the 1989 macquarie ridge earthquake seismic moment estimation from long period free oscillations we have analyzed the long period 1000 s part of the earth s free oscillation spectrum of the 1989 macquarie ridge earthquake this event stands out as the largest in the last decade and as the first great earthquake to have been recorded digitally on the new generation of very broadband seismic sensors around the globe its spectrum was modeled for five spectral peaks 0s4 0s6 0s9 1s8 and 5s3 taking into account the effects of lateral heterogeneity rotation and ellipticity which cause coupling within the spectral peaks self coupling and between different multiplets multiplet coupling the effects of self coupling were calculated following the formulation of woodhouse and girnius 1982 multiplet coupling only weakly affected the spectral peaks that were used a large discrepancy in phase and amplitude between the calculated spectrum and the data was observed which suggests that the seismic moment was 50 larger than that inferred from surface wave analysis satake and kanamori 1990 an error analysis was performed in order to detect inaccuracies that might have resulted in this discrepancy centroid time source finiteness correction and directivity effects are likely to cause a significant bias both in the moment and phase determination we saw no direct evidence that a precursory event could have been responsible for the additional energy release as was suggested by ihmle et al 1993

source finiteness of large earthquakes measured from long period rayleigh waves the source finiteness parameters of 11 large shallow earthquakes were determined from long period rayleigh waves recorded by the global digital seismograph network and international deployment of accelerometers networks the basic data sets are the seismic spectra of periods from 150 to 300 s in the determination of source process times we used furumoto s phase method and a linear inversion method in which we simultaneously inverted the spectra and determined the source process time that minimizes the error in the inversion these two methods yielded consistent results the source process times of the sumbawa indonesia colombia ecuador valparaiso chile and michoacan mexico earthquakes were estimated to be 79 118 69 and 77 s respectively from the linear inversion method the source process times determined from long period surface waves were in general longer than those obtained from body waves source finiteness of large shallow earthquakes with rupture on a fault plane with a large aspect ratio was modeled with the source finiteness function introduced by ben menahem the spectra were inverted to find the extent and direction of the rupture of the earthquake that minimize the error in the inversion for a rupture velocity of 2 5 km s 1 the estimated rupture was unilateral 100 km long and along the strike n26 w for the may 26 1983 akita oki japan earthquake 165 km and s57 e for the september 19 1985 michoacan mexico earthquake 256 km and n31 e for the december 12 1979 colombia ecuador earthquake and 149 km and s15 w for the march 3 1985 valparaiso chile earthquake the results for the august 19 1977 sumbawa indonesia earthquake showed that the rupture was bilateral and in the direction n60 e these results are in general consistent with the rupture extent inferred from the aftershock area of these earthquakes

teleseismic detection of a slow precursor to the great 1989 macquarie ridge earthquake low frequency spectra for the 1989 macquarie ridge earthquake magnitude 8 2 show an amplitude increase and a phase delay decrease below 6 millihertz that require a short term slow precursor this earthquake can be modeled as a compound event in which a fast rupturing ordinary earthquake was initiated by an episode of slow smooth deformation that began more than 100 seconds before the main shock the moment released in the slow precursor was large about 3 x 10 20 newton meters equivalent to an event of magnitude 7 6 the data are consistent with the precursor being generated in a region of the oceanic upper mantle below the main rupture

long period seismic radiation from the may 23 1989 macquarie ridge earthquake evidence for coseismic slip in the mantle long period seismic source parameters of the may 23 1989 macquarie ridge earthquake are determined using a surface wave inversion procedure that incorporates detailed source time functions obtained from shorter period body waves the seismic source model obtained using this method is consistent with observations of both body waves and long period rayleigh waves from the earthquake the macquarie ridge earthquake rupture has a centroid time 28 s and a right lateral strike slip fault mechanism with a rake of 175 on a vertical fault plane striking n38 e this mechanism is consistent with p wave first motions of the event inversions performed for various earth models demonstrate that the choice of surface wave attenuation model in particular affects the estimates of centroid depth and seismic moment significantly allowing for uncertainty in attenuation the long period rayleigh waves periods from 150 to 300 s indicate that the macquarie ridge earthquake had a seismic moment of 1 9 0 2 1021 n m and a corresponding centroid depth of 21 6 km the static stress drop calculated using the depth and seismic moment is 0 7 0 5 107 pa 70 50 bars the centroid depth combined with the lack of resolvable directivity of the earthquake rupture suggests that significant slip occurred beneath the moho which has a maximum depth of about 16 km in the epicentral region the mantle slip component may have preferentially radiated long period seismic energy given the shallow centroid depths and low moments determined for the event from the shorter period body wave observations

