the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

the meridional flow of source driven abyssal currents in a stratified basin with topography part i model development and dynamical properties the equatorward flow of source driven grounded deep western boundary currents within a stratified basin with variable topography is examined the model is the two layer quasigeostrophic qg equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic pg equations describing the abyssal layer on a midlatitude plane the model retains subapproximations such as classical stommel arons theory the nof abyssal dynamical balance the so called planetary shock wave balance describing the finite amplitude induced westward propagation of abyssal anomalies and baroclinic instability the abyssal height field can possess groundings in the reduced gravity limit a new nonlinear steady state balance is identified that connects source driven equatorward abyssal flow as predicted by stommel arons theory and the inertial topographically steered deep flow described by nof dynamics this model is solved explicitly and the meridional structure of the predicted grounded abyssal flow is described in the fully baroclinic limit a variational principle is established and is exploited to obtain general stability conditions for meridional abyssal flow over variable topography on a plane the baroclinic coupling of the pg abyssal layer with the qg overlying ocean eliminates the ultraviolet catastrophe known to occur in inviscid pg reduced gravity models the baroclinic instability problem for a constantvelocity meridional abyssal current flowing over sloping topography with present is solved and the stability characteristics are described

planetary shock waves abstract a number of general circulation models have recently been proposed that compute the steady state structure of the general circulation observation of 18 c water formation on the other hand suggest the need for a study of the time dependent large scale structure of the oceans in this paper the planetary geostrophic equations are used to compute the evolution of large thermal anomalies with a view toward understanding the variability in the general circulation caused by water mass formation events the evolution of a thermal anomaly is considered in the absence of wind forcing in this case the planetary geostrophic equations can be reduced to a first order equations the planetary geostrophic wave equation pgwe arbitrary initial conditions governed by the pgwe tend to steepen and under an assumed diffusive closure from shock waves the evolution of an initially columnar eddy is obtained and four different phases of shock propagation are identified the implications for heat transport p

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

estimation of dynamic rupture parameters from the radiated seismic energy and apparent stress we propose a new procedure for estimating the critical slip weakening distance dc by evaluating the spatio temporal distribution of the apparent stress calculated from the spatio temporal distribution of the slip velocity function on the fault plane obtained from a kinematic inversion of the earthquake source the idea is based on the fact that the apparent stress can be related to the difference between the earthquake average stress and the frictional dynamical stress on the fault plane during the rupture process from the cumulative slip apparent stress relationship we estimate the critical slip dc of the 1992 landers earthquake we find that the critical slip weakening plays an important role in controlling the rupture velocity in the case of the landers earthquake the very low rupture velocity in the central segment could be explained by the large dc value obtained for the main asperity on that segment

some characteristics of the stress field of the 1995 hyogo ken nanbu kobe earthquake we investigate the characteristics of the stress field associated with the 1995 hyogo ken nanbu kobe earthquake we use for the study a tomographic model of the fault slip inferred from the numerous near field recordings and we calculate the space and time evolution of shear stress on the fault during the earthquake we show that the spatial distribution of stress drop is very heterogeneous the apparent strength of the fault at the onset of the earthquake is low of the order of 1 mpa or less which indicates that the pre earthquake tectonic shear stress was close to the static friction over most of the fault at many locations the stress drop rotates significantly during sliding as was originally proposed by spudich 1992 we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake the pre earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 mpa average values of the initial and final shear stresses over the fault are 3 3 and 1 6 mpa respectively indicating that about half of the pre earthquake tectonic stress was released during the earthquake there is a relatively strong correlation between the initial and final stress distributions which suggests that intrinsic fault properties not modified by the earthquake control the spatial distribution of tectonic stress over the fault we show that on average over the fault the dynamic friction coefficient is equal to about 40 of the static friction coefficient diagrams depicting the evolution of shear stress as a function of slip are consistent with slip weakening behavior but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion

lithospheric response to volcanic loading in the southern cook islands abstract the response of the ocean lithosphere to volcanic island loading has been investigated for the southern cook islands of rarotonga mangaia and the aitutaki mauke group using 1 estimates of the change with distance from the load of uplift of islands located on the flexure created arch 2 geoid height perturbations as observed with the geos 3 and seasat radar altimeters and 3 bathymetric evidence for the moat and arch about the island loads the effective flexural rigidity for rarotonga is found to be higher than for the older and nearby islands of mangaia and aitutaki this together with an earlier result for tahiti 1 suggests that stress relaxation of the lithosphere has occurred and that this layer behaves as a viscoelastic material with an effective viscosity of about 5 10 10 25 poise the observations for the flexural rigidity do not indicate any dependence on the age of the lithosphere at the time the loading occurred this suggests that there are no significant differences in lithospheric properties for plates in the age range of 6 7 10 7 years altimeter passes to the east of mauke indicate that there may be an uncharted submarine extension of the aitutaki mauke chain

island arc character and late precambrian age of volcanics at wadi shwas hijaz saudi arabia geochemical and sr and nd isotopic evidence abstract a summary description of two volcanic sequences named the surgah and the shwas formation situated in the wadi shwas area at long 42 0 lat 20 0 in the arabian shield is given major and trace element concentrations including ree abundances are presented as well as sr and some nd isotope results one sequence the surgah formation shows a tholeiitic differentiation trend from basaltic to rhyolitic rocks whereas the shwas formation has calc alkaline affinities both sequences have low concentrations of incompatible elements notably rb nb zr and ti the coexistence of a tholeiitic and a calc alkaline suite along with these low concentrations is strongly reminiscent of some present day island arc environments an rb sr errorchron age of 721 55ma was obtained on the surgah formation with high initial87sr 86sr ratios around 0 705 the rare earth abundances are overall very low and the patterns show a relative depletion of the light ree in the least evolved rocks indicating a relatively light ree depleted mantle source a time integrated light ree depleted source is also indicated by the fairly radiogenic143nd 144nd ratios end values around 8 the nd model age of source depletion tchurnd is around 2 2 ga

dynamics of dip slip faulting explorations in two dimensions dynamic models of earthquake rupture and slip are a powerful method by which to investigate the physics of earthquakes owing to both conceptual and computational constraints dynamic earthquake models have largely been limited to cases with geometrical symmetry such as faults in unbounded media or vertical faults however there are both observational and theoretical reasons to believe that nonvertical dip slip faults behave differently from faults with more symmetrical geometries previous observations have shown greater ground motion from thrust reverse faults than normal faults and higher ground motion on hanging walls than on footwalls in the present work two dimensional dynamic simulations of thrust reverse and normal earthquakes show precisely these effects and also elucidate their causes for typical nonvertical dip slip faults the breakdown of symmetry with respect to the free surface allows radiated seismic waves to reflect off the free surface and to hit the fault again altering the stress field on the fault this process can lead to time dependent normal stress and a feedback between the friction rupture processes and seismic radiation this interaction leads to thrust reverse faults producing much higher fault and ground motion than normal faults with the same geometry and stress magnitudes the asymmetric geometry also directly leads to higher motion on the hanging walls of such faults than on the footwalls simulations show that these effects occur for a variety of dip angles but only for faults that either intersect or closely approach the free surface the results emphasize the strong effect that the free surface can have on the dynamics of fault rupture and slip

dynamic faulting under rate dependent friction we discuss the effects of rate dependent friction on the propagation of seismic rupture on active faults several physicists using burridge and knopoff s box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self similar stress distribution on the fault plane if this model proves to be correct it has important consequences for the origin of the complexity of seismic sources in order to test these ideas on a more realistic earthquake model we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate dependent friction we study rupture dynamics of models with single and twin asperities in our models asperities are places on the fault with a higher value of prestress othewise all fault parameters are homogeneous we show that for models with such asperities a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough for models with asperities we can also observe narrow slip velocity pulses qualitatively similar to the so called heaton pulses observed in some earthquake accelerograms we also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law

on the existence of a second scale of convection in the upper mantle summary this paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea floor deviates from the prediction of boundary layer theory for ages in excess of about 100 ma the energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large scale convective circulation associated with plate creation and destruction here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability if the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022poise as suggested by post glacial rebound data then the existence of the second scale is extremely unlikely on the other hand if a sufficiently pronounced low viscosity zone does exist under old sea floor then the development of such a second scale cannot be ruled out completely some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case

