theoretical investigation of the oceanic inverted barometer response dynamic tidal theory is generalized in order to predict the oceanic response to atmospheric pressure variations the oceans are found to exhibit distinct dynamic behavior when forced at periods less than 1 week depending on the harmonic type of forcing the amplitude of the response can differ by 20 or more from the static response even at roughly 2 months the traditional inverted barometer may not be sufficiently accurate for applications in which detection of long term sea level trends or vertical crustal motion is the goal for forcing at periods of order 1 year the oceanic response differs by at most a few percent from the static not the inverted barometer response thus calculations of meteorological excitation of the annual and chandler wobbles can employ the static approximation without reservation

propagation of signals in basin scale ocean bottom pressure from a barotropic model 1 the exchange of atmospheric plus oceanic mass between ocean basins is investigated using a global barotropic ocean model we find two particular cases of exchange between two basins at periods of 4 6 days the exchange is between the atlantic and pacific basins and represents a known oscillation forced by atmospheric pressure this mode represents a failure of the inverse barometer relationship due to the large scale and high frequency of atmospheric forcing and the presence of continents significant exchange between atlantic and pacific also occurs at longer periods the second case is most prominent at periods longer than 30 days strongest at periods longer than 100 days and represents a mass exchange between the southern ocean and the pacific the southern ocean part of this exchange is clearly related to the southern mode of fluctuations in antarctic circumpolar transport forced by southern ocean wind stress the reason for the exchange being with the pacific rather than other basins is explored and is found to be related to the balance of wind stress by form stress in drake passage exchange with the atlantic and indian oceans becomes dominant if drake passage topography is removed while recognizing the limitations of a barotropic model we contend that it is necessary to understand the barotropic adjustment process in order to make sense of longer timescale processes accordingly we end with speculation on the possible importance of the barotropic results for global sea level and tropical dynamics

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

characteristics of atmospheric divergence and convergence in the indian ocean inferred from scatterometer winds abstract large scale surface atmospheric convergence and divergence patterns in the indian ocean are mapped using high spatial resolution merged scatterometer wind vectors during 1991 2000 the convergence zone evolves to north of 15 s as a result of convection promoted by warm 28 c equatorial sea surface temperature sst and it exhibits strong intensity during boreal summer and winter a divergence zone evolves to the south of 15 s as a result of subdued convection caused by colder sst

temporal variation of large intraplate earthquakes in coupled subduction zones the focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large scale stress regimes induced by slab pull bending or unbending and lateral segmentation and undulations of the slab numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential and possibly even in earthquake forecasting a systematic analysis of 1130 focal mechanisms for intraplate earthquakes with m b 5 0 in the depth range 0 300 km is conducted for nine circum pacific subduction zones all of which are known to have large interplate thrust events the spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised the earthquake catalog assembled contains all published focal mechanisms and is probably complete for m b 6 5 for the years 1963 1986 for many of the localized regions considered in detail the catalog is complete to lower thresholds of m b 6 0 or m b 5 5 this analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity for the seismically coupled regions considered here outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps while outer rise tensional events generally only follow interplate ruptures in the intermediate depth range large down dip tensional events generally precede interplate thrusts and are often concentrated at the down dip edge of the coupled zone a transition to down dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events e g 1960 chile 1974 peru 1957 aleutian 1971 new britain these examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime assuming that this concept is correct the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity

body wave inversion moment tensors and depths of oceanic intraplate bending earthquakes this paper develops a moment tensor inversion technique suited for analysis of long period body waves from distant shallow earthquakes in addition to the moment tensor the procedure resolves precise source depths 5 km and estimates the duration of faulting the technique is applied to digital sro dwwssn recordings of outer rise intraplate earthquakes these earthquakes result from bending of oceanic plates prior to subduction bending induces horizontal tension near the top of the lithosphere and horizontal compression near its base a neutral surface separates compressional from tensional regimes an examination of 11 bending earthquakes in the period january 1981 to july 1982 indicates that the depth to the neutral surface is not geographically uniform although the seismicity sample is very small the depth of the neutral surface appears to correlate with the position of the trench arc in the evolutionary sequence of uyeda and kanamori 1979 in particular neutral surfaces at strongly coupled chilean type plate boundaries are elevated as much as 15 km above those at weakly coupled mariana type boundaries this correlation implies that nonuniformities in neutral surface height are created by variations in regional compressive stress resulting from contrasts in plate plate coupling to account for the observed heights maximum interplate compressive stresses must be comparable with stresses derived from bending

radar observations of a 3 day kelvin wave in the equatorial mesosphere mesospheric radars are used to investigate the characteristics of a kelvin wave from two equatorial sites jakarta indonesia in the western pacific and christmas island in the central pacific our study focuses on the time span from mid january through mid october 1993 a kelvin wave with a period near 3 days was detected throughout this 9 month duration although it underwent deep amplitude modulations on a 20 day timescale a fitting procedure is applied to study the phase amplitude behavior of the wave the vertical wavenumber was measured by the radars and found to be small wandering around zero with only a weak bias toward downward phase progression the long vertical wavelength suggests that the wave was predominantly zonal wavenumber 1 the amplitude of the wave measured by the jakarta meteor scatter radar was much larger than the amplitude measured by the mf partial reflection radar at christmas island the smaller wave amplitude at christmas island could at least partially be due to a measurement bias associated with mf radars the radar at jakarta is a vhf meteor scatter radar and is not susceptible to this bias however the mean velocities and the amplitudes of the tidal and quasi 2 day wave components were in good agreement at the two sites the estimated 9 month averaged zonal acceleration was 0 67 ms 1 day 1 over jakarta at 94 98 km and only about half as large over christmas island the magnitude of the zonal acceleration occasionally showed large enhancements which suggest the importance of refractive effects associated with vertical and temporal variations in the mean winds the larger 3 day wave amplitudes and inferred acceleration at jakarta may reflect its location in the western pacific a region of high convection and hence an excitation region for equatorial waves the relative phase of the wave between the two radar sites gradually shifted over a timescale of weeks these smooth variations in relative phase are suggestive of a superposition of waves with different zonal wavenumbers perhaps radiating preferentially from one longitude the phase of the wave as a function of altitude and time was much more disordered at jakarta than at christmas island the conjecture can be made that the more chaotic phase structure observed over jakarta is due to higher order zonal wavenumber components which weaken as they propagate eastward

the sea level response of a stratified ocean to barometric pressure forcing abstract the nature of the sea surface adjustment to atmospheric loading in a stratified ocean is examined for both midlatitude and equatorial regions using simple analytical solutions to the quasigeostrophic and equatorial plane equations while the interior response can have vertical structures ranging from oscillatory to surface trapped depending on the temporal and spatial scales of the forcing the sea surface reacts as an inverted barometer at most scales the inverted barometer or isostatic approximation breaks down only for narrow ranges of frequency and horizontal wavenumber values where the vertical dependence of solutions approaches that of the oceanic normal modes in these regions the sea surface adjustment can be both smaller and larger than the isostatic limit and is sensitive to frequency and wavenumber the vertical stratification introduces a number of nonisostatic regimes particularly in the equatorial regions not possible in a constant density ocean but the importance of these

quantitative estimates of interplate coupling inferred from outer rise earthquakes interplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones the spatial and temporal variations of such coupling control the patterns of subduction zone seismicity we calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact to quantitatively estimate the degree of interplate coupling for the tonga new hebrides kurile kamchatka and marianas subduction zones depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models we perform waveform modeling of body waves from the gdsn network to obtain reliable focal depth estimates for 24 outer rise earthquakes a propagator matrix technique is used to calculate outer rise stresses in a bending 2 d elastic plate floating on a weak mantle the modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate we estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location

intercomparison of recent ocean tide models one of the tremendous achievements of the topex poseidon t p satellite mission is the release of 12 new global ocean tide models during 1994 virtually all of these models are based on the high precision altimetry obtainable from the t p satellite and all of the models are superior to both the cartwright and ray 1990 1991 model as well as to the schwiderski 1980a b model when compared to tide gauges in the present intercomparison only global ocean tide models released during 1994 have been tested as t p continues to operate new ocean tide models will continue to be released and existing ocean models will be updated hence this intercomparison can only present a snapshot of the state of the art within ocean tide modeling however the intercomparison demonstrates the high accuracy and consistency of all these recent ocean tide models from a comparison with a common 104 tide gauge data set compiled by le provost and others unpublished data the general result is that six of the new ocean tide models have rms agreement better than 3 cm the obtainable accuracy with different tide gauge types raises the question about the accuracy of ground truth as ocean tide models become increasingly more accurate the accuracy of the harmonic constants derived from the tide gauge recordings becomes significant generally the rms agreement between models based on 2 years of t p altimetry is significantly better than the agreement between models based on 1 year of t p altimetry for models based on two years of t p altimetry the rms agreement has values around 2 0 1 4 1 0 and 0 7 cm for the m2 s2 k1 and o1 constituents respectively as a preliminary evaluation these values might be taken as a measure of the consistency of ocean tides estimated from the first part of the t p mission which is extremely promising for future ocean tide models derived from t p altimetry using 3 or more years of data this also suggests that further improvement using 3 or more years of data will only be marginal the differences arise when studying the ocean tides on continental shelves surrounding the ocean major differences are seen especially on the patagonian shelf as the continental shelves are critical for tidal dissipation it will be very important to improve tide models on the shelves in future global ocean tide models

the evolution of deep ocean pressure measurements in the uk abstract this paper is a brief review of the work carried out by the proudman oceanographic laboratory pol in measuring tides and sub surface pressure variations in the deep water of the ocean basins using bottom pressure recorders bprs it deals particularly with the work of david cartwright which he began in the early 1970s and carried out until he left pol in 1986 but is continued by his co workers of that time the paper mentions the early work in the north atlantic when instrument deployments were limited to one month duration and describes what was achieved from the measurements it concentrates mainly on measurements that were made since the early 1980s when it became possible because of developments in instrument technology to achieve individual measurements lasting for one year or more the results obtained from these measurements are described and some interesting features of the spectrum of the pelagic sub surface pressure are discussed as instrument technology further improved it became possible to study low frequency variations in sub surface pressure the contributions made by pol to studies of the atlantic and indian oceans and the contribution to ocean circulation studies during the world ocean circulation experiment are discussed this led to the development of an instrument capable of operating continuously at ocean depths for five years the preliminary results from a four year deployment of this instrument which was completed in november 1996 are included

a possible relationship between tropical ocean temperatures and the observed amplitude of the atmospheric 1 1 rossby normal mode space time spectral analysis was applied to 17 years of twice daily northern hemisphere sea level pressure grids the results for each individual year were compared with values of the sea surface temperature averaged over a large region in the tropical eastern pacific ocean this comparison revealed a positive correlation between the ocean temperature and the pressure variance at the zonal scales and frequencies generally associated with the gravest westward propagating zonal wave number 1 symmetric normal mode rossby wave i e the 1 1 mode variations in the surface temperature in the tropical pacific are known to be accompanied by large changes in the distribution of tropical precipitation thus the present results suggest that latent heat release may be a major excitation mechanism for the 1 1 rossby normal mode

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

interaction of low level flow with the western ghat mountains and offshore convection in the summer monsoon abstract seven year averaged values of percent frequency of occurrence of highly reflective cloud for the months june july and august indicate that offshore convection is a major component of the cloudiness of the southwest monsoon principal areas of convection occur off of the western coats of india burma thailand and the philippines this study concentrates on the area upstream of the western ghats mountains of india analysis of a special boundary layer mission flown during the wmo icsu summer monsoon experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection data obtained on this mission allow a large scale momentum budget computation for the subcloud layer which shows pressure deceleration to be significant the budget dominated by advection predicts an increase of average wind speed which is observed the pressure decelera

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

near 5 day nonisostatic response to atmospheric surface pressure and coastal trapped waves observed in the northern south china sea abstractoceanic nonisostatic responses to near 5 day rossby haurwitz atmospheric pressure waves have been observed in open oceans however such responses based on observations in marginal seas such as the south china sea have not been reported owing to the limited ocean bottom pressure pbot records the pbot measurements from pressure recording inverted echo sounders piess at sites in the northern south china sea revealed a nonisostatic like response near 5 days although the coastal trapped waves ctws appeared to obscure it because their broadband periods include the near 5 day band cross spectral analysis revealed that the pies pbot records and the sea level sl records of hong kong all correlate strongly with the atmospheric pressure and winds over the east china sea this is indicative of remotely forced ctws the pies pbot records showed higher coherence near 5 days with the zonal low pass wavelength filters applied to the atmospheric pressure and the phase analysis results strongly suggest

a very broad band inversion method for the recovery of earthquake source parameters abstract waveform studies of the earthquake source have traditionally been performed in narrow bands of the seismic spectrum waveform matching of p and sh waves emphasizes frequencies near and above the corner frequency normal mode and surface wave analyses on the other hand are performed on data which contain information only below the corner frequency a new method is presented which formally includes the result of a long period centroid moment tensor analysis in an inversion of broad band body wave phases a parametrized source model is used which allows for the recovery of a detailed source time function as well as the direction and magnitude of rupture propagation the technical details and the performance of the method are demonstrated in an analysis of the october 28 1983 borah peak idaho earthquake in particular it is shown how the combination of the long period and broad band data sets in the inversion improves the resolution of both the focal mechanism and the scalar moment the very broad band method is applied in an analysis of the september 19 1985 michoacan m w 8 0 earthquake two concentrations of moment release are identified in the source time function separated by 25 30 s in time the spatial distribution of moment is well explained both by a model of unilateral rupture towards the east with a propagation velocity of 2 4 km s 1 and by two point sources separated by 95 km it is shown that the p wave data are largely insensitive to the scalar moment and that source time functions obtained without consideration of the total moment release exaggerate the heterogeneity of slip on the fault plane source time functions which do contain the total moment show that a large fraction of the moment was released smoothly over the whole duration 80 s of the event the results suggest that the fault plane was characterized by two regions of high stress drop but that a large portion of slip occurred on surrounding weak portions of the plane

large scale response of the ocean to atmospheric forcing at low frequencies estimates of the time dependent sea surface topography on large space 500 2500 km and time 30 days to 2 years scales from altimetry and tide gauge data are compared with estimates of the surface meteorological fields winds and pressure on the same scales the comparisons are made in the north pacific and north atlantic despite the obvious remaining high noise level in the sea surface topography there is a significant correlation between the atmosphere and the time varying ocean circulation these relations are quantified using simple multichannel regression models with no time lags typically 50 of the sea level variance can be described by the wind curl and divergence and the pressure field some areas produce classical static inverted barometer effects but others suggest an amplified response the motions are believed to characterize most of the water column

relationship between surface atmospheric convergence over indian ocean and indian rainfall atmospheric convergence regions over the tropical indian ocean have been mapped for the first time using 31 years of vector wind data the convergence fields reveal that the energetic summer monsoon winds enhance convergence in the central arabian sea and in the eastern bay of bengal when compared with weak winter monsoon winds the summer monsoon is also effective in spatial migration of convergence regions in both basins area integrated convergence time series reveals an annual cycle with high amplitude during summer monsoon which occurs in phase with indian rainfall the study explores the prospects of using the convergence as one of the predictors of indian rainfall

source solutions and station residuals from long period waveform inversion of deep events deep earthquakes of up to moderate size have been analyzed by a long period waveform inversion of global digital seismographic network data where the use of a point source model proved to be adequate but the use of a standard earth model was inadequate without modification modifications include calculations in an earth model based on prem but with different source and receiver structures and the introduction of time delays which were determined together with the moment tensor components of the source by an iterative correlation method the moment tensor components are in accordance with the pattern of stress release in other work but results for the seismic moment differ somewhat and there is no compelling evidence for deviations from a double couple mechanism the time delays were inverted to find the source centroid depths and time shifts and the station residuals there is no baseline effect in the times of source duration which could be explained by a simple circular source model but there is such an effect in the station residuals together with a distance trend especially for s part of the baselines may be an effect of bias in origin times but the pattern of s residuals in particular suggests also possible modifications of the standard model

incoming plate faulting in the northern and western pacific and implications for subduction zone water budgets abstract the greatest uncertainty in the amount of water input into the earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench we explore the maximum depth of extension within the incoming plate at northwestern pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments we relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988 2011 by inverting teleseismic broadband p and sh waveforms we observe that the top 10 15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes 60 of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle 80 occur above 10 km within the mantle and 95 occur above 15 km there is evidence for variation throughout the different regions of study for example extensional earthquakes occur down to 20 km below the crust in the western aleutians and izu bonin we propose that the incoming plate mantle is most strongly hydrated in the upper 5 km and that partial serpentinization exists regionally within the incoming plate mantle to 15 km making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water we calculate that the total water carried into the northern and western pacific subduction zones is generally higher than previous estimates and is approximately 4 6 10 8 tg myr or 45 70 10 3 tg myr per kilometer of subduction zone

a comparison of source analysis methods as applied to earthquakes in tibet abstract basically two different methods have been used to determine simultaneously the intrinsic source parameters and the source location and or station residuals one is a linearized least squares method the other method uses a nonlinear correlation procedure the two methods have been applied here to the same long period 25 s gdsn data from a set of earthquakes under the tibetan plateau in these circumstances it is found that the nonlinear correlation method leads to more stable and reliable results which can be interpreted in a geophysically meaningful way the results show that all of the events are shallow focus 10 km and in accordance with east west extensional tectonics the time delays obtained by the correlation method were used to infer s p residuals associated with the near source structure although the number of data is rather small they do suggest a trend to high residuals in northern tibet 4 6 and 4 8 s relative to j b times corroborating earlier findings

importance of model parameterization in finite fault inversions application to the 1974 mw 8 0 peru earthquake the spatial and temporal slip distributions for the october 3 1974 mw 8 0 peru subduction zone earthquake and its largest aftershock on november 9 ms 7 1 are calculated and analyzed in terms of the inversion parameterization and tectonic significance teleseismic long period world wide standard seismograph network p and sh waveforms are inverted to obtain the rupture histories we demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source involving spatial variation in slip amplitude risetime and rupture time the inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time well located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone for the main shock a hinged fault is preferred having a shallow plane with a dip of 11 and a deeper landward plane with a dip of 30 the preferred nucleation depth lies between 11 and 15 km a bilateral rupture is obtained with two major concentrations of slip one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter for these source regions risetimes vary from 6 to 18 s our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity the slip distribution for the november 9 aftershock falls within a conspicuous hole in the main shock rupture pattern near the hypocenter of the main shock the november 9 event has a simple risetime function with a duration of 2 s aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending november 9 event and downdip from the largest main shock source region slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis the large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments and the relatively young age and high convergence rate of the subducted plate which results in good seismic coupling near the trench axis

seasat observations of flexure evidence for a strong lithosphere abstract outer rises seaward of subduction zones arise due to flexure of the oceanic lithosphere a mechanical model of this flexure is used to predict the shape of the associated geoid signal these predicted geoid signals are compared with seasat altimeter profiles over outer rises adjacent to nine major subduction zones the nine zones include the aleutian mariana izu bonin puerto rico lesser antilles peru chile middle america south sandwich philippine and kuril the comparisons with seasat data provide estimates of effective elastic thickness he and maximum bending moment mm for each zone it is shown that 1 1 these thicknesses hc are approximately proportional to the square root of lithospheric age 2 2 the maximum bending moments mm increase roughly as the cube of thickness hc these relations 1 and 2 are both very similar to those predicted by a layered rheology model brittle elastic ductile of the lithosphere derived from mechanical experiments on rocks according to this model thermally activated creep is the mechanism of ductile flow in the lower lithosphere hence an isotherm 740 50 defines the bottom of the lithosphere thus old oceanic lithosphere may have significant strength to depths in excess of 60 km or about half that of the thermal lithosphere

quasi periodic radar echoes from midlatitude sporadic e and role of the 5 day planetary wave using measurements of magnetic aspect sensitive radar echoes from midlatitude sporadic e collected over a two month period from tanegashima japan we show that while their occurrence duration from night to night did not exhibit any systematic variation that of the so called quasi periodic qp echoes varied sinusoidally with a period of 5 days we have interpreted this behavior in terms of effects produced by a planetary wave and identified its presence through neutral wind measurements made with a partial reflection drift radar located nearby at yamagawa we propose that the occurrence of qp echoes is affected both by a contribution of the wind to the dynamo electric field and by the direction of the neutral wind we argue that because the wind vector of the planetary wave is elliptically polarized at midlatitudes a preferred wind direction conducive to the generation of qp echoes occurs once every 5 days on the other hand this wave is linearly polarized and directed zonally over the geographic equator the fact that qp echoes are most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for qp echo production

zenisu ridge mechanical model of formation abstract seabeam seismic and submersible surveys took place during the kaiko project and revealed significant compressive deformation at the northeastern end of the philippine sea plate related to the recent collision of the izu ogasawara arc against central japan intraoceanic thrusting at the base of the zenisu ridge a linear topographic high running a few tens of kilometers south of the nankai trough is supported by tectonic magnetic and gravimetric data we investigate the formation of the zenisu ridge in terms of compressive mechanical failure of a thin elastic perfectly plastic plate subducting at a trench and subject to a regional compressive axial force the rheological envelope concept is used throughout the numerical calculations based on a detailed study of flexure of the present day bending far from the deformation zone we evaluate the bending forces involved the bulge is 120 to 150 m high and the compressive stress all along the nankai trough is about 100 mpa in the zenisu ridge area an additional compressive stress is superimposed due to the nearby collision at izu peninsula we compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20 25 km in the trench area and again at the surface 60 to 100 km seaward in the vicinity of the bulge the development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed the model predicts that the formation of the zenisu ridge did not occur before 4 ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the nankai trough

a non isostatic global sea level response to barometric pressure near 5 days changes in surface atmospheric pressure usually induce an isostatic response in the ocean characterized by an adjustment in sea level at the rate of approximately 1 cm hpa nonisostatic signals are however observed at many tropical tide gauges at periods near 5 days our analysis of satellite observations reveals the global nature of these signals involving nonisostatic sea level fluctuations with nearly uniform phase within separate basins and an out of phase oscillation between the atlantic and pacific oceans comparisons with a barotropic model further link the observed signals to forcing by a large scale pressure wave in the atmosphere and show that constrictions between basins prevent an isostatic response from being established the apparent forced nonresonant nature of the nonisostatic response contrasts with the predominantly resonant ocean dynamics at higher frequencies and may be relevant to explain the behavior of the long period tides

five day mesospheric waves observed in solar mesosphere explorer ozone solar mesosphere explorer sme ozone data were analyzed for short term periodic oscillations in the altitude region from 50 to 90 km a 5 day oscillation was found to be a dominant feature at high southern latitudes near the december solstice in the altitude range from 50 to 80 km the wave feature appears to travel westward with a speed consistent with that of a wave number one planetary scale oscillation circling the globe in 5 days the amplitude also appears to be consistent with the temperature oscillations predicted by geisler and dickinson 1976 this oscillation was not found to be a dominant feature in sme ozone during northern hemisphere summer

modeling the high frequency barotropic response of the ocean to atmospheric disturbances sensitivity to forcing topography and friction this study examines high frequency sea level variations forced by changes in surface atmospheric pressure and wind and their sensitivity to different forcing mechanisms bottom topography resolution and amount of friction in a barotropic ocean model optimal model performance defined in terms of the explained variance in satellite altimeter and bottom pressure data is found when using relatively strong friction equivalent to a damping timescale of only a few days over the deep ocean and topography with minimal smoothing spatial variations of the optimal friction parameter seem to reflect the roughness of bottom topography the model demonstrates skill in simulating the wind driven response as well as the nonequilibrium response to atmospheric pressure variations

deviation of long period tides from equilibrium kinematics and geostrophy new empirical estimates of the long period fortnightly mf tide obtained from topex poseidon t p altimeter data confirm significant basin scale deviations from equilibrium elevations in the low latitude pacific have reduced amplitude and lag those in the atlantic by 308 or more these interbasin amplitude and phase variations are robust features that are reproduced by numerical solutions of the shallow water equations even for a constant depth ocean with schematic interconnected rectangular basins a simplified analytical model for cooscillating connected basins also reproduces the principal features observed in the empirical solutions this simple model is largely kinematic zonally averaged elevations within a simple closed basin would be nearly in equilibrium with the gravitational potential except for a constant offset required to conserve mass with connected basins these offsets are mostly eliminated by interbasin mass flux because of rotation this flux occurs mostly in a narrow boundary layer across the mouth and at the western edge of each basin and geostrophic balance in this zone supports small residual offsets and phase shifts between basins the simple model predicts that this effect should decrease roughly linearly with frequency a result that is confirmed by numerical modeling and empirical t p estimates of the monthly mm tidal constituent this model also explains some aspects of the anomalous nonisostatic response of the ocean to atmospheric pressure forcing at periods of around 5 days

departures from the local inverse barometer model observed in altimeter and tide gauge data and in a global barotropic numerical model sea level data from the topex poseidon altimeter satellite and from subsets of the global tide gauge and bottom pressure recorder networks have been employed to study the worldwide distribution of departures from the simple local inverse barometer lib model from a combination of measurement techniques the lib model was found to provide a reasonable representation of the data within 5 10 outside of the tropics for timescales from several days to several weeks for most locations however several parts of the southern ocean and other higher latitude areas were seen from the altimetry to contain significant departures across the range of timescales accessible for study by the technique within the tropics the response of sea level to air pressure forcing was observed to be typically 50 of that of the lib model at timescales of several days with considerable spatial variation and to approximate the lib model more closely at timescales of several weeks a coarse grid global barotropic numerical model was found to reproduce qualitatively the global average response as a function of frequency to simulate many of the characteristics of the spatial variation of the response and to demonstrate the relative importance of air pressure and wind forcing at different timescales data sets from the permanent service for mean sea level and u k meteorological office were used to investigate lib departures at intraseasonal and longer timescales along continental coastlines and at ocean islands increasingly large departures were observed along many parts of the world coastline as timescales increased from months to seasons

upper stratospheric circulations in the two hemispheres observed by satellites a global analysis is made of upper stratospheric circulations with the aid of nimbus 5 and tiros n satellite observations in order to make a comparison of characteristics between the northern hemisphere nh and the southern hemisphere sh it is shown that the reversal of north south gradient of the zonal mean temperature at the stratopause level is always observed in higher latitudes of the sh winter while the temperature gradient of the nh winter is variable because of the occurrence of sudden warmings consequently due to the thermal wind relation the polar night westerlies of the sh stratosphere are generally stronger than those of the nh stratosphere in association with this difference the activity of transient planetary waves of wavenumber 2 is striking in the sh in contrast to the predominance of steady waves in the nh finally the interrelation between the zonal mean field and planetary waves in the upper stratosphere is briefly discussed

earthquake source parameters and stress distribution in the adak island region of the central aleutian islands alaska source parameters have been systematically determined for all earthquakes with mw 5 that occurred between 172 w and 179 w longitude in the adak island region of the central aleutian islands during 1977 march 1987 we relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination the first method uses arrival times of direct and prominent reflected phases primarily pwp the second method uses broadband p wave displacement seismograms in an inversion for source depth the analysis of these events provides a well constrained data set for the study of stress release along an active subduction zone three earthquakes which occurred seaward of the trench axis are located just below the crust mantle interface and show extension nearly perpendicular to the trench axis seven events occurred in the wadati benioff zone sixty four events are located in the main thrust zone and except for five unusual events are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle in cross section the thrust zone appears as a thin 10 15 km thickness interplate region which extends from 15 to 50 km depth a 10 15 average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the pacific and north american plates exists for these events the observed slip vectors are oriented more normal to the trench than is predicted by plate motions five earthquakes which occurred as aftershocks to the may 7 1986 earthquake mw 8 0 in the crust of the overriding plate have strike slip mechanisms consistent with right lateral motion on arc parallel fault planes observations of slip vectors along the whole aleutian arc show a similar trend to that observed in the adak island region the largest differences 30 between observed and predicted slip azimuths occur around 175 e we propose a model of plate interaction in which a portion of the along arc motion occurs along a weak strike slip shear zone in the upper plate near the volcanic line the slip azimuths in the main thrust zone fit this model well if the amount of transcurrent slip occurring in the upper plate is 60 of the arc parallel relative plate motion a consequence of the model is along arc extension of the overriding plate between the accretionary wedge and the volcanic line especially in the western part of the aleutian arc calculations based on a tectonic model by geist et al 1988 for the formation of arc summit basins through block rotation and translation suggest that along arc extension has been significant since late miocene or early pliocene

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

small scale mantle flows and induced lithospheric stress near island arcs summary this paper explores the middle ground between complex thermallycoupled viscous flow models and simple corner flow models of island arc environments the calculation retains the density driven nature of convection and relaxes the geometrical constraints of corner flow yet still provides semianalytical solutions for velocity and stress a novel aspect of the procedure is its allowance for a coupled elastic lithosphere on top of a newtonian viscous mantle initially simple box like density drivers illustrate how vertical and horizontal forces are transmitted through the mantle and how the lithosphere responds by trench formation the flexural strength of the lithosphere spatially broadens the surface topography and gravity anomalies relative to the functional form of the vertical flow stresses applied to the plate base i find that drivers in the form of inclined subducting slabs cannot induce selfdriven parallel flow however the necessary flow can be provided by supplying a basal drag of 1 5mpa to the mantle from the oceanic lithosphere these basal drag forces create regional lithospheric stress and they should be quantifiable through seismic observations of the neutral surface the existence of a shallow elevated phase transition is suggested in two slab models of 300 km length where a maximum excess density of 0 2 g m was needed to generate an acceptable mantle flow a north new hebrides subduction model which satisfies flow requirements and reproduces general features of topography and gravity contains a high shear stress zone 75mpa around the upper slab surface to a depth of 150 km and a deviatoric tensional stress in the back arc to a depth of 70 km the lithospheric stress state of this model suggests that slab detachment is possible through whole plate fracture

september 2019 antarctic sudden stratospheric warming quasi 6 day wave burst and ionospheric effects an exceptionally strong stationary planetary wave with zonal wavenumber 1 led to a sudden stratospheric warming ssw in the southern hemisphere in september 2019 ionospheric data from esa s swarm

the depth of the october 1981 off chile outer rise earthquake ms 7 2 estimated by a comparison of several waveform inversion methods the determination of source depth of the october 1981 ms 7 2 outer rise earthquake off the chilean coast has important implications for the stress state of the subducting plate we analyze the chile earthquake using three different waveform inversion methods in order to check the consistency of these methods and to estimate the source depth the methods we used are 1 1 honda and seno s 1989 inversion method in which we incorporate both long period p waves and surface waves 2 2 nab lek s 1984 body wave inversion methods for long period p and sh waves 3 3 seno and honda s 1988 deconvolution method in which we assume vertically distributed point sources and use long period p waves these methods give consistent results which suggest that the centroid depth and depth extent of the chile earthquake lies between 20 to 45 km measured from the sea surface this conclusion is contradictory to those obtained by the previous workers who prefer a shallow 25 km source depth we found that the gdsn long period data alone cannot constrain the source depth well the shape of deconvolved source time function is found to be strongly dependent on the assumption of point and distributed sources and the crustal structure used in the deconvolution the source depth obtained in this study shows that the 1981 chile outer rise earthquake possibly occurred because of the compression associated with the bending of the subducting plate

analysis of the trade off between hypocentral depth and source time function focal depth determination for shallow 60 km intermediate size ms 6 5 to 7 5 earthquakes continues to be problematic for these events the depth phases i e pp and sp are not distinctly separated in time and thus synthetic modeling of wave shapes becomes necessary unfortunately the uniqueness of the solution is not easily determined because of the trade off between various focal parameters we have investigated the trade off between depth and source function both theoretically and experimentally the changes in the green s function caused by an incorrect assumed depth can almost always be compensated by changes in the source function producing the same observed seismogram thus there is considerable trade off between depth and the source function perturbing the depth from the true depth leads to a more complicated source function the simplest source function then pinpoints the true depth we have used two different measures of simplicity the varimax norm v max and the half absolute moment time t 1 2 both measures successfully characterize the complications that appear in the source functions for incorrect assumed depths the true depth of an event can be interpreted from the resultant v max versus depth and t 1 2 versus depth curves thus we have a technique that determines the depth range of faulting in a systematic quantitative fashion our observations from the synthetic study have been applied to an outer rise event off the coast of central chile 16 october 1981 ms 7 2 to an event in southern mexico 24 october 1980 ms 7 0 and to the borrego mountain event in southern california 9 april 1968 ms 6 4 resulting in faulting depth ranges of 0 to 20 5 km 60 to 70 5 km and 10 to 14 5 km respectively

on the temporal variability of the transport through drake passage a recently obtained series of bottom pressure measurements from various positions in the drake passage dp has been used to estimate the transport variability of the antarctic circumpolar current acc under the assumption that the variability is predominantly barotropic and thus proportional to the north minus south pressure difference the standard deviations in derived transport range from 5 3 sv in 1993 to 8 9 sv in 1990 10 day filter 1 sv 106 m3 s 1 all values are less than the 10 sv 10 day filter obtained during the international southern ocean studies isos program at dp between the mid 1970s and early 1980s whitworth and peterson 1985 although some of this discrepancy is due to differences in procedure in dealing with gaps in the data our data suggest that the acc transport can be persistently less variable over several years than was previously thought in particular the isos data set contains two large amplitude changes in pressure difference over timescales less than 2 weeks while no similar events were observed in the more recent data although the more recent pressure gauges were deployed at different positions to those used in isos their reliability as indicators of acc transport changes has been established by examination of simultaneous pressure measurements within dp

inelastic models of lithospheric stress 11 implications for outer rise seismicity and dynamics summary outer rise seismicity and dynamics are examined using inelastic models of lithospheric deformation which allow a more realistic characterization of stress distributions and failure behaviour we conclude that thrust and normal faulting outer rise earthquakes represent substantially different states of stress within the oceanic lithosphere specifically the normal faulting events occur in response to downward plate bending which establishes the standard bending dominated state of outer rise stress and the thrust faulting events occur in response to an elevated level of in plane compression which develops only in response to exceptional circumstances this interpretation accounts for the observation that normal faulting outer rise earthquakes occur more frequently and are more widely distributed than their thrust faulting counterparts an observation for which the simple bending model offers no explanation in addition attributing both thrust and normal faulting outer rise earthquakes to plate bending implies that both classes of events should occur within relatively close lateral proximity to one another because both are allegedly a manifestation of the same bendingdominated stress distribution whereas in reality this is not observed we propose that the tendency for thrust faulting outer rise earthquakes to exhibit greater source depths than their normal faulting counterparts an observation that is frequently cited in support of the bending interpretation of the former is merely a consequence of the fact that bending induced tension is confined to the upper lithosphere our model predicts that outer rise in plane force variations may promote thrust faulting outerrise activity prior to an underthrusting interplate subduction earthquake and normalfaulting outer rise activity following such an earthquake but that both forms of outerrise activity are unlikely to be associated with the same subduction earthquake a corollary implication of our model is that subduction earthquakes are likely to be either preceded by or followed by an absence of large outer rise earthquakes levels of in plane compression necessary to generate thrust faulting outer rise earthquakes are attributed to stress concentrations within the subducting plate that are induced by relatively localized resistance to regionally distributed plate driving forces resistance of this nature may result from either the attempted subduction of relatively buoyant i e isostatically compensated bathymetric features or the existence of strong interplate

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

the 6 5 day wave and its seasonal variability in the middle and upper atmosphere 1 the zonal wave number 1 planetary wave of period near 6 5 days is a robust feature in the mesosphere and lower thermosphere mlt region with prominent seasonal variability as revealed by ground based and satellite observations this wave and its seasonal variability are well reproduced in a recent one model year run of the national center for atmospheric research thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm with its lower boundary specified according to the national centers for environmental prediction analysis year 1993 wavelet analysis of the model output shows that in the mlt region the wave maximizes before and after the equinoxes and minimizes at solstices the wave amplitudes at the equinoxes are smaller than the peaks before and after but are still larger than the wave amplitudes at solstices however at the lower boundary near 30 km the wave peaks are predominantly between fall and the following spring by examining the episodes of maximum and minimum wave amplitude and by conducting additional control experiments using the time gcm the structure of this planetary wave and the factors determining the wave characteristics and seasonal variability are studied in detail it is found that the wave source mean wind structure instability and the critical layers of the wave can all affect the wave response in the mlt region and can have a strong seasonal dependence before and after equinox the wave follows the waveguide and propagates from the stratosphere to the summer mesosphere mesopause where it may amplify due to baroclinic barotropic instability such instability is usually absent from the equinoctial atmosphere so that there is no wave amplification at equinox at solstice the wave decays significantly when propagating away from its winter source due to the strong eastward winter stratospheric jet in the summer side the westward jet is also strong and the meridional and vertical extension of the critical layer of the wave is large enough to enclose the instability in the summer mesosphere mesopause at middle to high latitudes the wave is thus reflected away and prevented from reaching and amplifying at the unstable region the seasonal variation of the quasi two day wave which has zonal phase speed similar to the 6 5 day wave is also studied using similar diagnostics it is further shown that within certain seasonal window periods the variability in the mlt especially the summer mlt may closely track the lower atmospheric variability associated with these waves

on subinertial oscillations trapped by the juan de fuca ridge northeast pacific spectra of currents measured along the 2100 m deep juan de fuca ridge in the northeast pacific have prominent tidal inertial and weather band 3 7 day period spectral peaks the weather band peak in particular has a number of interesting features it is strongest in close lateral proximity to the ridge and strongest near ridge depth intensification near the ridge is characteristic of trapped motion spectral peak intensities vary seasonally with largest amplitudes occurring in autumn and winter seasonal variation suggests that surface weather is forcing flow at depth together trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge at all frequencies forcing topography and stratification together shape current and hydrographic distributions near the ridge effects of those interactions for subinertial motion are quantified here using a primitive equation numerical model forcing period 1 10 days and friction are the principal dependencies examined results show that flow over the juan de fuca ridge can be amplified by factors of 3 4 for diurnal and up to a factor of 7 for weather band frequencies amplification is ridge trapped with effects extending many hundreds of meters upward and extending 5 10 km laterally for 5 day period flow the strength and area of amplification increase with increasing period over the 1 to 10 day period band oscillatory weather band flow leads to vertical velocities on the order of 0 3 cm s 1 on ridge flanks which in turn cause periodic temperature t and salinity s variations with amplitudes of as much as 0 05 c and 0 01 practical salinity units respectively the vertical motion and consequent vertical displacement 100 m lead to isotherms that plunge below the crest alternately each side of the ridge a distribution observed in conductivity temperature depth transect data near the ridge crest cross ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near geostrophic balance with the coriolis force associated with along ridge flow along ridge current amplification is therefore closely tied to isopycnal movement up and down ridge flanks since weather band oscillations lead to much larger cross ridge baroclinic pressure gradients than do diurnal motions of the same amplitude weather band forcing causes greater along ridge current amplification results vary strongly with forcing period and direction of forcing with respect to the ridge but depend only moderately on friction and turbulent mixing coefficients

centroid moment tensor solutions for 113 large earthquakes in 1977 1980 abstract the centroid moment tensor technique is applied to 113 large earthquakes of the period 1977 1980 including all the shallow events with magnitude mb ms or ml 6 5 we use digital data recorded by the global digital seismograph network gdsn and international deployment of accelerometers ida networks solutions are determined for three sequences of events close in time by inverting mantle wave data simultaneously for multiple sources

source inversion of seismic waveforms the koyna india earthquakes of 13 september 1967 abstract the treatment of the seismic source inverse problem when diverse forms of waveform data are available is simple and elegant using a moment tensor formalism if earth structure is known and its effects predictable in terms of vertically inhomogeneous elastic layered models then all types of wave phenomena e g surface waves body waves leaky modes etc for a purely deviatoric moment tensor point source may be represented by at most a sum of three green9s functions for an arbitrary symmetric moment tensor point source one additional green9s function is needed for the p sv system however elegant this formalism may be for posing the linear inverse problem the major difficulties lie in earth structure unknowns and resultant nonlinearities in the green9s functions which can cause significant trade offs with source parameters a hybrid inversion procedure is set up to gain insight into the probable unknowns in particular problems by incorporating both a linearized least squares gradient method for the moment tensor or double couple and smoothed time function parameters and a nonlinear systematic trial and error search for moment tensor or double couple parameters for several assumptions of green9s function the inversion technique is applied to near regional waveform data from a small earthquake associated with the koyna reservoir which occurred 13 september 1967 as the second in a group of three events with very similar waveshapes but differing amplitudes the magnitudes of the second and third events are smaller by 0 2 and 0 8 units respectively compared to the first the absolute magnitude for the first event is poorly constrained but is estimated to be 4 0 to 4 5 rather than the previously published value of 5 5 to 6 0 from the similarity of waveshapes all three events are inferred to have the same mechanism and occurred within about 2 km of the same hypocenter the results from moment tensor and double couple inversions for event 2 data indicate that source depth was 5 km and that left lateral faulting occurred on a plane with a strike of n20 e 5 dip of 90 15 and a rake of 0 35 the inferred far field time function is approximately 3 sec in duration unusually long for the seismic moment of 9 10 22 dyne cm yielding a possible stress drop of about 0 05 bars a fault map was constructed from landsat image interpretation and shows a predominance of nnw to nne striking faults in the koyna area which is used to infer the appropriate nodal plane in the inversion results these faults tend to define a broad en echelon zone which parallels the western ghats in this area

seismic moment tensors and source depths determined by the simultaneous inversion of body and surface waves we present a linear moment tensor inversion method that incorporates both body and surface wave data the source coordinates are fixed during the inversion and the source depth is determined by applying the inversion at several depths and taking the depth that produces the minimum residual the inversion is carried out in the time domain for body wave data and in the frequency domain for surface wave data the fundamental rayleigh waves at the period of 256 s from ida stations and the long period p waves from wwssn and gdsn stations whose angular distances are from 30 to 90 are used as the data the simultaneous inversion of both body and surface wave data sets has several merits because of the body wave constraints the present method gives reasonable solutions at shallow depths 30 km where long period surface wave inversions become unstable although the inversion using only body wave data is unstable in the sense that it allows for a variety of solutions with a slight change in the assumed depth probably because of the noise present and poor coverage of body wave data to determine the source mechanisms the inclusion of surface wave data makes the solution stable this is because the surface wave data constrain nodal plane strikes we may also expect better resolution in source depth by including the body wave data because the local crustal structure above the source can be taken into account to generate the green s function we applied our method to two large earthquakes one occurring at the triple junction off boso peninsula central honshu on 18 september 1984 and another at the outer rise area off the chile trench on 16 october 1981 the scalar moments of the boso oki and chile earthquakes are estimated to be 3 2 1026 dyn cm and 6 3 1026 dyn cm respectively the ratio of the minimum absolute eigenvalue of the moment tensor to the maximum which may measure the non double couple component is less than 10 for both earthquakes this suggests that their sources are essentially a double couple the source depths of both events are determined as 30 km the boso oki event is indicative of hinge faulting within the pacific slab at the triple junction the chile event is consistent with compressional bending stress in the deeper part of the lithosphere at the trench outer rise area off chile

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

depth extent of rupture of the 1981 chilean outer rise earthquake as inferred from long period body waves we have investigated how well the depth extent of rupture can be determined for a shallow dip slip earthquake from long period wwssn body waves the 16 october 1981 chilean outer rise earthquake mw 7 2 is analyzed as an example calculation of the resolution matrix for the vertical distribution of displacement rate on the fault plane shows that the depth extent of rupture cannot be resolved better than 20 km even when the data set consists of both p and s waves although the depth extent of rupture of the 1981 earthquake is too small to be resolved it is possible to infer some constraints from the point source depth in combination with the source duration while inversion for the depth extent of rupture can be formulated as a linear problem determination of the point source depth is nonlinear and a formal resolution study cannot be conducted the point source depth estimate for the 1981 earthquake is 28 to 32 km 80 confidence interval the source duration is 8 sec assuming a rupture velocity of 3 km sec we have explored several rupture models in a synthetic experiment to shed light on the relation between point source depth and centroid depth the experiments show that point source depths agree with the centroid depth given that there is poor resolution of the depth extent of rupture of the 1981 earthquake we conclude from the synthetic experiments that the earthquake may have ruptured updip to a depth as shallow as 17 km or downdip to a depth as deep as 43 km

determination of oceanic dynamic barometer corrections to atmospheric excitation of earth rotation the excitation of earth s rotation by atmospheric pressure fluctuations on timescales from days to years is modified by the oceanic response to the pressure load that response has often been approximated as a static inverted barometer ib change in sea level but evidence from theory and both tide gauge and altimetric data suggests that the ib poorly models the sea level response at short periods static models also fail to account for the currents induced by the pressure loading the actual dynamic response of the oceans to barometric loads depends on frequency and on the spatial structure of the loading this work presents a green s function approach to determining the rotational effects of that oceanic response the green s functions characterize the rotational effects of the oceanic response to pressure loads described spatially by individual spherical harmonics for each harmonic of forcing they were computed at nine periods ranging from 4 days to 18 years and interpolated at intermediate periods computations were based on solutions to the author s dynamic spherical harmonic ocean tide model the effects on rotation of the dynamic barometer oceanic response are obtained by a convolution type combination of the green s functions with actual atmospheric pressure data results for data spanning the past decade will be illustrated using harmonically decomposed national center for environmental prediction ncep and european centre for medium range weather forecasts ecmwf pressure fields

subionospheric vlf signal perturbations possibly related to earthquakes a likely vlf subionospheric signal effect related to seismic activity was first reported by hayakawa et al 1996a b in association with the great kobe earthquake we have analyzed similar data during periods around 10 other great earthquakes magnitude m 6 in order to understand the main features of such an effect the following characteristics emerged from our analysis the effect appears as a transient oscillation with a 5 to 10 day period which is initiated a few days before a large earthquake and decays over a few days to weeks after it it is mainly related to crustal earthquakes it appears when resonant atmospheric oscillations with periods in a range of 5 11 days exist before the earthquake the seismic influence on the vlf signal is probably explained by the generation of long period gravity waves during the earthquake process and their intensification at heights of 70 90 km

observations of the 5 day wave in the mesosphere and lower thermosphere the 5 day planetary wave has been detected in the winds measured by the high resolution doppler imager hrdi on the upper atmosphere research satellite uars in the mesosphere and lower thermosphere 50 110 kin the appearances of the 5 day wave are transient with a lifetime of 10 20 days in the two year data set the structures of selected 5 day wave events are in generally good agreement with the 1 1 rossby normal mode for both zonal and meridional components a climatology of the 5 day wave is presented for an altitude of 95 km and latitudes mainly between 40os and 40on

earthquake seismograms that show doppler effects due to crack propagation summary examples are presented of earthquake p wave pulses seen on broadband seismograms to show that on such recordings the pulse shapes are more clearly seen than on conventional short period and long period seismograms most of the broadband seismograms have been chosen because they show marked differences between the pulse lengths of p and those of the surface reflections in addition some of the pulses appear to have smooth onsets and abrupt trailing edges so that the onset of the pulse is difficult to observe and the largest amplitude arrivals seen on the seismogram coincide not with the onset of motion but with the termination of motion that is the large arrivals mark stopping phases of motion we assume that the differences in pulse length are due to the effects of a moving source that is a doppler effect and that the pulses with smooth onsets and abrupt trailing edges can be modelled simply by a source propagating on a line with low radiation amplitude at the start of motion a trial and error method guided by a published fault plane solution is then used to obtain a fit between observed and computed seismograms for one of the earthquakes this process leads to an estimate of the crack speed of about 1 4 times the 5 wave speed the errors that may arise in estimating source depths and orientation if stopping phases are not recognized as such is discussed

rupture process of the february 4 1965 rat islands earthquake the great rat islands underthrusting earthquake mw 8 7 of february 4 1965 represents subduction of the pacific plate beneath the north american plate along a 600 km segment of the western end of the aleutian islands body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the rat islands earthquake we have deconvolved world wide standard seismograph network long period teleseismic p wave seismograms to obtain source time functions directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s unilateral rupture in the direction 300 fault length of 420 km and average rupture velocity of 2 5 km s the three pulses of moment release are located along the fault and these regions of high moment release are interpreted as asperities the first asperity extends from the epicenter to 100 km to the wnw this is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s the second pulse of moment release is very jagged is less coherent between stations and is centered 200 km wnw of the epicenter the third pulse of moment release extends from 360 to 420 km wnw of the epicenter although the aftershock area is 600 km in length we can not resolve any moment release from the p waves beyond 420 km wnw of the epicenter the rat islands event was closely followed by a large tensional outer rise event on march 30 1965 ms 7 5 which is located oceanward of the largest moment release associated with the rat islands mainshock rupture detailed analysis of the p waves for this large outer rise event confine the depth extent to the upper 30 5 km of the crust the spatial and temporal association between the february 4 mainshock and the march 30 tensional outer rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter the overriding plate along the western aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins geist et al 1988 we suggest that the central undeformed parts of the blocks have the strongest coupling with the down going plate and hence are the sites of the largest moment release during an underthrusting earthquake the three asperities determined from the p waves correspond to the rat buldir and near tectonic blocks respectively hence the p wave seismic moment release of the rat islands earthquake is controlled by the lateral segmentation of the overriding plate

the 1977 sumba earthquake series evidence for slab pull force acting at a subduction zone the great 1977 sumba earthquake occurred at the eastern sunda trench just west of the collision of australian continental lithosphere with the island arc the length of the aftershock zone of this normal faulting earthquake is about 200 km aftershocks are concentrated 65 115 km east of the main shock epicenter with very few aftershocks in a 50 km long segment that spans the main shock epicenter relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere there is no evidence for thrust faulting of the deeper aftershocks these data imply that the neutral bending surface must be at least 35 40 km deep a second aftershock zone about 180 km northwest of the main shock became active immediately following the main shock but events were concentrated during days 50 52 this zone is a 70 km long lineation that trends toward the main shock epicenter and reflects right lateral strike slip faulting within the subducted oceanic plate seismicity exists to a depth of about 650 km in the very old plate beneath the sunda banda arc and that plate s negative buoyancy causes very large slab pull forces great interface thrust earthquakes are absent at the sumba region and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate this decoupling permits slab pull stresses to be guided updip to the region of the sumba main shock such shallow acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire sumba earthquake series in this model slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake postseismically the released oceanic plate undergoes a pulse of downdip strain returning the plate to a less extended state the moment of this downdip plate motion could exceed the seismic moment of the main shock

departures from the local inverse barometer model at periods of 5 days in the central south atlantic tide gauge data from ascension and st helena in the central south atlantic supplemented by altimeter data from the topex poseidon mission suggest that the local inverse barometer lib model for the forcing of sea level variability by means of air pressure changes is not applicable within a narrow band of the variability spectrum with periods of approximately 5 days this is in contrast to other sections of the wider frequency band spanning approximately 0 05 to 0 5 cycles d wherein the data are either consistent with approximate lib response or at least suggest no departures from the lib model of similar magnitude these results have similarities to findings from pacific gauges obtained over a decade ago but so far no comparisons have been to data from other ocean areas they are also qualitatively similar to predictions for the region from a recent global barotropic numerical model the anomalous part of the spectrum coincides in frequency with the madden and julian 1972 1973 5 day pressure wave in the atmosphere if that global scale forcing is responsible for both the atlantic and pacific departures from the lib model around 5 days then similar features should be observable in tide gauge data from other tropical areas

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

inelastic models of lithospheric stress i theory and application to outer rise plate deformation summary outer rise stress distributions determined in the manner that mechanical engineers evaluate inelastic stress distributions within conventional materials are contrasted with those predicted using simple elastic plate models that are frequently encountered in studies of outer rise seismicity this comparison indicates that the latter are inherently inappropriate for studies of intraplate earthquakes which are a direct manifestation of lithospheric inelasticity we demonstrate that the common practice of truncating elastically superimposed stress profiles so that they are not permitted to exceed laboratory based estimates of lithospheric yield strength will result in an accurate characterization of lithospheric stress only under relatively restrictive circumstances in contrast to elastic plate models which predict that lithospheric stress distributions depend exclusively upon the current load inelastic plate models predict that stress distributions are also significantly influenced by the plate loading history and in many cases this influence is the dominant factor in determining the style of potential seismicity e g thrust versus normal faulting numerous intuitive interpretations of outer rise earthquakes have been founded upon the implicit assumption that a unique relationship exists between a specified combination of plate curvature and in plane force and the resulting lithospheric stress distribution we demonstrate that the profound influence of deformation history often invalidates such interpretations finally we examine the reliability of yield envelope representations of lithospheric strength that are constructed on the basis of empirically determined frictional sliding relationships and silicate plastic flow laws although representations of this nature underestimate the strength of some major interplate faults such as the san andreas they appear to represent a reliable characterization of the strength of intraplate oceanic lithosphere

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

pressure variations on the atlantic equator records of more than a year s duration from three deep ocean pressure sensors and three island shallow pressure sensors all close to the equator between 30 w and 7 e are analyzed spectrally at frequencies below the diurnal tides the records were taken as integral parts of the focal sequal experiments in the equatorial atlantic 1983 1984 the low frequency gravitational tides at 1 2 3 cycles per month formed an important nearly simultaneous part of the signal and had to be removed the seasonal variations associated with equatorial upwelling are prominent in the shallow but not the deep pressure signals practically all the non tidal deep pressure signals at 0 1 0 7 cycles per day cpd are highly coherent between 20 w and 6 e some 2900 km and show no sign of zonal propagation a comparison of the coherent part of the signals between the equator and ascension island 8 s suggests a southward component of propagation at least in the region of the mid atlantic ridge these motions are evidently barotropic and are not trapped near the equator a close relationship to the atmospherically forced barotropic waves at 4 6 days period in the pacific identified by luther 1980 is suggested special consideration is given to a comparison of a 3 6 km deep pressure record at 6 e with a shallow pressure record from ilha sao tome supplied by j m verstraete their ordinary tidal components are practically identical as befits purely barotropic motion but the sub tidal continuum shows fairly low though significant coherence with higher energy in the island record this is attributed to the presence of internal wave modes of several meters internal amplitude theoretically the contribution of internal waves should give a much stronger signal in the surface pressure than in the deep pressure signal the relationships are demonstrated by calculations based on acoustic return times by two inverted echo sounder recorders placed close to deep pressure sites by e j katz deduced coherences agree reasonably with observed values and are consistent with internal waves dominated by mode 1 uncorrelated with the barotropic motion sensed in both deep and shallow pressure gauges

evidence of a 4 6 day barotropic planetary oscillation of the pacific ocean abstract spectral analysis of scattered island and coastal tide gage records from the pacific ocean reveals the presence of a coherent sea level fluctuation at 4 6 days period the oscillation is distinct from baroclinic inertia gravity wave fluctuations of sea level at the same periods that are trapped to the central pacific equatorial zone concomitant spectral analysis of island surface weather data demonstrates that sea level is forced by surface atmospheric pressure but does not respond statically like an inverted barometer the basinwide character and uniform westward propagation of the oscillation suggest the presence of a barotropic planetary wave s however the oscillation is strongly attenuated with an estimated energy e folding time of less than three days

near 5 day nonisostatic response of the atlantic ocean to atmospheric surface pressure deduced from sub surface and bottom pressure measurements 1 nonisostatic ocean responses to atmospheric pressure have been observed in tropical regions of the pacific and atlantic oceans at periods near 5 days barotropic ocean model simulations coupled to atmospheric forcing predicted that this nonisostatic ocean response is driven not regionally but globally by the rossby haurwitz wave to date little observational evidence has been provided to support the model simulations especially at extratropical latitudes here we present the basin scale nature of the nonisostatic response in the atlantic ocean near 5 day periods using four historical long term 1 5 year sub surface and bottom pressure measurements spanning from 16 s to 37 n joint analysis of them together with global gridded atmospheric pressure reveals a basin scale nonisostatic sea level fluctuation in the north and tropical atlantic oceans near 5 day periods with almost uniform phase it also confirms that the driving force for this near 5 day fluctuation is the westward propagating rossby haurwitz wave

quasi dynamic models of simple earthquakes application to an aftershock of the 1975 oroville california earthquake a class of kinematic rupture models for simple earthquakes is proposed these models incorporate results from both theoretical and numerical solutions to the mixed boundary value problem of a three dimensional frictional rupture the analytic form for the slip velocity is divided into two phases the rupture growth during which the slip distribution is self similar kostrov 1964 and the healing during which the slip velocity decreases monotonically to zero the arrival of a p wave stopping phase propagating into the interior of the rupture area from the fault perimeter initiates the healing the duration of healing may be varied to produce a gradual or abrupt stopping of the rupture which permits modeling a wide range of high frequency spectral behavior these models are used to generate synthetics which are fit to the velocity and the velocity squared pulse shapes obtained from strong motion accelerograms of the 0103 aftershock of the 1975 oroville california earthquake the aftershock ml 4 6 has a moment of 1 8 1023 dyne cm and a total radiated energy of 2 7 1019 dyne cm the fitted source model propagates updip with a rupture velocity of 0 85 and a dynamic stress drop of 210 bars the arrival times of five stopping phases correlated across the suite of accelerograms are used to determine the relative locations of the hypocenter and five stopping events the description of rupture determined from these stopping events both corroborates the source model obtained from the waveform modeling and establishes an important counterpoint to the model by providing a dynamic interpretation of the unmodeled complexity of the waveforms

ocean currents at axial volcano a northeastern pacific seamount 1 axial volcano is a magmatically active seamount that straddles the juan de fuca ridge jdfr with a summit depth of 1400 m the volcano intercepts deep ocean flows at a height well above that of the linear ridge adjoining it to the north and south multiyear yearlong observations of currents at axial show that mean flow circles the seamount in an anticyclonic sense consistent with dynamical constraints with means as large as 6 6 cm s 1 spectra of currents indicate prominent tidal inertial and in some cases weather band period 3 to 7 day peaks as has been seen elsewhere along the jdfr ridge for k1 o1 and s2 tidal currents at the seamount amplitudes are several times larger than the corresponding background amplitudes and rotation around the tidal current ellipses is anticyclonic m2 tidal currents show none of the same properties energy at the inertial frequency decreases by more than an order of magnitude from 100 m above to 400 m below the volcano s summit in contrast weather band wb period energy increases tenfold over the same depth range with wb energy all but unobservable at summit depth the presence of wb oscillations makes axial unlike other seamounts where long observations have been made the current meter observations also provide a picture of seasonal trends as expected during the period summer 1996 to spring 1997 largest amounts of wb energy are found during autumn and winter in contrast wb motion at axial volcano during winter 2000 was summer like in intensity a linear baroclinic model of forced damped azimuthal mode one oscillatory flow around an idealized axisymmetric seamount shows that amplification of diurnal flow should occur principally at the seamount summit while amplification of the wb should occur as the observations show on the seamount flanks model and measured phase relationships for temperature and current time series pairs circumscribing axial support the view that azimuthal mode one oscillations are prominent over the 0 8 to 6 day range for forcing period tc the model indicates that the principal topographic resonance as indicated by kinetic energy ke amplification occurs for a tc greater than 1 and less than 2 days with only mild dependence in that range on background stratification and seamount shape additional but weaker topographic resonances can occur at longer periods in model results but as tc becomes slightly larger than the frictional timescale topographic resonances no longer appear high dissipation and thus short frictional timescales reported by others in regions of rough sloping topography make topographic resonances at axial at periods greater than a few days somewhat unlikely the relatively broad wb spectral peaks at some axial sites are consequently more likely the result of spectral peaks in forcing than of topographic resonance

spectral and geographical variability in the oceanic response to atmospheric pressure fluctuations as inferred from dynamic barometer green s functions 1 a decade ago a novel theoretical approach was developed dickman 1998 for determining the dynamic response of the oceans to atmospheric pressure variations a response nicknamed the dynamic barometer db and the effects of that response on earth s rotation this approach employed a generalized spherical harmonic ocean tide model to compute oceanic green s functions the oceans fluid dynamic response to unit amplitude pressure forcing on various spatial and temporal scales and then construct rotational green s functions representing the rotational effects of that response when combined with the observed atmospheric pressure field the rotational green s functions would yield the effects of the db on earth s rotation the green s functions reflect in some way the geographical and spectral sensitivity of the oceans to atmospheric pressure forcing we have formulated a measure of that sensitivity using a simple combination of rotational green s functions we find that the db response of the oceans to atmospheric pressure forcing depends significantly on geographic location and on frequency compared to the inverted barometer ib the traditional static model the db effects differ slightly at long periods but become very different at shorter periods among all the responses the prograde polar motion effects are the most dynamic with large portions of the north atlantic and some of the north pacific no larger than one third of ib but most of the southern hemisphere oceans at least 50 greater than ib

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

mode of stress release within a subducting slab of lithosphere implication of source mechanism of deep and intermediate depth earthquakes we inverted the long period p wave seismograms of 28 deep and intermediate depth earthquakes including 16 tonga events to their sources the results are incorporated with our previous results for 18 deep and intermediate depth earthquakes to obtain a general picture of slab earthquakes a large event with seismic moment mo 5 1027 dyn cm tends to be a multiplet that consists of several subevents a small event with mo 5 1026 dyn cm remains a singlet stress drop of the largest subevent is 800 bar on average and significantly higher than 200 bar of the largest subevent for interplate thrust earthquakes stress drop tends to scatter for smaller earthquakes beyond the resolution limit of our analysis deep and intermediate depth shocks in the tonga region possess higher indicating a significant regionality in the mode of stress release within a subducting slab fault planes have in general preferred orientation according to the focal depth indicating alignment of weak planes within the slab the slab is therefore not simply elongated or shortened under down dip extension or compression but rather rotates by bulk simple shear we suggest that this bulk shear is a dominant mechanism for the deformation of a subducting plate including its bending and unbending near deep sea trenches

normal modes of the world ocean part ii description of modes in the period range 8 to 80 hours abstract we have calculated normal modes with period between 8 and 80 h for a domain consisting of the arctic atlantic indian and pacific oceans in this period range the numerical model has 56 modes of which 13 are topographic vorticity waves all slower than 30 h the trapping sites for these modes are the siberian shelf the icelandic plateau the grand banks the falkland plateau and patagonian shelf the kerguelen plateau the new zealand and fiji plateaus and the hawaiian ridge predominantly planetary vorticity waves do not appear in the model at periods less than 80 h the 41 modes found between 30 and 8 h include basin modes in the north atlantic indian and equatorial pacific quarter wave resonances in the arabian sea bay of bengal and gulf of guinea and kelvin waves on the antarctic coast the pacific north american coast and the new zealand coast several vorticity and gravity modes exhibit an eastward circumglobal flow of energy that is confined to equatorial latitudes except where defl

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

earthquake mechanisms and tsunami generation the kurile islands event of 13 october 1963 abstract investigations of tsunamigenesis on a spherically symmetric and elastic earth using normal mode summations and moment tensor line sources are extended to include general double couple mechanisms so that computed tsunami waveforms can be compared with specific observations amplitudes of 19 tide gauge records of the kurile islands sea wave of 13 october 1963 are reproduced within a factor of 2 by either a single line source 250 km in length or by a triplet of parallel line sources separated by 42 km both models predict a strong beaming of tsunami strength with wave heights toward the northwest and southeast three to eight times larger than those in the northeast and southwest for faults over 150 km in length 2 of the 6 moment tensor components dominate tsunami production this fact motivated the coining of a new parameter tsunami moment m t by cataloging tsunami fields produced by suites of fault mechanisms i find that m t is approximately proportional to the tsunamigenic potential of earthquakes despite the critical role of fault mechanism in tsunami generation many sources have nearly equal tsunami moment and nearly equal ability for sea wave excitation estimates of shallow source strength from measurements of elastic surface waves have questionable integrity at both long 100 sec and short 30 sec periods amplitudes of high frequency waves suffer from saturation due to source size and duration while low frequency amplitudes are unstable with respect to small perturbations in fault mechanism because of the influence of the earth9s surface many tsunami earthquakes may be artifacts of underestimated source strength resulting from the inefficiency of shallow nearly vertical dip slip faults to produce long period surface waves

a study of tidal and planetary wave periodicities present in midlatitude sporadic e layers the diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic e layers acting through their vertical windshear forcing of the long living metallic ions in the lower thermosphere also recent studies suggested that planetary waves play a role on sporadic e generation as well a fact that went unnoticed in the long going research of sporadic layers in this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic e critical frequencies foes in this approach standard analysis techniques used in neutral atmospheric dynamics are applied on foes time series obtained during summertime when sporadic e occurrence is nearly continuous it is shown that besides the dominant and known 24 hour and 12 hour tidal periodicities in foes there is often a weaker terdiurnal 8 hour oscillation present as well in addition there are planetary wave periodicites in foes with periods near the normal rossby modes that is 2 5 10 and 16 days it is also found that the tidal oscillations in foes undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data our results are in line with recent findings based on a single event study which suggested that sporadic e layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes the close relationship between neutral wave dynamics and midlatitude sporadic e periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere

lithospheric stress and deformation the study of the mechanisms magnitudes and modes of lithospheric stress and deformation occupies a central position in the discipline of tectonophysics it is therefore difficult in assembling a comprehensive review to place limits on what should be included for example one cannot discuss stress and deformation apart from considerations of the rheological models of the lithosphere which link the two and ultimately control the behavior of plates at their margins and in their interiors once admitting to the importance of rheological considerations in discussing stress and deformation one is then compelled to include information relating to the thermal state of the lithosphere since temperature is one of the foremost environmental variables controlling rheology furthermore lithospheric processes cannot be considered in isolation from those affecting the asthenosphere since surely there exists some coupling between the motion of the lithosphere and convection in the earth s interior in fact an understanding of lithospheric stress and deformation can be considered a prerequisite to solving problems in geodynamics because it is through the lithospheric filter that one detects the consequences of thermal and compositional anomalies at greater depth the topic of this report cannot even be cleanly separated from reviews in other disciplines such as seismology and geodesy the magnitude and orientation of lithospheric stress is often indicated by earthquakes while lithospheric strain can be observed geodetically the philosophy i have adopted here is that some overlap with other review papers in this series is unavoidable and probably desirable from the standpoint of emphasizing how recent work n other areas of geophysics has contributed to the advancement in our understanding of the lithosphere s state of stress and mechanisms of deformation i have however resisted the temptation to include extraterrestrial studies presumably covered in the planetary section despite the similar causes for deviatoric stress and deformational mechanisms operating on other terrestrial planets certainly ideas of how silicate materials behave and how lithosphere responds to internal forces benefit from the perspective of omparative planetology finally in defining the scope of this portion of the u s national report to the iugg i did not consciously exclude papers by non u s citizens work done at non u s institutions projects not funded by the u s government or however else foreign might be defined topics in geophysical research cannot be divided along political boundaries however i did not attempt to seek out articles written in languages other than english or published in journals not readily available in the united states and i apologise for this obvious shortcoming

the 6 5 day wave in the tropical stratosphere and mesosphere 1 the 6 5 day wave is a westward propagating zonal wave number 1 s 1 planetary wave that is prominent near equinox the spatial temporal structure of the 6 5 day wave seen in the united kingdom meteorological office ukmo data in the stratosphere is found to be consistent with that obtained from upper atmosphere research satellite high resolution doppler imager hrdi measurements of horizontal winds and temperatures in the mesosphere lower thermosphere mlt two wave events are commonly observed before and after equinox at low latitudes in both ukmo and hrdi data this paper focuses on a pair of such events in late 1994 the amplitude growth and phase decrease with altitude of the zonal wind 6 5 day wave appear to be continuous between the two data sets for the period analyzed both wave events appear to grow from the stratosphere to the upper atmosphere at a rate consistent with the suggested vertical phase velocity such consistency between the two data sets establishes that the wave previously observed in the mlt is propagating from the stratosphere further analyses of the ukmo data shows the presence of the 10 day s 1 and 4 day s 2 waves that appear to be either nonlinearly coupled with the 6 5 day wave or related through a common sourcing mechanism