Paul G. Richards

Scattering of elastic waves from depth-dependent inhomogeneities

We have studied scattered pulse shapes by modeling inhomogeneities as a sequence of infinitesimally thin homogeneous layers. With oblique incidence of plane P or SV waves, the reflected-converted-transmitted waves are obtained by taking the calculus limit for the sum of primary interactions of the incident wave with all layer boundaries. The resulting scattered waves thus present themselves naturally in the time domain. For an incident impulse, the scattered pulse shape is merely an analytic function of the depth from which scatter has taken place within the inhomogeneity. The direct application of this simple method appears to be new, and we have found it remarkably accurate when compared with methods in which higher-order boundary interactions are also retained (i.e., Haskell methods and an adaptation in the time domain which also keeps all multiples).In specific studies of P-waves incident (up to 30 degrees away from the vertical) upon a 5 km thick crust-mantle transition, between materials having impedance ratio 1:2.8, we find the scattered pulse shapes are given adequately by our theory, for the passband of short-period seismometers. Indeed, the theory remains remarkably accurate even for long periods, being in error by only 8 per cent at zero frequency.

Borovoye Geophysical Observatory, Kazakhstan

This is a story of surprises—of a geophysical observatory and a community of scientists who until about a year ago were unknown to scientists in the West, who now have new opportunities for collaboration with expert seismologists in Russia and Kazakhstan. The story began for us in the spring of 1990 at the annual workshop of the Incorporated Research Institutions for Seismology (IRIS), Seismologists from the Soviet Union attended and told us that they had permission to describe a geophysical observatory at Borovoye, near Kokchetav in Northern Kazakhstan, which had operated digital seismometers since 1966. They said there was a large tape archive and that much of the data were of excellent quality and would probably be made available on request—even the data for nuclear explosions.

DISCRIMINATION OF EARTHQUAKES AND EXPLOSIONS IN THE EASTERN UNITED STATES USING REGIONAL HIGH-FREQUENCY DATA

High-frequency regional records from small earthquakes (1.3 < magnitude < 4), and comparable magnitude explosions, are analyzed to find a reliable seismic discriminant in the eastern U. S. Over 500 digital, vertical-component seismograms recorded by the New York State Seismic Network in the distance ranges 10 to 610 km are used. Mean P/Lg spectral ratios in the band 1–25 Hz are about 0.5 and 1.25 for earthquakes and explosions, respectively, in the eastern United States. We find that the high-frequency P/Lg spectral amplitude ratio in the frequency band 5–25 Hz is a reliable and robust discriminant for classifying these events. A linear discriminant function analysis indicates that the P/Lg spectral amplitude ratio method provides discrimination power with a total misclassification probability of about 1%. Single-hole instantaneous explosions and ripple-fired quarry blasts have somewhat different P/Lg spectral ratios, but as a group are distinctly different from earthquakes.

Testing the nuclear test-ban treaty

A recent earthquake near a former Soviet nuclear test site has tested mechanisms for monitoring the test-ban treaty. Technical systems passed with flying colours, but relevant US agencies could have done better.

Variability of crustal attenuation in the northeastern United States from Lg waves

High-quality, digital seismograms from eight pairs of collocated earthquakes in the northeastern United States were analyzed to determine accurate source spectrum corner frequencies. This was accomplished by applying the empirical Greens function method to regional Pg and Lg (or Sg) phases recorded by vertical component seismographs of the U.S. National Seismographic Network (USNSN) and the Lamont-Doherty Cooperative Seismographic Network (LCSN) stations. The frequency band used was 0.5–16 Hz for USNSN and 1–30 Hz for LCSN records. The source spectrum corner frequencies for the eight larger earthquakes of the event pairs (magnitudes between mb(Lg) = 2.5 – 4.1) range from about 4.3 to 16.3 Hz. Based on the comer frequencies obtained independently from the empirical Greens function analysis, Sg or Lg wave displacement amplitude spectra up to 30 Hz were used to determine the crustal average Q factors along 87 event-station paths. These paths crossed diverse tectonic features in the northeastern United States and were in the epicentral distance range of 41 to 1394 km. We found that within the northeastern United States, the crustal average QLg we obtained was frequency dependent and showed spatial variability which correlated fairly well with the major tectonic features in the region. Our attenuation measurements indicated low Lg attenuation in the Adirondack Mountains with exposed Precambrian Grenville basement with QLg = 905 f0.40, high Lg attenuation in the central Appalachian Province with QLg = 561–586 f0.46–0.47, and an intermediate Lg attenuation in northern New England Appalachians with Q = 705 f0.41.

The dynamic field of a growing plane elliptical shear crack

Abstract The radiation for a three-dimensional problem of brittle fracture is investigated. A crack is presumed to nucleate at a point in an infinite pre-stressed elastic medium, and the crack subsequently grows steadily with subsonic rupture velocities, maintaining the shape of an ellipse. Shear stresses are relieved by the crack, and exact solutions are derived for the acceleration and stress-rate (at every point of the medium) in terms of single integrals and algebraic expressions. The solutions are evaluated analytically at wavefronts and singularities, and numerically, at different points in the medium, for different growth rates of the crack.

ELASTIC WAVE SOLUTIONS IN STRATIFIED MEDIA

Classical Thomson‐Haskell methods are proven applicable to the asymptotic wave solution in a stratified elastic medium which has both first and second order discontinuities in ρ, λ, and μ. This proof removes a weakness of the original method, which made it unsatisfactory for modeling velocity gradient anomalies. The generalized method, simple in application for a stack of inhomogeneous layers, is used here to derive P-SV reflection/conversion/transmission properties for a transition region in which ρ, [(λ+2μ)/ρ]1/2, and (μ/ρ)1/2 vary linearly. Although the solution method is approximate, its accuracy is confirmed by comparison with exact solutions.

North Korean nuclear test: Seismic discrimination low yield

North Korea carried out a widely reported nuclear explosion on 9 October 2006 at 0135 UTC at a location about 40 kilometers northwest of the city of Kilju (Figure 1). The location of the test determined from seismic signals recorded at 31 stations around the world was given as 41.294°N, 129.094°E in Quick Determination of Epicenters (QED) by the U.S. Geological Surveys National Earthquake Information Center (NEIC), very close to Mantap-san (Mount Mantap, 2205 meters, see Figure lc). Further, since the late 1990s, surveillance satellites have detected tunneling activity in this area suspected to be indicative of North Korea preparing to conduct nuclear tests at this site [Broad et al, 2005].

Lg-Wave Cross Correlation and Double-Difference Location: Application to the 1999 Xiuyan, China, Sequence

A surprising discovery has been made that in some cases the complex, highly scattered Lg wave is found to be similar for clusters of events. We analyze in detail a subset of 28 out of 90 events from the 1999 Xiuyan sequence. Cross correlations provide highly accurate differential travel-time measurements. Their error estimated from the internal consistency is about 7 msec. These travel-time differences are then inverted by the double-difference technique to obtain epicenter estimates that have location precision on the order of 150 m. The locations are computed with waveform data from four to five regional stations 500 to 1000 km away. The epicenter estimates are not substantially affected by the sparseness of stations or large azimuthal gaps. Comparison with a surface trace a few kilometers away and location estimates based on much more dense networks led us to conclude that the absolute positions are accurate to the 5-km level. Regional event locations must often be based on a small number of phases and stations due to weak signal-to-noise ratios and sparse station coverage. This is especially true for monitoring work that seeks to locate smaller magnitude seismic events with a handful of regional stations. Two primary advantages of using Lg for detection and location are that it is commonly the largest amplitude regional wave (enabling detection of smaller events) and it propagates more slowly than P waves or Sn (resulting in smaller uncertainty in distance, for a given uncertainty in travel time).

Lop Nor Revisited: Underground Nuclear Explosion Locations, 1976-1996, from Double-Difference Analysis of Regional and Teleseismic Data

We have used seismic signals recorded at regional and teleseismic distances to determine precise locations of 19 underground nuclear explosions (13 in vertical shafts, and 6 in horizontal tunnels) carried out between October 1976 and July 1996 at the Lop Nor test site in the southern Xinjiang province of China. In addition to first- and later-arriving phase-pick data from the International Seismological Centre and Chinese bulletins, we use waveform cross-correlation methods to measure relative arrival times between the explosions with an accuracy of about 10 msec. We adapted the double-difference algorithm to work with regional and global network data, and applied it to the combined Lop Nor data set to remove the effects of uncertainty in the Earth model. Specifically, we determined locations of more recent tests to the accuracy of the cross-correlation data while simultaneously determining the relative locations of the older tests, for which digital waveforms are not available, to the accuracy of the phase-pick data. In general, our locations are consistent, at the 90% confidence level, with previously published locations (when compared in a common reference frame), with anthropogenic features from satellite imagery, and with high-resolution elevation data. For four explosions, however, our results indicate that a particular explosion may have been carried out in a different shaft than previously noted, or previously associated features must be ruled out because of their location outside a particular error ellipse. Mislocations of explosions from associated satellite features are less than 1 km for all 13 shaft events. The pattern of tunnel-explosion locations falls within a region of suitable overburden required for containment. Eighteen Lop Nor locations have solution qualities at the GT2 level or better, and are well suited to calibrate IMS stations for the purpose of monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty. Manuscript received 4 September 2003.