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

seismic anisotropy and mantle creep in young orogens summary seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere we discuss the origin of anisotropy at various depths and relate it to tectonic stress geotherms and rheology the anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine and may result from ductile deformation dynamic recrystallization or annealing anisotropy beneath young orogens has been measured for the seismic phase pn that propagates in the uppermost mantle this anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event and thus provides information on the process of mountain building whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts pn anisotropy is aligned parallel to the structural axis we interpret this to indicate mountainparallel ductile i e creeping deformation in the uppermost mantle that is a consequence of mountain perpendicular compressive stresses the preferred orientation of the fast axes of some anisotropic minerals such as olivine is known to be in the creep direction a consequence of the anisotropy of strength and viscosity of orientated minerals in order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal escape or extrusion tectonics to the uppermost mantle we present rheological model calculations to support this hypothesis mountain perpendicular horizontal stress determined in the upper crust and mountain parallel seismic anisotropy in the uppermost mantle require a zone of ductile decoupling in the middle or lower crust of young mountain belts examples for stress and mountain parallel pn anisotropy are given for tibet the alpine chains and young mountain ranges in the americas finally we suggest a simple model for initiating mountain parallel creep

differences between site characteristics obtained from microtremors s waves p waves and codas we examine differences of empirical site characteristics among s waves p waves coda and microtremors using records at 20 sites in and around the sendai basin japan and interpret the differences theoretically at soft soil sites the horizontal to vertical spectral ratios hvrs for early p coda become different from hvrs for a p wave with increasing time and eventually converge on hvrs for microtremors the hvrs for an s coda become similar to hvrs for microtremors with increasing time in the frequency range lower than 3 hz at soft soil sites by contrast at a rock site and two hard soil sites hvrs for s coda agree well with hvrs for an s wave the soil to rock spectral ratios for horizontal hhrs and vertical vvrs components for early s coda are larger than those for an s wave at soft soil sites when we use the deep sedimentary structures above the bedrock before tertiary age theoretical hvr for the fundamental mode of rayleigh waves is consistent with observed hvr for microtremors and theoretical hvr for an obliquely incident sv wave is consistent with observed hvr for an s wave theoretical s wave site amplification factor explains well observed hhr for s wave but does not coincide with hvr for microtremors in general the frequencies of maximum peaks of hvrs for microtremors do not coincide with those of hvrs and hhrs for s wave however if we select hvrs with peak frequencies lower than 1 hz and peak amplitudes larger than three the peak frequencies of hvrs for microtremors roughly coincide with those of hvrs and hhrs for s wave even under these constraints their amplitudes do not coincide with each other concerning coda we conclude that the rayleigh wave contamination in coda is significant in the frequency range lower than 3 hz at soft soil sites

reflection polarity of the midcrustal surrency bright spot beneath southeastern georgia testing the fluid hypothesis a small reflection seismic experiment with an explosive source was conducted in southeastern georgia to determine the polarity of an unusually strong midcrustal reflector the surrency bright spot sbs which was found at a depth of approximately 16 km during earlier cocorp profiling in the region in addition to being very bright the sbs is notable for being unusually flat and horizontal for about half of its 4 km length as these characteristics are similar to those of fluid caused reflections at shallow depths it has been suggested that the sbs may be caused by in situ midcrustal fluids if caused by fluid enclosed in fracture porosity in solid rock the reflection would be expected to exhibit a negative polarity from the top of the porous zone the vibroseis data however gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal the new experiment consisted of four dynamite shots each recorded at three receiver stations by seismic group recorders sgr borrowed from amoco production company comparison of the dynamite records with geophone polarity tests indicate that the sbs is characterized by a positive reflection coefficient at its top this result itself does not negate the fluid hypothesis a fluid fluid interface could cause the positive reflection as well as the flat spot nature of the reflector waveform modeling shows that the sbs is caused by two or more thinly spaced reflectors a relatively high impedance layer about 120 m thick provided a good match to the observed dynamite data but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary 0 7 versus 1 0 a 150 m thick porous zone model also provided a relatively good fit to the observed dynamite data but in this model the polarity test requires that the initial sbs reflection of positive polarity be caused by a fluid fluid interface within the porous zone and that the top of the porous zone be relatively non reflective though these observations do not preclude the fluid hypothesis they certainly make the high impedance model the simpler of the two alternatives presented here the additional constraints imposed by the modeling also suggest that the sbs is more complex than these simple two interface models in either of these cases wavelet tuning contributes in part to the unusually large amplitude of the sbs reflection