radial mode observations from the 5 23 89 macquarie ridge earthquake the radial modes 1s0 5s0 axe clearly observable on several long period seismic records from the 5 23 89 macquarie ridge earthquake observations of 0s0 are weaker but stacking demonstrates significant excitation calculations suggest that for spherical earth models no simple double couple in the uppermost mantle can explain the excitation amplitudes of 1s0 5s0 relative to 0s0 calculations suggest that the anomalous amplitudes of 1s0 5s0 can be explained by coupling to other modes induced by plausible levels of deep earth lateral structure the coupling of 0s0 is much weaker and its observed amplitude suggests that roughly 4 of the total moment release inferred from surface wave studies deviated from pure strike slip geometry this can be modeled by 8 deviations from 0 90 in a simple double couple point source or by thrust subevents within the main shock

source characteristics of the nicaraguan tsunami earthquake of september 2 1992 a large ms 7 2 shallow earthquake took place off the pacific coast of nicaragua on september 2 1992 and set off large tsunamis the ground motion of the main shock was very weak along the whole nicaraguan coast a tsunami as high as 10 m was observed at el transito whereas a height of about 2 m is empirically expected for an earthquake of ms 7 2 the tsunami magnitude mt is estimated to be 7 9 from tide gage data the nicaraguan event is a tsunami earthquake which generates unusually large tsunamis for its earthquake magnitude the source mechanism is studied in detail by using waveforms of body waves and surface waves recorded on global broadband seismographs the possibility of single force source is ruled out from radiation patterns and the amplitude ratio of rayleigh and love waves the main shock is interpreted as a low angle thrust fault with strike 302 dip 16 and slip 87 the cocos plate underthrusting beneath the caribbean plate the seismic moment from surface wave analysis is 3 0 1020 nm mw 7 6 the source dimension is estimated to be 200 km 100 km from the aftershock area the inversion results of body waves suggest bilateral rupture with rupture velocity as low as 1 5 km s and duration time of about 100 s the source process time is unusually long indicating that the associated crustal deformation has a long time constant

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

effects of latent heat release at phase boundaries on flow in the earth s mantle phase boundary topography and dynamic topography at the earth s surface abstract mantle flow models that do not consider the effects of latent heat on phase boundaries typically predict dynamic surface topography too large to be compatible with observations here these effects were implemented in a mantle flow model and resulting changes in dynamic topography and topography of phase boundaries were computed inclusion of these effects was found to reduce the rms amplitude of dynamic topography by about 50 60 m still leaving a substantial misfit from observations in most cases considered generally it also leads to a slight improvement of correlation with dynamic surface and phase boundary topography constrained by observations results thus indicate that the model implemented here is applicable to the earth but that other effects need to be considered as well in order to fully explain phase boundary and dynamic surface topographies

slabs hotspots cratons and mantle convection revealed from residual seismic tomography in the upper mantle to further investigate the fate of slabs the structure of hotspots and the style of mantle convection we calculate residual tomographic maps in the upper mantle by excluding from the tomography the first order effects of conductive cooling of oceanic plates deep cratonic roots and cooling or partial melting associated with subducted lithosphere no correlation is found between residual tomography in the upper mantle and the 30 130 ma subduction the good correlations between residual tomography in the transition zone 400 650 km and 0 30 ma subduction at spherical harmonic degree l 2 can be explained either by slab accumulation in the transition zone beneath some subduction zones or by the poor lateral resolution of seismic tomography hotspots correlate with the residual tomography in the shallow 100 400 km and the lowermost mantle at degree l 2 however correlations of hotspots and seismic velocities in the middle mantle and correlations between residual tomography at shallow depths and lowermost mantle tomography are poor therefore the connection between residual tomography at shallow depths and seismic tomography in the lowermost mantle is not clear craton distribution correlates with distribution of hotspots and residual topography very well at degree l 6 this might indicate that cratonic roots may affect the locations of upwellings and thus modulate upper mantle convection or that the degree l 6 could be intrinsic in upper mantle convection with cratons and and other near surface features establishing the pattern

