crustal and upper mantle electrical conductivity structure in southern scotland summary magnetotelluric measurements were made during 1978 79 at nine sites on a linear traverse across south scotland extending from kinloch rannoch in the highlands to borthwickbrae in the southern uplands both one and two dimensional modelling of the apparent resistivities and phases indicate the existence of a low resisitivity 75 am zone at 20 90 km depth beneath the southern uplands rising closer to the surface beneath the midland valley to the north a zone of slightly higher resistivity 500s2m but still relatively conducting compared to the surrounding structure is shown to exist beneath the highlands the real part of the ratio of vertical to horizontal magnetic fields is shown to be suitably modelled by a combination of induction and conduction

asp an automated seismic processor for microearthquake networks abstract asp a low power in field automated seismic processor of microearthquake network data has been designed fabricated and deployed for initial operation in four different field areas to monitor seismicity associated with two geothermal areas the geysers california and cerro prieto mexico a nuclear explosion with its related collapse and aftershock sequence and late 1 year activity in a major aftershock series livermore california a second asp system with a high speed front end is used in acoustic emission 1 to 20 khz analysis for fracture monitoring each of the 15 channels of asp configured for up to 128 channels automatically detects measures times and amplitudes and computes and fits fft9s for both the p and s waves on data sampled at 100 samples sec these data from each channel are then processed with a central microprocessor for hypocenter location running b values source parameters event count and p wave polarities the system is capable of processing a 15 station detection in approximately 40 sec excluding printout the initial trials have demonstrated that in field real time analysis of data maximizes the efficiency of microearthquake surveys allowing flexibility in experimental procedures with a minimum of the traditional labor intensive postprocessing current efforts are directed toward improving efficiency of computation and data output and in expanding software capabilities

processing of regional seismic events using data from small aperture arrays abstract a regional on line array processing package ronapp has been developed and tested on data from a small aperture seismic array noress at norsar consisting of 12 sensors spaced with distances up to about 2 km the processing package automatically detects and locates regional seismic events in the distance range from a few kilometers to the maximum propagation range of regional s type waves sn or lg the package contains a conventional sta lta detector which works on a set of filtered beams deployed at typical phase velocities of pn and lg sn 8 0 and 4 5 km sec when a detection is found data within a certain time window around the detection time are used for estimates of refined arrival time and signal frequency and a wavenumber analysis is then performed following a proper prefiltering this gives in addition to arrival time and frequency estimates of phase velocity azimuth and signal to noise ratio for each detection following each processed detection an event location based on azimuth and s p travel times is performed if the last detection is identified as an s type wave phase velocity below 6 km sec and a preceding p wave phase velocity above 6 km sec has been detected with about the same direction of approach appropriate procedures are developed to allow for several detections within the p and s coda to keep track of used and unused detections and to identify epicenter locations that override preliminary ones as the analysis progresses through the s wave train of the event the processing package has been tested off line and on line on a number of data intervals for various experimental noress array configurations the automatic processing package provides detection results for regional events that compare favorably with those from the full norsar array and location results that deviate from locations by the fennoscandian network of seismological stations by typically 30 km which is of the order of the location uncertainty of the network

regional monitoring of infrasound events using multiple arrays application to utah and washington state summary in this paper we present an integrated set of algorithms for the automatic detection association and location of low frequency acoustic events using regional networks of infrasound arrays here low frequency acoustic events are characterized by transient signals which may arise from a range of natural and anthropogenic sources examples of which include but are not limited to earthquakes volcanic eruptions explosions rockets and bolides first we outline a new technique for detecting infrasound signals that works successfully in the presence of correlated noise we use an f statistic sequentially adapted to ambient noise conditions in order to obtain detections at a given statistical significance while accounting for real background noise at each array individual arrivals are then grouped together based on measured delay times and backazimuths each signal is identified as either a first or later arrival first arrivals at spatially separated arrays are then associated using a grid search method to form events preliminary event locations are calculated from the geographic means and spreads of grid nodes associated with each event we apply the technique to regional infrasound networks in utah and washington state in utah over a period of approximately 1 month we obtain a total of 276 events recorded at three arrays in a geographic region of 6 4 for four groundtruth explosions in utah the automatic algorithm detects associates and locates the events within an average offset of 5 4 km to the actual explosion locations in washington state the algorithm locates numerous events that are associated with a large coalmine in centralia washington an example mining explosion from centralia is located within 8 2 km of the mine the methodology and results presented here provide an initial framework for assessing the capability of infrasound networks for regional infrasound monitoring in particular by quantifying detection thresholds and localization errors

