Pg Waves Research Articles

Source Spectra of Pg and Lg Waves from North Korean Nuclear Tests Estimated Using a Nonmodel-Based Approach

ABSTRACT In this article, we demonstrate that source spectra for regional seismic waves generated by an underground explosion may be uniquely extracted using a simple linear regression method between the geometrical spreading corrected displacement spectra and epicentral distances as long as the recording stations are well distributed in distances. Using this method, source spectra of Pg and Lg waves from North Korean nuclear tests conducted between 2009 and 2017 are estimated, respectively, and are compared to test the conjecture of Fisk (2006) as well as to determine the relative size between the seismic moment of Lg waves and that of compressional waves. For a specified explosion, the resultant Pg and Lg source spectra approximately have equal low-frequency source level. Based on this result, it is estimated that for the explosions seismic moment of Lg waves would be about 35% of that of compressional waves. In addition, it is shown the Fisk conjecture approximately stands, as for a specified explosion the observed Pg and Lg source spectra share the same manner of high-frequency rolloff as f−2, the source corner frequency of Lg waves is smaller than that of Pg waves by a factor of about 1/3, and the Pg source spectrum exhibits a prominent overshoot, whereas it is not obvious in corresponding Lg source spectrum. The new method for source spectra estimation may have a promising application prospect in monitoring studies. As an example, it is shown that the yield and even buried depths of the North Korean nuclear explosions may be reasonably estimated by fitting the observed Pg source spectra using an MM71- (Mueller and Murphy, 1971) and DJ91 (Denny and Johnson, 1991)-based hybrid source model suggested by Ford and Walter (2013).

High‐Precision Relocation With the Burial Depths of the North Korean Underground Nuclear Explosions by Combining Pn and Pg Differential Traveltimes

AbstractIn seismic nuclear monitoring, an accurate source depth is an important prerequisite for reliably estimating the explosive yield. Relative location methods are often used for this purpose. However, the conventional method based on regional Pn waves usually fails to give satisfactory constraints on the source depth, mainly due to the strong trade‐off between the burial depth and origin time. This study explores a high‐precision relative location method to simultaneously determine the relative epicenter and source elevation by using differential traveltimes from both downward‐takeoff Pn waves and horizontal‐takeoff Pg waves. The properties of both Pg and Pn waves, including the consistency between waveforms, the reliability of differential traveltime measurements, and their sensitivities to epicenter and depth variations, are investigated. By jointly applying both types of waveform data, the proposed method significantly enhances the constraint on the source depth variation. This method is applied to regional seismic data collected from China, South Korea, and Japan to determine the relative epicenters, origin times and relative burial depths of six North Korean underground nuclear explosions. The source depths are then used to provide burial‐depth corrections to estimate the explosive yields. The depths of the six North Korean nuclear explosions detonated on October 2006, May 2009, February 2013, January 2016, September 2016, and September 2017 are determined to be 330, 540, 506, 468, 521, and 570 m, respectively, and their yields after burial depth corrections are 1.6, 5.7, 13.4, 12.6, 21.7, and 225.7 kt, respectively.

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years, this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions, such as offshore basins, west Shandong uplift, Tanlu fault zone and Jiangsu-Shandong orogenic belt. The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments. The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin; the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin. The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface, which reflects the high crustal velocity and stable structure in west Shandong uplift. The Pm and Pl wave are obviously complicated, which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt. The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale. Based on many geophysical phenomena such as electrical structure, density structure and terrestrial heat flow, it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

Study on the basement structure in the southeastern North China by blasting seismic

Since the Mesozoic, southeastern North China Craton has experienced intense crustal thinning and lithosphere destruction. Some of the responses of the deep activity in the upper crust crystalline basement have been retained in a series of tectonic evolution. The study of the upper crust velocity structure, especially the properties of the basement interface, is of great significance for studying the tectonic evolution and seismic hazard in the southeastern part of North China. In this study, we selected Pg waves of the blasting seismic data in the southeastern part of North China in recent years, which reflect the west Shandong uplift, offshore sedimentary basins and the Tanlu Fault zone and the Sulu orogenic transition zone, to study the structural and seismological characteristics of basement in North China Craton. The results of this study showed as follows: First, the obvious lag of Pg wave arrival time in Dongying depression and North Jiangsu basin reveals the thick sedimentary, low velocity and unstable basement structure. Second, the advance Pg wave arrival time with high apparent velocity, which reflects the basement structure of the west Shandong uplift, indicates the thin sediments and the shallow basement. Third, combined with many geophysical phenomena, such as electrical structure, density structure and terrestrial heat flow, we hold that the Tanlu tectonic belt and the Sulu orogenic belt have experienced great lithosphere destruction and there is shallow crust and the thinnest lithosphere in the vicinity of the Tanlu fault zone.

