test ban treaty verification with regional data a review this paper summarizes the use of regional 30 seismic data in a test ban context for detecting locating identifying and determining the yield of underground nuclear explosions in many areas of the world eastern north america africa eastern ussr lg is the largest amplitude wave recorded on standard seismograph systems and thus is the most appropriate phase for monitoring small magnitude events excellent location capability for near regional events has been demonstrated at the norwegian small aperture array noress using lg and p waves lg and other regional phases may contain information on source depth but such information has not been exploited to date fifteen classes of regional discriminants have been identified including 1 1 first motion 2 2 ms mb 3 3 p2 p1 ratio 4 4 excitation of short period sh waves 5 5 lg rg amplitude ratios 6 6 pn lg pg lg and pmax lg amplitude ratios 7 7 lg group velocity and energy ratios in lg 8 8 excitation of sn 9 9 third moment of frequency 10 10 generation of higher mode surface waves 11 11 peak amplitudes of love and rayleigh waves and long period surface wave energy density 12 12 prevailing period of long period love waves 13 13 spectral ratio long period s waves to rayleigh waves 14 14 spectral ratio long period love waves to rayleigh waves 15 15 frequency of the peak spectral amplitudes and spectral ratios in pn pg s and lg each of these proposed discriminants has with differing degrees of success separated some explosions from some earthquakes however most have been tested only on limited data usually from one geographic region and only one or two recording stations no systematic analyses have been done to determine the best individual discriminant or combination of them preliminary evaluation of the use of lg for yield determination suggests that regional waves hold promise in this application theoretical studies have contributed significantly to the understanding of propagation characteristics of regional waves but further studies are required emphasizing modeling for realistic anisotropic sources the major conclusion of this study is that a systematic and comparative evaluation of all the proposed regional discriminants is now required utilizing a common data base derived from all present day test sites this evaluation would suggest the optimal discrimination procedure using regional waves and would also define areas of needed research without such an integrated evaluation it is still possible to speculate using existing results on the most promising regional discriminants

a revised magnitude scale for south australian earthquakes a new local magnitude scale for south australian earthquakes has been developed from first principles the scale is defined by m l s a 0 7 log 10 a w a 1 10 log 10 0 0013 c i where awa is the equivalent wood anderson trace amplitude in millimeters is the epicentral distance in kilometers and ci is a station correction the formula accommodates the local attenuation function for south australia it is significantly less than richter s log a function but agrees closely with the recent results of boore and hutton 1984 for southern california the attenuation constants and the station corrections were determined by detailed statistical analysis of data from hundreds of earthquakes station corrections vary from 0 6 to 0 2 magnitude units and show a broad correlation with the regional geology south australian earthquake magnitudes are obtained from short period vertical component seismographs of dissimilar frequency response to that of the horizontal wood anderson instrument preliminary analysis of three component data from the longer period characteristic adelaide ade seismic station suggests that the departure from standard instrument type makes little difference to actual magnitude determinations

theoretical study of the excitation spectral characteristics and geometrical attenuation of regional seismic phases abstract we present a theoretical study of the generation and geometrical attenuation of regional crustal phases we do this through the computation of seismograms in the epicentral distance range from 60 to 500 km the geometrical attenuation of lg waves with epicentral distance is of the form r 0 83 pg wave amplitudes display a much stronger decay of the form r 1 5 the spectral density of the crustal transfer function for pg waves is relatively flat for frequencies between 0 1 and 5 hz while lg wave spectra strongly fall off beyond 2 to 3 hz the excitation of pg wave is insensitive to the depth of the source within the crust while the lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust the amplitudes of these two phases drastically decrease when the source is below the moho these results illustrate the important role of wave guide played by the crust for the propagation of lg and pg we find that the geometrical attenuation of pg and lg waves is independent of source mechanisms in the case of an explosion the excitation of pg is insensitive to the source depth the lg wave amplitude is small in comparison to pg and rayleigh waves and depends on the closeness of the source to an interface or to the free surface

regional seismic characteristics of the 9 october 2006 north korean nuclear test we investigate the regional seismic signature of the 9 october 2006 north korean nuclear test broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between december 2000 and no vember 2006 epicentral distances from the stations to the test site are between 371 and 1153 km we first use these regional events to calibrate the lg wave magnitude in the network then the network is used to calculate mblg 3 93 for the north ko rean nuclear explosion using a modified fully coupled magnitude yield relation the yield of the north korean nuclear test is estimated to be 0 48 kt because of large uncertainties in the source depth the estimate is preliminary the p s type spectral ratios pg lg pn lg and pn sn are calculated for the nuclear explosion and a group of earthquakes close to the test site at frequencies above 2 hz the network averaged p s spectral ratios clearly separate the 9 october 2006 explosion from the regional earthquakes our result indicates that a single blast explosion in the north korea re gion shows different seismic characteristics from an earthquake any well coupled single blast explosion detonated in this region with yield similar to that for the north korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study

