a coupled flow stress damage model for groundwater outbursts from an underlying aquifer into mining excavations abstract uncontrolled groundwater outbursts from underlying limestone aquifers into mining excavations present a significant safety challenge for underground coal mining in china although these mining hazards have been known for decades the mechanism for groundwater outbursts remains elusive a fully coupled flow stress damage model is presented to simulate the progressive development of fractures and the associated groundwater flow under incremental loading conditions resulting from mining processes the model is based on classical theories of porous media flow and damage mechanics and importantly links changes in permeability with the accumulation of damage in following the complete stress strain process this coupled flow stress damage model is applied to examine the influence of mining advance on the initiation extension and evolution of an outburst conduit as it develops adjacent to the mine panel fractures are shown to initiate both from the wings of the excavation in shear and from the center of the floor span in extension the growth of the extensile fractures is stunted by the presence of a high stress abutment but the wing fractures extend with one fracture becoming dominant as the dominant fracture develops into the underlying over pressured zone water pressures transmitted along the now open conduit reduce effective stresses and develop rapid heave displacements within the floor the result is a groundwater outburst the modeling is tuned to the results of laboratory experiments and follows the evolution of a viable outburst path observations corroborate with field measurements of permeability pre and post mining and are strong indicators of the veracity of the approach

earthquake induced clastic dikes detected by anisotropy of magnetic susceptibility clastic dikes form either by passive deposition of clastic material into preexisting fissures or by fracturing and injection of clastic material during seismic shaking or passive overpressure because of their similar final geometry the origin of clastic dikes is commonly ambiguous we studied the mechanisms of clastic dike formation within the seismically active dead sea basin where hundreds of clastic dikes crosscut soft rock of the late pleistocene lacustrine lisan formation we analyzed the anisotropy of magnetic susceptibility ams of clastic dikes of known origin and defined characteristic ams signatures of depositional or injection filling we discovered that passively filled dikes which contain brownish silt resembling local surface sediments are characterized by an oblate ams ellipsoid and vertical minimum susceptibility axis v3 dikes that contain green clayey sediment connected to a mineralogically identical detrital layer of the lisan formation are characterized by a triaxial ams ellipsoid well grouped subhorizontal and parallel to the dike walls maximum susceptibility axis v1 and subvertical intermediate susceptibility axis v2 field evidence and ams analysis indicate that most of these dikes were emplaced by injection inferred to be due to seismically triggered fluidization this novel application of the ams provides a petrofabric tool for distinguishing passively filled dikes from injection dikes and where appropriate for identifying the latter as seismites

coherence of seismic body waves from local events as measured by a small aperture array eight local earthquakes were recorded during the operation of a small aperture seismic array at pinyon flat california the site was chosen for its homogeneous granitic geology and its planar topography amplitude spectral ratios for the same signal measured at different stations had average values of less than 2 and maximum values of 7 magnitude squared coherences were estimated for all station pairs these estimates were highest for the p wave arrivals on the vertical component and lowest for the p wave recorded on the transverse component at 500 m station separation the p and s waves were incoherent above 15 hz and 10 hz respectively coherence for both the p and s waves decrease as frequency increases and as distance increases the coherence of signals from borehole sensors located at 300 and 150 m depth displays higher average coherence than equally spaced sites located on the surface the results here suggest that even for sites that appear to be very similar that is those which are located on a planar surface within a few meters of granite bedrock the measured seismic wavefield can be distorted substantially over scale lengths of 500 m coherence properties were calculated from synthetic seismograms which were computed for velocity models with exponential and self similar distribution perturbations standard deviations of 10 are not sufficient for the random velocity distributions to approximate the results from the small aperture array

can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals thesis s m massachusetts institute of technology department of earth atmospheric and planetary sciences 2015

preuplift joints in granites evidence for subcritical and postcritical fracture growth this study relates to jointing in granite that took place in the crust before uplift thus offering an insight into paleo fracture processes our investigation reveals a unique set of joints within granite of the south bohemian pluton exposed in the borsov quarry southeast of prague in the czech republic these joints occur in close proximity and exhibit a great variety of fracture surface morphologies the joints in the borsov granite may be divided into four groups based on fractographic features 1 partial mirror planes without fringes 2 mirror planes rimmed by en echelon fringes 3 complex en echelon fringes and 4 hackled fringes on the basis of theoretical experimental and fractographic data obtained in outcrops we have constructed a curve for the change in crack velocity v as a function of the stress intensity factor k i and we have plotted the various joints in the borsov granite on this curve the spread of the joints along the curve indicates that the joints terminated under widely different physical conditions possibly responding to repeated extensional events and different fracture mechanisms hence a new quantitative method of joint classification is introduced such a great variety of fractographic features is unknown from joint sets that occur in a single quarry in sedimentary rock because their formation always relates to a particular fracture mechanism we argue that the joints in the borsov granite formed before uplift in a fluid enriched environment while the granite responded to diverse local effective stress conditions