detailed evaluation of site response estimation methods across and along the sedimentary valley of volvi euro seistest using records from the euro seistest area near thessaloniki greece we studied empirically the amplification effects of a sedimentary valley one of the main advantages of this site is the very detailed knowledge of the near subsurface structure due to exhaustive geophysical and geotechnical measurements and data analyses jongmans et al 1998 the uniqueness of the seismological data set is the very dense network 31 stations were installed perpendicular and parallel to the valley axis with minimum interstation distances of 250 m the mean ampli fications along both axes estimated from 13 local events were imaged using the traditional spectral ratio technique variations of the mean amplifications due to the choice of different data subsets were tested by the use of up to 89 local events supplemented by four teleseismic events offering good low frequency resolution results obtained using the traditional spectral ratio technique were compared with results from a generalized inversion technique the h v ratio technique a coda wave technique and nakamura s technique advantages and disadvantages of each method are outlined especially from a practical point of view

a quantitative study of the site effects observed at the geress array site effects were studied for the individual sites of the 25 elements of the german experimental seismic system geress array based on the traditional spectral ratio tsr technique and horizontal to vertical ratios hvrs for earthquakes and microtremors commonly referred to as nakamura9s technique an understanding of these site effects is required for various applications in particular for the discrimination of seismic events the spectral ratio technique identifies three stations a0 a3 b3 as showing prominent peaks in the frequency band from 1 to 17 hz up to a factor of 30 with a0 and b3 having the largest site effect at a frequency of 7 hz these stations are situated in the immediate vicinity of the array center a number of other stations such as a1 b1 b4 b5 c1 c3 c5 d2 d5 and d7 d9 display a broad frequency range preferably above 10 hz where the spectral ratios indicate up to 10 times higher spectral amplitudes compared to the reference site c2 the remaining stations a2 b2 c6 c7 d1 d3 d4 and d6 show no noticeable site effects further study considers the appropriateness for the selection of c2 as reference site for the tsr technique and the comparison of tsr and hvr techniques for the three component elements of the array although typically site response refers to the site amplification of horizontal ground motions the assessed site response of vertical ground motions seems to yield comparable results the spectral variance a parameter based on the smoothness of stacked array spectra from the best 17 array elements of the geress array has been used successfully to distinguish between local earthquakes and mining explosions it is shown that the spectral variance estimate is considerably improved using all 25 array elements and applying a site response correction

seismic reflection investigations of the mount cayley bright spot a midcrustal reflector beneath the coast mountains british columbia an unusually high amplitude midcrustal reflector was discovered by a lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern british columbia canada the bright spot was observed adjacent to mount cayley a large quaternary volcanic center associated with the cascadia subduction zone magmatic arc characterization of the bright spot may advance our understanding of either the midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust the location and three dimensional geometry of the reflector were determined using travel time inversion the resulting suite of models consistently describe a northwest dipping reflector that lies 12 5 to 13 km beneath the mount cayley volcanic complex the areal extent of the reflective surface is 3 km by 1 km and the structure is less than 1 6 km thick the transition between the body and the country rock must be rapid 200 m and exhibit a large impedance contrast in order to generate reflections of such high frequencies 30 hz and relative amplitudes 8 0 db our preferred interpretation for the reflector is a fossil sill complex associated with the volcanic development of mount cayley however the data do not rule out the possibility that the reflective structure represents melt lenses or saline fluid produced by the dewatering of the subducted slab the mount cayley bright spot lies just below the brittle upper crust as do the midcrustal reflectors detected beneath japanese subduction arc volcanoes this correlation suggests that thermal and rheological properties just below the brittle ductile crustal transition may be favorable for reflector formation

identification of earthquakes and explosions using amplitude ratios the vogtland area revisited identification of seismic events is a major scientific issue in the framework of verification of the comprehensive nuclear test ban treaty ctbt of special interest in this context is the identification of the numerous low yield mining or blasting events especially those occurring in the same area as earthquakes such as the vogtland area in the border region of germany and the czech republic seismic events in this area were investigated by wuster 1993 1995 who achieved complete discrimination using measures of spectral decay and spectral variance at the geress array and a quadratic discrimination function