convective instability of thickening mantle lithosphere mantle lithosphere being colder and therefore denser than the underlying mantle is prone to convective instability that can be induced by horizontal shortening numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening thermal diffusion and gravitational instability in deforming the layer this analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate if viscosity is non newtonian the imposition of shortening decreases the lithospheric strength which causes perturbations to the lithosphere s temperature structure to grow exponentially with time once these perturbations become sufficiently large they then grow super exponentially with time eventually removing the lithospheric base because lithospheric viscosity is highly temperature dependent at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super exponential growth event after this event however a downwelling develops that removes material advected into the region of downwelling by horizontal shortening the magnitude of this persistent downwelling depends on the rate and duration of shortening if the total amount of shortening does not exceed 50 per cent doubling of crustal thickness then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening once shortening stops this downwelling is no longer replenished by the shortening process and should then detach due to its inherent gravitational instability the hottest 60 per cent of the mantle lithosphere could be removed in such an event which would be followed by an influx of hot buoyant asthenosphere that causes rapid surface uplift because more cold material is removed after the cessation of shortening than by the initial gravitational instability the former has a potentially greater influence on surface uplift the tibetan interior is thought to have been shortened by about 50 per cent in 30myr and afterwards at 8ma experienced a period of rapid uplift that may have resulted from the removal of a large downwelling finger of cold lithosphere generated by shortening

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

attenuation of coda waves in western mexico using local seismicity coda waves are an important instrument to study the elastic properties of the lithosphere the attenuation of coda waves in western mexico is analyzed using seismograms recorded during the mapping of the rivera subduction zone experiment the coda wave attenuation is measured by means of q within the single backscattering model and the single isotropic scattering model schemes the frequency dependence of q is analyzed using the power fit formulation q f q f the resulting q q and are then used to study the seismic attenuation properties in western mexico we compare our results with the crustal structure by means of the surface geology seismic activity crustal thickness and p wave velocity patterns in the study area there is a poor correlation of the attenuation parameters with such crustal features however an analysis of the variations of the p wave velocity shows that the seismic attenuation parameters correlate to some extent with seismic velocity variations previously noted in the study area where low attenuation and weak frequency dependence correlate with low velocity variation

rupture process of a multiple main shock sequence analysis of teleseismic local and field observations of the tennant creek australia earthquakes of january 22 1988 on january 22 1988 three large intraplate earthquakes with ms 6 3 6 4 and 6 7 occurred within a 12 hour period near tennant creek australia these earthquakes which occurred over a small interval of time and within a small volume of space present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms depths and complexity of rupture of each of the three main shocks hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible the interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release as locations of the main shock hypocenters progressively moved from west to east we infer that the first and third main shocks denoted as ms1 and ms3 produced the southeast northwest trending scarps observed at the western end the kunayungku fault and at the eastern end the east end of the lake surprise fault respectively of the rupture zone the epicenter of the only immediate foreshock was located in the gap between these two fault scarps ms1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6 5 km ms3 ruptured predominantly to the se at a depth of 4 5 km the second main shock ms2 is inferred to have produced the deformation of the southwest trending central scarp segment the western end of the lake surprise fault from the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes ms1 and ms3 in contrast the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps the rupture process of ms2 turns out to be commensurately complex and sufficiently explains the geological complexity ms2 consisted of three subevents the southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface choice of the north dipping plane as the rupture plane of the third subevent consistent with the surface deformation and coplanar with a second plane delineated by aftershocks would imply conjugate faulting the majority of the aftershocks are concentrated near the edges of the fault planes and there is an absence of activity in the center of the planes the areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred the rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture these characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release the first main shock nucleated at the deepest part of its aftershock zone and ruptured upward in contrast ms2 and ms3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones this variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