a coupled fluid fracture approach to propagation of clastic dikes during earthquakes abstract the propagation of clastic dikes was studied in ami az plain where hundreds of clastic dikes cross cut the soft rock of the late pleistocene lacustrine lisan formation within the seismically active dead sea basin two analytic models were established to estimate the dike propagation velocities the dike emplacement duration and the driving pressures that were associated with dike propagation either under laminar or turbulent flow conditions these models depend on the host rock and fluid properties dike width and length based on these properties and field limitations the results indicate that the dikes were propagated under turbulent flow conditions and propagation velocities of 4 65 m s the emplacement duration of the clastic dikes is between 0 8 and 2 s which is similar to the order of the acoustic wave duration that passes through the soft rock during an earthquake event this implies that the propagation of dikes could form during the passing acoustic waves and may thus be considered as an earthquake induced impact structures the calculated driving pressures that were generated during the earthquake events near the surface are 1 2 mpa one order of magnitude larger than the pressure due to the overburden on the source layer

tidal signals in ocean bottom magnetic measurements of the northwestern pacific observation versus prediction summary motional induction in the ocean by tides has long been observed by both land and satellite measurements of magnetic fields while these signals are weak 10 nt when compared to the main magnetic field their persistent nature makes them important for consideration during geomagnetic field modelling previous studies have reported several discrepancies between observations and numerical predictions of the tidal magnetic signals and those studies were inconclusive of the source of the error we address this issue by 1 analysing magnetometer data from ocean bottom stations where the low noise and high signal environment is most suitable for detecting the weak tidal magnetic signals 2 by numerically predicting the magnetic field with a spatial resolution that is 16 times higher than the previous studies and 3 by using four different models of upper mantle conductivity we use vector magnetic data from six ocean bottom electromagnetic obem stations located in the northwestern pacific ocean the obem tidal amplitudes were derived using an iteratively re weighted leastsquares irls method and by limiting the analysis of lunar semidiurnal m2 lunar elliptic semidinurnal n2 anddiurnal o1 tidalmodestothenight time usinga3 delectromagnetic induction solver and the tpx07 2 tidal model we predict the tidal magnetic signal we use earth models with non uniform oceans and four 1 d mantle sections underneath taken from kuvshinov and olsen shimizu et al and baba et al to compare the effect of upper mantle conductivity we find that in general the predictions and observations match within 10 70percent across all the stations for each of the tidal modes the median normalized percent difference npd between observed and predicted amplitudes for the tidal modes m2 n2 and o1were15percent 47percentand98percent respectively forallthestationsandmodels at the majority of stations and for each of the tidal modes the higher resolution 0 25 0 25 modelling gave amplitudes consistently closer to the observations than the lower resolution 1 1 modelling the difference in lithospheric resistance east and west of the izu bonin trench system seems to be affecting the model response and observations in the o1 tidal mode this response is not seen in the m2 and n2 modes thereby indicating that the o1 mode is more sensitive to lithospheric resistance

study of tidal periodicities using a transatlantic telecommunications cable we report the results of determinations of tidal periods from spectral analysis of the voltage induced across an unpowered length of the tat 7 telecommunications cable in the north atlantic ocean over an approximately 19 day interval particularly prominent spectral components are observed at several frequencies including those identified as corresponding to the diurnal declination luni solar k1 semi diurnal principal solar s2 semidiurnal principal lunar m2 and monthly lunar mm components in addition a large amplitude spectral component of high statistical significance was observed at a frequency of 3 0051 cpd approximately one third the sidereal day this component does not appear to be related to the solar component s3 and possibly corresponds to a declination luni solar k3 frequency although the amplitude is very much larger than that which might be expected other common lunar tides appear to be a minor component of the data such cable data can be used for studies and refinement on a global scale of large scale tidal ocean and ionosphere models

large scale electric field measurements on the earth s surface a review there exist only a few reported measurements of quasi stationary near dc electric potentials over very large spatial scales hundreds of kilometers or more on the earth s surface such measurements have typically been made using unpowered submarine telecommunications cables the measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents in addition there are possible interpretational problems from pervasive poorly understood low frequency electric fields induced by ocean water motion through the earth s stationary magnetic field nevertheless estimates of the magnitude of the electric field computed from large scale potential difference measurements made principally to date in the pacific ocean can be used to place a limit on the size of the toroidal magnetic field at the core mantle boundary under certain conditions on the earth s electrical conductivity profile thus large scale electric potential measurements can serve as an adjunct probe of the earth s dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the earth a review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude

evolution of the stress field near the arava basin located along the dead sea fault system as revealed by joint sets abstract to reveal the development of the stress field near the arava basin located along the dead sea fault system we characterized joint sets that were exposed mainly in the santonian and eocene chalk rock outcropping in the study area in total 2600 strike orientations of single layer joints slj and multilayer joints mlj mirrored four main stress fields 1 hnw j1 326 2 hnnw j2 343 3 hnne j3 033 and 4 hn j4 360 the results showed that the relatively lower values of the fracture spacing ratio fsr and their standard deviations in j1 implied that j1 was formed during the burial stage of the eocene chalk formations the inconsistency between the statistical parameters of the log normal distributions of joint spacing and fsr raised doubt about the applicability of the saturation joint density model in our studied area the joint sets characteristics suggest that the regional hnw operated during the eocene during the miocene the regional stress field changed clockwise to hnnw probably in association with the early stage of transform motion along the dead sea fault in the pliocene pleistocene the hnne and hn local stress fields both operated and were accompanied by a tensional stress field that can be identified in the intensive normal faulting evolved along the western arava basin margins

stability of underground mine development intersections during the life of a mine plan abstract the stability of mine developments is of utmost importance during the planned period of production or the life of a mine plan many canadian underground mines use transverse stoping with delayed backfill to extract tabular ore deposits these methods require access to the orebody through a number of sill drives and cross cuts which link the orezone to the haulage drift hence creating intersections on multiple levels this paper presents the results of a study on the stability of mine development intersections at garson mine of vale in sudbury on canada multi point borehole extensometers mpbx are used to monitor the rock deformations of an intersection as mining activities progress the monitoring results are used to calibrate a multi level flac3d numerical model which has been developed to assess the stability of the intersection it is shown that stope extraction causes a lateral shift to the intersection accompanied by high shear stress in the roof it is also shown that same level mining has stronger influence on the stability of the intersection than lower level mining

impact of oceanic warming on electromagnetic oceanic tidal signals a cmip5 climate model based sensitivity study in contrast to ocean circulation signals ocean tides are already well detectable by electromagnetic measurements oceanic electric conductivities from the coupled model intercomparison project phase 5 cmip5 climate simulations are combined with tidal currents of m2 and o1 to estimate electromagnetic tidal signals and their sensitivity to global warming ninety four years of global warming leads to differences of 0 3 nt in tidal magnetic amplitudes and 0 1 mv km in the tidal electric amplitudes at sea level locally the climate induced changes can be much higher e g 1 nt in the north atlantic in general all studied electromagnetic tidal amplitudes show large scale climate induced anomalies that are strongest in the northern hemisphere and amount to 30 of their actual values consequently changes in oceanic electromagnetic tidal amplitudes should be detectable in electromagnetic records electric and magnetic signals as well as tides of different frequencies contain complementary regional information

electromagnetic induction fields in the deep ocean off california oceanic and ionospheric sources summary a 56 day long measurement of seafloor electromagnetic field fluctuations is separated into parts coherent and incoherent with the horizontal magnetic field at a terrestrial observatory 1700 km distant but at the same geomagnetic latitude least squares methods show that the m2 and n2 ocean tides are significant in the seafloor data but not on land spectra of the remaining data show high seafloor electric to seafloor magnetic field coherences but markedly lower seafloor to land values especially for the horizontal electric field model studies show that the correlated parts of the seafloor spectra are consistent with induction by broad scale ionospheric sources while the uncorrelated parts are explained by a combination of a barotropic long wave continuum and an internal wave component the oceanic electromagnetic field is discussed in terms of its modal structure and wavelength dependence and implications for seafloor magnetotellurics may be profound

extracting ocean generated tidal magnetic signals from swarm data through satellite gradiometry ocean generated magnetic field models of the principal lunar m2 and the larger lunar elliptic n2 semidiurnal tidal constituents were estimated through a comprehensive inversion of the first 20 5 months of magnetic measurements from european space agency s esa swarm satellite constellation mission while the constellation provides important north south along track gradiometry information it is the unique low spacecraft pair that allows for east west cross track gradiometry this latter type is crucial in delivering an m2 estimate of similar quality with that derived from over 10 years of champ satellite data but over a shorter interval at higher altitude and during more magnetically disturbed conditions recovered n2 contains nonoceanic signal but is highly correlated with theoretical models in regions of maximum oceanic amplitude thus satellite magnetic gradiometry may eventually enable the monitoring of ocean electrodynamic properties at temporal resolutions of 1 to 2 years which may have important implications for the inference of ocean temperature and salinity

