was the 16 august 1997 seismic disturbance near novaya zemlya an earthquake the reviewed event bulletin for 16 august 1997 produced by the prototype international data center pidc reported a small seismic disturbance in the vicinity of the northern russian test site at novaya zemlya at about 02 11 utc initial reports suggested that this disturbance was caused by a small nuclear explosion whereas others identify the source as an earthquake using vertical component s p ratios however other authors show evidence that questions the validity of the s p method for this region and conclude that the disturbance cannot be positively identified as an earthquake using seismological evidence the apparent attenuation from the 16 august 1997 disturbance to spits spitsbergen distance 1280 km azimuth 317 is reported to be weak similar low attenuation is suggested from high frequency observations at kev kevo finland 1130 km azimuth 268 thus high frequency p and s waves propagate efficiently across the barents sea leading to the possibility that the radiation pattern of the source may be inferred we show using a grid search method that the high frequency sv sh amplitude ratios observed at spits and kev measured from the complex envelopes of the free surface corrected three component seismograms combined with simple p n signals with positive first motion at hfs hagfors sweden and nores norway are consistent with a double couple source with a range of orientations we compute synthetic seismograms for one such double couple source oriented at 255 115 120 similar to that reported for the 1 august 1986 kara sea earthquake the synthetic p n seismograms at hfs and nores and the relative sv sh amplitudes of the synthetic s n seismograms at kev and spits match those observed reasonably well our model also predicts 1 weak p n at nri norilsk russia suggesting that the emergent p n onset and low signal to noise ratio observed is related to the focal mechanism and 2 the negative p first motion and large sh sv ratio observed at amd amderma 360 km azimuth 152 thus we conclude that the 16 august 1997 seismic disturbance was an earthquake beneath the kara sea and that identification is possible using only data that would normally be available to the pidc assuming kev can be used as an alternative for the nearby station arces which was not working at the time of the disturbance

optimized seismic threshold monitoring part 1 regional processing continuous seismic threshold monitoring is a technique that has been developed over the past several years to assess the upper magnitude limit of possible seismic events that might have occurred in a geographical target area the method provides continuous time monitoring at a given confidence level and can be applied in a site specific regional or global context in this paper part i and a companion paper part 2 we address the problem of optimizing the site specific approach in order to achieve the highest possible automatic monitoring capability of particularly interesting areas the present paper addresses the application of the method to cases where a regional monitoring network is available we have in particular analyzed events from the region around the novaya zemlya nuclear test site to develop a set of optimized processing parameters for the arrays spits arces fines and nores from analysis of the calibration events we have derived values for beam forming steering delays filter bands short term average sta lengths phase travel times p and s waves and amplitude magnitude relationships for each array by using these parameters for threshold monitoring of the novaya zemlya testing area we obtain a monitoring capability varying between mb 2 0 and 2 5 during normal noise conditions the advantage of using a network rather than a single station or array for monitoring purposes becomes particularly evident during intervals with high global seismic activity aftershock sequences high seismic noise levels wind water waves ice cracks or station outages for the time period november december 1997 all time intervals with network magnitude thresholds exceeding mb 2 5 were visually analyzed and we found that all of these threshold peaks could be explained by teleseismic regional or local signals from events outside the novaya zemlya testing area we could therefore conclude within the confidence level provided by the method that no seismic event of magnitude exceeding 2 5 occurred at the novaya zemlya test site during this two month time interval as an example of particular interest in a monitoring context we apply optimized threshold processing of the spits array for a time interval around 16 august 1997 mb 3 5 event in the kara sea we show that this processing enables us to detect a second smaller event from the same site mb 2 6 occurring about 4 hours later this second event was not defined automatically by standard processing

anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

seismic location calibration of the european arctic a crustal velocity model has been developed for fennoscandia the baltic shield and adjacent areas this model represents a simplified average of various models developed for parts of this region we show that p wave travel times calculated with this model provide an excellent fit to observations at the fennoscandian krsc and iris station networks for a set of seismic events with known or very well constrained locations the station event paths cover large parts of western russia and the barents sea thus indicating that this model which we denote the barents model is appropriate for this entire region we show by examples that significant improvements in event location precision can be achieved compared to using the 1aspei model we finally use the barents model to calculate locations of recent small seismic events in the novaya zemlya region of interest in a ctbt monitoring context

the 11 october 2010 novaya zemlya earthquake implications for velocity models and regional event location characterizing the seismicity of novaya zemlya and the surrounding arctic seas requires accurate event location estimates low magnitude events in this region are currently observed only by a small number of stations in the european arctic with a large azimuthal gap making the accuracy of regional velocity models all the more important regional travel time calibration is difficult given the scarcity of sufficiently well constrained events on 11 october 2010 a magnitude 4 5 event occurred close to the northern tip of novaya zemlya this event is significant in that it is the first event in this region to have been recorded both on the relatively recent regional networks and arrays and also teleseismically with good azimuthal coverage we examine how well we can constrain the location and origin time using only teleseismic phases using only first teleseismic p arrivals we constrain the epicenter to approximately 76 25 n and 64 75 e but with no depth resolution clear depth phases notably on stations in the southern united states indicate a depth between 9 and 15 km this independent hypocenter and origin time estimate allow evaluation of regional phase travel time prediction using different models the predicted sn travel time appears to cause the greatest variability in regional location estimates the 3d regional seismic travel times models provide excellent pn travel time estimates for barents sea paths but may slightly overestimate sn travel times from this source region a modified regional 1d velocity model is defined which best predicts pn and sn observations at multiple stations up to 15 the significance of the regional travel time models for estimating location is demonstrated for a low magnitude event on or close to the northern island of novaya zemlya in march 2014 recorded with a satisfactory signal to noise ratio at only four stations

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

improvements to seismic monitoring of the european arctic using three component array processing at spits abstract the detectability of low magnitude seismic events in the european arctic is determined primarily by the small aperture international monitoring system arrays arces and spits in august 2004 the spits array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 hz most important however for the detection and location of small magnitude seismic events was the deployment of three component instruments at six of the nine sites detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances and in the absence of the strong lg phases typically observed for continental propagation paths multiple three component stations were deemed necessary to exploit the higher s phase amplitudes anticipated on the horizontal sensors we demonstrate improved signal to noise ratios snrs for s phases on horizontal beams for several events close to novaya zemlya horizontal component f k analysis improves direction estimates and phase classification for low snr signals we demonstrate secondary phases that are misidentified by vertical only f k analysis but which are correctly classified by three component array processing a significant problem with array processing at spits is the overlap in slowness space of regional p and s phases phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces considerations in the routine array processing of spits data are reviewed including the need for elevation corrections in slowness estimation and the need to take into account azimuth dependent variation of apparent velocity estimates for regional phases

identification of earthquakes and explosions using amplitude ratios the vogtland area revisited identification of seismic events is a major scientific issue in the framework of verification of the comprehensive nuclear test ban treaty ctbt of special interest in this context is the identification of the numerous low yield mining or blasting events especially those occurring in the same area as earthquakes such as the vogtland area in the border region of germany and the czech republic seismic events in this area were investigated by wuster 1993 1995 who achieved complete discrimination using measures of spectral decay and spectral variance at the geress array and a quadratic discrimination function

the seismic disturbance on 1986 august 1 near novaya zemlya a source of concern summary on 1986 august 1 a small seismic disturbance occurred to the east of a soviet nuclear test site at novaya zemlya reported attempts to identify the nature of the source using only short period seismological data recorded at regional distances were unsuccessful however by using teleseismic p wave data it is possible to identify the source as an earthquake which occurred within the crust at a depth of 24 km the source is shown to have a double couple mechanism with a well constrained fault plane solution the identification as an earthquake is supported by the mb ms evidence

