thermal stresses differential subsidence and flexure at oceanic fracture zones geosat geoid undulations over four pacific fracture zones have been analyzed after correcting for the isostatic thermal edge effect the amplitudes of the residuals are shown to be proportional to the age offset the shape of the residuals seems to broaden with increasing age both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults existing models for flexure at fracture zones cannot explain the observed anomalies a combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented our model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone the best fitting models have mechanical plate thicknesses that are described by the depth to the 600 700 c isotherms

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

on the strength of oceanic fracture zones and their influence on the intraplate stress field we use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere if fracture zones are weaker than the surrounding lithosphere they should be sites of concentrated intraplate seismicity and the lithospheric stress field should be perturbed in their vicinity the 77 earthquakes selected for the study have well determined focal mechanisms and epicenters in regions where fracture zones are well mapped we have searched for dependence of faulting style fault orientation or principal stress direction on the distance from the nearest fracture zone if fracture zones were generally weaker than the surrounding lithosphere one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner there is a slight tendency for earthquakes to occur near fracture zones and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress these patterns are not demonstrable at high significance however if oceanic transform fault zones are weak relative to the surrounding lithosphere these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere

an evaluation of seismic decoupling and underground nuclear test monitoring using high frequency seismic data an effective solution to the problem of the detection and identification of low yield coupled and fully decoupled underground nuclear explosions appears available via use of high frequency seismic data ranging up to 30 or 40 hz in order to evaluate detection identification capabilities when using such data it is necessary to estimate 1 spectral characteristics and relative amplitudes of both p and s waves from explosions and earthquakes over the frequency band from 5 to 40 hz 2 signal transmission characteristics over this band through pertinent types of earth structure and 3 recording system and ground noise characteristics over this frequency band in this study each of these topics is considered in turn as they relate to detection and discrimination of the signals from low yield coupled and decoupled explosions in the regional and teleseismic distance ranges estimates of the capabilities of specific hypothetical networks to detect and identify insofar as signal to noise ratio is an important factor in identification explosions within the ussr are then considered these estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability following the assessment of detection capabilities the problem of identification of small events is considered with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal to noise ratio the principal results and conclusions of this study are as follows 1 seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30 40 hz when using presently available hardware 2 average amplitudes of high frequency noise in a variety of geological environments are very low and change little with time or season 3 transmission of high frequency p and s wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 hz with effective q factors in shield areas being about 9000 and 4000 for high frequency pn and sn respectively while the effective q for pn waves in tectonic areas is about 1000 4 a properly designed and deployed network of 25 simple three component nonarray stations internal to the ussr and 15 similar stations surrounding the ussr is predicted to be capable of multistation detection at high signal to noise ratio of fully decoupled 1 kt explosions located at all potential decoupling sites within the ussr 5 by inference from the quantitative agreement of empirical observations and theoretical predictions when using lower frequency data over a great range of explosion yields we conclude that procedures based on the use of both detectable p and s waves will serve to identify explosion generated seismic signals at least as small as those expected from a fully decoupled 1 kt explosion

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

application of nuttli s method to estimate yield of nevada test site explosions recorded on lawrence livermore national laboratory s digital seismic system methods utilizing lg waves for estimating yields of underground nuclear explosions have attracted considerable attention for their accuracy claims of portability and suitability for low yield test ban verification in this study the method developed by nuttli is applied to a large dataset of nevada test site explosions recorded on livermore s digital seismic system the purpose was to check the reproducibility of nuttli s results and to verify the accuracy of mb lg to estimate yield at high and low yield levels portability of the method is examined from the perspective of variability of the magnitude yield relations as a function of station site and source geology the results of this study show accuracies of about a factor of 1 6 in yield at the two sigma level for shots below the water table in source regions exhibiting more or less uniform coupling at nevada test site there are three such source regions pahute mesa areas 19 and 20 northern yucca including areas 2 and 9 and southern yucca including areas 3 4 and 7 there is no apparent degradation in accuracies at low yields 1 to 10 kt variations in source coupling were observed as a function of shot location and as a function of the medium s gas porosity for shots in tuff the effect of gas porosity was not observed for shots in alluvium which have uniformly low coupling compared to shots in tuff variations in magnitude yield relations were observed from station to station while the four station network average magnitudes are more robust and agreed well with nuttli s magnitude yield relations thus portability to other test sites should be considered guardedly optimistic requiring geologic and seismic data related to coupling in each source region and sufficient numbers of stations four or five to estimate a robust network magnitude taken all together the findings of this study lend support to this method as a viable means of monitoring a threshold test ban or low yield threshold test ban

a comparison of regional phases from underground nuclear explosions at east kazakh and nevada test sites abstract analysis of regional data from east kazakh ussr and nevada test site nts underground nuclear explosions suggests possible mechanisms for the generation of both low and high frequency lg spectral characteristics of pn and lg phases from 25 soviet nuclear shots recorded at the chinese digital seismic network cdsn station wmq are analyzed for their dependence on parameters such as m b generally related to shot depth and spatial location shagan versus degelen test sites the observed results are compared with those from the nevada test site nts where the near source conditions are better known the average amplitude ratio pn lg is found to be stable with m b but to vary strongly with frequency for both shagan and yucca flat explosions of similar yield the reduction in amplitude over the frequency range of about 1 to 5 hz is considerably larger for lg than for pn the low frequency lg appears to be generated by the scattering of explosion generated rg into s especially for the shagan river explosions for which good agreement between observation and theory is observed this mechanism also provides a possible explanation for the stability of lg as an estimator of explosive yields because the scattering should be expected to occur over a large region of the shallow crust another important result supported by theory is that the higher frequency lg is strongly dependent on the source medium velocity at higher frequencies 3 to 7 hz the amplitude ratios pn lg for explosions from shagan degelen pahute mesa and yucca flat test sites show significant differences that appear related to differences in their source medium velocities the observed large differences in the relative spectral content of pn and lg from east kazakh and nts explosions are mainly due to the vast differences in the crustal structure of the two test sites

evolution and strength of pacific fracture zones previous studies have shown that pacific fracture zones are strong in some locations sustaining the stresses associated with differential subsidence across a locked fault while at other locations they show signs of weakness acting as preferential conduits for volcanism or supporting anomalously low shear stresses we find that half of all geosat crossings of central pacific fracture zones are inconsistent with a flexurally maintained scarp we test two hypotheses for the origin of such anomalous crossings 1 anomalous structures formed at the transform fault during times of changes in plate motions and preserved at strong fracture zones and 2 anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity we find that three quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions with the exception of several locations overprinted by hot spot volcanism these same crossings show no obvious correlation with regional tectonic processes these observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

numerical investigation of lake bed seepage patterns effects of porous medium and lake properties abstract three dimensional steady state numerical models were used to investigate the relative significance of several factors controlling lake bed seepage patterns lake depth lake bed slope orientation of an asymmetric lake with respect to a regional hydraulic gradient lake bed sediments and heterogeneity and anisotropy of the porous medium we considered both inflow and flow through lakes and our focus was on the details of seepage flux at the lake bed not on the surrounding porous medium while porous medium factors anisotropy and heterogeneity are important we found several conditions where lake bed factors were nearly as important in controlling the distribution of seepage varying lake bed slope from 0 013 to 0 04 in different simulations had a significant effect on shoreline seepage rates were 10 40 higher for lakes of low slope than lakes of steep slope also significantly elevated seepage in some cases a local maximum was observed offshore at the break in bed slope between the sloping side and flat central portions of the lake bed whenever the surrounding porous medium had a high anisotropy 1000 or 100 for flow through lakes in media of high anisotropy the annual volume of groundwater inseepage was significantly higher about 20 in lakes with steep bed slope compared to those with low slope this effect of slope was smaller at lower anisotropy for an asymmetric flow through lake a lake with a steep bed slope on one side moderate slope on the other the percentage of lake bed experiencing inseepage was greatest when the steep side was downgradient and the effect was larger at higher anisotropy these effects illustrate the complex interaction between lake bed slope and the anisotropy of the surrounding porous medium in controlling lake groundwater exchange adding low conductivity lake sediments and decreasing their conductivity shifted groundwater seepage further offshore in inflow lakes increasing the anisotropy of the surrounding porous medium had the same effect adding sediments and increasing anisotropy also decreased nearshore seepage in flow through lakes but without increasing offshore seepage in this case the net effect was a smaller annual volume of lake groundwater exchange at the same time the percentage of the lake bed experiencing inseepage increased in flow through lakes even as the annual volume of inseepage was decreasing thus for flow through lakes a larger area of inseepage may not be a good indicator of a greater volume of inseepage lake depth did not have a significant effect on the quantity or distribution of seepage to inflow or flow through lakes many of the physical factors investigated here influence the amount of lake groundwater exchange and the proportions of nearshore and offshore seepage therefore they are potentially significant to lake water quality and ecology in addition to hydrology

regional seismic recordings of the soviet nuclear explosion of the joint verification experiment during august and september 1988 the united states and the soviet union conducted a joint verification experiment in which each country was permitted to observe a nuclear explosion at the other country s test site using hydrodynamic means we have seismically recorded the soviet explosion at four stations at regional distances 750km and here report those observations the peak to peak particle motion in the p wave varied from 7 54 10 1 cm sec at 160 km to 1 02 10 2 cm sec at 740 km the average mb lg was 5 97 corresponding to a yield of 118 kt based on the nts mb lg yield relation of patton 1988 the average pn corner frequency of the soviet explosion was 4 hz with evidence of a second corner at about 0 7 hz both of these corners could be affected by spall near regional seismograms of the soviet jve explosion show a large arrival within the lg phase on the transverse component at all four sites this may correspond to tectonic release accompanying the explosion or to the effects of anisotropy on seismic wave propagation

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

modeling of three component lg waves in anisotropic crustal models abstract lg wave synthetic seismograms are presented in order to analyze the influence of anisotropy in the continental crust on their three component waveforms the effect of dry and liquid filled open cracks in the upper crust and petrological anisotropy in the lower crust on waves produced by surface explosions and crustal earthquakes is examined the polarization and especially the amplitude of the transverse component proves to be very sensitive to introduction of weak anisotropy in the crust comparisons of the modeling with lg wave data produced by explosions in southern scandinavia show that a realistic crustal anisotropy can explain observations that the transverse components of these waves are as large as their vertical ones a 10 km thick surficial layer with liquid filled open cracks is our perferred model for southern scandinavia

geophysical constraints on the shear stress along the marquesas fracture zone we examine bathymetry and gravity observations acquired by surface ships and deflection of the vertical data derived from the geosat radar altimeter for the purpose of determining if the bathymetric relief along the marquesas fracture zone in the central pacific ocean is caused by differential thermal subsidence across a locked fault if the fracture zone can maintain between 20 and 40 mpa of shear stress we would predict that a ridge and a parallel trough would develop to the south and north respectively of the fracture zone on account of flexural modification of the step in bathymetry locked into the plate along the fracture zone at the ridge transform intersection we find that only 2 out of 32 deflection of the vertical profiles from the geosat altimetric mission are consistent with this model while none of the 27 bathymetric profiles or 9 shipboard gravity profiles fits the theoretical predictions in cases where bathymetry and gravity can be modeled simultaneously it appears that the gravity anomaly is simply not large enough in magnitude to be consistent with the moho flexing conformably with the observed bathymetry as required if the relief is produced by deeper thermal variations across the locked fracture zone rather the potential field data allow shear stresses only 25 of those required to lock the fault and are more consistent with a model in which the relief on the moho is opposite in sign to that of the surface topography as would be produced by local or regional compensation of surface loads arising from volcanic activity along the fracture zone thus we conclude that the marquesas fracture zone in contrast to those other pacific fracture zones which fit the predictions of the locked fault high stress model may be unusually weak along almost its entire length

comparison of seismic and hydrodynamic yield determinations for the soviet joint verification experiment of 1988 abstract seismic magnitudes determined from surface and body waves for the soviet underground nuclear explosion of september 14 1988 are used to calculate the yield of that event from previously derived calibration curves the yield obtained by combining the two seismic estimates is 113 kilotons which is very close to those obtained by hydrodynamic measurements made on site this comparison substantiates previous conclusions about the sizes of past soviet weapons tests and compliance with the threshold test ban treaty the factor of uncertainty in the combined seismic yield is 1 28 at the 68 and 1 62 at the 95 confidence levels demonstrating that accuracies considerably better than a factor of 2 can be obtained by combining seismic determinations of yield

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

lg magnitudes of selected east kazakhstan underground explosions abstract the amplitude of 1 sec period lg waves as recorded on seismograms of wwssn stations in southern asia and scandinavia are used to determine m b lg values for 62 underground nuclear explosions in the eastern portion of the east kazakhstan test site the explosions were selected to be of m b p 5 5 or greater in the period 1965 to 1985 the yields of the explosions are estimated assuming that a nevada test site derived relation between m b lg and yield for water saturated rock is also applicable to the east kazakhstan test site assuming that lateral variations in crustal structure and in depth of emplacement of the explosion do not affect m b lg values the lg data indicate that the m b p bias between the nevada test site and east kazakhstan is 0 35 magnitude units

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

studies of high frequency seismic noise in eastern kazakhstan abstract three seismographic stations have been established in the region of the principal underground nuclear weapons test site in the republic of kazakhstan in the soviet union instrumentation includes both surface and borehole seismometers with a bandwidth of 100 hz and a system dynamic range of 180 db power spectra of the regional ambient ground acceleration peak between 4 and 6 hz and are on average frequency independent above 10 hz where levels are approximately equal to 10 15 to 10 16 m sec 2 2 hz 1 kazakh noise levels are roughly equivalent to those observed by the regional seismic test network between 1 and 20 hz and about 10 to 20 db higher than the very low noise model for lajitas texas in the same band noise observed by the borehole sensors is about 10 db less than that observed by the surface instruments above about 20 hz under low wind conditions at two of the three kazakh sites borehole improvement is greater under high winds the third kazakh site karasu is characterized by much higher surface noise levels 15 to 20 db higher than the borehole levels from 4 to 40 hz under both low or high wind conditions which is attributed to amplification by local site conditions and the exposed terrain power spectra of individual runs of ambient vertical ground acceleration measured down the borehole show that high frequency slopes between 10 and 80 hz vary between f 0 60 and f 0 22 depending on wind and site conditions

morphostructure of an incipient subduction zone along a transform plate boundary puysegur ridge and trench multibeam bathymetric and geophysical data reveal a major strike slip fault that extends along the summit of the puysegur ridge east of the puysegur trench the northward structural development of this ridge trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m y at the southern end of the trench where subduction has not yet started the puysegur ridge has a narrow 50 km steep sided cross section and the axial strike slip fault separates a shallow 125 625 m flat topped eastern crest from a deeper 400 1600 m western crest these characteristics indicate differential uplift during the initial stage of shortening on the lower plate an incipient 5 2 km deep trench developed in conjunction with normal and reverse faults suggesting strong interplate coupling across the trench northward the ridge broadens linearly to 80 km wide its western flank has locally collapsed and the ridge summit has subsided possibly by 1 5 km suggesting that the interplate coupling decreases and that a benioff zone is being formed concomitant to the northward ridge evolution the trench deepens to 6 2 km and normal fault throws increase along its outer wall indicating greater flexure of the downgoing plate

crustal structure in eastern kazakh u s s r from teleseismic receiver functions broadband receiver functions determined from teleseismic p waveforms recorded at two seismic stations in eastern kazakh u s s r are inverted for the vertical velocity structure beneath the stations the detailed broadband receiver functions are obtained by stacking source equalized radial components of teleseismic p waveforms a time domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat lying homogeneous layers the general features of the inversion results are a complex shallow crust velocities less than 6 km s in the upper crust a high velocity 6 9 7 5 km s lower crust and a moho that varies between 47 and 57 km depth these results compare favorably with a composite velocity model from deep seismic sounding data but show lower velocities in the upper crust and higher velocities in the lower crust the results indicate that the crust in this region is relatively uniform however the nature and depth of the moho changes significantly across the region the receiver function inversion structure for the kazakh sites is similar to a published receiver function structure for a site on the canadian shield

comment on numerical investigation of lake bed seepage patterns effects of porous medium and lake properties by genereux d and bandopadhyay i 2001 journal of hydrology 241 286 303

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

thin crust beneath ocean drilling program borehole 735b at the southwest indian ridge abstract a wide angle seismic experiment at the atlantis ii fracture zone southwest indian ridge together with geochemical analyses of dredged basalt glass samples from a site conjugate to ocean drilling program hole 735b has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735b the measured na8 composition of 3 3 0 1 corresponds to a melt thickness of 3 1 km a result consistent with rare earth element inversions which indicate a melt thickness of between 1 5 and 4 5 km the seismic crustal thickness to the north and south of the atlantis platform on which hole 735b is located is 4 1 km and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty beneath hole 735b itself the moho is at a depth of 5 1 km beneath the seafloor the seismic model suggests that on average about 1 km of upper crust has been unroofed on the atlantis platform however allowing for the inferred local unroofing of 2 km of upper crust at 735b the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor and is underlain by a 2 3 km thick layer of serpentinised mantle peridotite the p wave velocity of 6 9 km s for the serpentinised peridotite layer corresponds to a 35 10 vol serpentine content the moho beneath hole 735b probably represents a serpentinisation front