Velocity structure of the western North China basement from high-resolution wide-angle reflection/refraction profiles

Two 1420 km-long, high-resolution wide-angle reflection/refraction profiles were produced for the western part of the North China Craton. Twenty-one shots were fired along the two profiles and seismic data was collected by 700 portable three-component digital seismographers. The crust-mantle structure was investigated in different regions, including the Ordos Block, Haiyuan arcuate tectonic region, Alashan Block, and Yinchuan Basin. First-arrival Pg waves were analyzed using the finite-difference method of first-arrival traveltime tomography to construct the 2D interface morphology and velocity structure of the crystalline basement. The study identified the location and shape of major faults, and notable variations in basement depth and velocity structure in different regions. The crystalline basement in the Ordos Block is typically stable with a depth of 4.5–5.5 km and exhibits only minor fluctuations in the lateral velocity gradient. This is evidenced by Pg wave features, including stable travel time, long tracing distance and high velocity. Between the Helan Mountains and the Yinchuan Basin, notable variations in topography, basement depth, and velocity structure were identified, indicating strong divergence in vertical movement and deformation. In the Yinchuan Basin, the Pg wave traveltime curve revealed the complex structure of the crystalline basement and variations in the velocity gradient. Since the Cenozoic Era, the western North China Craton has experienced strong and frequent tectonic activity causing strong deformation on the periphery of the Alashan Block. The presence of small, low-velocity, near-surface basins, and variations in the crystalline basement interface and velocity isolines provides evidence of strong tectonic deformation of the basement near Bayan Ura Hill, on the eastern margin of the Alashan Block. The Haiyuan arcuate tectonic region is complex and interacts with the Qilian Mountains, Alashan Block, and Ordos Block. Basement depth extends to 6.0 km and varies considerably from the adjacent uplift zone, which has a depth of 2.0 km. The highly deformed and unstable basement structure at the northeastern margin of the Tibetan Plateau is probably related to the collision and squeezing of the Indian Plate and the Eurasian Plate. Seismology data indicates differences between the crystalline basement interface and the velocity structure, as well as strong transverse inhomogeneity. Differences in basement depth in the Ordos Block, together with significant lateral heterogeneity, compressional fold deformation, and the presence of fault zones in the marginal basement, block the continuous compressive movement of the Tibetan Plateau in a northeast direction.

Spatio-temporal variations in the structure of the attenuation field of the S-wave in the region of Nevada nuclear test site

The characteristics of the attenuation field of short-period shear waves in the region of Nevada nuclear test site (NNTS) are studied. The seismograms of underground nuclear explosions (UNEs) and earthquakes recorded by three seismic stations in 1975–2012 at the epicentral distances of up to 1000 km are processed by the methods based on the analysis of the amplitude ratios of Sn to Pn and Lg to Pg waves, as well as the S-coda envelopes for close events. It is shown that the structure of the attenuation field in the Earth’s crust and upper mantle in the NNTS region experienced significant temporal variations during the interval of nuclear operations. The strongest variations were associated with UNEs conducted in the Pahute Mesa area, which held about two-thirds of the most intense explosions. Our data indicate that temporal variations in the structure of the attenuation field are related to the migration of deep fluids. A comparison of the general characteristics of the attenuation field in the regions of the three large nuclear test sites is presented.

Depth determination of small shallow earthquakes in eastern Canada from maximum power Rg/Sg spectral ratio

For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined focal depth caused by the error in the epicentral distance is in the order of 0.1 km. The error in the focal depth when using a default focal mechanism is in the order of 0.5 km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies show that the MPSR method can be used to successfully determine focal depths for events as small as m N 1.6.

