rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

the effect of crustal anisotropy on reflector depth and velocity determination from wide angle seismic data a synthetic example based on south island new zealand when deriving velocity models by forward modelling or inverting travel time arrivals from seismic refraction data a heterogeneous but isotropic earth is usually assumed in regions where the earth is not isotropic at the scale at which it is being sampled the assumption of isotropy can lead to significant errors in the velocities determined for the crust and the depths calculated to reflecting boundaries laboratory velocity measurements on rocks collected from the haast schist terrane of south island new zealand show significant up to 20 compressional p wave velocity anisotropy field data collected parallel and perpendicular to the foliation of the haast schist exhibit as much as 11 p wave velocity anisotropy we demonstrate using finite difference full wavefield modelling the types of errors and problems that might be encountered if isotropic methods are used to create velocity models from data collected in anisotropic regions these reflector depth errors could be as much as 10 15 for a 10 km thick layer with significant 20 p wave velocity anisotropy the implications for south island new zealand where the problem is compounded by extreme orientations of highly anisotropic rocks foliation which varies from horizontal to near vertical are considered finally we discuss how the presence of a significant subsurface anisotropic body might manifest itself in wide angle reflection refraction and passive seismic datasets and suggest ways in which such datasets may be used to determine the presence and extent of such anisotropic bodies d 2002 elsevier science b v all rights reserved

stress measurements and tectonic implications for fennoscandia abstract the main results from the fennoscandian rock stress data base frsdb are presented rock stress data from sweden are now being selected to enter the data base of the world stress map project of the international lithosphere programme it is suggested that ridge push from the mid atlantic ridge is the major stress generating mechanism the scatter in stress magnitude and direction for fennoscandia is due to the change in the stress field near faults this is illustrated by means of simple 2 dimensional distinct element analyses of blocky rock masses subjected to a homogeneous far field stress regime earthquakes are assumed to appear at fault intersections where stresses are concentrated

kinematic gps as a source for airborne gravity reduction in the airborne gravity survey of switzerland an airborne gravity survey of switzerland was performed in a joint project between the swiss federal institute of technology zurich and lacoste and romberg gravity meters inc the survey was flown in a twin otter aircraft equipped by the swiss federal directorate of cadastral surveying the aircraft was outfitted with three global positioning system receivers one for navigational purpose and two as sources of positioning velocity and vertical acceleration for airborne gravity reduction four receivers were installed on the ground as reference stations the gravity data were recorded with a modified lacoste and romberg marine gravimeter at a sampling rate of 1 s modifications included anti alias filters and an absolute encoder for the measuring screw the flights were performed at a barometric altitude of 5100 m above sea level an airborne bouguer anomaly map computed at flight altitude with topographic corrections of up to 167 km and a density of 2670 kg m3 is presented bouguer anomaly values at crossing points between lines flown at comparable altitudes h 10 m show differences varying between 1 1 and 2 7 mgal 10 5 m s2 comparison of the ground bouguer anomaly map upward continued to 5100 m and the one computed from airborne data shows a strong similarity both in amplitude and wavelength

appalachian stress study 1 a detailed description of in situ stress variations in devonian shales of the appalachian plateau we describe an experiment to measure variations in the state of stress within a horizontally bedded devonian shale sandstone limestone sequence in western new york a total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline supported hydraulic fracturing system the stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales this drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in sh and sh of 3 5 and 9 mpa respectively above the sands thrust regime conditions prevail although the amount by which sh exceeds sv is undetermined since instantaneous shut in pressures isips were clipped at the level of sv due to fracture rotation below the sands the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography the magnitude of sh in the sand beds themselves and a lower limestone remains at least as great as sv despite the decline in shale stress hence stress contrasts between these beds and neighboring shales become pronounced with depth the contrast in sh and sh between the lowermost sand and the immediately underlying shale is at least 6 and 14 5 mpa respectively sh levels in the lower strike slip regime are about 1 75 times greater than sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces for the upper thrust regime and the sand and limestone beds however the inferred lower bound on sh is close to the slippage threshold for a coulomb friction coefficient of 0 6 the orientation of sh is ene with a standard deviation of 20 fracture traces were usually splayed occasionally spanning 30 of well bore no systematic correlation between mean orientation and lithology is evident significantly different orientations were obtained for adjacent tests in which almost identical islps were observed suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction similarly vertical traces were observed in those tests where isips apparently reflect sv suggesting that rotation to horizontal was rapid