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

lg wave attenuation in the australian crust abstract we estimate the lg wave quality factor q across the australian continent from vertical component lg waveforms a tomographic inversion is performed to construct an lg attenuation model for 58 frequencies between 0 05 and 10 0 hz the available spatial resolution is approximately 1 5 1 5 for the 0 5 2 0 hz band at 1 0 hz the lg wave q over the whole island continent varies from 50 to 1250 with an average value of 850 significant regional variations in the lg wave q images tie well with many geological features and boundaries in australia the cratons in western northern and southern parts of australia usually have higher q values 700 1250 while the volcanic regions sedimentary basins and orogenic areas in eastern australia are characterized by increased attenuation lower q values 50 650 we determine the frequency dependent q of lg waves for different blocks across australia and find the frequency dependence of q is much more complex than the traditional single power law representation when combined with the assumed geometrical spreading relation the lg q maps provide a new way of assessing potential ground motion across the continent for any event location

seismic crustal structure of the proterozoic north australian craton between tennant creek and mount isa seismic recording along a 600 km traverse across the southern part of the north australian craton indicates that lateral velocity variations exist in the middle lower crust taken together with other data from the craton there is evidence for midcrustal velocities increasing from east to west and from south to north these trends could result from a greater proportion of high velocity granulites being emplaced at higher levels in the crust between tennant creek and mount isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites near surface 0 5 km p wave velocities on the tennant creek inlier increase from 5 47 km s at the surface to about 6 2 km s at 6 km depth and correspond with low grade metamorphics of the warramunga group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains in the mount isa geosyncline velocities of 6 03 6 15 km s correspond to the leichhart metamorphics interspersed with granites in the middle crust p wave velocities of 6 85 km s occur at a depth of 26 km near tennant creek whereas such velocities are not evident until depths of about 37 km near mount isa there is therefore a trend for midcrust velocities to increase from east to west the lower crust is characterized by velocities of 7 3 7 5 km s and the upper mantle velocities of 8 16 8 20 km s are reached at depths of 51 54 km exposed granulite facies rocks in central australia are low in heat producing elements and are compatible with the low surface heat flow generally measured in precambrian australia these data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust through hornblende granulite to garnet granulite in the lower crust

a novel technique for measuring lg attenuation results from eastern canada between 1 to 10 hz spectral amplitudes of regionally recorded lg waves are studied in detail between 0 6 and 10 hz using vertical component velocity seismograms of the eastern canada telemetered network stations and a supplementary seismic research observatory type station at glen almond gac quebec we find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest furthermore they are found to be strongly frequency dependent consequently it is essential that they be taken into consideration in studies of lg wave attenuation and lg source spectra of regionally recorded seismic events we present a new method of measuring interstation surface wave attenuation which is closely related to the conventional two station method while retaining all the desirable features of the conventional two station method the new technique which we will call the reversed two station method allows simple direct one parameter determination of the lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error the reversed two station method is successfully tested over weakly attenuating short 53 to 210 km interstation paths in eastern canada a difficult experimental condition by normal standards the lg attenuation coefficient 0 6 to 10 hz in eastern canada is found to be frequency dependent and of the form f 0 0008 f 0 81 km 1 at higher frequencies the lg attenuation appears to be essentially frequency independent this latter finding is preliminarily interpreted as evidence that regionally recorded lg waves in the canadian shield are as in the case of lg waves propagating through the structurally complex appalachian province contaminated by the high frequency coda of sn waves the lg contamination over the shield paths becomes severe starting at 14 hz twice the frequency above which the lg signal propagating over the appalachian province becomes completely dominated by non lg arrivals

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

evolution of the late cenozoic tectonic stress regime in the shanxi rift central north china plate inferred from new fault kinematic analysis abstract the shanxi rift an intracontinental belt along the eastern margin of the ordos block north china plate has undergone multi stage extensional deformation during the late cenozoic in this study new fault kinematic analysis of boundary faults in the shanxi rift constrained by geochronological data defines a three stage tectonic stress regime since the late miocene providing new structural evidence for deciphering the northeastward growth of the tibetan plateau the earliest stage involved a transtensional stress regime of nw se extension and ne sw compression in the late miocene early pleistocene ca 10 1 8 ma and characterized by mean principal stress axes of 1 at 344 7 87 2 2 at 217 6 01 1 and 3 at 128 1 01 0 the early transtensional stress regime was responsible for the opening of the shanxi rift and was very likely associated with intensive right lateral pull apart activity in the eastern ordos block due to the northeastward growth of the tibetan plateau during the second stage the stress regime changed to less intense ne sw extension with mean stress orientations of 323 3 85 2 for 1 127 5 02 9 for 2 and 214 9 02 6 for 3 in the early late pleistocene ca 1 8 0 11 ma this stage is marked by a distinct shift from fluvio lacustrine to fluvio piedmont sedimentary facies at the end of the early pleistocene and the widespread eruption of basalt in the datong basin north of the shanxi rift this short lived extension appears to be the result of relaxation in the midst of crustal shortening during growth of the tibetan plateau the third stage was transtensional characterized by nnw sse extension and wnw ese compression 1 at 060 88 9 2 at 074 9 03 9 and 3 at 164 00 since the late pleistocene ca 0 11 ma causing dextral shear deformation in the shanxi rift and the disappearance of its paleo lake this stage resulted from regional ene wsw shortening due to northeastward growth of the tibetan plateau

passive seismic imaging of a craton edge central australia abstract passive seismic recordings of teleseismic p wave arrivals and their immediate coda along a dense profile of stations can be used to image the reflection structure beneath the profile the process exploits the autocorrelation of the seismic signals that extracts the reflection response from the transmitted signals recorded at the surface which can be migrated to provide a depth image this common reflection point crp imaging exploits the same portion of the seismic record used in receiver function studies but is much less influenced by multiple problems in the presence of sediments this style of passive seismic imaging is applied to a dense line of 68 stations at 3 5 km spacing at the northern edge of the gawler craton in south australia in the mal experiment with nine months of passive seismic monitoring it has been possible to image fine scale structural variations throughout the lithosphere there is a strong variation in crustal thickness as the mal profile crosses the inferred location of the craton edge under sedimentary cover a transitional zone between crust and mantle can be identified that extends beneath the receiver function moho pinching out gently at the craton edge the crp imaging reveals distinct reflectivity in the lithospheric mantle with horizontal scales of 10 30 km and vertical scales of no more than a few kilometres the character of this reflectivity appears to change across the craton edge there are indications of a semi continuous mid lithosphere discontinuity at around 90 km depth

the shape of ground motion attenuation curves in southeastern canada the shape of the attenuation curve of the fourier spectrum of acceleration is established for southeastern canada from empirical data the dataset is comprised of 1200 digital vertical component seismograms from 100 earthquakes recorded on 30 stations of the eastern canada telemetered network the distance range of the observations is 10 to 1700 km focal depths are from 5 to 30 km results indicate that the attenuation curve has three distinct sections at distances less than 70 km corresponding to attenuation of the direct wave amplitudes decay slightly more rapidly than 1 r r 1 1 between 70 and 130 km where the direct wave is joined by sms spectral amplitudes are approximately constant beyond 130 km corresponding to the lg phase amplitudes decay at a rate that is consistent with r 0 5 and q 670 f 0 33 the findings support the hypothesis of burger et al 1987 that postcritical reflections from the moho discontinuity play an important role in determining the shape of the attenuation curve however the influence of sms is subtle allowing the shape to be approximated by simple functional forms for example the commonly assumed bilinear form r 1 r 0 5 with frequency dependent q fits well if the transition from direct wave to lg spreading is placed at approximately 60 km a linear form r 1 over all distances with constant q 2000 also provides a satisfactory fit overall we find little evidence for the dependence of attenuation on focal depth or tectonic province

intensity relations for australian earthquakes although intensity data are essentially derived from qualitative observations they can be used to estimate basic earthquake parameters such as hypocentres and magnitudes particularly when instrumental observations are not available in this study the intensity information was used from 138 earthquakes to derive relations which enable earthquake magnitudes to be determined from intensity data the earthquakes ranged in size from ml 1 3 to ml 7 2 and the felt areas ranged from 8 km2 to 2 5 106 km2 we obtained the following results 1 ml 0 35 0 12 log rp 2 0 63 0 41 log rp 1 87 0 36 2 ml 0 38 0 19 log riv 2 0 49 0 66 log riv 2 38 0 57 3 ml 1 35 0 34 0 57 0 06 io 4 ml 0 024 0 014 k2 0 061 0 029 k 1 51 1 46 where io is the maximum observed modified mercalli mm intensity k the sponheuer earthquake parameter rp the equivalent radius of the mmiii isoseismal riv the equivalent radius of the mmiv isoseismal and ml the richter magnitude quadratic terms are necessary in relations 1 2 and 4 because of the breakdown of the simple kanai relation at distances close 10 km to the epicentre rather surprisingly it was found that for earthquakes smaller than ml 6 eqns 1 and 3 are more consistent with richter s results for california than with those obtained by nuttli et al for the eastern united states there is excellent agreement between our results and those obtained by sibol et al for central and eastern north america for estimating ml eqn 1 is preferred because the observations have a lower rms deviation about the line of least squares fit

source spectra for the 1988 saguenay quebec earthquakes abstract source spectra are obtained for the 1988 saguenay quebec earthquake m n 6 5 its foreshock m n 4 8 and largest aftershock m n 4 1 using recordings from analog and digital seismographs on rock sites at epicentral distances from 40 to 700 km the source spectra of the foreshock and the aftershock are well matched by a single corner frequency 2 model with seismic moments of 2 2 10 22 and 2 2 10 21 dyne cm respectively and a stress parameter of 65 bars for both events by contrast the spectrum of the mainshock differs dramatically from this simple shape a stress parameter of 500 bars is required to match the observed high frequency amplitudes using the seismic moment determined from teleseismic observations as a constraint this single corner frequency model however overestimates amplitudes at frequencies near 1 hz by a factor of about 2 the relative radiation of short and long period energy for the mainshock is inconsistent with that from other intraplate earthquakes the fore and aftershocks appear typical in this regard the extraction of source spectra from the regional data required a careful examination of attenuation azimuthal variations were observed with the attenuation being most severe to the southeast normal to the st lawrence valley and the dominant structural grain of the region the st lawrence river coincides in part with the boundary between the grenville and appalachian tectonic provinces and the difference in attenuation may be related to the difference in tectonic province the distribution of data however makes it difficult to obtain good estimates of the attenuation in each province lumping the data from both provinces and the three earthquakes together results in an anelastic attenuation factor given by q 755 f 0 5 for frequencies f from 0 6 to 18 hz using data at distances r beyond 100 km from the sources and assuming 1 r geometrical spreading this attenuation is similar to that obtained in a number of other studies in the general area three component recordings were available at a few stations and from these the following frequency dependent ratio of horizontal to vertical motion was obtained hiv 1 14 0 118 f 0 00638 f 2 for frequencies from 0 2 to 15 hz different methods of correcting observations for attenuation in order to derive source spectra were examined source spectra derived by simple regression analyses of observed data assuming a uniform medium are very similar to those based on a more complicated synthetic seismogram correction which accounts for wave propagation in a layered earth

crustal structure of western tibet revealed by lg attenuation tomography abstract the lateral lg wave attenuation in western tibet and surrounding regions is studied a total of 706 two station method tsm and 533 reverse two station method rtsm pairs are generated from 282 regional earthquakes the earthquakes are recorded at 48 seismic stations operated across western kunlun and western tibet regions the lg phases are extracted from the vertical component of seismograms with magnitude greater than 4 6 mb and focal depth less than 50 km to measure the tsm and rtsm lg q at 1 hz q0 the lgq0 tomography map is generated with grid size 0 5 0 5 using the least squares orthogonal factor decomposition lsqr algorithm lateral variation of lg attenuation is found to exist across the region the results provide new insights into the attenuation properties of the crust and its linkage with regional geodynamics the low lgq0 200 is revealed in the central part of lhasa terrane in western tibet from both tsm and rtsm images the high lgq0 values complemented with the high pn velocities beneath the central part of lhasa terrane reveal the trace of underthrusting indian lithosphere beneath the western tibet

constraints on locations of large intraplate earthquakes in the northern territory australia from observations at the warramunga seismic array three earthquakes of ms 6 3 6 4 and 6 7 near tennant creek northern territory australia on january 22 1988 produced 35 km of surface rupture in the precambrian shield the surface deformation consists of low angle 20 30 thrust faulting on 2 main scarps separated by a 7 km gap the kunayungku western fault and the eastern end of the lake surprise eastern fault dip to the south while the western end of the lake surprise fault dips to the north although these earthquakes are among the largest ever recorded on the australian continent there is no record of earthquakes larger than magnitude 5 within 500 km prior to 1987 data from the warramunga wra seismic array located 30 km east of the rupture show no local activity between 1965 and 1986 two small events in the fault area were observed in february 1986 and were followed by a series of 6 mb 4 to 5 2 earthquakes in january 1987 and their aftershocks the 1986 1987 and january 22 1988 mainshocks all nucleated within a zone about 5 km across from north to south but elongated radially away from wra by up to 30 km although the distribution of these events is in the direction of poorest hypocentral resolution we can place meaningful constraints on the epicenters by examining the range of solutions that fit the p arrival time data the 1986 and largest 1987 events were restricted to the western part of the fault zone the three mainshocks progressed from west to east suggesting that the kunayungku scarp was produced by the first mainshock and the west and east ends of the lake surprise scarp by the second and third respectively

lg coda q and its relation to the structure and evolution of continents a global perspective tomographic maps of lg coda q q lg c variation are now available for nearly the entire african eurasian south american and australian continents as well as for the united states q lg c at 1 hz q 0 varies from less than 200 to more than 1000 and q lg c frequency dependence varies between 0 0 and nearly 1 0 q 0 appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region a plot of q 0 versus time since that activity indicates that a single q 0 time relation approximates most mean q 0 values those that deviate most from the trend lay in australia the arabian peninsula and the east african rift the increase in q 0 with time may be due to a continual increase in crustal shear wave q q caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity extrapolated values of q lg c at 5 hz using q 0 and values measured at 1 hz and assuming that is constant in all regions between 1 and 5 hz show a similar percentage wise increase with times that has elapsed since the most recent activity other factors that can reduce q 0 in continental regions include thick accumulations of sediment especially sandstone and shale of mesozoic age and younger severe velocity gradients at the crust mantle transition and possibly lateral variations in the depth thickness and severity of those gradients severe and large increases of q in the mid crust of some regions can cause relatively large values of even if the frequency dependence of q is small

the upper mantle s and p velocity structure beneath northern australia from broad band observations abstract good coverage of seismic wave propagation through the upper part of the mantle can be obtained for sources in the earthquake belt through indonesia and new guinea recorded at broad band seismometers in the northern territory of australia the midpoints of the propagation paths lie along the northern margin of the australian continent most of the observations come from a guralp cmg3 seismometer at the warramunga array but these have been supplemented by field deployments using comparable instrumentation two groups of sources 1 along the flores arc indonesia with propagation under northwestern australia and 2 in new guinea with paths to the nne of the array have been used to construct composite record sections for p sv and sh waves over the distance range 1250 3600 km the choice of the two source zones was based on earlier short period studies and the broad band observations confirm the presence of noticeable differences in p velocity profiles for paths whose turning points are separated by about 1000 km the s waves recorded on the radial sv and tangential sh components are of comparable quality because the hard rock recording sites minimise the influence of coupling to p on the radial component as the p and s sections are prepared from the same events we are able to make direct estimates of p s velocity ratios for the mantle transition zone this ratio increases from 1 77 at the top of the mantle to 1 85 at the 410 km transition indicating that the s wave velocity gradients are rather lower than might be expected however through the transition zone down to 660 km the s wave gradients increase and the p s velocity ratio is less than 1 81 below the 660 km transition there is a significant difference in the frequency content of p and s waves beyond 2000 km the p waves remain high frequency but the s waves returned from the transition zone and below are of intermediate period 0 25 0 3 hz associated with significant attenuation zone for s beneath 210 km

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

finite difference modelling of lg blockage summary a p sv finite difference method is used to calculate seismic wave propagation in the crust and upper mantle the anelastic effect is excluded from the calculation and therefore any wavefield changes can be attributed to either geometrical or structural effects snapshots of displacement distrihutions within and synthetic seismograms at the free surface of different models revealed that the inclusion of a water column over an appropriate section of the crust is necessary to explain the severe lg attenuation widely observed in many parts of the world in particular for an oceanic propagation path they clearly show that the effect of a water column over the crust is an important factor in blocking lg piupagation a propagation path of more than 100 km over an oceanic crust can completely block the transmission of lg waves originating in a neighbouring continental crust the energy of the lg wavetrain is redistributed at continental margins most of the incoming energy is either converted to waves which propagate into the mantle or is backscattered to form waves which propagatc in the continental crust in the opposite direction very little energy is presented at the expected arrival times at the other side of the oceanic crust which is consistent with the lg blocking phenomena simplified 2 d models of the north sea central graben are used to simulate lg propagation in a laterally varying continental path the inclusion of a water column over the crust enhances the attenuation of lg the deeper the water column the greater the attenuation the undulation of sea bottom can further enhance the attenuation of lg