A simple smoothed velocity model of the uppermost Earth’s crust derived from joint inversion of Pg and Sg waves

The paper presents some results of seismic experiments carried out on the territory of northern Moravia and Silesia, roughly delimited by the coordinates 16°E–19°E and 49°N–51°N. The experiments were aimed at compiling a velocity model of the uppermost Earth’s crust using the database of arrival times of Pg and Sg waves recorded at a fairly large number of seismic stations, which enabled us to produce a simple 1D-layered velocity model of the region. The velocity model was computed using the traditional tomographic iterative process composed of consecutive solutions of linear equations. Based on the analysis of velocity distribution, it was found that the velocities of Pg and Sg waves increase from about 5.9 and 3.3 km/s at the surface, to about 6.1 and 3.5 km/s at a depth of 11 km, respectively.

Recenti risultati sulla velocità delle onile sismiche in Italia Centrale

The seismic phenomena which happened near Cerreto di Spoleto (December 1974) have been studied by data collected from the National Network and the seismic mobile station. The latter had been placed in the epicentral zone. As we could know good hypocentral parameters, we determined travel times for Pg, Sg, Pn and Sn waves. For short travel distances it lias been possible to point out strong azimuthal velocity anomalies, connected with systematic errors in the epicentral determination. This work allows us to foresee the possibility of improving (lie knowledge of the crustal characteristics in our Country, as far as recording techniques will improve.

Joint inversion for crustal and Pn velocities and Moho depth in Eastern Margin of the Tibetan Plateau

A classical problem in seismic tomography is the trade-off between a discontinuity location and the velocity of the medium. For that reason, crustal tomography and Pn tomography are typically done separately by isolating first-arriving Pg and Pn waves. Although this avoids the contamination of secondary arrivals because of the existence of the Moho, lots of good data are discarded and, as a result, the lower crust is particularly poorly constrained. Here we design an iterative scheme to invert jointly for the velocities of the whole crust and the Pn waves as well as the Moho depth. We use a spherical pseudo-bending ray tracing method for the heterogeneous crust and include secondary Pg waves at large distances. We applied the method to the eastern and southeastern margin of the Tibetan Plateau, where this method is particularly useful because of the rapid variation of crustal structure and Moho depth. Our data include arrival times from Chinese provincial and national earthquake bulletins and our own hand picks. The results show significant features, including slow upper crust but fast lower crust under Sichuan Basin, rapid Moho variation, and slow Pn in the plateau and fast Pn in the borderland separated by major boundaries and faults. The mid–lower crust in the plateau is relatively slow compared to the average model and Sichuan Basin and Poisson's ratio is anomalous (exceeding 0.3), which is consistent with weak mid–lower crust in the plateau.

Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole, Structural Interpretation Supported by Magnetic, Gravity and Petrophysical Data

The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01 profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels, were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the “slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity image down to 15 km. Although the contact of Saxothuringian and the Tepla-Barrandian Unit (TBU) is collateral with the S01 profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular, the Moldanubian and TBU contact beneath the Western Krusne hory/Erzgebirge Pluton, the buried contact of the Lusatia unit and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs beneath the Doupovske hory and the Ceske středohoři volcanic complexes. The intermediate mid-crustal P-wave velocity lows are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic and petrophysical data.

Local crustal structures of southern Korea from joint analysis of waveforms and travel times

A joint inversion technique of waveforms and travel times is applied to broadband seismic data from a local earthquake in order to estimate local crustal velocity structures of southern Korea. Combining the waveform and travel time inversion techniques, we can surmount the demerits of the techniques: the distortion of deep velocity structure in the waveform inversion caused by low signal-to-noise ratio (SNR), the velocity-depth trade-off, and errors in picking phases in the travel time inversion. The purpose of this study is not only for verifying whether the technique is performed well in the local areas where the number of stations is limited, but also for estimating the velocity structures of the areas that have been little investigated. We adopted the genetic algorithm (GA) as a search algorithm, since we could not expect appropriate initial models due to little a priori information about crustal velocity structure. Both broadband waveforms bandpassed between 0.05 Hz and 0.3 Hz and travel times of Pg, Pn, and PmP waves from the 26 April 2004 Daegu earthquake (ML=3.9) were used as input data. We performed the joint inversion ten times or more for each local area, and adopted the averaged model of optimal models to acquire credible crustal structure. Synthetic waveforms and travel time curves obtained from the estimated velocity models were generally agreed with observed seismograms, and the estimated source depths from the velocity models of the three local areas are similar to and consistent with each other. Therefore, we believe that the joint inversion technique is still applicable to local areas where the number of stations is limited.

Upper crust structure of eastern A’nyemaqên suture zone: Results of Barkam-Luqu-Gulang deep seismic sounding profile

Barkam-Luqu-Gulang deep seismic sounding profile runs from north of Sichuan Province to south of Gansu Province. It is located at the northeastern edge of Tibetan Plateau and crosses eastern A’nyemaqen suture zone. The upper crust structures around eastern A’nyemaqen suture zone and its adjacent area are reconstructed based on the arrival times of refracted Pg and Sg waves by using finite difference method, ray tracing inversion, time-term method and travel-time curve analysis. The results show that the depth variation of basement along profile is very strong as indicated by Pg and Sg waves. The basement rose in Zoige basin and depressed in eastern A’nyemaqen suture zone, and it gradually rose again northward and then depressed. The results also indicate that eastern A’nyemaqen suture zone behaves as inhomogeneous low velocity structures in the upper crust and is inclined toward the south. Hoh Sai Hu-Maqen fault, Wudu-Diebu fault and Zhouqu-Liangdang fault are characterized by low velocity distributions with various scales. The distinct variation in basement depth occurred near Hoh Sai Hu-Maqen fault and Zhouqu-Liangdang fault, which are main tectonic boundaries of A’nyemaqen suture zone. Wudu-Diebu fault, located at the depth variation zone of the basement, possibly has the same deep tectonic background with Zhouqu-Liangdang fault. The strongly depressed basement characterized by low velocity distribution and lateral inhomogeneity in A’nyemaqen suture zone implies crushed zone features under pinching action.

Application of stacking and inversion techniques to three-dimensional wide-angle reflection and refraction seismic data of the Eastern Alps

SUMMARY We present new methods for the interpretation of 3-D seismic wide-angle reflection and refraction data with application to data acquired during the experiments CELEBRATION, 2000 and ALP 2002 in the area of the Eastern Alps and their transition to the surrounding tectonic provinces (Bohemian Massif, Carpathians, Pannonian domain, Dinarides). Data was acquired on a net of arbitrarily oriented seismic lines by simultaneous recording on all lines of seismic waves from the shots, which allows 2-D and 3-D interpretations. Much (80%) of the data set consists of crossline traces. Low signal to noise (S/N) ratio in the area of the young orogens decreases the quality of travel time picks. In these seismically heterogeneous areas it is difficult to assign clearly defined arrivals to the seismic phases, in particular on crossline record sections. In order to enhance the S/N ratio, signal detection and stacking techniques have been applied to enhance the Pg-, Pn- and PmP phases. Further, inversion methods have been developed for the interpretation of WAR/R-data, based on automated 1-D inversion (Pg) and the application of the delay time concept (Pn). The results include a 3-D velocity model of the crust based on Pg waves, time and depth maps of the Moho and a Pn-velocity map. The models based on stacked data are robust and provide a larger coverage, than models based on travel time picks from single-fold (unstacked) traces, but have relatively low resolution, especially near the surface. They were used as the basis for constructing models with improved resolution by the inversion of picks from single-fold data. The results correlate well with geological structures and show new prominent features in the Eastern Alps area and their surrounds. The velocity distribution in the crust has strong lateral variations and the Moho in the investigation area appears to be fragmented into three parts.

Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment

We study the azimuthal velocity variation of Pg waves in the Moldanubian, which is a crystalline segment within the Bohemian Massif in the Czech Republic. We use the data from a multi-azimuthal common-shot experiment performed as part of the ALP 2002 refraction experiment, complemented by profile refraction data from the CELEBRATION 2000 experiment. We analyze the travel times of waves recorded by 72 portable seismic stations deployed along two circles with radii of 35 and 45 km around a shot. The observed travel times display an azimuthal variation indicating anisotropy of 2%. The minimum and maximum velocity values are 5.83 and 5.95 km/s, respectively. The direction of the maximum velocity is ∼N50°E. These values characterize horizontal anisotropy of the uppermost crust down to 3 km. The strength and orientation of uppermost crustal anisotropy in the Moldanubian is consistent with the overall upper crustal anisotropy in the entire Bohemian Massif. The high-velocity direction is roughly perpendicular to the present-day maximum compressive stress in the Bohemian Massif and Central Europe and coincides with the orientation of structures formed by the main Variscan tectonic events in the area. This indicates that the anisotropy is caused predominantly by alignment of textural elements and minerals in the rocks, which developed in early geological stages rather than by a preferred orientation of cracks or microcracks due to present-day stress. If the crack-induced anisotropy is present in the medium, then its strength should not exceed 1% and the cracks should be water saturated.

Crustal anisotropy in the Bohemian Massif, Czech Republic: Observations based on Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000

We study the azimuthal velocity variation of Pg waves in the Bohemian Massif using data collected during Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. We analyze travel times of waves generated by 28 shots and recorded by 256 portable and 19 permanent seismic stations deployed on the territory of the Czech Republic and in adjacent areas. We use recording offset ranging from 30 to 190 km with azimuths covering the whole interval of angles. The observed travel times are inverted for parameters of a velocity model formed by an isotropic low‐velocity subsurface layer with a varying depth lying on a homogeneous transversely isotropic half‐space with a horizontal axis of symmetry. The recovered velocity displays a systematic azimuthal variation indicating a regional‐scale intrinsic or effective anisotropy in the Bohemian Massif. The mean, minimum and maximum values of the velocity are vmean = 6.03 km/s, vmin = 5.98 km/s, vmax = 6.10 km/s, respectively, indicating an anisotropy of 1.5–2.5%. The direction of the maximum propagation velocity is ∼N35°E being approximately perpendicular to the present maximum compression in the Earth crust in central Europe. The observed anisotropy cannot be induced by stress‐aligned cracks in the crust, because the crack models predict azimuthal velocity variations completely inconsistent with the observed one. Therefore we suggest the crustal anisotropy to be induced by a preferred orientation of rock‐forming minerals and large‐scale intrusion fabrics developed during a tectonic evolution of the Bohemian Massif.

A Study on Velocity Distribution in Upper Crust and Tectonics of Basement and Cover in South China

AbstractFive representative wide‐angle seismic profiles in South China are analyzed to study the fine velocity structure. The interpolation was made by fitting the travel times of first arrive Pg waves, and combining with other geophysical data and geological data, the synthesis explanations for the tectonics of basement and cover are performed. It is proved that some fault basins infilled with Mesozoic volcanic rock above the basement are distributed in the eastern region of Zhejiang, Fujian, and Guangdong provinces and the thickness of the volcanic rock layer is 1 000–4 000 m. In the region of western Zhejiang, southern Anhui, Hunan, and Jiangxi Provinces, the basement surges obviously and the cover is mainly Palaeozoic group, and some Mesozoic Cretaceous smaller basins controlled by boundary faults develop there. The Chuxiong basin in western South China is a deep depression with complex structure, in which the thickness of Jurassic‐Cretaceous systems is over 10km. This basin is divided into two parts of eastern ascent and western depression, and the western depression is farther divided by a center uplift into two smaller deep depressions. Some Main faults in the researched region are reflected clearly in the contour velocity diagrams. Jiangshan‐Shaoxing fault, Lianhuashan fault and Wuchuan‐Sihui fault in the eastern region, and Chenghai fault, Luzhijiang fault in the western region are all the important tectonic boundaries, also flock basement faults are primarily the boundaries of the basins and the uplifts.