study of the time space strength relation for mining seismicity at laohutai coal mine and its prediction mining seismicity is akin to an arti cial earthquake caused by mining activities mining seismicity has a long history in chinese coal mines among which laohutai coal mine in fushun city which is located in liaoning province northeast of china is a typical one 1 laohutai mine is located at the south edge of fushun city its geographical coordinates are east longitude 123o54 42 123o58 17 north latitude 41o51 07 41o52 10 the mine is 4 95 km from east to west and about 2 km from south to north so the mine area is about 10 km2

on measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method magnetotelluric sounding reveals a conductive zone beneath the pacific ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone further resolution can be obtained by controlled source electrical methods the simplest of these are the galvanic techniques dipole dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity a viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere the curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere the depth of penetration is of the order of half the transmitter receiver separation magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole modern instrumentation modified for the ocean floor can detect such signals at a range of 20 km at a frequency of about 0 02 hz averaged over several hours

electromagnetic study of the atlantic continental margin using a section of a transatlantic cable the induction of currents in an approximately 1200 km length of the tat 7 transatlantic telecommunications cable by geomagnetic field fluctuations is studied analyses of the induced voltage drop across the essentially geomagnetically east west oriented cable and of magnetic field data obtained at field sites at the cable head tuckerton new jersey and in new hampshire show a very high correlation over a wide frequency range between the induced currents in the cable and the east west magnetic field fluctuations the observations do not correspond to the classical ocean coast effects discussed in the literature the results of the analyses suggest that there are telluric currents flowing in an approximately north south direction off the east coast of the united states the electrical current flow could be primarily confined in the ocean water or channeled in a conductivity structure within the earth s crust and upper mantle if the latter situation exists as may be likely because of the wide range of frequencies over which the effect is seen then the region of the induced current flow could be related to the east coast magnetic anomaly ecma in this case these results would indicate that in addition to being a static magnetic anomaly the ecma is a structure with significant electrical characteristics such as higher conductivity

geoelectric power spectra over oceanic distances we present initial results of simultaneous measurements of the geoelectric potential as measured across two long cable routes in the pacific guam to philippines and california to hawaii separated by 6h in local time and 30 o in latitude over the fre quency range 10 5 to 1 67 x 10 3 hz the power levels tend to be enhanced near local noon along the two routes especially during geomagnetic quiet intervals during the seven days analyzed geoelectric power levels up to 104 mv km 2 km were measured on a geomagnetically disturbed day the power levels at the longest periods 17m to 150m are gen erally found to be largest along the route at lower geomagnetic latitudes guam to philippines when comparisons are made at equivalent local times in con trast the power levels for periods in the hydromagnetic wave region periods 5m tend to be larger at all local times for the higher geomagnetic latitude route the nightside geoelectric power levels along the higher lati tude route tend to increase with increasing geomagnetic activity

orthogonal cross joints do they imply a regional stress rotation abstract orthogonal cross joints extend across intervals between systematic joints in brittle sedimentary strata and abut the systematic joints at about 90 angles these joints typically form a ladder like pattern if viewed on a bedding surface a common interpretation is that orthogonal cross joints define the orientation of the regional stress field during their formation least compressive stress perpendicular to the joints it follows that they indicate a rotation of regional principal stresses by 90 after the formation of the systematic joints using a three dimensional boundary element code poly3d we considered a simple geologic case of vertical systematic fractures developing in horizontal strata under a triaxial remote load with the maximum principal tensile stress being horizontal and perpendicular to the strike of the fractures the intermediate principal stress being horizontal and parallel to the strike of the fractures and the least principal tensile stress i e maximum compressive stress being vertical the results show that the local maximum principal stress is first perpendicular and then parallel to the strike of the systematic fractures as the ratio of fracture spacing to height changes from greater than to less than a critical value when the horizontal remote principal stress ratio the ratio of the intermediate remote principal stress to the maximum remote principal stress under the sign convention of positive for tensile stresses is greater than a threshold value 0 2 thus the fracturing process changes from infilling of systematic fractures to the formation of orthogonal cross fractures this provides an alternative mechanism for the formation of orthogonal cross joints that does not require a systematic rotation of the regional stress field by 90 the critical spacing to height ratio for the local principal stress switch is independent of the least remote principal stress i e overburden it increases nonlinearly with increasing ratio of the horizontal remote principal stresses and decreases nonlinearly with increasing poisson s ratio of the material