seismicity and tectonics of the stiegler s gorge area tanzania a microearthquake network with six stations and a diameter of about 50 km has been in operation in the stiegler s gorge area rufiji basin tanzania between 1978 and 1981 the network which used radiotelemetry and digital recording at 30 hz based on a triggering system has recorded a large number of teleseismic and regional earthquakes in addition to several thousand local ones within the network most of the locally recorded earthquakes are in the magnitude range 0 4 measured on a duration magnitude scale which has been developed for the area a scale which also satisfactorily covers regional earthquakes with distance up to 25 an active and well defined seismicity zone has been delineated in the stiegler s gorge area striking n140 e and dipping 45 ne with events distributed between 0 and 40 km in depth the time and space development of the local seismicity shows a fairly stable pattern and a detailed focal mechanism study within the most active zone shows a predominance for normal faulting with east west extensional movements this is consistent with results from local geological studies even though there is no clear correlation between the seismicity and locally mapped faults and it is also consistent with our knowledge of the regional tectonics of the east african rift system

a new constraint on the composition of the topmost continental mantle anomalously different depth increases of p and s velocity summary compared to clear crustal p and s waves as well as a strong refracted pn wave propagating in the topmost mantle i e 10 to 15 km below the moho an anomalously weak appearance of the corresponding refracted shear wave sn is observed on refraction seismic profiles in sw germany france coastal maine ne united states and fennoscandia it is possible that the observation of pn pmp ratios close to unity combined with small sn sms ratios i e small sn amplitudes is typical for the continental uppermost mantle since this was observed for quite different types of continental crust it is shown that the observed phenomenon is not a source effect several possible causes to explain the observations are studied also with the aid of synthetic seismograms the effects of temperature attentuation and anisotropy have been investigated their influence cannot explain the observations the most likely candidate is a depth increasing vp vs ratio an increase in the order of 0 027 over a depth range of 10 km forcing p and s waves to propagate on different paths in the topmost mantle the observed phenomenon provides new constraints on the change of composition with depth for the crust mantle transition zone with basalt depletion as the most likely explanation

sources of short term fluctuations in the seismic noise level at noress short term fluctuations in the seismic noise field at the noress array have been investigated by analyzing the two year output of an automatic short term to long term average sta lta power detector a remarkable feature is the occasional occurrence of a vast increase in the number of noise detections sometimes lasting for several days from slowness analysis the majority of these noise bursts of a few seconds duration are found to originate in the azimuth range 95 105 from noress apparent velocities less than 3 0 km s 1 suggest that they are propagating rayleigh waves the temporal distribution of the noise detections originating in the azimuth range 95 105 shows a strong correlation with the water flow in the nearby river glomma although there is in some cases a time lag of 3 5 days between peak water flow and peak detection rates a detailed analysis of a typical six hour period has revealed the presence of a continuous source generating rayleigh waves with azimuth 99 5 0 8 apparent velocity 2 83 0 03 km s 1 and dominant frequencies between 2 5 and 2 9 hz there is a strong possibility that these rayleigh waves are caused by the operation of a nearby hydroelectric power plant although the source and generation mechanisms of these disturbances have not been explicitly determined