misalignment of the orientation of fractures and the principal axes for p and s waves in rocks containing multiple non orthogonal fracture sets summary azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with diverent polarizations if the source polarization is not aligned with the principal axes of the medium in the presence of two or more non orthogonal sets of vertical fractures the symmetry of the medium may be approximated as monoclinic with a horizontal plane of mirror symmetry if in the absence of fractures the rock is transversely isotropic ti with the symmetry axis perpendicular to the bedding plane for such a medium the fast and slow polarization directions for vertically propagating shear waves are not parallel or perpendicular to any of the fracture planes but lie in directions given by the principal axes of a second rank fracture compliance tensor this tensor is independent of the normal compliance of the fractures it follows that for vertical propagation of shear waves in a vertically fractured ti medium any number of arbitrarily oriented vertical fracture sets is equivalent to two mutually perpendicular fracture sets provided that the seismic wavelength is large compared to the size of the fractures for ovsets typical of surface seismic acquisition the p wave velocity at fixed ovset varies with azimuth as cos 2 w w 0 where w is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave w 0 depends on both the normal and the shear compliance of the fractures and may diver from zero if the ratio of the normal to shear compliance of the fractures varies significantly between fracture sets if this ratio is similar for all fractures w 0 0 and the principal axes of the variation in p wave velocity with azimuth for fixed ovset are determined by the principal axes of the second rank fracture compliance tensor

shallow seismic profiling of the exhumed punchbowl fault zone southern california the relationship between seismic velocity and internal fault structure was investigated through a shallow seismic refraction experiment across the punchbowl fault devil9s punchbowl los angeles county park california the punchbowl fault is a northwest striking large displacement fault of the san andreas system that is exhumed to several kilometers depth and places crystalline basement against arkosic sandstone of the punchbowl formation seismic refraction profiles using hammer and impulsive shear wave sources along a 300 m long line reveal the velocity structure of the fault zone beneath a thin deposit of alluvium we determine a velocity model assuming the alluvial layer is fairly uniform in velocity and thickness consistent with geologic observations and p wave travel times raytracing with damped least squares inversion of travel times of p and s waves indicate that the punchbowl fault is best modeled as a zone several tens of meters wide with velocities reduced by 10 25 from wall rock velocities v p 3 2 km sec for granitic basement and v p 2 9 km sec for punchbowl formation thickness of the low velocity zone and the variation in seismic velocity across the zone are qualitatively consistent with expectations based on the observed distribution of fault related fracturing and alteration apparent crack densities calculated from measured seismic velocities using o9connell and budiansky 1974 formulation for a cracked medium range from about 0 4 in the core to a background crack density of 0 1 in the host rock the variation in calculated crack density across the fault is similar to observed variations in microfracture density in the punchbowl formation sandstone along traverses across the fault an estimate of the poisson9s ratio near the fault is about 0 25 suggesting that open cracks in the shallow part of the punchbowl fault zone are dry consistent with the geologically inferred location of the groundwater table although the seismic data do not completely constrain the velocity structure the seismic velocity model determined by raytracing and inversion of travel times is admissible on the basis of structural data

estimates of velocity dispersion between seismic and ultrasonic frequencies it is generally accepted that acoustic velocities in fluid saturated rocks vary with frequency evidence comes from experimental measurements and from theoretical causality arguments we have developed a simple analysis technique that gives estimates of total velocity dispersion between zero frequency and any measurement frequency the technique requires compressional p and shear s wave velocity measurements on dry and fully saturated rock assuming that the dry velocities are independent of frequency the biot gassmann equations are used to calculate the zero frequency velocities in the fully saturated rock any difference between the measured velocities and the calculated zero frequency velocities is interpreted as evidence of dispersion application of this analysis technique to a variety c ultrasonic data sets gives consistent results in many rocks dispersion between zero frequency and ultrasonic frequencies is on the order of 10 percent at low effective stress and it decreases to only a few pe

retarded compaction due to overpressure deduced from a seismic velocity depth conversion study in the danish central trough north sea as a result of overpressuring below the upper miocene seismic velocities below this level are low in the danish central trough north sea compared with areas outside the trough using the top middle miocene as a datum velocity depth functions match those functions established east of the study area and lead to improved depth conversion it is proposed that the present day velocity depth porosity depth profiles below the top middle miocene reflect the frozen pre late miocene profiles the compaction effect of late cenozoic burial was retarded with the exception of areas where the sealing capacity of the lower tertiary sediments was reduced due to permeable sandy deposits or possibly due to fracturing caused by salt diapirism in the study area the thick late cretaceous danian chalk group constitutes a significant high velocity layer depth conversion of maps covering the danish central trough down to the top middle jurassic an important deep exploration target should include at least four layers the post chalk group cenozoic excluding danian the chalk group and the cromer knoll group lower cretaceous and the upper jurassic analysis of time depth data from 76 wells shows that velocities in the study area vary as a function of depth thickness lithology presence of gas and overpressure caused by rapid late cenozoic deposition and by gas generation for wells drilled away from salt diapirs optimum depth conversion with constant velocity is obtained with velocities of 2050 4400 2950 and 2800 m s for the four layers the relative success of different depth conversion methods is evaluated by comparing the mean absolute error on the estimated thicknesses for the layers individually assuming each well in turn to be undrilled velocity anomaly depth conversion or the v0 k method is shown to be an effective way to convert travel times to depth when interval velocity increases with depth of burial in this method velocity is assumed to increase linearly with depth and laterally velocity is calibrated to wells velocity variations in the upper jurassic impose the major problem for depth conversion in the area

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

airborne gravimetry from a small twin engine aircraft over the long island sound in january 1990 a test of the feasibility of airborne gravimetry from a small geophysical survey aircraft a cessna 404 was conducted over the long island sound using a bell aerospace bgm 3 sea gravity meter gravity has been measured from large aircraft and specially modified de havilland twin otters but never from small standard survey aircraft the gravity field of the long island sound is dominated by an asymmetric positive 30 rngal anomaly which is well constrained by both marine and land gravity measurements using a trimble 4000 gps receiver to record the aircraft s horizontal position and radar altimeter elevations to recover the vertical accelerations gravity anomalies along a total of 65 km were successfully measured the root mean square rms difference between the airborne results and marine measurements within 2 km of the flight path was 2 6 mgai for 15 measured values the anomalies recovered from airborne gravimetry can also be compared with the gridded regional free air gravity field calculated using all available marine and land gravity measurements the rms difference between 458 airborne gravity measurements and the regional gravity field is 2 7 mgai this preliminary experiment demonstrates that gravity anomalies with wavelengthsas short as 5 km can be measured from small aircraft with accuracies of 2 7 mgal or better the gravity measurements could be improved by higher quality vertical and horizontal positioning and tuning the gravimeter s stabilized platform for aircraft use

the alpine swiss french airborne gravity survey summary in february 1998 a regional scale airborne gravity survey was carried out over the french occidental alps within the framework of the geofrance 3 d research program the survey consisted of 18 ns and 16 ew oriented lines with a spacing of 10 and 20 km respectively covering the whole of the western french alps total area 50 000 km2 total distance of lines flown 10 000 km the equipment was mounted in a medium size aircraft dehavilland twin otter flowing at a constant altitude of 5100 m a s l and at a mean ground speed of about 280 km h 1 gravity was measured using a lacoste romberg relative air sea gravimeter type sa mounted on a laser gyro stabilized platform data from 5 gps antennae located on fuselage and wings and 7 ground based gps reference stations were used to determine position and aircraft induced accelerations the gravimeter passband was derived by comparing the vertical accelerations provided by the gravimeter with those estimated from the gps positions this comparison showed that the gravimeter is not sensitive to very short wavelength aircraft accelerations and therefore a simplified formulation for computing airborne gravity measurements was developed the intermediate and short wavelength non gravitational accelerations were eliminated by means of digital exponential low pass filters cut off wavelength 16 km an important issue in airborne gravimetry is the reliability of the airborne gravity surveys when compared to ground surveys in our studied area the differences between the airborne acquired bouguer anomaly and the ground upward continued bouguer anomaly of the alps shows a good agreement the rms of these differences is equal to 7 68 mgal for a spatial resolution of 8 km however in some areas with rugged topography the amplitudes of those differences have a striking correlation with the topography we then argue that the choice of an appropriate density reduction by a factor of 10 per cent for computing the ground topographic corrections over the highest mountains results in significantly reducing the differences between airborne and ground upward continued bouguer anomalies which shows that some of the misfit stems from errors in the ground data

seismic and geodetic evidence for extensive long lived fault damage zones during earthquakes slip is often localized on preexisting faults but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes seismic and geodetic data from the calico fault in the eastern california shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli using seismic traveltimes trapped waves and interferometric synthetic aperture radar observations we document seismic velocities reduced by 40 50 and shear moduli reduced by 65 compared to wall rock in a 1 5 km wide zone along the calico fault observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures no large earthquake has broken the calico fault in historic time implying that fault damage persists for hundreds or perhaps thousands of years these fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces and throughout much of the seismogenic zone a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems

an airborne gravity study of eastern north carolina the naval research laboratory nrl has developed a prototype airborne gravity measurement system the core of the system is a lacoste and romberg air sea gravity meter mounted on a three axis stable platform corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters the original prototype system was designed for use over oceanic areas we recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained the radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces vertical accelerations due to horizontal velocity over a curved rotating earth the eotvos correction and precise two dimensional positions are determined from a texas instrument p code global positioning system the updated system was tested

sources of solutes and heat in low enthalpy mineral waters and their relation to tectonic setting new zealand abstract approximately 90 of the low enthalpy spring systems in new zealand are distributed along the present day convergence of the pacific and australian plates which varies from subduction to oblique plate collision forearc subducted waters rising in the hikurangi accretionary prism consist of pore waters containing labile organic material water of hydration from clays and seawater these subducted waters are traceable in the north island axial ranges and the northern marlborough fault system in the south island albeit modified by temperature mixing with surface and heated meteoric waters and protracted interaction with the rock the signature of subducted marine sediments in the hikurangi trench is retained in the chemical and isotopic compositions of the taupo volcanic zone magmatic arc type waters the concentrations of b ba br ca cl i k li mg na rb and sr solutes found in high concentrations in forearc subducted waters increase from northeast to southwest towards the incipient subduction zone in the puysegur trench southwest of south island most thermal mineral waters in the south island are expelled along the alpine fault zone where plate collision results to rapid uplift of the southern alps elevated geothermal gradients and fault shearing of rocks in the alpine fault zone variations in temperatures 175 30 c and mineral water compositions are affected by elevation hydrological gradient intensity of tectonic activity permeability fluid volume and rate of throughput and rates of uplift as the main trace of the alpine fault zone is approached concentrations of cl and other solutes generally increase suggesting increasing contributions from ascending cl rich metamorphic waters outside the active convergent zone low enthalpy springs discharge diagenetically modified ancient forearc waters na cl sedimentary formation waters and heated seawater na hco 3 cl waters and na hco 3 deep circulating heated meteoric waters the total surface heat flow in the north island low enthalpy mineral waters is more than 10 higher than in the south island with projected subsurface temperatures as high as 250 c this is because of the widespread occurrence of shallow magmatic and mantle sources associated with pliocene to holocene volcanism and back arc continental rifting the sources of heat and solutes in low enthalpy mineral waters outside the taupo volcanic zone and ngawha are decoupled except for rapidly ascending hot pore waters in the hikurangi accretionary prism and low temperature serpentinization ca cl waters in fiordland

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

advanced seismic imaging techniques characterize the alpine fault at whataroa new zealand the plate bounding alpine fault in new zealand is a 850 km long transpressive continental fault zone that is late in its earthquake cycle we have acquired and processed reflection seismic data to image the subsurface around the main drill site of the deep fault drilling project dfdp 2 the resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the alpine fault zone the most prominent feature is a strong reflector at depths of 1 5 2 2 km with an apparent dip of 48 to the southeast below the dfdp 2 borehole which we assume to be the main trace of the alpine fault above the main reflector parallel reflectors suggest the presence of a 600 m wide damage zone additionally subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone conjugate faults imaged by the data show the complexity of the subsurface the derived p wave velocity model reveals a 300 600 m thick sedimentary layer with velocities of 2 3 km s above a schist basement with velocities of 4 5 5 5 km s a low velocity layer can be observed within the basement at 0 8 2 km depth a small scale low velocity anomaly appears at the top of the basement that can be correlated to the fault zone the results provide a reliable basis for a seismic characterization of the dfdp 2 drill site that can be used for further structural and geological investigations of the architecture of the alpine fault in this area

small scale faulting as an indicator of deformation mechanism in the tertiary sediments of the northern danish central trough abstract the tertiary sediments in the northern part of the danish central trough are characterized by a large number of small faults 25 50 m offset models for the accommodation of the deformation of the sediments are presented and it is shown quantitatively that a bending model best explains the observed strain distribution from this we infer that the internal deformation of the tertiary sediments is due to differential subsidence along older fault trends and not caused by applied external stresses on the tertiary sediments the effect of rock rheology and excess fluid pressure is discussed in order to explain the size of the throw and the smaller number of reverse faults compared to normal faults although the rocks are brittle they may in regions of shortening suffer more grainscale creep deformation than in regions of extension the strain distribution is determined on seismic sections and the main limitations of the method is thus the vertical and horizontal resolution of the seismic surveys used

in situ stress from breakouts in the danish sector of the north sea abstract borehole breakouts from 26 wells situated in the danish central graben and one well situated in the northwestern section of the ringkobing fyn high have been studied the data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4 7 km statistics were used to obtain a mean azimuth of breakouts for each well and each individual breakout was weighted by its length data quality of breakouts is assigned according to the world stress map quality ranking scheme the breakout data reveal that s h orientations in the danish central graben are extremely scattered no regional trend in mean s h orientation can be identified as a result of the large standard deviation 77 similar scatterings of s h orientations have been reported from adjacent regions of the north sea the results indicate that the state of stress in the sedimentary cover of the danish central graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the post chalk group chalk group and pre chalk group

damage patterns stress rotations and pore fluid pressures in strike slip fault zones 1 active faults unfavorably oriented with respect to the regional maximum compressive stress have been labeled as weak the seismic hazards posed by these faults make understanding this apparent weakness a priority stress rotations in these fault zones together with an increase in mean stress could enable high pore fluid pressures to weaken a fault zone such a model requires a foundation in the physics and mechanics of damage this paper presents a new model for stress rotations in fault zones by combining the effective medium theory with anisotropic poroelasticity this approach enables the quantitative characterization of crack damage and the prediction of progressive changes in the elastic properties of rocks across the fault zone the processes of fault growth and wear will lead to distinct patterns of crack damage with different effects on the elastic properties elevated pore fluid pressures have long been known to change the effective normal and shear stresses of anisotropic rocks and this work incorporates these effects into a multilayer fault zone model it is shown that high pore fluid pressures in the anisotropic rocks of the core zone can generate large stress rotations i e more fault parallel and increases in mean stress sufficient to weaken the fault stress rotations in the damage zones of unfavorably oriented faults tend to be away from the fault i e more fault normal for likely combinations of damage patterns and pore fluid pressure

acoustic evidence for a broad hydraulically active damage zone surrounding the alpine fault new zealand abstract we present an analysis of the wireline sonic logs acquired through alpine fault hanging wall rocks during the second phase of the deep fault drilling project dfdp 2 due to inaccuracy in the initial tool processed slowness logs a more robust reprocessing of the data using the slowness time coherence method was required the reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness the average slowness values of compressional and shear waves are 68 s ft and 110 s ft respectively corresponding to velocities of 4 5 km s and 2 8 km s with some excursions related to fractures borehole conditions and tool settings but no systematic downhole trends the log values are compared to laboratory measurements of p and s wave velocity for samples of alpine schist and mylonitic rocks to examine the scale dependence of the measurement the average log velocity is 2 to 18 slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale using effective medium theory we estimate the crack density in the hanging wall host rock to be 0 4 comparison to field scale seismic studies indicates that there is a broad 500 600 m wide outer damage zone surrounding the intensely deformed inner damage zone of the alpine fault this fractured outer zone facilitates fluid flow through the hanging wall and around the alpine fault

analysis of gravimetric anomalies in furnas caldera sao miguel azores abstract furnas volcano sao miguel island azores is a steep sided caldera complex resulting from several collapses in this paper we analyse and interpret the gravity survey carried out by the instituto de astronomia y geodesia spain and the gravity data published by the instituto nacional de meteorologia e geofisica portugal the resulting bouguer anomaly reveals the presence of a regional trend several local features and a component of noise in separating the regional trend we use a robust polynomial fitting further a covariance analysis and a least squares prediction are applied to filter the noise and to model the local anomaly a first signal map represents the main local anomaly while the secondary signal map shows superficial anomalies this distribution reveals a local main minimum close to the 1630 a d and gaspar craters several possible alignments are shown that lead to a model mainly with sw ne and se nw orientations of the negative anomaly with the pattern of the low density zones showing a relationship with the volcanic features applying a stabilized linear method of gravimetric inversion we obtain a density contrast distribution of the sources of the filtered local bouguer anomaly considering as unknowns the anomalous densities of a fixed discrete distribution of rectangular prisms which represents the subsoil volume a mixed minimization criteria for the weighted residuals and the weighted density contrast allow us to obtain a solution as a discrete three dimensional density contrast model the obtained anomalous densities model shows a clear relation with the structure of the caldera complex and tectonic trends with the principal body of negative contrast density being associated with the subsurface structure of the caldera

san andreas fault damage at safod viewed with fault guided waves 1 highly damaged rocks within the san andreas fault zone at parkfield form a low velocity waveguide for seismic waves giving rise to fault guided waves prominent fault guided waves have been observed at the san andreas fault observatory at depth safod site including a surface array across the fault zone and a borehole seismograph placed in the safod well at a depth of 2 7 km below ground the resulting observations are modeled here using 3 d finite difference methods to fit the amplitude frequency and travel time characteristics of the data the models require a downward tapering 30 40 m wide fault core embedded in a 100 200 m wide jacket compared with the intact wall rocks the core velocities are reduced by 40 and jacket velocities by 25 based on the depths of earthquakes generating guided waves we estimate that the low velocity waveguide along the fault at safod extends at least to depths of 7 km more than twice the depth reported in pervious studies

hydraulic and acoustic properties of the active alpine fault new zealand laboratory measurements on dfdp 1 drill core abstract we report on laboratory measurements of permeability and elastic wavespeed for a suite of samples obtained by drilling across the active alpine fault on the south island of new zealand as part of the first phase of the deep fault drilling project dfdp 1 we find that clay rich cataclasite and principal slip zone psz samples exhibit low permeabilities 10 18 m 2 and that the permeability of hanging wall cataclasites increases from c 10 18 m 2 to 10 15 m 2 with distance from the fault additionally the psz exhibits a markedly lower p wave velocity and young s modulus relative to the wall rocks our laboratory data are in good agreement with in situ wireline logging measurements and are consistent with the identification of an alteration zone surrounding the psz defined by observations of core samples the properties of this zone and the low permeability of the psz likely govern transient hydrologic processes during earthquake slip including thermal pressurization and dilatancy strengthening

multi scale compressional wave velocity structure of the san gregorio fault zone 1 understanding fault architecture at multiple scales is crucial to delineate in situ fault zone physical properties and rupture dynamics through modeling and geophysical imaging monitoring an exposure of the active large offset strike slip san gregorio fault at moss beach ca provides a unique field site to relate the well mapped fault zone architecture with compressional wave velocity vp structure measured at centimeter to meter scales laboratory ultrasonic velocities of fault zone samples adjusted for fluid related frequency and structural dispersion indicate that i a seismic velocity reduction of 30 characterizes the central smectite rich clay gouge relative to the rocks 100 m away in the relatively undeformed host rocks and ii the across fault velocity profile trends for the seismic to ultrasonic bandwidth correlate almost exactly to the previously mapped macroscale fault zone structure these results highlight the value of conducting multiscaled investigations when measuring fault zone properties defined by physical elements at multiple scale lengths

late interseismic state of a continental plate bounding fault petrophysical results from dfdp 1 wireline logging and core analysis alpine fault new zealand 1 we present a geophysical characterization at 0 1 100 m scales of a major plate bounding continental fault in a late interseismic state the alpine fault produces mw 8 earthquakes every 200 400 years and last ruptured in 1717 ad wireline geophysical logs and rock cores extending from one side of the alpine fault to the other were acquired in two boreholes drilled in 2011 at gaunt creek during the first phase of the deep fault drilling project dfdp 1 these data document ambient conditions under which the next alpine fault earthquake will occur principal component analysis of the wireline logging data reveals that 80 of the variance is accounted for by electrical acoustic and natural gamma properties and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements the fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone superimposed on different protolith compositions our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0 1 100 m scales to that of the crustal scale orogen imaged by remote geophysical methods on the basis of the fault zone scale asymmetry the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred alpine fault rupture on going characterization of the structural and hydraulic architecture of the alpine fault will improve our understanding of the relationship between in situ conditions earthquake rupture processes and the hazards posed by future earthquakes

interferometric global positioning system three dimensional positioning source for airborne gravimetry the naval research laboratory nrl and the national geodetic survey recently collaborated on an experiment to test interferometric mode global positioning system gps tracking as a source of altitude and vertical acceleration for airborne gravity measurement reduction a texas instruments ti 4100 gps receiver and an extremely precise radar altimeter were installed in an nrl p 3 orion aircraft a second ti 4100 was located at a fixed base station of known location carrier phase data from four gps satellites were recorded from the fixed and airborne receivers and postprocessed to produce three dimensional displacements of the airborne station relative to the base station differences in gps and radar altimeter coordinate systems were modeled and removed the resulting gps and radar altitudes were compared the two unfiltered 1 hz data sets differed by an rms of 20 cm when aircraft dynamics were controlled by autopilot in straight and level flight during hand flown maneuvers such as turns climbs and descents rms differences varied from 40 to 80 cm a significant portion of the differences can be attributed to remaining coordinate system discrepancies and to uncompensated geometric effects due to aircraft motion we have also examined vertical acceleration time series produced by doubly differencing both data sets by applying a low pass filter which allows resolution of features with wavelengths of 20 km and greater at nominal aircraft speeds of 100 m s the rms difference between the radar and gps derived vertical accelerations was less than 2 mgal 2 10 5 m s2 during straight and level flight it should be possible to improve this accuracy and wavelength resolution with future improvements in measurement and processing techniques the results of this experiment show that the incorporation of interferometric mode gps tracking into the airborne gravity measurement system will make possible rapid and relatively inexpensive gravity surveys over any type of terrain while simultaneously providing accurate measurements of ground height profiles beneath the aircraft

what can airborne gravimetry contribute to geoid determination this paper evaluates the suitability of airborne gravimetry for geoid determination spectral analysis of gravity data is used to derive the high frequency geoid spectrum and to compare it to the error spectrum of the geoid as derived from an airborne gravity system gravity values in various areas with typical topographic features are used for the spectral analysis the achievable accuracy of airborne gravimetry in geoid determination is studied through an assessment of inertial navigation system ins and global positioning system gps errors in the spectral ranges of interest the results of these computations are presented in a number of graphs which allow the following conclusions to determine geoid undulations with an accuracy of 1 cm rms the minimum wavelength to be resolved is 14 km in flat terrain and 5 km in mountainous terrain using a flight speed of 300 km h at an altitude of 2 5 km or less above ground current hardware sensitivity is sufficient to resolve the geoid spectrum for such wavelengths the total gps ins induced geoid undulation error rms in this case is less than 1 cm for wavelengths between 5 km and 100 km and less than 10 cm for wavelengths between 5 km and 500 km the cumulative rms geoid error is less than 30 cm for wavelengths up to 1000 km the accuracy for relative geoid determination is 0 1 ppm for wavelengths up to 100 km and 0 2 ppm for 500 km the results were verified by analyzing the data collected during an airborne test the results indicate that airborne gravimetry has the potential to determine geoid undulation with centimeter level accuracy and to fill the gaps in the global gravity map in an efficient manner and with an accuracy superior to current terrestrial methods

fault zone guided wave generation on the locked late interseismic alpine fault new zealand fault zone guided waves fzgws have been observed for the first time within new zealand s transpressional continental plate boundary the alpine fault which is late in its typical seismic cycle ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone distinctive dispersive seismic codas 7 35 hz have been recorded on shallow borehole seismometers installed within 20 m of the principal slip zone near the central alpine fault known for low background seismicity fzgw generating microseismic events are located beyond the catchment scale partitioning of the fault indicating lateral connectivity of the low velocity zone immediately below the near surface segmentation initial modeling of the low velocity zone indicates a waveguide width of 60 200 m with a 10 40 reduction in s wave velocity similar to that inferred for the fault core of other mature plate boundary faults such as the san andreas and north anatolian faults

airborne gravimetry using a strapped down lacoste and romberg air sea gravity meter system a feasibility study marine gravimeters mounted on stabilized platforms are commonly used in aircraft to perform airborne gravity measurements the role of the stabilized platform is to level the sensor mechanically whatever the aircraft attitude however this compensation is generally insufficient due to the sensitivity of modern gravity sensors correcting the offlevel error requires that an offlevel correction calculated from positioning data be added to gravimeter measurements which complicates not only the processing but also the assessment of precision and resolution this paper is a feasibility study describing the levelling of a completely strapped down lacoste and romberg gravimeter for airborne gravimetry operation by means of gps positioning data it focuses on the calculation of the sensor offlevel correction needed for the complete gravity data processing the precision of the offlevel correction that can be achieved in terms of gps data precision and gravity wavelengths is theoretically studied and estimated using the gravity and gps data acquired during the alpine swiss french airborne gravity survey carried out in 1998 over the french western alps while a 1 cm precision of gps determined baseline coordinates is sufficient to achieve a 5 mgal precision of the offlevel correction we maintain that this precision has to reach 1 mm to ensure a 1 mgal precision of the offlevel correction at any wavelength without a stabilized platform the onboard instrumentation becomes significantly lighter furthermore the correction for the offlevel error is straightforward and calculated only from gps data thus the precision and the resolution of airborne gravity surveys should be estimated with a better accuracy

comparison of airgrav and gt 1a airborne gravimeters for research applications airbornegravimetryhasplayedavitalroleincontributing to our knowledge of the subglacial environment in polar regions previous programs have produced extensive gravity data sets inantarctica but the resolution and accuracy of the datahavebeenlimited wehaveevaluatedtherelativeperformance and suitability of two different airborne gravimeters for research applications from flight tests over the canadian rocky mountains near calgary survey design mission profiles and demands on the performance of an airborne gravimeter are different for the remote polar environment thanformostcommercialexplorationsurveys bothsystems theairgrav and gt 1a can produce higher resolution data with improved flight efficiency than can the bgm 3 and lacoste romberg gravimeters used inantarctica theairgrav and gt 1asystems are capable of draped flying of airborne gravity allowing new applications for polar use both systems could provide the academic community with a significant increase in accuracy and horizontal resolution to enablemajoradvancesinunderstandingthesubglacialenvironment compared to the gt 1a system the airgrav system hasalowernoiselevelandhigheraccuracy anditislesssensitive to changing flight conditions in particular vertical accelerationsduringturbulentflights

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

gravity measurements in an airplane using state of the art navigation and altimetry in 1976 the u s naval oceanographic office made experimental gravity measurements over the atlantic in an airplane using a lacoste and romberg shipboard gravity meter a honeywell electrostatic gyro inertial system a rosemont pressure port altimeter a honeywell radar altimeter loran c and other electronic navigational aids preliminary results were reported at the 47th annual international seg meeting in calgary canada in 1977 these preliminary results were compared with sea gravity data furnished by the naval oceanographic office and were found to be in general agreement however there were some unexplained systematic errors of about 15 mgal and the airplane gravity profiles were considerably more noisy than helicopter gravity profiles reported by william gumert 1977 personal communication after the paper was given we found that corrections for schuler oscillations in the inertial system significantly reduced the systematic errors we also found that by making full use of a half second samp

airborne gravity measurements over mountainous areas by using a lacoste romberg air sea gravity meter this paper introduces a new approach to airborne gravity data reduction well suited for surveys flown at high altitude with respect to gravity sources mountainous areas classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western french alps by using a lacoste romberg air sea gravity meter the part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft this finding is theoretically argued by solving the equation of the gravimetric system gravity meter and stabilized platform the transfer function of the system is derived and a new formulation of airborne gravity data reduction which takes care of the sensitive response of spring tension to observable gravity field wavelengths is given the resulting gravity signal exhibits a residual noise caused by electronic devices and short wavelength eotvos effects the use of dedicated exponential filters gives us a way to eliminate these high frequency effects examples of the resulting free air anomaly at 5100 m altitude along one particular profile are given and compared with free air anomaly deduced from the classical method for processing airborne gravity data and with upward continued ground gravity data the well known trade off between accuracy and resolution is discussed in the context of a mountainous area

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

in situ stress orientations in the witch ground graben north sea revealed by borehole breakouts preliminary results directions of minimum horizontal stress h derived from borehole breakouts are presented for seven boreholes in the witch ground graben north sea we have found a clear tendency for a dominant n s more accurately a nnw sse direction of h approximately perpendicular to the graben axis however significant variations from this general trend were encountered even within this relatively small area especially within the boreholes from the tartan and highlander fields contrasts in local structure and lithology within the area may explain these differences although such contrasts have yet to be quantified broad regional trends of h are apparent throughout northwest europe although previously published data show a certain amount of variability it has been shown that the minimum compressive stress direction changes significantly across many of the graben structures within the north sea basin and this also appears to be the case within the witch ground graben the geological society 1992

seismic tomography of central sao miguel azores abstract we determine the three dimensional distribution of p and s wave velocities for central sao miguel island azores portugal by tomographic inversion of local earthquake arrival times we use p and s phases from 289 earthquakes recorded by a network of 20 seismometers the model shows good resolution in the shallowest 5 6 km as illustrated by different resolution tests there are several velocity anomalies interpreted as pyroclastic deposits intrusive bodies geothermal fields and the effects of tectonics a low vp zone marks furnas caldera probably evidencing volcaniclastic sediments with development of intense geothermal activity another low vp zone extends in correspondence of the highly fractured area between fogo and the north coast conversely strong positive anomalies are found south of fogo and northwest of furnas they are interpreted in terms of high density deposits and remnants of a plutonic intrusion these interpretations are supported by the distribution of vp vs and are consistent with previous geological geochemical and geophysical data