or possibly the horizontal to vertical ratio receiver function and nakamura esti mates might be due mainly to laterally propagating waves

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

paleomagnetic correlation of columbia river basalt flows using secular variation a statistical method is developed to evaluate stratigraphic correlations that are based on the similarity of paleomagnetic directions it involves comparison of the probabilities that such directions could have arisen by either simultaneous or random samplings of the ancient magnetic field to calculate these probabilities one must estimate the effects of paleomagnetic errors and paleosecular variation respectively the method is tested successfully on sections of 8 10 grande ronde basalt columbia river basalt group flows at sentinel gap and umtanum ridge which are separated by only 25 km and contain two flows thought to be the same on geological grounds more significantly for widely separated sections at sentinel gap and butler canyon the analysis again shows that simultaneous magnetization is a much more likely explanation of the general similarity of paleomagnetic directions than chance agreement of ancient field directions the simplest interpretation of this result is that many of the flows at the two sites are the same implying that series of grande ronde flows extended over distances of several hundred kilometers

use of small amplitude paleomagnetic fluctuations for correlation and dating of continental climatic changes abstract recent advances in field and laboratory methods have made possible the study of paleomagnetic fluctuations recorded with high resolution in lake sediments of relatively small amplitude and short period 102 years to 104 years foremost among these methods are sediment coring techniques that allow azimuthally oriented long cores 25 m to be raised with small orientation errors 3 and the use of cryogenic magnetometers to measure weakly magnetized m 10 9 a m2 sediment samples accurately with these methods records of inclination declination and relative paleointensity nrm arm have been obtained in north america that span the last 20 000 years substantial agreement in declination and inclination features is seen on a scale of 103 km in north america for the last 10 000 years these paleomagnetic features can be useful in determining the relative age of paleoclimatic features particularly in those situations where 14c dating is unreliable similar agreement is seen for the last 4000 years between relative paleointensity estimates obtained from pennsylvania lake sediments and absolute paleointensity estimates obtained from 14c dated lava flows and potsherds from the western united states similar paleointensity fluctuations to those occurring from 0 to 4000 yr b p in the western united states are observed for the last 12 500 yr b p in lake sediments from new york and pennsylvania such relative paleointensity records from lake sediments if independently confirmed considerably extend the range and resolution of paleointensity data and have important implications for the study of secular variation models variations in atmospheric radiocarbon concentration and the proposed relationships between geomagnetism and climate an absolute secular variation chronology for the varved sediments of elk lake minnesota can be used to date magnetically the secular variation records of unvarved sediments elsewhere in north america

construction of type curves of geomagnetic secular variation for dating lake sediments from east central north america type curves of palaeomagnetic secular variations of declination and inclination through the last 12 000 radiocarbon years have been constructed by combining data from the minnesota lakes with paral

remanent magnetization of ash from the 18 may 1980 eruption of mount st helens ash from the may 1980 eruption of mount st helens deposited from air faithfully records the direction of the local geomagnetic field in eastern washington whereas ash settled from suspension in water in fluvial environments exhibits significant inclination and current rotation errors in magnetic direction similar to those reported in other subaqueously deposited sediments the current rotation errors are associated with partial alignment of the major axes of magnetic susceptibility in the direction of water currents subaerial deposition produces strong stable remanent magnetization in ash with or without postdepositional wetting by rain

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

magnetotelluric experiment over the rose area seafloor observations in natural electromagnetic fluctuations diffusing into the earth have been made in the vicinity of the pacific rise at 12 sup 0 n and 21 sup 0 n to gather information on the electrical conductivity structure of the oceanic basement near a spreading ridge at 21 sup 0 n the close proximity of land baja california introduces a noticeable distortion of the em fields and consequently the magnetotelluric sounding for this area must be interpreted with caution at both locations the conductance of the upper 200 km appears exceptionally high and the conductivity rises rapidly below reaching 1 s m sup 1 or more at 350 km the statistical significance of a prominent conductivity increase near a depth of 22 5 km detected in the inverted data from 12 sup 0 n and of a less developed one at 30 km in the case of 21 sup 0 n is discussed records of the magnetic variations of 21 sup 0 n taken above the spreading center compared to records from 40 and 120 km to the west reveal a conspicuous enhancement of the high frequency horizontal variations in a direction slightly west of magnetic north we interpret this distinctive feature as the signature of an electric current sheet concentrated in an extremely highmore conductance relatively shallow layer electrically bridging the two seafloor areas adjacent to the spreading center an approximate calculation suggests a conductance in excess of 3000 s per meter of ridge crest in the close vicinity of the accretion center equivalent to 1 to 8 km of molten basalt depending on temperature 1000 sup 0 1200 sup 0 c and depth 2 10 km this structure is consistent with seismic refraction information for the same area and with stratigraphic reconstruction of a fossil accretion center in samail ophiolite an interpretation in terms of a well developed magma chamber is irresistable although confirmation of the existence of this feature as well as improvement of its resolution by means of additional em data would be of major interest less