multimode rayleigh wave attenuation and qβ in the crust of the barents shelf the multimode method of cheng and mitchell 1981 is used to study the attenuation properties of the crust throughout the barents shelf the sensitivity of the spectra to various source factors and to propagation factors which might characterize a shelf region are thoroughly studied for plane layered models the method is very sensitive to q in the upper crust and to q of sedimentary layers if those layers are thick and highly attenuating it is far less sensitive to q in the lower crust and to q the spectra for shallow strike slip earthquakes are very insensitive to changes in strike slip and dip angles of the fault throughout the entire period range of interest but are very sensitive to variations in focal depth except in the long period range of the fundamental mode and the short period range of the higher modes the fundamental mode but not the higher mode spectra for one path across the barents shelf are apparently distorted at short periods by lateral heterogeneities along the travel path simple two layer trial and error inversions yield a model of the barents shelf with an upper crustal q value of about 80 three layer inversions with a sedimentary layer however yield an upper crustal q of about 250 if the sediments are 1 km thick and are characterized by q values of 40 shear velocity models for the barents shelf show variations between about 3 0 and 3 6 km s in the upper crust along different paths the lower crust shows smaller regional differences but velocities vary between 3 5 and 4 0 km s through the depth range 25 35 km the variation of shear velocities in the upper mantle may correlate with those of the upper crust and vary regionally between about 4 3 and 4 8 km s

regional phase development of the non proliferation experiment within the western united states abstract to effectively monitor a comprehensive test ban treaty seismologists must be able to confidently detect and identify low yield explosions this requires the use of short period regional phases which can be extremely complicated the non proliferation experiment npe was a low yield chemical explosion detonated at the nevada test site nts and recorded at more than 50 broadband seismic stations located throughout the western united states these data were used to investigate the development of lg and pn two seismic phases used in regional discriminants the frequency dependent attenuation for lg recorded at 43 stations is described by the relation q lg vertical 238 f 1 28 the amplitude decay for pn recorded at 38 stations is proportional to 1 29 0 05 f for the frequency window of 1 to 6 hz after removing the effects of distance and attenuation we found the values of the spectral ratio lg 4 6 2 4 to group according to the geologic terrain associated with the path traveled stations located within the basin and range had lower ratio values than stations located outside the basin and range however for the spectral ratio pn 2 4 4 6 geologic terrain had an indirect effect pn is affected by moho structure associated with the backazimuth causing an azimuthal dependence of course moho structure can be a function of geologic terrain furthermore the pn arrival may be large or small independent of azimuth the values of the discriminant phase ratio pn 1 2 lg 2 4 have an order of magnitude more scatter than lg lg or pn pn nonetheless the values are both a function of geologic terrain the lg contribution as well as moho structure pn contribution

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

attenuation of surface waves across the arabian peninsula abstract amplitude spectra of fundamental mode rayleigh and love waves are used to determine shear wave q q models for the upper crust of the arabian peninsula the observed spectra are compared to theoretical spectra computed for earthquakes with known fault plane solutions shear wave q in the upper crust can be obtained by trial and error modeling to fit the the slope of the amplitude spectra at short periods 5 30 s if the source time function is less than about 6 s and if the depth of the earthquake is known to within about 3 km shear velocity q values obtained in this way vary from about 60 along the margin of the red sea to 100 150 in the central part of the peninsula to 65 80 in the eastern folded region these values are unusually low for a stable region and suggest that observed anelasticity there is affected by tectonic and epeirogenic activity which has occurred over the past 30 m y the shear wave q models derived from surface waves at periods of 5 30 s closely predict reported values of lg q at 1 hz shear wave q therefore appears to be independent of frequency in this region at periods between about 1 and 30 s

