local site effects in atakoy istanbul turkey due to a future large earthquake in the marmara sea summary since the 1999 izmit and duzce earthquakes in northwest turkey many seismic hazard studies have focused on the city of istanbul an important issue in this respect is local site effects strong amplifications are expected at a number of locations due to the local geological conditions in this study we estimate the local site effects in the atakoy area southwestern istanbul by applying several techniques using synthetic data hybrid 3 d modelling and 1 d modelling and comparing to empirical data we apply a hybrid 3 d finite difference method that combines a complex source and wave propagation for a regional 1 d velocity model with site effects calculated for a local 3 d velocity structure the local velocity model is built from geological geotechnical and geomorphological data the results indicate that strongest spectral amplifi cations sa in the atakoy area occur around 1 hz and that amplification levels are largest for alluvial sites where sa reaching a factor of 1 5 2 can be expected in the case of a large earth quake we also compare our results to h v horizontal to vertical component of the recorded signal spectral ratios calculated for microtremor data recorded at 30 sites as well as to ambient noise synthetics simulated using a 1 d approach because the applied methods complement each other they provide comprehensive and reliable information about the local site effects in atakoy added to that our results have significant implications for the southwestern parts of istanbul built on similar geological formations for which therefore similar sa levels are expected

geologic and seismic setting pertinent to dam safety review of duncan dam le barrage duncan est un ouvrage de b c hydro construit sur la riviere duncan dans le sud est de la colombiebritannique au canada entre 1965 et 1967 le barrage repose sur une succession complexe de plus de 380 m de sediments detritiques et fluvio glaciaires dont certains sont permeables et compressibles quelques unites sableuses peuvent presenter un danger de liquefaction en particulier une couche de sable unite 3c ayant jusqu a 23 m d epaisseur les regles sismiques en vigueur actuellement a b c hydro pour les barrages exigent que le barrage duncan soit capable de resister au seisme maximum envisageable mce sans deversement catastrophique du reservoir cet article decrit les conditions geologiques et sismiques dans la region entourant le barrage et le choix des parametres pour les mouvements sismiques du sol des methodes probabilistes ont ete appliquees pour reconstituer les mouvements du terrain correspondant au mce on a estime que l on pourrait avoir un pic d acceleration horizontale du terrain de 0 12 g provenant d un tremblement de terre d amplitude m 6 5 a une distance d environ 50 km pour les analyses dynamiques des sols on a choisi plusieurs cas historiques tires du passe et presentant des caracteristiques semblables a celles du seisme previsionnel

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

high latitude lower thermospheric neutral winds at eiscat and sondrestrom during ltcs 1 the incoherent scatter radar located at sondre stromfjord greenland 67 n 51 w 74 5 and the eiscat incoherent scatter facility located in northern scandinavia 69 5 n 19 e 66 3 both obtained e and f region measurements during the first campaign of the lower thermosphere coupling study ltcs 1 september 21 25 1987 neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components a number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results forbes 1982 the semidiurnal amplitudes and phases although somewhat consistent between the two radars are not well represented in equinox tidal model results forbes and vial this issue results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km during a period of impulsive magnetospheric forcing september 22 23 winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at sondrestrom comparison with the results of a national center for atmospheric research thermosphere ionosphere general circulation model tigcm simulation of the ltcs 1 interval shows generally better agreement with the observations at eiscat than at sondrestrom during the period of activity on september 22 23 the tigcm is reasonably successful at simulating the eastward surge near midnight in the eiscat but not in the sondrestrom observations and the equatorward flow after midnight in the sondrestrom results the observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn

qlg tomography in colombia the crustal attenuation of lg waves in colombia was estimated and analyzed using local seismological data from the national seismological network of colombia rsnc the selected dataset comprises 510 crustal earthquakes with a total of 2928 ray paths this large dataset allowed us to invert for a regional average of qlg tomographic images for colombia and simultaneously source size for each event and site term at the seismological stations the computed regional average for qlg in the frequency band 0 5 5 0 hz was found to agree with the previously reported values in neighboring regions in order to resolve the lateral variation in the attenuation of lg waves in the crust independent tomographic inversions for 26 frequencies between 0 5 and 5 0 hz were conducted the resulting maps confirm that heterogeneities in the crust exist and that they are related to the large scale tectonic features in the country the highest attenuation in the region is linked to the presence of active volcanic arcs where the crust is weakened by partial melting relatively high attenuation is also found in zones where the crust is composed of accreted oceanic rocks and in regions with near surface low velocity sedimentary layers 2002 elsevier science b v all rights reserved

multiyear observations of tidal oscillations in oh m 3 1 rotational temperatures at south pole antarctica 1 tidal amplitudes and phases deduced from austral winter observations of oh meinel 3 1 rotational temperatures at south pole antarctica over the period 1994 2007 are presented here dominant periodicities in temperature data are seen at 24 12 and 8 h the amplitude of these wave oscillations shows significant variability during the months analyzed herein april through september in contrast to 12 and 8 h waves the 24 h wave is stronger more persistent and coherent at south pole the observed amplitude variability in 24 12 and 8 h waves during the majority of the years analyzed is shown to be correlated phase variations of 24 and 8 h waves with longitudes indicate that these waves are standing zonal wave number 0 waves

moment magnitude local magnitude calibration for earthquakes off canada s west coast local magnitude m l values of earthquakes off canada9s west coast are known to be underestimated by at least 0 5 magnitude units compared with other magnitude scales moment magnitude m w derived from moment tensor analysis provides the most robust estimate of the magnitude of earthquakes moment tensor analysis of regional seismic data in western canada is now possible due to the installation of more than 40 three component broadband stations in western canada the u s pacific northwest and southeast alaska moment tensor solutions are now possible down to m 4 0 more than 230 regional moment tensor solutions have been calculated off canada9s west coast at the geological survey of canada for 1995 2002 these solutions along with 14 previous solutions by oregon state university in 1994 1995 and 13 harvard solutions for 1984 1993 allow a systematic m w m l calibration for earthquakes in this region the study area extends from the queen charlotte islands region in the north to the area off the west coast of southern vancouver island at the northern end of the study area where there is little oceanic crust in the source receiver travel path m w is systematically larger than m l by 0 28 0 08 magnitude units at the southern end of the study area where there is a significant amount of ocean crust in the source receiver travel path m w is systematically larger than m l by 0 62 0 08 magnitude units calibration of m l with m w will allow the western canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis

the acquisition of source path and site effects from microearthquake recordings using q tomography application to the united kingdom source site and propagation parameters are inverted from a u k database of weak motion events 2 0 m l 4 7 this results in the complete spectral parameterization of over 3200 velocimetric records of 273 events from the year 1992 to 2006 the s wave is extracted from the vertical records and is processed using a multitaper fourier transform we initially use a nonlinear least squares log space optimization to obtain estimates of the attenuation parameter for each spectrum the estimates of t are then used to geometrically constrain a depth dependent q model using a technique adapted from velocity tomography we then invert for the remaining frequency dependent parameters and a collective amplitude parameter from the velocity spectra while fixing the newly computed attenuation parameters based on raytracing through our q model the resultant amplitude parameters are then split into source moments apparent geometrical spreading and site correction factors we find a frequency independent depth dependent q structure a linear relationship proportional to 0 7 m l between moment magnitude m w and local magnitude m l is found in the range of 2 4 7 m l the majority of stress drops are found to range on the order of 0 1 10 mpa a multiple segment apparent geometrical spreading model is found to best describe the amplitude decay with distance accounting for factors such as geometrical spreading and scattering along with multiple phase interference in the analysis window site response functions are found to broadly correlate with regional geology mean amplification occurring in the cenozoic sedimentary rock sites to the southeast of england relative to the harder palaeozoic rock sites of wales and scotland we use a bootstrap analysis technique to analyze the dependence of our results on the data in order to estimate the variance of the results and check the robustness of different inversions synthetic spectra are also computed in order to obtain minimum variance and bias of model parameters associated with the method in applying a geometrical q constraint through the use of q tomography we find that the robustness of the results is significantly increased a thorough analysis of the trade offs involved in the inversion is performed using synthetic datasets we find strong trade offs between the parameters but we are able to show that this covariance is reduced when adopting the q tomography approach