joints in the mesozoic sediments around the bristol channel basin abstract analysis has been carried out at four locations on the edges of the bristol channel basin to illustrate the later phases of deformation of a sedimentary basin and to illustrate the control on joint patterns of subtle changes in the stress system the characteristics of the joints are described and influences on joints are determined including the roles of faults folds and beds there is a low coefficient of correlation between joint spacing and bed thickness except in very thin limestone beds which have a high density of joints the lengths and spacings of earlier joint phases are usually greater than those of later phases later joints normally abut against earlier joints the joints abut the latest faults but are not displaced by them so the joints post date the main alpine contraction the joints formed in five main phases during reduction of the alpine stresses phase 1 joints are sub parallel to the regional compression direction 160 180 phase 2 joints are perturbed by faults often curving towards points of stress concentrations along the faults phase 3 joints are sub parallel to the earlier e w striking fold axes phase 4 joints are cross joints and phase 5 joints form polygonal patterns within joint bound blocks phases 2 and 3 do not occur in the absence of faults and folds and correspond with a reduction in horizontal compression and an increase in the importance of local factors phases 4 and 5 occur at all locations

geomagnetic induction on a transatlantic communications cable the phenomenon of geomagnetically induced currents in long conductors has been recognized since the mid nineteenth century when the use of the telegraph became prevalent in north america and europe1 powering systems of telegraph and telephone cables are now engineered to eliminate large current surges although the occasional significant geomagnetic disturbance such as those produced by the february 19582 august 19723 flare events can still be a problem however induced voltages can be used with complementary geophysical and geological data to deduce features of the earth s crust and upper mantle13 14 improved scientific understanding of mag netospheric phenomena means that the geomagnetic induction process in long conductors and thus in the earth are better understood for example the cable outage problem associated with the august 1972 geomagnetic storm interval was qualitatively attributed not to the line currents associated with normal auroral current systems but rather to the significant enhanced currents flowing on the magnetosphere boundary at the time of an extreme compression of this boundary in greatly enhanced solar wind conditions3 we have measured the geomagnetic induction phenomenon on modern day long cables to get better assessments of the induced currents in various types of geomagnetic variations our initial work was concerned with induction in the transatlantic telecommunications cable no 6 tat 6 which is laid between green hill rhode island 41 4 n 71 7 w geographic and st hilaire de riez france 46 7 n 1 9 w 4 we concentrate quantitatively here on induction by the solar induced quiet day currents sq in the ionosphere and find that calculated values agree well with observed values of sq

why no anomaly is visible over most of the continent ocean boundary in the global crustal magnetic field abstract it is generally believed that an anomaly should be expected over the continent ocean c o boundary in the global magnetic anomaly maps however such maps prepared at satellite altitude do not show an anomaly over most of the c o boundary we address this issue by a forward modelling technique we use a geographic information system gis technique to integrate information of the various geological units of continents and oceans the seismic crustal structure and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility vis model in addition a remanent magnetisation model is used for the oceanic crust combining the induced and remanent magnetisation models the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16 80 the results show that an anomaly is indeed not predicted over the c o boundaries where the oceanic crust is flanked by continental regions phanerozoic in age we demonstrate that largely a c o anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1 80 confirming that it is not a long wavelength feature masked by the core field an anomaly however is predicted over some parts of the c o boundary where the flanking geological provinces over the continents are precambrian in age we argue that the vis values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the c o boundary

induction of currents in long submarine cables by natural phenomena the influence of induced currents on long submarine cables has been observed for nearly a century the currents originate from time variations in the magnetic field of external origin magnetospheric atmospheric tides and earth and ocean tides steady earth currents of internal origin have also been proposed as a possible source the induced currents will depend upon the length depth and geomagnetic location of the cables present understanding of the phenomena is reviewed also discussed are potential scientific problems which might be addressed with data from such long cables and possible limitations on the information obtained such problems include studies of 1 the deep conductivity structure of the earth 2 telluric currents flowing in mid oceanic ridges 3 the spatial extent of external source fields and 4 a possible role of electromagnetic eddy currents in fluctuations of the earth s spin rate

determination of actual limit angles to the surface and their comparison with the empirical values in the upper silesian basin czech republic abstract this study deals with the determination of the actual limit angles in the long term formed subsidence basin in the czech part of the upper silesian basin the remaining part lays in poland here there was the most extensive underground exploitation of black coal in the north east of the czech republic a limit angle is absolutely decisive in the determination of deep mining influences in relation to the current and future utilisation of this territory the two csm and paskov mine localities on the edge of the subsidence basin were selected for this study since they could not have been affected by mining in neighbouring mines on the grounds of the long term levelling monitoring of points on the surface which started prior to mining the horizontal distance between the mined out area and the undisturbed ground surface was identified the impact of limit angle on the surface was calculated by means of the horizontal distance and the established mining depth such identified actual limit angles reached the values of 36 at the csm mine and 25 for the paskov mine the calculated values are significantly lower when compared with the currently applied empirical values of 57 for the csm mine and 56 for the paskov mine this means that the extent of the undermining effects is unambiguously wider by 611 m at the csm mine and by 883 m at the paskov mine this study thus establishes the existence of a significantly vaster area influenced by coal deep mining than previously expected the identified facts lead to the conclusion that regular revisions and follow up updates of the empirically determined parameters are recommended

3 d modelling the electric field due to ocean tidal flow and comparison with observations 1 the tidal motion of the ocean water through the ambient magnetic field generates secondary electric field this motionally induced electric field can be detected in the sea or inland and has a potential for electrical soundings of the earth a first goal of the paper is to gain an understanding of the global distribution of the electric signal due to tidal ocean flow we simulate the electric signals for two tidal constituents lunar semidiurnal m2 and diurnal o1 tides we assume a realistic earth s conductivity model with a surface thin shell and 1 d mantle underneath simulations demonstrate that in some coastal regions the amplitudes of the electric field can reach 100 mv km and 10 mv km for m2 and o1 tides respectively the changes of lithosphere resistance produce detectable changes in the tidal electric signals we show that our predictions are in a good agreement with observations

relative fracture velocities based on fundamental characteristics of joint surface morphology two different joint surface morphologies plumes and rib marks characterize joint surfaces but the mechanical conditions that lead to the formation of either of the morphologies are not understood well we studied two orthogonal joint sets that cut the same santonian chalk beds in the judea desert israel joints of the j1 set are systematic relatively long characterized by almost exclusively by plumes and predate the shorter non systematic joints of the j2 set that are characterized by rib marks joints of the j1 set formed at high stress during deformation of the syrian arc folding in the late senonian joints of the j2 set formed at lower stress that occurred because of stress relaxation after the formation of the j1 joints a mechanical analysis indicates that the j1 joints propagated at subcritical velocities several orders of magnitude faster than the j2 joints based on previously published data of laboratory tests the plumes and the rib marks are semiquantitatively placed on the subcritical part of the fracture velocity vs stress intensity factor diagram

the east coast magnetic anomaly a model is proposed for the east coast magnetic anomaly that for the first time incorporates an observed basement feature a normal fault that consistently lies below the landward flank of the anomaly the east coast magnetic anomaly is similar to slope anomalies found over passive continental margins in many parts of the world thus the proposed model can be used to interpret slope anomalies in general

analogue simulation of natural orthogonal joint set formation in brittle varnish abstract two orthogonal joint sets are commonly observed in exposed sedimentary rocks with a wide variety of abutting and cross cutting relationships brittle varnish analogue models are carried out in conjunction with field studies in order to classify the different orthogonal fracture patterns and constrain the mechanical basis of orthogonal joint development the results suggest that the stress which creates the second joint set can result from i relaxation effects ii slight tension due to warping of the bands defined by the first parallel fractures and iii local and or regional reversals between 2 and 3 a ladder pattern formed by the combination of an initial set of long parallel joints and associated non cross cutting joints of the second set is obtained if the shear strength of the initial joints is low during the development of the second set a grid pattern where both sets mutually cross cut occurs when the shear strength of the initial joint set is high possibly the result of a high normal stress or healing and could result from two independent stress events an intermediate pattern comprising cross cutting and abutting orthogonal cross joints can form if the shear strength of the initial joints was intermediate and or variable during the development of the second set mutually abutting joint sets are observed within each pattern and could result from stress reversals or low differential stress during the final stages of joint development the classical presentation of joint data using rose diagrams cannot distinguish between the wide variety of orthogonal joint patterns maps of fracture intersections should compliment the orientation data we discuss methods to estimate the joint pattern where intersection data are absent

ground movements over longwall workings in the kamptee coalfield india abstract subsidence investigations were conducted over three superimposed longwall panels in the kamptee coalfield of central india the extracted contiguous seams were lying below a water bearing unconfined aquifer filling of voids with sand hydraulically was practised to avoid a sudden inrush of water into the under ground workings through ground movement induced cracks the non effective width of extraction varied between 0 55 and 0 61 times the thickness of intact rockmass in the overburden the maximum subsidence slope compressive and tensile strains were 9 of extraction thickness 6 1 6 6 and 5 3 mm m 1 respectively the angle of draw was influenced by the presence of a fault and composition of superincumbent strata the ground movements lowered water table by 6 5 m in the unconfined aquifer due to the mining

joint nucleation in layered rocks with non uniform distribution of cavities abstract characterization of critical flaws that initiate into systematic joints is essential for understanding the process of fracturing in sedimentary rocks in this study advantage is taken of the well developed surface morphology of joints in the dolomite layers of the judea group central israel and the role played by spherical cavity shaped flaws during nucleation and growth of joints is analyzed an analysis of cavity geometry and distribution reveals that joint initiation points are governed by such isolated relatively large cavities preferably located close to the bedding top and by stress gradients during joint nucleation in the absence of macroscopic cavities joint initiation points are located along bedding interfaces consequently joints typically show two forms of growth depending on the abundance and spatial distribution of cavities within the layers in layers with plentiful cavities joints preferably nucleate at critical cavities propagate vertically toward the bedding interfaces and horizontally toward adjacent joints and form elliptical fractures in layers free of cavities joints nucleate at bedding interfaces commonly propagate downward toward the layer base and adjacent joints and form semi elliptical fractures in both cases bedding interfaces between the dolomite layers impedes joint propagation fractographic methods based on joint surface morphology yield satisfactory first order estimates of the driving paleostress required to initiate joints in the judea group

florida a jurassic transform plate boundary magnetic gravity seismic and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern florida during the jurassic on the basis of this integrated suite of data the pre cretaceous florida bahamas region can be divided into the pre jurassic north american plate jurassic marginal rift basins and a broad jurassic transform zone including stranded blocks of pre mesozoic continental crust major tectonic units include the suwannee basin in northern florida containing paleozoic sedimentary rocks a central florida basement complex of paleozoic age crystalline rock the west florida platform composed of stranded blocks of continental crust the south georgia rift containing triassic sedimentary rocks which overlie block faulted suwannee basin sedimentary rocks the late triassic jurassic age apalachicola rift basin and the jurassic age south florida bahamas and blake plateau marginal rift basins the major tectonic units are bounded by basement hinge zones and fracture zones fz the basement hinge zone represents the block faulted edge of the north american plate separating paleozoic and older crustal rocks from jurassic rifted crust beneath the marginal basins fracture zones separate mesozoic marginal sedimentary basins and include the blake spur fz jacksonville fz bahamas fz and cuba fz bounding the blake plateau bahamas south florida and southeastern gulf of mexico basins the bahamas fz is the most important of all these features because its northwest extension coincides with the gulf basin marginal fault zone forming the southern edge of the north american plate during the jurassic the limited space between the north american and the south american african plates requires that the jurassic transform zone connecting the central atlantic and the gulf of mexico spreading systems was located between the bahamas and cuba fz s in the region of southern florida our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the gulf of mexico southern florida and the bahamas indicate that the gulf was sealed off from the atlantic waters until callovian time by an elevated florida bahamas region restricted influx of waters started in callovian as a plate reorganization and increased plate separation between north america and south america africa produced waterways into the gulf of mexico from the pacific and possibly from the atlantic

ocean cable measurements of the tsunami signal from the 1992 cape mendocino earthquake the movement of the seawater across the earth s magnetic field produces a large scale motional electric field using the point arena california to hanauma bay hawaii unpowered haw 1 cable we have studied the geopotential across this distance to look for possible tsunami induced fields that might have been produced following the april 1992 cape mendocino earthquake we have used a ten day interval prior to and including the earthquake as a reference for geopotential signals and for geomagnetic activity we have also used geomagnetic data from point arena honolulu and boulder as reference data the results of the analyses show that there are tsunami related effects in the cable geopotential data these are a larger voltage prediction errors residuals for the interval following the main shock b enhanced compared to the 10d reference interval geopotential spectral power following the main shock two enhancements are larger than geomagnetically induced spectral power enhancements in the same time interval and c strong evidence for an 30 min echo in the cable geopotential signal following the main shock

satellite observations of magnetic fields due to ocean tidal flow the ocean is an electrically conducting fluid that generates secondary magnetic fields as it flows through earth s main magnetic field extracting ocean flow signals from remote observations has become possible with the current generation of satellites measuring earth s magnetic field here we consider the magnetic fields generated by the ocean lunar semidiurnal m2 tide and demonstrate that magnetic fields of oceanic origin can be clearly identified in satellite observations