crustal bright spots and anisotropy from multi component p and s wave measurements in southern germany abstract controlled shear wave experiments have focussed on deep crustal targets that were earlier recognized by dekorp p wave surveys in southern germany black forest upper palatinate a prominent p wave reflection bright spot at 8 km depth at the ktb deep drilling site is not seen on a comparable s wave reflection section in contrast to deeper reflections this behaviour can be explained either by purely compositional effects layer with anomalously high quartz content or by fluid accumulation reservoir in a fracture zone a 20 km deep 10 km long prominent p wave reflection in the southwestern black forest is only partially accompanied by corresponding s wave reflections the influence of the physical state of the rock e g stress related cracking in addition to possible compositional variations is suggested to explain the contrasting p and s wave reflection strength seismic anisotropy of 10 magnitude revealed by s wave splitting in vertical seismic profiling data at the ktb to more than 4 km depth is attributed to sub vertically dipping and nw striking foliation

lower crustal thinning in the rhinegraben implications for recent rifting seismic near vertical and undershooting experiments carried out in the southern upper rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings a 12 to 14 km thick reflective lower crust beneath a 15 km thick transparent upper crust of the eastern graben shoulder black forest probably originated during permo carboniferous reequilibration of thickened variscan orogenic crust thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder the dinkelberg block is interpreted to be related to cenozoic extensional faulting a discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust in the transitional dinkelberg block such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area

an interpretation of wide angle compressional and shear wave data in southwest germany poisson s ratio and petrological implications shear wave velocity especially when used with compressional wave velocity is an important constraint on estimates of the composition and physical state of the continental lithosphere we present shear velocity s models derived from high quality s wave data along three profiles in southwest germany which combine with existing p wave velocity p models to give poisson s ratio in the crust and uppermost mantle we then construct a laterally varying petrological model of the crust by comparing our p and s models with published laboratory measured rock velocities in general poisson s ratio is 0 25 in the uppermost crust 0 22 0 23 in the mid crust but reaching a low of 0 15 in the black forest low p zone and varies laterally from 0 24 to 0 30 in the lower crust the average poisson s ratio of the whole crust is everywhere about 0 25 the laminated signature of the lower crust prominent on reflection and p wave refraction data is absent on the s wave refraction data implying a strong high low alternation of poisson s ratio in the lower crust this suggests that the lower crustal laminae represent mafic silicic compositional layering for example basic sills in an originally more quartz rich lower crust rather than alternating zones of high and low pore pressure there is no evidence for a shear wave low velocity zone lvz in the black forest mid crust despite the presence of a strong p wave lvz this suggests high quartz content and the presence of pore fluids at low not high pore pressure a clear p to s converted moho reflection pm s is visible on several shots indicating that the moho is at least locally a first order discontinuity no shear wave refractions from the upper mantle are present even where strong p wave refractions are observed possibly indicating an increase of poisson s ratio with depth in the upper mantle the petrological modeling indicates that the upper and middle crust are probably of granitic composition that the mid crustal low p zones have high quartz content and that the lower crust probably consists of granulites of laterally varying bulk composition from granodioritic to gabbroic the lower crust is more mafic in areas of high reflection density on seismic reflection profiles suggesting that the laminations are caused by mafic intrusions into the lower crust either prior to the variscan orogeny or during tertiary volcanic activity

seismic observations of meteors coupling theory and observations to date coupling of acoustic waves directly with the earth s surface represents the bulk of the observed meteor related seismic signals while precursory and impact related seismic waves remain an observational rarity with proliferation of infrasound and seismic monitoring systems new opportunities exist to explore the relationship between earth s atmosphere andsurface continued studyofmeteorseismologywillleadto new methods to constrain energies sizes and fluxes for moderately cm to m sized meteoroids on earth and potentially on mars

classification of local and regional events in central europe based on estimates of s wave spectral variance summary the vogtland region in the border region of germany and the czech republic is of special interest for the identification of seismic events on a local and regional scale since both earthquakes and explosions occur frequently in the same area and thus are relevant for discrimination research for verification of the comprehensive nuclear test ban treaty previous research on event discrimination using spectral decay and variance from data recorded by the geress array indicated that spectral variance determined for the s phase for the seismic events in the vogtland region seems to be the most promising parameter for event discrimination because this parameter provides for almost complete separation of the earthquake and explosion populations almost the entire set of vogtland events used in this research and more than 3000 local events detected in germany in 1998 and 1999 were analysed to determine spectral slopes and variance for the p and s wave windows from stacked spectra of recordings at the geress array the results suggest that small values for the spectral variance are associated not only with earthquakes in the vogtland region but also with earthquakes in other parts of germany and neighbouring countries while mining blasts show larger spectral variance values mininginduced events yield a wide range of values for example in the lubin area a threshold based identification scheme was applied almost all events classified as earthquakes are found in seismically active regions while the earthquakes are uniformly distributed throughout the day events classified as explosions correlate with normal working hours which is when blasting is done in germany in this study spectral variance provides good event discrimination for events in other parts of germany not only for the vogtland region showing that this identification parameter may be transported to other geological regions

seismicity and structural complexity of the dinkelberg block southern rhine graben summary a detailed analysis of the dinkelberg seismicity is presented the seismic activity of the central dinkelberg block starts at about 10 km and increases with depth to a maximum between 18 and 20 km joint hypocentre determinations clearly prove that no hypocentres are located within the lower crust which starts at a depth of about 20 km according to results from recent reflection seismic surveys the seismic activity is concentrated just above the top of the lower crust fault plane solutions of the dinkelberg events show that the strike of the nodal planes is parallel to the directions of near surface tectonic features a correlation also exists between the main fault directions and the spatial distribution of the hypocentres in the pratteln earthquake cluster at the southern end of the dinkelberg block the occurrence of negative p residuals from local earthquakes for special propagation directions of waves crossing the dinkelberg block is modelled by a westward dipping layer this postulated upper crustal structure together with observations of preferred s wave polarizations are discussed in regard to recent tectonic models and geological findings

a study of site effects in ilan taiwan based on attenuation relationships of spectral acceleration abstract the purpose of this paper is to study the site effects for ilan in northeastern taiwan based on attenuation relationships of spectral acceleration sa over 2660 accelerograms recorded from 92 shallow earthquakes with m w magnitude ranging from 4 0 to 7 7 are used to develop these regional attenuation relationships site effects are investigated using the site residuals obtained from the sa attenuation relationships the results show the standard deviations of residuals decrease by 1 7 16 4 in horizontal sa after incorporating the site effect term v s 30 in the attenuation relationship of periods greater than 0 3 s clearly the attenuation relationships are improved by including the site effect term especially for the longer periods in addition from the contour maps of intraevent site residuals relative to an attenuation equation of our model 2 phase 1 for horizontal sa we find high amplification factors of short period 0 1 0 2 s reaching 2 7 in nanao with hard schist rocks high amplification factors of intermediate periods 0 3 1 0 s reach 1 9 and 2 0 respectively in the city of ilan and zhuangwei high amplification factors of long period 1 0 3 0 s reach 2 1 in zhuangwei where there is the greatest sediment thickness the results call for special attention to the seismic design of structures in these locations furthermore the intraevent residuals of the four site classes converging the 0 15 0 2 s period range are in agreement with the findings by aki 1988 accordingly site classes d and e have the largest ground motion amplification factor in the 0 15 0 5 s and 0 5 s period ranges respectively finally the predominant period contour maps deduced from v s 30 v s 75 and sediment depth in the ilan plain fall in the ranges of 0 4 1 0 1 0 1 5 and 1 5 8 0 s respectively this suggests that construction of buildings taller than nine stories in the zhuangwei area should pay special attention to avoid resonance of seismic excitation online material contour maps of relative corrected site amplification factors at 10 periods and tables of site parameters list of earthquakes intraevent site residuals and corrected site amplification factors

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

discrimination of chemical explosions and earthquakes in central europe a case study both natural and artificial seismicity is observed in the vogtland area at near regional distance 180 km from the geress array this setting offers the opportunity to test various discrimination criteria using a uniform data set of 39 earthquakes and 22 chemical explosions strip mine and quarry blasts from a single epicentral region mostly eliminating wave path and distance effects discriminants used include lg pg and lg rg maximum amplitude ratios spectral peaks and slopes extracted by autoregressive moving average arma modeling master event correlation and the search for time independent frequency structures in sonograms although no single discrimi nant achieves a perfect separation of the two populations a joint discrimination scheme employing simple majority voting reduces misclassifications to 2 future research will focus on possible generalization and the physical basis of the discriminants

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

shear wave evidence for an anisotropic lower crust beneath the black forest southwest germany abstract controlled shear wave sources a horizontal vibrator and dynamite were used in near vertical reflection studies to explore the nature of the reflective laminated lower crust in southwest germany the data reveal strong s wave reflections from the lower crust these lower crustal s reflections were not observed by previous wide angle surveys in the same region a laminated crustal model with isotropic layers and changing poisson s ratio cannot explain the differing near and far offset s response by introducing alternating anisotropic and isotropic lamellae this discrepancy may be resolved this seismic model is consistent with a petrological model of deformed amphibolites containing 10 30 of preferentially orientated hornblende observed amplitude differences in p and s wave reflections from the same depth in the lower crust suggest regions with a changing poisson s ratio