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

rupture process of the macquarie ridge earthquake of may 23 1989 broadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the may 23 1989 macquarie ridge earthquake the centroidal solution strike 211 degree dip 86 degree rake 180 degree and depth of 10 km below the seafloor indicates shallow rupture with pure right lateral strike slip motion along the pacific australia plate boundary in agreement with motion predicted by plate tectonic models the total seismic moment is 13 4 times 10 sup 20 nm 80 of which was released in the first 24 s of the rupture process modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions the estimated fault length is quite short about 90 km and the derived stress drop of 180 bar and average displacement of 17 m are unusually high the bathymetry in the epicentral region shows topographic segmentation of the ridge possibly indicating fault segmentation which confines ruptures to short segments

receiver function analysis of the north american crust and upper mantle summary data from 27 permanent broad band stations spread over north america and sampling diverse tectonic regimes of varying ages from the archaean to the phanerozoic were analysed using the receiver function approach receiver functions a few hundred at most of the locations provide valuable constraints on the composition of the crust shaped by diverse processes characteristic of their time of formation the chemically inert and stable nature of the archaean domains of canada is well manifested in their low poisson ratio 0 26 felsic lower than global averages for precambrian age crust accompanied by a thicker than global average 35 km archaean crust data from the canadian cordillera and innuitian orogens of phanerozoic times indicate the possible operation of two distinctly different crustal processes resulting in thin crusts 35 km with dominantly felsic and more mafic compositions respectively these processes are restricted to the region above 410 km without disturbing the integrity of the mantle transition zone mtz which is evident from the remarkably homogeneous nature of the 410 and 660 km discontinuities beneath the canadian landmass contrastingly the thermally complex phanerozoic southern california region suggests the presence of a heterogeneous mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410 overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent

the 23 may 1989 macquarie ridge earthquake a very broad band analysis we use data from the geoscope global seismograph network to determine source parameters for the 23 may 1989 macquarie ridge earthquake by moment tensor inversion of long period surface waves we obtain a seismic moment of 1 8 2 4 1021 n m which makes this the largest seismic event to have occurred worldwide since the 1977 sumba earthquake the focal mechanism shows pure right lateral strike slip motion on a plane parallel with macquarie ridge and the pacific australian plate boundary the mechanism is consistent with the strike slip motion predicted for this area from relative plate motion models we invert broad band teleseismic p and sh waveforms to determine source complexity using the results from the long period analysis as a priori soft constraints the broad band waveforms show that the release of seismic energy was concentrated in the first 30 s of seismic rupture but fault slip continued for more than 60 s lack of directivity in the long period and broad band waveforms indicates that the rupture propagated bilaterally or vertically from the nucleation point

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

centroid moment tensor solutions for july september 1992 abstract centroid moment tensor solutions are presented for 224 earthquakes that occurred during the third quarter of 1992 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

moho map of south america from receiver functions and surface waves 1 we estimate crustal structure and thickness of south america north of roughly 40 s to this end we analyzed receiver functions from 20 relatively new temporary broadband seismic stations deployed across eastern brazil in the analysis we include teleseismic and some regional events particularly for stations that recorded few suitable earthquakes we first estimate crustal thickness and average poisson s ratio using two different stacking methods we then combine the new crustal constraints with results from previous receiver function studies to interpolate the crustal thickness between the station locations we jointly invert these moho point constraints rayleigh wave group velocities and regional s and rayleigh waveforms for a continuous map of moho depth the new tomographic moho map suggests that moho depth and moho relief vary slightly with age within the precambrian crust whether or not a positive correlation between crustal thickness and geologic age is derived from the pre interpolation point constraints depends strongly on the selected subset of receiver functions this implies that using only pre interpolation point constraints receiver functions inadequately samples the spatial variation in geologic age the new moho map also reveals an anomalously deep moho beneath the oldest core of the amazonian craton