growth of exfoliation joints and near surface stress orientations inferred from fractographic markings observed in the upper aar valley swiss alps abstract granitic rock mass of the upper aar valley grimsel area switzerland contains distinct generations of exfoliation joints which formed during different stages of the pleistocene subparallel to distinct glacial valley palaeotopography the bulk of exfoliation joints shows prominent common fractographic features 1 radial plumose structures with distinct plume axes 2 arrest marks superimposed by plumose striations and 3 gradually developing en echelon fringe cracks multiple arrest marks reveal that exfoliation joints formed incrementally and together with the absence of hackle fringes suggest stable i e subcritical fracturing conditions smooth transitions from plumose structures on the parent plane to en echelon fringe cracks combined with non systematic stepping senses of fringe cracks suggest local vs temporal stress field variations assuming that plume axes formed parallel to the maximum principal compressive stress 1 enables us to infer near surface palaeostress orientations and compare them with classical borehole based in situ stress data the majority of plume axes suggest 1 persistently subhorizontal to slightly inclined 1 orientations at trough valley slopes and 2 near surface variability of 1 orientations originating from topographic perturbation caused by glacial valley erosion superimposed on the regional stress field our investigations of fracture surface morphologies yield unique insights into exfoliation fracture formation such as directional trends of fracture propagation and associated palaeostress orientations within alpine valley slopes

lithosphere conductivity structure using the cam 1 lisbon madeira submarine cable summary voltage measurements made in the cam 1 submarine cable located between sesimbra lisbon and madeira island have been used in combination with magnetic observations carried out in the geomagnetic observatory of guimar canary islands to estimate the deep geoelectrical structure beneath the ocean apparent resistivities and phases were calculated from those data sets published geomagnetic deep sounding results were used to correct the cam 1 response for static shift distortion magnetic transfer function tipper and invariant apparent resistivity and phase obtained from the determinant of the impedance tensor z det on shore in sw iberia were used in the modelling in order to better constrain some model parameters the results obtained by 1 d occam s inversion and 2 d trial and error modelling reveals a resistive zone 300 1200 m between 10 and 200 km depth followed by a decreasing in resistivity with depth the order of the integral resistivity of the lithosphere beneath the ocean was found lying in the 9 10 7 6 10 8 m 2 range the estimated depth conductivity profile was compared with those obtained from studies in the pacific ocean indicating that the atlantic lithosphere between lisbon and madeira is more resistive than that in the pacific the most probable explanation for such observation is the long term stability or very low activity of the sampled atlantic region a comparison of the depth conductivity profile with published theoretical results based on olivine properties shows that our result is compatible with an isotropic and undersaturated mantle

reliability of hoek brown estimates of rock mass properties and their impact on design this technical note examines the reliability of a slope stability calculation and a tunnel support design calculation

ocean tidal signals in observatory and satellite magnetic measurements 1 ocean flow moves sea water through the earth s magnetic field inducing electric fields currents and secondary magnetic fields these motionally induced magnetic fields have a potential for the remote sensing of ocean flow variability a first goal must be to gain a better understanding of magnetic field generation by tidal ocean flow we predict the motionally induced magnetic fields for the six major tidal constituents and compare their amplitudes with the spectra of night time observatory and satellite magnetic measurements for the indian ocean the magnetic variations at the solar s2 kl and p1 periods turn out to be dominated by unrelated external fields in contrast observed lunar m2 and n2 tidal signals are in fair agreement with predictions from motional induction the lunar diurnal o1 signal visible at some observatories could be caused by ocean flow but disagrees in amplitude with our predictions

on the electrical conductivity of the oceanic lithosphere abstract below the bottom of the sediments the oceanic lithosphere is difficult to sound electromagnetically because the resistivity is high compared to over and under lying structures there are three methods which can contribute information 1 1 magnetotelluric methods provide the impedance as a function of frequency for signals of sufficiently low frequency to penetrate through the ocean in the deep sea the upper frequency limit is a few cph 2 2 a quasi static electric field can be measured at the ocean floor the field is generated by charges built up by the motion of ocean water currents through the geomagnetic field by providing a current path below the ocean the lithospheric conductivity tends to reduce this field in order to interpret the measurements one needs to know the spatial character of the water motions 3 3 an artificial em source which transmits at frequencies near 1 hz can be used at the ocean floor energy propagation through the lithosphere has been detected on the ocean floor to horizontal distances of 18 km detection is aided by the very low noise level at the high frequencies concerned estimates of the typical conductivity distribution throughout the ocean crust are given