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

thermal structure of the low velocity zone derived from laboratory and seismic investigations laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone the measurements carried out at high temperature and high confining pressure can be extrapolated to upper mantle conditions we compare laboratory velocities with seismic observations and estimate temperatures and partial melt fractions in the low velocity zone the comparison gives 1310 c and 7 vol partial melt at 45 km depth in 0 5 my asthenosphere beneath the iceland plateau and 1450 c and 3 vol partial melt at 85 km depth in 0 20 my asthenosphere beneath the pacific ocean the laboratory results show that up to a 6 velocity drop in the low velocity zone relative to the lithospheric velocity does not necessarily require the existence of partial melt this implies that partial melt may not exist in the low velocity zone older than 5 my and 50 my in the iceland plateau and the pacific ocean respectively

the level of deterrence provided by data from the spits seismometer array to possible violations of the comprehensive test ban in the novaya zemlya region summary the yield threshold at which a fully decoupled explosion can be identified has been a recurring issue in the debate on whether the comprehensive nuclear test ban ctb can be adequately verified here we assess this yield threshold for the novaya zemlya nz and kola peninsula regions by analysing seismograms from six small body wave magnitude mb 3 5 seismic disturbances recorded at regional distances 1050 1300 km by the seismometer array at spitsbergen spits multiple filter analysis of the seismograms shows clear high frequency pn f 14 hz except from a calibration explosion on the kola peninsula from four of the disturbances studied we observe clear high frequency sn the explosion showed no clear high frequency sn and the data from the remaining disturbance was potentially contaminated by a data glitch frequency domain analysis indicates that the pn and sn attenuation across the barents sea is similar to that observed across stable tectonic regions shields we define a spectral magnitude for the 2 5 3 5 hz passband that is tied to teleseismic mb from nz explosions the six disturbances considered have 2 3 mb 3 5 three component data are available from spits for four of the disturbances considered including the explosion from the explosion the s p ratios on the vertical z radial r and tangential t components in the 3 0 6 0 hz passband are all less than unity the s p ratios for the same passband on the z component from the remaining three disturbances are less than unity but the ratios on the r and t components are significantly greater than unity we argue that s p ratios 3 0 6 0 hz passband of less than unity on all of the z r and t components at spits may indicate a potential treaty violation in the kola peninsula and nz regions the temporal variation of seismic noise in the 3 0 6 0 hz passband at spits suggests that our three component s p criterion will be effective 95 per cent of the time for disturbances with mb 2 8 we suggest that mb 4 25 b log10w where w is the explosive yield in kilotons kt with b 0 75 for w 1 and b 1 0 for w 1 is suitable for conservatively estimating the yield threshold of a potential violation of the ctb in the nz region from this we infer that a 35 ton fully coupled explosion in the nz region is likely to be identified as suspicious under the ctb using the three component s p criterion simulations show that the low frequency decoupling factor df for a fully decoupled nuclear explosion in hard rock is about 40 suggesting that such an explosion with a yield of 1 6 kt in the nz region is likely to be identified using data from spits the conservatism likely to be employed by a potential violator and uncertainties in the dfs for nuclear explosions in hard rock cavities together with data from stations other than spits within 2000 km of the nz region suggest that the yield at which a potential violator of the ctb could confidently escape detection using decoupling in the nz region is in reality probably less than 0 5 kt

study of regional surface waves and frequency dependent ms mb discrimination in the european arctic accurate discrimination of seismic events with a regional network requires detailed knowledge of the propagation characteristics of seismic waves in the region at present such propagation characteristics are reasonably well known for p and s waves in the european arctic however much work remains to be done regarding surface wave propagation and magnitude estimation regional long period or broadband seismic data in digital form has been available in the european arctic for only a few years in order to assess regional surface wave propagation and in particular to evaluate the m s m b discriminant at regional distances it is therefore necessary to take advantage of the historic analog recordings the station apa in apatity forms a unique source of such data with high quality long period seismic recordings of regional earthquakes and nuclear explosions dating back about 30 years

observed characteristics of regional seismic phases and implications for p s discrimination in the european arctic in this paper we use data from seismic stations operated by norsar the kola regional seismological centre krsc and iris to study the characteristics of regional phases in the european arctic with emphasis on the p s ratio discriminant while the detection and location capability of the regional station network is outstanding source classification of small seismic events has proved very difficult for example the m b 3 5 seismic event near novaya zemlya on 16 august 1997 has been the subject of extensive analysis in order to locate it reliably and to classify the source type we consider the application of the p s discriminant in the context of this event and other events observed at regional distances in the european arctic we show that the p s ratios of novaya zemlya nuclear explosions measured in the 1 3 hz filter band scale with magnitude indicating a need for caution and further research when applying p s discriminants using mainly data from the large norsar array we note that observed p s amplitude ratios in the european arctic show large variability for the same source type and similar propagation paths even when considering closely spaced observation points this effect is most pronounced at far regional distances and relatively low frequencies typically 1 3 hz but it is also significant on closer recordings around 10 degrees and at higher frequencies up to about 8 hz our conclusion from this study is that the p s ratio at high frequencies e g 6 8 hz shows promise as a discriminant between low magnitude earthquakes and explosions in the european arctic but its application will require further research including extensive regional calibration and detailed stationsource corrections such research should also focus on combining the p s ratio with other short period discriminants such as complexity and spectral ratios

broadband observations of upper mantle seismic phases in northern australia and the attenuation structure in the upper mantle abstract sources in the earthquake belt through indonesia and new guinea recorded at a broadband guralp cmg3 seismometer at the warramunga array in the northern territory of australia give good coverage of propagation through the upper part of the mantle the midpoints of the propagation paths lie along the northern margin of the australian continent beyond 18 there is a significant difference in the frequency content of p and s waves the p waves remain high frequency but the s waves returned from the transition zone and below are of intermediate period 0 1 0 5 hz which would be difficult to record without a broadband instrument and a quiet site the later branches associated with the 410 and 660 km discontinuities are clearly seen in individual seismograms the s waves recorded on the radial sv and tangential sh components are of comparable quality because the hard rock recording site minimizes the influence of coupling to p on the radial component we interpret the observed difference in frequency content of p and s waves at transition zone distances as the effect of a layer under the seismically fast lid which possesses a large degree of shear dissipation we quantify the observation in terms of the decay rate of the spectral ratio of s and p waves that quantity can be interpreted in terms of path integrals of the difference in inverse q for s waves and p waves t s t p we analyse 22 seismograms from the wra broadband instrument and a further four from portable broadband instruments deployed near warramunga the measured slope of the s p spectral ratio is consistently small out to 18 epicentral distance where it increases dramatically assuming that shear dissipation dominates over bulk dissipation these measurements are consistent with an average quality factor for s waves in the lid of the order of q s 1400 q p 2800 on top of a highly attenuative asthenosphere of 200 km thickness with q s 100 q p 200 which is underlain by a transition zone with q s 600 q p 1200 the low q zone in the asthenosphere can equally well be modelled with a thinner layer with a lower q e g a layer of 100 km thickness with q s 60 frequency dependence of q may render the above estimates of q about 25 too large these results compare fairly well with published results from 0 3 hz body wave observations under the eurasian shield notable exceptions are that we obtain a somewhat lower value for q s in the asthenosphere and a much higher value in the lid the lid under northern australia is unusually

anelasticity of the iceland plateau from surface wave analysis we study the anelasticity of the asthenosphere beneath the youngest seafloor of the iceland plateau region using spectral analyses of surface waves the epicenters and station geometry naturally define a fan shooting experiment with pure path propagation in regions with different seafloor ages both rayleigh and love waves are used in this study to help constrain the source parameters needed for a reliable assessment of q the inversion results indicate an approximately 130 km thick layer of q of about 7 for the asthenosphere of the youngest region of 0 5 ma for the region of age 5 10 ma q is about 50 and increases to over a hundred for the oldest zone 10 ma these attenuation values when combined with laboratory experiments indicate a partial melt of 3 in the asthenosphere near the spreading ridge

