influence of the lower crustal structure on the early coda of regional seismograms we use synthetic seismograms for a flat layered earth model to study the influence of the widely observed strong reflectivity of the lower crust on the propagation of seismic regional phases the lower crustal layering induces a clear increase in the duration of the pg and lg phases on both the vertical and radial components beyond the group velocity of their terminations in a model with a homogeneous lower crust the transverse component is weakly affected the amplitude and the spectral content of the early lg coda are strongly dependent on the distribution of layer thicknesses the predominance of the low frequency components below 2 hz observed in the lg coda of real data is correctly accounted for with 1 km thick layers in the lower crust from particle motions at various depths and the analysis of the dispersion curves of normal modes we hypothesize that the late arrivals result from the interaction of plate type disturbances of the high impedance layers these waves leak energy in the mantle at a low rate this mode of generation of the coda waves explains the puzzling observation that the spreading coefficient of lg seems to be applicable to the early coda of regional seismograms the study of the influence of the lamellae on the average q factor of the continental crust shows that the lower crust layering does not account for the observed low values of the apparent qs another origin is needed for this strong attenuation for example scattering by randomly distributed heterogeneities

guided wave propagation in laterally varying media ii lg waves in north western europe summary lg wave observations at norsar from a sequence of explosions straddling the central graben in the north sea show that the phase is almost extinguished by passing through this structure for models of the graben structure based on seismic refraction experiments with crustal thinning beneath a zone of thickened sediments numerical modelling of lg propagation shows very poor transmission the inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity the theoretical effect of a crustal pinch is to redistribute s energy over a wide range of groups velocities a pattern that fits well with the observations three components recordings at norsar of explosive charges fired in water covering a wide range of azimuths show significant transverse components to the seismograms the transverse energy builds up with travel time and for lg is of the same order as the vertical component these results are consistent with the theoretical prediction of progressive net transfer of energy from rayleigh to love modes as the wavetrain propagates through a three dimensionally heterogeneous medium

mapping of crustal heterogeneity in the north sea basin via the propagation of lg waves summary the propagation of lg waves within the continental area in and around the north sea basin shows strong dependence on the path between source and receiver paths lying within the british isles and norway show very clear lg phases but paths which cross the graben zone lying in the middle of the north sea basin have very weak lg over 150 paths have now been studied across the region and the character of the lg wave has been described by comparison with the size of the sn phase for shallow events this gives a stable measure of the efficiency of lg propagation the regions which appear to block the transmission of lg are quite localized in the middle of the north sea in the region with graben structures and extend on into the netherlands a weak zone of poor transmission appears to be associated with the oslo graben with the relatively dense areal coverage provided by paths crossing the basin with a wide range of azimuths it is possible to attempt to invert for the pattern of crustal heterogeneity which gives rise to the observed character of the lg propagation an interative scheme has been devised to find the propagation properties in cells of a 1 x 1 grid and a well defined map of the heterogeneity is obtained the strongest heterogeneities correlate very well with the major tectonic features of the viking and central grabens running north south through the basin where the path coverage is greatest the position of the heterogeneity can be tightly constrained and lies within a rather narrow zone around 100 km wide despite the number of paths employed it is not possible to obtain this level of resolution over the whole region seen in the light of theoretical studies of lg wave transmission the heterogeneity map gives an indication of the strength of horizontal gradients in seismic properties in the crust these are most likely to be associated with crustal thinning beneath the graben structures

lg wave propagation in a laterally varying crust and the distribution of the apparent quality factor in central france the aim of this study is to evaluate the sensitix ity of lg waves to lateral changes of the earth s structure considering a simple model of uplift of the moho numerical simulations show that the geometrical attenuation of lg is not much affected by a smooth anomaly of the moho depth on the other hand the passing of the lg wave through the region of the moho uplift results in a clear deterioration of the wave shapes which confirms the occurrence of mode conversions the presence of an overlying sedimentary basin causes a local amplification of lg above the basin itself and the appearance behind the basin of a secondary surface wave guided in the sediments the effect of the basin on the lg wave is found to be reasonably taken into account by applying a local amplification function to the data we use a data set consisting of records of lg phases in france to test the conclusions of our numerical study the mapping of the apparent quality factor computed from lg at different frequencies confirms the interpretation of the strong attenuation of s waves around i hz in terms of scattering and shows the weak sensitivity of the amplitude of lg to smooth changes in the depth of the moho

numerical modelling of lg wave propagation across the north sea central graben summary in an effort to study the effect of strong crustal lateral variations on the propagation characteristics of lg waves we perform a numerical modelling of these waves at 1 hz in a graben structure we use a coupled mode method and a simplified 2 d model of the north sea central graben we show that at 1 hz the energy reflected by the structure is negligible and that on the average only 20 per cent of the lg incoming energy is converted to waves propagating into the mantle the energy remaining in the lg wavetrain is subject to a strong redistribution with depth which has an effect at the surface of the model which is very dependent on source type and depth multiplying the surface amplitude by a factor of 0 6 for shallow sources and 0 9 1 4 for mid crustal sources on the average the phase of the wavetrain is shown to be stable enough with period to avoid a destructive interference of the signal crossing the graben perpendicularly or at an angle leads to equivalent results lg wave propagation appears to be surprisingly robust in crustal structures with strong lateral variations the large scale geometric features of the graben prove insufficient to explain the strong attenuation of the lg waves which has been observed in the north sea area

mapping crustal heterogeneity using lg propagation efficiency throughout the middle east mediterranean southern europe and northern africa in this paper we describe a technique for mapping the lateral variation of lg characteristics such as lg blockage efficient lg propagation and regions of very high attenuation in the middle east north africa europe and the mediterranean regions lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the comprehensive nuclear test ban treaty ctbt these applications can give significantly biased results if the lg phase is reduced or blocked by discontinuous structure or thin crust mapping these structures using quantitative techniques for determining lg amplitude attenuation can break down when the phase is below background noise in such cases lg blockage and inefficient propagation zones are often mapped out by hand with our approach we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of lg the visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest we trace lg rays for each event station pair which is simply the great circle path and attribute to each cell a value equal to the maximum value of the lg p coda amplitude ratio for all paths traversing that particular cell the resulting map from this empirical approach is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone this map can then be used to screen out events with blocked lg prior to performing q tomography and to avoid using lg based methods of event identification for the ctbt in regions where they cannot work for this study we applied our technique to one of the most tectonically complex regions on the earth nearly 9000 earthquake station raypaths traversing the vast region comprised of the middle east mediterranean southern europe and northern africa have been analyzed we measured the amplitude of lg relative to the p coda and mapped the lateral variation of lg propagation efficiency with the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of lg propagation we observe that the propagation characteristics of lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region for example clear strong lg arrivals are observed for paths crossing the stable continental interiors of northern africa and the arabian shield in contrast weakened to absent lg is observed for paths crossing much of the middle east and lg is absent for paths traversing the mediterranean regions that block lg transmission within the middle east are very localized and include the caspian sea the iranian plateau and the red sea resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations lg propagation is best resolved within the middle east where regions of crustal heterogeneity on the order of 100 km are imaged e g south caspian sea and red sea crustal heterogeneity is resolvable but is poorest in seismically quiescent northern africa

the potassic granites of the igbeti area further evidence of the polycyclic evolution of the pan african belt in southwestern nigeria abstract field geochemical and isotopic studies on granitic rocks in the igbeti area of southwestern nigeria indicate that this pan african belt was affected by two episodes of granite plutonism represented by an older augen gneiss and a younger relatively undeformed coarse grained porphyritic granite these rocks have similar major element characteristics but their trace element compositions are sufficiently different to warrant a suggestion that they are probably of different origin rb sr whole rock isotopic analysis give an eburnean age of 1904 18 ma for the augen gneisses and a pan african age of 617 37 ma for the coarse grained porphyritic granite a low initial 87 sr 86 sr ratio 0 7005 0 0025 for the augen gneiss suggests derivation from the mantle or a low rb sr source whereas a value of 0 7120 0 0016 for the coarse grained porphyritic granite indicates a significantly older crustal component in the magma alkaline magmatism may have been an important component of orogenic magmatism during the eburnean event

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

effects of crustal structure under the barents and kara seas on short period regional wave propagation for novaya zemlya explosions empirical relations abstract short period seismic recordings at regional and upper mantle distances from underground explosions at novaya zemlya demonstrate that propagation across the continental shelf under the barents and kara seas appears to modify the partitioning of energy between l g and s n phases relative to purely continental paths in the eurasian crust while the underwater segments of the paths are relatively short variations in bathymetric characteristics from path to path influence the regional wave field with systematic behavior that can be used to establish empirical amplitude corrections for regional phases we analyze a large set of eurasian recordings to explore the relationship between regional phase energy partitioning and bathymetric characteristics maximum water depth along the path is the most influential factor for the novaya zemlya data it has strong linear correlations with the logarithmic rms amplitude of l g and the ratios s n l g and p l g the maximum water depth probably reflects the extent of necking of the crustal wave guide under the continental margin which may disrupt l g modes resulting in l g to s n scattering but there is surprising sensitivity to small variations in bathymetry empirical relations like those found here may be useful for nuclear yield estimation and discrimination for regions such as the korean peninsula and persian gulf where many seismic phases traverse water covered continental shelf with poorly known crustal structure

a north sea southern norway seismic crustal profile summary in this paper results are presented from a seismic refraction experi ment canobe carried out in southern norway ten explosions fired at sea were recorded on land by shifting 13 recording instruments along a pro file with an average station spacing of 5 km the main line runs in a north easterly direction from the south coast at lista along the western margin of the oslo graben into the norsar array with a total length of 5 15 km a separate leg runs across the graben just north of oslo for the first time allow ing a direct comparison of seismic records in the graben with those in the adjacent precambrian shield area the laterally varying crustal structure along the profile is examined by modelling of travel times and amplitudes of p wave arrivals the moho which is the major discontinuity in the lowermost crust appears to sink beneath the coast from 27 km on the seaward side to 34 km onshore the two dimensional modelling procedure adopted proves invaluable in explaining the characteristic amplitude pattern observed in this area beyond the coastal area our picture of the crust is that of a relatively homogeneous one as expected for a shield area there are no indications of significant discontinuities in the crust along the first 300 km of the profile although from the large pg amplitudes within this distance range we infer a strong velocity gradient in the lower crust two structural models are pro posed for the oslo graben where the moho appears to be elevated to between 25 and 29 km

physical description of lg waves in inhomogeneous continental crust summary the view of lg waves as a superposition of seismic surface waves is very useful for explaining the amplitudes of the late parts of seismograms from regional earthquakes we have recently made computations for rayleigh waves interpreting the special attenuation of lg waves in the north sea as being caused by 1 the distribution of energy over longer time intervals combined with 2 the upwards or downwards pumping of wave energy in the diverent crustal structures combined with 3 anelastic absorption no special discussion is necessary of the claim that anelastic absorption in a particular area gives a reduction in wave amplitude whilst a physical description of the geometrical evect is warranted a physical description of the wave propagation phenomena involved is given in the present paper by classifying the mode conversion that is involved in the propagation from one crustal structure into another the physics is described via relations between the phase velocities group velocities and energy dependences on depths of the various surface wave modes that exist in the diverent crustal structures we introduce the concept of energy depth similarity a parameter that is almost as easily computed as the ratios of phase velocities or group velocities but is a far better measure of the transition character the crustal structures in question are representative of 1 the north sea and its surroundings in general and 2 the viking and central grabens in the middle of the north sea wave propagation is fundamentally diverent for diverent wave frequencies it is coherent for long period waves and incoherent for short period waves the propagation may generally be termed well behaved for wave periods longer than several seconds that is energy cross coupling between incoming and outgoing modes follows some regular pattern this regularity is highly dependent on the similarity of the two modes on either side of a discontinuity and also on the similarity of their neighbouring modes a characteristic of the regularity is that there is no mode conversion for very longperiod modes in contrast a pattern cannot be recognized for waves of shorter period and one may use the term ill behaved the lg waves in the continental crust are in the short period range because their wavelengths are short compared to thickness changes as well as to the thicknesses of crustal structures this leads to the view that lg waves are superpositions of many modes changing into even more modes at any discontinuity since rayleigh and love waves are coupled at non perpendicular incidence and the earth has many discontinuities the superposition of modes is constantly changing in relative proportions rayleigh and love modes being intermingled

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

variations in regional phase propagation in the area around japan the high level of seismicity and dense network of short period stations in japan allows a detailed characterization of regional phase propagation there are substantial differences depending on the location of the source and stations despite the structural complexity lg is clearly seen in many parts of japan lg is particularly well developed in the western part of the main island honshu and will propagates through some volcanic zones with loss of high frequency components there are also zones of lg blockage in northeastern honshu and for subduction zone events with significant oceanic paths for which the mantle phases pn and sn are particularly clear the oceanic region in the sea of japan blocks lg propagation paths for events in mainland asia at many stations but there are clear lg propagation corridors through korea to stations in the west kyushu western honshu and in the north into hokkaido the lg phase carries substantial energy for large events and the differences in efficiency of propagation influences the intensity of ground shaking thus the 1995 hyogo ken nanbu kobe earthquake shows intensity contours extended to the west in the region of efficient lg wave propagation the rg phase and the fundamental mode of love waves are very significant for the shallow 2000 tottori ken seibu event and the effect of the combination of lg with these slightly lower frequency surface waves would help to explain the discrepancy between the japan meteoro logical agency magnitude from regional stations mj 7 3 and the moment magnitude from distant observations m w 6 6

propagation and attenuation characteristics of the crustal phase lg thelg wave consists of the superposition ofs waves supercritically reflected and thus trapped in the crust this mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed the numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models in regions with strong lateral variations the influence of large scale heterogeneities on thelg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

waveform inversion for lateral heterogeneities using multimode surface waves summary we propose a waveform inversion scheme that can be used to invert strong structural heterogeneities the method is based on a ray approximation for surface waves strong structural lateral variations are modelled by vertical discontinuities therefore a geologically heterogeneous region is partitioned into a number of lateral homogeneous subregions synthetic seismograms are calculated by modal summation over incident and transmitted modes with this method the complete waveforms of surface waves including mode cross branch coupling multipathing and scattering are considered in the inversion we test the method with several geophysically realistic structural models an example of waveform inversion for real data a wave path across the iberian peninsula and the oceanic structure off the french coast owing to the gibraltar earthquake ms 5 7 is presented the method allows us to obtain the information represented by different set of structures along the same source receiver minor arc through which the waves have propagated the method is applicable to different models of lateral heterogeneity in its present development it is most appropriate for inverting structures around tectonic scenarios such as continental margins grabens and discontinuities between major plates

developments toward computations of synthetic seismograms in laterally inhomogeneous anelastic media observations of short period surface waves lg waves panza and calcagnile 1974 indicate that there are large differences between propagation properties in various geographic regions one example of this is the north sea shown in fig 1 we have evaluated the possibility of explaining these observations by making computations of theoretical seismograms in various laterally homogeneous models by summation of as many as 205 rayleigh wave modes we have generated synthetic seismograms for those s and p waves that arrive at times later than a specified phase velocity the procedure which has been thoroughly described by panza 1985 works up to frequencies of 10 hz marson et al

seismicity and observations of lg wave attenuation in greenland abstract earthquakes of local magnitudes between 2 1 2 and 5 1 2 have occurred in greenland in the last few decades most of the earthquakes are located near the coasts some of the earthquake zones are connected to known tectonic features in and near greenland the magnitude determinations are based on lg wave amplitudes and are calibrated using the m b determinations of the international seismological centre the attenuation of the lg waves differs from region to region along some paths crossing younger geologic features the attenuation is large compared to that in the paths of the old crystalline shield rocks the lg wave propagation is influenced by the greenland ice cap all the long paths go through the interior of greenland with its 2 3 km ice cap thickness and they exhibit relatively large lg wave attenuation

investigation of teleseismic lg blockage and scattering using regional arrays abstract incoherent beamforming and continuous frequency wavenumber polar scan analysis have been applied to study the characteristics of p coda and lg waves recorded at the regional arrays noress and arcess from presumed explosions on the northwestern russian platform and at test sites of novaya zemlya and eastern kazakh also recordings at the graefenburg array near the new geress regional array of novaya zemlya events and an eastern european platform white sea event have been compared with the norwegian array recordings of the same events the white sea explosion at distances of 14 and 22 from noress and graefenburg respectively exhibits lg rms amplitudes on incoherent beams that exceed those of sn however for explosions at novaya zemlya at 20 from noress and eastern kazakh at 30 from arcess the lg waves have much smaller amplitudes than sn in the case of novaya zemlya the lg wave is almost completely blocked at both noress and arcess but is well recorded at graefenburg this reduction of the lg amplitudes relative to those from events at longer distances cannot be accounted for by anelastic attenuation and spreading comparison of propagation paths suggests that the reduction in amplitudes may be due to blockage and scattering of lg in the barents sea analysis of sn coda waves on incoherent beams and continuous frequency wavenumber analysis polar scans suggests that they can result from lg to sn scattering from the coastline between the barents sea and scandinavia recent geologic studies have revealed significant variations in crustal structure in the barents sea basin which may cause the blockage of lg waves from novaya zemlya in the case of eastern kazakh explosions arcess incoherent beams show much reduced lg amplitudes relative to the p coda waves compared with noress even though the distance to arcess 31 is less than that to noress 38 this reduction in lg amplitude at both noress and arcess is accompanied by enhancement of p and sn coda the timing of the coda waves and the propagation paths suggest that lg waves may be partially blocked and scattered by the ural mountains and that the p and sn coda waves may result from lg to pn pn to sn sn to lg and lg to sn conversions from lateral heterogeneities in the crust beneath the urals the blockage is greater for the eastern kazakh to arcess path because it crosses the middle part of the urals whereas the eastern kazakh to noress path crosses the southern part of the urals chain these blockage effects seem to result from sudden lateral variations in shallow crustal structure such as variations in sediment thicknesses in the sedimentary basins of the russian platform and adjacent to the urals these blockage effects can be predicted from geologic studies and need to be accounted for when using lg amplitudes to estimate absolute yields of explosions and for event identification

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

efficiency of lg propagation from sms dynamic ray tracing in three dimensionally varying crustal waveguides the effects of variations in crustal and basin thicknesses on the efficiency of lg propagation in central asia are investigated by comparing the predictions of ray theoretical modeling of lg as multiple critically reflected sms rays with observations at the knet cnet and ilpa arrays ray modeling predicts that strong spatial gradients in crustal thickness within 10 to 50 km of either the source or receiver can affect lg efficiency over paths exceeding 500 km variations in the efficiency of lg propagation to knet including instances of strong variations across the aperture of the array are consistent with predicted blockage of sms by high slopes of moho topography associated with the hindu kush and pamir mountains at cnet and ilpa the blocking effects of moho slopes associated with the caucausus and zagros mountain belts and black and caspian seas are accentuated by the effects of thick basins thick basins greater than 2000 m are found to have large effects on the efficiency of lg propagation from shallow 1 km crustal sources blocking either a portion or all of the lg phase along paths crossing these basins the basin effects are strongly sensitive to source depth nearly vanishing for sources 15 km and deeper

numerical simulation of high and low frequency lg wave propagation summary lg waves are sometimes extinguished when traversing geological structures such as mountain ranges or grabens previous investigations have applied numerical full waveform simulations at relatively low frequencies on the order of 1 hz to investigate the relationship of different structural models and the amplitude anomalies of the lg waves these studies often fail to link specific known structures to the amplitude anomalies however the observed waveforms also often contain data of significantly higher frequency than the full waveform simulations we compare such low frequency simulations computed with the boundary integral equation bie technique with synthetic seismograms generated by dynamic ray tracing drt beginning with a plane layered model of the crust in central france we show that the general characteristics of the lg wave train over distances ranging from 100 km to around 500 km can be modelled by computing a relatively small number of rays reflected between the free surface and the moho matching the details of the waveforms is more difficult but can be improved by the addition of reflections between other interfaces in the model considering a one layer model of the crust with a step like change in moho depth that leads to a rapid decrease in crustal thickness we show again that the bie and drt solutions are very similar in character significantly both solutions predict that the lg wave will not be extinguished on traversing the moho step finally we consider a more complete and detailed model of the pyrenees though it is more difficult to match exactly the ray and integral equation solutions the general trends continue to be the same and the lg waves are still predicted to cross the tectonic structures associated with the mountains therefore it appears that observed amplitude changes must be caused by something other than the general structure of the pyrenees such as scattering by smaller scale features within the lower crust

asymmetric evolution of el nino and la nina the recharge discharge processes and role of the off equatorial sea surface height anomaly observed oceanic and atmospheric anomalies in the tropical pacific are analyzed to understand the symmetric and asymmetric characteristics between el nino and la nina evolutions it is noted that the evolutions are largely symmetric for el nino and la nina prior to and in their mature phase but become asymmetric afterwards it is further demonstrated that this asymmetry is due to the fact that on average the discharge process associated with el nino is stronger than the recharge counterpart associated with la nina the symmetric and asymmetric evolution for different phases of el nino southern oscillation enso is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies sshas it is suggested that the evolution of the meridional gradient of sshas in the central and eastern equatorial pacific is different between el nino and la nina years due to relatively different rates of the ssha changes on and off the equator during the decay of a warm event the equatorial ssha quickly switches from positive to negative meanwhile the off equator areas are occupied by positive sshas such changes on the equator and in the off equatorial regions make the concavity of the meridional ssha favorable for growth and persistence of westward surface zonal current anomaly in the equatorial pacific then for the enso phase transition during the decay of a cold event the slow decrease of the negative ssha on the equator combined with the disappearance of the negative ssha in the off equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition thus the asymmetric evolution of enso is associated with both the atmosphere and ocean anomalies on the equator and off the equator