geotherms evolution of the lithosphere and plate tectonics abstract for the purpose of calculating precambrian geotherms modern average geotherms need to be defined curves calculated from estimated plate sections are close to those of clark and ringwood 1964 thicknesses of lithospheric plates at any time in the past can be estimated from fossil geotherms because subduction of oceanic plates depends upon their density average densities of precambrian plates should place important constraints on the beginning of subduction this method however only indicates that in all probability subduction did not begin before 1000 ma ago geological data suggest a date of about 800 or 900 ma ago formation of archean continents would have proceeded from partial melting of basaltic crust and the end of that phase of earth s evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub continental mantle the beginning of subduction was also thermally controlled and probably accompanied the increasing stability of eclogite in the upper mantle

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

proterozoic plate tectonics in canada with emphasis on evidence for a late proterozoic rifting event abstract several early proterozoic fold belts in the canadian shield have been interpreted in terms of plate tectonic processes the most impressive evidence comes from those preserved on the margins of archaean cratons such as the wopmay orogen and the penokean fold belt there are striking similarities in the stratigraphic successions of many widely separated early proterozoic fold belts within the canadian shield most of these are currently best interpreted in terms of some modified version of plate tectonics post grenville rocks contain several independent lines of evidence that support a major episode of continental fragmentation stratigraphic and sedimentological evidence from the cordilleran region records a dramatic change from widespread development of essentially platformal marine sediments l mackenzie mountains supergroup through rift related evaporites and clastics u mackenzie mountains supergroup to a largely resedimented sequence ekwi or windermere supergroup that has been interpreted as a continental margin facies a closely comparable succession of facies can be documented in other parts of the world but perhaps the closest analogue is found in s e australia it has been suggested that these two regions may formerly have been juxtaposed and represent the sundered fragments of a late proterozoic supercontinent relevant to these interpretations are geochemical data such as 34s values from late proterozoic evaporites and 87sr 86sr measurements from carbonates and processes involved in precipitation of glacially related iron formations such as those of the rapitan group in the n cordillera preservation of late proterozoic ophiolites in north africa provides additional evidence of plate tectonic processes at that time these several lines of evidence all suggest a significant period of late proterozoic continental breakup which produced many of the continental margins that were and are the loci of subsequent phanerozoic orogenic activity

perturbation analysis of convective instability of oceanic lithosphere and initiation of subduction zones a linear perturbation analysis is made on convective instability of the oceanic lithosphere to understand the occurrence of subduction on the earth the oceanic lithosphere is modeled by the upper part of top thermal boundary layer in a convecting newtonian temperature dependent viscosity fluid thickening of oceanic lithosphere with its age due to cooling at the surface is modeled by thickening of the thermal boundary layer with time its thickening rate l is a measure of the rate with which temperature in the thermal boundary layer decreases with time and hence is a measure of the rate with which viscosity in the thermal boundary layer increases with time the linear perturbation analysis shows the possible excitation of plural eigenmodes to give rise to an instability in the thermal boundary layer depending on a threshold value rc of the local rayleigh number r of the thermal boundary layer further rc is shown to be a linearly increasing function of l which is treated as a free parameter from estimates of r and l appropriate for the major oceanic lithosphere denoted as robs and l obs respectively i find that rc l 0 robs rc l l obs the flow pattern of mantle convection depends strongly on which eigenmodes are excited by the convective instability in thermal boundary layer 1 when r is lower thanrc only the fundamental and the first higher modes at most are excited the resulting mantle convection manifests itself as the secondary convection 2 when r is higher than rc the modes higher than the first higher mode are also excited the resulting mantle convection involves lithospheric instability leading to large local deformation in the oceanic lithosphere and finally to subduction of the oceanic lithosphere this result implies that 1 subduction does not begin newly in the major oceanic lithosphere at present since lithospheric instability is suppressed by thickening of the major oceanic lithosphere neither by its high viscosity nor by its buoyancy 2 subduction will newly begin in the major oceanic lithosphere due to lithospheric instability in future when the oceanic lithosphere becomes so old that its thickening rate l decreases to sufficiently small value the latter result suggests that lithospheric instability is an important agent that maintains multiplate tectonics on the earth

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

interannual planetary wave breaking in the presence of ekman pumping and mean flow a two layer planetary geostrophic model is adopted to study the breaking of planetary waves in the presence of ekman pumping and the associated mean flow the governing equation for the interface is a quasi linear equation which is solved analytically by the method of characteristics the waves are forced by annual or interannual upwelling or downwelling along the eastern boundary of a subtropical gyre it is found that the time and position at which breaking occurs is mainly determined by the speed and depth of the eastern boundary perturbation while the intensity of a breaking front is mainly determined by the amplitude of the perturbation

back arc spreading in the proterozoic a theoretical approach abstract the early proterozoic earth probably had a hotter mantle and a lower residence time for oceanic plates at the surface than the modern one the hotter mantle may have led to thinner oceanic plates young ocean floor has low negative buoyancy and therefore exerts low slab pull forces in subduction zones this effect would have been greater if the plates were thinner since they would have warmed up more quickly as they descended into the mantle thus the negative buoyancy of slabs and the slab pull forces were probably substantially lower in the early proterozoic calculations show that this applies even if subduction rates were higher and the crust was thicker lower slab pull leads to lower rates of rollback in subduction zones and lower rates of retreat of upper plates from trenches the limited age range of oceanic crust and the more rapid heating of descending slabs in subduction zones probably leads to a smaller range of slab profiles since this range is controlled by thermal properties of the slab and therefore to more stability in subduction zones thus many of the proposed mechanisms for back arc spreading may have been less effective and backarc spreading may have been less common in early proterozoic subduction zones this may explain the apparent absence of ophiolite complexes of that age

modeling dynamic rupture in a 3d earthquake fault model we propose a fourth order staggered grid finite difference method to study dynamic faulting in three dimensions the method uses an implementation of the boundary conditions on the fault that allows the use of general friction models including slip weakening and rate dependence because the staggered grid method defines stresses and particle velocities at different grid points we preserve symmetry by implementing a two grid row thick fault zone slip is computed between points located at the borders of the fault zone while the two components of shear traction on the fault are forced to be symmetric inside the fault zone we study the properties of the numerical method comparing our simulations with well known properties of seismic ruptures in 3d among the properties that are well modeled by our method are full elastic wave interactions frictional instability rupture initiation from a finite initial patch spontaneous rupture growth at subsonic and supersonic speeds as well as healing by either stopping phases or rate dependent friction we use this method for simulating spontaneous rupture propagation along an arbitrarily loaded planar fault starting from a localized asperity on circular and rectangular faults the shape of the rupture front is close to elliptical and is systematically elongated in the in plane direction of traction drop this elongation is due to the presence of a strong shear stress peak that moves ahead of the rupture in the in plane direction at high initial stresses the rupture front becomes unstable and jumps to super shear speeds in the direction of in plane shear another interesting effect is the development of relatively narrow rupture fronts due to the presence of rate weakening friction the solutions for the thick fault boundary conditions scale with the slip weakening distance do and are stable and reproducible for do greater than about 4 in terms of 2t t ax finally a comparison of scalar and vector boundary conditions for the friction shows that slip is dominant along the direction of the prestress with the largest deviations in slip rate direction occurring near the rupture front and the edges of the fault

effects of physical fault properties on frictional instabilities produced on simulated faults laboratory studies of large scale simulated faults show that physical properties of the fault specifically normal stress and fault roughness significantly influence the unstable shear failure behavior of the fault in addition the experiments provide insights into important length scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension stick slip shear failures have been generated along a 2 m long simulated fault in a block of sierra white granite experiments were conducted at normal stresses between 0 6 and 4 mpa with two different prepared roughnesses of the simulated fault a smooth fault with a profilometer measured roughness of 0 2 m and a rough fault with a measured roughness of 80 m high speed records of shear strains and slip velocities clearly show both the propagation of the stick slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved both the dynamic stress drops of the stick slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress although critical slip weakening displacements are insensitive to normal stress they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault slip velocities in these experiments are influenced by roughness and are proportional to stress drop although the onset of slip is sharply defined the termination of slip is more gradual without a clear stopping phase measured rupture velocities on the rough fault are lower than those on the smooth fault when a length scale based on rupture nucleation dimensions directly proportional to critical displacements is introduced the data suggest that at fault lengths close to the nucleation lengths rupture velocity and slip velocity increase with increasing fault length

