the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

attenuation relationships of peak ground acceleration and velocity for crustal earthquakes in taiwan strong seismic ground motion data obtained by the taiwan strong motion instrumentation program tsmip and central mountain strong motion array cmsma are used to derive new attenuation relationships for the vertical and horizontal peak ground acceleration pga and peak ground velocity pgv for crustal earthquakes in taiwan more than 7900 three component accelerograms recorded from 51 crustal earthquakes in taiwan with m w magnitudes ranging from 4 0 to 7 1 have been analyzed to study the dependence of peak ground motion parameters on magnitude distance regional and local site effects through attenuation relationships we first found that for both pga and pgv the attenuation relationships decay faster with distance for the vertical component than for the horizontal component also the attenuation relationships decay faster with distance for the vertical pga than for the vertical pgv we further compared the attenuation relationships for three subregions chy iwa and nto and the whole taiwan region twn it is found that the chy area has higher ground motion either in pga or pgv than the other areas especially at near source distances this is because the chy area is located on a thick recent alluvial plain comparison of our new attenuation relationships with strong motion data from the 1999 chi chi earthquake m w 7 7 and the 2003 cheng kung earthquake m w 6 8 shows that the attenuation relationships developed for m w 7 can be extrapolated to make reasonable estimates of strong motion from larger m w 7 earthquakes finally we analyzed the residuals to investigate variations of pga and pgv with respect to site conditions the results show that 1 the residual contour maps especially for the pgv have high consistency with the regional geology and topography of taiwan 2 the pgv residual contours reveal that taipei basin changhua plain chianan plain pingtung valley ilan plain and taitung longitudinal valley have high residual values note that most major metropolitan areas all fall in high residual areas 3 the site classification based on geologic criteria by lee et al 2001 can be simplified into three classes that is class e for soft soils a combination of classes c and d for dense and stiff soils and class b for rocks these three site classes are mostly distributed respectively in the alluvial plains at an elevation less than 50 m in terraces and hills at an elevation less than 1000 m and in high mountainous areas

low vp vs ratios in the crust and upper mantle beneath the sea of okhotsk inferred from teleseismic pmp smp and sms underside reflections from the moho 1 teleseismic ground motion recordings of deep earthquakes often contain clear seismic wave reflections from the free surface above the source pp sp ss with isolated precursors pmp smp and sms produced by underside reflections from the moho the crust mantle boundary below the free surface reflection points the waveforms of the depth phases and their precursors are readily modeled providing constraints for steep angles of incidence on integrated two way crustal travel times and moho shear and compressional impedance contrasts above deep earthquake source regions we model surface reflection waveforms in stacks of three component long period wwssn and digital broadband seismograms for signals traversing the crust and upper mantle beneath the sea of okhotsk which overlies deep earthquakes in the subducted kurile slab the crust beneath most of the sea of okhotsk is 19 25 km thick indicating a submerged continental margin with minor crustal thinning toward the kuril back arc basin where thin oceanic crust is found modeling pmp and sms phases that sample the same subregion provides tight constraints on the vp vs ratio in the crust and uppermost mantle assuming common density structure and consistent crustal thickness for p and s waves the absolute velocities are not constrained by the data we find unusually low vp vs ratios in the range 1 6 1 7 for the uppermost mantle under the sea of okhotsk and comparably low average vp vs ratios for the crust in several regions the presence of fluids and extensive silica enrichment possibly involving low temperature veining are viable explanations for the anomalous vp vs ratios this may represent an important process of continentalization taking place landward from the volcanic arc in subduction zones

direct measurement of the deep circulation within the brazil basin abstract in support of the deep basin experiment part of the world ocean circulation experiment a large number of neutrally buoyant floats were released within the brazil basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior three levels corresponding to the three major subthermocline water masses were selected and results from the deeper two north atlantic deep water nadw and antarctic bottom water aabw are described at this writing processing of acquired tracking data is incomplete hence this paper reports on the progress of the observational program and gives our initial conclusions it appears that the flow in the deep brazil basin is unlike previous conjectures in which the circulation patterns can be characterized as being primarily meridional both along the western boundary and in the interior the existence of a deep western boundary current dwbc is quite clear in the float data at the nadw level but less prominent in the aabw and the interior flow is dominantly zonal with unexpectedly small meridional space scales integral time scales are long of order 20 30 days and eddy kinetic energy levels are low of order 1 cm2 s2 in spite of the low energy levels a surprising number of our floats became caught up in vortices a line of seamounts extending offshore near 20 s known as the vitoria trindade seamounts interrupts the dwbcs and is the location for eddy formation and apparent flow away from the boundary into the interior although it has been speculated that this could feed a narrow zonal current of nadw the namib col current our float trajectories suggest a return to the western boundary rather than a continuation to the east

geostrophic transport in the benguela current region historical data from the region between the greenwich meridian and the african continental shelf are used to compute the offshore geostrophic transport of the benguela current at 32 s the benguela current is located near the african coast transporting about 21 sv 1 sv 106 m3 s 1 of surface water toward the north relative to a potential density surface lying between the upper branch of circumpolar deep water and the north atlantic deep watar two warm core eddies of probable agulhas current origin an observed west of the benguela current at 32 s near 30 s the benguela current turns toward the northwest and begins to separate from the eastern boundary it carries about 18 sv of surface water across 28 s the current then turns mainly toward the west to flow over a relatively deep segment of the walvis ridge south of the valdivia bank a surface current with northward surface of about 10 cm s 1 flows along the western side of the valdivia bank while another northward surface current flows at about 20 cm s 1 some 300 km west of the bank about 3 sv of surface now do not leave the cape basin south of the vaidivia bank but instead drift northward as a wide sluggish flow out of the northern end of the cape basin because of the more southerly seaward extensions of most of the benguela current there are no deep reaching interactions observed between this current and the cyclonic gyre in the angola basin east of the greenwich meridian beneath the surface layer about 4 5 sv of antarctic intermediate water are carried northward across 32 and 28 s by the benguela current essentially all of which turns westward to cross the greenwich meridian south of 24 s

lagrangian circulation of antarctic intermediate water in the subtropical south atlantic this study combines float data from different projects collected between 1991 and 2003 in the south atlantic to describe the flow of antarctic intermediate water aaiw velocity space time averages are calculated for various grid resolutions and with cells deformed to match the bathymetry f h or f h with h being the water depth and h being the thickness of the aaiw layer when judged by the degree of alignment between respective isolines and the resulting average velocity fields the best grid is based on a nominal cell size of 3o latitude by 4o longitude with cell shapes deformed according to f h using this grid objectively estimated mean currents and their associated errors as well as meridional and zonal volume transports are estimated results show an anticyclonic subtropical gyre centred near 36os and spanning from 23o 1 s to 46 1os the south atlantic current meanders from 33os to 46os and shows a mean speed of 9 6 7 8 cm s 1 8 5 sv 3 5 sv 1 sv 1 106 m3 s 1 the northern branch of the subtropical gyre is located between 22os and 32os and flows westward with a mean speed of 4 7 3 3 cm s 1 9 3 sv 3 4 sv evidence of a cyclonic tropical gyre divided in two sub cells is visible on the stream function

southern ocean surface characteristics from fgge buoys abstract in this analysis of satellite tracked drifting surface buoys released in the southern ocean buoy velocities are averaged along trajectories for 90 days to determine the mean circulation and eddy kinetic energy is computed using perturbations about the 90 day mean the 90 day mean is based upon wind driven circulation theory southern ocean wind spectra cumulative averages of mean buoy velocity and energy and plots of the resulting mean circulation excluding the antarctic circumpolar current acc the spatial distribution of the eddy kinetic energy is consistent with previous distributions based upon spatially averaged ship drift and surface drifting buoy data reported in the literature in the acc values of eddy kinetic energy are generally lower than found elsewhere the analysis indicates that the spatial structure of perturbations about the 90 day mean is zonally extended and meridionally compressed the meridional compression may be due to which acts as the restoring force for pla

probability for simulating future earthquakes with mw 6 0 in taiwan for seismic hazard for the earthquake catalog from 1900 to 2008 in this study we use extreme value theory based on gumbel equation derivations to estimate the gutenberg richter a and b parameters for taiwan data are from the augmented homogenized in terms of moment magnitude historic catalog for taiwan the island is divided into grids of 0 2 latitude by 0 2 longitude and gumbel type 1 statistical analysis is applied the values of a and b are then used to determine the probability of large earthquakes m w 6 0 occurring at each grid the results show two relatively high probability paths for large earthquakes one extending from hsinchu southward to taichung chiayi and tainan in western taiwan and the other from ilan southward to hualian and taitung in eastern taiwan both of which are characterized by low b values it indicates that future earthquakes can be expected along these paths characterized by low b values additionally maximum peak ground acceleration and maximum peak ground velocity determined from respective attenuation laws and a gridding regimen of 0 1 latitude by 0 1 longitude for taiwan follow similar paths to that of the low b values

test of seismic hazard map from 500 years of recorded intensity data in japan abstract maximum seismic intensity maps for japan are constructed using the recorded intensity data from 1498 to 2007 and are used to test the probabilistic seismic hazard map pshm by the headquarters for earthquake research promotion japan the historical intensity maps are compared with the hazard maps of probable maximum seismic intensity for a 475 yr return period assuming a poisson distribution 10 in 50 yrs we look at cases that include all events only subduction zone earthquakes and all events excluding subduction zone earthquakes the megathrust earthquakes in the subduction zones produce large bands of high intensities along the pacific coast side while onshore crustal earthquakes create a patchy distribution of large intensities over all of japan the maximum recorded intensity map for the past 500 yrs and the maximum predicted intensity map for the return period from the pshm are very similar for the cases including all events and the subduction zone earthquakes while there is poor correlation for the third category that includes mostly onshore crustal earthquakes if we consider only the amount of area not the specific locations the recorded intensity map and the pshm using the maximum case have a high degree of correlation for japanese meteorological agency jma intensity higher than 4 for all of the cases statistically the present hazard maps for japan seem to agree with the past intensity distributions and can be regarded as appropriate hazard maps even though there may be strong dependencies on uncertain model parameters for the pshms

are long period body wave coda caused by lateral heterogeneity summary data from two broad band arrays in western europe nars and grf are used to study the character of long period body wave coda the events studied are at epicentral distances of 40 to 60 in the hindu kush region and on the mid atlantic ridge sampling the upper mantle to a depth of about lo00 km the periods studied are 5 50 s the long period coda of p at grf interstation distance about 10 km are strongly coherent whereas the long period coda of pp ppp and s waves are incoherent this indicates that the latter coda consists of scattered waves to investigate the nature of the scattering process the data of grf are analysed for slowness and azimuth variations in the coda intervals a new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals the method is based on backus gilbert inverse theory the results show that the incoherent long period coda intervals almost entirely consist of surface waves these waves are scattered from the preceding body waves some calculations with linearized theory for body wave to rayleigh wave conversion at topography at the surface or at the moho show that realistic scatterers can account for the observed constant coda level the beamforming results show that the phases in the p coda all arrive along the great circle as scattering calculations point out that body wave to body wave scattering is inefficient it is concluded that the long period p coda does not contain a significant amount of scattered energy synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves for events in the hindu kush region an upper mantle with a thick lid overlying a pronounced low velocity zone lvz is necessary to explain the character of the p coda at grf such an upper mantle agrees with previous studies of similar great circle paths the strong coherence of the p coda at grf is lost on the scale of nars station separation about 200 km this suggests lateral variations in the upper mantle at a scale of about 200 km it appears from previous studies of the upper mantle under europe that these variations must be sought in the lvz it is shown that the long period p coda is sensitive to variations in the p velocity structure of the lvz this suggests the p coda i e pdp phases as a tool for monitoring lateral variations in the lvz and possibly to prove the existence or absence of a lvz in the p velocity

estimated seismic intensity distributions for earthquakes in taiwan from 1900 to 2008 in this study we recreated peak ground accelerations pga and peak ground velocity pgv distributions for taiwan by applying the attenuation relations of liu and tsai 2005 to calculate the pga and pgv values for 1989 mw 5 0 earthquakes in a catalog of earthquakes from 1900 to 2008 with homogenized magnitude mw chen and tsai 2008 we further combined the pga and pgv values to obtain corresponding modified mercalli intensity mmi values wald quitoriano heaton kanamori et al 1999 and their spatial distributions and recurrence intervals we adopted a logarithmic functional form analogous to the gutenberg richter relation for seismicity to represent the annual frequency of seismic intensity parameters log10n a blog10pga log10n a blog10pgv and log10n a bi the regions with high pga and pgv values are often associated with low b values in these equations as it is well known that the mw 7 45 chi chi earthquake of 21 september 1999 had produced high pga values in excess of 0 9g and pgv values in excess of 300 cm s we used these relations to estimate the poisson probability distributions in taiwan for mmi viii i e pga 485g for recurrence intervals of 30 50 and 100 years the results show a wide range of differences in the poisson probability of mmi viii among different areas of taiwan for example for a 50 year interval this probability at 10 major cities in taiwan is as follows taipei 0 67 hsinchu 2 15 taichung 5 24 chiayi 24 35 tainan 1 61 kaohsiung 0 04 hengchun 4 94 ilan 17 67 hualien 37 04 and taitung 9 82 these esti mates should be of interest to city planners especially for earthquake preparedness planning

intermediate water in the brazil malvinas confluence zone a lagrangian view the subsurface flow within the subantarctic and subtropical regions around the brazil malvinas falkland confluence zone is studied using daily hydrographic and kinematic data from four subsurface floats and a hydrographic section parallel to the south american shelf the float trajectories are mapped against sea surface flow patterns as visible in concurrent satellite sea surface temperature sst images with focus on the november 1994 and october november 1995 periods the unprecedented employment of lagrangian s diagrams enables us to trace the advection of patches of fresh antarctic intermediate water aaiw from the confluence zone into the subtropical region the fresh aaiw consists of a mixture of subtropical aaiw and malvinas current core water within the subtropical gyre these patches are discernible for extended periods and drift over long distances reaching north to 34 s and east to 40 w the cross frontal migration of quasi isobaric floats across the confluence zone from the subtropical to the subantarctic environment is observed on three occasions the reverse process float migration from a subpolar to a subtropical environment was observed once these events were located near 40 s 50 w the site of a reoccurring cold core feature subsurface float and sst data comparison reveals similarities with analogous observations made in the gulf stream rossby 1996 where cross frontal processes were observed close to meander crests the limited number of floats of this study and the complex structure of the brazil malvinas confluence zone however restricts the analysis to a description of two events

on the structure paths and fluxes associated with agulhas rings in january 1993 two agulhas rings were sampled in the eastern south atlantic during the world ocean circulation experiment one time hydrographic section a11 the first of these ring 1 was sampled at 36 s 4 e in the subtropical frontal zone and was smaller and less energetic than other more typical agulhas rings owing partly to its interactions with the north subtropical front the other ring 2 was sampled at 33 s 10 e in the subtropics this ring was average in size and reasonably representative of the rings observed in the southeast atlantic during the early 1990s the path of the rings is found to be dominated by advection in the mean flow rather than self induced velocities of eddies under the effect we observe significant differences in reduced gravity and available heat and salt anomalies in the rings from these we infer temporal variability in the stratification of rings upon their formation at the agulhas retroflection this temporal variability in the structure of rings will affect the fluxes associated with them data collected within ring 2 show that neglecting consideration of the water beneath the 10 c isotherm underestimates the volume flux by half and the potential temperature and salinity flux by in excess of one third and one half respectively using a base of the 3 5 c isotherm and including the high salinity intermediate water from the indian ocean is found to be more appropriate with these conditions six agulhas rings per year entering the subtropical south atlantic would equate to a volume flux of 9 sv and an absolute potential temperature and salinity flux of 84 sv c and 322 sv psu 1 sv psu 106 kg s 1 the significant flux of intermediate water from the indian ocean may be as important in the return path of the thermohaline circulation as the intermediate water that enters the south atlantic through drake passage further the water associated with agulhas rings and cooler than 10 c potentially contributes a larger volume flux to the warm water return path than the corresponding surface and upper thermocline water

a kinematic analysis of the indian atlantic interocean exchange we analyze the indian atlantic interocean exchanges in an eddy permitting numerical simulation focusing on the cape basin a region of the southeastern atlantic bounded on the northwest by the walvis ridge and on the south by the agulhas ridge near 40 s to quantify the relative importance of the different dynamical mechanisms involved in the interocean exchange we separated the climatological mean circulation from the transients the analysis indicates that agulhas eddies influence not only the transient but also fluxes associated with the mean circulation eddy fluxes for example supply most of the energy of the benguela current a distinctive characteristic of the eddy variability within the cape basin is the co existence of cyclonic and anticyclonic vortices in dipole structures that resemble the heton model of hogg and stommel proc r soc london a 397 1985 1 anticyclones are surface intensified vortices that in spite of their baroclinic structure reach to deep layers cyclones are bottom intensified vortices with dominant barotropic structure that projects into the upper layer the propagation of both cyclones and anticyclones is strongly affected by the bottom topography our analysis shows that the walvis ridge and the vema seamount block the passage of bottom intensified cyclones and changes the trajectories of the upper intensified anticyclones although most anticyclones are able to escape the basin the deep compensation generated by the ridge generates an energy loss of approximately 30 and a change of the eddy trajectory to a more westward direction

discrete eddies in the northern north atlantic as observed by looping rafos floats abstract rafos float trajectories near the 27 5 density level were analyzed to investigate discrete eddies in the northern north atlantic with the objective of determining their geographical distribution and characteristics floats that made two or more consecutive loops in the same direction loopers were considered to have been in an eddy overall 15 24 float years of the float data were in loopers one hundred and eight loopers were identified in 96 different eddies roughly half of the eddies were cyclonic 49 and half were anticyclonic 51 although the percentages varied in different regions a few eddies were quasi stationary for long times one for over a year in the iceland basin and many others clearly translated often in the direction of the general circulation as observed by non looping floats several floats were trapped in eddies in the vicinity of the north atlantic current just upstream west of the charlie gibbs 52 n and faraday 50 n fracture zones which seem to be preferred routes for flow crossing the mid atlantic ridge five floats looped in four anticyclones that translated southwestward away from the eastern boundary near the goban spur 47 50 n these could have been weak meddies forming from remnants of warm salty mediterranean water advected northward along the eastern boundary

eddy and wave dynamics in the south atlantic as diagnosed from geosat altimeter data almost 3 years of geosat data were analyzed to ascertain the characteristics of the sea surface height variability in the south atlantic the nature of the sea surface height field is shown to be nonstationary heterogeneous and anisotropic in particular the wavenumber spectral characteristics of the sea surface height field such as spectral slopes shape breaks in spectral slopes and energy density values vary with latitude and longitude within the basin the spectral slopes exhibit a spectral dependence approaching k 5 in the high energy areas and k 3 in the low energy areas along track characteristic wavelengths are found to decrease from north 500 km to south 300 km and from west to east the eddy available potential energy exhibits maximum values in the highly energetic eddy shedding regions of the malvinas brazil confluence and the agulhas leakage region mean frequency spectra calculated from time series of sea surface height variability at crossover points reveal evidence of annual and semiannual signatures with energy levels reaching 1000 cm2 in the high energy western and eastern systems these values represent 20 of the energy found for periods of 100 days or less which is the time scale of the mesoscale variability baroclinic rossby waves with periods of 400 500 days are found to propagate away from the eddy shedding region of the agulhas leakage into the atlantic the phase energy of these planetary waves propagates toward the west southwest west northwest from their source near the southern tip of africa correlation functions in space both in the zonal and along track directions and time lag corroborate these findings zonal and meridional wavenumbers were calculated using a simple wavenumber projection technique which takes into a count the anisotropic property of the wave field the resultant wavelengths approximately 250 km are consistent with those calculated from the dispersion relation for baroclinic rossby waves these waves propagate with a group speed of 0 4 cm s 1 and provide a net energy flux from the agulhas leakage into the atlantic toward the western boundary and the equator across 25 s

origins and variability of the benguela current a subset of the benguela sources and transports best 1992 1993 data is analyzed to study the magnitude and variability of the large scale transports in the area the data consist of inverted echo sounder series and conductivity temperature depth stations the mean 16 month transport values for the upper 1000 m indicate that of the 13 sv northward transport within the benguela current 50 is derived from the central atlantic which from geometry may be chiefly south atlantic water 25 comes from the indian ocean which may be chiefly agulhas water and the remaining 25 may be a blend of agulhas and tropical atlantic water a simple schematic of the transport pattern with a somewhat restricted corridor for agulhas eddies translation is envisioned to the west of the eddy corridor flows the south atlantic source for the benguela current to the east is the agulhas indian ocean source the corridor is breached by south atlantic and indian water as the transient eddy field stirs these water masses

interannual variability of south atlantic circulation from 4 years of topex poseidon satellite altimeter observations variability of large scale and regional south atlantic circulation is investigated using topex poseidon sea level observations interannual variations are identified from empirical orthogonal functions of gridded sea level fields year to year fluctuations of root mean square sea level variability and variability of agulhas eddies evaluated from the along track data two modes of variability are identified a basin scale mode indicates that sea level in the eastern south atlantic underwent a transition from a state of high sea level and enhanced gyre scale geostrophic circulation in 1993 and 1994 to a state of lower sea level and more sluggish circulation in 1996 the dominant mode of basin scale zonal wind has the same temporal signature suggesting a link between the observed variation of gyre scale circulation and the regional wind forcing time variations of this mode also coincide with a transition from a broad agulhas eddy corridor observed in 1993 and 1994 to a narrower corridor observed in 1996 the input of salt and vorticity to the south atlantic subtropical gyre via agulhas eddies may therefore be partially controlled by interannual variations of the wind forced large scale circulation a second mode isolates interannual variations in the brazil malvinas confluence region during 1993 eddy variability along the brazil current extension was relatively strong and variability along the continental slope was weak the opposite pattern was observed in 1995 these variations may be related to interannual variations of the latitude of the confluence while variations associated with both modes are smaller than those observed on seasonal timescales these interannual variations contribute significantly to the total south atlantic variability

interannual to decadal changes in the western south atlantic s surface circulation 1 a combination of surface drifters and altimetry is used to analyze the seasonal to interannual variability of the surface velocity field in the brazil malvinas confluence of the western south atlantic ocean longer term changes are inferred from wind and sea surface temperature fields during the period october 1992 to december 2007 a southward shift of 0 6 to 0 9 decade 1 is found in the confluence latitude of the brazil and malvinas currents a comparable trend is found in the latitude of the maximum wind stress curl averaged across the south atlantic basin allowing a proxy for the confluence latitude to be calculated for the prealtimeter time period this longer time series suggests that the recent trend may be part of a longer term oscillation which has returned to values last sustained in the early 1980s this variation does not appear to be related to the multidecadal trend in the southern annular mode but instead is inversely related to long term variations in the sea surface temperature anomaly in the agulhas benguela pathway of the eastern south atlantic subtropical basin

upwelling filaments off cap blanc interaction of the nw african upwelling current and the cape verde frontal zone eddy field 1 the hydrographical and dynamical properties of the upwelling filaments forming off cap blanc mauritania are investigated using remotely sensed and in situ data collected in april may 2009 during the strongest upwelling season the area is situated at the southern edge of the nw african upwelling system where the cape verde frontal zone cvfz separates warmer saltier north atlantic central water nacw and cooler fresher south atlantic central water sacw sea surface temperature images indicated the presence of an upwelling filament extending 280 km offshore rooted over the cap blanc promontory and entrained around a warm core anticyclonic eddy after this filament started to decay a new cold filament developed at the approximate same location high resolution moving vessel profiler mvp and acoustic doppler current profiler adcp surveys of these mesoscale structures revealed that both filaments were carrying south atlantic central water sacw offshore through an intense jet like flow similarity of the relative vorticity structure across the filament with that of the tangent eddy suggested that the latter was responsible for the offshore current tracking of this eddy in altimetric data demonstrated that it originated from the cvfz as implied by its hydrographic structure altimetric data also revealed that another anticyclonic structure centered over the cap blanc promontory was responsible for the northwestward advection of sacw into the base of the filament the results support the idea that some upwelling filaments can be produced by the interaction of an external eddy field including topographic eddies with the upwelled water

pn attenuation for a spherically symmetric earth model this paper presents the results of a theoretical study of the sensitivity of pn attenuation to the elastic and anelastic structure of the uppermost mantle for frequencies between 1 and 15 hz and distances between 200 and 1000 km synthetic seismograms are computed using wavenumber integration for an elastic model consisting of a crastal layer over an upper mantle with a slight linear velocity gradient approximately equal to the earth flattening transformation of a spherically symmetric homogeneous upper mantle these are compared to synthetic seismograms for a crustal layer over a mantle halfspace pure elastic head wave we find that earth sphericity alone causes a significant departure in pn attenuation from that of a canonical head wave almost two orders of magnitude at 1000 km and 15 hz and that high frequencies attenuate less rapidly than low frequencies if the spherical earth velocity gradient is greater than or equal to zero this implies that methods that assume frequency independent pn geometric spreading will produce biased estimates of the anelasticity of the upper mantle this is demonstrated using synthetic seismograms computed for an anelastic spherical earth model these results show that the velocity structure must be carefully considered when relating observed pn spectral amplitudes to the anelastic structure of the upper mantle

surface circulation and kinetic energy distributions in the southern hemisphere oceans from fgge drifting buoys abstract trajectories of approximately 300 satellite tracked drifting buoys deployed throughout the southern hemisphere oceans during the fiat garp global experiment fgge have been analyzed to infer the mean surface circulation and kinetic energy distributions of the surface flow the resulting picture of mean surface circulation is consistent with expectations based on compilations of historical observations of currents and the density field the distribution of kinetic energy of the mean flow is highly zonal in character with high values in the persistent zonal flows of the antarctic circumpolar current and the south equatorial currents and low values in the broad intervening zone using statistics based on 5 squares the western boundary currents do not emerge as dominant features of the mean flow on the other hand these currents do appear to be a major source of eddy kinetic energy most eddy kinetic energy appears to be due to fluctuations with periods less than one month both mean and eddy kin

nucleation points of recent mainshocks in southern iceland mapped by b values the earthquake catalog in southern iceland between 20 25 w and 21 3 w is complete down to approximately mw 0 and shows no evidence for mag nitude shifts as a function of time since 1991 this means that 6902 earthquakes m 0 1 depth 20 km are available for studies of seismicity patterns preceding the two ms 6 6 mainshocks in 2000 in their source volumes and vicinity 20 25 w and 20 9 w the power law of the frequency magnitude relationship holds down to m0 detailed mapping including 3d mapping of the b value of the frequency magnitude distribution shows that variations from b 0 6 to 1 4 exist over distances of about 2 km this suggests that the tectonic fabric in southern iceland is hetero geneous on a small scale the hypocenters of the two ms 6 6 mainshocks and one additional m 4 5 mainshock are associated with low b values 0 6 0 8 statistically significantly different from volumes in their vicinity therefore one can expect future mainshocks in southern iceland to also emanate from volumes characterized by low b values however the small dimensions of these asperities may render it difficult to identify them before the mainshocks occur low values of estimated local recur rence times 500 2000 years map the two fault zones that ruptured in 2000 against a background of local recurrence times longer than 5000 years the first of the two m 6 6 mainshocks emanated from a volume of short estimated local recurrence time this supports the hypothesis that asperities with short local recurrence times control locations of major ruptures mapping of b values in cross sections shows anomalies of high b at the bottom of the seismogenic crust correlating with the change of its thickness in the middle of the study area these high b value anomalies and other pockets of such anomalies at shallower depths are best interpreted as caused by high pore pressure because evidence of fluids under high pressure is ubiquitous in this area

main eddy vertical structures observed in the four major eastern boundary upwelling systems in the four major eastern boundary upwelling systems ebus mesoscale eddies are known to modulate the biological productivity and transport near coastal seawater properties toward the offshore ocean however little is known about their main characteristics and vertical structure this study combines 10 years of satellite altimetry data and argo float profiles of temperature and salinity and our main goals are i to describe the main surface characteristics of long lived eddies formed in each ebus and their evolution and ii to depict the main vertical structure of the eddy types that coexist in these regions a clustering analysis of the argo profiles surfacing within the long lived eddies of each ebus allows us to determine the proportion of surface and subsurface intensified eddies in each region and to describe their vertical structure in terms of temperature salinity and dynamic height anomalies in the peru chile upwelling system 55 of the sampled anticyclonic eddies aes have subsurface intensified maximum temperature and salinity anomalies below the seasonal pycnocline whereas 88 of the cyclonic eddies ces are surface intensified in the california upwelling system only 30 of the aes are subsurface intensified and all of the ces show maximum anomalies above the pycnocline in the canary upwelling system 40 of the aes and 60 of the ces are subsurface intensified with maximum anomalies extending down to 800 m depth finally the benguela upwelling system tends to generate 40 50 of weak surface intensified eddies and 50 60 of much stronger subsurface intensified eddies with a clear geographical distribution the mechanisms involved in the observed eddy vertical shapes are discussed

north brazil current retroflection and transports 1 a subset of data collected as a part of a larger program the north brazil current rings nbcr experiment is analyzed to study the variability of the transport of the north brazil current nbc and its relation with the shedding of rings it is concluded that there is a direct relation between the latitude of penetration the number of rings shed and the intensity of the nbc the data set consists of dynamic height time series derived from three inverted echo sounders and a shallow pressure gauge deployed along a section perpendicular to the south american coast between the continent and 7 n and between 48 and 45 w velocity and hydrographic data collected during the nbcr cruises are also analyzed and used to validate the results the 15 month mean transport of the nbc is 16 2 sv the 18 month mean of the retroflected southeastward flow is 22 2 sv both flows display considerable variability the retroflected southeast flow reaches its maximum value during september 1999 near the time when the climatological north equatorial countercurrent necc reaches its maximum strength and it is minimum when the climatological necc reverses or is not present in the basin the mean difference between the nbc flow and the retroflected flow during august december 1999 when the necc is fully established is 7 sv the excess in the retroflected flow is due to north atlantic water joining the retroflected flow from the south atlantic the combination of both flows constitutes the necc

a catalog of taiwan earthquakes 1900 2006 with homogenized mw magnitudes taiwan has a relatively complete catalog of earthquakes since the first seismograph was installed in 1897 however due to changes in seismographic characteristics network coverage and observational practice the definition and procedure for magnitude determination were different during different time periods recognizing a complete catalog of earthquakes with consistent magnitudes is essential for delineating seismicity patterns and assessing seismic hazards for taiwan efforts have been made to convert the original magnitudes of earthquakes in taiwan based on various magnitude scales to a common m l or m s magnitude scale unfortunately the m l or m s magnitude scales chosen for previous studies all are subject to a fundamental limitation of saturation toward large earthquakes besides these studies are nearly two decades old in order to avoid this limitation and to follow the current trend we have chosen in this study to convert original magnitudes of various scales to a common m w scale in this study we used two independent methods for magnitude conversion in the first method we converted the original magnitudes to m w through empirical relations between these magnitudes and m w this magnitude is called the old m w in the second method we used the best fitting a and b values to convert the original magnitudes to m w this magnitude is called the new m w the converted m w magnitudes from both methods have resulted in significant improvements over the original magnitudes and are in good agreement with each other nevertheless by examining the log10 n versus m w plots we found better linearity and tighter overlap among different time periods for the new m w than for the old m w thus we chose the new m w as the unified magnitude for the catalog of taiwan earthquakes finally a list of 899 earthquakes from 1900 to 2006 with m w 5 5 is presented graphic 1 see the earthquake lists available in the electronic supplement to this article the list is considered complete for the whole period from 1900 to 2006 1 embed inline graphic 1 gif

rossby wave radiation in the cape verde frontal zone abstract radiating baroclinic rossby waves excited through instability of the cape verde frontal zone are proposed as a mechanism for the generation of mesoscale variability at middepth 1000 m in the southeastern north atlantic basin linear quasigeostrophic theory is applied to an idealized front representative of the cape verde frontal zone to demonstrate that the front is unstable to modes that may radiate away from the frontal region as baroclinic rossby waves evidence for the existence of these waves is obtained from an eddy resolving basin scale general circulation primitive equation model in addition the model fields are used to identify characteristic signature of the waves in terms of quantities that may be directly observed in the ocean lagrangian trajectories reynolds stress eddy kinetic energy and frequency spectra taken from sofar float and current meter records are all in good agreement with the amplitude and distribution implied by the wave radiation in both the linear theory and

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

characteristics of intermediate water flow in the benguela current as measured with rafos floats abstract seven floats not launched in rings crossed over the mid atlantic ridge in the benguela extension with a mean westward velocity of around 2 cm s between 22s and 35s two agulhas rings crossed over the mid atlantic ridge with a mean velocity of 5 7 cm s toward 285 this implies they translated at around 3 8 cm s through the background velocity field near 750 m the boundaries of the benguela current extension were clearly defined from the observations at 750 m the benguela extension was bounded on the south by 35s and the north by an eastward current located between 18s and 21s other recent float measurements suggest that this eastward current originates near the trindade ridge close to the western boundary and extends across most of the south atlantic limiting the benguela extension from flowing north of around 20s the westward transport of the benguela extension was estimated to be 15 sv by integrating the mean westward velocities from 22s to 35s and multiplying by the 500 m estimated thickness of intermediate water roughly 1 5 sv of this are transported by the 3 agulhas rings that cross the mid atlantic ridge each year as observed with altimetry this value of the benguela extension transport is the first one to have been obtained from long term two year observations and across the full width of the benguela extension

agulhas eddies a synoptic view using geosat erm data abstract warm core rings formed in the agulhas retroflection transfer water from the indian ocean to the south atlantic in an attempt to measure the strength of this exchange a combination of satellite altimeter and hydrographic data are used to examine agulhas eddy paths and decay rates in the south atlantic because the surface dynamic height of a warm core eddy is higher than surrounding waters the rings are visible in satellite altimeter measurements over 20 agulhas eddies have been tracked from maps of anomalous sea surface height ssh derived from the geosat exact repeat mission erm dataset the correlation r2 of dynamic height referenced to 2000 dbar and anomaly ssh for one coincidentally sampled area is 97 within an agulhas eddy dropping to a fraction of that outside of it indicating that the ssh anomaly signal is a reliable measure for strong features like agulhas eddies the sizes and distribution of the agulhas eddies in the erm record compare favorably with those in recent hydrogr

estimates of large magnitude late cambrian earthquakes from seismogenic soft sediment deformation structures central rocky mountains a variety of unusual early post depositional deformation structures exist in grainstone and flat pebble conglomerate beds of upper cambrian strata western colorado including slide scarps thrusted beds irregular blocks and internally deformed beds thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined 15 to 40 ramp locally producing hanging wall lenses with fault bend geometries thrust plane orientations are widely distributed and in some cases nearly oppositely oriented in close proximity indicating that they did not form as failures acted upon by gravity forces irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces these features represent parts of beds that detached moved up onto and some distances across the laterally adjacent undisturbed bed surfaces deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm induced deformation because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks finally internally deformed beds are characterized by large blocks fitted fabrics of highly irregular fragments and contorted lamination which represent heterogeneous deformation such as brecciation and liquefaction the deformation structures were produced by earthquakes linked to the reactivation of mesoproterozoic crustal scale shear zones in the central rockies during the late cambrian analysis of the deformation structures indicates very large body forces and calculated earthquake generated ground motion velocities of ca 1 6 m sec 1 these correspond to moment magnitudes of ca 7 0 or more and a mercalli intensity of x these are the only known magnitude estimates of phanerozoic other than quaternary large intensity earthquakes for the rocky mountain region and they are as large as or larger than previous estimates of proterozoic earthquakes along these major shear zones of the central rockies

estimating seismic hazards from the catalog of taiwan earthquakes from 1900 to 2014 in terms of maximum magnitude maximum expected earthquake magnitude is an important parameter when designing mitigation measures for seismic hazards this study calculated the maximum magnitude of potential earthquakes for each cell in a 0 1 0 1 grid of taiwan two zones vulnerable to maximum magnitudes of m w 6 0 which will cause extensive building damage were identified one extends from hsinchu southward to taichung nantou chiayi and tainan in western taiwan the other extends from ilan southward to hualian and taitung in eastern taiwan these zones are also characterized by low b values which are consistent with high peak ground shaking we also employed an innovative method to calculate at intervals of m w 0 5 the bounds and median of recurrence time for earthquakes of magnitude m w 6 0 8 0 in taiwan

intermediate depth circulation of the indian and south pacific oceans measured by autonomous floats as part of the world ocean circulation experiment 306 autonomous floats were deployed in the tropical and south pacific ocean and 228 were deployed in the indian ocean to observe the basinwide circulation near 900 m depth mean velocities seasonal variability and lateral eddy diffusivity from the resultant 2583 float years of data are presented area averages local function fits and a novel application of objective mapping are used to estimate the mean circulation patterns of mean circulation resemble those at the surface in both basins well developed subtropical gyres twice as strong in the indian ocean as in the pacific feed western boundary currents tropical gyres are separated by eastward flow along the equator in both hemispheres of both basins although the indian subcontinent splits the north indian tropical gyre the antarctic circumpolar current acc and west wind drifts are prominent in both basins generally tending slightly southward but deviating to the north behind the del cano kerguelen and campbell plateaus and of course south america remarkably the eastern boundaries of the southern subtropical gyres in all three basins apparently occur in the ocean interior away from land the indian ocean s subtropical gyre and perhaps part of the south atlantic s reaches east to a retroflection just upstream of the campbell plateau south of new zealand seasonal variability at 900 m is focused around the equator with weaker variability found near certain bathymetric features there is a remarkable agreement between the observed seasonable variability and that predicted by the jet propulsion laboratory jpl estimating the circulation and climate of the ocean ecco data assimilating numerical model aside from seasonal effects eddy variability is greatest along the equator in tropical and subtropical western basins and along the acc integrals of velocity across regional passages tasman sea mozambique channel provide useful reference for hydrographic analyses of transport across whole ocean basins however the uncertainty associated with the appropriate continuity relation for horizontal flow e g geostrophy vs nondivergence is comparable to the mean flow

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

interannual variability of tehuantepec eddies 1 topex poseidon satellite altimeter observations and the naval research laboratory layered ocean model simulations show interannual variability in the number and intensity of tehuantepec eddies off the mexican southwest coast analysis of the results illustrates that downwelling coastally trapped waves which are generated in the equatorial pacific play a crucial role in the modulation and generation of tehuantepec eddies and a dominant role in tehuantepec eddy interannual variability this introduces a new paradigm in which the generation and modulation of tehuantepec eddies is not exclusively explained in terms of the strong and intermittent tehuantepec wind events in fact the results show anticyclonic eddy formation during periods of calm tehuantepec winds that is specifically exemplified by the formation of two anticyclonic tehuantepec eddies during a 5 month period of weak gulf of tehuantepec winds during summer of 1997 furthermore the satellite observed and nlom simulated proliferation of tehuantepec eddies during el nino years is explained by the corresponding increase in downwelling coastally trapped waves and a lack of increase in the number and strength of tehuantepec wind events during el nino years

observations of mesoscale eddies in the south atlantic cape basin baroclinic and deep barotropic eddy variability 1 anticyclones and cyclones in the eastern south atlantic are characterized based on data collected during january 2003 to march 2005 along a jason 1 satellite altimeter ground track as part of the agulhas south atlantic thermohaline transport experiment large and small cyclones and anticyclones were ubiquitous in the deep ocean of the eastern south atlantic as well as in the upper ocean eddy structures jointly corotating in the upper and deep water column were common most of the time 94 these were not axially aligned as they copropagated the agulhas rings and cyclones that populate the region generally carry both a steric component baroclinic and a mass loading component deep barotropic structure average translation speeds were 7 5 cm s 1 for baroclinic eddies and twice as fast for barotropic eddies irrespective of polarity translation speeds were higher than advection by the mean background flow field in addition large mixed baroclinic barotropic rings crashed into the agulhas ridge and nearby seamounts and split into two or more parts some ring parts were also observed to fuse together deep cyclones as well as interactions with topography were observed to play a role in the fission process of agulhas rings these processes can increase the population of agulhas rings and their remnant eddies which took three pathways from the agulhas and into the cape basin 1 a deep pathway between the continental slope and erica seamount 2 a shallower pathway over or near the agulhas ridge and schmitt otto seamount and 3 a deep seaward pathway around the agulhas ridge

the intermediate depth circulation of the western south atlantic the subsurface oceanic circulation is an important part of the earth climate system subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances occasionally supplemented by current meter measurements neutrally buoyant floats however now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western south atlantic here our combined data set provides unprecedented observations and quantification of key flow patterns such as the subtropical gyre return flow 12 sv 1 sverdrup 10 6 m 3 s 1 its bifurcation near the santos plateau and the resulting continuous narrow and swift northward intermediate western boundary current 4 sv this northward flowing water passes through complex equatorial flows and finally enters into the north atlantic

sofar float observations of an intermediate depth eastern boundary current and mesoscale variability in the eastern tropical atlantic ocean two neutrally buoyant sofar floats vigorously looped and meandered at depths of 950 1150 m in the eastern tropical atlantic ocean the float trajectories illustrate a poleward flow along the tropical eastern boundary and significant intermediate depth mesoscale variability in the low latitude eastern basin one float caught within an energetic cyclonic eddy near the eastern boundary looped cyclonically 14 times while translating 600 km northward parallel to the african coastline a second float launched near the mid atlantic ridge meandered eastward with a lagrangian zonal wavelength of 400 km and meridional amplitude exceeding 200 km satellite infrared imagery indicates that horizontal shear associated with the system of near surface zonal equatorial currents may contribute to the observed intermediate depth variability

comparison of the historical record of earthquake hazard with seismic hazard models for new zealand and the continental united states we compare the historical record of earthquake hazard experienced at 78 towns and cities sites distributed across new zealand and the continental united states with the hazard estimated from the national probabilistic seismic hazard psh models for the two countries the two psh models are constructed with similar methodologies and data our comparisons show a tendency for the psh models to slightly exceed the historical hazard in new zealand and westernmost continental united states interplate regions but show lower hazard than that of the historical record in the continental united states intraplate region factors such as non poissonian behavior parameterization of active fault data in the psh calculations and uncertainties in estimation of ground motion levels from historical felt intensity data for the interplate regions may have led to the higher than historical levels of hazard at the interplate sites in contrast the less than historical hazard for the remaining continental united states intraplate sites may be largely due to site conditions not having been considered at the intraplate sites and uncertainties in correlating ground motion levels to historical felt intensities the study also highlights the importance of evaluating psh models at more than one region because the conclusions reached on the basis of a solely interplate or intraplate study would be very different

comparison of the upper mantle shear wave velocity structure of the indian shield and the tibetan plateau and tectonic implications summary travel times and waveforms of long period sh waves recorded at distances of 10 30 and some ss waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the indian shield and the tibetan plateau the shear velocity in the uppermost mantle beneath both the indian shield and the tibetan plateau is high and close to 4 7 km s 1 the indian shield has a fairly thick high velocity lid and the mean velocity between 40 and 250 km is between 4 58 and 4 68 km s 1 in contrast s wave travel times and waveforms of s waves as well as a few for ss show that the mean velocity between 70 and 250km beneath the central and northern part of the tibetan plateau is slower by 4 per cent or more than that beneath the indian shield and probably is between 4 4 and 4 5km s 1 no large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of sh phases crossing tibet or the indian shield these results show that the structure of tibet is not that of a shield and imply that the indian plate is not underthrusting the whole of the tibetan plateau at the present time

three agulhas rings observed during the benguela current experiment a field program to study the circulation of the benguela current and its extension into the southeastern atlantic ocean has completed the survey and instrument deployment phase we report here new observations of three agulhas rings north and west of cape town south africa three mesoscale anticyclonic rings initially identified by means of topex poseidon altimetry were surveyed with expendable bathythermographs conductivity temperature depth oxygen profiles direct current measurements from a lowered acoustic doppler current profiler a hull mounted acoustic doppler current profiler and satellite tracked surface drifters characteristics of the rings are presented and their origins are discussed two are typical agulhas rings surveyed at different times after their generation the third agulhas ring has an anomalous water mass structure whose most likely origin is the subtropical front

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

complete pp waveform modelling for determining crust and upper mantle structure summary we analyse the pp phase at upper mantle distances 25 to 60 to quantify its propagation characteristics and potential for determining crust and upper mantle velocity structure upper mantle distance pp waveforms are very complex involving interference between a variety of arrivals traversing different depth ranges in the earth even for laterally homogeneous structures complete synthetic seismograms such as provided by reflectivity calculations are required to adequately model the full suite of arrivals that contribute to upper mantle pp phases the complexity of pp waveforms can be exploited to extract crust and upper mantle structural information for relatively homogeneous paths while pp p differential traveltimes and pplp amplitude ratios provide constraints on average upper mantle velocities above the transition zone early pp coda comprized of crustal reflections and conversions and precursors to the pp phase comprized of multiple moho underside reflections can provide information on crustal thickness and velocity gradients in the uppermost mantle positive velocity gradients below the crust generate large amplitude moho underside reflections that are particularly visible for short period and broad band seismograms at distances from 25 to 38 we demonstrate the potential use of the pp phase for determining upper mantle structure by modelling long period signals traversing relatively pure paths across north america a continuum of p wave velocity models with varying lid structure is appropriate for the upper mantle beneath north america the highest velocities are found beneath the northern canadian shield with the lid becoming progressively slower beneath the southern shield continental platform and tectonically active regions respectively most of the heterogeneity in p velocity structure is concentrated above a depth of 250 km although some data are consistent with models that are relatively slow at greater depths

upper mantle p velocity structure of the canadian shield waveform modeling of the phases pnl p and pp is used to determine the p wave upper mantle structure beneath the canadian shield long period recordings at world wide standard seismograph network long range seismic measurement and canadian network stations in north america are used with earthquakes within and just outside the shield as sources the p data set is incomplete due to the scarcity of large events within the shield and the inclusion of pp provides much more data using the wkbj approach to calculate the mantle response the pp waveforms can be modeled almost as well as the p the phase pnl persists to large distances in shield regions and the interference of pnl with triplication range p helps constrain the structure in the upper 400 km important features of our model which is designated s25 are 1 a thick high q high velocity lid to 165 km 2 a deep low velocity zone and 3 discontinuities of 5 at 405 km and 4 at 660 km the lid and low velocity zone create an unusual b branch arrival not seen in other regions which persists well beyond 30 only slight lateral variation is noted within the shield region the major features of the model are similar to those of grand and helmberger s s wave model for the canadian shield