spectral evidence for source multiplicity in explosions application to regional discrimination of earthquakes and explosions abstract spectral and cepstral analysis were performed on regional array noress recordings of mining explosions in scandinavia and russia in order to detect the effects of delayed explosions or ripple firing commonly used in mine blasting practice noress array stacked spectra corrected for instrument and noise were computed in windows on pn pg sn and lg waves from the mine blasts by averaging individual channel spectra frequency by frequency across the array for comparison spectra were also computed for six western norway earthquakes located near the blasjo and titania norwegian mines also stack spectra of p coda sn and lg were computed using norsar seismograms for presumed peaceful nuclear explosions pnes located at near regional distances 10 in western russia the norsar spectra were determined in the same manner as the noress spectra except that an individual source correction was applied to the spectra comparison of western norway mine blast spectra and nearby earthquakes shows that the earthquake pn spectra are peaked in the 8 to 16 hz band with little energy below 6 hz whereas the explosion spectra have flatter spectra from 3 to 16 hz this difference in the pn spectra may be caused by the earthquakes occurring at greater depths than the explosions differences in the near source media or source mechanism effects in the earthquake spectra the main difference between earthquake and explosion spectra is that the explosion spectra exhibit marked scalloping or modulation patterns not observed in the earthquake spectra the modulation patterns are identical in spectra of all phases indicating that they are caused by multiple shot sequences or ripple firing simulated multiple source spectra determined using the noress seismograms for one of the western norway earthquakes resulted in spectra that resembled many of the explosion modulation patterns based on the simulation studies and cepstral analysis of the spectra the mine explosions appear to be composed of two to three separate explosions delayed by between 80 to 150 msec moreover the norsar spectra of pnes indicate that these signals are also produced by multiple explosions although they have larger delay times of between 0 9 to 1 5 sec these results show that spectral modulations in seismogram spectra of economic explosions including mine explosions and nuclear explosions fired for peaceful economic purposes can be used to distinguish them from earthquakes and nuclear explosion weapons tests

electrical resistivity structure of the senegal basin as determined from magnetotelluric and differential geomagnetic soundings summary the methods of magnetotelluric mt and differential geomagnetic soundings dgs have been applied to study the electromagnetic response of the structure of the senegal sedimentary basin the measurements at 10 sites were carried out along a profile running perpendicularly across a zone of north south flexures and faults over the period range 10 1000 s variation of periods up to 10000 s were obtained at two sites the anomalous geomagnetic variation field across the major fault zone is characterized by an increase in the magnetic eastward component two dimensional modelling of the apparent resistivities and phases reveals a highly anomalous upper crustal structure involving a low resistivity zone in the central part of the basin less than 30 am to satisfy the additional long period data at two sites a deep conductor is also required at a depth below 300 km with a resistivity less than 10 am the crustal discontinuity located 200 km off the coast is possibly related to the opening of the atlantic ocean the difficulty in resolving the question of the nature of the crust in this region results in part from the limitations of the methods used at present for this determination

deep crustal geoelectric structure beneath the northumberland basin in terms of lateral variations in conductivity structure the southern southern uplands and northumberland basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor the response data are used to investigate the deep geoelectric crustal structure of the region three appropriate sets of response data have been subjected to two construction algorithms for 1 d inversion the geoelectric profiles recovered identify a deep crustal conducting zone underlying the northumberland basin the zone modelled as a layered structure dips steeply from mid crustal depths underneath the northumberland basin to lower crustal depths to the nw the structure thus correlates in location and geometry with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile

classification of local and regional events in central europe based on estimates of s wave spectral variance summary the vogtland region in the border region of germany and the czech republic is of special interest for the identification of seismic events on a local and regional scale since both earthquakes and explosions occur frequently in the same area and thus are relevant for discrimination research for verification of the comprehensive nuclear test ban treaty previous research on event discrimination using spectral decay and variance from data recorded by the geress array indicated that spectral variance determined for the s phase for the seismic events in the vogtland region seems to be the most promising parameter for event discrimination because this parameter provides for almost complete separation of the earthquake and explosion populations almost the entire set of vogtland events used in this research and more than 3000 local events detected in germany in 1998 and 1999 were analysed to determine spectral slopes and variance for the p and s wave windows from stacked spectra of recordings at the geress array the results suggest that small values for the spectral variance are associated not only with earthquakes in the vogtland region but also with earthquakes in other parts of germany and neighbouring countries while mining blasts show larger spectral variance values mininginduced events yield a wide range of values for example in the lubin area a threshold based identification scheme was applied almost all events classified as earthquakes are found in seismically active regions while the earthquakes are uniformly distributed throughout the day events classified as explosions correlate with normal working hours which is when blasting is done in germany in this study spectral variance provides good event discrimination for events in other parts of germany not only for the vogtland region showing that this identification parameter may be transported to other geological regions

electromagnetic evidence for lateral inhomogeneities within the earth s upper mantle abstract the composition of the upper mantle is of great significance to our understanding of plate tectonics and global evolution information about the physical properties of the earth at upper mantle depths including lateral variations in electrical conductivity can be deduced from measurements of the electric and magnetic fields at the earth s surface electromagnetic methods appear to give poorer resolution than do some other methods for example seismics but as they are sensitive to quite different properties of a medium they provide a different and complementary class of information the basic theory of electromagnetic sounding methods is briefly reviewed below and evidence regarding lateral conductivity inhomogeneities in the earth s upper mantle is examined while lateral electrical conductivity inhomogeneities appear to be the rule rather than the exception the interpretation of electromagnetic data still presents difficulties and the results from many regions are not as yet unambiguous where the data are of sufficient resolution a rapid increase in electrical conductivity can usually be identified within the upper mantle the depth to this highly conductive zone is different in different tectonic environments but is broadly consistent between analogous but widely separated tectonic environments a comparatively shallow conducting region is found beneath the ocean lithosphere the depth of this region is dependent on lithospheric age many of the more shallow conducting regions in both continental and oceanic environments are associated with high heat flow values and seismic low velocity zones these highly conducting regions may be zones of partial melt

hypocenter constraint with regional seismic data a theoretical analysis for the natural resources defense council network in kazakhstan ussr source depth and epicenter determination remains a simple but key method for discrimination between earthquakes and nuclear explosions at near regional distances a variety of secondary body wave arrivals can be observed that may be useful for constraining source depth and epicenter in this paper we study the source location problem using regional arrival time and arrival azimuth data from a sparse seismic network a theoretical analysis of the source location inverse problem provides estimates of the stability and uncertainty of the solution for a particular suite of data as a function of epicenter and depth as well as evaluating the importance of each datum to the solution an empirical analysis with actual data complements the theoretical analysis our approach is applied to the three station natural resources defense council soviet academy of sciences network in kazakhstan the theoretical analysis indicates that reasonable constraint on depth and epicenter is possible whenever three distinct phases p pn pg pzp and pmp or equivalent s phases are observed at two or three stations our theoretical analysis for two station locations shows that the epicenter constraints are extremely poor for sources along the line passing through the two observing stations the use of arrival azimuth in two station locations results in improvement in epicenter constraint for events occurring in the vicinity of the line connecting the two stations but there is no significant improvement elsewhere an empirical analysis of data from two of the september 1987 chemical explosions and data from an explosion in an active mining area supports the theoretical analysis showing that two station locations both epicenter and depth are quite reliable furthermore the theoretical analysis also indicates that even with data from a single observing station satisfactory results are obtained if three or more phases plus arrival azimuth are available

source parameters of seismic events at heinrich robert mine ruhr basin federal republic of germany evidence for nondouble couple events abstract a three component digital seismic station is in operation at heinrich robert coal mine in the ruhr basin at a depth of 890 m a cluster of seismic events occurred at the beginning of 1987 at a distance of about 5 km from the station and continued in 1988 source parameters of 46 tremors selected from the cluster with local magnitude from 1 2 to 2 3 are studied the ratio of p wave to s wave corner frequency is variable implying its dependence on the angle of observation and in turn the variability of focal mechanism with time the scaling relations indicate that characteristic tremors with a fault length of about 200 m are possibly generated in the area but the evidence is not conclusive the seismic energy for a given seismic moment can vary by a factor of 20 and the ratio of s wave to p wave energy ranges from 1 5 to 30 stress release estimates show low values not exceeding a few bars typical for tremors induced by mining a comparison of the apparent stress with the static stress drop divides the selected tremors into high and low apparent stress events the latter events are characterized by high p wave energy and almost constant and large ratio of p wave to s wave corner frequency which differs from the lower ratios of other events implying similar focal mechanism these high p wave energy tremors are the most likely candidates for nondouble couple events

the interpretation and tectonic implications of the geoelectric structure of southern scotland summary recent magnetotelluric observations in a traverse across southern scotland from the grampian highlands across the midland valley to the southern uplands have led to a two dimensional model in which layers of anomalously high conductivity exist at varying depths along the traverse from available heat flow measurements and thermal conductivity estimates of probable crustal and upper mantle materials temperature depth curves have been derived for the three regions and these in turn have been used to interpret the electrical conductivity in terms of the possible causes of high conductivity in the highlands and midland valley the conducting zones have been identified with ionic or electronic conduction in hydrated rocks and the process of dehydration at the boundary between amphibolitic and granulitic facies the increase in depth of the conducting zone towards the north lends support to the hypothesis that the highland boundary fault is the location of an ancient subduction zone the thick conductor under the southern uplands is also interpreted as arising from both electronic conduction in hydrated rocks and dehydration at the amphibolitic granulitic transition but with the additional possibility of a partial melt zone at depths of about 70 km

a new three component detector and automatic single station bulletin production a new three component 3c seismic signal detector is introduced that has enabled us to produce a preliminary single station bulletin in an automatic mode we consider automatic station bulletins as very important because in the high frequency range local events mostly chemical explosions like quarry blasts are truly numerous say 20 to 50 per day and cannot be handled efficiently by an analyst in the conventional way a short term average long term average sta lta test statistic is used either alone or in combination with 3c signal polarization for p phases which allows a lowered threshold an added advantage with the polarity estimate is that a corresponding slowness vector estimate is obtained the detector is operated in three consecutive steps 1 the detector is run on a suite of bandpass filtered records 2 the initial detection log is compressed while retaining parameters from the best filter and 3 the detected and identified phases are grouped into event families and epicenter parameters are estimated the 3c detector has been tested on 6 days of recordings from the station c4 in the noress array located in se norway and results were compared to the array bulletin for teleseismic signals the array has an advantage in snr of about 15 db for local and regional events distance 1500 km the 3c snr in the frequency range 5 to 15 hz is about 6 to 8 db below that of the array the 3c detector picks practically all s phases detected by the array but occasionally misses the preceding pn however some new events missed by the array were also found at local and regional distances the 3c azimuth and distance estimates are generally within 20 and 20 km respectively of the noress solution and the origin time estimates are within a few seconds the 3c epicenter solutions are less stable than that of the array mainly due to phase identification problems and pn azimuth errors at low snr naturally the benefit of analyst screening and re processing would improve the 3c bulletin quality although the performance is considered satisfactory at the present stage of development

teleseismic event detection using the noress array with special reference to low yield semipalatinsk explosions abstract the small aperture noress array in norway has been designed for improving signal to noise ratios at high frequencies 1 to 15 hz while the main motivation for this has been to enhance the capabilities for detecting and characterizing weak seismic events at regional distances it has been found that the array is also very effective in the teleseismic distance range in particular for eurasia this results from a combination of two factors 1 due to high q paths eurasian earthquake and explosion recordings generally show high dominant frequencies typically in the range of 1 5 to 4 0 hz where the noise level at the site is low and 2 the array provides a particularly high signal to noise ratio gain of 12 to 14 db 0 6 to 0 7 m b units at these frequencies analysis of noress on line detection performance shows that there are large regional variations even within small epicentral areas this is interpreted as resulting mainly from signal focusing effects underneath the source areas in analogy to the receiver focusing effects earlier observed across large arrays a detailed study of noress recordings of underground nuclear explosions at the semipalatinsk test site has shown that relative to worldwide m b noress has a significant positive m b bias it is highest 1 0 m b units for the eastern part shagan river and somewhat lower 0 4 m b units for the western parts degelen konystan although there is a fair amount of scatter within each area analyzing observed signal to noise ratios at the noress site for a set of low yield nuclear explosions at semipalatinsk with published yields available in the soviet literature it is estimated that fully coupled explosions of yields as low as 0 1 kt would be detectable by noress under normal noise conditions however to give a precise threshold is difficult because of the low number of reference events available and the significant amount of extrapolation involved it is emphasized that this detection level will not be achieved in cases when the noise level is abnormally high e g in the coda of a large earthquake or if coupling conditions are not optimal e g in the case of full or partial cavity decoupling it must also be noted that the event identification threshold is necessarily higher than the signal detection threshold

detection and characterization of seismic phases using continuous spectral estimation on incoherent and partially coherent arrays summary seismic arrays are employed in the global monitoring of earthquakes and explosions because of their superior ability to detect and estimate the direction of incident seismic arrivals traditional beamforming and f k analysis require waveform semblance over the full array aperture and cannot be applied in many situations where signals are incoherent between sensors the norsar and mjar arrays are two primary ims stations where this is the case for high frequency regional phases large intersite distances and significant geological heterogeneity at these arrays result in waveform dissimilarity which precludes coherent array processing in the frequency bands with optimal snr multitaper methods provide low variance spectral estimates over short time windows and seismic arrivals can be detected on single channels using a non linear spectrogram transformation which attains local maxima at times and frequencies characterized by an energy increase this detection procedure requires very little a priori knowledge of the spectral content of the signal the transformed spectrograms can be beamformed over large aperture arrays or networks according to theoretical time delays resulting in an incoherent detection system which does not require waveform semblance at any frequencies we outline a real time automatic detection system for regional phase arrivals on the norsar array and demonstrate how stable and accurate slowness and azimuth estimates can be obtained for quite marginal signals in the case of partially coherent arrays the procedure described may provide stable if low resolution estimates which can subsequently be refined using coherent processing over subsets of sensors in particular we illustrate how the spectrogram beamforming method facilitates a stable and accurate slowness estimate for the incoherent high frequency pn arrival at the mjar array in japan from the 2006 october 9 underground nuclear test in north korea

spatial noise field characteristics of a three component small aperture test array in central italy in order to evaluate detection and localization capabilities of a future array in the upper tiber valley northern apennines italy an irregularly configured test array was installed near citta di castello cdc for a period of two weeks consisting of nine sites with intersensor distances between 150 and 2200 m this test array installation is particular in its use of three component sensors at all array sites which allows the application of all array specific analyses techniques for the full seismic wave field that is also for horizontal component data in this study we investigate the intersensor coherence of the seismic noise field for the test array area in addition to the classic noise analysis where noise cross correlation values are calculated at single vertical instruments without relative time shifts between the traces we extend the study by a dynamic approach which accounts for possible slowness characteristics of the noise field furthermore we investigate how the noise characteristics are dependent on the chosen component of the seismic sensors by analyzing the noise coherence not only between vertical components but also on the radial and the transverse components the coherence found for noise observed by the different sensors of the test array shows strong azimuthal variation on all components which is most pronounced for noise within the frequency passband of 1 5 4 0 hz and an apparent velocity of 1 5 km sec rg waves the calculated correlation lengths of noise observed for the cdc array are about half of the values found for the nores array in southern norway therefore a future permanent array installation should be planned for minimum intersensor distances between 150 and 200 m

magnetic variation anomalies in northern england and southern scotland single station transfer functions linking the time variations of the vertical and horizontal components of the magnetic field at stations in northern england and southern scotland have been compiled into a uniform data set from hypothetical event analysis we show here that there are two distinct anomalies in the borders region one runs south west to north east immediately to the south east of the southern uplands fault the second follows the northumberland basin and seems to exist because the conductive sedimentary rocks filling the basin create a link between the irish and north seas if the lapetus suture is marked by a conductivity anomaly as has been suggested1 2 these results place it beneath the southern uplands unless it is masked by the surface conductor in the northumberland basin

average crustal travel times in sweden re examined a long range scandinavian seismic profile of august 1979 is subjected to a preliminary evaluation by means of the records of the permanent swedish network of stations the resulting travel times confirm those earlier adopted in the calculation of swedish earthquakes for the period 1951 1976

seismo and neotectonics in finnmark kola and the southern barents sea part 2 seismological analysis and seismotectonics abstract a detailed review and analysis of seismological data acquired since 1979 from finnmark kola and the southern barents sea has provided a much improved delineation of the earthquake activity in that region in the barents sea the seismicity is confined only to the western areas while the finnmark kola coastal areas show some seismicity only north and east of murmansk in the rybachi cape tereberski region further inland there are several clear seismicity lineations that to some extent can be correlated with postglacial faults including the ne sw striking stuoragurra fault in finnmark the spatial correlation between seismicity and mapped faults is interpreted in general as a reflection of broad zones of weakness that respond to the present day stress field a detailed analysis of the earthquakes in this area has also shown a dynamic consistency in that they reveal a reverse faulting pattern that is consistent with a detailed geological study that includes a core drilling the dominant horizontal stress direction in this region is nnw sse consistent with the ridge push effect

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

investigations of sn and pn phases in the area of the upper rhine graben and northern switzerland summary 22 regional earthquakes in sw germany and northern switzerland recorded with the southern black forest digital seismic network are studied for pn and sn phases in contrast to the results of some seismic refraction studies a large number of these earthquake recordings show clearly correlatable sn phases so far there are two remarkable trends concerning the appearance or disappearance of sn 1 earthquakes with large focal depths z 20 30 km preferentially show sn phases 2 earthquakes from the sw with rays crossing the southern rhine graben show no sn energy regardless of focal depth to investigate the amplitude behaviour of the sn phase synthetic seismograms for varying focal parameters as well as for different structural parameters submoho velocity gradient and attenuation of s waves are calculated using the reflectivity method the synthetics show that the sn amplitude is sensitive especially to the fault plane orientation and the submoho velocity gradient after correction for the radiation pattern the missing of the sn phase from earthquakes in the south western part of the studied area seems to be associated with structural effects in the uppermost mantle a small submoho s wave gradient or a high upper mantle attenuation are the most likely explanations another very interesting observation relates to the pn velocities the azimuthal variation of the velocity as well as the high maximum pn velocity v 8 6 8 8 km s 1 itself point to an anisotropic uppermost mantle the velocity variation of 0 6km s 1 and the directions of minimum and maximum velocities are in accordance with earlier results from seismic refraction experiments and their petrological interpretation the derived sn velocities have a mean value of 4 71 km s 1 up to now the quality and number of the sn recordings do not allow us to resolve a possible s wave anisotropy of the uppermost mantle

azimuth estimation capabilities of the arcess regional seismic array abstract the effect of local geology and noise conditions on the performance of a small regional array is investigated by comparing the regional p n backazimuth estimation capabilities of the arcess array in northern norway to the noress array a broadband frequency wavenumber estimator was used to calculate backazimuths from the p n arrival for each of 203 regional events recorded at arcess while varying element spacing frequency band and time window most of the errors in backazimuth are less than 20 when appropriate parameter combinations are used and mean backazimuth errors are close to zero the best results are obtained using a 13 element configuration that has a 1 4 km aperture and a maximum station spacing of about 600 m with the 13 element configuration and the data filtered to include frequencies between 3 and 10 hz the mean errors for the 203 event data set are less than 0 9 and s d are as small as 16 9 there are differences seen in the backazimuth estimation capabilities of arcess and noress with specific parameter combinations the larger aperture configurations 10 and 17 elements have smaller means at arcess although the precision is about the same the estimates using unfiltered data at arcess are poor because of local noise conditions that increase the level of background noise at low frequencies overall the precision is better at noress but both regional arrays have the best results using the 13 element configuration and filtering the data in the middle frequency range 3 to 10 hz other factors investigated include snr and source region backazimuth estimation statistics improve if only events with 5 db of snr are included in the data set at both arcess and noress the mean errors move closer to zero and standard deviations decrease the differences between the two arrays are not as pronounced there are some path effects from different source regions around the arcess array however combinations of small aperture configurations and middle 3 to 10 hz frequency bands work well for events over the entire distance range of 30 to 1200 km arcess and noress have similar backazimuth estimation capabilities even though there are differences in the local geology and noise conditions because a 13 element configuration produces reliable results for both arrays it would be reasonable to reduce the number of elements in a regional array this in turn will reduce the costs associated with building and deploying small regional arrays

reply to comment by t c birkett and r r miller on the role of hydrothermal processes in the granite hosted zr y ree deposit at strange lake quebec labrador evidence from fluid inclusions