Analysis of Lg spectral ratios from NTS explosions: Implications for the source mechanisms of spall and the generation of Lg waves

Spectral ratios between normal-depth and over-buried nuclear explosions are investigated for purposes of detecting the effects of spall on the amplitude spectra of regional seismic signals. Lg and Pg spectral ratios in the frequency band 0.2 to 2.0 Hz are found to correlate with the strength of spall for explosions on Yucca Flats. The Lg ratios are characterized by spectral scalloping and a pronounced null at 0.55 Hz. The spectral null does not shift in frequency for explosions with significantly different spall dwell times. This observation, and the fact that a corresponding null is absent in the spectral ratios of Pg waves, are strong evidence against an interference effect in the spall time function as the cause of the spectral null. We examine the effects of Green's function excitation and spall source mechanism for models in which a normal-depth explosion is assumed to be a linear superposition of a spherical source (monopole) and spall. A tensile crack model for spall cannot reproduce the salient features of the spectral ratios. The only linear, equivalent elastic model in general agreement with the observations is one where spall is represented by a compensated linear vector dipole (CLVD), and Lg is generated by near-source scattering of Rg waves into body waves, which become trapped in the crust. In this model, the Rg spectrum is imprinted onto the scattered P and S waves. A spectral null at 0.55 Hz is caused by an excitation null for Rg waves for a CLVD source buried in the upper 500 m of the crust. While monopole and spall sources can excite Lg waves directly, the results of this study suggest that scattering of large-amplitude Rg waves in the near field is the most important source of Lg waves with frequencies near 1 Hz for explosions on Yucca Flats. This model is discussed in light of several outstanding observations documented for regional discriminants using Lg waves.

High‐frequency seismic observations in eastern Kazakhstan, USSR, with emphasis on chemical explosion experiments

Two temporary three‐station seismic networks, deploying surface and 100‐m borehole high‐frequency seismometers, of the order of 200 km from the Kazakh test site in the USSR and the Nevada test site in the United States are discussed, with emphasis on chemical explosion experiments. Seismograms attained from the detonation of three buried explosions (10 t, 20 t, 10 t) in eastern Kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1–80 Hz. Observed signal‐to‐noise (S/N) ratios were high, reaching a maximum of 400 for Pg waves and 200 for Lg waves. Good signal‐to‐noise levels persisted to high frequencies; S/N = 2 at about 50 Hz for Lg waves about 250 km from the source, and at about 14 Hz at 680 km distance. For Pg waves, S/N = 2 at about 50 Hz 270 km from the source. Shapes of displacement amplitude spectra were similar, characterized by a broad maximum in signal‐to‐noise levels between 4–8 Hz, and a decay at higher frequencies (e.g. above 10 Hz) of about f−3.5 − f−4.1 for Lg waves, and f−3.1 − f−4.5 for Pg, unconnected for distance. Magnitudes estimated from Lg time domain amplitudes for the 10 t explosion are between 2.8 and 3.3, depending on the magnitude relation used. Spectral characteristics are used to put some constraint on Lg Q. Pg Q is poorly constrained by the data. A similar experiment in southern Nevada showed much lower Pg and Lg signal‐to‐noise levels above 1 Hz, although Kazakh and Nevada absolute noise levels are comparable.

Attenuation in western Nevada: Preliminary results from earthquake and explosion sources

We present preliminary results from a study of the attenuation of regional seismic waves at frequencies between 1 and 15 Hz and distances up to 250 km in Western Nevada. Following the methods of Anderson and Hough (1984) and Hough et al. (1988), we parameterize the asymptote of the high frequency acceleration spectrum by a two‐parameter model. We relate the model parameters to a two‐layer model for Qi and Qd, the frequency‐independent and the frequency dependent components of the quality factor. We comparing our results to previously published Q studies in the Basin and Range and find that our estimate of total Q, Qt, in the shallow crust is consistent with shear wave Q at close distances as determined by Chavez and Priestley (1985). For depths greater than 5 km, we obtain a Qt which is consistent with previous estimates of coda Q (Singh and Herrmann, 1983) and LgQ (Chavez and Priestley, 1986), suggesting that both coda Q and LgQ are insensitive to near‐surface contributions to attenuation.In addition to our attenuation results, we compare the high frequency content of explosions with that of earthquakes. Our results from S waves are similar to results for Pg waves by Chael et al. (1987), and Chael (1988), who find that the Pg wave spectra from explosions are depleted in high frequency energy with respect to earthquake spectra. Taylor et al. (1988) look at spectral energy at 1–2 Hz compared to energy at 6–8 Hz for earthquakes and explosions and find, especially for Lg waves, relative depletion of the higher frequencies for explosion sources. This result is also consistent with the results from this study.