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

average qlg qsn and observation of lg blockage in the continental margin of nova scotia the term lg blockage refers to the sudden disappearance of the lg phase along a particular propagation path which is commonly seen at continental oceanic transition zones in this paper we present observational evidence of lg blockage across the continental margin of nova scotia in eastern canada regional lg and sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2 5 and 4 7 are inverted simultaneously for the source spectrum site amplification and average attenuation the vertical displacement spectra were estimated between 0 9 and 10 75 hz the assumptions include a fixed frequency independent geometric spreading rate for lg and a frequency dependent spreading model for the sn estimates for the apparent regional attenuations are qlg f 615 25 f0 35 0 04 and qsn f 404 23 f0 45 0 03 results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region based on the observation of a strong trade off between sn and lg amplitudes which have different attenuation characteristics we conclude any attenuation study based on measuring amplitude of a package of several different phases without taking into consideration the propagation characteristics of individual waveforms at the region of study may bias the estimation of average regional q

frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations campillo m and plantet j l 1991 frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations phys earth planet inter 67 48 64 we processed digital records from 431 earthquakes at 25 stations of the laboratoire de detection geophysique network in france we computed spectral amplitudes of pg and lg in different group velocity windows this large data set aliowed us to compute a map of mean crustal attenuation through simultaneous consideration of source amplitude site response and apparent q our inversion procedure combines an iterative reconstruction technique with an adjustment of source amplitudes and site responses at each step the stability of the result is tested with different subsets of data and the resolution is evaluated maps of apparent attenuation at frequencies between 1 5 and 10 hz were computed from pg lg and early coda of lg these results are in a good agreement with the predictions of the singie scattering model both for frequency dependence and for qs qp ratios the comparison between the attenuation anomaly found in the yariscan belt and results from a deep reflection seismic profile confirms the prominent part played by scattering in the apparent attenuation of seismic waves in the crust

on the nature of regional seismic phases ii on the influence of structural barriers summary the blockage of the lg wave by crustal barriers such as continental margins and graben structures has long been recognized as providing a very useful tool for mapping large scale lateral crustal variations along the propagation path numerical simulation of lg wave propagation in complex anelastic media using the pseudospectral method provides insight into the nature of the propagation process using both snapshots of the wavefield and synthetic seismograms a variety of 2 d structures have been investigated including the influence of sediments crustal thickness and attenuation thick sedimentary basins covering a graben structure can have a major influence since they remove lg energy by generating p conversion and scattering the principal mechanisms for strong lg attenuation across a graben the reduction of the lg energy is reinforced by anelastic attenuation in the sediments as well as the influence of the gradually thinning crustal waveguide associated with an elevated moho the extinction of lg in a sequence of explosions fired across the central graben of the north sea can be simulated by numerical calculations for the structure derived from refraction experiments

the ml scale in southern california measurements 9 941 of peak amplitudes on wood anderson instruments or simulated wood anderson instruments in the southern california seismographic network for 972 earthquakes primarily located in southern california were studied with the aim of determining a new distance correction curve for use in determining the local magnitude m l events in the mammoth lakes area were found to give an unusual attenuation pattern and were excluded from the analysis as were readings from any one earthquake at distances beyond the first occurrence of amplitudes less than 0 3 mm the remaining 7 355 amplitudes from 814 earthquakes yielded the following equation for m l distance correction log a 0 log a 0 1 110log r 100 0 00189 r 100 3 0 where r is hypocentral distance in kilometers a new set of station corrections was also determined from the analysis the standard deviation of the m l residuals obtained by using this curve and the station corrections was 0 21 the data used to derive the equation came from earthquakes with hypocentral distances ranging from about 10 to 700 km and focal depths down to 20 km with most depths less than 10 km the log a 0 values from this equation are similar to the standard values listed in richter 1958 for 50 200 km the effect at close distances is consistent with that found in several other studies and is simply due to a difference in the observed 1 r geometrical spreading for body waves and the 1 r 2 spreading assumed by gutenberg and richter in the construction of the log a 0 table m l s computed from our curve and those reported in the caltech catalog show a systematic dependence on magnitude small earthquakes have larger magnitudes than in the catalog and large earthquakes have smaller magnitudes by as much as 0 6 units to a large extent these systematic differences are due to the nonuniform distribution of data in magnitude distance space small earthquakes are preferentially recorded at close distances relative to large earthquakes for large earthquakes however the difference in the two magnitudes is not solely due to the new correction for attenuation magnitudes computed using richter s log a 0 curve are also low relative to the catalog values the differences in that case may be due to subjective judgment on the part of those determining the catalog magnitudes the use of data other than the caltech wood anderson seismographs the use of different station corrections or the use of teleseismic magnitude determinations whatever their cause the departures at large magnitude may explain a 1 0 0 7 proportionality found by luco 1982 between m l s determined from real wood anderson records and those from records synthesized from strong motion instruments if it were not for the biases in reported magnitudes luco s finding would imply a magnitude dependent shape in the attenuation curves we studied residuals in three magnitude classes 2 0 m l 3 5 3 5 m l 5 5 and 5 5 m l 7 0 and found no support for such a magnitude dependence based on our results we propose that local magnitude scales be defined such that m l 3 correspond to 10 mm of motion on a wood anderson instrument at 17 km hypocentral distance rather than 1 mm of motion at 100 km this is consistent with the original definition of magnitude in southern california and will allow more meaningful comparison of earthquakes in regions having very different attenuation of waves within the first 100 km

results and implications of seismic performance studies for duncan dam screening level studies indicated large zones of the foundation soil beneath duncan dam could be triggered to liquefy under the design earthquake resulting in flow slides that could lead to breaching of the dam in these studies estimates of the key soil parameters were obtained from an indirect procedure using penetration tests and seed s chart which is based on field experience during past earthquakes together with empirical correction factors for high confining stress and static bias this is state of the practice procedure in north america and is used because of the difficulty in retrieving and testing undisturbed samples of sand with the high confining stresses present at the site the indirect approach gave very low values of the key liquefaction properties of the soil and the use of such low values in analyses indicated that major and costly remedial measures were required thus a more direct approach was undertaken this involved both direct and semidirect determination of the key soil parame

interpretation of in situ test results from the canlex sites one of the primary objectives of the canadian liquefaction experiment canlex project was to evaluate in situ testing techniques and existing interpretation methods as part of the overall goal to

propagation of lg waves in the north australian craton influence of crustal velocity gradients abstract aftershocks of three large earthquakes near tennant creek in the northern territory of australia provide a unique opportunity to study regional seismic phases in the north australian craton three portable digital seismographs were operated near the source zone to provide control on earthquake locations and seven recorders formed a line between tennant creek and the seismic array at alice springs which is 430 km to the south eleven earthquakes of sufficient size for analysis were recorded during a one week deployment two months following the main shocks the group velocity observed for lg waves along this profile of 3 7 km sec is at the high end of the normal range and appropriate for a stable continental interior we estimate attenuation in discrete frequency bands by measuring the decay of amplitude as a function of distance from the source and assuming the common r 5 6 relationship for geometrical spreading however this assumption leads to frequency dependent q values q 230 36 f 0 66 0 12 that are abnormally low for a stable continental region the apparent q is also not consistent with other observations such as the high lg group velocity the positive bias in m l estimates obtained for other parts of australia using standard attenuation relations the size of felt areas for large central australian earthquakes nor with the high frequency content of seismic reflection data collected near our profile some crustal models for northern australia derived from seismic refraction work show a gradient zone between 30 and 55 km rather than a sharp moho discontinuity we therefore investigate the effect of gradient zones at the crust mantle transition on the decay of lg amplitudes using synthetic seismograms calculated with the wavenumber integral method the modeling suggests that some of the s wave energy that would be trapped in a crustal wave guide with a sharp lower boundary can leak out in the presence of a gradient and so enhance the attenuation in addition there are focusing effects due to the formation of caustics from the gradient zones which cannot be represented by a single analytic amplitude correction

sondrestrom incoherent scatter radar observations during the lower thermosphere coupling study september 21 26 1987 the incoherent scatter radar located at soendre stroemfjord greenland obtained e and f region measurements during the first lower thermosphere coupling study ltcs 1 september 21 26 1987 lower thermospheric neutral winds deduced from these measurements show that the neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing during an interval which was relatively quiet geomagnetically september 23 24 a semidiurnal oscillation dominated the neutral motion the model equinox tidal amplitudes and phases of forbes 1982 for the diurnal tide are roughly in agreement with the observed diurnal oscillation for the first four days of the experiment vertical variations in the observed diurnal phases are consistent with the results of forbes and hagan 1988 and may provide evidence of dissipation of the propagating 1 1 tidal mode the semidiurnal motion observed during this period is not well represented by the recent theoretical results for the amplitude and phase of the semidiurnal tide forbes and vial this issue neutral winds obtained during a geomagnetically active interval september 25 26 displayed a flow pattern that was significantly distorted from that observed during the preceding relatively quiet interval although the changes in the zonal winds throughout this active interval were consistent with the directionmore of the ion drag force at 115 km and above the variations in the meridional winds suggest that other forces such as pressure gradients driven by joule heating need to be considered to explain the observations less

lg wave modelling for the north sea summary theoretical seismograms for inhomogencous 2 d media are computed via summation of surface wave modes and use of transmission coefficient matrices in an attempt to explain the observed lg wave barrier within thc continental structure of the north sea where other scientists have been able to trcat lg wave propagation in the north sea for one frequency many surface wave modes and a complicated structural model of the north sea grabens our method takes into account many frequencies and is consequentiy limited to a few significant modes and a relatively simple structural model

highland boundary fault zone tectonic implications of the aberfoyle earthquake sequence of 2003 the highland boundary fault zone hbfz is one of the major faulted tectonic boundaries in great britain historically seismicity has occurred in this zone around the town of comrie but an earthquake sequence that occurred in 2003 near the village of aberfoyle ml 1 3 3 2 was the first significant activity to be recorded in the hbfz since the installation of modern seismograph networks in the 1970s this study describes detailed analysis of these data the waveform signals of the events were almost identical and by applying a cross correlation technique combined with multiple event location the alignment of the events was found to be wsw ene this alignment matches one of the nodal planes determined by joint focal mechanism analysis the fault plane dips to the northwest and shows oblique sinistral strike slip with normal movement the orientation of the event alignment matches the direction and orientation of observed features in the hbfz hence it is concluded that the wsw ene striking nodal plane was the causative fault that is associated with the hbfz the orientation of maximum compressional stress is rotated from the regional average expected due to the mid atlantic ridge push force this rotation is possibly explained by stresses due to postglacial rebound smaller events in the sequence were used as empirical green s functions and deconvolved from the larger events to determine source time functions the corresponding corner frequencies matched results from spectral fitting showing that the events were of relatively low stress drop

frequency dependent lg wave attenuation in northern morocco abstract frequency dependent attenuation q 1 in the crust of northern morocco is estimated from lg wave spectral amplitude measurements every quarter octave in the frequency band 0 8 to 8 hz this study takes advantage of the improved broadband data coverage in the region provided by the deployment of the iberarray seismic network earthquake data consist of 71 crustal events with magnitudes 4 m b 5 5 recorded on 110 permanent and temporary seismic stations between january 2008 and december 2013 with hypocentral distances between 100 and 900 km 1274 high quality lg waveforms provide dense path coverage of northern morocco crossing a region with a complex structure and heterogeneous tectonic setting as a result of continuous interactions between the african and eurasian plates we use two different methods the coda normalization cn analysis that allows removal of the source and site effects from the lg spectra and the spectral amplitude decay sad method that simultaneously inverts for source site and path attenuation terms the cn and sad methods return similar results indicating that the lg q models are robust to differences in the methodologies larger errors and no significant frequency dependence are observed for frequencies lower than 1 5 hz for distances up to 400 km and the frequency band 1 5 hz 4 5 the model functions q f 529 22 23 f 1 5 0 23 0 06 and q f 457 7 7 f 1 5 0 44 0 02 are obtained using the cn and sad methods respectively a change in the frequency dependence is observed above 4 5 hz for both methods which may be related to the influence of the sn energy on the lg window the frequency dependent q 1 estimates represent an average attenuation beneath a broad region including the rif and tell mountains the moroccan and algerian mesetas the atlas mountains and the sahara platform structural domains and correlate well with areas of moderate seismicity where intermediate q values have been obtained

characteristics of semidiurnal tides observed by the kyoto meteor radar and saskatoon medium frequency radar we have compared seasonal variations of semidiurnal tides in the upper middle atmosphere observed by the kyoto meteor radar and the saskatoon medium frequency partial reflection radar in 1983 1985 amplitudes of semidiurnal tides are generally larger at saskatoon than at kyoto seasonal variations of phases at 90 km agree fairly well for both zonal and meridional components between kyoto and saskatoon the seasonal variation is mainly composed of two fundamental states appearing in summer centered april may september october and winter centered months november february march which are characterized by long and relatively short vertical wavelengths respectively clear transitions between the two states occur in equinoxes february april and october november using determinations at kyoto and saskatoon with 4 to 5 day observation intervals we have studied the behavior of semidiurnal tides during the transitions in october november in 1983 1985 and february april in 1985 precise comparisons have shown that the transitions are slightly delayed at saskatoon in both vernal and autumnal equinoxes abrupt changes in phases of semidiurnal tides coincide with reduction and pulsating fluctuations of their amplitudes suggesting the existence of interference between fundamental and higher modes which are dominant in summerlike and winterlike months respectively

amplitude tomography of lg waves in xinjiang and its adjacent regions amplitude tomography of lg waves at frequencies from 1 to 5 hz are investigated in this work to reveal the lateral attenuation heterogeneities of the study area the xinjiang uygur autonomous region of china and its adjacent regions by using a large set of broadband seismic waveform data first we inverted the average q lg at each of 10 frequencies from 0 5 to 5 hz then the study area was divided into evenly spaced 1 1 grid cells after conducting both a checkerboard test and a resolution test finally the high resolution q lg tomographic images at the 10 frequencies were obtained the moment magnitudes of the events and the site terms of the stations we used were obtained simultaneously the attenuation pattern of q lg at each frequency shows obvious lateral heterogeneities in xinjiang and its adjacent regions which are coincident with the geological features of the area the tarim basin and the middle and the eastern part of the southern tian shan belong to high q regions the northeastern margin of the pamir plateau the northwestern margin of the tibetan plateau the western part of the southern tian shan and northern tian shan are prominent low q areas the attenuation pattern of lg waves shows that large scale tectonic boundaries zones with rapid topographic or crustal thickness gradient are in good accordance with the boundaries of the high and low q lg most of the historical strong earthquakes occurred on the interior or edge of low q lg areas

thermospheric extensions of the classical expansion functions for semidiurnal tides solutions to the thermospheric tidal equations are described that define the extensions into the thermosphere of normalized wind and temperature structures associated with the 2 2 2 3 2 4 and 2 5 semidiurnal propagating tidal modes sample figures are presented that illustrate the degree of alteration of vertical structures with latitude and the change in horizontal shapes with height corresponding to semidiurnal oscillations in northerly westerly and vertical velocity and temperature in the thermosphere additional figures and tables covering the 80 to 400 km altitude region for five levels of solar activity at 6 latitude increments are available on microfiche for potential users the structures can be used to extend meteor wind 80 100 km and partial reflection drift 60 100 km measurements to heights above 100 km for consistency checks with tidal winds and temperatures from thomson scatter measurements at possibly different latitudes or to simultaneously fit data covering these height regions for modeling purposes supplementary figures and tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20002 document j82 003 1 00 payment must accompany order

mesosphere thermosphere tidal coupling during the september 21 25 1987 ltcs 1 campaign the thermosphere ge 90 km is dynamically coupled to the mesophere and lower atmospheric regions by gravity waves tides and planetary waves which propagate upward from below and often undergo dissipation in the 80 to 150 km region this paper focuses on the mesosphere thermosphere coupling due to the solar semidiurnal atmospheric tide during the first lower thermosphere coupling study ltcs campaign interval of september 21 25 1987 during this period determinations of semidiurnal eastward u and northward v velocity components and temperature delta t are available within the 80 to 150 km region from a total of 13 incoherent scatter meteor and spaced antenna radars a new method is developed and reported here for assimilating such data sets into a self consistent global empirical description of the tidal dynamics between 80 and 150 km using a set of numerically generated hough mode extension hme functions for u v and delta t corresponding to the 2 2 2 3 2 4 and 2 5 semidiurnal tidal modes a least squares fit to the radar data is performed the fit results in a single complex normalizing factor for each hme from which globally continuous and internally consistent semidiurnal oscillations in u v and delta t as well as vertical velocity and density canmore be derived the quality of fit to these data from various height regions including both northern and southern hemispheres establishes the internal consistency between these measurements as well as the applicability of the method to future data sets the resulting empirical model reproduces the data significantly better than recent theoretical models suggesting use of the method for deriving tidal boundary conditions for thermospheric general circulation models less

effect of low q sediments and crustal q on lg attenuation in the united states q values for lg and its coda exhibit significant lateral variations across the united states some of which correlate with known variations in the thickness of shallow sedimentary layers the possibility that all variations in lg attenuation are caused by changes in the thickness of shallow sediments is investigated by computing multi mode synthetic seismograms for crustal models with various thicknesses of low q sediments using crustal velocity and q models obtained in previous studies and reasonable values for the thicknesses and intrinsic q values of sediments in several regions we find that the variation of lg q values between the appalachian and rocky mountains can easily be produced by varying accumulations of sandstone and shale of mesozoic age and younger by contrast throughout most of the western united states neither the low lg q values which have been observed nor the regional variation of those values can be explained by accumulations of low q sediments instead their explanation appears to require low and laterally varying values of q in the crystalline crust

on the coupling between the lower and the upper thermosphere during the first lower thermosphere coupling study simultaneous observations of the neutral winds and temperatures and electron densities in the upper and lower thermosphere are presented the measurements were made at arecibo and millstone hill during the first lower thermosphere coupling study ltcs 1 campaign in september 1987 the observations show a great deal of day to day variability possibly due to the geomagnetic activity which occurred during the period storm effects are discernible in the data at low altitudes near 120 km even at low latitudes near 18 n theoretical simulations of the period were attempted by the national center for atmospheric research thermosphere ionosphere general circulation model a novel development was the use in the model of lower boundary conditions representative of the upward propagating semidiurnal tides based on the ltcs 1 mesosphere lower thermosphere observations the upward propagating diurnal 1 1 mode was also included in the model simulations of the low and mid latitudes did not substantially improve with the use of lower boundary conditions derived from the measurements the model was also unable to reproduce the observations in both the lower and upper thermosphere this may be partially due to the effects of geomagnetic activity on the atmospheric fields since the observational period included two intervals of geomagnetic disturbances in the lower thermosphere resolution of the discrepancies between the data and model predictions may require the extension of the model to lower altitudes at present the model predictions in the lower thermosphere are strongly dependent on the lower boundary conditions the model predicts that diurnal waves are not negligible in analysis of lower thermosphere data indeed near 100 km at the model location of arecibo the diurnal wave amplitudes of the meridional winds were predicted to be greater than the semidiurnal investigation of the model predictions awaits the development of techniques to obtain observations of at least 24 hours duration in the lower thermosphere and the extension of the theoretical models to lower altitudes

nonmigrating semidiurnal tide over the arctic determined from timed doppler interferometer wind observations received 25 june 2009 revised 30 october 2009 accepted 5 november 2009 published 30 march 2010 1 the timed doppler interferometer tidi on the nasa thermosphere ionosphere mesosphere energetics and dynamics timed satellite has been measuring horizontal winds in the mesosphere and lower thermosphere mlt since 2002 because of the high inclination of the timed orbit tidi measures the horizontal winds from pole to pole every orbit this paper presents the first assessment of the spatial structure and temporal evolution of the nonmigrating semidiurnal tides over the arctic determined from the tidi wind measurements and a comparison of the structure of the nonmigrating semidiurnal tide between the arctic and antarctic the nonmigrating semidiurnal tides were determined as a 60 day average based on the yaw cycles of the spacecraft the nonmigrating semidiurnal tidal wind field over the arctic comprises mainly the westward propagating zonal wave numbers1 w1 and3 w3 andstandingzonalwavenumber0 s0 modes thew1modeis the most prominent maximizing above 90 km poleward of 60 n during the yaw interval ranging from mid march to mid may while this mode exhibits a slight amplitude increase toward the north pole during this interval its phase is nearly constant with latitude the s0 mode is enhanced over two yaw intervals ranging from mid january to mid may but its amplitude decreases toward the north pole compared to the w1 semidiurnal tide over the antarctic that over the arctic is smaller in amplitude of less extended duration achieves maximum amplitudes at higher altitudes by 10 km and exhibits a weaker amplitude increase toward the pole these differences likely result from differences in excitation mechanisms and efficiency and or in propagation conditions in the two responses for the nonmigrating semidiurnal tides between the arctic and antarctic

climatology of semidiurnal lunar and solar tides at middle and high latitudes interhemispheric comparison the semidiurnal lunar and solar tides obtained from meteor radar measurements spanning from 2009 until 2013 observed at davis 69 s and rio grande 54 s are presented and compared to the northern hemisphere ones at andenes 69 n and juliusruh 54 n mean tidal differences for both intra and inter hemispheric scenarios are analyzed tidal behavior is also compared against numerical simulations during 2009 and 2013 sudden stratospheric warming ssw time periods possible influences in the southern hemisphere from the local stratosphere are also investigated using modern era retrospective analysis for research and applications version 2 merra 2 datasets the main features of the mean zonal wind are similar in both hemispheres i e stronger amplitudes over mid latitude locations eastward winds during winter and westward below 90 km with eastwards higher up during corresponding summer times on the other hand the semidiurnal solar tides observed in the southern hemisphere show clear differences when compared to the northern hemisphere and between mid and high latitude locations at the same hemisphere these differences are even larger for the semidiurnal lunar tide which shows stronger amplitudes from october to march and march to october over davis and rio grande respectively our results indicate that the lunar tides over the southern hemisphere mid latitudes are more prone to react to the northern hemisphere stratospheric polar vortex influences in agreement with numerical simulations particularly for the time of the 2013 ssw

coordinated incoherent scatter radar observations of the semidiurnal tide in the lower thermosphere the incoherent scatter radars at arecibo 18 3 degree n millstone hill 42 6 degree n and sondrestrom 67 degree n simultaneously observed the thermal structure and dynamics of the earth s lower thermosphere during the period september 21 25 1987 as part of the lower thermosphere coupling study plasma spectral measurements were used to derive ion temperatures and horizontal neutral wind velocities in the altitude region 90 140 km and semidiurnal harmonics were determined from the data sets the daily mean temperatures were found to be within 40 degree k at the three stations and the largest semidiurnal temperature amplitudes exceeding 70 degree k above 115 km were recorded at arecibo mean winds were about plus minus 30 m s at most altitudes shifting from eastward to westward directions at 115 km and the semidiurnal amplitudes were in the range 20 70 m s phase variations with height reveal reasonable consistency among the three stations within 3 4 hours and indicate an average vertical wavelength of 40 70 km however a strong variation is seen in the vertical structure in the 110 to 130 km region suggesting the presence of different tidal modes comparison with the most current tidal propagation model by forbes and vial this issue indicates that the model tends to underestimate the amplitudes for this time period particularlymore above 110 km by virtue of the latitudinal distribution of the three radars their coordinated observations of the lower thermosphere serve to constrain numerical models and improve their understanding of this important yet complex region of the atmosphere less

satellite observations of mean winds and tides in the lower thermosphere 2 wind imaging interferometer monthly winds for 1992 and 1993 1 satellite observations of monthly i e 60 d averages centered on the 15th of each month zonal mean and migrating tide winds in the region of 90 120 km and 40 s 40 n from the wind imaging interferometer are presented the zonal mean zonal winds are characterized by annually varying eastward winds at midlatitudes with a maximum in summer and semiannually varying westward winds in the tropics with maxima at equinox below 105 km the zonal mean meridional winds are characterized by a summer to winter flow below 100 km at solstice and by a cell like structure at equinox the latter is the first such global observation of this tidally driven structure two distinct types of diurnal tide are observed an upward propagating one and an evanescent one of comparable magnitude the propagating tide which is dominant in the subtropics below about 105 km exhibits a semiannual variation with a primary maximum in march april and a secondary maximum in september october the evanescent tide which is dominant poleward of 20 n s and above 100 km has an annual variation with a maximum in the summer months the coupling of the two produces a double peaked structure in the vertical profile of the diurnal amplitude the semidiurnal tide is generally weaker than the diurnal tide the maximum semidiurnal winds are at 30 40 n s and at 105 110 km both zonal and meridional components are stronger in april september than in october march in both hemispheres but in the equatorial region in april september the meridional component has a maximum whereas the zonal component has a minimum

solar tides as revealed by measurements of mesosphere temperature by the mls experiment on uars abstract temperatures between 25 and 86 km measured by the microwave limb sounder mls experiment on the upper atmosphere research satellite uars are analyzed to delineate diurnal semidiurnal and terdiurnal tidal structures and stationary planetary waves these fourier components are determined from temperatures averaged in bins covering 5 latitude 30 longitude and 1 h in local time this study confirms the presence of diurnal nonmigrating tides with zonal wavenumbers s 0 2 3 s 0 s 0 implying westward eastward propagation and semidiurnal tides with s 1 and 3 and some components of lesser importance that were previously determined from uars wind measurements near 95 km the seasonal latitudinal and height structures of these components are now revealed and utilized to aid in interpreting their behaviors and ascertaining their origins new discoveries include the terdiurnal s 2 and s 4 components and trapped nonmigrating diurnal tides with s 0 and s 2 the former are like

coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during january 20 30 1993 a multi instrument global campaign involving incoherent scatter medium frequency and meteor wind radars was conducted during the period of january 20 30 1993 to study the dynamics of the mesosphere and lower thermosphere data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 10 day campaign spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48 hour wave near 90 km at latitudes between 40 n and 40 s that is present up to 108 km at 18 n the semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km while the diurnal tide is observed to be particularly important in the upper mesosphere near 40 latitude a least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48 hour waves comparison with national center for atmospheric research thermosphere ionosphere mesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes

in situ ground freezing to obtain undisturbed samples of loose sand in situ ground freezing with liquid nitrogen can be an excellent technique for obtaining high quality undisturbed samples from a wide range of loose sand deposits with their in situ conditions preserved this paper describes the techniques used during the canlex project to carry out in situ ground freezing and undisturbed sampling of the loose sand deposits encountered at six test sites specific details with respect to the effects of difficult subsoil or ground water conditions encountered at the sites are provided modifications made to the ground freezing and sampling system to accommodate difficult ground conditions are described the actual advance of the freezing front and the consumption of liquid nitrogen are compared with theoretical predictions the dry coring technique utilized to recover undisturbed frozen sand samples is described the void ratios measured for the frozen core samples are compared with void ratio measurements obtained using geophysical logging in boreholes located within a 5 m radius of the freeze pipe the costs associated with in situ ground freezing utilizing liquid nitrogen and coring of the frozen sand at six test sites are presented

lunar tidal winds in the upper atmosphere over jakarta 1 winds have been measured at heights from 70 to 120 km by means of a meteor wind radar near jakarta 6 4s 106 7e from 1993 to 1997 these data have been analysed for the lunar semidiurnal tide using a least squares fitting method generally lunar tidal amplitudes in the winds are about 2 m s 1 though the northward wind amplitude reaches 6 m s 1 in southern summer variations with height and season are shown and compared with the global scale wave model gswm often the agreement with the model is good but there is a difference in phase of the eastward wind in southern winter the gswm predicts a more rapid increase in northward wind magnitude with height than is observed year to year changes are examined for different seasons and these reveal sometimes consistent results and sometimes a variation in phase from year to year often the phase relations between eastward and northward winds at jakarta are more characteristic of a northern hemisphere tide as predicted by classical tidal theory particularly in southern spring and summer

sedimentary basins and the blockage of lg wave propagation in the continents the phenomenon of lg blockage where lg is strongly attenuated by crustal heterogeneities poses a serious problem to ctbt monitoring because lg is an important seismic phase for discrimination this paper examines blockage in three continental regions where the lg blockages may be caused by large enclosed sedimentary basins along the propagation path the barents sea basin blocks lg propagation across the barents sea from the russian nuclear test sites at novaya zemlya to scandinavian stations also early lg waves are observed in sn codas on norsar noress and arcess recordings of novaya zemlya explosions where direct lg is blocked early lg waves may have resulted from sn to lg mode conversion at the contact between the barents basin and the kola peninsula the northern and southern caspian sea basins also block lg waves from pnes and earthquakes perhaps due to thick low velocity low q sediments replacing the granitic layer rocks in the crust lg blockage has also been observed in the western mediterranean levantine basin due to low q sediments and crustal thinning a basin capture model is proposed to explain lg blockage in sedimentary basins in this model shear waves that reverberate in the crust and constitute the lg wave train are captured delayed and attenuated by thick low velocity sediments that replace the granitic layer rocks of the upper crust along part of the propagation path sn waves which propagate below the basin would not be blocked and in fact the blocked lg waves may be diverted downward into sn waves by the low velocity sediments in the basin

simultaneous mesosphere lower thermosphere and thermospheric f region observations during geomagnetic storms 1 simultaneous mesosphere lower thermosphere mlt and thermospheric f region observations made during geomagnetic storms are presented using the mu radar 35 n 136 e in alternate meteor and incoherent scatter is modes mf radar 45 n 136 e optical mesosphere thermosphere imagers omti 35 n 136 e and over 1000 gps receivers in japan the observations were made during one of the mtec s mesosphere thermosphere experiments for coupling studies campaigns conducted during 23 march to 2 april 2001 when a major storm with dst reaching 358 nt and a moderate storm with dst reaching 112 nt occurred and solar activity varied from high to very high f10 7 220 to 270 the campaign has provided zonal and meridional wind velocities at mlt altitudes 80 95 km over the mu and mf radars meridional wind velocity in the upper thermosphere 220 450 km and ionospheric electron density 150 600 km peak height and plasma drift velocity over the mu radar all sky airglow intensities at 557 7 nm 630 0 nm and 777 4 nm wavelengths over the mu radar location and vertical gps tec all over japan study of the data indicates that following the major storm the meridional wind in the upper thermosphere over the mu radar 35 n becomes weak less poleward during daytime and its diurnal amplitude reduces the winds and tides at mlt altitudes show changes during both nonstorm times and storm times but this study is not able to confirm any statistically significant changes related to the storms in the ionosphere following the onset of the major morning storm the electron density increases at altitudes near and above the ionospheric peak which after about 5 hours is followed by large increase in density at all altitudes above about 200 km on the following night though the density is generally low there is a large increase in density in the bottomside for about 3 hours centered at 0230 lt during this time a rare intense optical activity has occurred at all wavelengths 557 7 nm 630 nm and 777 4 nm over the low midlatitude location 35 n and an associated large scale traveling ionospheric disturbance lstid has propagated from north to south at f region altitudes

comparison of observed diurnal and semidiurnal tropospheric winds at christmas island with tidal theory four years of hourly averaged atmospheric wind measurements from the christmas island vhf wind profiler were processed into typical days to show the vertical phase structure of the diurnal and semidiurnal tropospheric zonal winds the winds from the lowest resolvable height were compared with both the surface winds calculated from a three year record of surface pressure using the equations of motion on a rotating frictionless sphere and the winds from a tidal model it is concluded that at the lowest height the observed diurnal and semidiurnal zonal winds have tidal 1 1 and 2 2 mode structures respectively further analysis is necessary to evaluate the tidal mode s of the zonal wind throughout the troposphere

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

single station attenuation measurements of high frequency rayleigh waves in scotland summary amplitude spectra derived from fundamental mode rayleigh waves generated by small underground and underwater explosions in scotland are analysed using a technique for measuring attenuation for a single sourcestation path the procedure estimates the average qi profile as a function of depth along an isoleted propagation path by correcting for known effects and mapping the function representing the remaining phenomena onto the corresponding data two methods of inversion are employed both giving similar results the technique is applied to explosion data recorded on the lispb and lownet arrays in scotland with epicentral distances of up to 85 km this yields well resolved single station qj values of between 0 02 and 0 09 in the top 400 m of the crustal area traversed by the waves the values at greater depth are weakly constrained but the qj estimates imply a decrease to 0 01 between 400 m and 800 m depth followed by an increase to 0 04 below 800 m the underground explosion spectra are modelled most accurately by a source time action in the form of an exponential step with an inverse timeconstant of 4s being optimum the mean source strength value of 1012nm agrees with a theoretical prediction based on the explosive yield a model consisting of a shock wave and ensuing negative pressure of 0 8 s duration is optimal for the underwater explosion data the mean source strength value is 107nm in close agreement with the theoretical prediction using known pressure data in both cases attenuation coefficients calculated using the qj models estimated from this analysis are consistent with the coefficients previously obtained in scotland using surface waves for pure geological provinces

lunar tidal winds at adelaide and saskatoon at 80 to 100 km heights 1985 1990 winds data from the two sites were analyzed for lunar tides in 2 year groups the variation with height of the phase and amplitude of the tide showed considerable variability from year to year yet in general seasonal variations at particular heights were similar to those determined previously phases of the tide at the two sites were compared in order to estimate whether the tide was symmetric or antisymmetric for this purpose it was found necessary to use only 1 year data sets the predominance of symmetric tides during solstice months and antisymmetric tides at the equinox came as a surprise yet the result is in broad agreement with phase relations of the lunar geomagnetic tide

lg wave q tomography in central america summary the lateral variation of lg wave spectral attenuation in central america was studied through tomographic inversion at various frequencies between 0 5 and 5 hz separately for each frequency the input data set consisted of 558 travel paths recorded on short period and broadband seismic stations in the region the size and quality of the data set were sufficient to resolve significant lateral variation in the lg wave quality factor q lg the average dependence of q lg with frequency was found to be q lg f 182 f 0 84 corresponding to high attenuation low q lg values in the nicaraguan depression presented the most significant variation from the average the strong attenuation of lg waves in the nicaraguan depression was also observed in the visual analysis of the seismograms this observation is possibly explained by the nearsurface low velocity layers in the depression low q lg values were also found along the chain of volcanos due to increased scattering and partial melting beneath the volcanic belt this study is the first attempt to determine q lg for central america and to provide knowledge on attenuation of lg waves needed for the prediction of ground motion during future earthquakes

long term oscillations of the wind field in the tropical mesosphere and lower thermosphere from hawaii mf radar measurements 1 wind measurements in the mesosphere and lower thermosphere obtained with a medium frequency mf radar in hawaii 22 n 160 w spanning 16 years are employed to examine the intra annual and interannual variability of the mean and tidal motions at altitudes between 84 and 94 km intra annual periodicities range from 3 to 12 months with significant coherence in altitude and between the zonal and meridional components of each motion field interannual variations confirm the dominant periodicities identified previously at this site and elsewhere in particular the significant diurnal and less significant semidiurnal tidal responses at periods of 28 and 48 months amplitudes of these long period oscillations of the diurnal tide increase with altitude below 92 km and are larger than the amplitudes of the 12 month oscillations above 90 km phases of the 28 and 48 month oscillations show a downward progression with a slightly larger altitude variation in the meridional diurnal tide for the 28 month oscillation and a significantly larger altitude variation in the zonal diurnal tide for the 48 month oscillation the long and nearly continuous hawaii data set also enables characterization of the responses of the wind fields to the 11 year solar cycle both wind and tidal fields exhibit this periodicity but these responses display interesting and different relations to the phase of the solar cycle the 11 year oscillation of the meridional wind is nearly in phase with the solar cycle while the 11 year oscillation of the zonal wind is in approximate quadrature with the solar cycle the 11 year oscillations of the semidiurnal tidal amplitudes and the meridional diurnal amplitude are all in approximate quadrature with the solar cycle with the tidal amplitudes leading by 29 to 37 months while the 11 year oscillation of the zonal diurnal amplitude is somewhat nearer an antiphase than a quadrature relation to the solar cycle leading by 54 months

semidiurnal tidal climatology of the e region theoretical predictions of global monthly and seasonally averaged semidiurnal temperatures and winds between 80 and 150 km are presented and compared with seasonal means from measurements at the arecibo chatanika millstone hill and sondrestrom incoherent scatter radars the model values above 100 km are obtained by utilizing a set of tabulated hough mode extension functions to extrapolate the 2 2 2 3 2 4 and 2 5 propagating tidal modes at 100 km from a spectral model not valid in the e region due to the way molecular dissipation is parameterized to higher altitudes and by superimposing a component driven by in situ excitation important mainly above 140 km the monthly simulations demonstrate the close coupling that exists between seasonal and latitudinal structures and argue for an observational program emphasizing adequate seasonal and latitudinal coverage despite constraints imposed by the distributions of existing data with respect to month and year comparisons with the model reveal several trends that appear to be real 1 the model amplitudes are usually 20 50 smaller than those observed 2 vertical wavelengths are well predicted by the model but model phases generally lead the observations at middle and low latitudes by approximately 2 3 hours at all heights for both winds and temperatures 3 semidiurnal temperatures at middle and low latitudes in both the observations and model indicate significant seasonal changes in the phase of the temperature oscillation but not in the winds and 4 the seasonal trends in the model and observations are consistent with summer leading winter at millstone hill while the reverse is true at arecibo the sources of the absolute amplitude and phase discrepancies in the model which are probably connected with the adopted mean wind and thermal excitation specifications need to be investigated in addition a concerted effort must be made within the coupling energetics and dynamics of atmospheric regions cedar program to better delineate the seasonal latitudinal thermal structure and dynamics of the e region following the example of the extensive f region observational data base which has been accumulated over the last decade

the canadian liquefaction experiment an overview the canadian geotechnical engineering community has completed a major collaborative 5 year research project entitled the canadian liquefaction experiment canlex the main objective of the project was to study the phenomenon of soil liquefaction which can occur in saturated sandy soils and is characterized by a large loss of strength or stiffness resulting in substantial deformations in many areas of canada large structures are constructed on or comprise sandy soils e g some major hydroelectricity earth dams and many tailings impoundments in the mining industry the behaviour of loose sandy soils can be difficult to predict but can have a significant financial impact on these types of engineering structures consequently the intent of the canlex project was to improve the overall understanding of soil liquefaction this paper provides an overview of the canlex project outlining the project objectives major achievements and conclusions four companion papers describe different aspects of the pr

atmospheric tide 2 the solar and lunar semidiurnal components solar and lunar semidiurnal wind and temperature oscillations from the surface to 400 km are simulated for average solar activity conditions utilizing the numerical tidal model described in part 1 hough mode decomposition of the solstitial solar semidiurnal tide excited by h2o and o3 insolation absorption below 80 km indicates that it is characterized by a predominance of 2 2 below 60 km about equal contributions of 2 2 2 3 and 2 4 between 70 and 90 km mostly 2 4 with some 2 5 and 2 2 between 90 and 120 km and a predominance of 2 2 above 140 km with secondary contributions from 2 3 and 2 5 the solstitial lunar semidiurnal tide is characterized by a predominance of 2 2 below 70 km 2 4 with secondary contributions from 2 2 2 3 and 2 5 between 80 and 110 km and by 2 2 with a fairly strong secondary contribution from 2 3 above 120 km similar descriptions for both the solar and lunar oscillations apply at equinox except that the asymmetric 2 3 and 2 5 modes are absent relative to the solar component lunar temperature and wind amplitudes are sufficiently large 20 in the lower and upper thermosphere to account for some of the observed variability previously attributed to the solar tide amplitudes of the 2 4 and 2 5 modes above 100 km are significantly affected by mesospheric eddy diffusion coefficients in excess of 2 106 cm s 1 the total solar semidiurnal exospheric temperature oscillation for average solar conditions ranges from 10 20 k poleward of 40 n to 40 60 k equatorward of 30 n and has its origin in three sources of excitation all of comparable importance 1 euv solar radiation absorption in the lower thermosphere 100 200 km 2 in situ momentum coupling due to the interaction of diurnal winds and diurnally varying ion drag and 3 tides propagating upwards from below 100 km in addition the calculations are compared with rocket meteor radar partial reflection thomson scatter and satellite data to evaluate the consistency of the model with observations

tidal oscillations of the arctic upper mesosphere and lower thermosphere in winter the upper mesosphere and lower thermosphere of the northern polar cap was investigated using fabry perot interferometer measurements of the horizontal wind velocity in the lower thermosphere meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere and michelson interferometer records of the infrared meinel oh bands brightness and rotational temperature in the upper mesosphere tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new moon intervals november 9 22 1993 december 7 20 1993 and january 4 17 1994 in eureka 80 n tidal harmonics were uncovered with an iterated least chi squared fit and their existence in the data tested using a goodness of fit probability against a null hypothesis of no oscillations no single dominant tide was found at all times and altitudes of the winter arctic upper mesosphere and lower thermosphere the horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide with the largest amplitude oscillations shown by the diurnal component of the meridional wave speed the atmospheric layer near the mesopause that contributes to the oh airglow emission experienced terdiurnal oscillations throughout the winter season theoretical polarization relations for evanescent tides considerations of energy density propagation with altitude and a day by day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on november was an evanescent zonally symmetric tide other data indicate propagating tides migrating tides or a mixture of both

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

lg wave attenuation in britain summary the lg wave quality factor qlg in britain has been modelled using data from the uk seismic network operated by the british geological survey the data set consists of 631 vertical mostly short period recordings of lg waves from 64 earthquakes 2 7 4 7 ml and 93 stations we have inverted for both regional average qlg and tomographic images of qlg and simultaneously a source term for each event and a site term for each station for 22 frequencies in the band 0 9 10 0 hz the regional average model is 266 f 0 53 between 1 0 and 10 0 hz and indicates that attenuation in britain is slightly higher than in france and significantly higher than in eastern north america and scandinavia tomographic inversions at each frequency indicate that qlg varies spatially broadly speaking southeastern england the lake district and parts of the east irish sea basin and a small region between the highland boundary fault and the southern uplands fault are characterized by higher than average attenuation southwestern england eastern central england and northwestern scotland are regions of relatively low attenuation to some extent these regions correlate with what is known about the tectonics and structure of the crust in the uk

moment magnitude determination for local and regional earthquakes based on source spectra we investigated the use of an automated routine to determine moment magnitudes from the displacement spectra of local and regional earthquakes two algorithms a genetic algorithm and a converging grid search were developed and tested with earthquake data from mexico norway and deception island antarctica it was found that compared with manual analysis the algorithms give reliable automatic moment magnitude m w estimates in the range 1 m 8 the converging grid search appeared to be more cost effective than the genetic algorithm m w at local and regional distances seems superior to amplitude based magnitudes that saturate for large earthquakes the application of the automated algorithm in near real time may help to obtain a nonsaturated magnitude estimate in the case of a large earthquake immediately after the earthquake has occurred also the method can be useful for processing large amounts of data manuscript received 27 july 2001

a local magnitude scale ml for the united kingdom we have developed a new local magnitude scale ml for the united kingdom uk to replace the hutton and boore 1987 scale developed for southern california which has been used in the uk until now the new uk scale is developed from 1482 observations of 85 earthquakes on 50 stations located across the british isles and ireland most of the observations are from epicentral distances of less than 600 km and only few from greater distances up to 900 km the distance range of the scale is therefore 0 600 km the amplitude observations were used to invert for the parameters defining distance dependence in the ml scale and station corrections synthetic tests showed that the inversion was robust the new ml scale for the uk is given by ml loga 0 95logr 0 00183r 1 76 in which a is horizontal component ground displacement amplitude in nanometers the amplitudes are measured on traces that are filtered to simulate the wood anderson seismograph r is the hypocentral distance in km the uk scale is intermediate between scales determined for california and those of other intraplate areas such as norway or the northeastern united states the absolute station corrections found are all less than 0 5 the scale derived for the uk helps to reduce the overall variance of the mean magnitude estimates by 30 much of this improvement is due to the use of station corrections applying the uk scale to the database of recorded earthquakes results in a reduction of magnitude for earthquakes above ml 2 and a slight increase in magnitude for earthquakes below ml 2 the biggest change to the ml computation is likely to be for small earthquakes with few amplitude readings where the use of station corrections makes a significant difference

an improved uk local magnitude scale from analysis of shear and lg wave amplitudes the amplitudes of shear and lg waves recorded at uk seismograph stations from local earthquakes in the range 0 600 km have been subjected to an analysis of variance allowing separation of the effects of earthquake size distance and station corrections for each recording site the propagation paths sample mainly the central to western part of the uk with good north south coverage the analysis of 385 amplitude readings at 28 stations from 40 earthquakes showed that the effects of both distance and station site corrections were statistically significant tables of corrections for both distance and station effects have been derived to allow local magnitude ml to be determined more accurately from horizontal and vertical component records one set of tables allows the estimation of an ml which is consistent with the original richter definition of ml with a standard deviation which is smaller than that produced by the theoretical attenuation curve defined according to attenuation in southern california which has customarily been used to calculate ml in the uk the reduction in standard deviation is mainly due to the incorporation of station terms to correct for station effects since the theoretical and observed variations of attenuation with distance are similar this similarity implies that southern california and the uk show a similar variation of distance dependent attenuation for lg waves which was not expected in view of the differences in geology and tectonics another set of tables is provided to calculate an ml which is consistent with the body wave magnitude mb determined by the international data centre from station records of the comprehensive test ban treaty organization s global monitoring network the decay with distance of the predominantly lg wave amplitude values gives a value of 440 50 for lg q at a frequency of 3 hz in agreement with estimates obtained from spectral displacement amplitudes of lg waves in the uk

post liquefaction stability and deformation analysis of duncan dam duncan dam is a 39 m high zoned embankment and is founded on a deep deposit of unconsolidated sediments consisting of sands silts and gravels that are susceptible to liquefaction during earthquake loading a comprehensive program of field laboratory and analytical investigations was conducted to evaluate the liquefaction potential of the foundation soils and performance of the dam during an earthquake this paper describes the evaluation procedures and analyses and presents results of the post liquefaction stability and deformation analysis of the dam analyses were carried out based on the total stress approach using two methods namely the lab method and seed s method the lab method is a site specific direct laboratory based approach utilizing soil parameters from laboratory tests on undisturbed soil samples obtained in situ after freezing the ground seed s method is indirect and is based on field penetration data and past experience from earthquakes the lab method indicates that the residual

geophysical images of the deep crust the iapetus suture the iapetus suture arguably the most fundamental lineament of british and irish structure has been previously identified on the birps winch deep seismic reflection profiles offshore as a nw dipping feature new depth migrated interpretations of these and other reflection data show good correlation of structure for 100 km along strike in the northern irish sea towards magnetotelluric stations onshore in the north of england and southern uplands of scotland three different methods of inverting data from these sites all reveal a thin nw dipping slab of high conductivity a contour map of the suture trace dipping at 15 25 nw can be constructed down to and through the moho at 28 km depth this map suggests crustal stretching below the solway basin the mylonitic suture zone is characterized by good seismic reflection layering high conductivity presumably fluid produced and a steep increase of p wave velocity over the few kilometres of its width crustal refraction experiments lispb and cssp provide useful velocities but locally misleading structure so that the combination of the seismic reflection and geoelectric methods may be the most powerful means of determining deep crustal structure with a good degree of both vertical and lateral resolution

lg coda q in australia and its relation to crustal structure and evolution we have determined lg coda q lg c from ground motion recorded at seven broadband stations in australia using a stacked spectral ratio method in spite of the relatively small number of events and less than optimum station coverage we were able to use those data to obtain a tomographic map q lg c and its frequency dependence at 1 hz for almost the entire island continent

liquefaction analysis of duncan dam using strength ratios b c hydro performed extensive seismic safety studies for duncan dam in the late 1980s and early 1990s the results of these studies indicated that duncan dam was reasonably safe from liquefaction induced failure a recently proposed liquefaction analysis procedure that uses strength ratios to assess liquefaction triggering and post triggering stability was used to revisit the liquefaction analysis of duncan dam when compared to the detailed field and analytical studies performed by b c hydro the proposed liquefaction analysis procedure predicts i similar zones of liquefaction within a contractive unit of foundation sand ii similar shear strengths at the triggering of liquefaction yield shear strength and at large strain liquefied shear strength and iii similar post triggering factors of safety for slope stability the good agreement suggests that the strength ratio based liquefaction analysis approach is relatively robust and is suitable for screening level studies for large projects or m

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

radar wind profiler observations of solar semidiurnal atmospheric tides semidiurnal solar tides in the mid latitude tropo sphere are investigated using harmonic analysis of 404 mhz radar profiler wind data obtained from a wide longitude zone in the u s the tides are apparent above a 1000 m deep surface layer and increase in amplitude with height attaining speeds of 0 5 0 7 rn s at 5 7 km observed wind characteristics agree well with tidal characteristics obtained with a dynamical model driven by observed global semidiurnal horizontal pressure gradients

frequency dependent attenuation in the crust beneath central france from lg waves data analysis and numerical modeling abstract the quality factor of shear waves in the crust beneath central france is evaluated by the combined use of data analysis and theoretical modeling the measurement is made from lg wave records of 18 earthquakes at 22 short period vertical seismic stations of the french seismic network the geometrical attenuation of lg waves has been calculated through a numerical simulation in a perfectly elastic model the station responses including site effects have been evaluated from lg waves and from records of the seismic noise q has been calculated for a set of 11 frequencies between 0 5 and 10 hz and has been found to be in the form q 290 f 0 52 where f is the frequency the numerical simulation leads to the interpretation of q of lg waves as the mean value of q within the crust theoretical and experimental decay of energy with distance are in very good agreement the lg waves present a different sensitivity to a low q zone whether this zone is associated with the sediments or with scattering in the lower crust the lower crust beneath central france appears from our experimental results to be associated with high values of q

evaluation of liquefaction potential of foundation soils at duncan dam a comprehensive program of field laboratory and analytical investigations was carried out to evaluate the potential of liquefaction for the foundation soils at duncan dam duncan dam was completed in 1967 under the columbia river treaty in southeastern british columbia the 39 m high zoned embankment dam is founded on a sequence of sands silts and gravels some of the foundation soils may liquefy during earthquake loading and this would affect the stability and performance of the dam the liquefaction studies were carried out in two phases to characterize the engineering properties of the foundation soils and to assess its potential for triggering liquefaction using the total stress approach this paper describes methods of assessment of liquefaction potential using soil parameters based on field penetration data seed s method and laboratory testing of undisturbed soil samples obtained in situ after freezing the ground lab method and presents the results of triggering analysis influence of confin

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust

variability of crustal attenuation in the northeastern united states from lg waves high quality digital seismograms from eight pairs of collocated earthquakes in the northeastern united states were analyzed to determine accurate source spectrum corner frequencies this was accomplished by applying the empirical green s function method to regional pg and lg or sg phases recorded by vertical component seismographs of the u s national seismographic network usnsn and the lamont doherty cooperative seismographic network lcsn stations the frequency band used was 0 5 16 hz for usnsn and 1 30 hz for lcsn records the source spectrum corner frequencies for the eight larger earthquakes of the event pairs magnitudes between mb lg 2 5 4 1 range from about 4 3 to 16 3 hz based on the comer frequencies obtained independently from the empirical green s function analysis sg or lg wave displacement amplitude spectra up to 30 hz were used to determine the crustal average q factors along 87 event station paths these paths crossed diverse tectonic features in the northeastern united states and were in the epicentral distance range of 41 to 1394 km we found that within the northeastern united states the crustal average qlg we obtained was frequency dependent and showed spatial variability which correlated fairly well with the major tectonic features in the region our attenuation measurements indicated low lg attenuation in the adirondack mountains with exposed precambrian grenville basement with qlg 905 f0 40 high lg attenuation in the central appalachian province with qlg 561 586 f0 46 0 47 and an intermediate lg attenuation in northern new england appalachians with q 705 f0 41

simultaneous lidar measurements of the sodium layer at the air force geophysics laboratory and the university of illinois simultaneous lidar measurements of the mesospheric sodium layer were made at the air force geophysics laboratory afgl and university of illinois at urbana champaign uiuc on the nights of august 28 october 9 and october 16 1985 on these nights the layer centroid height was 1 to 3 km lower at afgl in october the altitude of the layer bottomside was near 80 km at afgl compared to 85 km at uiuc gravity wave activity was present in the layer at both sites on all 3 nights in october the wave amplitudes were significantly larger at afgl the sodium profiles were compared with similar measurements obtained at the goddard space flight center in october 1981 and at the white sands missile range in october 1984 the data from all four sites indicate that there is a significant longitudinal variation in the layer structure between 70 w and 90 w it is suggested that these structural differences may be due to dynamics the most promising mechanism capable of producing the large 1 to 3 km differences in the layer centroid heights between uiuc and afgl is a small difference in the mean vertical wind velocity because the typical residence time for a sodium atom in the layer is approximately 5 8 days a 0 4 cm s 1 difference in the mean vertical wind velocities at the two sites is capable of inducing a 2 km difference in the centroid height of the sodium layer

latitude and seasonal dependence of the semidiurnal tide observed by the high resolution doppler imager the high resolution doppler imager hrdi on the upper atmosphere research satellite uars has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere mlt since november 1991 the observed dynamical patterns are dominated by the 1 1 diurnal tide but semidiurnal perturbations are also very prominent in the hrdi data particularly at latitudes greater than 40 this paper presents a preliminary study of the semidiurnal tide and discusses latitudinal and seasonal variations in the phenomenon regular seasonal changes are clearly detected and significant interhemispheric asymmetries have been identified in general the observed features are consistent with previous measurements and model results but some discrepancies have been found this new technique which brings a global perspective to the observation of mlt dynamics is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models

recognition of tectonic events in undeformed regions contrasting results from the midland platform and east midlands shelf central england apatite fission track and vitrinite reflectance data from central england demonstrate how these techniques can reveal otherwise unrecognized tectonic and or palaeothermal episodes in apparently tectonically stable areas the results document the transition from an inverted basinal region in the north east midlands shelf to a tectonically stable platform in the south midland platform afta data from the region reveal two discrete cooling episodes in the early and late tertiary maximum palaeotemperatures from afta and vr data in outcrop samples define a consistent increase from 50 c in lower cretaceous and upper jurassic units in the se to around 80 90 c in triassic and older units in the nw these early tertiary palaeotemperatures reflect a combination of deeper burial and elevated basal heat flow results from the rufford 1 well define an early tertiary palaeogeothermal gradient of 40 5 c km 1 32 50 c km 1 at 95 confidence limits corresponding to deeper burial by 1450 m of additional section 1 1 2 2 km at 95 confidence limits subsequently removed by tertiary erosion in contrast geological considerations suggest a maximum overburden of 800 900 m above the base of the lower jurassic in the vicinity of rugby where palaeotemperatures at outcrop are similar to those near the rufford 1 location the discrepancy between stratigraphic and palaeo thermal reconstruction of former burial depths often noted in earlier studies remains unresolved the late tertiary episode is much less well constrained but results from rufford 1 may require between 910 and 1650 m of eroded section thus much of the total amount of removed overburden may have been removed during the late tertiary results from the apley barn borehole oxfordshire reveal a late tertiary palaeothermal episode characterized by a highly non linear palaeotemperature profile which probably reflects local heating due to passage of hot fluids despite stratigraphic evidence for some early tertiary erosion results from this borehole show no evidence of early tertiary effects major early and late tertiary exhumation was limited to regions underlain by older palaeozoic basins while regions overlying palaeozoic basement were more stable experiencing significantly less exhumation we suggest this reflects the preferential reactivation of the weaker basinal regions as a result of compressional events at plate margins our results emphasize the importance of incorporating results from both inverted and non inverted areas in understanding the causal mechanisms of uplift and inversion and highlight the importance of testing apparent stability using palaeo thermal methods

seasonal variations of semidiurnal tidal perturbations in mesopause region temperature and zonal and meridional winds above fort collins colorado 40 6 n 105 1 w 1 on the basis of colorado state university csu na lidar observations over full diurnal cycles from may 2002 to april 2006 a harmonic analysis was performed to extract semidiurnal perturbations in mesopause region temperature and zonal and meridional winds over fort collins colorado 40 6 n 105 1 w the observed monthly results are in good agreement with mf radar tidal climatology for urbana illinois and with predictions of the hamburg model of the neutral and ionized atmosphere hammonia sampled at the csu na lidar coordinates the observed semidiurnal tidal period perturbation within the mesopause region is found to be dominated by propagating modes in winter and equinoctial months with a combined vertical wavelength varying from 50 km to almost 90 km and by a mode with evanescent behavior and longer vertical wavelength 100 150 km in summer months most likely due to dominance of 2 2 and 2 3 tidal hough modes the observed semidiurnal tidal amplitude shows strong seasonal variation with a large amplitude during the winter months with a higher growth rate above 85 90 km and minimal amplitudes during the summer months maximum tidal amplitudes over 50 m s for wind and 12 k for temperature occur during fall equinox a detailed comparison with hammonia predictions shows excellent agreement in semidiurnal phases hammonia predicted semidiurnal amplitudes generally agree well with observations however hommonia underestimates temperature amplitudes in some of the nonsummer months as well as zonal wind and meridional wind amplitudes in april and september but overestimates them in february to reveal the effects of the atmospheric background on vertical propagation of tidal modes and their relative importance in the composite semidiurnal tide during different seasons we use the lidar observed monthly mean temperature and zonal wind from the same data set as well as hammonia output to calculate the vertical wave number seasonal variations of the major tidal modes of the migrating semidiurnal tide this leads to a qualitative understanding of the lidar observed and hammonia predicted seasonal variation of the semidiurnal tidal perturbation