detecting slow long duration slip of large earthquakes using very long period orbital surface waves summary in this paper we present a simple yet effective technique for measuring the source time delay spectrum from very long period world circling surface waves without relying on assumed earth models or calculation of synthetic seismograms by using autocorrelation and self convolution functions derived from whole seismograms of large earthquakes recorded at a seismic station it is possible to isolate the source effects and precisely 5 5 s determine the source delay spectrum at periods as long as 600 s measurements at such long periods are necessary for detecting slow precursory slip and slow creep after the main fast seismic phase of large earthquakes we apply this technique to five recent large earthquakes the 1989 macquarie ridge m 8 2 the 1992 nicaragua m 7 6 the 1993 mariana islands m 7 8 and the 1994 deep events in fiji m 7 6 and bolivia m 8 3 our results are largely in agreement with the results obtained by other investigators the mariana islands fiji and bolivia earthquakes had a normal rupture duration the nicaragua earthquake exhibits a source time delay spectrum with the delay increasing from 31 s at the 100 s period to 50 s at the 600 s period indicating an unusually slow rupture process the macquarie ridge earthquake is anomalous by the discrepancy between the love wave and rayleighwave estimates love waves in the period band of 150 to 600 s show a normal flat source delay spectrum with a delay of about 28 s the rayleigh wave spectrum agrees with the love wave spectrum for periods smaller than 200 s and greater than 400 s however it shows a significantly shorter source delay about 15 s in the 250 to 350 s period range the unusual nature of the macquarie ridge earthquake s surface wave spectrum was pointed out by ihmli harabaglia jordan 1993 who also provided an explanation in terms of a slow precursory moment release this explanation is not consistent with the flat love wave spectrum determined here

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

teleseismic search for slow precursors to large earthquakes some large earthquakes display low frequency seismic anomalies that are best explained by episodes of slow smooth deformation immediately before their high frequency origin times analysis of the low frequency spectra of 107 shallow focus earthquakes revealed 20 events that had slow precursors 95 percent confidence level 19 were slow earthquakes associated with the ocean ridge transform system and 1 was a slow earthquake on an intracontinental transform fault in the east african rift system these anomalous earthquakes appear to be compound events each comprising one or more ordinary fast ruptures in the shallow seismogenic zone initiated by a precursory slow event in the adjacent or subjacent lithosphere

elasticity of single crystal olivine at high pressures and temperatures abstract elasticity of single crystal san carlos olivine has been derived from sound velocity and density measurements at simultaneous high pressure temperature conditions up to 20 gpa and 900 k using in situ brillouin spectroscopy and single crystal x ray diffraction in externally heated diamond anvil cells these experimental results are used to evaluate the combined effect of pressure and temperature on full elastic constants of single crystal olivine to better understand its velocity profiles and anisotropies in the deep mantle analysis of the results shows that the shear moduli display strong concave behaviors as a function of pressure at a given high temperature while the longitudinal modulus c 11 and the off diagonal moduli c 12 and c 13 exhibit greater temperature dependence at higher pressures than at relatively lower pressures using a finite strain theory and thermal equation of state modeling for a pyrolitic mantle composition along an expected mantle geotherm our results show that the magnitude of the v p and v s jumps at the 410 km depth are 6 and 6 4 respectively which are greater than that found in seismic observations suggesting a mantle olivine content of 40 50 vol which is less than what is expected for the pyrolite model our modeled velocity profiles for a metastable olivine wedge in the subduction slabs along a representative cold slab geotherm are 6 and 10 lower than those of wadsleyite and ringwoodite respectively at corresponding depths of the normal mantle our modeled results also show that metastable olivine in the cold slabs could have strong v p and v s anisotropies the maximum v p anisotropy is estimated to be 19 22 at transition zone depth whereas the maximum v s splitting is 13 23 and increases with depth as a result the presence of a metastable olivine wedge at the transition zone depth would exhibit a seismic signature of low velocity and strong seismic anisotropy which are consistent with recent seismic observations for various locations of the slabs and can be used as mineral physics constraints for future seismic detections of the metastable olivine wedges in the deep mantle

centroid moment tensor solutions for july september 1993 abstract centroid moment tensor solutions are presented for 186 earthquakes that occurred during the third quarter of 1993 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward

quantifying the relationship between short wavelength dynamic topography and thermomechanical structure of the upper mantle using calibrated parameterization of anelasticity oceanic residual depth varies on 5000 km wavelengths with amplitudes of 1 km a component of this short wavelength signal is dynamic topography caused by convective flow in the upper 300 km of the mantle it exerts a significant influence on landscape evolution and sea level change but its contribution is often excluded in geodynamic models of whole mantle flow using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure here we remove this cooling signal from global surface wave tomographic models revealing a correlation between positive residual depth and slow residual velocity anomalies at depths 300 km to investigate whether these anomalies are of sufficient amplitude to account for short wavelength residual depth variations we calibrate an experimentally derived parameterization of anelastic deformation at seismic frequencies to convert shear wave velocity into temperature density and diffusion creep viscosity asthenospheric temperature anomalies reach 150 c in the vicinity of major magmatic hotspots and correlate with geochemical and geophysical proxies for potential temperature along mid ocean ridges locally we find evidence for a 150 km thick low viscosity asthenospheric channel incorporating our revised density structure into models of whole mantle flow yields reasonable agreement with residual depth observations and suggests that 30 km deviations in local lithospheric thickness account for a quarter of total amplitudes these predictions remain compatible with geoid constraints and substantially improve the fit between power spectra of observed and predicted dynamic topography this improvement should enable more accurate reconstruction of the spatio temporal evolution of cenozoic dynamic topography

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

imaging a slow bilateral rupture with broadband seismic waves the september 2 1992 nicaraguan tsunami earthquake the spatio temporal rupture history of the september 2 1992 nicaragua tsunami earthquake ms 7 2 mw 7 6 is analyzed using long period 157 288 s rayleigh and love wave spectral inversions and an empirical green function analysis of very broadband 10 250 s body and surface waves the event has a rupture duration exceeding 110 s with a slow 0 6 2 3 km s rupture velocity and involves shallow dipping 6 10 thrust faulting at a shallow depth 10 km with the latter being primarily responsible for the strong tsunami excitation two subevents dominate the source radiation resulting from a slow asymmetric bilateral 100 160 km long rupture long period surface wave directivity reveals a predominant rupture azimuth of 140 30 consistent with asymmetric extension of the bilateral rupture at an azimuth of 125 30 resolved by the source time function analysis an inverse radon transform of the source time functions confirms the asymmetric bilateral rupture characteristics

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

the macquarie ridge earthquake of 1989 summary a study of the seismicity on the central portion of the macquarie ridge before and after the 1989 may 23 macquarie ridge earthquake mw 8 2 and the solution of the inverse problem to obtain the moment release time history during the earthquake are presented the aftershock study shows that there were remarkably few and small aftershocks on the main fault the aftershocks were distributed along a 220 km portion of the india australia pacific plate boundary and indicate that the motion was bilateral on a vertical fault in addition the earthquake reactivated a 175 km section of a fault to its west which had been dormant since at least 1964 and 44 per cent of the aftershocks occurred on this feature moreover the largest aftershocks in the five month period following the event occurred not on the main fault plane but on the reactivated one based on available bathymetry gravity and magnetic anomaly data from the region this reactivated feature is interpreted as being an old oceanic fracture zone and hence a pre existing zone of weakness the aftershock distribution and their focal mechanisms are interpreted in terms of the tectonics of the region and suggest that a triangular piece of crust to the west of the macquarie ridge is being squeezed to the north and west the centroid moment tensor solution was obtained using both gdsn and geoscope data simultaneously it shows that the motion was primarily strike slip on a vertical fault with a moment of mo 1 5 x lo2 n m and a centroid depth of 19 5 km the moment release distribution and time history were obtained by inversion of broad band body waves recorded digitally by the gdsn and geoscope networks the rupture is found to be bilateral with the propagation first starting towards the north east and a few seconds later towards the south west the moment release was low in the hypocentral area with the higher areas of moment located towards the north east and south west of this the average rupture speed was approximately the shear wave speed no simple relation between the moment release pattern and the distribution of aftershocks on the fault plane was found instead the aftershock distribution is seen to be dominated by the intersection of the reactivated fault and the main fault plane with most aftershocks occurring to the north of this intersection

tsunami earthquakes slow thrust faulting events in the accretionary wedge the november 20 1960 peru october 20 1963 kurile and june 10 1975 kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude long period surface wave analysis indicates double couple faulting mechanisms for all three events rather than single force mechanisms indicative of submarine landslides the earthquakes have shallow depths 15 km and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events with dip angles of 6 8 surface wave spectral amplitudes and deconvolution of sh waveforms suggests anomalously long source durations and large seismic moments relative to ms specifically the 1963 kurile event ms 7 2 shows a duration of 85 s and a moment of 6 0 1027 dyn cm mw 7 8 the 1975 kurile event ms 7 0 shows a duration of 60 s and a moment of 2 0 1027 dyn cm mw 7 5 and the 1960 peru event ms 6 75 shows a time function consisting of four subevents with a total duration of 110 130 s and a seismic moment of 3 4 1027 dyn cm mw 7 6 estimated rupture velocities are about 1 km s or less but there is no evidence of unusually low stress drops the august 1 1968 philippines event previously classified as a tsunami earthquake shows none of the anomalous source properties and teleseismic tsunami height measurements are sparse we do not consider this event a tsunami earthquake most of the anomalous tsunami excitation results from underestimation of earthquake size by ms due to the long source duration the tsunami heights are not significantly anomalous relative to seismic moment the slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism standard scaling laws when adjusted for the slow seismic velocity in the source region show an mw ms relationship similar to that observed for the tsunami earthquakes and predict ms saturation at about 7 3 rather than 8 0 for typical events

the mechanism of the deep bolivia earthquake of june 9 1994 we analyzed the iris broad band seismograms for the june 9 1994 bolivia earthquake the largest deep focus earthquake ever recorded we inverted the record sections with a duration of 330 sec which include p pp and sp phases and obtained a dip slip mechanism dip rake strike 13 4 1 or 89 103 95 with a scalar moment of mo 2 9 10 21 nm mw 8 2 this mechanism is similar to that obtained from long period 175 to 250 sec rayleigh and love waves 83 100 90 mo 3 0 10 21 nm mw 8 3 the main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1 2 10 20 nm mw 7 3 the main rupture starts at about 20 km east or northeast of the initial break and propagates horizontally in the east to northeast direction the source duration is 40 sec and the rupture dimension is about 40km if a fault area of 40 40km is assumed the stress drop is 110 mpa an inversion of body waves p pp and sp with 6 basis moment tensors yielded a best fit solution with a 10 implosive component this result is of marginal significance and its confirmation would require more detailed information on the structures near the source region pp bounce points and stations the observed amplitude of the radial mode 0s 0 1226 sec is consistent with a double couple model with less than 2 isotropic component

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

diversity of solutions of the problem of earthquake faulting inversion application to sh waves for the great 1989 macquarie ridge earthquake abstract we use the sh waves generated by the great 1989 macquarie ridge earthquake to investigate the non uniqueness of the inverse problem of earthquake faulting the details of the method of linear programming developed for this problem of solution of a constrained set of equations is described protective measures necessary to remove loss of accuracy when solving the problem for large numbers of unknowns and constraints are developed it is shown that solutions which look very different fit the data almost equally well a method is presented for choosing solutions with desirable properties e g with the most uniformly distributed moment along the fault or with the most concentrated moment release on the fault from among available solutions the results show that the data may equally well be interpreted as representing an asperity model of faulting or a propagating crack model of faulting the common feature of all the models is that the hypocentral region is a region of low moment release with regions of higher release being located to its northeast and its southwest along the fault clearly this earthquake did not initiate by the rupture of an asperity

the zagros core deformation of the continental lithospheric mantle 1 the zagros of iran form one of the youngest collisional orogenic belts on earth at shallow depths shortening across the zagros is accommodated by folding in the sediments high angle thrust faulting in the basement and thickening of the lower crust but how shortening is accommodated by the lithospheric mantle has been uncertain largely because the upper mantle seismic structure has been poorly known we map the lateral variations in upper mantle shear wave speed beneath this region using a large multimode surface wave data set the upper mantle is slow for most of the middle east but a high shear wave speed lid extending to 225 km depth exists beneath the zagros we use at vs z relation to convert the shear wave speed profiles to temperature profiles and fit these with geotherms to identify the base of the lithosphere the upper mantle temperatures from the seismic model are consistent with temperatures derived from geochemical modeling the lithosphere is less than 120 km thick over the region except for a thick lithospheric root beneath the zagros implying that shortening in the mantle is accommodated by lithospheric thickening the composition of the volcanic rocks from above the area of the thickened lithosphere has depleted magma source regions with densities 60 kg m 3 less than the morb source elsewhere in the middle east the volcanic source regions have compositions and densities similar to that of morb the shortening across the zagros is accommodated by lithospheric thickening but the cool thickened lithosphere has been stabilized from delamination by depletion

lateral heterogeneity in the convecting mantle thermally induced lateral heterogeneity in the earth s mantle has been studied in terms of idealized two dimensional constant viscosity numerical models of mantle convection steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity by examining the changes in spectral signature produced by varying the rayleigh number the degree of internal heating and the depth in the convection cell it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer the spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography the forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data

estimate of the tsunami source of the 1992 nicaraguan earthquake from tsunami data the nicaraguan earthquake of september 2 1992 excited large tsunamis which caused significant damage along the nicaraguan coasts some of the inhabitants felt only minimal shock before the tsunami arrived indicating that excitation of short period seismic waves was very small numerical simulation with the initial tsunami source estimated by the seismic data is carried out to study the characteristics of the tsunami source by comparing the calculated data with the measured data along the nicaraguan coast finding of the comparison shows that the dislocation of the fault estimated from the measured data is 5 6 to 10 0 times larger than that from seismic data the tsunami source area which is 200km in length 100km in width is used to explain the distribution of measured tsunami heights along the coast and the wave period as shown in the tide record at corinto the effect of rise time on tsunami excitation indicates a slow process which corresponds with the seismic waves this event falls under the tsunami earthquake category to produces anomalously large tsunamis relative to earthquake magnitude

spatial and temporal patterns of cenozoic dynamic topography around australia 1 despite its importance the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known we present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding australia our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf these anomalies were determined from a combined dataset of 200 seismic reflection and wide angle images of well sedimented oceanic crust they have amplitudes of between 1 km and 0 5 km and their spatial variation is broadly consistent with long wavelength free air gravity and shallow seismic tomographic anomalies along the northwest shelf a regional depth anomaly of 300 to 700 m intersects the adjacent continental shelf the temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform which fringes the shelf and records a dramatic switch from progradation to aggradation during neogene times three dimensional seismic mapping calibrated by boreholes was used to calculate water loaded subsidence histories at rollover points of clinoforms along the shelf at 9 3 ma the rate of subsidence increases from 5 to up 75 m myr 1 generating a subsidence anomaly of 300 to 700 m the amplitude of this anomaly varies along the shelf and cannot be generated by glacio eustatic sea level variation instead we propose that a regional subsidence episode which affects both the proximal shelf and the distal oceanic basin was generated by convective drawdown by combining our results with other published estimates of uplift and subsidence a map of australia which shows the spatial and temporal pattern of dynamic topography is presented most but not all of australia s epeirogeny can be attributed to rapid northward motion of the australian plate over a pre existing pattern of convective circulation

fault parameters and tsunami excitation of the may 23 1989 macquarie ridge earthquake the macquarie ridge earthquake of may 23 1989 is one of the largest events in the last decade furthermore it is the largest strike slip earthquake ever recorded instrumentally we analyzed long period surface waves and body waves recorded at global networks gdsn iris geoscope and ida to estimate the fault parameters and computed tsunamis from the estimated parameters the centroid moment tensor cmt solution from surface waves shows that the mechanism is almost pure strike slip with one of the nodal planes parallel to the strike of the macquarie ridge the seismic moment is 1 6 10 21 nm 10 28 dyn cm and the corresponding moment magnitude is m w 8 1 teleseismic p and s waves from 10 stations with good azimuthal coverage are used to model the temporal and spatial distribution of the subevents four subevents are located sequentially from south to north along the ridge system in about 30 sec all of them have a mechanism similar to the cmt solution the fault length is estimated to be about 120 km from the subevent and the aftershock distributions the rapture propagates from south to north at a relatively high speed the average slip on the fault depends on the estimate of the fault width but is probably larger than 9 m a large strike slip earthquake like this event produces significant vertical displacements on the ocean bottom and excites tsunamis computation of tsunamis using the estimated fault parameters predicts that small tsunamis are expected at australia and new zealand in fact small tsunamis were observed on the southern coast of australia

mechanism of the 1992 nicaragua tsunami earthquake the 1992 nicaragua earthquake generated larger tsunamis than expected from its surface wave magnitude ms 7 2 and is known as a tsunami earthquake seismological studies showed that the duration was very long for its size about 100 s other studies have shown that the seismic moment estimated from tsunamis is an order of magnitude larger than that from seismic waves even after the long duration is accounted for numerical computations of tsunamis from various fault models are made to reconcile this discrepancy comparison of calculated waveforms with tide gauge records shows that the fault width is 40 km much narrower than the aftershock area and extends only into the upper 10 km of the ocean bottom slip amount on the fault is estimated to be 3 m from amplitude comparisons the fault length is estimated to be 250 km slightly longer than the aftershock area from comparison of the tsunami height distribution the rigidity around the shallow fault may be smaller than that of a standard underthrust fault and the seismic moment is estimated as 3 1020 nm consistent with the seismic observations a slow rupture on the shallow fault presumably in the subducted sediments is responsible to the unusually large tsunami excitation

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

source times and scaling relations of large earthquakes source time is a kinematic fault parameter corresponding to the duration of seismic source time functions and is accurately determined from phase spectra of long period surface waves source times are determined for 36 large earthquakes during the last three decades scaling relations among source times seismic moments and fault dimensions are derived seismic moment is proportional to the cube of source time and fault dimension is proportional to source time source times for low angle thrust earthquakes along deep sea trenches are found to be longer than those of other types such as intraplate shocks and deep shocks they are also significantly longer than the rupture times expected from a haskell model suggesting that generally there exists an introductory stage of faulting that precedes the main stage of the rupture propagation

an experiment in the retrieval of depth and source mechanism of large earthquakes using very long period rayleigh wave data abstract the possibility of obtaining an estimate of depth for large earthquakes from low frequency mantle wave data alone is investigated using moment tensor formalism after illustrating the sensitivity to depth of the eigenfunctions in the period range of 160 to 350 sec we give several examples of application of the two step inversion procedure proposed earlier for shorter period rayleigh wave data romanowicz 1982 we find that for earthquakes below a depth of about 50 km we are able to resolve depth even when a spherically symmetric average earth model is used for propagation corrections this is particularly interesting in the case of large subduction zone earthquakes for which an estimate of the vertical extent of faulting can thus be obtained independently of aftershock studies in a very fast and simple manner which does not involve any synthetic seismogram calculations or lateral heterogeneity modeling for shallower earthquakes especially those with one steeply dipping nodal plane the resolution of depth is less precise taking lateral heterogeneity into account by using available regionalized phase velocities improves the results marginally we anticipate that accounting more accurately for the low order harmonics in the worldwide phase velocity distribution should prove most helpful since these interfere the most with the theoretical radiation patterns of earthquakes the determination of depth depends more critically on the source process time which has to be accounted for especially for horizontally propagating faults