source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

discovery of outcrop scale fine grained sediment waves in the lower halang formation an upper miocene submarine fan succession in west java abstract we report the first discovery of outcrop scale fine grained sediment waves in the left side muddy overbank deposits relative to the down current direction of the adjacent channel deposits in the lower halang formation turbidite system in a late miocene back arc basin west java the present outcrop examples are characterized by an average wavelength of 10 7 m and an average wave height of 0 13 m and the aspect ratio is similar to that of modern large scale fine grained sediment waves the dimension and asymmetry of waveforms decrease in the down current direction in association with the increase in wave steepness in particular aggradation and up current migrating of waveforms are commonly observed in the proximal part of overbank deposits furthermore the stoss side deposits locally contain thicker and coarser sandstone and coarse siltstone beds beds up to 5 cm thick than those of the lee side deposits the basal part of individual waveforms does not contain any sandy bedform and or irregularity as a precursor for developing asymmetrical waveforms from fine grained turbidity currents these spatial variations in geometry and component deposits of the waveforms are interpreted to have been formed under antidune or cyclic step and standing wave conditions from the proximal to distal overbank environments respectively the present outcrop scale examples can fill the gap in dimension and formative processes between laboratory scale bedforms and modern large scale fine grained sediment waves for elucidating scale independent processes which are responsible for the development of fine grained waveforms in a deep water environment

downfan transformation from turbidity currents to debris flows at a channel to lobe transitional zone the lower pleistocene otadai formation boso peninsula japan transformation of turbidity currents into debris flows across a channel to lobe transition zone was recognized on the basis of lateral mapping of a falling stage systems tract deposit in the lower pleistocene otadai formation submarine fan sequence on the boso peninsula japan the most proximal outcrops comprise 1 to 4 m thick turbidites which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow such turbidites and encased debrites were deposited from single flow events the most distal outcrops comprise only thinner 2 to 40 cm thick turbidites debrites contain many siltstone clasts and finer grained sediments which are characterized by deep water faunas and clay mineral composition similar to those of interbedded hemipelagites therefore the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer grained sediment particles into the precursor turbidity currents through erosion of muddy substrates subsequently turbulence in the precursor turbidity currents was suppressed and near bed flow with higher sediment concentration developed intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle fan channels the precursor turbidity currents or turbidity currents generated by dilution of debris flows produced a turbidite deposit which was subsequently overrun by the debris flow in a proximal lobe environment the debris flows are interpreted to have subsequently been transformed into turbidity currents which are recorded by turbidite deposition in the more distal lobe environment debrites can characterize the proximal portion of an attached lobe deposit in a channel to lobe transition zone such debrites can be significant heterogeneities for fluid flows in sand prone successions from middle fan channel to attached lobe deposits

coarse grained sediment waves in hyperpycnal clinoform systems miocene of the austral foreland basin argentina in this paper we present an outcrop example of coarse grained sediment waves generated by hyperpycnal discharges at the toe of miocene clinoform systems exposed in the northeast atlantic coast of tierra del fuego argentina an individual set of these sediment waves is formed during the waxing and waning phases of the hyperpycnal discharge during the waxing phase accumulation of coarse grained sediments with generation and migration of dune bedforms occurs at the base of the clinoforms these deposits show transitions of tractive sedimentary structures with evidence of bedload transport and a continuous reworking by the overpassing hyperpycnal flow the continued erosion and reworking during the waxing phase affect mainly the top and the stoss side of the dunes resulting in isolated lenses having upcurrent aggradation structures during the waning phase these isolated lenses are covered by fine grained heterolithic strata that are thicker on the stoss side than on the lee side constituting a differential draping the pulsating and sustained character of the hyperpyc nal flows and the steep depositional slope clinoforms controlled the stacking pattern of the coarse grained sediment waves during successive hyperpycnal events in this way small scale coarse grained sediment waves with wavelengths of 10 40 m and up to 5 m amplitude and a stacking pattern showing an up current growth are generated by high density hyperpycnal flows on slopes having steep gradients

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

possible resonance effect in the distribution of earthquake damage in mexico city the destruction caused by the earthquake of 19 september 1985 in mexico city shows three remarkable features1 first a concentration of damage in the former lake bed second a peculiar distribution of high and low damage areas alternating within a few city blocks and third a selectivity for buildings between five and fifteen storeys high while the last point is understood by civil engineers2 and the first confirms the relevance of the soft ground the second feature is the most puzzling to a physicist such a pattern evokes the idea of a standing wave with low and high damage areas corresponding to nodes and antinodes this suggests a resonance phenomenon of the waterlogged ground moving collectively from the navier equations of classical elasticity it is known3 that a transfer of the incoming seismic wave energy to longitudinal waves takes place if a sharp change from hard to soft ground exists solving then the two dimensional poisson equation for the longitudinal waves within the ancient lake boundaries we show that for the observed seismic wave period of 2 s a standing wave solution exists which explains qualitatively the damage distribution pattern

strontium isotope systematics of mixing groundwater and oil field brine at goose lake in northeastern montana usa groundwater surface water and soil in the goose lake oil field in northeastern montana have been affected by cl rich oil field brines during long term petroleum production ongoing multidisciplinary geochemical and geophysical studies have identified the degree and local extent of interaction between brine and groundwater fourteen samples representing groundwater surface water and brine were collected for sr isotope analyses to evaluate the usefulness of 87 sr 86 sr in detecting small amounts of brine

shallow erosion beneath turbidity currents and its impact on the architectural development of turbidite sheet systems erosion by turbidity currents changes the morphology of the sea floor the relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition this could in turn affect flow and deposition patterns in subsequent events this study investigates shallow centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the oligocene macigno formation of north west italy where erosion and deposition are variably coupled at the bed scale in a net aggradational setting the research focus was on i the recognition of scour edges and erosive surfaces ii quantification of spatial differences in the amount of erosion and iii an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces where they can be observed scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded a consequence of this de laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges giving the appearance of normal conformable bed bases despite their cryptic nature such bedding parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs here 16 bed parallel scour surfaces were identified in a succession comprising 95 beds different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief it is found that differential erosion is on average under compensated by differential sedimentation in some cases the overlying deposits anti compensate being thinner at the location where more erosion has occurred unequal spatial distribution of differential erosion in the study area combines with sedimentary under compensation to result in a trend of accumulating section thickness differences over multiple beds in one ca 25 m thick package the maximum cumulative change in lateral gradient during some 20 events reached 0 17 before being reset by a single event this process can be interpreted either as a lobe compensation effect or as a scour enhancement effect depending on the orientation of the palaeohorizontal datum if allowed to proceed the latter process could force the system past a channellization threshold prompting a change from sheet to channelled architecture this type of shallow substrate scouring and differential deposition is likely to be an important process in the build up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea floor gradients

large scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows large scale 20 m to 7 km wavelength bedforms are common on the seafloor yet there is a lack of consensus on how they form and thus what to call them we conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments the data form three distinct groups 1 small scale 20 300 m wavelength sediment waves with mixed relief made of medium sand to cobble sized sediment that form in confined settings which we call small sediment waves 2 large scale 300 7000 m wavelength sediment waves with mixed relief made of fine grained sediment that form in relatively unconfined settings which we call large sediment waves and 3 large scale fully enclosed depressions in the seafloor which we call scours there is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact this data gap probably results from the environments in which sediment waves form being either confined e g channel or canyon or unconfined e g open slope bedform migration direction is available for 36 of the data and includes small and large scale sediment waves of these examples all are shown to migrate up current up current migration is indicative of supercritical flow thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves therefore we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size the migration direction for scours remains unknown scours may form from similar processes to small and large sediment waves or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre existing defects in the seafloor this novel statistical analysis of a global database shows that up current migrating bedforms associated with supercritical flow are unusually widespread and are recognised at two distinct scales

small scale hummocky cross stratification in turbidites a form of antidune stratification small scale hummocky cross stratification occurs in upper cretaceous calciclastic turbidites exposed in the western basque pyrenees facies associations and microfossil assemblages indicate slope to base of slope bathyal depositional environments it is developed in the fine grained portion of beds and displays spacings mostly between 0 2 and 0 7 m the beds fine upward with no sharp grain size breaks or mud partings suggesting that deposition occurred during a single flow event hummocky intervals are 0 1 0 8 m thick and consistently grade laterally and vertically into flat planar laminations of bouma b divisions suggesting that deposition occurred under upper flow regime conditions they have wave like geometries with laminae continuous across crests and troughs and display a ratio of wavelength to estimated underflow thickness of 11 3 12 8 combining the above observations and inferences these examples of small scale hummocky cross stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner denser underflow main body tail of the turbidity current and an overlying thicker low density layer this occurrence is further evidence that small scale hummocky cross stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment

combined measurements of velocity and concentration in experimental turbidity currents abstract three different sets of experimental turbidity currents were run in which velocity and concentration were measured simultaneously for several different heights above the bed one set with cohesive sediment had an initial volumetric concentration of 16 kaolinite and the other two sets with non cohesive sediment had concentrations of 28 and 4 silica flour velocity was measured at 104 122 hz using an ultrasonic doppler velocimetry profiler and concentration was measured at 10 hz using an ultrasonic high concentration meter the similarity of changes in velocity and concentration at the same measurement heights are described and it is shown that the similarity depends on flow concentration and position in the flow the measurements are analysed using cross correlations and wavelet analysis velocity measurements are compared with analytical solutions for flow around a semisphere and flow around a half body measurements and analyses indicate that turbulence is diminished by stratification decoupling of regions where turbulence is generated and by reduction of vertical flow in the turbidity currents

strontium isotope stratigraphy of enewetak atoll sup 87 sr sup 86 sr ratios determined for samples from a 350 m core of neogene lagoonal shallow water limestones from enewetak atoll display a remarkably informative trend like the recently published data for deep sea drilling project dsdp carbonates sup 87 sr sup 86 sr at enewetak increases monotonically but not smoothly from the early miocene to the pleistocene the data show intervals of little or no change in sup 87 sr sup 86 sr punctuated by sharp transitions to lower values toward greater core depths the sharp transitions correlate with observed solution disconformities caused by periods of subaerial erosion whereas the intervals of little or no change in sup 87 sr sup 86 sr correspond to intervals of rapid accumulation of shallow water carbonate sediments when converted to numerical ages using the published dsdp 590b trend the best resolved time breaks are at 282 m 12 3 to 18 2 ma missing and 121 6 m 3 0 to 5 3 ma missing below the lagoon floor at enewetak sr isotopes offer a stratigraphic resolution for these shallow marine neogene carbonates comparable to that of nannofossil zonation in deep sea carbonates 0 3 3 m y in addition the correlation of times of sr isotope breaks at enewetak with times of rapid sr isotope change in the dsdp 590b samples confirms the importance ofmore sea level changes in the evolution of global marine sr isotopes and shows that the sr isotope response to sea level falls is rapid less

orbitally tuned sr isotope chemostratigraphy for the late middle to late miocene we present a sr chemostratigraphic reference section for the late middle to late miocene 14 5 ma from ocean drilling program site 926 on the ceara rise this site combines a precise orbitally tuned timescale with a high sedimentation rate 15 m m y continuous deposition and excellent biostratigraphic control the sr isotope curve is based on measurements of cleaned planktonic foraminifera at 100 200 kyr sample intervals and it illustrates periods of rapid change in 87sr 86sr alternating with periods of little change chemostratigraphically defined ages for these intervals can be determined within 0 8 m y and 1 6 m y respectively there is excellent correlation with the published curve for site 588 hodell and woodruff 1994 however the curve for site 747 oslick et al 1994 exhibits less structure which may be due to small errors in age estimates related to slow sedimentation rates high latitude fauna and an interval of complicated magnetics late miocene data compare favorably with data from site 758 farrell et al 1995

laboratory experiments and numerical simulation of sediment wave formation by turbidity currents abstract this paper presents the results of laboratory experiments and numerical simulations carried out in order to understand the mechanism of sediment wave formation by turbidity currents experimental turbidity currents were generated in a 10 m long laboratory flume so as to investigate the topographic effects of slope and ridges on turbidite deposition the results indicate that preferential deposition occurs on the upstream side of the ridges this preferential deposition is considered to result in possible upstream migration of the topography a common feature of deep sea sediment waves provided that such deposition were repeated the preferential deposition occurred under a subcritical turbidity current implying that antidune flow conditions 0 844

reply to comment by j l mateos o novaro t h seligman and j flores on can horizontal p waves be trapped and resonate in a shallow sedimentary basin summary using a complete mathematical formulation we show that the trapping of horizontal p waves in a very soft shallow alluvial layer is a minor effect these waves do not have a stable way of propagation since in order to exist they require an incident wave and are therefore incapable of resonating in the lateral direction when confined in a basin of limited extent

quantification of variability in bedform geometry we analyze the variability in bedform geometry in laboratory and field studies even under controlled steady flow conditions in laboratory flumes bedforms are irregular in size shape and spacing also in case of well sorted sediment our purpose is to quantify the variability in bedform geometry we use a bedform tracking tool to determine the geometric variables of the bedforms from measured bed elevation profiles for each flume and field data set we analyze variability in 1 bedform height 2 bedform length 3 crest elevation 4 trough elevation and 5 slope of the bedform lee face each of these stochastic variables is best described by a positively skewed probability density function such as the weibull distribution we find that except for the lee face slope the standard deviation of the geometric variable scales with its mean value as long as the ratio of width to hydraulic radius is sufficiently large if the ratio of width to hydraulic radius is smaller than about ten variability in bedform geometry is reduced an exponential function is then proposed for the coefficients of variation of the five variables to get an estimate of variability in bedform geometry we show that mean lee face slopes in flumes are significantly steeper than those in the field the 95 and 98 values of the geometric variables appear to scale with their standard deviation the above described simple relationships enable us to integrate variability in bedform geometry into engineering studies and models in a convenient way

isotopic composition of recent shark teeth as a proxy for environmental conditions abstract the o c and sr isotope compositions of teeth from ten species belonging to five families and three orders of sharks were measured to determine the influence of habitat diet and possible species specific fractionation effects on the isotopic composition of biogenic phosphate from fish the sharks were recently caught in subtropical waters off the kwazulu natal kzn coast of south africa as well as from cold waters in prince william sound pws alaska and victor bay vb nunavut canada 18 o values of tooth phosphate 18 o p range from 20 9 to 23 5 for the kzn sharks for most species the range in measured 18 o p values is about 0 6 but it may be as high as 1 1 for different teeth from a single shark dentine and enameloid within individual teeth have no apparent differences in 18 o p values the 18 o p values of the kzn shark teeth reflect the typical habitat of the studied species primarily the thermal structure of the water column off kzn at depths between 20 and 280 m the 18 o p values of teeth from different greenland sharks from vb and pacific sleeper sharks from pws are very homogeneous averaging 25 8 and 24 7 respectively these values appear to be in equilibrium with deep 500 m ocean waters in each case at temperatures of about 0 3 c or less there is little discernable evidence for species specific fractionation effects for the oxygen isotope composition of phosphate in the studied marine fish the oxygen isotope composition of carbonate in apatite averages about 9 1 higher than corresponding 18 o p values in agreement with equilibrium fractionation between carbonate and phosphate but with a large variance 1 1 5 18 o c values also vary by up to 1 between enameloid and dentine within single teeth but in a non systematic way differences in 13 c values between carbonate in enameloid and dentine is also large up to 8 but the 13 c values vary systematically enameloid is always enriched in 13 c compared to dentine and the 13 c content increases with developmental stage of the teeth 13 c values measured for enameloid 1 6 to 4 8 appear to approach equilibrium with dissolved inorganic carbon in seawater in contrast 13 c values for dentine range from 6 4 to 2 3 for kzn sharks and 9 0 to 10 8 for the cold water sleeper sharks and are compatible with a predominantly dietary carbon source the 87 sr 86 sr ratios of teeth from kzn sharks as well as those from pws and vb are uniform averaging 0 709167 sr content varies from 1270 to 2100 ppm a range that is similar to that in well preserved fossilized teeth seawater sr is thus clearly incorporated in vivo concentrations of sm and nd are in the ppb range and contrast the ppm range in fossilized teeth indicating a postmortem incorporation of rare earth elements in apatite of the teeth

are coarse grained sediment waves formed as downstream migrating antidunes insight from an early pleistocene submarine canyon on the boso peninsula japan abstract gently undulating waveforms were identified in conglomerates and pebbly sandstones of the lower pleistocene higashihigasa formation which represents an infill of a submarine canyon on the boso peninsula japan on the basis of dimension geometry and texture these waveforms are interpreted as bedforms that are analogous to coarse grained sediment waves in modern deep water environments the studied waveforms exhibit symmetrical or dune like asymmetrical forms in association with minor antidune like asymmetrical forms with the lee sides being longer than the stoss sides the waveforms are also characterized internally by bedding gently inclined in the downstream direction to the northeast laterally washout dune and humpback dune deposits are locally developed in association with planar bedding the coarse grained sediment wave deposits are interpreted to have developed as downstream migrating antidunes in the upper flow regime condition of turbidity currents on the basis of the empirical relationship between the wavelengths and mean flow depths of downstream migrating gravel antidunes the waveforms are interpreted to have been formed by waves at the interface between the denser bottom flow and the less dense upper flow within a turbidity current it is possible that some of the coarse grained sediment waves in modern deep water environments may have been formed as downstream migrating antidunes regardless of their plan view geometry

sticky stuff redefining bedform prediction in modern and ancient environments the dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments however the effect of physical cohesion imparted by cohesive clay within mixed sand mud substrates has not been examined despite its recognized influence on sediment stability here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow the results show that bedform dimensions and steepness decrease linearly with clay content and comparison with existing predictors of bedform dimensions established within cohesionless sediments reveals significant over prediction of bedform size for all but the lowermost clay contents examined the profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross sets in turbidites the results therefore offer a step change in our understanding of bedform formation and dynamics in these and many other sedimentary environments

bedform development in mixed sand mud the contrasting role of cohesive forces in flow and bed abstract the majority of subaqueous sediment on earth consists of mixtures of cohesive clay and cohesionless sand and silt but the role of cohesion on the development and stability of sedimentary bedforms is poorly understood the results of new laboratory flume experiments on bedform development in cohesive mixed sand mud beds are compared with the results of previous experiments in which cohesive forces in high concentration clay flows dominated bedform development even though both series of mixed sand mud experiments were conducted at similar flow velocities the textural and structural properties of the bedforms were sufficiently different to permit the designation of key criteria for identifying bedform generation under cohesive flows against bedform generation on cohesive substrates these criteria are essential for improving bedform size predictions in sediment transport modelling in modern sedimentary environments and for the reconstruction of depositional processes in the geological record the current ripples developing on the cohesive mixed sand mud beds with bed mud fractions of up to 18 were significantly smaller than equivalent bedforms in noncohesive sand moreover the bedform height showed a stronger inversely proportional relationship with initial bed mud fraction than the bedform wavelength this is in contrast with the bedforms developing under the cohesive clay flows which tend to increase in size with increasing suspended clay concentration until the flow turbulence is fully suppressed selective removal of clay from the mixed beds i e clay winnowing was found to be an important process with 82 100 clay entrained into suspension after 2 h of bedform development this winnowing process led to the development of a sand rich armouring layer this armouring layer is inferred to have protected the underlying mixed sand mud from prolonged erosion and in conjunction with strong cohesive forces in the bed may have caused the smaller size of the bedforms winnowing was less efficient for the bedforms developing under the cohesive clay flows where bedforms consisting of muddy sand were more characteristic the winnowed sand was also found to heal irregularly scoured topography thus reestablishing classic quasitriangular bedform shapes in cohesive flows the bedforms had more variable shapes and the healing process was confined to lower transitional plug flows in which strong turbulence is only present close to the sediment bed furthermore the bedforms on the cohesive beds tended to form angle of repose cross lamination whereas low angle cross lamination was more common in bedforms under cohesive flows in general terms erosional bedforms prevail when cohesive forces in the bed dominate bedform dynamics whereas depositional bedforms prevail when cohesive forces in the flow dominate bedform dynamics empirical relationships between the proportion of cohesive mud in the mixed sand mud bed and the development rate and size of the bedforms are defined for future use in field and laboratory studies

resonant response models for the valley of mexico i the elastic inclusion approach summary the model developed by bard and bouchon to study the earthquake response of open ended 2 d sediment filled valleys with simple geometries is generalized to describe completely enclosed basins with arbitrary geometries by solving the basin form that corresponds to the valley of mexico it is possible to understand via a two step process model several of the observed features of the 1985 september 19 earthquake that devastated downtown mexico city here a general approach that allows the treatment of the particular seismic behaviour of the valley of mexico which is a large sediment filled valley containing an extremely soft ground urban area with a very high water content using collective resonance methods that vary from elastic to acoustic models is established this will lead through a series of papers to a theory that takes into account the role played by any valley s collective modes for microzonation and seismic damage predictions

resonant response models for the valley of mexico ii the trapping of horizontal p waves summary the strong ground motion recorded during the 1989 april 25 earthquake ms 6 9 shows many of the previously observed features in the soft lacustrine clays over which downtown mexico city is built a dominant response frequency around 0 4 hz enormous time prolongation of the seismic signal with characteristic beating phenomena and conspicuous differential amplification following the two step process suggested in part i seligman et al 1989 we add two new theoretical aspects the generation of horizontally propagating sp waves in the layered structure beneath mexico city for incoming sv waves beyond critical incidence and the subsequent lateral trapping of these sp waves in the lacustrine clays the time evolution of the lateral resonant pattern produced by these waves is calculated for the realistic geometry of the lake beds synthetic accelerograms and fourier spectra at different sites within the clay region are thus obtained our results can explain the monochromaticity and the long duration of the strong motion observed systematically in the soft clays of the valley of mexico

comment on can horizontal p waves be trapped and resonate in a shallow sedimentary basin by f j sanchez sesma and f luzon summary we show that most of the arguments in the above paper are either incorrect or irrelevant to the point the authors are trying to make we show that their results have no bearing on the model proposed by our group as they claim they discuss the seismic response of a valley with a 2 d trapezoidal cross section in a vertical plane whereas we dealt with a closed basin with a 2 d cross section but of arbitrary geometry and in the horizontal plane even more significantly the width of the valley they use is much smaller than the wavelength of the horizontal p waves that can resonate thereby precluding any possibility of them being trapped therefore their arguments do not clarify the issue posed in the title of their article

ground motion in mexico city during the april 25 1989 guerrero earthquake abstract instrumental observations of ground motion in mexico city during the april 25 1989 guerrero earthquake were analyzed our aim was to understand various aspects of the seismic response of the valley that had not been completely resolved such understanding of the basic mechanisms that control the seismic behavior of the valley sediments is crucial in any modeling attempt the study of vertical motion for this event which was shown to be practically unaffected by site conditions lead to the identification of a prominent long period rayleigh wave this together with the availability of absolute time for some stations allowed the establishment of a common time reference for all recordings horizontal motion in contrast was significantly amplified with large increases in duration at lake bed sites in order to interpret the observed complexity of ground motion we studied two simplified models of soft alluvial valleys one of these is two dimensional and it is excited by plane s waves with variable polarization and incidence angles this model allows three dimensional response the other is a three dimensional axi symmetric flat valley with a rigid base computations were performed in the frequency domain by means of a boundary integral method for the two dimensional model and using a collocation least squares technique for the three dimensional one seismograms were obtained through fourier synthesis it was found that the irregular soft layer response produces polarization patterns which are similar to the observations suggesting that the latter are a consequence of three dimensional effects

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

on the nature of regional seismic phases iii the influence of crustal heterogeneity on the wavefield for subduction earthquakes the 1985 michoacan and 1995 copala guerrero mexico earthquakes summary the most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the lg phase this arrival represents trapped s wave propagation within the crust as a superposition of multiple reflections and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path in an environment where the events occur in a subduction zone such as the western coast of mexico quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone the great 1985 michoacan earthquake mw 8 1 which occurred in the mexican subduction zone was one of the most destructive earthquakes in modern history and its notable character was that at mexico city located over 350 km from the epicentre there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake the nature of the propagation process in this region can be understood in part by using the detailed strong motion records from the 1995 copala guerrero mw 7 4 earthquake near the coast to the south of mexico city which also had an enhanced amplitude in the valley of mexico numerical modelling of both p and s seismic waves in 2 d and 3 d heterogeneous crustal models for western mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface a comparison of different models allows the influences of different aspects of the structure to be isolated 2 d and 3 d modelling of the 1985 michoacan and 1995 copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the sn and lg wave trains these s wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the cocos plate beneath the mexican mainland the amplitude and duration of the lg coda is also strongly reinforced by transmission through the mexican volcanic belt from the amplification of s waves in the low velocity surficial layer associated withs to p conversions in the volcanic zone the further amplification of the large and long lg wave train impinging on the shallow structure in the basin of mexico city with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast gives rise to the destructive strong ground shaking from the mexican subduction earthquakes

geochemical and c o sr and u series isotopic evidence for the meteoric origin of calcrete at solitario wash crater flat nevada usa calcite rich soils calcrete in alluvium and colluvium at solitario wash crater flat nevada usa contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi arid southwestern united states nevertheless a ground water discharge origin for the solitario wash soil deposits was proposed in a series of publications proposing elevation dependent variations of carbon and oxygen isotopes in calcrete samples discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at yucca mountain nevada site of a proposed high level nuclear waste repository new geochemical and carbon oxygen strontium and uranium series isotopic data disprove the presence of systematic elevation isotopic composition relations which are the main justification given for a proposed ground water discharge origin of the calcrete deposits at solitario wash values of 13c 4 1 to 7 8 per mil 18o 23 8 17 2 87sr 86sr 0 71270 0 71146 and initial 234u 238u activity ratios of about 1 6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at yucca mountain and are incompatible with a ground water origin for the calcrete variations in carbon and oxygen isotopes in solitario wash calcrete likely are caused by pedogenic deposition from meteoric water under varying quaternary climatic conditions over hundreds of thousands of years

sediment slides and turbidity currents on the laurentian fan sidescan sonar investigations near the epicenter of the 1929 grand banks earthquake the epicenter area of the 1929 grand banks earthquake on the continental slope south of newfoundland has been investigated using sea marc i a deeply towed midrange sidescan sonar with a 4 5 khz subbottom profiler shallow slides pass downslope into debris flows on the muddy continental slope east of the epicenter at the head of the eastern valley of the laurentian fan west of the epicenter arcuate slide scars cut undisturbed upper slope sediment and lead downslope to a lineated erosional seabed at a water depth of about 1600 m this erosional seabed passes into extensive fields of 100 m wavelength gravel waves situated on the broad irregular valley floor the gravel waves become better developed downslope and extend at least to water depths of 3000 m all these morphological features appear fresh on the sidescan sonograms suggesting that they date from the 1929 earthquake event and the distribution of slides corresponds to the area of instantaneous cable breaks in 1929 the upper limit of erosion on valley walls suggests that the 1929 turbidity current was less than 300 m thick timing of cable breaks downfan suggests that flow velocities were sufficient to rework gravel deposits into large bedforms during waning flow stages over elevated areas of the valley floor similar cross bedded coarse sands and gravels are common in ancient channel deposits

sediment dynamics driven by contour currents and mesoscale eddies along continental slope a case study of the northern south china sea abstract based upon 3 d seismic reflection data published drill core studies oceanographic observations and numerical modelling this study identifies a small scale contourite depositional system that is locally confined by canyons slope failures erosional terraces and erosional slopes on the jianfeng slope northern south china sea 1500 to 2300 m water depth possible oceanic driving mechanisms for the spatial heterogeneity in sedimentation and erosion within the quaternary deposits in this complex slope system are investigated numerical simulations show that the quasi steady south china sea deep water circulation which affects the study area is characterised by low velocities maximum 4 cm s with a slight modulation by tides 1 cm s this quasi steady deep water thermohaline circulation is able to keep sediment particles in suspension rather than to erode seafloor sediment the bottom hydrodynamic regime becomes energetic when mesoscale eddies horizontal scale of 10 to 100 km approach our modelling study indicates that both surface and bottom mesoscale eddies are essential to account for the spatial heterogeneity in sedimentation pattern along the northern south china sea continental slope according to simulation the eddy front contains the highest flow velocity over a mesoscale eddy cycle 45 days exceeding the threshold for resuspension of unconsolidated sediment 15 cm s as a consequence massive resuspension is produced at various sites and redistributed by sub mesoscale horizontal scale of 1 to 10 km circulations originated from an eddy topography interaction in contrast to the surrounding areas which are subject to erosional forcing for a relatively long part 7 of an eddy cycle both the locally confined drift and a further upstream large elongated mounded drift experience little erosion

late cenozoic sea level and the rise of modern rimmed atolls this paper is not subject to u s copyright the definitive version was published in palaeogeography palaeoclimatology palaeoecology 451 2016 73 83 doi 10 1016 j palaeo 2016 03 018

the messinian succession of the crotone basin southern italy ii facies architecture and stratal surfaces across the miocene pliocene boundary the upper messinian to lower zanclean succession of the crotone basin a forearc basin located along the ionian side of the calabrian arc southern italy consists of a turbidite bearing succession the petilia policastro formation overlying evaporites and abruptly passing upward into coarse grained continental deposits the carvane conglomerate which are in turn overlain by distal shelf to slope mudstones the cavalieri marl the marked environmental changes from subaqueous to continental and vice versa documented by the studied succession are inferred to be the expression of basin scale tectonics leading to uplift and exposure superposed on the mediterranean scale base level rise that followed the acme of the messinian salinity crisis the upper part of the succession documents a transgressive trend probably initiated within the carvane conglomerate and further testified by the presence of a ravinement surface marked by a transgressive lag at the base of the zanclean cavalieri marl pointing to a non catastrophic deepening occurred near the end of the messinian

tb or not tb banding in turbidite sandstones recognition and interpretation of sedimentary structures is fundamental to understanding sedimentary processes banded sandstones are an enigmatic sedimentary facies comprising alternating mud rich as matrix and or mud clasts and cleaner sand layers the juxtaposition of hydrodynamically different grain sizes contradicts established models of cleaner sand bedform development here outcrop subsurface core and petrographic data from three deep water systems with well constrained paleogeographic contexts are used to describe the range of sedimentary textures bedform morphologies and facies associations and to quantify the mud content of banding banding can occur in any part of a bed base middle or top but it typically overlies a structureless basal sandstone or mud clast conglomerate lag and is overlain by clean parallel laminated sandstone and or ripple cross lamination banding morphology ranges from sub parallel to bedforms that comprise low angle laminae with discontinuous lenses of mudstone or asymmetric bedforms comprising steeply dipping foresets that transition downstream into low amplitude bedwaves or steeply dipping ripple like bedforms with heterolithic foresets this style of banding is interpreted as a range of bedforms that form progressively in the upper stage plane bed flow regime via tractional reworking beneath mud laden transitional plug flows the balance of cohesive and turbulent forces and the rate of flow deceleration aggradation rate govern the style of deposit banded sandstones and linked debrites are rarely found juxtaposed together in the same bed because they are distributed preferentially in proximal and distal settings respectively understanding the origins of banding in turbidite sandstones the conditions under which it forms and its distribution across deep water systems and relationship to linked debrites is important for it to be used effectively as a tool to interpret the geological record

seismic coupling of interface modes in sedimentary basins a recipe for disaster abstract coupling between an evanescent p wave and a rayleigh wave can occur when the two modes share the same phase velocity at an interface the result is a new distinctive monochromatic mode of large amplitude and finite duration that may reverberate in a bounded layer generating stationary patterns due to constructive and destructive interference such modes may be responsible for earthquake damage in the valley of mexico and perhaps in other sedimentary basins vulnerable to seismic disasters on soft ground

generation of inhomogeneous p waves in a layered medium abstract the generation of inhomogeneous horizontally propagating p waves in a stratified elastic medium were analyzed for incident sv waves beyond the critical angle using realistic values of s and p wave velocities and considering both clay and alluvial sedimentary layers overlying a bedrock half space we obtained enormous amplifications for the horizontal displacements at the free surface these amplifications are of the order of several thousands for the fundamental mode of the system which systematically dominates all other frequencies

generation and migration of coarse grained sediment waves in turbidity current channels and channel lobe transition zones large scale sediment waves composed of gravels and sands have been studied using deep water sidescan systems new data are presented from submarine channels off the canary islands and from canyon mouths off portugal data from other areas are briefly reviewed including a re interpretation of data from laurentian fan in order to summarise the varied morphology and setting of these bedforms coarse grained sediment waves are found in the proximal dominantly bypassing areas of deep water turbidite systems within canyons channels and channel lobe transition zones wave heights are in the region of 1 10 m and wavelengths are up to several hundred metres the distribution of waves and sparse sedimentological evidence from modern and ancient sediment wave fields suggests that initial transport and deposition of coarse sediment occurs within a high density turbidity current and not as a non newtonian debris flow in some cases the development of pronounced wave asymmetry and evidence of wave disruption and reworking suggests that the wave morphology is at least partially controlled by a later phase of low density turbidity flow grain size also appears to exert some control on wave morphology for example gravel rich waves have a greater height for the same wavelength than sand rich waves coarse grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents and are equally difficult to recognise in outcrop because of their large size

experiments on oscillatory flow and combined flow bed forms implications for interpreting parts of the shallow marine sedimentary record abstract analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm like oscillatory and combined flow experiments were carried out in a large wave tunnel and used a range of sand sizes fine and very fine wave periods 10 5 and 8 s oscillatory velocities 0 to 125 cm s and unidirectional velocities 0 to 25 cm s at low unidirectional velocities 10 cm s addition of an increasing collinear oscillatory flow caused the bed to evolve from small scale wavelength 100 cm symmetric orbital ripples to plane bed at higher unidirectional velocities 10 cm s a similar trend was noted but ripples were more asymmetric three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions these diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent distinctive features of small scale asymmetric combined flow ripples generated are a 3d planform round crest and convex up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a boxy appearance similarly large scale asymmetric combined flow ripples had broad and round crests convex up stoss sides and compressed profiles due to scouring at the toe of the stoss side hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed form scale and symmetry hummocks were more prevalent at longer oscillatory periods and in finer grained sediment stratification produced by synthetically aggrading hummocky bed profiles closely resembles hummocky cross stratification with the introduction of only a small unidirectional flow component hummocks evolve into downstream migrating large scale asymmetric ripples and the resultant cross stratification becomes similar to that produced by unidirectional flow dunes accordingly these experiments suggest that much of the hummocky cross stratification observed in the stratigraphic record is produced by storm generated long period oscillatory dominant combined flows inasmuch as long period high energy waves require deep wide basins to form hummocky cross stratification may therefore serve as a useful indicator of deposition in unrestricted open water conditions

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained deposits

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

experimental and numerical study of topographic effects on deposition from two dimensional particle driven density currents abstract topographic effects on deposition from particle driven density current were investigated the laboratory experiments were carried out on topography consisting of a ramp and a series of humps the results show a localized increase in deposit distribution downstream of the slope break and on the upslope of a hump a numerical model is developed to predict the topographic effects on deposit distribution the model based on layer averaged navier stokes equations is applied to the experiments and the process of the topographic influence is analyzed the preferential deposition downstream of the slope break is interpreted to result from deceleration of the flow which increases deposit distribution through loss of capacity of the flow and longer duration of the flow passage the former effect is more influential than the latter while additional effects of a hydraulic jump may be imposed increased deposit on the upslope of a hump is attributed to partial blocking of the flow at the hump crest as well as to the differential deposition due to deceleration on the upslope

allogenic vs autogenic controls on megaflute formation abstract megaflutes are large concave up erosional scours within turbidite successions and have been described from modern and ancient settings they are interpreted to represent the passage of erosive largely bypassing turbidity currents here two categories of megaflute are distinguished 1 allogenic corresponding to the case above where erosion is associated with sediment bypass leading to deposition farther into the basin e g the classical megaflutes of the ross formation 2 autogenic where the erosion occurs during aggradation of the host bed both types are illustrated with examples from the carboniferous succession of northern england the distinction is important because the formative processes are different leading to distinct facies associations for each group for example allogenic flutes are interpreted to form in systems with large variations in flow magnitude and imply significant down dip sediment accumulation whereas autogenic megaflutes are thought to be formed by unsteady flows and imply less significant bypass autogenic megaflutes occur above inversely graded poorly sorted sandstones which improve in sorting and decrease in mud content upwards to the megaflute surface which is overlain by a well sorted mantle of the coarsest sediment found within the bed detailed grain size profiles suggest that autogenic megaflutes develop in response to waxing of flows from a depositional to an erosional state cannibalizing the already deposited sand bed as the flow wanes reverting to a depositional state the coarsest grained fraction of its remaining load is deposited first forming the basal part of a variably well developed normally graded and overall finer grained interval thus it is interpreted that flow unsteadiness promotes the formation of these features which has implications for the reconstruction of paleoenvironmental conditions specifically the generation mechanisms return periods flow routing into the basin and the resultant flow characteristics of turbidity currents

meteoric diagenesis marine diagenesis and microporosity in pleistocene and oligocene limestones enewetak atoll marshall islands abstract microporosity occurs in pleistocene and oligocene limestones cored on enewetak atoll in cores of pleistocene limestone 10 80 m deep mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution chalkification leaving a porous meshwork of aragonite needles microporous aragonite is most abundant in 5 10 m thick intervals below zones of intense diagenesis associated with paleo freshwater lenses extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite most calcitized aragonite neomorphic calcite formed by precipitation of sparry calcite in over microporous aragonite aragonitic fossils were not converted to microporous microcrystalline calcite therefore data from the pleistocene of enewetak suggest that widespread microporous microcrystalline calcite does not form directly from pure aragonite at least not in open meteoric systems or mixing zones microporous micrite occurs in oligocene wackestones 930 m deep deposited in a slope environment the microporous micrite is characterized by equant and prismatic crystals of low magnesium calcite lmc equant crystals are anhedral to euhedral and 2 10 m in diameter prismatic calcite crystals are subhedral 5 10 m wide and 20 50 m long stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high magnesium calcite hmc micrite in deep seawater undersaturated with respect to hmc but supersaturated with respect to lmc probably near the sediment water interface mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the thamama group cretaceous persian gulf see moshier 1989 equant microcrystals of calcite in the oligocene micrites are similar to those in microporous parts of the thamama group widespread microporosity in lmc limestones could form by alteration of hmc micrite in mixing zones or in seawater undersaturated with respect to hmc but supersaturated with respect to lmc

architecture and morphodynamics of subcritical sediment waves in an ancient channel lobe transition zone in modern systems submarine channel lobe transition zones show a well documented assemblage of depositional and erosional bedforms in contrast the stratigraphic record of channel lobe transition zones is poorly constrained because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings several locations from an exhumed fine grained base of slope system unit b laingsburg depocentre karoo basin south africa show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions the regional stratigraphic context lack of a basal confining surface wave like morphology in dip section size and facies characteristics support an interpretation of subcritical sediment waves within a channel lobe transition zone setting some sediment waves show steep 10 to 25 unevenly spaced 10 to 100 m internal truncation surfaces that are dominantly upstream facing which suggests significant spatio temporal fluctuations in flow character their architecture indicates that individual sediment wave beds accrete upstream in which each swell initiates individually lateral switching of the flow core is invoked to explain the sporadic upstream facing truncation surfaces and complex facies distributions vertically within each sediment wave variations in bedform character are related to the axial to marginal positions within a channel lobe transition zone the depositional processes documented do not correspond with known bedform development under supercritical conditions the proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture

hydraulic jumps in sediment driven bottom currents laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of a slope transition both sediment laden and saline hydraulic jumps were produced the vertical structure of the currents was found to depend on flow regime the saline and turbid hydraulic jumps showed similar characteristics the amount of water entrained by the flows through a jump was small the change in flow regime caused a marked reduction of the bed shear stress downstream of the jump in nature a turbidity current experiencing a hydraulic jump will drop most of its bedload immediately downstream from the jump while the suspended load will respond more gradually to the change in flow regime and will deposit sediment over a distance far exceeding 1 000 times the jump height

the full range of turbidite bed thickness patterns in submarine lobes controls and implications a widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop and in core and well logs is a thickening and or coarsening upward profile lobe deposits from the laingsburg depocentre sw karoo basin demonstrate that a full range of bed thickness patterns exists within lobes when lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes five types of bed stacking patterns are identified thickening upward thinning upward thickening then thinning upward thinning then thickening upward and constant the abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels the stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller scale lobe elements rather than lobe wide initiation and progradation as implied by a thickening upward only pattern the full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements the evolution of sediment deposition through time and space and the relative movement of depocentres

topography effect at the critical sv wave incidence possible explanation of damage pattern by the whittier narrows california earthquake of 1 october 1987 abstract buildings damaged by the whittier narrows california earthquake of 1 october 1987 were concentrated in several areas most of which were located 8 to 10 km from the epicenter corresponding to the critical incidence for sv waves and were near topographic irregularities such as the northern part of whittier just south of puente hills the purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when sv waves are near critical incidence first we examined the accelerograms obtained at the usgs station 7215 bright ave nearest to downtown whittier and conclude that the dominantly eastwest motion observed at the station is due to the influence of the building in which accelerograms were recorded we next demonstrate that strong ground motions recorded by 12 stations in the epicentral area show a polarization direction pattern that is primarily sv type which is consistent with a simple thrust fault located at the hypocenter then we calculated the response of a two dimensional hill with the height 0 3 km and the width 2 4 km to 1 a plane sv wave with a nearly critical angle of incidence 2 a horizontal line force 3 a haskell type 2d dislocation source and 4 a bouchon type 2d multiple crack source the results show that the amplification due to the hill relative to the flat surface is more than 1 5 for all the source models since this amplification is nearly independent of the source type and spectrum we conclude that the combined effect of the topographic irregularity and critically incident sv waves might be responsible for the concentration of damage observed during the whittier narrows earthquake

destructive strong ground motion in mexico city source path and site effects during great 1985 michoacan earthquake simultaneous consideration of source path and site effects on ground motion during the michoacan earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in mexico city by the rupture process the mode of propagation of the waves between the epicentral zone and mexico city and the local amplification in contrast to the horizontal component which showed dramatic amplification for the 2 to 3 sec motion at lake sediment sites we observe almost identical vertical displacement seismograms containing ripples with 2 to 3 sec period throughout the mexico city valley whether the recording site is on the lake sediments or on hard rock we therefore conclude that the 2 to 3 sec motion responsible for the destruction of mexico city was present in the incident field after performing a phase analysis we interpret the signal as the superposition of long period rayleigh waves and short period lg with a dominant period of about 3 sec the analysis of the teleseismic records indicates that the radiation of this event is enhanced for waves around the 3 sec period except in the case of stations for which an anomalous path effect is suspected the records present ripples appearing a few seconds after the beginning of the signal the characteristics of near fault records show that the rupture process consists of the growth of a smooth crack the numerical simulation indicates that the 3 sec period ripples can be explained by a series of changes of the rupture front velocity we examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in mexico city our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in mexico city this interpretation leads to a very coherent scenario of what happened from the start of the failure on the fault up to the destruction in mexico city this example illustrates the need to consider simultaneously source path and site effects in order to understand strong ground motions

analysis of wave propagation in the valley of mexico from a dense array of seismometers a dense array of seismometers was installed and operated in the botanical garden of unam within the valley of mexico during april and may 1994 the slow surface velocities at the array require an approach to array analysis with the capability of measuring arrival time differences between stations of less than the sampling interval of the data applying the method to principal component seismograms from earthquakes at local and regional ranges from a number of directions provides the first quantitative information on the composition of the wave field incident upon mexico city phases that propagate through the entire crust such as the initial p and lg waves cross the array with the expected backazimuth on the other hand surface waves in the 2 5 to 5 sec period band which are sensitive to uppercrustal structure show systematic rotations of backazimuth these suggest changes in upper crustal structure or point scatterers located north and south of mexico city for earthquakes outside of the valley of mexico no significant energy was observed propagating across the array from the east from the lake zone thus although it is clear that seismic waves resonate within the lake zone little if any of this energy is transmitted into the hill zone where the array was located however long duration ground motions are observed at the array with evidence of multipathing a back projection of these late off azimuth arrivals indicates that their sources correlate with the boundaries of the quaternary volcanics of the trans mexican volcanic belt

