change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

geotechnical site characterisation in the umbria marche area and evaluation of earthquake site response in this research a site classification is proposed for the umbria marche area central italy based on geotechnical and geomorphological criteria two empirical techniques to evaluate site effects are used namely the generalised spectral inversion and the horizontal to vertical spectral ratio the soil transfer functions obtained by the two techniques are compared in order to capture the features common to each class and verify the accuracy of the classification we used strong ground motions recorded during the umbria marche seismic sequence september 1997 may 1998 and we selected 25 sites that were classified into four groups the results of the empirical techniques have also been tested using theoretical 1 d or 2 d methods that are commonly used to simulate linear soil behaviour the results expressed in terms of amplitude transfer functions are shown according to each soil class and confirm the consistency of the site characterisation the empirical techniques are efficient in detecting the fundamental frequency of vibration but seldom the higher harmonics conversely the peak amplitude of the transfer functions is not consistently determined especially for sites located in deep sedimentary basins where 2 d effects occur and simplified methods such as the horizontal to vertical spectral ratios hvsr or 1 d theoretical models frequently fail significant amplifications are also found for 2 d rock structures such as rock crests or cliffs

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

on the horizontal to vertical spectral ratio in sedimentary basins the horizontal to vertical spectral ratio hvsr has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil one of its variants is that proposed by nakamura 1989 in which records of microtremors are used usually the analysis is aimed to obtain the predominant period of the site under study in this work we explore what can be achieved by using this method we study the response of different configurations under incident waves coming from an explosive source using the indirect boundary element method ibem we investigate two cases low and high velocity contrast holding constant the physical properties inside the basin and changing only the properties of the bedrock then we compute the seismic response using the horizontal sediment to bedrock spectral ratio sbsr at various locations on the free surface of the basins and compare it with the one calculated by the hvsr at the same locations the comparison shows that in general the predominant period computed with the hvsr is not the same as that obtained by the sbsr in all the locations on the other hand the hvsr approximation can reasonably well predict the fundamental local frequencies when the impedance contrast between the basin and the bedrock is low however hvsr cannot be used in sedimentary basins having a high impedance contrast with respect to the bedrock below

preliminary magnetostratigraphy of a thick eolian red clay loess sequence at lingtai the chinese loess plateau magnetic remanence and bulk susceptibility measurements are reported for a 305 m eolian loess red clay sequence at lingtai the chinese loess plateau polarity zonation defined by about 680 remanence data yields a basal age of about 7 05 ma for the section which represents the oldest wind blown dust deposited continuously in north central china during the late cenozoic the resultant magnetostratigraphic scheme implies a nearly linear accumulation rate about 2 98 cm ka of atmospheric dust on the loess plateau during the period of 7 05 to 2 58 ma comparison of the bulk susceptibility record with the pedogenic characteristics of the red clay observed in the field suggests that magnetic susceptibility in the red clay is not as useful as in the loess soil sequence to indicate summer monsoon variations

determination of elastic properties of shallow sedimentary deposits applying a spatial autocorrelation method abstract the capability of the spatial autocorrelation method for the determination of the elastic properties of shallow sedimentary deposits has been investigated at a test site in which the subsurface structure is known the technique has been shown very innovative and convenient for this kind of study as no artificial sources are needed and also overcomes the problems of soil invasive techniques the method uses the three components of the seismic microtremor recorded simultaneously at different configuration arrays with the possibility to obtain dispersion curves of rayleigh and love waves these curves together with the horizontal to vertical spectral ratio of the seismic microtremors can be used to calculate the s wave velocities of the sediments in order to test the goodness of this procedure a specific experiment was performed with real data the underground model obtained from this methodology is compared with the geotechnical information available our results are in a good agreement with those models obtained previously by s wave refraction experiments and borehole measurements