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

seismic anisotropy of the crystalline crust what does it tell us the study of the directional dependence of seismic velocities seismic anisotropy promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of p and s wave velocities the alignment of specific minerals by ductile rock deformation for instance causes specific types of seismic anisotropy which can be identified by appropriate field measurements vice versa the determination of anisotropy can help to discriminate between different rock candidates in the deep crust seismic field measurements at the continental deep drilling site ktb s germany are shown as an example that anisotropy has to be considered in crustal studies at the ktb the dependence of seismic velocity on the direction of wave propagation in situ was found to be compatible with the texture composition and fracture density of drilled crustal rocks

microtremor measurements used to map thickness of soft sediments the observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers the most suitable method for this determination is nakamura s technique which is the ratio of the horizontal component noise spectrum and that of the vertical component i i v spectrum the frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site using an extensive database of microtremor measurements carried out in the western lower rhine embayment germany it was possible to show that this correlation is clearly valid for a wide range of thickness namely from tens of meters to more than 1000 m a simple formula was derived that for the sediments to be found in the area investigated directly calculates the cover thickness from the frequency of the main peak in the h v spectrum a comparison with calculated resonant frequencies sug gests the relation derived from the noise measurements depending on the velocity depth function of the shear wave classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the res onant frequency of the subsoil the practical relevance of the investigation is illus trated by means of cross sections constructed from results of the microtremor anal yses which provide a convincing image of the surficial structure of the areas investigated

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

the structure and genesis of weichselian to early hologene aeolian sand sheets in western europe abstract weischselian to early holocene aeolian sands are widespread in the lowlands of western europe to a large extent these deposits occur in the form of sheet like coversands with slipfaced dunes being much rarer whereas the latter type of landform corresponds to a single facies with dune foreset cross bedding aeolian facies 1 two structurally different facies are distinguishable in the sand sheets this paper concentrates on the two sand sheet facies which are referred to as aeolian facies 2 and aeolian facies 3 data on these two types are based on 1 a survey of existing literature and 2 a detailed analysis of lacquer peels and larger scale exposures in england the netherlands and the federal republic of germany aeolian facies 2 is defined by the spatial position of its beds which may be either horizontal inclined or oppositely dipping in the case of structures formed by fluctuating winds horizontal bedding is by far the most common type inclined bedding is related either to small isolated dome dunes or to scoop shaped deflation surfaces the structures resulting from fluctuating winds are rare and do not represent an inherently large measure of directional variability of the wind regime the internal structure of the beds is dominated by aeolian planebed lamination with or without concordantly infilled wind scours degradation of this stratification type because of interference by coarse particles is discussed aeolian facies 3 is uniquely typified by the alternation of coarser and finer grained horizontal thin beds that are either wavy or even in shape depositional models of facies 3 are given at both the local and regional scales in both cases the coarser grained layers result from tractional deposition of saltating and creeping grains whilst the finer grained strata were laid down by settling from suspension periodic changes in surface wind speed the presence of a damp depositional surface and the availability of both sand and silt in the source area are necessary conditions for the working of the models the large scale model is associated with specific environmental conditions of the weichselian upper pleniglacial the local scale model accounts for the fact that facies 3 is also found in units of weichselian late glacial or early holocene age the regional scale model involves stepwise tractional transport over long distances so that grains of distant provenance were mixed with material from sources nearer to the receiving site this process was an important control on the mineralogical composition of the resultant deposits the prevalence of aeolian planebed lamination in both facies 2 and the coarser grained layers of facies 3 is attributed to the interaction of three factors viz the rarity of topographic barriers the sparseness of vegetation cover and a high ratio between wind energy and sand availability during the processes of transport and deposition in the lowlands of europe sand sheets are gradually replaced in an easterly direction by coeval wind dunes the possible causes of this phenomenon are considered

lower crustal petrology from wide angle p and s wave measurements in the black forest abstract the lower crust of the black forest in southwest germany is strongly reflective for compressional waves but almost non reflective for shear waves if the reflectivity for p waves is caused by alternating high and low velocity rocks the s wave observations require that the low velocity rocks have a low poisson s ratio and vice versa a purely compositional petrological model for lower crustal rocks can fit the observed p and s wave velocity data provided a rather high quartz content 50 is acceptable for the lower crust