attenuation of high frequency shear waves in the crust measurements from new york state south africa and southern california we compare the attenuation of high frequency 3 30 hz shear waves for crustal paths in new york state south africa and southern california over source receiver distances of about 10 400 km the data consist of digital recordings of s waves 5 100 km and lg waves 100 400 km produced by earthquakes we use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the s and lg waves over the entire distance range studied 10 400 km the amplitude decay of 3 hz shear wave energy is considerably less for the tectonicaily stable areas of new york and south africa than for the tectonicaily active region of southern california high frequency 30 hz s wave attenuation is significantly less for new york and south africa than for southern california for distances between 15 and 90 km we parameterize the decay with distance r of coda normalized shear wave amplitudes with a frequency independent q and geometrical spreading exponent where geometrical spreading is proportional to r for new york state the s wave amplitude decay 3 30 hz is well described by a frequency independent q of 2100 330 490 and of 1 3 0 1 the decay of lg wave amplitudes from 3 to 15 hz in the new york state region is fit with a frequency independent q of 1600 280 330 of 0 70 0 2 the s wave amplitudes 3 30hz in south africa yield a q of 1500 190 380 and of 1 3 0 1 fixing the geometrical spreading at r 0 5 produces an lg wave q estimate at 3 hz in south africa of 360 50 80 this lg wave q is low considering that south africa is a cratonic tectonicaily stable area the s wave amplitudes from southern california are described with a frequency independent q of 800 150 240 and a large geometrical spreading exponent of of 1 9 0 2 we find an lg wave q at 3 hz of 260 30 for southern california after constraining the geometrical spreading at r 0 5

attenuation of ground motion in the eastern united states abstract frequency dependent attenuation of ground motion in the eastern united states is investigated for both vertical and horizontal components of motion this is accomplished by digitizing and analyzing three component short period long range seismic measurements data from over 200 earthquake station pairs with epicentral distances less than about 1000 km pseudo relative velocity as well as velocity response spectra for 5 per cent damping are obtained for the frequency range of 0 5 to 7 hz a multi variate least squares inversion is used to derive the station and source terms as well as the mean q analysis of data for the entire eastern united states indicates q values for the vertical and horizontal components to be indistinguishable substantial increase in q with frequency a factor of about 2 from 1 to 7 hz large as much as a factor of 20 in amplitude site terms for both the vertical and horizontal components and the mean ratio of the peak horizontal to peak vertical component to be a stable characteristic of the site large regional variations in q are also indicated by subsets of data such as for the central united states and the appalachian mountains region

measurement of frequency dependent lg attenuation in the great basin measurement of the spatial decay of spectral amplitudes of the higher mode seismic surface wave train lg indicates that in the great basin the apparent seismic quality factor q is a function of frequency observations along a 300 km profile recording of a nuclear explosion yield the function q 206 0 68 over the band 0 3 10 0 hz similar analysis using numerous recordings of earthquakes along paths which provide a good average sampling of the great basin crust gives q 214 15 0 54 0 09 for 0 3 5 0 hz if the crustal sampling by lg energy from nuclear explosions surface sources is primarily in the shallow crust as has been suggested then these results indicate a greater frequency dependence of apparent q there than at depth

lg attenuation tomographic models of himalaya and southern tibet abstract we investigate the lg attenuation structure of the crust beneath the tectonically complex himalaya and southern tibet regions adopting a tomographic regionalization method a total of 1671 earthquake waveforms registered at 38 seismic stations operated in the region are selected for the initial lgq measurements using the standard two station method q0 1 hz lgq values of 76 high quality interstation paths are finally considered as input for the tomographic inversion the estimates of q0 exhibit distinct variations in the crustal attenuation from north to south across the whole region the zones of lowest q0 values

aftershocks of the 1988 january 22 tennant creek australia intraplate earthquakes evidence for a complex thrust fault geometry summary on 1988 january 22 three earthquakes of ms 6 3 6 7 occurred in a 12 hr period near tennant creek northern territory in the proterozoic north australian craton and produced 32 km of surface rupture on two main scarps the lake surprise scarp is shaped like a boomerang with the east arm trending ese and the west arm wsw the kunayungku scarp lies 7 km to the northwest and trends ese locations of 211 aftershocks determined using data from portable seismograph arrays define a 10 km by 40 km aftershock zone elongated parallel to the trend of the surface ruptures in the western and eastern part of the fault zone aftershocks occur only south of the kunayungku and eastern lake surprise scarps respectively in the central section in contrast aftershocks lie primarily to the north of the western arm of the lake surprise scarp focal depths range from the surface to 8km in all sections the shallowest earthquakes lie closest to the scarps whereas deeper events lie further away in cross section the aftershocks lie in three distinct inclined zones which delineate fault surfaces ruptured by the main shocks aftershocks are concentrated near the edges of the inferred fault surfaces suggesting that stress was relieved in the centre of the faults by rupture during the main shocks the inferred fault plane dips are 45 to the ssw for the kunayungku fault 55 to the nnw for the western lake surprise fault and 35 to the ssw for the eastern lake surprise fault these dip directions are consistent with the sense of thrust movement inferred from surface deformation the reversal of fault dip and sense of movement between the central and end segments requires a complicated fault geometry the aftershock data would allow a second south dipping plane in the central section that does not intersect the surface this blind fault may be responsible for much of the moment release interpreted from the modelling of broad band teleseismic waveforms choy bowman 1989 rupture of the north dipping plane may have been controlled by a pre existing fault marked at the surface by a quartz ridge 5 km long and may be of secondary importance although the quartz ridge suggests previous faulting the absence of significant topographic relief together with estimates of erosion rates suggest an average repeat time in excess of 104yr preliminary interpretation of water withdrawal histories and water level data from a borefield adjacent to the eastern lake surprise scarp does not support a causal relationship between the borefield and the occurrence of the earthquakes the nearby warramunga wra seismic array provides a record of local earthquake activity from 1965 to the present the fault area was essentially aseismic until two small earthquakes in 1986 february and a series of six magnitude 4 5 earthquakes in 1987 january which were followed by more than 1000 aftershocks the frequency of occurrence of aftershocks declined very slowly in 1987 with p 0 2 in the modified omori formula whereas for the 1988 aftershock sequence p 1 the low p during 1987 may have been diagnostic of high stress in the source and water authority provided partial financial support for j r b and g g

attenuation of ground motion spectral amplitudes in southeastern australia a dataset comprising some 1200 weak and strong motion records from 84 earthquakes is compiled to develop a regional ground motion model for southeastern australia sea events were recorded from 1993 to 2004 and range in size from moment magnitude 2 0 m 4 7 the decay of vertical component fourier spectral amplitudes is modeled by trilinear geometrical spreading the decay of low frequency spectral amplitudes can be approximated by the coefficient of r 1 3 where r is hypocentral distance within 90 km of the seismic source from approximately 90 to 160 km we observe a transition zone in which the seismic coda are affected by postcritical reflections from midcrustal and moho discontinuities in this hypocentral distance range geometrical spreading is approximately r 0 1 beyond 160 km low frequency seismic energy attenuates rapidly with source receiver distance having a geometrical spreading coefficient of r 1 6 the associated regional seismic quality factor can be expressed by the polynomial log q f 3 66 1 44 log f 0 768 log f 2 0 058 log f 3 for frequencies 0 78 f 19 9 hz fourier spectral amplitudes corrected for geometrical spreading and anelastic attenuation are regressed with m to obtain quadratic source scaling coefficients modeled vertical component displacement spectra fit the observed data well amplitude residuals are on average relatively small and do not vary with hypocentral distance predicted source spectra i e at r 1 km are consistent with eastern north american ena models at low frequencies f less than approximately 2 hz indicating that moment magnitudes calculated for sea earthquakes are consistent with moment magnitude scales used in ena over the observed magnitude range the models presented represent the first spectral ground motion prediction equations developed for the southeastern australian region this work provides a useful framework for the development of regional ground motion relations for earthquake hazard and risk assessment in sea

differences in attenuation among the stable continental regions 1 there are systematic differences in the attenuation of damaging earthquake ground motions between different stable continental regions scrs seismic intensity and weak motion data show that the attenuation in seismic waves for eastern north america ena is less than for india africa australia and northwest europe if ena ground motion attenuation relations are used in seismic hazard models for other scrs as is commonly done then the estimated ground motions and resulting hazard may be too large if an attenuation model that averages observations from ena and the other scrs is used to estimate the magnitudes of large historical earthquakes in ena as is the case for recent estimates of m for the 1811 1812 new madrid missouri and the 1886 charleston south carolina events then the magnitude estimates for these events will be too large as will be the resulting hazard

comparison of earthquake source spectra and attenuation in eastern north america and southeastern australia abstract the paucity of ground motion data in stable continental regions scrs remains a key limitation when developing relations that seek to predict effects of strong ground shaking from large damaging earthquakes it is desirable to combine data from more than one scr in order to increase database size but this raises questions as to whether the source and attenuation properties of the scrs are equivalent we merge recently compiled spectral amplitude databases from small to moderate events moment magnitudes 2 0 m 5 0 in both southeastern australia and eastern north america in order to compare the key characteristics of ground motion in these two regions both are scrs but are widely separated spatially and in tectonic history we statistically compare ground motions by plotting mean and standard deviations of spectral amplitudes for data grouped in magnitude and distance bins these comparisons show that the source and attenuation properties of the two regions are very similar particularly at shorter hypocentral distances