far source long period displacement response spectra in the po and venetian plains italy from 3d wavefield simulations abstract design elastic displacement spectra for seismic codes and zoning studies of seismic hazard for long period structures are becoming of utmost importance in modern earthquake engineering the increase of displacement based design approaches together with the availability of good quality digital strong motion data allow the creation of reliable spectral shapes for seismic design of up to 10 s nevertheless to characterize the site amplification effects during large levels of shaking additional studies are needed especially in deep sedimentary basins and plains that can be strongly excited by large amplitude long period waves data within deep basins is rare because accelerometric seismic networks are normally deployed on rock or stiff soil and modeling of waveforms is hence necessary for bridging this gap focusing on the numerical modeling of the 3d wavefield in 1d flat layered and 3d structures in the po and venetian plains northern italy and on earthquakes with magnitudes m 6 6 and hypocentral distances greater than 50 km in this study we 1 estimate the displacement response spectra drs from 1 to 10 s in the po and venetian plains 2 compare the drs from numerical simulations using flat layered and heterogeneous 3d models against the currently available uniform hazard drs for italy and 3 evaluate the influence of deep sediments on drs shape and thus on seismic codes finally 1d and 3d simulated drs are compared with drs obtained from recordings in the kanto basin where similar long period displacement is observed

the main features of the local geological conditions can explain the macroseismic intensity caused in xiji langfu beijing by the ms 7 7 tangshan 1976 earthquake the special geological conditions in the xiji langfu area are the main reason for the anomalous high macroseismic intensity caused by the tangshan 1976 earthquake the area is formed by deep deposits mainly alluvium sands and clays poorly consolidated and with high water content that have been trapped by the xiadian fault from simulated ground motion we have computed quantities commonly used for engineering purposes like the acceleration maximum amplitude amax and the total energy of ground motion w which is related to the arias intensity the thick low velocity deposits are responsible for the large increment of the values of amax and w inside the basin on the two sides of the xiadian fault amax and w can vary by 200 and 700 respectively and these variations are quite stable with varying thickness of the sedimentary deposit used in the models with the existing relationships between acceleration amax and macroseismic intensity i our results can explain the large values of i observed in the xiji langfu area in connection with the tangshan earthquake

c3 c4 vegetation evolution over the last 7 0 myr in the chinese loess plateau evidence from pedogenic carbonate δ13c the stable carbon and oxygen isotopic compositions of soil carbonate were measured on an eolian loess and red clay sequence at lingtai the chinese loess plateau this sequence is composed of 130 m of tertiary red clay deposits with a basal age of 7 05 ma overlain by 175 m of pleistocene loess in the field we identified ca 110 carbonate nodule horizons in the red clay and 27 nodule horizons in the loess these carbonate nodule horizons are formed by leaching and re precipitation of carbonate from the eolian material the d13c record of soil carbonate indicates a major expansion of c 4 plants at ca 4 0 myr in the loess plateau this event is comparable in timing with the expansion of c 4 plants in northern north america cerling et al 1997 nature 389 153 158 but is ca 3 million years later than the c 4 biomass expansion in pakistan quade et al 1989 nature 342 163 166 the pedogenic characteristics of the soils and the d18o record in the red clay suggest that the c 4 plant expansion in the loess plateau was not driven by local climatic changes which may support cerling et al s 1997 assertion that the decline of atmospheric co 2 levels in the neogene is responsible for this global vegetation change our record also implies that the tibetan plateau could have been uplifted to a critical height in the late miocene thus resulting in the formation of the atmospheric great east asia trough 2000 elsevier science b v all rights reserved

a hybrid method for the estimation of ground motion in sedimentary basins quantitative modeling for mexico city to estimate the ground motion in two dimensional 2d laterally heterogeneous anelastic media a hybrid technique has been developed that combines modal summation and the finite difference method in the calculation of the local wave field owing to a seismic event both for small and large epicentral distances it is possible to take into account the source path and local soil effects as a practical application we have simulated the ground motion in mexico city caused by the michoacan earthquake of september 19 1985 by studying the one dimensional 1d response of the two sedimentary layers present in mexico city it is possible to explain the difference in amplitudes observed between records for receivers inside and outside the lake bed zone these simple models show that the sedimentary cover produces the concentration of high frequency waves 0 2 to 0 5 hz on the horizontal components of motion the large amplitude coda of ground motion observed inside the lake bed zone and the spectral ratios between signals observed inside and outside the lake bed zone can only be explained by 2d models of the sedimentary basin in such models the ground motion is mainly controlled by the response of the uppermost clay layer the synthetic signals explain the major characteristics relative amplitudes spectral ratios and frequency content of the observed ground motion the large amplitude coda of the ground motion observed in the lake bed zone can be explained as resonance effects and the excitation of local surface waves in the laterally heterogeneous clay layer also for the 1985 michoacan event the energy contributions of the three subevents are important to explain the observed durations

simulation of the seismic response of a 2d sedimentary basin a comparison between the indirect boundary element method and a hybrid technique abstract the seismic response of a two dimensional sedimentary basin is studied with the indirect boundary element method ibem and compared with the results of a hybrid technique that combines modal summation and the finite difference technique to perform this numerical experiment we used the observed ground motion at the edge of the basin computed independently with the hybrid technique this ground motion is used to estimate the apparent incoming wave field the estimation is based on a simplified polarization analysis that led us to select time windows for various incident plane wave types the numerical simulations with ibem gave results in the time and frequency domain that compare well with those of the hybrid approach it suggests that a realistic simulation of strong ground motion is possible using two dimensional wave propagation modeling together with an estimate of the incident wavefield this estimate can be obtained with a polarization analysis of observed ground motion at a station located on bedrock as long as the different wave types can be separated in time and frequency

wind blown origin of the pliocene red clay formation in the central loess plateau china abstract in order to determine the depositional processes of the pliocene red clay formation deposited directly beneath the plio pleistocene loess in the chinese loess plateau four red clay sections spanning over 400 km are studied grain size analysis of closely spaced samples in the sections shows that the particles of the red clay are very fine with the sand fraction 63 m being negligible and that all the curves indicating changes in different grain size parameters have a similar pattern the grain size records in the upper part of the four sections are almost identical with the median grain size centered at 4 8 m the ree patterns of 28 samples from the jiaxian red clay section are all characterized by lree enrichments relatively flat hree and slight negative eu anomaly being similar to those of eolian loess and the average upper continental crust these lines of evidence point to a wind blown origin of the red clay deposits accumulation of the loess soil sequences in the chinese loess plateau during the past 2 6 ma can be therefore regarded as the continuation of the pliocene atmospheric dust deposition observations of spatial grain size changes in the loess plateau suggest that the eolian red clay might be transported mainly by the westerlies differing significantly from the overlying loess that was transported essentially by the east asia winter monsoonal winds

complex polarization analysis of particle motion knowledge of particle motion polarization aids in identifying phases on three component seismograms the scheme of montalbetti and kanasewich 1970 is extended to analytic three component seismograms where the imaginary part of the signal is the hilbert transform of the real part this scheme has only one free parameter the length of the time window over which the polarization parameters are estimated so it can be applied in a routine way to three component data the azimuth and dip of the direction of maximum polarization and the degree of elliptical polarization as a function of time for the seismograms are obtained polarization analysis of strong motion data from the 1971 san fernando earthquake aids in the discrimination between wave types which is important for the understanding of the complicated earthquake induced shaking observed in basins most arrivals are incident on the receivers in the direction of the back azimuth to the epicenter which suggests that despite the complicated motions two dimensional finite difference methods are sufficient to understand the effect on seismic waves of the los angeles and san fernando basins vidale and helmberger 1986b

fossil horses and carbon isotopes new evidence for cenozoic dietary habitat and ecosystem changes in north america abstract the transformation from low crowned to high crowned horse teeth during the miocene has traditionally been interpreted as an adaptive response to the spread of savanna grasslands during the middle miocene by about 15 m y ago carbon isotope data from 50 north american horse teeth spanning eocene to pleistocene age indicate that savanna grasslands presumably dominated by c 4 grasses as they are today first became widespread much later than do horses with high crowned teeth prior to 7 m y horses had a c 3 based diet and after 7 m y horses started eating c 4 grasses this change in diet occurred when the major drop in the diversity of horses occurs during the late miocene the change in vegetation reflected in horses diet may be related to a significant reduction in atmospheric co 2 level toward the end of the miocene which provided the c 4 grasses with an adaptive advantage and led to their expansion at the expense of c 3 plants

a study on the response of a soft basin for incident s p and rayleigh waves with special reference to the long duration observed in mexico city abstract the responses of two types of soft basins for incident sh sv p and rayleigh waves in a 2 d elastic half space are investigated with special reference to the long duration of strong motions observed in mexico city during the michoacan mexico earthquake of 1985 first the difficulty for simple 1 d models to reproduce the later part of the accelerogram observed in mexico city is shown then the effect of a 2 d deep basin structure of a relatively large scale 10 km wide and 1 km deep and a moderate impedance ratio 2 5 is studied the results show that surface waves generated at the edges of the basin are clearly propagating inside the basin back and forth however the increase in duration by these surface waves is 20 sec at most introducing another much softer layer inside the basin we found that the calculated durations and envelop shapes become very similar to the observed ones because of the slowness of surface wave propagation these theoretical studies suggest that the exceptionally long duration observed in mexico city might be caused by a strong constructive interaction of soft surface layers with a deep basin structure beneath the city

structure of alluvial valleys from 3 d gravity inversion the low andarax valley almeria spain test case this paper presents a gravimetric study based on 382 gravimetric stations in an area about 32 km2 of a nearly flat basin the low andarax valley this alluvial basin close to its river mouth is located in the extreme south of the province of almeria and coincides with one of the existing depressions in the betic cordillera the paper presents new methodological work to adapt a published inversion approach growth method to the case of an alluvial valley sedimentary stratification with density increase downward the adjusted 3d density model reveals several features in the topography of the discontinuity layers between the calcareous basement 2 700 kg m3 and two sedimentary layers 2 400 and 2 250 kg m3 we interpret several low density alignments as corresponding to se faults striking about n140 145 e some detected basement elevations such as the one previously known by boreholes in viator village are apparently connected with the fault pattern the outcomes of this work are 1 new gravimetric data 2 new methodological options and 3 the resulting structural conclusions

expansion of c4 grasses in the late miocene of northern pakistan evidence from stable isotopes in paleosols stable isotopic clay mineralogic and bulk chemical analyses were conducted on paleosols of the neogene siwalik sections in northern pakistan in order to reconstruct floodplain environments over the past 17 ma the stable carbon isotopic composition of soil carbonate mean 13c pdb 10 2 and associated organic matter mean 13c pdb 24 1 in paleosols representing 17 7 3 ma reveal that floodplain vegetation was dominated by c3 plants at 7 3 ma a shift toward more positive carbon isotopic values began signaling the gradual expansion of c4 grasses onto the floodplain from 6 ma to present carbon isotopic values for paleosol carbonate mean 13c pdb 0 6 and organic matter mean 13c pdb 14 4 are uniformly enriched in 13c indicating the presence of nearly pure c4 grassland the scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1 0 1 25 m yr or less for the entire 17 ma of record paleosols in the lower portion of the section lack organic a horizons but have reddish b horizons often containing secondary iron oxide nodules leaching depths of soil carbonate in these older paleosols are typically greater than those in the plio pleistocene part of the section where organic a horizons are common and b horizons are markedly more yellow the combined evidence suggests that the mature paleosols in the pre 7 3 ma part of the record are dominantly calcareous alfisols or mollisols that once underlay nearly pure c3 vegetation perhaps trees and shrubs while calcareous mollisols underlying c4 grassland dominate the upper part of the record the carbon and oxygen isotopic trends in the paleosol record in pakistan are also evident in the diet of fossil mammals and in paleosols from nepal thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent wide local effects such as the development or intensification of the asian monsoon driven by uplift of the tibetan plateau may have led to the expansion of c4 grasses if however the expansion of c4 grasses proves globally synchronous then a larger scale cause such as a marked decrease in co2 may be the driving mechanism

late miocene atmospheric co2 concentrations and the expansion of c4 grasses the global expansion of c4 grasslands in the late miocene has been attributed to a large scale decrease in atmospheric carbon dioxide co2 concentrations this triggering mechanism is controversial in part because of a lack of direct evidence for change in the partial pressure of co2 pco2 and because other factors are also important determinants in controlling plant type distributions alkenone based pco2 estimates for the late miocene indicate that pco2 increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago the estimates presented here provide no evidence for major changes in pco2 during the late miocene thus c4 plant expansion was likely driven by additional factors possibly a tectonically related episode of enhanced low latitude aridity or changes in seasonal precipitation patterns on a global scale or both

simulation of the seismic response of sedimentary basins with vertical constant gradient velocity for incident sh waves the simulation of the seismic response of heterogeneous sedimentary basins under incident plane waves is computed using the indirect boundary element method ibem to deal with these kinds of basins we used approximate analytical expressions for the two dimensional green s functions of a medium with constant gradient wave propagation velocity on the other hand for the homogeneous half space underlying the sedimentary basin the full space green s functions were used the response of semi circular heterogeneous basins under incident sh waves is explored by means of the displacements in the frequency space diagrams and synthetic seismograms moreover we compared these results with those obtained for other homogeneous semi circular models the principal differences among them are pointed out this simulation provided interesting results that displayed a complex amplification pattern in a rich spectrum of frequencies and locations the maximum amplitudes levels were found around the edges of the heterogeneous sedimentary basins in time domain some features characterize the seismic response of the basin which include enhanced generation and trapping of surface waves inside the sediments and the reduced emission of seismic energy to the hard rock in the heterogeneous models the lateral reflections of surface waves greatly influence the total displacements at the free surface in comparison with the homogeneous models where the displacements have a shared influence among both vertical and lateral reflections

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

sensitivity of climate to late cenozoic uplift in southern asia and the american west numerical experiments experiments with general circulation models with no mountains half mountains and full modern mountains show the sensitivity of atmospheric circulation patterns to progressive amounts of uplift the experiments were motivated by geologic evidence for large kilometer scale uplift in the late cenozoic in southern asia and the american west particularly within the last 10 m y in january the amplitude of the mid latitude upper tropospheric planetary waves increased with uplift and the low level winds were progressively blocked or diverted around the topographic features in july the progressive uplift caused monsoonlike circulations to develop in the vicinity of the colorado and tibetan plateaus atmospheric heating rates midtroposphere vertical motion and upper tropospheric planetary wave amplitudes varied approximately linearly with progressive uplift these results suggest that geologically recent kilometer scale uplift may have had climatic consequences comparable in magnitude and pattern to those of earlier stages of uplift comparison of the simulated climatic effects of uplift with the geologic evidence for late cenozoic climatic change especially for the last 10 m y is given in accompanying papers

study of the propagation and amplification of seismic waves in caracas valley with reference to the 29 july 1967 earthquake sh waves the antiplane response of a 2 d model of the valley of caracas venezuela a ns cross section through the palos grandes district to seismic excitations such as those due to antiplane line point sources and incident plane sh waves is investigated it is found that the steeply sloped northern edge of the valley is a more efficient generator and reflector of surface waves than the midly sloped southern edge also the spectral amplification and time response characteristics of this asymmetric wedge shaped valley are a function of the direction of incidence of the excitation waves e g incidence from the north versus incidence from the south versus vertical incidence the same 2 d model of the valley is used to simulate also the antiplane response of the valley to synthetic strong motions equivalent in amplitude frequency content and duration to those that most likely were generated by the 29 july 1967 caracas earthquake this event has been found to consist of four subevents of which the second subevent located at a distance of 25 km from the valley caused the damage in caracas the source spectrum of each of the subevents used in the generation of the synthetic time histories was assumed to be a simple square model the locations relative times of occurrence seismic moments and mechanisms of the subevents were available from an inversion study of teleseismic data it is found that peak acceleration was fairly uniform across the valley varying from 0 15 to 0 21 g on the contrary peak velocities and peak rotational torsional strains varied considerably factor of 2 to 3 across the valley with the maximum values being 25 cm sec for velocity and 1 5 10 4 for rotational strain for sites in palos grandes spectral accelerations sa reached values as high as 0 4 g for periods in the range 0 4 t 1 4 sec and damping ratio 5 our estimates of the intensity of motion on rock and on the sediments are higher by a factor of 3 compared with estimates proposed by previous investigators who assumed that all the seismic energy was released from a point located at a distance of 55 km from caracas various pieces of evidence e g seismoscope response recorded on rock are presented that suggest that the intensity of motion was at least as high as estimated in this paper

a numerical experiment on the horizontal to vertical spectral ratio in flat sedimentary basins in this paper we study the seismic response of flat sedimentary basins and carry out numerical experiments to determine the extent to which we could go using the horizontal to vertical spectral ratio hvsr for a given site the hvsr has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil and one of its variants that proposed by nakamura 1989 in which records of microtremors are used is one of the most applied in recent years we study the response of different configurations under incident waves coming from an explosive source using the indirect boundary element method ibem and we investigate two cases low and high velocity contrast we compute the seismic response using the hvsr technique at various locations in the free surface of the basins and compare it with the response calculated with the horizontal sediment to bedrock spectral ratio sbsr and with the horizontal component hc of the transfer function for the displacement at the same locations the comparison shows that in general hvsr cannot provide the predominant period of a site due to the fact that this technique cannot predict accurately the spectral amplification levels on the other hand the hvsr provides an erroneous response in the sedimentary basins which have a low impedance contrast with respect to bedrock and with shape ratios like the one studied here whereas it can reasonably well predict the fundamental local frequency when there is a high impedance contrast except in the center of the basin

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less