prediction of lateral variability in fracture intensity using multicomponent shear wave surface seismic as a precursor to horizontal drilling in the austin chalk summary a method of multicomponent multisource shear wave interpretation utilizing the output of the alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling the method exploits a fundamental difference between the two particle motion stacks when vertical aligned fractures are the source of the shear wave splitting the fast shear wave s1 section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures hence reflectivity on the s1 cdp stacked section should be likewise insensitive to fractures the slow shear wave s2 section however possesses particle motion perpendicular to the fracture planes so variation in lateral fracture intensity will affect velocity and reflectivity examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the s1 section for the austin chalk the horizon of interest but laterally inconsistent reflector strength for the same horizon on the s2 section in the study area reflector dimming on the s2 section was interpreted as indicating fracture intensity maxima horizontal drilling confirmed both fracture azimuth and intensity predictions and resulted in significantly higher initial hydrocarbon production than for surrounding wells

geomorphic response to seasonal variations in rainfall in the southwest united states the interaction of the north american monsoon with watershed hydrology and landscape response is evaluated by observing geomorphic characteristics of hillslopes hydrology and stream channels in two mountain ranges with contrasting intensity of precipitation the study compares watersheds in the hualapai and santa catalina mountains in arizona which are similar in lithology elevation tectonic setting vegetation and annual precipitation but differ in the proportion of precipitation received in summer thunderstorms the hualapai mountains receive most of their precipitation in winter whereas rainfall in the santa catalina mountains occurs mostly in summer drainages in the santa catalinas are more variable in local relief and exhibit much more exposed bedrock and higher drainage density the trunk channel of a major drainage in the santa catalina mountains has measured discharges several orders of magnitude greater than a channel draining a comparable area in the hualapai mountains the alluvial segment of the channel in the santa catalina piedmont additionally displays greater concavity smaller width to depth ratios and a larger caliber of bed material in contrast to conventional interpretation summer monsoonal precipitation is not primarily responsible for generating most of the discharges that modify channels in large scale drainages in the monsoonal climate regime of the santa catalinas small basins flood most often in summer whereas larger drainages exhibit peak discharges in response to low intensity winter precipitation we attribute the paradoxical discharge response of larger drainages to the small spatial scale of summer thunderstorms which fail to deliver enough precipitation to generate floods in these larger basins but do prime hillslopes by stripping colluvium and lowering hillslope infiltration rates making the large drainages more responsive to areally extensive winter storms

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

shear wave splitting and anisotropy from the aftershocks of the nahanni northwest territories earthquakes two large earthquakes ms 6 6 6 9 occurred in 1985 near the north nahanni river in the northwest territories canada the first on october 5 and the second on december 23 the earthquakes were followed by long aftershock sequences aftershocks between m 1 0 and 3 9 were recorded by five to eight three component stations during three field experiments in october 1985 and in january and september 1986 two distinct shear waves were generally observed in september 1986 the plane of polarization of the first arriving shear wave was well defined and averaged 156 8 measured clockwise from north the stress direction was inferred from the initial shear wave polarizations recorded at one station from a widespread aftershock distribution but was limited to no more than 10 km around it the delay between first and second arriving shear waves was up to 190 ms assuming that the two shear waves result from anisotropy and that the entire ray paths varying in length from 7 to 23 km contributed the maximum velocity anisotropy is 7 however the distribution of the anisotropy is not uniform the pressure axes determined from fault plane solutions for the two main shocks are nearly e w normal to the compressive direction found in the september 1986 field survey from shear wave splitting data four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey

shear wave birefringence a new tool for evaluating fractured reservoirs propagation characteristics of shear waves make them particularly well suited to exploration for and definition of fractured reservoirs the department of geophysics at colorado school of mines conducted a three component p and s wave seismic survey in order to determine whether shear wave polarization and splitting birefringence caused by fracture induced azimuthal anisotropy could be measured over a naturally fractured reservoir results indicate that surface seismic recordings of shear waves can be used to calculate azimuthal anisotropy and that seismic observation of azimuthal anisotropy can provide information about the location orientation and intensity of subsurface fractures data obtained from this investigation indicate that measurement of shear wave birefringence is a potentially powerful tool for predicting fracture orientation and permeability trends

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

observation of shear wave splitting from natural events south moat of long valley caldera california june 29 to august 12 1982 three component seismograms from local earthquakes recorded by three university of wisconsin stations in the south moat of long valley caldera in 1982 have been analyzed for shear wave splitting the average polarization direction of the first arriving shear waves observed at the two stations south of the resurgent dome is nnw sse time delays between the fast and slow shear waves are proportional to hypocentral distances and the maximum velocity anisotropy inferred is 6 4 the average shear wave polarization observed at the station above the resurgent dome is north south the time delays there are not a function of hypocentral distance and the normalized time delays ms km from events at the southwest boundary of the caldera are larger than those at other azimuths indicating maximum 9 6 velocity anisotropy the east west to ene wsw variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the sierra nevada range front faults under the generally wnw ese tension expected from the regional tectonics

coevolution of nonlinear trends in vegetation soils and topography with elevation and slope aspect a case study in the sky islands of southern arizona 1 feedbacks among vegetation dynamics pedogenesis and topographic development affect the critical zone the living filter for earth s hydrologic biogeochemical and rock sediment cycles assessing the importance of such feedbacks which may be particularly pronounced in water limited systems remains a fundamental interdisciplinary challenge the sky islands of southern arizona offer an unusually well defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type granite and tectonic history here we compile high resolution spatially distributed data for effective energy and mass transfer eemt the energy available to drive bedrock weathering above ground biomass soil thickness hillslope scale topographic relief and drainage density in two such mountain ranges santa catalina scm pinaleno pm strong correlations exist among vegetation soil topography variables which vary nonlinearly with elevation such that warm dry low elevation portions of these ranges are characterized by relatively low above ground biomass thin soils minimal soil organic matter steep slopes and high drainage densities conversely cooler wetter higher elevations have systematically higher biomass thicker organic rich soils gentler slopes and lower drainage densities to test if eco pedo geomorphic feedbacks drive this pattern we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales with rates explicitly dependent on vegetation density the model self organizes into states similar to those observed in scm and pm our results highlight the potential importance of eco pedo geomorphic feedbacks mediated by soil thickness in water limited systems

seismic velocities in fractured rocks an experimental verification of hudson s theory flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures these cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir several theories including hudson s claim to predict the seismic effects of cracks although hudson s theory has already been used to calculate crack densities from seismic survey s the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution this paper describes an experimental verification of the theory the rock used carrara marble was chosen for its uniformity and low porosity so that the effect of cracks would not be obscured by other influences cracks were induced by loading of laboratory specimens velocities of compressional and shear waves were measured by ultrasound at 0 85 mhz in dry and water saturated specimens at high and low effective pressures the cracks were then counted in polished sections of the specimens in dry specimens with both dry and saturated cracks hudson s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated the theory made poor predictions for fully saturated specimens shear wave splitting caused by anisotropy duemore to both crystal and crack alignment was observed cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading the results demonstrate that hudson s theory may be used in some cases to determine crack and fracture densities from compressional and shear wave velocity data less

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

an integrated study of seismic anisotropy and the natural fracture system at the conoco borehole test facility kay county oklahoma a significant body of published work has developed establishing fracture related seismic anisotropy as an observable effect to further the understanding of seismic birefringence techniques in characterizing natural fracture systems at depth an integrated program of seismic and geologic measurements has been conducted at conoco s borehole test facility in kay county oklahoma birefringence parameters inferred from the seismic data are consistent with a vertical fracture model of density 0 04 striking east northeast that direction is subparallel to a fracture set mapped both on the surface and from subsurface data to the in situ maximum horizontal stress and to the inferred microfabric

comparison between natural fractures and fracture parameters derived from vsp summary the feasibility of resolving a subsurface fracture system using p wave and cross polarized shear wave vertical seismic profiling vsp was examined in situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer bhtv and by measurements of core sample velocities the bhtv images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part two sets of dominant parallel fractures are observed by bhtv in the fracture rich depth interval of the shallower part the orientations of fracture strikes in the two sets are almost the same directing to nw whereas the average dip angles of the fractures in the two sets are 73 and 46 respectively the fracture orientations in the fracturepoor depth interval of the deeper part are found to be random the degree of anisotropy of the core sample velocity is so small that the present vsp measurement cannot resolve the expected anisotropy from the core sample measurement p and two shear wave velocities vp v and v are determined from the downgoing phases of three component vsp with a zero offset p wave source and with zero offset shear wave sources polarized in two orthogonal directions respectively the vsp results are as follows 1 the shear wave polarization anisotropy is not detected within the accuracy of 4 per cent 2 v and v average of v and v for the fracture rich interval are 94 per cent and 86 per cent respectively of those for the fracture poor interval the vsp results are consistent with expected velocity changes due to an isotropic distribution of cracks however a biplanar crack model cannot satisfy the vsp results whereas the bhtv images show two such sets of parallel fractures it is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present vsp experiment

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

fracture detection using crosshole surveys and reverse vertical seismic profiles at the conoco borehole test facility oklahoma summary shear waves in combined crosshole surveys chss and reverse vertical seismic profiles rvsps at the conoco borehole test facility oklahoma are analysed to examine the relationship between shear wave propagation and natural fracture systems the surveys cover a range of azimuths and angles of incidence and give the possibility of studying fracture induced azimuthal anisotropy in some detail the data display several features characteristic of seismic anisotropy shear wave splitting displaying substantial time delays up to 7 ms for source geophone distances of less than 50 m suggest large fracture densities estimates of fast shear wave polarizations and time delays confirm the n75de 10d fracture strike deduced from other geological and hydrological observations analyses of chs data including modelling with synthetic seismograms suggest that at least the dominant fracture set is rotated by 20d from the vertical one of the most important results of this study is the positive identification of guided waves propagating between wells along interfaces with sufficient velocity constrast in chs profiles it appears that combinations of chss and rvsps offer unique possibilities in evaluating fracture parameters at shallow depths this could be important for the study of fluid flow at shallow depths in hydrological investigations

seismic tomography at a fire flood site a crosshole seismic experiment was conducted to locate and characterize a firefront at an enhanced oil recovery eor pilot project the reservoir engineers involved in the project were interested in finding out why the burnfront apparently had stalled between two wells 51 m apart in a noisy producing environment good quality seismic data were recorded at depths ranging from 710 to 770 m the frequency range of the data 500 to 1500 hz allows resolution of the velocity structure on a scale of several meters the moveout of first arrivals indicates that there are large velocity variations in the study region a high amplitude late arriving channel wave points to the existence of a low velocity channel connecting the boreholes using an iterative nonlinear scheme which incorporates curved ray tracing and least squares inversion in each iteration the first arrival times were inverted to obtain a two dimensional model of the compressional seismic velocity between the boreholes the velocities range from

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

micromorphological study of irrigation canal sediments landscape evolution and hydraulic management in the middle gila river valley phoenix basin arizona during the hohokam occupation the hohokam were a long lived 300 1450 ad agrarian society in the deserts of southern arizona with an efficient irrigation system and a complex socio political organization dependent on environmental conditions they coped with hydrological and geomorphic changes which potentially had an impact on their cultural evolution irrigation canals were regularly maintained and or abandoned and their fills record secondary signatures of fluvial dynamics landscape change and land use micromorphological studies allow for the creation of a typology of canal fills based on semi quantitative measurements a synchronous and diachronic analysis of canal systems can then be undertaken by means of pedosedimentary correlation as part of cultural resource management projects on the gila river indian reservation south of phoenix studies have been conducted on the prehistoric santan irrigation system in use from the 10th to the 15th century ad phases of low flow were identified followed by alluvial fan flooding and seasonal deposition between the 10th 14th century ad sedentary early classic period during the 14th 15th century ad late classic period evidence of active erosion is initially recorded in the sediments followed by seasonal water supply and ultimately canal abandonment despite probable efficient irrigation

estimating the internal structure of reservoirs with shear wave vsps in the past 10 years vertical seismic profiles vsps have played an increasingly important role in identifying and delineating in situ layers such vsps have usually recorded p wave data three component shear wave trains carry three or four times more information about the source and structure of the rocks along the raypath than the equivalent p wave train so in principle shear wave vsps should be a particularly valuable technique for examining in situ layers

changes in shear wave polarization azimuth with depth in cymric and railroad gap oil fields shear wave s wave polarization azimuths although consistent over large depth intervals changed abruptly and by large amount of various depths in nine component vertical seismic profiling vsp data from the cymric and railroad gap oil fields of the southwest san joaquin basin a simple layer stripping technique made it possible to follow the polarization changes and determine the s wave birefringence over successive depth intervals because the birefringence and polarization azimuth are related to in situ stresses and fracture information from such analysis could be important for reservoir development near offset vsp data from cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers the upper layer from the surface to 800 ft 240 m had vertical s wave birefringence as large was about 6 percent down to 1300 ft 400 m in the upper layer the polarization azimuth of the fast s wave was n 60 e while in the lower layer it was about n 10 e refinement of the laye

tectonic footprints and landscape evaluation along kulur river valley kumaun lesser himalaya india abstract tectonic reactivation along the major boundary thrusts and subsidiary thrust faults in the himalaya during the quaternary period has resulted in the rejuvenation of the landforms considering the role of neotectonic activity in the hazardous kumaun region we aim to study the neotectonically formed landscape along the kulur river valley a non glacier fed tributary of the saryu river in the zone of the berinag thrust bt the field evidences encompassing a variety of geomorphic indicators e g unpaired fluvial terraces palaeo river channels alluvial fans active and stabilized landslides lineaments waterfalls alignment of springs deeply dissected hills straight meandering course of rivers streams v shaped valleys triangular fault facets and water logged river ponding along with integrated values of eight morphometric parameters and stream steepness index ks indices characterize the modification of landscape in the kulur river valley the computed morphometric parameters in the kulur river drainage and its tributaries show that the basin has been tilted and uplifted forming v shaped valleys along with deformed and disequilibrium longitudinal profile thereof suggesting it to be controlled by lithology and tectonics based on the response of active tectonics in form of geomorphic features and quantitative measurements of river channels the entire basin has been divided into three segments in which the northern and southern segments are active compared to the middle sector a possible nne ssw trending lineament fault running parallel to the kulur river has been observed in the upper catchment and is characterized by fluvial terraces river ponding and deflection and offsetting of drainage with rectangular pattern etc this may have given rise to the widening and straightening of river courses which provide the favorable room for river ponding resulting in deposition of about 2 m thick lacustrine mud deposit

neotectonics of the marmara sea region of turkey seismological investigations in western anatolia nw turkey have identified linear patterns of earthquake epicentres outlining a wedge shaped block in the area of the marmara sea this block shows different seismic characteristics from the rest of western anatolia and appears to act as a separate tectonic unit earthquake fault plane mechanisms show that the marmara block is being rotated and sheared in order to accommodate the right lateral motion of the north anatolian fault and the extensional tectonics of the southwestern anatolia investigations of the behaviour of seismic shear waves suggest that neotectonic stress aligns water filled microcracks throughout the upper brittle 10 20 km of the crust and that in situ stress directions can be monitored by analysis of shear waves recorded on three component instuments

vsp normal moveout stretch the vertical seismic profiling common depth point vspcdp transformation is a technique used to image the reflected waves in an offset vsp this coordinate transformation procedure maps reflection amplitudes from receiver depth and reflection times to lateral offset and reflector depth wyatt and wyatt 1981 the process involves the application of moveout correction followed by stacking this moveout correction results in a distortion of the original waveform and a smear of the reflection event that consequently degrades the lateral resolution of the transformation owusu 1991 therefore to determine the optimum processing parameters for the transformation we need to understand the effects of the vsp moveout stretch

suggestions for a consistent terminology for seismic anisotropy seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand i suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings it is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non specialist this not a manual of anisotropy and it is not intended for theoreticians it is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

an analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath shikoku japan implications in effective depth extent of seismic anisotropy shear wave splitting has been detected in three component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the shikoku area japan the faster shear waves are polarized nearly in e w at the stations in the central part of shikoku and in nen sws at the southern stations the arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations the faster shear wave polarizations at the stations in the central part of shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress induced microcracks the difference in polarization directions suggests that the tectonic stress state beneath the southern shikoku region is significantly different from that beneath the central part of shikoku a quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust nevertheless the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out the average aspect ratio of crack might be greater than of the order of 10 2 or the cracks may not be purely vertical but have some fluctuations in the dip angle

anisotropy survey for reservoir definition summary in the context of franco soviet cooperation a seismic survey was carried out on the romashkino reservoir in the tatar republic ussr this survey involved evaluating azimuthal anisotropy parameters from s wave vsps recorded on three wells and from a surface seismic line the latter in order to monitor any lateral variations in the parameters in this paper we present results which indicate in particular that for the two wells in which oil production has been established the anisotropy rate appears to be proportional to production figures this seems to confirm the hypothesis that variations in production are asociated with the presence of subvertical fractures which cause s wave birefringence by processing the reflected converted p s waves of a surface seismic line it is possible to monitor lateral variations in the anisotropy parameters and well ties effectively confirm differences revealed by the s wave vsp data

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

a holocene climatic record for the sonoran desert from pollen analysis of montezuma well arizona usa abstract pollen and macrofossil analyses of a radiocarbon dated core 1125 cm long from montezuma well elev 1125 m yavapai county arizona provide a record of summer precipitation similar to that of other monsoonal regions of the world precipitation was least from 4000 to 5000 yr b p and generally was greater than today before 8400 yr b p during the early holocene moist period oak and grass pollen are abundant and the best modern analogs are within the arizona monsoon boundary so increased summer precipitation is indicated climatic parameters are estimated with the technique of best modern analogs for fossil samples temperature was highest ca 3400 and 5700 yr b p and generally was cooler than today before 6800 yr b p brief cold intervals in the holocene match periods of global cooling and of positive 14 c and 10 be anomalies

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

ray series modeling of seismic wave travel times and amplitudes in three dimensional heterogeneous anisotropic crystalline rock borehole vertical seismic profiling seismograms from the mojave desert california a 600 m borehole vertical seismic profiling vsp survey conducted in crystalline rock in the mojave block stress province southern california yielded 100 oriented three component seismograms from a fan shooting geometry with radius of 100 m using both vertical and horizontal vibrators and a vertical impact source the seismograms show up to 12 ms of shear wave splitting and 5 lateral velocity heterogeneity first and second order ray tracing was used to model the travel times and amplitudes of p sv and sh waves in a three dimensional mildly heterogeneous medium with elastic anisotropy induced by aligned fractures a least squares inverse procedure was used to improve model parameters determined by forward modeling the final root mean square travel time residual was 2 ms for purposes of amplitude computation we assume that the wavelets observed in a vsp section derive from a smooth intermediate interface base of weathering layer or sedimentary basement interface that is illuminated by a narrow range of ray angles originating at a point source on the surface source derived amplitude complications are thus minimized our results strongly indicate a population of vertical cracks with an average crack density of 0 035 oriented in the n31 5 w direction such a fracture population is consistent with regional principal strain and stress determinations ray series geometric amplitudes were consistent with the observation that wave motion parallel to the aligned fractures decayed more slowly than wave motion transverse to the aligned fractures we estimate that q 75 100 we were unable to measure a polarization dependent q

late pleistocene holocene alluvial stratigraphy of southern baja california mexico abstract a late pleistocene to holocene alluvial stratigraphy has been established for the basins of la paz and san jose del cabo in the southern tip of the baja california peninsula mexico six discrete alluvial units qt1 through qt6 were differentiated across the region using a combination of geomorphologic mapping sedimentological analysis and soil development these criteria were supported using radiocarbon optically stimulated luminescence and cosmogenic depth profile geochronology major aggradation started shortly after 70 ka qt2 and buildup of the main depositional units ended at 10 ka qt4 after deposition of qt4 increasing regional incision of older units and the progressive development of a channelized alluvial landscape coincide with deposition of qt5 and qt6 units in a second incisional phase all units consist of multiple 1 3 m thick alluvial packages deposited as upper flow stage beds that represent individual storms main aggradational units qt2 qt4 occurred across broad 2 km channels in the form of sheetflood deposition while incisional stage deposits are confined to channels of 0 5 2 km width continuous deposition inside the thicker 10 m pre qt5 units is demonstrated by closely spaced dates in vertical profiles in a few places disconformities between these major units are nevertheless evident and indicated by partly eroded buried soils the described units feature sedimentological traits similar to historical deposits formed by large tropical cyclone events but also include characteristics of upper regime flow sedimentation not shown by historical sediments like long 10 m wavelength antidunes and transverse ribs we interpret the whole sequence as indicating discrete periods during the late pleistocene and holocene when climatic conditions allowed larger and more frequent tropical cyclone events than those observed historically these discrete periods are associated with times when insolation at the tropics was higher than the present day conditions determined by precessional cycles and modulated by the presence of el nino like conditions along the tropical and northeastern pacific the southern baja california alluvial record is the first to document a precession driven alluvial chronology for the region and it constitutes a strong benchmark for discrimination of direct tropical influence on any other alluvial record in southwestern north america

shear wave polarizations and subsurface stress directions at lost hills field 2 2 s wave data matrix accomplished by computationally rotating sources and receivers although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth considerable improvement in consistency resulted from analytically stripping off a thin near surface layer whose fast s wave polarization direction was about n 6 e s wave birefringence for vertical travel averaged 3 percent in two zones 200 700 ft and 1200 2100 ft 60 210 m and 370 640 m which had closely similar s wave polarizations between those zones the polarization direction changed and the birefringence magnitude was not well defined s wave polarizations from two concentric rings of offset vsps were consistent in azimuth with one another and with polarizations of the near offset vsp this consistency argues strongly for the robustness of the s wave polarization technique as applied in this area the s wave polarization pattern in offset data fits a model of vertical cracks striking n 55 e

nonstretching nmo correction of prestack time migrated gathers using a matching pursuit algorithm abstractwide azimuth long offset surveys are becoming increasingly common in unconventional exploration plays where one of the key routine processes is maintaining data fidelity at far offsets the conventional nmo correction that processes the data sample by sample results in the well known decrease of frequency content and amplitude distortion through stretch which lowers the seismic resolution and hinders and amplitude variation with offset and azimuth avaz analysis of the long offset signal to mitigate the stretch typically associated with large offsets we use a matching pursuit based normal moveout correction mpnmo to reduce nmo stretch effects in long offset data mpnmo corrects the data wavelet by wavelet rather than sample by sample thereby avoiding stretch we apply our technique 1 to a set of synthetic gathers and 2 as part of a residual velocity analysis workflow to a prestack time migrated data volume acquired over the northern chicontepec basin mexico test results show th

evidence for younger dryas global climate oscillation and human response in the american southwest whether or not abrupt younger dryas climate change affected regional paleoenvironments and late pleistocene hunter gatherer populations is an important topic in the archaeology of the american southwest this paper reviews multiple age resolved proxy evidence to gauge the magnitude and direction of younger dryas chronozone ydc environmental changes in different settings and systems there is no record of ydc pluvial lake highstands in arizona or new mexico but there are impressive records of vegetation faunal stable isotope and geomorphological change coincident with the ydc these correlate with important adaptive changes in human hunting and land use as revealed in the analysis of the spatiotemporal distribution of late pleistocene hunting technologies clovis and folsom projectile point distributions do not support extant models of paleoenvironmental conditions in these interpretations significant cultural changes that coincide with the ydc include the clovis to folsom transition the demise of mammoth hunting and the development of a highly successful emphasis on bison increased regionalization and the abandonment of the northwestern chihuahuan and the sonoran deserts by mobile big game hunters

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

reservoir management using 3 d seismic data the geologic detail needed to properly develop most hydrocarbon reservoirs substantially exceeds the detail required to find them this obvious but compelling precept has fueled the steadily increasing application of 3 d seismic analyses to reservoir management a measure of the increase is that 3 d surveys now account for half of the seismic activity in the offshore gulf of mexico and north sea and the percentage has been rising steadily year by year since commercial 3 d surveys were first shot in these areas in 1975 figure 1 three dimensional seismic surveying in other offshore areas and on land likewise is growing rapidly the fall 1988 seg distinguished lecture addressed the general subject of managing reservoirs using 3 d seismic data and this article is a condensed version of that lecture there are three parts to the article a definition of reservoir management a discussion of the various kinds of 3 d seismic analyses that can impact the development and production of a field and a synopsis

thrust related mudstone fabrics from the barbados forearc a backscattered scanning electron microscope study backscattered electron imaging on the scanning electron microscope has been used to examine the nature of microfabrics in mudstones from the barbados forearc samples from two important tectonic junctions in hole 671b of ocean drilling program leg 110 have been examined in detail at thrust a 128 m depth clays and clasts are pervasively aligned at about 50 to the thrust surface and a macroscopic scaly fabric is defined by anastamosing fractures in the decollement zone 500 550 m depth there is no preferred alignment of clays or clasts even though an irregular macroscopic scaly fabric is present scaly fabric in hole 671b is in part an artifact of specimen desiccation but may also reflect the release of anisotropic elastic strain relating to in situ stress conditions the disclosure that scaly fabric does not relate to grain alignment has profound implications for the nature of the barbados decollement seismic reflector the reflector does not correspond to a fabric anisotropy and is most likely to relate to the documented fluid flow anomaly in the decollement in contrast to scaly fabrics the microfabrics are the product of sediment deformation history the differences between the microfabrics at thrust a and those in the decollement probably relate to contrasts in deformation conditions in the decollement deformation probably occurred by paniculate flow under high pore fluid pressures where the individual panicles were aggregates of phyllosilicates at thrust a high pore fluid pressures were not maintained due to surface proximity enabling preconsolidation stresses to be exceeded as a result a pervasive phyllosilicate alignment has developed

middle and late wisconsin paleobotanic and paleoclimatic records from the southern colorado plateau usa the colorado plateau is a distinct physiographic province in western north america which presently straddles the transition between summer wet and summer dry climatic regimes to the south and northwest respectively in addition to climate the diversity of environments and plant communities on the colorado plateau has resulted from extreme topographic diversity desert lowlands as low as 360 m elevation are surrounded by forested plateaus and even higher peaks greater than 3800 m elevation this environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of north america although the colorado plateau harbours numerous potential sites the paleoecological record of the plateau is poorly known potential deposits for analysis include packrat middens alluvial and cave sites at lower elevations and lake bog and wetland sites at higher elevations forty six sites have been analysed across the nearly 337 000 km2 region of which 27 contain records that span marine oxygen isotope stage is 2 data with is 3 information coming from only 12 sites most is 2 and 3 sites are clustered along the lowland regions of the colorado river corridor and the uplands of the mogollon rim area we compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern colorado plateau for the middle and late wisconsin during the middle wisconsin mixed conifers covered middle elevations presently dominated by ponderosa pine pinus ponderosa and juniper juniperus woodland grew at elevations today covered by blackbrush coleogyne and sagebrush artemisia desert during the late wisconsin boreal conifers primarily engelmann spruce picea engelmannii replaced the mixed conifer association estimates of mean annual temperatures mat during is 3 were at least 3 4 c cooler than today whereas is 2 mat estimates are at least 5 c colder our investigation of millennial scale climatic variability within the region provided equivocal results the packrat midden sequence could not distinguish vegetation changes that might be associated with heinrich events in the north atlantic from the lake records however many heinrich events were associated with generally drier intervals often with elevated sagebrush pollen concentrations future paleoecological investigations should concentrate on the northern colorado plateau as well as the eastern and western margins additional sites along with closer spaced sampling in regions already studied will be important in determining the history of important climatic phenomena such as the timing of the arizona monsoon

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

deep borehole log evidence for fractal distribution of fractures in crystalline rock summary sonic velocity and electrical resistivity logs run to a depth of 3 5 km in crystalline rock near the san andreas fault at cajon pass in southern california correlate over scale lengths both small sub metre and large tens to hundreds of metres no such correlations are seen with the more lithologically sensitive natural gamma intensity log the correlation between the sonic velocity and electrical resistivity logs suggests that a non lithologic property of the crystalline rock controls fluctuations in situ fracture intensity is a logical candidate for the controlling rock property the fluctuations of the individual sonic velocity and electrical resistivity logs are examined with the hurst rescaled range parameter over borehole log intervals 1 5 m l 1500 m for log fluctuations arising from a scale invariant physical process the hurst rescaled range scales with data interval as lh 0 h 1 a purely random sequence of in situ fractures produces a scaling exponent h 0 50 fluctuations in the cajon pass sonic velocity and electrical resistivity logs yield h 0 70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals the tendency for fracture clustering over log intervals 1 5 m l 1500 m suggests that fracture formation is a fractal process independent of length scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals seismic reflectivity derived from the borehole sonic velocity log is also scale independent over the range of data intervals 1 5m l 1500 m with a hurst exponent h 0 21 if we associate fracture clustering with crustal fault formation the cajon pass borehole sonic velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension d 2 30 the equivalent b value for earthquake size distribution is b d 2 1 15 on this hypothesis observed b values 1 15 indicate a tendency for earthquakes to cluster on existing weak faults a scale independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting thus in crustal regions of active but low strain fracture anisotropy is observed to be aligned with the inferred maximum principal stress while in actively faulted crust fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process

temporal frequency distributions of alluvium in the american southwest taphonomic paleohydraulic and demographic implications abstract the use of radiocarbon frequency distributions to reconstruct prehistoric human and animal populations must account for taphonomic loss and other factors that affect the archaeological and paleontological records surovell et al jas 36 1715 1724 have recently proposed a correction for taphonomic bias that is based on the radiocarbon frequency of a global sample of volcanic deposits analysis of 717 radiocarbon dates sampled from the alluvium of the san pedro and santa cruz rivers and their tributaries in southeastern arizona shows that discovery and scientific biases also play an important role in the creation of radiocarbon frequency distributions and that the rate of taphonomic bias in prehistory is not predicted by the radiocarbon frequency of volcanic deposits the latter principle is further argued using a sample of 123 pliocene to clovis age proboscideans from the san pedro valley we propose an alternative model that is based on the nature of the stratigraphic record with discovery bias scientific bias taphonomic loss and the shape of the calibration curve all operating to influence the temporal frequency distribution of radiocarbon dated prehistoric phenomena

paris basin vsps case history establishing combinations of fine layer or lithologic anisotropy and crack anisotropy from modelling shear wavefields near point singularities summary this paper examines shear wave splitting in multi offset vsps at a borehole site in the paris basin and demonstrates that the sedimentary structure must have at least orthorhombic anisotropic symmetry it is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine layer or lithologic anisotropy and crack anisotropy transition zones close to shear wave singularities where there are rapid changes of shear wave polarizations and delays can account for the anomalous observed waveforms observations of large variations in shear wave polarizations in propagation directions close to point singularities are probably the first positive identification of the effects of singularities in sedimentary basins

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

millennial scale dynamics of valley fills over the past 12 000 14c yr in northeastern new mexico usa we studied the alluvial histories of eight small 2 watersheds in the uplands of northeastern new mexico the data come from radiocarbon dated sections exposed in the banks of arroyos and permanent channel streams results show that nine periods of valley aggradation separated by incision episodes occurred over the past 12 000 14 c yr these millennial scale cycles occurred in rough synchrony within different drainages parts of some cycles coincided with well known climatic fluctuations for example aggradation occurred during the younger dryas chronozone 11 000 10 000 14 c yr b p and valley fills persisted in incised states during the early holocene peak in milanko vitch summer insolation incision occurred early in the medieval warm period ca a d 1000 1300 aggradation during the little ice age a d 1300 1880 and incision during the last century changes in precipitation tied to the strength of the north american monsoon system probably drive these cycles of aggradation and incision when the system is strong more frequent summer thunderstorms cause increased flooding in these small watersheds wetter summers over periods of decades to centuries allow forest vegetation to spread which reduces sediment input from hillslopes at the same time floods are eroding valley fills aggradation of valley fills probably occurs when summer thunderstorms are less frequent and large floods are correspondingly rare prolonged summer drought reduces forest cover which increases erosion from slopes and causes sediment to accumulate in valleys in turn the strength of the north american monsoon system is probably controlled by fluctuating sea surface temperatures the geomorphic cycles we describe may reflect a previously unrecognized millennial scale climate oscillation that is important in shaping the landscapes of the southern great plains

pressures and hydrogeology in petroleum basins abstract in most deep sedimentary basins in the world there is a layered arrangement of at least two superimposed hydrogeological systems the shallowest hydrogeological system can extend to great depths however in many basins it extends from the surface down to only about 10 000 ft greatest historical depth of burial the shallow hydrogeological systems generally are basin wide in extent and exhibit normal pressures the pore water apparently is free to migrate approximately in accordance with the theory of fluid migration proposed by munn 1909 and quantified by hubbert 1940 and 1953 the deeper hydrogeological systems usually are not basin wide in extent and exhibit abnormal pressures they generally consist of a layer of individual fluid compartments which are sealed off from each other and from the overlying system in some basins mainly in the onshore u s there is an even deeper near normally pressured noncompartmented section the compartmented layer in those basins generally is in the sequence of rocks which were deposited during the period of most rapid deposition the underlying noncompartmented layer where present usually is in prebasin shelf deposits and basement rock the uppermost noncompartmented layer usually is in rocks which were deposited during the last 10 000 ft in basin filling correlations of petroleum in reservoirs with the rocks from which the petroleum was generated has imposed a system of mainly upwards paths for the first several thousand feet of migration of petroleum from source rocks largely within the compartmented layer to pools generally in the shallow hydrogeological system the migration paths of petroleum and water may not be coincident

late quaternary eolian and alluvial response to paleoclimate canyonlands southeastern utah began at ca 6 ka and ended by ca 3 2 ka followed by a shift to drier modern conditions localized mobilization of dune sand has persisted to the present these interpretations are similar to those of studies at the chaco dune fi eld new mexico and the tusayan dune fi eld arizona and are consistent with paleoclimate interpretations of pollen and packrat middens in the region a period of rapid deposition and infi ltration of eolian dust derived from distant igneous source terranes occurred between ca 12 and 8 ka before ca 17 ka and apparently back to at least 45 ka paleosols contain little or no such infi ltrated dust after ca 8 ka either the supply of dust was reduced or the more arid climate inhibited translocation of dust into the soils

on shear wave splitting in the los angeles basin three component seismograms at the three usc stations pvp gfp and dhb have been examined most earthquakes with magnitudes ranging from 1 4 to 5 0 within a period from 1985 to 1988 show evidence of shear wave splitting the preferred polarization of the first split shear wave arrivals at pvp is nearly in n s which is consistent with both regional maximum horizontal compressive stress direction and local subsurface fault strike showing that shear wave splitting is caused by liquid filled cracks or fractures associated with the n s faulting the polarizations of first shear wave arrivals at gfp are roughly divided into two almost perpendicular directions ene wsw and nnw sse which are parallel or perpendicular to the strike of the geology or topography near the station because gfp is near the foothills of santa monica mountains the shear wave arrivals may be disturbed by topographic irregularities and by subsurface dipping interfaces two examples at dhb clearly display shear wave splitting their polarizations of shear wave are in the direction of n s which agree with the fragmentary surface and fracturing direction there from some relatively reliable delay times the crack densities at three stations are given that is 0 025 at pvp 0 020 at gfp and 0 045 at dgb no systematic change of shear wave polarization is discovered in this study

tectonic and climatic controls on holocene channel migration incision and terrace formation by the rio grande in the palomas half graben southern rio grande rift usa in order to evaluate potential effects of tectonics and climate change on the behaviour of the axial rio grande in the rio grande rift a 16 5 km stretch of modern floodplain and holocene terraces were mapped in the tectonically active palomas half graben south central new mexico usa in addition 51 cores and natural exposures were logged and 20 radiocarbon dates were obtained from charcoal bulk organic matter mollusc shells and pedogenic calcite the holocene alluvium comprises four terraces above the modern floodplain each of which formed by a period of river incision followed by stability and renewed floodplain construction to a level below that of the previous terraces estimated times of incision between terraces i and ii ii and iii and iii and iv are after 12 400 8040 to 5310 and 760 to 550 yr bp respectively whereas the incision between terrace iv and the modern floodplain occurred within the last 260 years although there is some evidence for tectonic control on river behaviour in the southern part of the basin terrace formation is interpreted as being related to climate change with periods of incision corresponding to times of increased aridity and low sediment water discharge ratio in the rio grande this process may have resulted from a reduction in intensity and magnitude of summer storms which supply sediment to the axial river coupled with an increase in spring discharge peak caused by snowmelt in upstream mountain catchments

seismic anisotrdpy in granuulte facies rocks of the arunta block central australia using travel times from explosive sources recorded at distances less than 36 km evidence for p wave anisotropy was obtained in an area of central australia where granulite facies rocks from the lower crust are exposed at the surface these rocks form the hanging wall of one of the most prominent ancient thrust structures seen anywhere in a continental interior the azimuthal variation in seismic velocity for rocks at depths less than about 1 km is about 5 and is comparable in magnitude to values reported for both continental and oceanic upper mantle the maximum velocity east west is parallel to the predominant strike direction of gneissic foliation observed in outcrop in the survey region which was produced during granulite facies metamorphism at about 1 800 ma

tropical squall lines of the arizona monsoon abstract squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern arizona and northwestern mexico initial thunderstorm formation is over the continental divide in the late afternoon and the systems become organized within a few hours satellite imagery cloud to ground lightning strike data and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than the environmental flow at all levels below 20 kpa so that most of the anvil clouds lag behind the synoptic scale circulation is anomalous with a strong ridge located over the western united states and a deep trough located over the eastern united states west to northwest winds are found in the boundary layer over southern arizona and northwest mexico while a deep layer of east winds are observed above as a result most of the environmental wind shear is confined to the lowest 2 5 km above the ground

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

optimum field development with seismic reflection data petroleum engineers and production geologists generally build their reservoir models from smooth interpolation of petrophysical parameters obtained at well locations well information is usually accurate but is unevenly and sparsely distributed in heterogeneous formations these interpolated models can be erroneous with costly consequences on field development expenditures in addition the volume of reservoir rock physically investigated in a field by core analysis and wireline logging generally is on the order of one part per billion yet development geologists and petroleum engineers need to produce one or more detailed 3 d model s of the reservoir to estimate reserves design an effective drilling program and optimize resource recovery

sub basalt decoupled walkaway vsp imaging walkaway vertical seismic profile vsp data acquired in basalt covered areas can be used to improve knowledge of the sub basalt structure a synthetic example and a case study from the north atlantic uk show that elastic two way downward continuation migration combined with the stationary phase principle is well suited to the processing of vsp data vector data are processed using decoupled elastic migration algorithms in both isotropic and anisotropic media to illustrate the value of decoupled imaging equations conventional pp imaging is carried out on the enhanced vsp data and compared with the decoupled scheme decoupled vector migration operates directly on the displacement vector and uses various wave modes downgoing waves are migrated to image basalt lava flows and measure their anisotropy upgoing waves are used for high resolution sub basalt imaging

episodic sediment delivery and landscape connectivity in the mancos shale badlands and fremont river system utah usa abstract the fremont river drains about 1000 km 2 of mancos shale badlands which provide a large percentage of the total sediment load of its middle and lower reaches factors controlling sediment movement include weathering that produces thin paralithic soils mass movement events that move the soil onto locations susceptible to fluvial transport intense precipitation events that move the sediment along rills and across local pediments and finally fremont river floods that move the sediment to the main stem colorado river a forty year erosion pin study has shown that down slope creep moves the weathered shale crust an average of 5 9 cm yr weather records and our monitoring show that wet winters add large slab failures and mudflows recent sediment trap studies show that about 95 of sediment movement across pediments is accomplished by high intensity summer convective storms between 1890 and 1910 a series of large autumn floods swept down the fremont river eroding its floodplain and transforming it from a narrow and meandering channel to a broad braided one beginning about 1940 the fremont s channel began to narrow sequential aerial photos and cross sections suggest that floodplain construction since about 1966 has stored about 4000 to 8000 m 3 of sediment per kilometer per year these data suggest that it will take two centuries to restore the floodplain to its pre 1890 condition which is in line with geologic studies elsewhere on the colorado plateau the various landscape elements of slope pediment and floodplain are semi independent actors in sediment delivery each with its own style accelerated mass movement on the slopes has an approximate 20 year recurrence sediment movement from slope across pediments to master stream is episodic and recurs more frequently the slope to pediment portion of the system appears well connected however sediment transport through the floodplain is not well connected in the decadal time scale but increases in the century and millennial time scales and changes over time depending on the cycle of arroyo cutting and filling

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

shear wave splitting above small earthquakes in the kinki district of japan abstract we have identified shear wave splitting diagnostic of the effective anisotropy induced by aligned microcracks in the wavetrains of micro earthquakes at four stations of the abuyama network in the kinki district of japan we find that the directions of polarization of the faster split shear waves are nearly parallel for all azimuths of arrival and for all angles of incidence less than the critical angle at three of the four stations these directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault plane mechanisms and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination these results suggest that crack induced anisotropy is present in the brittle upper crust beneath japan as has been found elsewhere but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field or results from the general trends of surface geology although the delay times between faster and slower shear waves are difficult to estimate reliably because of their high sensitivity to internal interfaces the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0 04 the consistency or lack of consistency of the directions of the shear wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear wave polarizations

vertical open fractures and shear wave velocities derived from vsps full waveform acoustic logs and televiewer data fracture or stress related shear wave birefringence or azimuthal anisotropy from vertical seismic profiles vsps is commonly observed today but no attempt is made to fit the observations with observed in situ fractures and velocities with data from a hard rock limestones dolomites and anhydrites region of michigan fast and slow shear wave velocities have been derived from a nine component zero offset vsp and compared to shear wave velocities from two full waveform acoustic logs to represent the shear wave birefringence that affects the shear wave s vertical propagation a propagator matrix technique is used allowing a local measurement independent of the overburden layers the picked times obtained by using a correlation technique have been corrected in the birefringent regions before they compute the fast and slow velocities although there are some differences between the three velocity sets there is a good fit between the velocities from the shear wave vsp and those from the two logs the authors suspect the formations showing birefringence to be vertically fractured to support this they examine the behavior of the stoneley wave on the full waveform acoustic logs in the formations in addition they analyze the borehole televiewer data from a nearby well there is a good fit between the fracturesmore seen from the vsp data and those seen from the borehole less

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

shear wave polarizations near the north anatolian fault i evidence for anisotropy induced shear wave splitting summary the turkish dilatancy projects tdp1 in 1979 and tdp2 in 1980 recorded small earthquakes near the north anatolian fault with closely spaced networks of three component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear wave propagation this paper examines the polarizations of shear wavetrains recorded in the shear wave window immediately above the earthquake foci abrupt changes in the orientation and or ellipticity of the shear wave polarizations are almost always observed during the first few cycles following the initial shear wave arrival on each seismogram the horizontal projections of the polarizations of the first shear wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites with one exception it is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms we demonstrate that the shear wave splitting is likely to be the result of anisotropy in the region above the earthquake foci which could produce polarizations displaying the observed alignments the temporal change of the azimuth of alignment observed at one locality between 1979 and 1980 may be due to the release of a local stress anomaly by a very near earthquake

isotopic analysis of wetland development in the american southwest the analysis of stable isotope and elemental fractions of organic material collected from san bernardino cienega was used to understand the history of vegetation composition and climate change within this desert wetland a 4000 yr record of sediment buildup based on four 14c measurements provides unique opportunities for the study of environmental conditions within an arid landscape and documents climate shifts from drier to wetter conditions in the late holocene 13c 15n and c n values were measured from a 3 8 m deep sedimentary section to understand the dynamics of vegetation and hydrology in desert wetlands through this section we observe 13c and c n values indicating a shift in the dominant source of organic matter within the section prior to 850 cal yr bp below 60 cm aquatic vascular plants and occasionally terrestrial vegetation were the primary organic sources whereas freshwater algae were the dominant organic matter source above this level these values indicate that while condition