tectonic cycles and the history of the earth s biogeochemical and paleoceanographic record an integrated approach to earth history for the last 2 5 b y reveals the strong interdependence of all geologic variables three earth history components are identified and interrelated secular trends as the earth ages recurring events of a previously proposed supercontinent megacycle worsley et al 1984 1985 and nonrecurring environmental shifts declining secular trends are endogenic heat production atmospheric carbon dioxide levels and surface temperatures continental differentiation enzyme efficiency atmospheric oxygen levels and solar luminosity are increasing secular trends four phases of a supercontinent cycle supercontinent fragmentation maximum continent dispersal continental assembly and supercontinental stasis recur with a 0 5 b y period and are correlated with tectonism cratonic sediment preservation atmospheric and hydrospheric evolution and the distribution of marine platform stable isotopes these correlations reflect coupling among the various oceanic and continental constituents during consecutive supercontinent assemblies and fragmentations nonrecurring shifts are unique and irreversible events in earth history e g development of photosynthesis occurrence of banded iron formations and deposition of detrital uraninite that represent periods of rapid geochemical adjustment to biospheric evolution earth s tectonobiogeochemical history is then synthesized by integrating secular trends and nonrecurring events with the supercontinent cycle emphasizing the preferentially preserved periods of fragmentation and continental dispersal paleoceanographic consequences of the megacycle are considered to reflect the stepwise coupling of secular trends and nonrecurring events to a biotically controlled phosphorus organic carbon climate linkage required to maintain earth s post archean surface temperature

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

heterogeneous distribution of the dynamic source parameters of the 1999 chi chi taiwan earthquake 1 the spatial and temporal distribution of the stress on the fault plane of the 1999 chi chi taiwan earthquake is calculated from kinematic inversion results using a three dimensional finite difference method for solving the elastodynamic equations we analyze the relations between stress and slip for all grid positions on the fault and use these relations to infer the friction law for the rupture the dynamic source parameters were also determined our results show that for most of the points on the fault the relation between stress and slip was consistent with the slip weakening law during the rupture process especially for those points with large slip however consistency with the velocity weakening law is not clear from the observed relation between stress and slip velocity the distributions of the dynamic parameters on the fault are very heterogeneous the peak value of the static stress drop is 35 mpa in general high stress drop occurred in the areas with large slip the estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock the slip weakening distance dc and the fracture energy gc are proportional to the final slip the aftershock activity correlates with the spatial distribution of dynamic source parameters usually the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop and there were few aftershocks in regions which had large values of critical slip weakening distance dc

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

dynamic stress drop and rupture dynamics of the october 15 1979 imperial valley california earthquake abstract a dynamic faulting model is constructed to determine the dynamic stress drop and relative strength distribution for the october 15 1989 imperial valley earthquake and to study the dynamics of rupture propagation for this event reproduction of the kinematic quantities of the strike slip rate strike slip final offset rupture time and rupture duration bu varying the dynamic parameters of dynamic stress drop and yield strength is attempted the fault plane is discretized in 1 5 km square grids the model incorporates first order corrections for the effect of the free surface and material property heterogeneity on the calculation of rupture at depth the strike slip particle velocities final offsets and rupture durations of the kinematic model are matched within about 40 everywhere below about 5 km except in the hypocentral region and in a high displacement region 20 km north of the epicenter the strike slip rate of the dynamic model is half and rupture duration double those of the kinematic model in the epicentral region the rupture durations and strike slip final offsets are two thirds those of the kinematic model in a high stress drop region beneath the el centro crossing array two slip concentrations correspond to stress drop concentrations one of about 80 bar at 20 km from the epicenter and at 8 km depth and the other of about 70 bar at 30 km from the epicenter and 10 km depth most of the dynamic stress drop occurs between the depths of 9 and 11 km averaging about 60 bar stress drops in the 7 9 km range vary between 25 and 35 bar while stress drops in the 5 7 km depth range vary between 10 and 15 bar this depth dependence of stress drop was a consistent feature across nearly 20 km of the fault stress drop appeared to be well correlated with rock type negative stress drop was found in the sediment regions and the regions at depth while positive stress drop was found in the metasediment regions between the depths of 5 and 11 km yield strength and rupture velocity are inversely related high yield strength is found in regions of low rupture velocity and vice versa rupture was found to terminate at the northern end of the fault by collision with a high strength barrier a region of supersonic rupture velocity was found to occur in a region of high stress drop and low fracture strength the kinematic rupture velocity at the northern end of the fault was found to be unreasonably low results show that this region reaches its peak stress before the kinematic break time yield strength increased with depth in the seismogenetic zone until the base of the zone was reached below which it tapered off slightly

computer simulation of strong ground motion near a fault using dynamic fault rupture modeling spatial distribution of the peak ground velocity vectors computer simulation was used to study the nature of the strong ground motion near a strike slip fault the faulting process was modeled by stress release with fixed rupture velocity in a uniform elastic half space or layered half space the fourth order 3 d finite difference method with staggered grids was employed to compute both ground motions and slip histories on the fault the fault rupture was assumed to start from a point and propagate circularly with 0 8 times shear wave velocity in the present paper we focused on the spatial pattern of ground velocity vectors i e the direction of strong motions in the case of bilateral rupture propagation the strong fault parallel ground motion appeared near the center of the fault the fault normal motions of ground velocity appeared near the edges of the fault in the case of unilateral rupture the fault parallel motion appeared near the starting point however the amplitude was lower than that for the bilateral rupture case the fault normal motion was predominant near the terminal point of the rupture the results were applied to the earthquake damage data especially the directions that simple bodies overturned and wooden houses collapsed caused by the 1927 tango the 1930 kita izu and the 1948 fukui earthquakes the spatial distributions of the direction data were found to reflect the strong ground motions generated from the earthquake source process

scaling of the shear rupture process from nucleation to dynamic propagation implications of geometric irregularity of the rupturing surfaces a series of systematic high resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces on the basis of the experimental results it has been discussed theoretically how consistently scale dependent physical quantities inherent in shear rupture are scaled the experiments led to conclusive results that the nucleation process consists of two phases phase i an initial quasi static phase and phase ii a subsequent accelerating phase and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces in phase i the rupture grows at a slow steady speed which is independent of the rupture growth length l in contrast during phase ii the rupture develops at accelerating speeds v which increase with an increase in l obeying a power law v vs l c n where vs is the shear wave velocity c is the characteristic length representing the geometric irregularity of the fault surfaces and and n are constants 8 87 10 29 and n 7 31 scale dependency of scale dependent physical quantities including the nucleation zone size and its duration is commonly ascribed to scale dependency of the slip dependent constitutive law parameter dc which is in turn governed by c it has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip dependent law

the role of komatiites in plate tectonics evidence from the archaean and early proterozoic crust in the eastern baltic shield abstract the early archaean continental crust of the eastern part of the baltic shield had been submitted to repeated rifting during the archaean and the early proterozoic the newly formed rifting belts evolved by sagduction and seemingly subduction in the archaean and subduction and collision in the lapland fold belt and in the svecokarelides of the central baltic shield the comparative evolution of these domains suggests that sagduction compatible with plate motion is a process competitive with subduction it is postulated what the sagduction subduction duality characterizing the archaean is equivalent to the intracontinental marginal orogenies encountered in proterozoic and phanerozoic times and that both represent at different periods variations in crustal interaction within an overall plate tectonic framework

stability of the oceanic tectosphere a model for early proterozoic intercratonic orogeny resistance of modern oceanic tectosphere to the initiation of subduction can be evaluated by considering its flexural rigidity shear resistance in the subduction zone and the negative buoyancy force after mckenzie 13 these depend on the effective elastic thickness the depth to which shear stresses are appreciable and the tectospheric thickness respectively evaluation of these thicknesses and their variation with age allows development of a model in which oceanic tectosphere becomes progressively less stable with increasing age and spontaneously subducts at an age of 200 ma the same model applied to early proterozoic oceanic tectosphere distinguished only by having sub tectosphere temperatures 100 c higher than in the modern rquires that proterozoic ocean tectosphere subduct spontaneously at ages of about 75 ma the subduction of universally young oceanic tectosphere inhibits the production of juvenile magmas due to the release of volatiles from the descending slab at low pressures near the trench the model therefore accounts for the scarcity of subduction related magmas in many early proterozoic orogens it also predicts that ocean basins opening in the early proterozoic should have generally limited width that blueschist metamorphism should be rare and that early proterozoic oceanic crust should rarely be preserved at the earth s surface with progressive cooling of the mantle the proterozoic tectonic scheme may have changed gradually into the modern one by early phanerozoic time although parts of the pan african system may exhibit the characteristics of still unstable oceanic tectosphere

the meridional flow of source driven abyssal currents in a stratified basin with topography part ii numerical simulation abstract a numerical simulation is described for source driven abyssal currents in a 3660 km 3660 km stratified northern hemisphere basin with zonally varying topography the model is the two layer quasigeostrophic equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic equations describing the abyssal layer on a midlatitude plane the source region is a fixed 75 km 150 km area located in the northwestern sector of the basin with a steady downward volume transport of about 5 6 sv sv 106 m3 s 1 corresponding to an average downwelling velocity of about 0 05 cm s 1 the other parameter values are characteristic of the north atlantic ocean it takes about 3 2 yr for the abyssal water mass to reach the southern boundary and about 25 yr for a statistical state to develop time averaged and instantaneous fields at a late time are described the time averaged fields show an equatorward flowing abyssal current with distinct up and downslope groundings with decr

dynamic propagation and interaction of a rupture front on a planar fault we investigate the propagation and interaction of a rupture front that propagates on a planar fault using a boundary integral equation method we show first that the rupture velocity is controlled by a delicate balance between consumed fracture energy and supplied elastic strain energy a very sharp boundary in parameter space separates models in which ruptures stop spontaneously from those in which rupture propagates at super shear speeds the transition zone or bifurcation is shown to be stable with reference to small scale heterogeneities of the stress field using the relations derived from this analysis we examined the mechanism to generate high slip rate when two rupture fronts collide we found that collision at slow rupture velocities causes abrupt stress drop and generates high slip rates however these features tend to be moderated by large slip weakening distances finally we simulated rupture front focusing at the initial stages of an earthquake a phenomenon that may cause high slip rate pulses and therefore generate high frequency seismic waves we assume a pre slip region in which stress has decreased quasi statically to the dynamic friction level due to this pre slip strong stress concentration has developed around the pre slip area and a dynamic rupture starts at a certain point on the rim of the pre slip region we observe rupture front focusing that generates high slip rate pulses we also studied a double pre slip model in which two pre slip regions exist close to each other before the earthquake and found that multiple pre slips enhance the focusing effects

geochemistry of precambrian basal gneisses in lofoten vesteralen northern norway abstract the basal gneiss complex of lofoten vesteralen consists of an older ca 2 7 ga migmatite complex believed to be largely supracrustal in origin and a younger ca 1 8 ga cover complex of clearly supracrustal origin the major element geochemistry of both groups is that of an igneous suite of generally calc alkaline character similarities with the volcanic rocks of modern continental margin environments extend to trace element contents and ree patterns a complex multistage evolution is inferred an orthopyroxene isograd bisects the area element fractionation between amphibolitefacies and granulite facies rocks appears to be limited to some trace elements ree patterns are not affected by the metamorphism

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature

passive margin evolution initiation of subduction and the wilson cycle we have constructed finite element models at various stages of passive margin evolution in which we have incorporated the system of forces acting on the margin depth dependent rheological properties and lateral variations across the margin we have studied the interrelations between age dependent forces geometry and rheology to decipher their net effect on the state of stress at passive margins lithospheric flexure induced by sediment loading dominates the state of stress at passive margins this study has shown that if after a short evolution of the margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favourable for transformation into an active margin although much geological evidence is available in support of the key role small ocean basins play in orogeny and ophiolite emplacement evolutionary frameworks of the wilson cycle usually are cast in terms of opening and closing of wide ocean basins we propose a more limited role for large oceans in the wilson cycle concept