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

sr isotopic variation in shallow water carbonate sequences stratigraphic chronostratigraphic and eustatic implications of the record at enewetak atoll sr isotope data from two boreholes within the lagoon at enewetak atoll have been used to evaluate the use of such data to correlate date and monitor sea level change in shallow water carbonate sequences correlative stratigraphic intervals of relatively invariant 87sr separated by abrupt transitions to lower 87sr with increasing depth are recognized in both boreholes conversion of 87sr values to age via calibration with the seawater 87sr trend with age indicates that correlative and synchronous deposition of atoll sediments occurred at 0 4 1 2 and 2 1 ma in contrast a 5 m y hiatus is recognized in one borehole but not the other sr isotope stratigraphy sis is a powerful stratigraphic and chronostratigraphic tool in shallow water carbonate sequences only when significant secular variation of 87sr occurs and retention of depositional 87sr values is demonstrated the latter is best demonstrated when 87sr data are integrated with 18o 13c sr content data and petrographic observations several diagenetically altered intervals have greater 87sr values low 13c values and low sr ca ratios relative to adjacent intervals a combination that is consistent with open system meteoric diagenesis calcite cements from these intervals have early pleistocene 1 2 ma 87sr values despite their occurrence well within the late pliocene 2 1 ma sequence thus local sedimentological and diagenetic processes have produced intralagoon variability in the sis of the two boreholes complicating subsurface stratigraphic correlations the occurrence of anomalously young calcite cement relative to adjacent limestone is a direct response of the interaction of sea level change and meteoric phreatic diagenesis whereby overlying metastable carbonates with greater 87sr values are dissolved during periods of atoll emergence and sea level lowstand liberating sr and soil gas co2 to the pore fluid which is then incorporated into downflow meteoric phreatic diagenetic calcite the stratigraphic relation between the sites of carbonate dissolution and diagenetic calcite precipitation suggest an early pleistocene 1 2 ma sea level fall of between 34 and 64 m moreover the stratigraphic position and inferred age of paleophreatic lens calcite when backtracked for subsidence suggest that early pleistocene 1 2 ma sea level was 72 81 m range of 60 100 m below present sea level the enewetak record of early pleistocene sea level lowstand elevation agrees reasonably well with proxy sea level records inferred from sequence stratigraphy and deep sea foraminiferal 18o data however the enewetak record of the amplitude of early pleistocene sea level change is more consistent with the deep sea foraminiferal 18o proxy than the sequence stratigraphy proxy

froude supercritical flow processes and sedimentary structures new insights from experiments with a wide range of grain sizes recognition of froude supercritical flow deposits in environments that range from rivers to the ocean floor has triggered a surge of interest in their flow processes bedforms and sedimentary structures interpreting these supercritical flow deposits is especially important because they often represent the most powerful flows in the geological record insights from experiments are key to reconstruct palaeo flow processes from the sedimentary record so far all experimentally produced supercritical flow deposits are of a narrow grain size range fine to medium sand while deposits in the rock record often consist of a much wider grain size distribution this paper presents results of supercritical flow experiments with a grain size distribution from clay to gravel these experiments show that cyclic step instabilities can produce more complex and a larger variety of sedimentary structures than the previously suggested backsets and scour and fill structures the sedimentary structures are composed of irregular lenses mounds and wedges with backsets and foresets as well as undulating planar to low angle upstream and downstream dipping laminae the experiments also demonstrate that the froude number is not the only control on the sedimentary structures formed by supercritical flow processes additional controls include the size and migration rate of the hydraulic jump and the substrate cohesion this study further demonstrates that froude supercritical flow promotes suspension transport of all grain sizes including gravels surprisingly it was observed that all grain sizes were rapidly deposited just downstream of hydraulic jumps including silt and clay these results expand the range of dynamic mud deposition into supercritical flow conditions where local transient shear stress reduction rather than overall flow waning conditions allow for deposition of fines comparison of the experimental deposits with outcrop datasets composed of conglomerates to mudstones shows significant similarities and highlights the role of hydraulic jumps rather than overall flow condition changes in producing lithologically and geometrically complex stratigraphy

some aspects of source characteristics of the 19 september 1985 michoacan earthquake and ground motion amplification in and near mexico city from strong motion data abstract the fourier amplitude ratio of the 19 september m s 8 1 to the 21 september 1985 m s 7 6 michoacan earthquakes obtained from data in and near mexico city increases from about 3 5 at 0 5 hz to about 10 at 0 2 hz this increase is neither seen in the coastal data nor in the teleseismic broadband gdsn p wave spectra it is also not predicted by 2 or gusev9s scaling laws we tentatively attribute the increase at frequencies of less than 0 5 hz to a special path and or a depth of energy release effect for the 19 september earthquake the damage to mexico city is mostly related to 0 5 hz energy since at f 0 5 hz the fourier amplitude ratios in and near mexico city along the coast and at teleseismic distances are nearly equal we attribute this energy to the source characteristics of the 19 september earthquake the evidence suggests that the 19 september was about twice more energetic at f 0 5 hz than expected from scaling laws and from spectral regression of coastal earthquakes recorded in mexico city the analysis of strong motion data recorded in mexico city shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in ciudad universitaria the frequency at which the maximum relative amplification occurs varies from site to site and lies between 0 2 and 0 7 hz the data suggest that f o 4 h where f o frequency at which maximum relative amplitude occurs and and h are the shear wave velocity and the thickness of the clay layer respectively relatively small if any nonlinear behavior of the clay is found at strains of about 0 2 per cent during the great 19 september earthquake for a given lake bed site maximum relative amplitude and f o are roughly constant independent of magnitude depth and azimuth of sources located more than about 200 km ground motion at lake bed sites in mexico city with f o 0 5 hz appear to be amplified 75 times with respect to hard rock coastal sites at equal distances from the source this is also confirmed by m l calculation of the 19 and 21 september earthquakes for the 19 and 21 september earthquakes the average values of m l at hard rock coastal sites 100 distance 250 km are 7 4 and 6 7 whereas they are 8 7 and 8 4 in the lake bed zone distance 300 km respectively

can horizontal p waves be trapped and resonate in a shallow sedimentary basin summary it has been proposed recently that horizontal p waves can be trapped and resonate laterally in a shallow sedimentary basin in this work it is shown both theoretically and numerically that in a sedimentary basin of finite lateral extent trapped horizontal p waves if they exist are not a stable way of propagation and therefore cannot resonate