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

comparison of location procedures the kara sea event of 16 august 1997 various location procedures and velocity models are compared for the ml 3 3 kara sea event of 16 august 1997 this event has been the subject of considerable discussion because of its geographical position and the difficulty in obtaining a reliable focal depth estimate a comprehensive data set was extracted by re reading the records from all available stations these readings have then been used in a sequence of location experiments to examine the effect of using different velocity models to describe the travel times of the phases and also to compare the use of a fully nonlinear scheme shakena sambridge and kennett 2001 and a linearized location algorithm hyposat schweitzer 2001 2002 a standard least squares misfit criterion has been used for direct comparisons between the two methods the results confirm both the importance of s wave information in assessing the depth of regional events and the need to apply a reliable velocity model to place the strongest constraints on the location of the event even with only a limited data set but an adequate velocity model it is possible to find the position of the kara sea event close to the most probable locations however there is then no depth resolution reported error ellipses from standard data centers tend to have relatively small error ellipses with the commonly made assumption that the reading errors and the a posteriori residuals have an unbiased normal distribution such inversion results may indicate an unreasonably high resolution and accuracy of the solution the epicenter estimates for the whole data set using the range of different tech niques agree quite well with some overlap of the estimated confidence regions the observed seismic source was most likely an earthquake in the middle or lower crust at about 10 30 km depth

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

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

geotherms in the pacific ocean from laboratory and seismic attenuation studies seismic methods have been used extensively to measure attenuation coefficients in various regions of the earth attenuation is inversely related to the quality factor q which may be measured directly a low velocity and high attenuation zone in the upper mantle is commonly reported1 3 until now there has been no direct means to derive thermal structures from such seismic investigations recently q values have been determined in the laboratory for a dry peridotite an upper mantle rock for pressures and temperatures of up to 0 73 gpa and 1 280 c respectively4 the results indicate a quantifiable relation between q and temperature here we use these results to derive geotherms from q measurements for a region beneath the pacific ocean the geotherms are consistent with temperatures obtained from heat flow observations5 6 and imply that the oceanic upper mantle is rather dry in asthenos phere older than 40 myr geotherms for this region are sub solidus and no partial melt is expected

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

crustal q structure in the united states from multi mode surface waves determination of q structures are made using a new method which matches observed surface wave spectra with theoretical spectra computed for earthquakes with known fault plane solutions the importance of the method lies in its ability to utilize relatively short paths between a single source and receiver both of which are preferably located within a single geological province the method has been applied to study the regions of the eastern united states the colorado plateau and the basin and range province q models have been obtained through trial and error procedures and with an inversion scheme developed for that purpose models for each region obtained from these two procedures are in good agreement q values in the upper crust as obtained from inversions are 275 for the eastern united states 160 for the colorado plateau and 85 for the basin and range province q values in the lower crust although not well determined must be much higher than upper crustal values in all three regions the conclusions are consistent with those of previous work in which q values in the upper crust of a broad region of the western united states are lower than those of the eastern united states these new results show that substantial variations of q occur in different regions of the western united states the upper crustal q values show an inverse relationship to heat flow values

study of magnitudes seismicity and earthquake detectability using a global network abstract based on 10 yr of observations reported in the international seismological centre bulletins 1971 1980 m b magnitudes for about 70 000 earthquakes have been recomputed using a maximum likelihood estimation technique reportings from a network of 115 globally distributed stations were used in these calculations comparison to conventional m b estimates show that the network magnitude bias problem is quite significant at low and intermediate magnitudes recurrence statistics based on the revised m b estimates give b values consistent with those obtained from lasa and norsar data and indicate that the number of earthquakes worldwide of m b 4 0 averages about 7 500 annually this number is much lower than previous estimates the teleseismic detection capability of the network requiring at least four detecting stations has been evaluated based on recurrence statistics the estimated 90 per cent incremental m b threshold ranges from 3 9 to 4 5 in the northern hemisphere and from 4 2 to 4 8 in the southern hemisphere this is consistent with results obtained by the networth approach

waveform inversion of surface wave data test of a new tool for systematic investigation of upper mantle structures summary in most tomographic inversion of surface wave data the long period seismograms are first interpreted in terms of dispersion and or attenuation curves before performing an inversion in terms of laterally and depth varying properties an alternative to this approach is to perform a direct waveform inversion or as in cara lev que 1987 to use another set of secondary observables built up from the seismograms in this paper we systematically test with actual rayleigh wave records this latter technique by considering laterally homogeneous models we use for this purpose a set of recent long period digital records from the geoscope station dumont d urville antarctica for surficial events in the south indian ocean and the southeast pacific ocean and for intermediate depth events in the vanuatu and kermadec trenches in addition to the obvious advantages of being able to deal with situations where overtones are present in the seismogram it is found that the waveform inversion procedure allows us to reach a better depth resolution than in classical inversion of dispersion curves even when only the fundamental mode is present in the seismogram quite good resolution is obtained to depth as large as 300km for s velocity when using surficial events located at a few thousand kilometres from the station while classical surface wave studies do not allow us to resolve s velocity at depth larger than 150 km for these events when intermediate depth events are used at distances of about 5000 km the presence of overtones in the signal allows us to get resolution to depth as large as 600 km for s velocity poorer resolution is obtained in both situations for the attenuation factor s velocity appears furthermore to be more robust than attenuation at depth where good resolution is achieved due to the great sensitivity of surface wave amplitude to departure from the assumption of lateral homogeneity more sophisticated direct modelling would be required to get more confidence in the inverted attenuation models

study of low magnitude seismic events near the novaya zemlya nuclear test site abstract a study of available seismic data shows that all but one of the 42 known underground nuclear explosions at novaya zemlya have been detected and located by stations in the global seismic network during the past 30 years only one seismic event in this area has been unambiguously classified as an earthquake 1 august 1986 m b 4 3 several other small events most of which are thought to be either chemical explosions or aftereffects of nuclear explosions have also been detected since 1990 a network of sensitive regional arrays has been installed in northern europe in preparation for the global seismic monitoring network under a comprehensive nuclear test ban treaty ctbt this regional network has provided a detection capability for novaya zemlya that is shown to be close to m b 2 5 three low magnitude events have been detected and located during this period as discussed in this article 31 december 1992 m b 2 7 13 june 1995 m b 3 5 and 13 january 1996 m b 2 4 to classify the source types of these events has proved very difficult thus even for the m b 3 5 event in 1995 we have been unable to provide a confident classification of the source as either an earthquake or explosion using the available discriminants a study of m b magnitude in different frequency bands shows as expected that the calculation of m b at regional distances needs to take into account source scaling effects at high frequencies thus when comparing a 4 to 8 or 8 to 16 hz filter band to a teleseismic 2 to 4 hz band the smaller events have relatively speaking significantly more high frequency energy up to 0 5 m b units than the larger events this suggests that a p wave spectral magnitude scale might be appropriate the problem of accurately locating small events using a sparse array network is addressed using the 13 january 1996 event which was detected by only two arrays as an illustrative example our analysis demonstrates the importance of using accurately calibrated regional travel time curves and at the same time illustrates how array processing can be used to identify an interfering phase from a local disturbance thereby avoiding location errors due to erroneous phase readings

characteristics of the deforming bed till properties on the deglaciated antarctic continental shelf contemporary ice stream flow is directly linked to conditions at the ice bed interface yet this environment is logistically difficult to access instead we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated antarctic continental shelf we test currently accepted hypotheses surrounding subglacial processes and till properties with a ross sea dataset till shear strengths indicate a continuum of simultaneous processes acting at the bed rather than discrete deformation and lodgement end members we identify a threshold water content representing saturated pore spaces leading to basal sliding and meltwater channelization based on observations of till properties relative to glacial landforms we challenge the assumption that low shear strength is linked to intense deformation spatial variability in landform morphology reflects variability in deforming processes at the sub ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line regional till properties generally correlate with seafloor geology and deglacial history the western ross sea is characterized by higher and more variable shear strengths and water contents while lower shear strength till was preserved in the eastern basin these observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves