the direct wave root in marine seismology abstract the direct wave root is an evanescent wave which is observed below the sea floor when the direct water wave is incident on the sea floor from above at supercritical angles for shallow borehole receivers it is the largest amplitude arrival on large offset seismograms the wave has been studied and observed in electromagnetics and acoustics but this paper is the first report of the direct wave root in marine seismology theoretically an expression for the direct wave root can be obtained from a branch line integral in the complex ray parameter plane for the transmission problem between two acoustic half spaces synthetic seismogram analysis based on the discrete wavenumber or reflectivity method can be applied to study the more realistic cases of elastic bottoms gradients beneath the sea floor and intrinsic attenuation steep gradients below the sea floor give amplitude depth relations which are not exponential attenuation q 40 affects amplitudes less than 1 db observations of the direct wave root have been made at the deep sea drilling project site 504b in the costa rica rift area and the observed properties are consistent with theory it is proposed that more detailed measurements of the decay of the direct wave root amplitude with depth be used to obtain high resolution measurements of upper crustal elastic properties at seismic frequencies

middle permian cephalopods from central peninsular malaysia implications for faunal migration through the southern tethys abstract a wordian middle permian cephalopod fauna consisting of four ammonoid species tauroceras aff scrobiculatum gemmellaro agathiceras sp bamyaniceras orientale n sp and pronoritidae gen and sp indet and two nautiloid species tainoceras sp and orthocerida fam indet were recovered from the bera south area southern pahang peninsular malaysia the outcrop represents the southernmost extent of the bera formation bamyaniceras and tainoceras are recorded in malaysia for the first time the presence of t aff scrobiculatum suggests a regional correlation with an ammonoid bearing bed of sungai cheroh western pahang and permits global correlations with strata of northeastern iraq northern oman and sicily italy a possible eastwards faunal migration through the southern tethys is suggested

velocity structure in upper ocean crust at hole 504b from vertical seismic profiles hole 504b provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust providing a baseline for comparison with seismic measurements elsewhere we determine p and s velocities from vertical seismic profiles vsps obtained on ocean drilling program odp legs 111 and 148 four issues are considered the location of the seismic layer 2 3 boundary p to s wave conversion by scattering transverse isotropy and poisson s ratio as an indicator of lithology porosity and structure 1 in the p velocity profile the change in slope marking the layer 2 3 boundary coincides with the top of the sheeted dike unit seismic layer 2 is composed of the extrusives and the lithologic transition zone the layer in which flows and dikes interfinger 2 even in these normal incident vsps several second arrivals with velocities indicative of vertically polarized shear energy are observed p to s wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis 3 vertical velocities determined from the vsp differ by 10 from horizontal velocities obtained from the oblique seismic experiment ose on deep sea drilling project dsdp leg 92 the p wave velocity structure is determined by small and intermediate 1 cm pore structure with no measurable anisotropy the large scale well oriented vertical fractures which are formed tectonically do not have a detectable effect on compressional wave velocities 4 high poisson s ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows low poisson s ratio at 850 1150 m below seafloor mbsf coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes relatively large aspect ratio cracks are required to produce such low values of poisson s ratio the cracks were likely created by hydraulic fracturing when hot dikes encountered low temperature seawater

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

modeling the effect of san andreas fault structure on receiver functions using elastic 3d finite difference abstract teleseismic p wave data collected at the san andreas observatory sao seismic station show strong frequency dependent nonrectilinear particle motions consisting of off azimuth horizontal displacements that are sometimes larger than the vertical displacements the azimuthal dependence of the horizontal components amplitude is directly related to the symmetry of the fault zone a 3d finite difference program frankel and vidale 1992 is used to examine the response of the san andreas fault zone velocity structure to incident teleseismic p waves synthetic seismograms are formed using a 3d representation of an assumed 2d fault zone structure the fault zone velocity model produces large amplitude phases that contain combinations of refracted and internally reflected p to s converted phases diffractions and possible p to rayleigh converted phases the largest amplitude phases observed in the data are nondispersive and have complicated particle motions confined to the vertical plane perpendicular to the fault zone the data are fit best by synthetic receiver functions for field points similar to the location of sao relative to the san andreas fault zone since the velocity model used in the computations is nonunique sensitivity studies were performed by modifying the structure synthetics match the timing and nature of the observed phases at sao showing that assumption of plane layered structure is invalid in the area of this station

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

moist teleconnection mechanisms for the tropical south american and atlantic sector abstract teleconnections have traditionally been studied for the case of dry dynamical response to a given diabatic heat source important anomalies often occur within convective zones for instance in the observed remote response to el nino the reduction of rainfall and teleconnection propagation in deep convective regions poses theoretical challenges because feedbacks involving convective heating and cloud radiative effects come into play land surface feedbacks including variations of land surface temperature and ocean surface layer temperature response must be taken into account during el nino descent and negative precipitation anomalies often extend across equatorial south america and the atlantic intertropical convergence zone analysis of simulated mechanisms in a case study of the 1997 98 el nino is used to illustrate the general principals of teleconnections occurring in deep convective zones contrasting land and ocean regions comparison to other simulated events shows similar behavior t

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

middle permian brachiopods from the tumenling formation in the wuchang area southern heilongjiang ne china and their palaeobiogeographical implications abstract this paper describes nine brachiopod species in eight genera from the tumenling formation in the wuchang area southern heilongjiang northeast china the tumenling fauna is constrained to a wordian capitanian guadalupian middle permian age based on the correlation of the faunas and previously published data from the same horizons in the same area this middle permian brachiopod fauna is characterized by an admixture of elements of boreal affinity kochiproductus sp yakovlevia sp stenoscisma margaritovi gypospirifer volatilis spiriferella keilhavii spiriferella lita and alispiriferella neimongolensis with palaeoequatorial forms spinomarginifera sp vediproductus sp and leptodus nobilis the wuchang fauna as a whole is comparable to several contemporaneous faunas described from inner mongolia in northern china jilin in northeast china south primorye in eastern russia and the hida gaien belt of japan in terms of its index elements and its boreal palaeoequatorial mixture during the middle permian these areas or blocks were referred to the inner mongolia japan transition zone or the northern transition zone between the palaeoequatorial and boreal realms the overwhelming majority of the boreal elements reveal that the wuchang area was probably situated in the northern subzone of this transitional zone which belongs to the southern margin of the bureya block these east and northeast asian blocks acted as migratory stepping stones bridging faunal migration between the palaeoequatorial and boreal realms during the middle permian

interhemispheric influence of the atlantic warm pool on the southeastern pacific abstract the atlantic warm pool awp is a large body of warm water comprising the gulf of mexico caribbean sea and western tropical north atlantic the awp can vary on seasonal interannual and multidecadal time scales the maximum awp size is in the boreal late summer and early fall with the largest extent in the year being about 3 times the smallest one the awp alternates with the amazon basin in south america as the seasonal heating source for circulations of the hadley and walker type in the western hemisphere during the boreal summer fall a strong hadley type circulation is established with ascending motion over the awp and subsidence over the southeastern tropical pacific this is accompanied by equatorward flow in the lower troposphere over the southeastern tropical pacific as dynamically required by the sverdrup vorticity balance it is shown by analyses of observational data and ncar community atmospheric model simulations that an anomalously large small awp during the boreal summer f

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

seismic energy partitioning and scattering in laterally heterogeneous ocean crust we present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust the finite difference formulation is a 2 d solution to the elastic wave equation for heterogeneous media and implicitly calculates p and sv propagation compressional to shear conversion interference effects and interface phenomena random velocity perturbations with gaussian and self similar autocorrelation functions and different correlation lengths a are presented which show different characteristics of secondary scattering heterogeneities scatter primary energy into secondary body waves and secondary stoneley waves along the water solid interface the presence of a water solid interface in the model allows for the existence of secondary stoneley waves which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models

distribution and characteristics of mixed transitional mid permian late artinskian ufimian marine faunas in asia and their palaeogeographical implications abstract the permian marine biogeography of the asian region is characterised by the development of three broad realms boreal tethyan and gondwanan in the early early permian asselian to early artinskian there seems to have existed sharp biogeographical boundaries with no transitional zones between the realms presumably due to the effect of gondwanan glaciation and sharp climatic zonation the mid permian late artinskian to ufimian of the asian region is distinguished by the rise of two broad transitional zones in which mixed marine faunas flourished the northern transitional zone north and northeast china sikhote alin central and northeast japan is characterised by an admixture of both warm water cathaysian and cold temperate boreal genera the southern transitional zone arabian peninsula iran afghanistan the karakorum southeast and central pamir salt range central and northwest tibet shan thai terrane timor and western irian jaya on the other hand is distinguished by incorporating faunal elements of both gondwanan and cathaysian origin the mixed faunas of both transitional zones share several antitropically distributed genera e g monodiexodina lytvolasma and spiriferella and are succeeded by palaeotropical tethyan faunas of late permian age it has been postulated that the mixed mid permian fauna of the northern transitional zone was probably formed in an epicontinental sea which was connected to both the asian tethys and the arctic which facilitated the migration of cathaysian and boreal forms into the epicontinental sea and subsequent faunal mixing the formation of the mixed faunas of the southern transitional zone on the other hand is probably related to the breaking off of the cimmerian blocks from northern gondwana and subsequent drifting into a lower latitudinal zone with geographical proximity to both gondwana and cathaysia during the mid permian

impact of the atlantic warm pool on the summer climate of the western hemisphere abstract the atlantic warm pool awp is a large body of warm water that comprises the gulf of mexico the caribbean sea and the western tropical north atlantic located to its northeastern side is the north atlantic subtropical high nash which produces the tropical easterly trade winds the easterly trade winds carry moisture from the tropical north atlantic into the caribbean sea where the flow intensifies forming the caribbean low level jet cllj the cllj then splits into two branches one turning northward and connecting with the great plains low level jet gpllj and the other continuing westward across central america into the eastern north pacific the easterly cllj and its westward moisture transport are maximized in the summer and winter whereas they are minimized in the fall and spring this semiannual feature results from the semiannual variation of sea level pressure in the caribbean region owing to the westward extension and eastward retreat of the nash the ncar community atmospher

geoacoustic scattering from seafloor features in the rose area a strong refraction branch diffraction has been observed on ocean bottom hydrophone data from the rivera ocean seismic experiment rose modeling of the interaction between seismic acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features seismic acoustic wave propagation was computed for models of marine seismic refraction lines using a 2d heterogeneous formulation of the elastic wave equation these models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave traveling through the upper crust are incident upon a structure since much of the diffracted energy travels through the crust the diffracted arrivals appear on the refraction branch of the seismograms at large ranges energy partitioning at the seafloor of the two incident wave types produce both compressional p and shear s diffracted phases in

toe of slope of a cretaceous carbonate platform in outcrop seismic model and offshore seismic data apulia italy synthetic seismic models of outcrops in the early cretaceous slope of a carbonate platform on the gargano promontory southern italy were compared to an offshore seismic section south of the promontory outcrops of the same age on the promontory have the same sequence stratigraphic characteristics as their offshore equivalent and are the only areas where the transition from platform to basin of early cretaceous is exposed on land two adjacent outcrop areas were combined into one seismic scale lithologic model with the aid of photo mosaics measured sections and biostratigraphic data velocity density and porosity measurements on spot samples were used to construct the impedance model seismic models were generated by vertical incidence and finite difference programs the results indicate that the reflections in the seismic model are controlled by the impedance contrast between low porous intervals rich in debris from the platform and highly porous intervals of pelagic lime mudstone nearly devoid of debris finite difference seismic display showed best resemblance with the real seismic data especially by mapping a drowning unconformity

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

seismic propagation across the east pacific rise finite difference experiments and implications for seismic tomography we use a full waveform acousto elastic finite difference technique to investigate seismic propagation across the east pacific rise at 9 30 n for a two dimensional velocity model based on that proposed by vera and others 1990 the primary feature of the model is an upper crustal low velocity region corresponding to the axial magma chamber which includes a small magma body located 1 6 km beneath the seafloor at the rise axis the high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber a qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9 30 n shows generally good agreement for paths that cross the rise axis the first arrival in the finite difference solutions diffracts above the magma chamber this phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment in such cases the first arrival with significant energy is a diffraction from below the magma chamber a high amplitude moho turning pmp phase which results from the large velocity change across the moho beneath the rise axis is apparent in both the finite difference solutions and the observations ray theoretical calculations of the paths of the diffracted arrivals are very unstable and for the diffractions above the magma chamber no solution can be found with a single precision algorithm synthetic delay time inversions using an approximate ray tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body synthetic inversions of spectral estimates of t show that q 1 models are compromised not only if the ray paths are incorrect but also if t estimates include significant contributions from more than one phase deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

structural analysis and vein episode of the penjom gold deposit malaysia implications for gold mineralisation and tectonic history in the central belt of malaysia abstract gold mineralisation at the penjom gold deposit in the central belt of peninsular malaysia is dominantly hosted in structurally controlled quartz carbonate veins that display a variable degree of overprinting events under a ductile brittle regime mineralised veins are categorised into two main episodes fold and thrust related veins mainly comprised of bedding and penjom thrust parallel shear veins and associated extension veins developed during a d2 event in addition ne to nne trending splays and penjom thrust parallel reverse dextral and locally nnw to ns reverse sinistral faults overprint earlier vein events forming a new episode of extensional veins during d3 event mostly at the fault contacts of intrusives either as shallow dipping or vertical sheeted stockwork or brecciated veins at least two episodes of faulting d4 d5 are evident which postdates the main episodes of veining and mineralisation this is indicated by a series of late episodes ns trending sinistral and oblique to normal faults which transect and displace the penjom thrust and the folded footwall sequence at low angles and responsible for intensive vein deformation along the penjom thrust and displacement of ore body structural interpretations of mineralised veins suggest that the veins were formed under a compressional to transpressional regime with later post vein displacement in a transtensional to extensional regime vein formation in response to deformation observed at penjom reflects regional structural deformation within the central belt of peninsular malaysia and specifically along the bentong raub suture zone

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

the bentong raub suture zone abstract it is proposed that the bentong raub suture zone represents a segment of the main devonian to middle triassic palaeo tethys ocean and forms the boundary between the gondwana derived sibumasu and indochina terranes palaeo tethyan oceanic ribbon bedded cherts preserved in the suture zone range in age from middle devonian to middle permian and melange includes chert and limestone clasts that range in age from lower carboniferous to lower permian this indicates that the palaeo tethys opened in the devonian when indochina and other chinese blocks separated from gondwana and closed in the late triassic peninsular malaysia segment the suture zone is the result of northwards subduction of the palaeo tethys ocean beneath indochina in the late palaeozoic and the triassic collision of the sibumasu terrane with and the underthrusting of indochina tectonostratigraphic palaeobiogeographic and palaeomagnetic data indicate that the sibumasu terrane separated from gondwana in the late sakmarian and then drifted rapidly northwards during the permian triassic during the permian subduction phase the east malaya volcano plutonic arc with i type granitoids and intermediate to acidic volcanism was developed on the margin of indochina the main structural discontinuity in peninsular malaysia occurs between palaeozoic and triassic rocks and orogenic deformation appears to have been initiated in the upper permian to lower triassic when sibumasu began to collide with indochina during the early to middle triassic a type subduction and crustal thickening generated the main range syn to post orogenic granites which were emplaced in the late triassic early jurassic a foredeep basin developed on the depressed margin of sibumasu in front of the uplifted accretionary complex in which the semanggol formation rocks accumulated the suture zone is covered by a latest triassic jurassic and cretaceous mainly continental red bed overlap sequence

a simple atmospheric model of the local and teleconnection responses to tropical heating anomalies abstract a minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected the model is linearized about some idealized and realistic background wind fields and forced with a localized heating for illustration in the tropics the baroclinic responses are familiar from the matsuno gill model these excite barotropic responses by advective interactions with vertical background wind shear the barotropic signals are in turn transmitted to high latitudes only in the presence of barotropic background westerly winds for an el nino like equatorial heating the barotropic response has anticyclones to the north and south of the heating reinforcing opposing the anticyclonic cyclonic baroclinic gyres in the upper lower troposphere with realistic background flows the model reproduces the hemispheric asymmetry

teleconnection mechanisms for tropical pacific descent anomalies during el nino abstract teleconnection mechanisms in relative descent zones are examined using the quasi equilibrium tropical circulation model qtcm the regions of anomalous descent neighboring the warm sea surface temperature sst anomalies in the eastern and central pacific ocean during the 1997 98 el nino are used as an example and results are verified for three other el nino warm events 1982 1987 and 1991 by analyzing the moisture and moist static energy budgets over these regions it is found that the physical processes responsible for the descent anomalies are different for each region anomalous dry advection is the dominant mechanism for the reduced precipitation to the north of the el nino warm region on the other hand the relative descent to the south of the el nino warm ssts is caused by reduced surface wind speed and reduced air sea contrast and resulting weaker than normal evaporation budget analysis based on the national aeronautics and space administration seasonal to interannual prediction p

barotropic wave propagation and instability and atmospheric teleconnection patterns abstract a global barotropic model linearized about the 300 mb climatological mean january flow is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics structures which resemble the observed pacific north american and east atlantic teleconnection patterns noted by wallace and gutzler 1981 tend to recur in the responses similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes it is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally varying climatological basic state in the absence of damping this mode has an e folding time of about a week and a period close to 50 days in localized regions the instantaneous growth rates can be competitive with those of baroclinic instability these episodes of rapid local barotropic growth are interspersed with intervals in which the local

structural mapping using palsar data in the central gold belt peninsular malaysia abstract the central gold belt cgb of peninsular malaysia has been investigated to map structural elements associated with gold mineralization using the phased array type l band synthetic aperture radar palsar satellite remote sensing data gold mineralization in this belt is structurally controlled and associated with steeply dipping faults and fold hinges adaptive local sigma and directional filters were applied to palsar data for tracing structural elements associated with gold mineralization structural features along the bentong raub suture zone have been identified as highly potential areas for gold prospecting four sets of lineaments trending n s ne sw nnw sse and ese wnw were identified results of this study demonstrate the applicability of palsar remote sensing data to assist gold exploration in the cgb particularly in reducing costs related to exploration for epithermal and polymetallic vein type mineralization in tropical environments

organization of extratropical transients during el nino abstract four observed el nino southern oscillation enso events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter a parallel analysis of a gcm s response to el nino is performed in order to assess if similar mechanisms are operative in the model atmosphere the observed stationary wave anomalies over the pacific north american pna region are found to he similar during the four winters despite appreciable differences in sea surface temperatures the anomalous transient vorticity fluxes are remarkably robust over the north pacific during each event with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40 n 150 w this forcing is in phase with the seasonal mean aleutian trough anomaly suggesting the importance of eddy mean flow interactions by comparison the intersample variability of the gcm response over the pna region is found to exceed the observed inter el nino variabi

interhemispheric teleconnections from tropical heat sources in intermediate and simple models themechanismsthatcontroltheinterhemisphericteleconnectionsfromtropicalheatsourcesareinvestigated using an intermediate complexity model a quasi equilibrium tropical circulation model qtcm and a simple linear two level model with dry dynamics illustrating the interhemispheric teleconnection process with an atlantic warm pool principal case the heat source directly excites a baroclinic response that spreads across the equator then threeprocesses involvingbaroclinic barotropicinteractions shearadvection surfacedrag and vertical advection forcea cross equatorial barotropicrossbywaveresponse ananalysis ofthese processes in qtcm simulations indicates that 1 shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics 2 surface drag has large amplitude and spatial extent and can be very effective in forcing barotropic motions around the globe and 3 vertical advection has a significantcontribution locally and remotely where large vertical motions and vertical shear occur the simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted by adding surface drag and vertical advection and comparing each to shear advection the effects of the three mechanisms on the generationandpropagationofthebarotropicrossbywavesareshowntobequalitativelysimilartotheresultsin qtcm it is also found that the moist processes included in the qtcm can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent and by inducing remote precipitation anomalies by interaction with the basic state

the chanthaburi terrane of southeastern thailand stratigraphic confirmation as a disrupted segment of the sukhothai arc a permo triassic volcanic arc system the sukhothai arc is recognised between the indochina and sibumasu continental blocks the chanthaburi terrane is here interpreted as a fault detached highly disrupted southern segment of the sukhothai arc occupying part of southeastern thailand and extending into cambodia the klaeng tectonic line is defined as the boundary between the chanthaburi terrane and sibumasu block the stratigraphy of the chanthaburi terrane is compared with that of the sukhothai terrane in northern thailand the late palaeozoic mesozoic sequences of these two volcanic arc terranes in the sukhothai zone share important similarities but show marked contrasts to those of the sibumasu and indochina blocks where the late permian triassic is largely absent due to the indosinian i unconformity western indochina or is dominantly carbonates with little terrigenous clastic input sibumasu there is no clear evidence of pre carboniferous sedimentary rocks for either the sukhothai or chanthaburi terranes late permian lyttoniid brachiopod shale near klaeng in the chanthaburi terrane was revisited the brachiopod previously reported as leptodus is re identified to oldhamina the genus previously known elsewhere in southeast asia only in the huai tak formation of the sukhothai terrane oldhamina in thailand is confined to the sukhothai arc the marine stratigraphy of the sukhothai arc is represented by a permian triassic lithological succession of mixed carbonates and siliciclastics with common volcanic material the late permian and triassic litho and biostratigraphy of the chanthaburi terrane are comparable with the upper ngao and lampang groups of the sukhothai terrane in particular they share similar successions from oldhamina brachiopod bearing shale to palaeofusulina colaniella foraminifer bearing limestone in the latest permian marine depositional conditions were terminated on the sukhothai arc by end triassic times later than on the indochina block late permian but earlier than on the sibumasu block jurassic cretaceous

compressional and shear wave structure of the east pacific rise at 11 20 n constraints from three component ocean bottom seismometer data we report the analysis of seismograms from two orthogonal refraction lines in the rose area of the east pacific rise at 11 20 n recorded by a three component ocean bottom seismometer obs converted shear waves are clearly observed on the horizontal seismometers for both lines including paths crossing the rise axis neither a travel time delay nor measurable attenuation is observed for shear or compressional waves which have passed beneath the rise crest at depths down to 4 km on the basis of these observations we conclude that no large crustal magma chamber is present beneath the rise at this latitude we invert the travel times of p and converted s waves to infer the crustal structure in the vicinity of the rise and we use p wave amplitudes to constrain further the range of solutions the p wave velocity distribution does not differ markedly from that of normal young oceanic crust but the shear velocity distribution results in a poisson s ratio 0 32 that may be more typical of highly fractured crust in rise axis regions a thin 80 m layer of sediment is inferred to be present beneath the obs on the basis of the measured particle motions the observation of an sp phase and the reverberatory character of the seismograms a combined consideration of the results of this experiment and other observations of rise crossing crustal phases elsewhere in the rose area leads us to conclude that any magma within the crust beneath the rise axis of this region must be confined to narrow dikes or pipes or to small isolated bodies with a vertical thickness of less than about 1 km

the generation of global rotational flow by steady idealized tropical divergence abstract tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent the horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field a vorticity equation model is used to diagnose this relationship it is shown that because of the advection of vorticity by the divergent component of the flow the rossby wave source can be very different from the simple fd source often used in particular an equatorial region of divergence situated in easterly winds can lead to a rossby wave source in the subtropical westerlies where it is extremely effective this part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence a divergence field which is asymmetric about the equator can lead to a quite symmetric rossby wave source for a steady frictionless flow the rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is under cert

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure

middle permian brachiopods from central peninsular malaysia faunal affinities between malaysia and west cambodia a moderately diverse permian brachiopod fauna is described from a new rock unit the bera formation in the bera district central pahang peninsular malaysia the fauna consists of 19 taxa including 14 genera and 17 both identified and unidentified typically tethyan species the fauna appears to be correlative on the basis of brachiopods with the neoschwagerina yabeina fusulinid zones in indochina and south china in particular it has strong linkage to member c yabeina beds of the sisophon limestone west cambodia this is indicated by three of the bera species urushtenoidea chaoi ching spyridiophora gubleri termier and termier and transennatia termierorum sp nov being shared with the cambodian fauna a possible early capitanian middle permian age is proposed for the bera brachiopod fauna

the leptodus shales of central peninsular malaysia distribution age and palaeobiogeographical affinities the terms leptodus shales and leptodus beds have been used to describe a rich brachiopod bearing unit within the permian argillaceous facies of the central belt of peninsular malaysia to date there has been no formal description of this unit regarding its age spatial distribution or faunal composition a review of previous literature backed by recently collected data from our field surveys and biostratigraphical studies reveals that there is a sequence of fossiliferous assemblages within the leptodus shales which range in age from middle permian to possibly early late permian and extend geographically from southern kelantan to southern pahang peninsular malaysia these assemblages are found in argillaceous sediments which are often highly tuffaceous and in northern pahang are associated with pyroclastic volcanics of probable island arc origin the faunas are of palaeo equatorial affinity and are taxonomically close to faunas in indochina such as the sisophon fauna in cambodia typical elements include vediproductus cf punctatiformis chao transennatia gratiosa waagen t termierorum sone leman and shi uncuninellina timorensis beyrich leptodus richthofeni kayser l cf tenuis waagen leptodus nobilis waagen gubleria aff ninglangensis fang and jiang and spyridiophora gubleri termier and termier

finite difference seismograms for laterally varying marine models summary the method of finite differences is applied to the elastic wave equation to generate synthetic seismograms for laterally varying seafloor structures the results are compared with borehole seismic data from the gulf of california deep sea drilling project site 485 in which lines were shot over flat and rough topography the significant new phenomenon observed in both the synthetic seismograms and the field data is the generation of a double head wave due to the interaction of the incident wavefront with the side of a hill and the flat seafoor adjacent to the hill in these models the hills are on the order of a seismic wavelength in height and steep velocity gradients occur over distances comparable to wavelengths ray theoretical methods would not be suitable for studying such structures true amplitude record sections are obtained by the finite difference method which show for these models that the head wave generated at the flat seafloor adjacent to the hill is lower in amplitude than if the hill were not present and is lower in amplitude than the head wave generated at the hill a second feature which is important for borehole receivers is the existence of the direct wave root in the upper basement this energy occurs below the sharp interface when the direct wave impinges on the interface from above there is no corresponding snell s law ray path for this energy and the energy is evanescent with depth in the lower medium the properties of both the double head wave and the direct wave root are clearly demonstrated in the finite difference snapshot displays

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

a review of finite difference methods for seismo acoustics problems at the seafloor understanding seismic wave propagation in realistic seafloor environments is essential for many problems in marine seismology finite difference methods are becoming increasingly popular in solving propagation problems as the limitations of other techniques which apply only at high frequency or for flat lying media become fully appreciated the seafloor problem a high contrast in poisson s ratio at a rough sharp interface is particularly challenging and many published formulations fail to solve it accurately the purpose of this paper is to summarize the published finite difference formulations for the elastic wave equation and to outline their applicability to seafloor problems