tectonic release from the soviet joint verification experiment the regional broadband seismic recordings of the soviet joint verification experiment underground nuclear explosion show evidence of tectonic release in the form of love waves at long periods and high frequency s sub m s phases on the transverse components the authors present the results of some simple forward modeling of this data in order to quantify both the moment and mechanism of the tectonic release using 10 20 second love wave amplitude and phase data they estimate the tectonic release moments to be 3 5 times 10 sup 15 n m for a north striking right lateral strike slip fault and 7 0 times 10 sup 15 n m for a northwest striking reverse fault using the results of these end member tectonic models the rayleigh waves can be satisfied with explosion moments of 1 7 times 10 sup 16 n m f 0 31 and 2 4 times 10 sup 16 n m f 0 44 respectively to resolve the ambiguity in tectonic mechanism the authors used the reflectivity method to generate synthetics and compare the relative amplitudes of the sh and sv parts of the s sub m s phase while the results favor the strike slip mechanism of tectonic release application of source inversion techniques to this data and other regional data will be needed to evaluate more realistic tectonic releasemore models involving for example oblique fault slip and dip less

three dimensional groundwater flow to a shallow pond an explicit solution summary steady groundwater flow near a shallow disk shaped pond the bottom of which is in direct hydraulic contact with a confined homogeneous aquifer is studied analytically by the fabrikant method of l operators for the case of a unidirectional flow at infinity a rigorous and explicit expression for the hydraulic head and specific discharge components are obtained depending on the ambient specific discharge aquifer conductivity pond radius and the pond bottom head three regimes gaining losing and flow through of pond water groundwater interaction are explicitly demarcated the hinge line on the pond bottom for the flow through regime is shown to be a straight line the only stagnation point in the gaining regime is found from a simple nonlinear equation closed form expressions for the total pond seepage rate and inflow outflow rates are presented streamlines and isochrones of groundwater travel time are found by a particle tracking method the capture and release zones of the pond are outlined

the effects of spall on regional seismograms the effects of spall on regional signals are investigated using synthetic seismograms to frequencies of 20 hz depending on the scaling relationships used it is found that spall may have an important effect on regional body waves and especially surface waves the frequency range in which the spall effect dominates is dependent upon the characteristic times of the spallation process but generally occurs in the frequency range of about 0 2 to 2 hz because of the peaked nature of the radiated spall spectra effects at frequencies outside the 0 2 to 2 hz frequency range will be dominated by the explosion source the frequency range and strength of the spall effect depend in a complicated manner upon the yield depth of burial near source structure and material strength uncertainties in the spall scaling relationships need to be better understood before the effects of spall on regional discrimination and yield estimation can be properly quantified copyright american geophysical union 1989

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

changes in plate motions and the shape of pacific fracture zones geosat passes the new 2 min global gravity grid smith and sandwell 1995 and shipboard bathymetry across central pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions these features are not predicted by current locked fault fracture zone models during a change in spreading direction that induces tension across the transform fault large offset greater than 500 km transforms develop multiple parallel faults spaced 50 to 100 km apart the gravity signature of small offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model seafloor formed during a period of change usually marks a transition between structural styles for example between multiple fracture zone strands and a narrower single fault fracture zone widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to but not across the old transform fault

anatomy of regional phases and source characterization of the soviet joint verification experiment underground nuclear explosion broadband data from the nrdc deployment of stations in kazakhstan at karkaralinsk kkl and bayanual bay are subjected to detailed waveform modeling to understand the nature of regional wave propagation and to determine the nature of the explosion time function and sources of low and high frequency tectonic release minor modification of a crustal model inferred from dss studies in the region is very effective in explaining the arrival times amplitudes and waveshapes of the pn and pmp arrivals at both stations the sh wave data are remarkable however for showing a large high frequency arrival after the expected and observed arrival of sms from the explosion source these secondary s arrivals imply a secondary source 1 to a few kilometers to the south of the shot point shear waves from this source dominate the seismic data at high frequency but are minor at low frequency where shear waves and surface waves from the inferred relaxation of the explosion cavity dominate the wave field the secondary s wave source is inferred to represent explosion driven faulting with no net moment significant overshoot in the rdp is required to match the p wave data it is found that von seggern blandford and haskell source functions are incapable of producing the observed overshoot however the mueller murphy source model is very effective in matching the clear overshoot effects seen in the p waves spall effects are also considered but appear to be minor and serve to require even more overshoot in the rdp the excitation and dispersion of rayleigh waves in the period band of 1 to 10 sec is seen to be a sensitive but nonunique function of both vs and vp in the upper crustal layer high poisson ratios in this layer are needed to explain the rayleigh waveform

seismic properties of the eltanin transform system south pacific abstract we present a compilation of the seismic properties of the eltanin transform fault tf system compelled by the recent discovery of the hollister ridge and the possibility of a change of plate kinematics pattern in the region the hollister ridge is a major volcanic system located on the western flank of the pacific antarctic ridge par immediately south of the eltanin fracture zones fzs we find or confirm two anomalous characteristics the occurrence of seven normal faulting events on the transform segments expressing extension across the plate boundary in the azimuth n26 w and more than 90 deficiency in the seismic moment released during strike slip events on the transforms as compared to the rate expected from kinematic models other seismic properties are typical of the seismicity of a fast spreading mid ocean ridge mor system in particular we could not document a single teleseismically recorded event on the hollister ridge earthquakes are confined to narrow tfs with no activity present on the ridge segments the transform events have regular frequency moment statistics and we could not document any significantly slow sources these seismic properties generally support conventional plate tectonics models such as nuvel 1 and cannot be reconciled with a proposed reorientation of the pacific plate 4 ma ago

underthrusting at the hjort trench australian pacific plate boundary incipient subduction 1 the southernmost portion of the macquarie ridge complex mrc comprises the hjort ridge trench and plateau the topographic expression of the australian pacific plate boundary south of new zealand on the basis of marine geophysical swath bathymetry reflectivity seismic reflection gravity magnetic data teleseismic data and gravity modeling we argue that the australian plate is actively underthrusting the pacific plate along the hjort trench but self sustaining subduction does not yet appear to have commenced we interpret a crustal discontinuity in the trench as a shallowly dipping thrust splay off a sub vertical transform fault in the crest of the hjort ridge hjort fault the hjort fault separates oceanic crust generated at the southeast indian ridge seir crust located to the south from oceanic crust generated at the extinct australian pacific spreading center located to the north mrc crust oblique convergence on the transform has initiated a significant plate boundary decollement in the trench but our estimate of 50 km of underthrusting does not support the existence of an eastwardly dipping australian slab below 20 km depth for the length of the hjort ridge and trench system oblique convergence is partitioned between the decollement in the trench and the hjort fault south of 57 5 s the trench decollement accommodates thrusting north of 57 5 s the boundary parallel component of convergence becomes dominant the trench gradually shallows and the trench decollement evolves into a low angle oblique slip fault the trench decollement and strike slip system in the hjort ridge are structural boundaries that contain tectonic slivers of inferred seir crust of the western flank of the hjort ridge that currently belong to neither the australian nor the pacific plates

nonlinear viscoelasticity and the formation of transverse ridges transverse ridges are regions of anomalously high uplift that run parallel to many of the transform fattits at mid ocean ridges previous models of the formation of these ridges generally fail to explain the magnitude of uphft between 1 and 8 km or account for geophysical observations which indicate that the ridges are dynamically uphfted by neutral or heavy but not buoyant material however the potentially large maxwell relaxation time of the lithosphere and the high strain rates and finite deformation of transform faults imply that nonlinear viscoelasticity may play an important role in the formation of these ridges we present simple theoretical and experimental models of viscoelastic flow beneath a transform fattit to show that the purely horizontal motion of the fattit can generate vertical uplift typical of transverse ridges the theoretical model is of an infinite half space of fluid driven from above by two plates moving parallel and in opposite directions to one another the constitutive relation for the viscoelastic rheology is for a second order fluid i e nonlinear viscoelastic effects are treated as perturbations to newtonian purely viscous flow the theoretical model not only predicts the two transverse ridges that are often observed on either side of the transform fault but the central transform valley that separates them as well for typical hthospheric flexural rigidity and deformation associated with transverse ridges the viscoelastic effect provides sufficient vertical stress to produce the observed uplift further the dynmnic uplift is a purely mechanical effect i e it does not involve buoyant materim which corresponds to geophysical observations the experimental model is comprised of a viscoelastic fluid 2 aqueous solution of high molecular weight carboxymethylcellulose driven by a rotating plate this is analogous to a t ransk rm fattit because the boundary of a spiroting circular tectonic plate surrounded by stationary plates is one continuous transform fattit the experiment produces a large ridge of fluid along the edge of the plate with a shght trough outside of the ridge this result is qualitatively described by a cyhnch ical version of of the above theoretical model in conclusion both experimental and theoretical models show that when nonlinear

hypocenter constraint with regional seismic data a theoretical analysis for the natural resources defense council network in kazakhstan ussr source depth and epicenter determination remains a simple but key method for discrimination between earthquakes and nuclear explosions at near regional distances a variety of secondary body wave arrivals can be observed that may be useful for constraining source depth and epicenter in this paper we study the source location problem using regional arrival time and arrival azimuth data from a sparse seismic network a theoretical analysis of the source location inverse problem provides estimates of the stability and uncertainty of the solution for a particular suite of data as a function of epicenter and depth as well as evaluating the importance of each datum to the solution an empirical analysis with actual data complements the theoretical analysis our approach is applied to the three station natural resources defense council soviet academy of sciences network in kazakhstan the theoretical analysis indicates that reasonable constraint on depth and epicenter is possible whenever three distinct phases p pn pg pzp and pmp or equivalent s phases are observed at two or three stations our theoretical analysis for two station locations shows that the epicenter constraints are extremely poor for sources along the line passing through the two observing stations the use of arrival azimuth in two station locations results in improvement in epicenter constraint for events occurring in the vicinity of the line connecting the two stations but there is no significant improvement elsewhere an empirical analysis of data from two of the september 1987 chemical explosions and data from an explosion in an active mining area supports the theoretical analysis showing that two station locations both epicenter and depth are quite reliable furthermore the theoretical analysis also indicates that even with data from a single observing station satisfactory results are obtained if three or more phases plus arrival azimuth are available

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

lithospheric stress near a ridge transform intersection oceanic lithosphere at a ridge transform intersection has distinct structural characteristics the spreading axis and normal fault scarps which parallel the ridge axis away from the intersection locally curve towards the transform fault this implies that the least compressive stress direction becomes oblique to the spreading direction near a ridge transform intersection this stress field may be the result of a tensile ridge axis stress resisting plate separation and a transform shear stress resisting relative plate motion a plane stress plate model with prescribed plate boundary stresses is formulated to explore this hypothesis the orientation of the least compressive horizontal stress near an intersection depends strongly on the ratio of the applied boundary stresses thus ridge axis curvature may place an important constraint on plate boundary forces

source characterization using simplified waveforms tests on earthquakes and nuclear explosions in xinjiang china abstract complete waveforms with periods longer than 30 sec recorded at teleseismic and regional distances are used to characterize the source of several earthquakes and underground nuclear explosions in the lop nor area xinjiang china propagation effects of earth9s heterogeneities on data for several auxiliary events with well constrained source mechanisms in the vicinity of the primary event are calibrated by frequency domain division of each pair of observed and synthetic waveforms with the synthetics being computed for a reference earth model the frequency dependent deconvolution filters obtained for the auxiliary events are interpolated to construct a predicted deconvolution filter which is convolved with observed waveforms for the primary event to remove the empirically determined propagation effects and to obtain simplified waveforms that are intrinsically well predicted by the reference earth model these waveforms are then inverted for the source mechanism and centroid time location and depth of the primary event the inversion yields a centroid depth of 13 km for the 7 september 1994 earthquake m b 5 1 significantly shallower than the 33 km depth reported previously from routine determinations the centroid depth of the 5 october 1993 nuclear explosion plus tectonic release is estimated at approximately 4 km while the depth of the 21 may 1992 explosion plus tectonic release is less well resolved and may be deeper than 5 km reflecting limited resolution of the passband used here with extension to shorter period energy the method holds promise for reliable source inversion for small events in a calibrated region

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

yield estimates of nevada test site explosions obtained from seismic lg waves a methodology is presented for determining the yield of underground nuclear explosions from lg wave amplitudes the methodology is applied to nevada test site nts explosions for which the data from short period vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water saturated rock source conditions the latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the united states and in the french sahara suggesting that they may be applicable to all continental areas if so they also can provide an estimate of the bias of mb p magnitudes between different continental sites for example the lg data from nts explosions indicate a 0 31 0 02 magnitude unit bias between nts and eastern north america similar to the approximately 0 33 unit bias found between western and eastern north america previously by use of earthquake data

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces