field test of sub basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data the recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping however this technology also has potential for structural mapping applications particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data this paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128 km long profile the profile extends across the trough between the faroe and shetland islands in the north sea here we focus on how 2 5d inversion can best recover the volcanic and sedimentary sections a synthetic test carried out with 3d anisotropic model responses shows that vertically transverse isotropy 2 5d inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections we find the 2 5d inversion works well despite moderate 3d structure in the synthetic model triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided ac onstant resistivity contour that most closely matched the true base of the volcanic flows for the field survey data triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation the structural boundaries estimated by the 20 mc ontour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11 with a mean error of 1 2 compared with the interpreted base of the volcanic horizon both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low resistivity prospective siliciclastic sediments and higher resistivity non prospective volcaniclastic sediments beneath the volcanic section

lithofacies analysis of colluvial sediments an aid in interpreting the recent history of quaternary normal faults in the basin and range province western united states inferring the frequency and magnitude of past earthquakes from the stratigraphy in exposures of normal faulted sediments is difficult because colluvial lithofacies assemblages adjacent to faults are complex similarities in facies assemblages adjacent to young fault scarps in arid to semiarid areas such as the basin and range province allow lithofacies to be grouped into two genetic architectural elements debris and wash elements upper and lower facies associations can commonly be recognized within each element a lithofacies code scheme similar to those used in the analysis of fluvial and glacial lithofacies sequences provides a concise way of illustrating lithofacies relations in fault exposures the source lithology of colluvial lithofacies is shown in the code and soil horizon symbols can be included the architecture of lithofacies assemblages near fault scarps in semiarid areas is explained by a model of colluvial sedimentation in response to a single surface faulting event analysis of lithofacies assemblages exposed in three trenches across normal faults in the eastern basin and range shows how the model can be used to interpret fault histories similar facies analysis methods may be useful in interpreting colluvial sequences formed by non tectonic processes

integrated seismic hazard analysis of the wasatch front utah we examined the combined effects of different sources that influence earthquake hazard of the populated wasatch front utah we first evaluated the fault stress interaction of the two largest historic earthquakes of the intermountain seismic belt isb the 1959 hebgen lake montana m s 7 5 and the 1983 borah peak idaho m s 7 3 earthquakes which experienced multisegment normal faulting ruptures estimates of the static stress change for these events revealed an increase in coulomb failure stress in areas of extended aftershocks these observations suggested that fault stress analysis is applicable in evaluating the spatial distribution of aftershocks after large normal faulting earthquakes in the same extensional stress regime of the eastern basin range including the wasatch front utah which encompasses the 370 km long wasatch fault and surrounding faults on the basis of this result we applied the modeling technique to the historically seismically quiescent wasatch fault and examined the relation between the pattern of stress change and the space time distribution of paleoearthquakes ages and locations of the wasatch fault paleoseismic data imply 17 single segment or 11 multisegment ruptures in the past 5 6 kyr we prefer the multisegment model because almost all of the large historical normal faulting earthquakes in the isb were multisegment fitting the along fault displacements by an analytic half ellipse function provides a first order distribution of the displacement over an entire rupture length and allows new estimates of fault slip rates and seismic moment with these data we also estimated the occurrence rate of m w 6 6 wasatch paleoearthquakes and showed that the rate was about three times higher than that inferred by the historical seismicity this result along with new information from a megatrench on the wasatch fault near salt lake city that revealed no scarp forming earthquakes between ca 9 0 and 15 5 ka suggests that large wasatch fault events in the past 5 6 kyr may be clustered the recurrence rate of large earthquakes estimated by the geodetic measured strain on the other hand is three to four times higher than that estimated by the long term fault slip rate this difference together with the observation of a low regional strain rate from historical seismicity suggests that cumulative aseismic deformation may be significant on the wasatch fault we also examined a major consideration for along strike segmentation namely how stress contagion could affect the probability of failure of adjacent faults including paleoearthquake derived fault slip rates global positioning system derived geodetic moment rates and the effect of stress contagion in the earthquake hazard estimation for a specific location in the salt lake valley revealed an increase in the annual frequency of peak ground acceleration 0 25 g by a factor of 1 4 4 0 and 5 4 respectively compared with that derived from historical seismicity only manuscript received 31 may 2001

resolution of reservoir scale electrical anisotropy from marine csem data a combination of 1d and 3d forward and inverse solutions is used to quantify the sensitivity and resolution of conventional controlled source electromagnetic csem data collected using a horizontal electric dipole source to transverse electric anisotropy located in a deep water exploration reservoir target because strongly anisotropic shale layers have a vertical resistivity that can be comparable to many reservoirs we examined how csem can discriminate confounding shale layers through their characteristically lower horizontal resistivity forward modeling indicated that the sensitivity to reservoir level anisotropy is very low compared with the sensitivity to isotropic reservoirs especially when the reservoir is deeper than about 2 km below the seabed however for 1d models where the number of inversion parameters can be fixed to be only a few layers both vertical and horizontal resistivity of the reservoir can be well resolved using a stochastic inversion we found that the resolution of horizonta

determining the orientation of marine csem receivers using orthogonal procrustes rotation analysis electromagnetic receivers deployed to the seafloor for csem surveys can have unknown orientations because of the unavailability of compass and tilt recordings in such situations only the orientation independent parameters derived from the measured csem field vector can be interpreted and this may result in less structural resolution than possible when the sensor orientations are known an orthogonal procrustes rotation analysis opra technique can be used to estimate the full 3d receiver orientation for inline and off line csem receivers the generality of this method allows it to be easily embedded into nonlinear csem inversion routines so that they iteratively search for both the receiver orientation and a seafloor electrical conductivity model compatible with the data synthetic tests using the opra method jointly with a 1d inversion demonstrate that it can recover the rotation and tilt angles to about one degree accuracy for 1d data and to within a few degrees for 2d data application of this method to real survey data shows good agreement with a previous orientation method that is suitable only for determining the horizontal rotation of inline receivers csem data collected over the pluto gas field offshore the northwest coast of australia were used to demonstrate how the opra method can be used to orient csem receivers prior to inversion of only the inline electric and crossline magnetic field components

marine csem of the scarborough gas field part 1 experimental design and data uncertainty we describe the planning processing and uncertainty analysis for a marine csem survey of the scarborough gas field off the northwest coast of australia consisting of 20 transmitter tow lines and 144 deployments positioned along a dense 2d profile and a complex 3d grid the purpose of this survey was to collect a high quality data set over a known hydrocarbon prospect and use it to further the development of csem as a hydrocarbon mapping tool recent improvements in navigation and processing techniques yielded high quality frequency domain data data pseudosections exhibit a significant anomaly that is laterally confined within the known reservoir location perturbation analysis of the uncertainties in the transmitter parameters yielded predicted uncertainties in amplitude and phase of just a few percent at close ranges these uncertainties may however be underestimated we introduce a method for more accurately deriving uncertainties using a line of receivers towed twice in opposite directions comparing the residuals for each line yields a gaussian distribution directly related to the aggregate uncertainty of the transmitter parameters constraints on systematic error in the transmitter antenna dip and inline range can be calculated by perturbation analysis uncertainties are not equal in amplitude and phase suggesting that inversion of these data would be better suited in these components rather than in real and imaginary components one dimensional inversion showed that the reservoir and a confounding resistive layer above it cannot be separately resolved even when the roughness constraint is modified to allow for jumps in resistivity and prejudices are provided indicating that this level of detail is beyond the single site csem data further when range dependent error bars are used the resolution decreases at a shallower depth than when a fixed error level is used

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

effects of micromilling on δ18o in biogenic aragonite 1 extraction of small aliquots g mg from aragonite samples such as corals sclerosponges bivalves and otoliths is required for bulk oxygen isotope analyses however aragonite is a metastable polymorph of calcium carbonate that undergoes transformation to calcite which is the more thermodynamically stable polymorph of caco3 this polymorph transformation occurs via heating and or stress and such conditions may occur during drilling during this polymorph transition exchange with atmospheric or organic c and o is possible therefore extracting aliquots during drilling without modification or contamination via isotope exchange is imperative for accurate isotopic analysis using a micromill aliquots were taken from aragonite cod otoliths that had been grown in the laboratory under stable temperature conditions x ray diffraction analysis showed that although 6 of the sample converts to calcite during drilling there is no significant effect on 18o

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

marine dipole dipole controlled source electromagnetic and coincident loop transient electromagnetic experiments to detect seafloor massive sulphides effects of three dimensional bathymetry seafloor massive sulphides smss are regarded as a potential future resource to satisfy the growing global demand of metals including copper zinc and gold aside from mining and retrieving profitable amounts of massive sulphides from the seafloor the present challenge is to detect and delineate significant sms accumulations which are generally located near mid ocean ridges and along submarine volcanic arc and backarc spreading centres currently several geophysical technologies are being developed to detect and quantify sms occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock here we use a short fixed offset controlled source electromagnetic csem system and a coincident loop transient electromagnetic tem system which in theory allow the detection of sms in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings in 2016 csem and tem experiments were carried out at several locations near the trans atlantic geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor measurements were conducted in an area that contains distinct sms sites located several kilometres off axis from the mid atlantic ridge some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased based on the quality of the acquired data both experiments were operationally successful however the data analysis indicates bias caused by three dimensional 3d effects of the rough bathymetry in the study area and thus data interpretation remains challenging therefore we study the influence of 3d bathymetry for marine csem and tem experiments focusing on shallow 3d conductors located beneath mound like structures we analyse synthetic inversion models for attributes associated with 3d distortions of csem and tem data that are not sufficiently accounted for in conventional 1d tem and 2d csem interpretation schemes before an adequate quantification of sms in the region is feasible these 3d effects need to be studied to avoid over underestimation of sms using the acquired em data the sensitivity of csem and tem to bathymetry is investigated by means of 3d forward modelling followed by 1d tem and 2d csem inversion of the synthetic data using realistic error conditions subsequently inversion models of the synthetic 3d data are analysed and compared to models derived from the measured data to illustrate that 3d distortions are evident in the recorded data sets

electrical resistivity structure of the valu fa ridge lau basin from marine controlled source electromagnetic sounding summary in december 1995 we carried out a comprehensive controlled source electromagnetic survey of the valu fa ridge at 22u25k si n the lau basin the valu fa ridge is ab ack arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity seismic studies have imaged a melt lens at an average depth of 3 2 km below the seafloor surrounded by a zone of lowered seismic velocity interpreted as a region of partial melt in the crust the electromagnetic experiment was part of a multidisciplinary study which included wide angle and reflection seismics bathymetry and potential field measurements electromagnetic signals at frequencies between 0 25 and 40 hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea bottom receivers at ranges of up to 20 km from the source over 80 hr of data consisting of the magnitude of the horizontal electric field at the seafloor were collected these data have extremely low scatter compared to similar data from previous surveys the data were interpreted using a combination of 1 and 2 d forward modelling and inversion the vertical resistivity gradient in the upper crust at the valu fa ridge is abnormally low with resistivities of less than 10 v m observed throughout layer 2 of the crust to a depth of 3 km this is significantly more conductive at depth than the axis of the slow spreading reykjanes ridge at 57u45kn and the fastspreading east pacific rise at 13un where similar data sets have been collected in the past although the structure of layer 2 is well constrained by the electromagnetic data its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it and hence the data are not sensitive to the structure in layer 3 in particular the structure of the melt lens or surrounding low velocity zone the seismic velocity structure of the valu fa ridge determined from the coincident wide angle seismic study is similar to that observed at other mid ocean ridges with a steep seismic velocity gradient through layer 2 although overall velocities are slightly lower the seismic velocity anomaly calculated relative to an average off axis structure is also small this suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity however hot and or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity since the conductive region extends unbroken from 3 km depth to the seafloor it is probable that these fluids circulate to or close to the magma chamber itself

reevaluation of holocene faulting at the kaysville site weber segment of the wasatch fault zone utah the 1978 kaysville utah trench excavated by swan and others 1980 across a large graben of the weber segment of the wasatch fault zone was reexcavated in 1988 to reevaluate the timing and nature of holocene faulting relogging of the trench reveals evidence for five or six faulting events younger than the provo phase of lake bonneville circa 13 000 14c years b p geometric reconstruction of net vertical offset in the last three events suggests a variation in coseismic vertical displacement at this site ranging from a net of 1 4 3 4 m per event the three latest faulting events occurred at shortly before 0 6 0 8 ka 2 8 0 7 ka and circa 3 8 7 9 ka earlier events cannot be directly dated because older graben fill sediments yielded thermoluminescence ages older than the time of deposition and some scarp derived colluvial wedges beneath the trench floor were not exposed the two younger faulting events we recognize at kaysville correlate reasonably well with faulting events on the same segment 25 km north near east ogden utah at circa 0 8 1 2 ka and 2 5 3 0 ka forman and others 1991 whereas the earlier kaysville event is significantly older than the earliest 3 5 4 0 ka event dated at east ogden the 3 5 4 0 ka ground rupture recognized at east ogden may have died out at a subsegment boundary between the two trench sites within the 61 km long weber segment

seismically regularized controlled source electromagnetic inversion marine controlled source electromagnetic csem data can be highly sensitive to the presence of resistive hydrocarbon bearing layers in the subsurface yet due to the relatively poor depth resolution of csem data and the smoothness constraints imposed by electromagnetic em inversion methods the resulting resistivity models are often highly smoothed out typically underestimating the reservoir resistivity and overestimating its thickness conversely seismic full waveform inversion fwi can accurately recover the depths of seismic velocity changes yet is relatively insensitive the presence of hydrocarbons in spite of their low depth resolution csem data have been shown to be highly sensitive to the resistivity thickness product of buried resistive layers suggesting that if the thickness of a target layer can be constrained a priori very accurate resistivity estimates may be obtained we developed a method for leveraging the high depth resolution of fwi into a standard csem inversion algorithm so

decadal cyclicity of regional mid holocene precipitation evidence from dominican coral proxies 1 the stable carbon 13c and oxygen 18o isotopic compositions of four holocene specimens of montastraea sp and siderastrea sp coral skeletons from the dominican republic were analyzed to examine decadal scale fluctuations in regional precipitation the specimens range in length from 47 to 197 years individually and range in age from 7 2 to 5 2 kyr oscillations in the 13c and 18o of coral skeletons from the holocene enriquillo valley occur with a seemingly consistent frequency of 15 20 years for one coral 6 ka this decadal to multidecadal mode of variability is statistically significant the decadal to multidecadal mode of stable isotopic variability may represent cyclic fluctuations in mid holocene migration of the intertropical convergence zone itcz or in the hydrogeology of the region as governed by fluctuations in the passage of tropical storms and hurricanes regional temperature is believed to act as only a secondary control on the 18o values of these corals however itcz migration and or oscillations in storm frequency at this site may ultimately reflect decadal scale patterns in greater tropical north atlantic sea surface temperatures the length and resolution of climate variability represented in this study indicate a persistent decadal to multidecadal mode of atlantic climate fluctuation throughout the mid holocene that is indistinguishable from observed historical climate

bayesian inversion of marine csem data with a trans dimensional self parametrizing algorithm summary the posterior distribution of earth models that fit observed geophysical data convey information on the uncertainty with which they are resolved from another perspective the non uniqueness inherent in most geophysical inverse problems of interest can be quantified by examining the posterior model distribution converged upon by a bayesian inversion in this work we apply a reversible jump markov chain monte carlo method to sample the posterior model distribution for the anisotropic 1 d seafloor conductivity constrained by marine controlled source electromagnetic data unlike conventional gradient based inversion approaches our algorithm does not require any subjective choice of regularization parameter and it is self parametrizing and trans dimensional in that the number of interfaces with a resistivity contrast at depth is variable as are their positions a synthetic example demonstrates how the algorithm can be used to appraise the resolution capabilities of various electromagnetic field components for mapping a thin resistive reservoir buried beneath anisotropic conductive sediments a second example applies the method to survey data collected over the pluto gas field on the northwest australian shelf a benefit of our bayesian approach is that subsets of the posterior model probabilities can be selected to test various hypotheses about the model structure without requiring further inversions as examples the subset of model probabilities can be viewed for models only containing a certain number of layers or for models where resistive layers are present between a certain interval as suggested by other geological constraints such as seismic stratigraphy or nearby well logs

a marine controlled source electromagnetic survey to detect gas hydrates in the qiongdongnan basin south china sea abstract the qiongdongnan basin qb is an important prospecting area for gas hydrates in the south china sea scs however it is difficult to infer gas hydrates using a bottom simulating reflector bsr under the influence of flat seabed multiples and bubble effects in the qb the upper limit of the gas hydrate stability zone ghsz and the distribution of gas hydrates in the shallow seabed are still not well understood a marine controlled source electromagnetic mcsem survey was completed in order to collect electric resistivity information along a 4 5 km profile electromagnetic data were acquired and processed at nine sites the seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis opra method as a result of isotropic vertical transverse isotropic vti and triaxial anisotropic inversion of electric field pmax data a 2 d resistivity image of the seabed was presented preliminary results show that the shallow seabed can be divided into four electric layers according to thermodynamic conditions in the qb the estimated thickness of ghsz is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area the bsr and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data the mcsem result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the ghsz

coulomb stress evolution in a diffuse plate boundary 1400 years of earthquakes in eastern california and western nevada usa diffuse plate boundaries are characterized by deformation distributed over a wide area in a complex network of active faults and by relatively low strain rates these characteristics make it difficult to understand the spatial and temporal distribution of seismicity the area east of the sierra nevada between longitudes 121 degrees w and 116 degrees w is part of a diffuse plate boundary at least 17 major surface rupturing earthquakes have happened here in the last 1400 years our purpose is to determine whether these events influence each other or whether they are randomly distributed in time and space we model the evolution of coseismic and postseismic coulomb failure stress changes delta cfs produced by these earthquakes and we also model interseismic stresses on the entire fault network our results show that 80 of the earthquake ruptures are located in areas of combined coseismic and postseismic delta cfs 0 2 bar this relationship is robust as shown by the control tests that we carried out using random earthquake sequences we also show that the fish lake valley pyramid lake and honey lake faults have accumulated 45 37 and 27 bars respectively of total delta cfs i e coseismic postseismic interseismic in the last 1400 years such values are comparable to the average stress drop in a major earthquake and these three faults may be therefore close to failure

surface faulting accompanying the borah peak earthquake and segmentation of the lost river fault central idaho on the morning of 28 october 1983 the ms 7 3 borah peak earthquake struck central idaho and formed a y shaped zone of surface faults that is divided into a southern a western and a northern section the total length of the surface faults is 36 4 3 1 km and the maximum net throw is 2 5 to 2 7 m the near surface net slip direction determined from the rakes of striations in colluvium averaged 0 17 m of sinistral slip for 1 00 m of dip slip the 20 8 km long southern section is the main zone of surface faulting and coincides with the thousand springs segment of the lost river fault it has the largest amount of net throw most complex rupture patterns and best evidence of sinistral slip the surface faults include zones of ground breakage as much as 140 m wide en echelon scarps with synthetic and antithetic displacements and individual scarps that are nearly 5 m high the 14 2 km long western section diverges away from the lost river fault near the northern end of the southern section the net throw on this section is generally less than 0 5 m but locally is as much as 1 6 m the new ruptures are poorly developed across the crest and north flank of the willow creek hills they are mostly downhill facing arcuate scars perhaps incipient landslides that may overlie a deeper zone of tectonic movement the northern section at least 7 9 km long is on the warm spring segment of the lost river fault and has a maximum net throw of about 1 m the pattern of surface faulting on this section is simple compared to the other sections a 4 7 km long gap in 1983 surface faults separates the northern and southern sections but contains an older scarp of late pleistocene age geologic seismologic and geodetic data from the earthquake suggest that barriers confined the primary coseismic rupture to the thousand springs segment of the fault the rupture propagated unilaterally to the northwest from a hypocenter near the southeastern end of the segment the southeastern boundary of the segment is marked by an abrupt bend in the range front a 4 km long gap in late quaternary scarps and transverse faults of eocene age that intersect the lost river fault the northwestern boundary of the thousand springs segment is at the junction of the willow creek hills and the lost river fault here the southern and western sections of surface faults diverge and there is a gap in the 1983 scarps during the first few weeks after the main shock the large magnitude and large stress drop aftershocks clustered near this barrier later aftershocks were mainly northwest of the barrier on the warm spring and challis segments and showed that strain adjustments eventually affected the entire northern part of the lost river fault fault scarp morphology and the bedrock geology suggest that the boundary between the thousand springs and warm spring segments has probably ruptured less frequently and had less net slip during much of the late cenozoic than the interior of the adjacent segments the 1983 faulting shows that although segment boundaries can stop or deflect primary ruptures secondary surface faulting can occur on adjacent segments of the main fault a late pleistocene scarp in the 1983 gap suggests that infrequent earthquakes perhaps larger than the 1983 event might break through a segment boundary and thus release strain on two adjacent segments

first results from a marine controlled source electromagnetic survey to detect gas hydrates offshore oregon 1 submarine gas hydrate is a hazard to drilling a potential hydrocarbon resource and has been implicated as a factor in both submarine slope stability and climate change bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data we conducted a marine controlled source electromagnetic sounding at hydrate ridge oregon usa in august 2004 electromagnetic fields transmitted by a deep towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers positioned 600 m apart to produce a dense coverage in the recorded electric field data results are presented in simple form by apparent resistivity pseudosections which produce an approximate image of lateral resistivity variations across the study region resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors archie s law based on pseudosection apparent resistivities predicts volumetric hydrate concentrations vary from 0 30 across the ridge

transient marine electromagnetic responses of 3d resistive structures implications for navigation the marine controlled source electromagnetic method has become a popular technique for mapping the electrical resistivity structure of the seafloor electromagnetic fields produced by an electric dipole transmitter diffuse through the earth and are recorded on the seabed by nodal or cable based electric dipole receivers accurate information on transmitter and receiver geometry is extremely important for proper interpretation errors in the position and orientation of the transmitter and or the receivers propagate into errors in the predicted seafloor resistivity we consider the special situation where a receiver cable is towed in the vicinity of a transmitter which is located on the seafloor with a known position and orientation we theoretically and numerically examine the response of this system for a selection of 1d 2d and 3d models and show that although the electromagnetic field recorded at early times contains information about the subsurface resistivity the late time decay of the transient response is only a function of the seawater conductivity and receiver position for all the models we consider in fact the shape of the late time decay is almost exactly the same as that of a dipole in a wholespace having the same resistivity as seawater this shows that late time transient information can be used to determine the position of the receiver cable whereas early time information can be used to determine the resistivity structure of the seafloor

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

seafloor massive sulphide exploration using deep towed controlled source electromagnetics navigational uncertainties deep towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the sub seafloor we address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data we show that the data error is laterally variable due to irregular motion during deep towing but also due to lateral variations in conductivity including those resulting from topography to address this variability and quantify the data error prior to inversion we propose a two dimensional perturbation study our workflow enables stable and geologically reliable results for multi component and multi frequency inversions an error estimation workflow is presented which comprises the assessment of navigational uncertainties perturbation of navigational parameters and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous sub seafloor conductivity model some navigational uncertainties are estimated from variations of direct measurements other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation some navigational parameters show direct correlation with the measured electric fields for example the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field for the perturbation study each standard deviation is added to the navigational parameters forward models are run for each perturbation amplitude deviations are summed in quadrature with the stacking error for a total laterally varying data error the error estimation is repeated for a heterogeneous sub seafloor model due to the large conductivity range several orders of magnitude which affects the forward model the approach enables us to utilize data from several components multiple electric fields frequencies and receivers in the inversion to constrain the final model and reduce ambiguity the final model is geologically reasonable in this case enabling the identification of conductive metal sulphide deposits on the seafloor

estimating surface rupture length and magnitude of paleoearthquakes from point measurements of rupture displacement we present a method to estimate paleomagnitude and rupture extent from measurements of displacement at a single point on a fault the variability of historic ruptures is summarized in a histogram of normalized slip then scaled to give the probability of finding a given displacement within a rupture for any magnitude considered the histogram can be inverted assuming any magnitude earthquake is as likely as another yielding probability density functions of magnitude and rupture length for any given displacement measurement to improve these distributions we include a term to account for the probability that the earthquake would cause ground rupture and two alternative distributions of earthquake magnitude the gutenberg richter magnitude distribution predicts shorter rupture lengths and smaller magnitudes than does a uniform distribution where any magnitude earthquake is considered equally likely longer ruptures and larger magnitudes than the uniform model are predicted by an alternative magnitude distribution designed to return site average displacement this model is a generalization of the characteristic earthquake model and reasonably describes paleoseismic findings on the southern san andreas fault where slip is accommodated by average displacements of a few meters and earthquake recurrence times of 100 250 years our results should increase the value of paleoseismic displacement measurements for hazard assessment in particular they quantify probability estimates of earthquake magnitude and rupture length where point observations of rupture displacement are available and so can contribute to probabilistic seismic hazard analyses online material rupture profiles used to sample rupture variability

electromagnetic detection of plate hydration due to bending faults at the middle america trench water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism geochemical indicators suggest that the nicaraguan slab is anomalously wet yet the mechanism of slab hydration remains poorly constrained extensional bending faults on the incoming oceanic plate of the middle america trench offshore nicaragua have been observed to penetrate to mantle depths suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation here we use controlled source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile crossing the trench offshore nicaragua along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults furthermore a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor the observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle this implies that most serpentinisation happens at the trench with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration this observation explains why the heavily faulted trench offshore nicaragua is associated with an anomalously wet slab whereas other sections of the middle america trench containing fewer bending faults have less fluid flux from the subducting slab

the feasibility of reservoir monitoring using time lapse marine csem monitoring changes in hydrocarbon reservoir geometry and pore fluid properties that occur during production is a critical part of estimating extraction efficiency and quantifying remaining reserves we examine the applicability of the marine controlled source electromagnetic csem method to the reservoir monitoring problem by analyzing representative 2d models these studies show that csem responses exhibit small but measureable changes that are characteristic of reservoir depletion geometry with lateral flooding producing a concave up depletion anomaly curve and bottom flooding producing a concave down depletion anomaly curve lateral flooding is also revealed by the spatial temporal variation of the csem anomaly where the edge of the response anomaly closely tracks the retreating edge of the flooding reservoir measureable changes in csem responses are observed when 10 of the resistive reservoir is replaced by conductive pore fluids however to avoid corrupting the relatively small signal changes associated with depletion the acquisition geometry must be maintained to a fraction of a percent accuracy additional factors such as unknown nearby seafloor inhomogeneities and variable seawater conductivity can mask depletion anomalies if not accounted for during repeat monitoring measurements although addressing these factors may be challenging using current exploration csem practices straightforward solutions such as permanent monuments for seafloor receivers and transmitters are available and suggest the method could be utilized with present day technology

paleoseismic evidence of characteristic slip on the western segment of the north anatolian fault turkey we have conducted a paleoseismic investigation of serial fault rupture at one site along the 110 km rupture of the north anatolian fault that produced the mw 7 4 earthquake of 17 august 1999 the benefit of using a recent rupture to compare serial ruptures lies in the fact that the location magnitude and slip vector of the most recent event are all very well documented we wished to determine whether or not the previous few ruptures of the fault were similar to the recent one we chose a site at a step over between two major strike slip traces where the principal fault is a normal fault our two excavations across the 1999 rupture reveal fluvial sands and gravels with two colluvial wedges related to previous earthquakes each wedge is about 0 8 m thick considering the processes of collapse and subsequent diffusion that are responsible for the formation of a colluvial wedge we suggest that the two paleoscarps were similar in height to the 1999 scarp this similarity supports the concept of characteristic slip at least for this location along the fault accelerator mass spectrometry ams radiocarbon dates of 16 charcoal samples are consistent with the interpretation that these two paleoscarps formed during large historical events in 1509 and 1719 if this is correct the most recent three ruptures at the site have occurred at 210 and 280 year intervals

unusually low rates of slip on the santa rosa range fault zone northern nevada the santa rosa range fault zone srrfz is one of the most topographically prominent normal fault systems in the northern basin and range province of the western united states it has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central nevada seismic belt cnsb we use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large magnitude paleoearthquakes on the southern part of the srrfz at a trench site near orovada nevada coseismic vertical displacements ranged from 1 to 2 8 m for each of the last four events luminescence ages provide time limits for the last three events of 125 155 ka 90 108 ka and 11 16 ka these data yield recurrence intervals of 17 65 k y and 74 97 k y and an elapsed time of 11 16 k y since the youngest event slip rate determinations at the orovada site are complicated by multiple fault strands but rates calculated from a variety of data are surprisingly low 0 01 0 16 mm yr given the topographic prominence of the santa rosa range a lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb

fault segmentation new concepts from the wasatch fault zone utah usa the question of whether structural segment boundaries along multisegment normal faults such as the wasatch fault zone wfz act as persistent barriers to rupture is critical to seismic hazard analyses we synthesized late holocene paleoseismic data from 20 trench sites along the central wfz to evaluate earthquake rupture length and fault segmentation for the youngest 3 ka and best constrained earthquakes differences in earthquake timing across prominent primary segment boundaries especially for the most recent earthquakes on the north central wfz are consistent with segment controlled ruptures however broadly constrained earthquake times dissimilar event times along the segments the presence of smaller scale subsegment boundaries and areas of complex faulting permit partial segment and multisegment e g spillover ruptures that are shorter 20 40 km or longer 60 100 km than the primary segment lengths 35 59 km we report a segmented wfz model that includes 24 earthquakes since 7 ka and yields mean estimates of recurrence 1 1 1 3 kyr and vertical slip rate 1 3 2 0 mm yr for the segments however additional rupture scenarios that include segment boundary spatial uncertainties floating earthquakes and multisegment ruptures are necessary to fully address epistemic uncertainties in rupture length we compare the central wfz to paleoseismic and historical surface ruptures in the basin and range province and central italian apennines and conclude that displacement profiles have limited value for assessing the persistence of segment boundaries but can aid in interpreting prehistoric spillover ruptures our comparison also suggests that the probabilities of shorter and longer ruptures on the wfz need to be investigated

resistivity image beneath an area of active methane seeps in the west svalbard continental slope the arctic continental margin contains large amounts of methane in the form of methane hydrates the west svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone the presence of bottom simulating reflectors bsrs on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area resistivity obtained from marine controlled source electromagnetic csem data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments we acquired two csem lines in the west svalbard continental slope extending from the edge of the continental shelf 250 m water depth to water depths of around 800 m high resistivities 5 12 m observed above the bsr support the presence of gas hydrate in water depths greater than 600 m high resistivities 3 4 m at 390 600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone ghsz of the continental slope in addition high resistivities 4 8 m landward of the ghsz are coincident with high amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas pore space saturation estimates using a connectivity equation suggest 20 50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments a free gas zone beneath the ghsz 10 20 per cent gas saturation is connected to the high free gas saturated 10 45 per cent area at the edge of the continental shelf where most of the seeps are observed this evidence supports the presence of lateral free gas migration beneath the ghsz towards the continental shelf

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

paleoseismic results from the alpine site wasatch fault zone timing and displacement data for six holocene earthquakes at the salt lake city provo segment boundary

navigating marine electromagnetic transmitters using dipole field geometry the marine controlled source electromagnetic csem technique has been adopted by the hydrocarbon industry to characterize the resistivity of targets identified from seismic data prior to drilling over the years marine controlled source electromagnetic has matured to the point that four dimensional or time lapse surveys and monitoring could be applied to hydrocarbon reservoirs in production or to monitor the sequestration of carbon dioxide marine controlled source electromagnetic surveys have also been used to target shallow resistors such as gas hydrates these novel uses of the technique require very well constrained transmitter and receiver geometry in order to make meaningful and accurate geologic interpretations of the data current navigation in marine controlled source electromagnetic surveys utilize a long base line or a short base line acoustic navigation system to locate the transmitter and seafloor receivers if these systems fail then rudimentary navigation is possible by assuming the transmitter follows in the ship s track however these navigational assumptions are insufficient to capture the detailed orientation and position of the transmitter required for both shallow targets and repeat surveys in circumstances when acoustic navigation systems fail we propose the use of an inversion algorithm that solves for transmitter geometry this algorithm utilizes the transmitter s electromagnetic dipole radiation pattern as recorded by stationary close range 1000 m receivers in order to model the geometry of the transmitter we test the code with a synthetic model and validate it with data from a well navigated controlled source electromagnetic survey over the scarborough gas field in australia

generic along strike segmentation of afar normal faults east africa implications on fault growth and stress heterogeneity on seismogenic fault planes understanding how natural faults are segmented along their length can provide useful insights into fault growth processes stress distribution on fault planes and earthquake dynamics we use cumulative displacement profiles to analyze the two largest scales of segmentation of 900 normal faults in afar east africa we build upon a prior study by manighetti et al 2009 and develop a new signal processing method aimed at recovering the number position displacement and length of both the major i e longest and the subordinate secondary segments within the faults regardless of their length age geographic location total displacement and slip rate 90 of the faults contain two to five major segments whereas more than 70 of these major segments are divided into two to four secondary segments in each hierarchical rank of fault segmentation most segments have a similar proportional length whereas the number of segments slightly decreases with fault structural maturity the along strike segmentation of the afar faults is thus generic at its two largest scales we summarize published fault segment data on 42 normal reverse and strike slip faults worldwide and find a similar number two to five of major and secondary segments across the population we suggest a fault growth scenario that might account for the generic large scale segmentation of faults the observation of a generic segmentation suggests that seismogenic fault planes are punctuated with a deterministic number of large stress concentrations which are likely to control the initiation arrest and hence extent and magnitude of earthquake ruptures

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

stable oxygen isotopic composition of corals from the gulf of guinea as indicators of periods of extreme precipitation conditions in the sub sahara stable oxygen isotopic analyses of scleractinian coral skeletons from the gulf of guinea in the eastern atlantic reveal that the corals from this region can be used to identify periods of severe drought and above average precipitation in sub sahara africa data presented in this paper show an inverse correlation between precipitation in the sahel and the 18 o values of a coral skeletons of the species siderastrea spp collected from the island of principe in the gulf of guinea this is opposite to the correlation expected as previous work has suggested that higher sea surface temperatures occur in the gulf of guinea during periods of low rainfall in the sahel such an association would lead to a positive correlation between sahel precipitation and skeletal 18 o the explanation for the observed inverse correlation is that the salinity of the gulf of guinea is strongly influenced by the outflow from the niger and congo rivers these periods of high freshwater input also correlate with periods of higher rainfall in the sub sahara and therefore affect the 18 o values of the coral skeleton the correlation between 18 o values of the coral skeleton and temperature in the northern subtropical atlantic ocean r 0 34 the magnitude of the dipole r 0 45 and the latitudinal position of the intertropical convergence zone r 0 37 illustrate that the 18 o values in the coral skeleton reflect climate dynamics of the region that affect the precipitation patterns in sub sahara africa

extensional reactivation of thrust faults accompanied by coseismic surface rupture southwestern wyoming and north central utah extensional reactivation of thrust faults north of the uinta mountains in utah and wyoming is accompanied by quaternary surface faulting tilting and monoclinal folding the 40 km long bear river fault zone consists of down to the west right stepping en echelon scarps each about 3 0 to 3 5 km in length striking n20 w to n20 e scarp derived colluvial deposits record two holocene surface ruptures with net vertical tectonic displacements ranging from 5 m per event west 1989 in press radiocarbon ages indicate surface faulting events occurred at about 4 6 ka and 2 4 ka the length of surface rupture and net slip per event in the bear river fault zone imply a seismic potential comparable to the lost river fault hebgen lake fault and the wasatch fault zone a late quaternary normal scarp is also coincident with the leading edge of the absaroka thrust fault 7 2 km west of the bear river fault zone scarp derived colluvial deposits record one surface faulting event over a length of at least 5 0 km the age of surface rupture is coeval with the bear river fault zone 2 4 ka normal faults displacing pleistocene surfaces deposits and tectonic tilt indicate that the hogsback thrust east of the bear river fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the bear river fault zone cumulative normal throw of 200 m on the hogsback fault may be responsible for postulated tectonic separation of the bear river and green river drainage basins neotectonic deformation results from east west extension superposed on the hogsback and absaroka thrust plates extensional reactivation of thrust faults caused propagation of new normal faults over stress points particularly at the hogsback ramp to flat transition and at the thrust leading edges the ages of surface rupture and cumulative throws indicate normal reactivation of the hogsback and absaroka thrusts and propagation of the intervening bear river fault zone proceeded from east to west opposite initial development of the thrusts in late cretaceous to paleocene time normally reactivated thrust faults and propagation of listric normal faults accompanied by surface rupture have significant implications for the tectonic seismogenic development of the eastern basin and range transition zone

calibration of stable oxygen isotope signatures in galapagos corals a 2 year 1993 1994 study was conducted in the galapagos islands ecuador to determine the relationship between 18o in skeletal carbonate and sea surface temperature sst in three species of reef building corals pavona clavus pavona gigantea and porites lobata coral samples were grown at 3 10 and 3 m depth at bartolome island champion iisland and urvina bay isabela island respectively hourly measurements of sst and sea surface salinity sss were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth in addition surface waters were sampled periodically bimonthly to monthly at each site to determine variation in 18o seawater results indicate the mean annual ssts were similar between sites varying from 22 9 c at champion to 23 8 c at urvina bay comparisons of monthly sst averages between instrumental and remote sensing satellite 1 1 grid data show a high correspondence r2 ranging from 0 84 to 0 94 indicating that remote sensing data are useful for interpreting the 18o record in corals when instrumental data are lacking here 18ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of sst in galapagos in general higher resolution coral sampling year resolved more of the monthly variation in sst up to 97 at a sampling resolution of 1 4 samples per millimeter of linear skeletal growth comparisions of the 18ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking sst in addition comparisons between sites were highly concordant with some differences reflecting local variation in sst seasonal patterns however were essentially the same over the entire region thus we conclude that the 18ocoral signal from coral skeletons in galapagos can be used to interpret regional changes in sst variation

environmental dose rates and radioactive disequilibrium from some australian luminescence dating sites abstract determination of geological archaeological ages by luminescence dating requires the measurement of the rate of delivery of radiation dose from the sample and its environment for all age determinations it is necessary to assess whether the dose rate has been constant during the time that the dose has been accumulating and to make allowance for it if it was not we report here results from a number of australian sites to illustrate the information that can be found about element concentrations about their distribution and about dose rates tests for radioactive disequilibrium form part of these measurements such tests take the form of comparison of independent measurements of dose rates and or individual elements in general there is little or no evidence for disequilibrium in aeolian dune systems even when the age of the system exceeds 500 ka however disequilibrium has been found in several sites subject to wet conditions

an introduction to marine controlled source electromagnetic methods for hydrocarbon exploration early development of marine electromagnetic methods dating back about 80 years was driven largely by defense military applications and use for these purposes continues to this day deepwater frequency domain electric dipole dipole controlled source electromagnetic csem methods arose from academic studies of the oceanic lithosphere in the 1980s and although the hydrocarbon exploration industry was aware of this work the shallow water environments being explored at that time were not ideally suited for its use low oil prices and increasingly successful results from 3d seismic methods further discouraged investment in costly alternative geophysical data streams these circumstances changed in the late 1990s when both statoil and exxonmobil began modeling studies and fieldtrials of csem surveying in deep water around 1000 m or deeper specifically for characterizing the resistivity of previously identified drilling targets trials offshore angola in 2000 2002 by both these companies showed that

review paper instrumentation for marine magnetotelluric and controlled source electromagnetic sounding we review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry the principal sensors magnetometers and nonpolarizing electrodes are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea floor but also fall off more rapidly with increasing frequency as a result magnetotelluric signals approach the noise floor of electric field and induction coil sensors 0 1 nv m and 0 1 pt at around 1 hz in typical continental shelf environments fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect seafloor magnetotelluric data down to 40 100 s controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea bed achieving output currents of 1000 a or more limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship the maximum source receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments the position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging while sea floor receivers need additional measurements of orientations from compasses and tiltmeters all equipment has to be packaged to accommodate the high pressure up to 40 mpa and corrosive properties of seawater usually receiver instruments are self contained battery powered and have highly accurate clocks for timekeeping even when towed on the sea floor or in the water column behind a transmitter

palaeoseismicity of the oquirrh fault utah from shallow seismic tomography the magnitude and frequency of normal fault palaeoearthquakes are usually determined by trenching studies that ascertain the size and number of colluvial wedges along the fault such information can be invaluable in predicting the seismic hazard and potential for a future earthquake in that region digging trenches across normal faults however is environmentally intrusive expensive and limited in the penetration depth to overcome these problems we propose the use of 3 d seismic tomography as a means to identify the shapes and sizes of colluvial wedges along normal faults as an example 2 d and 3 d seismic surveys were conducted across the oquirrh fault utah with the purpose of imaging the normal fault structure to a depth of about 10 m results show that the 3 d tomogram clearly delineates the fault zone and a colluvial wedge both of which correlate extremely well with the geological cross section interpreted from an adjacent trench the thickness of the colluvial wedge image is used in conjunction with a seismic section to compute an estimate of a 6 8 moment magnitude earthquake for the most recent event on this fault which is in close agreement with the 7 0 estimate based on a nearby trenching study these tomographic results demonstrate for the first time that seismic imaging methods can be used in some cases to estimate unambiguously the shapes of colluvial wedges and the sizes of prehistoric earthquakes thus seismic tomography has the possibility of providing cheaper deeper and wider but less resolved images of fault systems than the intrusive excavation of trenches across faults

from coseismic offsets to fault block mountains in the basin and range extensional province of the western united states coseismic offsets under the influence of gravity display predominantly subsidence of the basin side fault hanging wall with comparatively little or no uplift of the mountainside fault footwall a few decades later geodetic measurements gps and interferometric synthetic aperture radar insar show broad 100 km aseismic uplift symmetrically spanning the fault zone finally after millions of years and hundreds of fault offsets the mountain blocks display large uplift and tilting over a breadth of only about 10 km these sparse but robust observations pose a problem in that the coesismic uplifts of the footwall are small and inadequate to raise the mountain blocks to address this paradox we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift which is predicted to take place within one to two decades after each large earthquake thus the best preserved topographic signature of earthquakes is expected to occur early in the postseismic period

mapping thin resistors and hydrocarbons with marine em methods insights from 1d modeling the use of marine controlled source electromagnetic em csem sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling we examine the sensitivity of the csem method to such layers with forward and inverse modeling in one and three dimensions the 3d modeling demonstrates that if both source and receivers are over a tabular 3d target 1d modeling predicts the observed response to very high accuracy experimental design can thus be based on 1d analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure modeling in three dimensions shows that the vertical electric field response is largest over the edges of a 3d target the 3d modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by csem sounding a simple air wave model energy propagating from source to receiver via the atmosphere allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range frequency and seafloor resistivity unlike dc resistivity sounding the marine csem method is not completely t equivalent and in principle can resolve resistivity and thickness separately smooth inversion provides an estimate of the method s resolving power and highlights the fact that although the radial csem fields contain most of the sensitivity to the thin resistive target inverted alone they produce only increasing resistivity with depth inclusion of the radial mode csem data forces the recovery of the thin resistor but magnetotelluric data can be used more effectively to achieve the same result

estimating prehistoric earthquake magnitude from point measurements of surface rupture abstract we have developed a method for estimating the magnitude of prehistoric earthquakes using displacement data that usually can be collected from paleoseismic investigations this method is necessary because essentially all current magnitude estimates for prehistoric events rely on determining the total length of coseismic surface rupture which is rarely measurable or rely on segmentation scenarios for which uncertainties cannot be quantified although surface rupture length is a better predictor of magnitude than displacement for historic earthquakes paleoseismic investigations are better at providing measurements of the amount of displacement at a site along a fault the key to our method is to incorporate the variability in displacement observed in 14 modern events which allows a formal uncertainty in magnitude to be assigned to prehistoric ruptures we show how multiple measurements along a preserved fraction of a rupture can be combined to reduce the uncertainty in the estimate of magnitude our analysis shows that uncertainty asymptotically approaches the natural variability of ruptures so 5 to 10 displacement measurements are sufficient to characterize paleomagnitude we conclude that sampling of scarps with lengths of even 10 of the original rupture can provide magnitude values that reasonably estimate the earthquake tests of the method using randomly sampled data from the 1992 m w 7 3 landers and 1954 m s 6 8 dixie valley earthquakes provide close approximations of the actual magnitudes

joint inversion of navigation and resistivity using a fixed transmitter and a towed receiver array a transient marine csem model study a typical marine controlled source electromagnetic system consists of an electric dipole transmitter and one or more electric dipole receivers the objective of a survey is to determine the seafloor resistivity by recording the electromagnetic transients which diffuse through the earth from the transmitter to the receivers accurate knowledge of system geometry is crucial for proper interpretation errors in the position and orientation of the transmitter and or the receivers propagate into errors in the predicted seafloor resistivity we show theoretically that for certain multireceiver set ups and crustal electrical profiles that the geometry and the seafloor resistivity may be determined independently a specific example is an experiment proposed in association with neptune canada here we have already deployed an electric dipole transmitter with a known orientation in a known location a cabled streamer of receivers may be towed by a survey vessel in the vicinity of the transmitter on a known heading for this configuration an eigenparameter analysis of two seafloor models consisting of 1 a halfspace and 2 a resistive layer buried within a halfspace shows that the resistivity structure of the seafloor can be independently resolved from the cable location further studies of these two models also indicate that the position of the streamer must be roughly known in advance on the order of a hundred metres to be used as a suitable starting model in a non linear inversion the crucial information is contained in the parts of the pulse which travel through the seawater and which act as a calibration path such information is absent for a static dc method

estimation of seafloor electromagnetic receiver orientation abstractif the orientation of a seafloor electromagnetic em instrument is unknown it has to be estimated afterwards relatively early in the processing here we present a method to estimate the horizontal and vertical orientation of seafloor electromagnetic receivers three component magnetic recordings of the naturally occurring magnetotelluric field are utilized to find the tilt pitch and azimuth angle relative to a given reference estimation of angles is controlled by minimum difference criteria precautionary measures are necessary to eliminate the subsurface response in the recordings of the em field components the proposed method is applied to synthetic and real data demonstrating that even in difficult geologic settings the simple characteristics of the incident natural em field allow us to quickly estimate the orientation of the instruments

ground penetrating radar investigation of active faults along the dead sea transform and implications for seismic hazards within the city of aqaba jordan abstract ground penetrating radar gpr was used in an effort to locate a major active fault that traverses aqaba city jordan measurements over an exposed trenched cross fault outside of the city identify a radar signature consisting of linear events and horizontal offset flexured reflectors both showing a geometric correlation with two known faults at a control site the asymmetric linear events are consistent with dipping planar reflectors matching the known direction of dip of the faults however other observations regarding this radar signature render the mechanism generating these events more complex and uncertain gpr measurements in aqaba city were limited to vacant lots seven gpr profiles were conducted approximately perpendicular to the assumed strike of the fault zone based on regional geological evidence a radar response very similar to that obtained over the cross fault was observed on five of the profiles in aqaba city although the response is weaker than that obtained at the control site the positions of the identified responses form a near straight line with a strike of 45 although subsurface verification of the fault by trenching within the city is needed the geophysical evidence for fault zone location is strong the location of the interpreted fault zone relative to emergency services military bases commercial properties and residential areas is defined to within a few meters this study has significant implications for seismic hazard analysis in this tectonically active and heavily populated region

rupture of the pitaycachi fault in the 1887 mw 7 5 sonora mexico earthquake southern basin and range province rupture kinematics and epicenter inferred from rupture branching patterns during the 3 may 1887 mw 7 5 sonora earthquake surface rupture end to end length 101 8 km an array of three north south striking basin and range province faults from north to south pitaycachi teras and otates slipped sequentially along the western margin of the sierra madre occidental plateau this detailed field survey of the 1887 earthquake rupture zone along the pitaycachi fault includes mapping the rupture scarp and measurements of surface deformation the surface rupture has an endpoint to endpoint length of 41 0 km dips 70 w and is characterized by normal left lateral extension the maximum surface offset is 487 cm and the mean offset 260 cm the rupture trace shows a complex pattern of second order segmentation however this segmentation is not expressed in the 1887 along rupture surface offset profile which indicates that the secondary segments are linked at depth into a single coherent fault surface the pitaycachi surface rupture shows a well developed bipolar branching pattern suggesting that the rupture originated in its central part where the polarity of the rupture bifurcations changes most likely the rupture first propagated bilaterally along the pitaycachi fault the southern rupture front likely jumped across a step over to the teras fault and from there across a major relay zone to the otates fault branching probably resulted from the lateral propagation of the rupture after breaching the seismogenic part of the crust given that the much shorter ruptures of the otates and teras segments did not develop branches

displacement and geometrical characteristics of earthquake surface ruptures issues and implications for seismic hazard analysis and the process of earthquake rupture there now exist about three dozen historical earthquakes for which in vestigators have constructed maps of earthquake rupture traces accompanied by de scriptions of the coseismic slip observed along the fault strike the maps and slip distributions are compiled here to place observational bounds on aspects of seismic hazard analysis and fault mechanics analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults the observations also give support to the ideas that there exists a process zone or volume of about 3 4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes to this may be added the observation that the form of earthquake surface slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike online material maps of surface ruptures digitized values and curve fits to surface slip distributions and notes and references for tables 1 and 2