Travel-time curves for a low-velocity channel in the upper mantle

1964 ◽  
Vol 54 (6A) ◽  
pp. 1981-1996 ◽  
Author(s):  
John Dowling ◽  
Otto Nuttli
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Body Wave ◽  
Shadow Zone ◽  
Low Velocity ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Time Distance ◽  
Velocity Channel

abstract Velocities within the earth can be determined from body wave time-distance (T-D) data by the Herglotz-Wiechert method provided the velocity does not decrease too rapidly with depth. Until the present time, the properties of T-D curves for rapid decreases of velocity with depth have been considered only qualitatively. This paper presents a technique for calculating a T-D curve for any velocity distribution, including continuous and discontinuous increases and decreases of velocity with depth. Some properties of T-D curves are quantitatively studied by systematically varying the characteristics of a single model and noting the corresponding variations in the calculated T-D curves. From this it is concluded that a significant low-velocity channel may not be evidenced by a shadow zone but rather by an overlapping of two distinct branches of the T-D curve. It is further concluded that the presence of a shadow zone implies a very gentle velocity gradient below the low-velocity channel. By fitting a calculated T-D curve to observed data one can determine velocity as a function of depth even when the velocity decreases rapidly with depth, when a low-velocity channel exists. Observed T-D data for two underground nuclear explosions (gnome and bilby) measured in four different azimuths were fitted with T-D curves calculated for assumed velocity distributions. It is concluded that these data can be satisfied by a low-velocity channel for P waves in the upper mantle. The character of this channel (depth, thickness and velocity) was determined in each azimuth. The depth to its top was shallow (70 ± km) in the western U.S. and deep (125 ± km) in the eastern U.S. The velocity gradient below the channel is sharp enough to produce no prominent shadow zones. There are significant lateral changes in upper mantle velocities in the western U. S.

Reflections from discontinuities beneath antarctica

1971 ◽  
Vol 61 (5) ◽  
pp. 1441-1451
Author(s):  
R. D. Adams
Keyword(s):  
North American ◽  
Upper Mantle ◽  
Deep Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
The Earth ◽  
Velocity Channel ◽  
Upper Boundary

abstract Early reflections of the phase P′P′ recorded at North American seismograph stations from nuclear explosions in Novaya Zemlya are used to examine the crust and upper mantle beneath a region of eastern Antarctica. Many reflections are observed from depths less than 120 km, indicating considerable inhomogeneity at these depths in the Earth. No regular horizons were found throughout the area, but some correlation was observed among reflections at closely-spaced stations, and, at many stations, reflections were observed from depths of between 60 and 80 km, corresponding to a likely upper boundary of the low-velocity channel. Deeper reflections were found at depths of near 420 and 650 km. The latter boundary was particularly well-observed and appears to be sharply defined at a depth that is constant to within a few kilometers. The boundary at 420 km is not so well defined by reflections of P′P′, but reflects well longer-period PP waves, arriving at wider angles of incidence. This boundary appears to be at least as pronounced, but not so sharp as that near 650 km. The deep structure beneath Antarctica presents no obvious difference from that beneath other continental areas.

Evidence of tectonic release from underground nuclear explosions in long-period P waves

1983 ◽  
Vol 73 (2) ◽  
pp. 593-613
Author(s):  
Terry C. Wallace ◽  
Donald V. Helmberger ◽  
Gladys R. Engen
Keyword(s):  
Upper Mantle ◽  
High Stress ◽  
Test Site ◽  
Strike Slip ◽  
Body Waves ◽  
Release Time ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Long Period

abstract In this paper, we study the long-period body waves at regional and upper mantle distances from large underground nuclear explosions at Pahute Mesa, Nevada Test Site. A comparison of the seismic records from neighboring explosions shows that the more recent events have much simpler waveforms than those of the earlier events. In fact, many of the early events produced waveforms which are very similar to those produced by shallow, moderate-size, strike-slip earthquakes; the phase sP is particularly obvious. The waveforms of these explosions can be modeled by assuming that the explosion is accompanied by tectonic release represented by a double couple. A clear example of this phenomenon is provided by a comparison of GREELEY (1966) and KASSERI (1975). These events are of similar yields and were detonated within 2 km of each other. The GREELEY records can be matched by simply adding synthetic waveforms appropriate for a shallow strike-slip earthquake to the KASSERI observations. The tectonic release for GREELEY has a moment of 5 ՠ1024 dyne-cm and is striking approximately 340°. The identification of the sP phase at upper mantle distances indicates that the source depth is 4 km or less. The tectonic release time function has a short duration (less than 1 sec). A comparison of these results with well-studied strike-slip earthquakes on the west coast and eastern Nevada indicate that, if tectonic release is triggered fault motion, then the tectonic release is relatively high stress drop, on the order of several hundred bars. It is possible to reduce these stress drops by a factor of 2 if the tectonic release is a driven fault; i.e., rupturing with the P velocity. The region in which the stress is released for a megaton event has a radius of about 4 km. Pahute Mesa events which are detonated within this radius of a previous explosion have a substantially reduced tectonic release.

Strain effects of nuclear explosions in Nevada

1971 ◽  
Vol 61 (1) ◽  
pp. 55-64 ◽  
Author(s):  
Gary Boucher ◽  
Stephen D. Malone ◽  
E. Fred Homuth
Keyword(s):  
Body Wave ◽  
High Sensitivity ◽  
Test Site ◽  
Strain Effects ◽  
Underground Nuclear Explosions ◽  
Static Strain ◽  
Nuclear Explosions ◽  
Shot Point ◽  
A Minor ◽  
The University

abstract The University of Nevada's three-component quartz-rod strain meter installation at Round Mountain, Nevada (38°42.1′N, 117°04.6′W) has recorded a number of underground nuclear explosions at the Nevada Test Site, beginning with the megaton-sized JORUM event September 16 1969. Both that explosion and the larger HANDLEY event on March 26 1970 produced static strain offsets of a few parts in 109 at Round Mountain. These offsets did not decay within the first few hours after the explosions. In both cases, the strain offsets were in the sense of ground extension radial to the shot point, which is inconsistent with the assumption of a pure compressive source of strain. The strain-change ellipse for the HANDLEY event was found to have a major strain axis of 11 × 10−9 extensional, oriented N 34°W, and a minor axis of 7.4 × 10−9 compressional. A single-component strain meter at Mina, Nevada, (38°26.3′N, 118°9.3′W) was operated for the HANDLEY event, and recorded a strain offset of 2.6 × 10−9 in the direction N 74°E. Strain offsets at the time of the largest collapse events following HANDLEY were observed at Round Mountain. These offsets had the same sense on each component as those following the explosion itself. This is interpreted as support for the hypothesis that the strain changes are tectonic in origin, and the explosion initiates the strain release. Small offsets were observed for three smaller explosions out of a total of 13 studied. The relationship between body-wave magnitude mb and maximum dynamic strains at Round Mountain may be described empirically by the equation Log S = − 13.4 + 1.10 mb. Because of its high sensitivity and stability, the Round Mountain strain meter is capable of obtaining useful measurements of dynamic and static strain effects of intermediate- to large-sized explosions, at distances ranging from 160 to 200 km.

Effects of Secondary Sources of Underground Nuclear Explosions on the mb : Ms Criterion and Implications for Discrimination of the DPRK’s Nuclear Tests

2020 ◽  
Author(s):  
Henglei Xu ◽  
Sidao Ni ◽  
Ping Jin ◽  
Shiban Ding ◽  
Hongchun Wang
Keyword(s):  
Body Wave ◽  
Test Site ◽  
Nuclear Test ◽  
Source Spectrum ◽  
Secondary Source ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Secondary Sources ◽  
Nuclear Tests ◽  
Recognition Criterion

ABSTRACT The mb :  Ms (mb vs. Ms) relationship is an important criterion for screening explosions from earthquakes and has been widely adopted in seismological monitoring by the Comprehensive Nuclear-Test-Ban Treaty Organization. In general, the earthquakes have larger Ms than the underground explosions with equivalent mb. However, it has been reported that this recognition criterion failed to identify some explosions at the North Korea nuclear test site. In this study, we investigate the potential effects of secondary source components, including the compensated linear vector dipole (CLVD) and double-couple (DC) sources, on mb and Ms magnitude measurements and the physical mechanism of the mb :  Ms recognition criterion by calculating synthetic seismograms. The results show an apparent critical body-wave magnitude of 5 when using the mb :  Ms method to discriminate North Korean underground nuclear explosions. The Ms measurements decrease as the CLVD components increase, whereas the effects from the DC source can be neglected. Small events, such as the first five North Korean nuclear tests, generate weak CLVD components, leading to the failure of mb :  Ms-based discrimination, whereas the last event, with a larger magnitude, caused extensive damage and hence can be successfully discriminated. In addition, the large difference between the source spectrum of explosions and those of earthquakes might be another important factor in the successful mb :  Ms-based discrimination of the sixth North Korean nuclear test.

scholarly journals Vertical force scaling in seismic source models of underground nuclear explosions

2020 ◽  
Vol 221 (1) ◽  
pp. 251-264
Author(s):  
Michael Howe ◽  
Göran Ekström ◽  
Paul G Richards
Keyword(s):  
Surface Wave ◽  
Body Wave ◽  
Scaling Factor ◽  
Body Waves ◽  
Burial Depth ◽  
Vertical Force ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Seismic Source Models ◽  
Force Scaling

SUMMARY We have reanalysed observations of body waves and surface waves for 71 well-recorded underground nuclear explosions (UNEs) that were conducted between 1977 and 1989 at the Balapan subregion of the Semipalatinsk Test Site in Kazakhstan. To reconcile differences between body-wave and surface-wave amplitudes, we solve for a scaling factor between vertical and horizontal forces in the explosion model. We find that the estimated scaling factor is anticorrelated with the scaled depth of burial for the subset of UNEs at Balapan that have published depths. The observed anticorrelation and the inferred variations in force scaling suggest that recorded surface-wave amplitudes are significantly influenced by UNE burial depth as well as by previously recognized tectonic release. As part of our analysis, we revisit the relationship between teleseismic mb(P) and yield for UNEs at Balapan, and discuss the physical basis for effectiveness of the mb–MS discriminant.

scholarly journals 2.5 dimensional model of mantle heterogeneities under the Ukrainian shield according to the gradients of the velocities of seismic waves

2020 ◽  
Vol 29 (2) ◽  
pp. 431-441
Author(s):  
Liudmyla O. Shumlianska ◽  
Yurii I. Dubovenko ◽  
Petro H. Pigulevskyy
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Seismic Waves ◽  
Deep Structure ◽  
Ukrainian Shield ◽  
Depth Range ◽  
Tectonic Structures ◽  
P Waves ◽  
Useful Signal ◽  
Background Density

We analyze the basic techniques for the investigation of the deep structure of the mantle and the shortcomings of the models of mantle structures derived from them. Thus, we reveal that there is no analysis of the velocity field by means of analytical transformants. Therefore, we developed and tested a new approach to define the mantle boundaries based on the calculations of the sequence of P-waves velocity derivatives. As a result, we obtain some new set of velocity gradient distributions for the principal tectonic structures of the Ukrainian Shield along the composite profile. The boundaries of the mantle discontinuities according to the velocity gradient we define in a special manner to eliminate the false anomalies and the fluctuations of the velocity curves that occur due to the conversion of the hodograph into the mean velocities. The smoothing of the velocity curve we perform with a previously defined wavelength step being equal to 50 km. We treat the calculated velocity gradient anomalies as the useful signal response above the appropriate sections, which have different velocity accelerations levels inside the upper mantle. We assume that the mantle anomalies have the same physical background (density/viscosity distributions, temperature gradients etc.) within each range with the equal acceleration value. However, the singular points determined by the inflections of the gradient curve could be the possible boundaries of additional inhomogeneities within the mantle. We calculate both the 1st and the 2nd derivatives for the velocity curves obtained. The excesses 2.5-D model of the 1-th and 2-th gradient curves (the acceleration of the gradvp itself) determine the position of the max / min anomalies of gradvp at the consolidated seismic profile within the Ukrainian Shield. Finally, we analyze in detail the distribution of velocity gradients of P-waves within the upper mantle in the depth range of 50–750 km. It results in the identification of a series of additional gradient velocity boundaries within three principal structural horizons of the upper mantle (under ~ 200–300 km, ~ 410–500 km, and ~ 600–650 km respectively).

On the velocity of P in the upper mantle

1964 ◽  
Vol 54 (4) ◽  
pp. 1097-1103
Author(s):  
I. Lehmann
Keyword(s):  
Upper Mantle ◽  
Travel Times ◽  
Uppermost Mantle ◽  
Abrupt Increase ◽  
Low Velocity ◽  
Time Curve ◽  
Time Distance ◽  
Low Velocity Layer ◽  
Straight Lines ◽  
Velocity Layer

Abstract The travel times of P in the upper mantle are considered. Usually the phases are more clearly recorded at epicentral distances beyond 15° and here the time-distance curves have appreciable curvature. At smaller distances the time-curves are nearly straight lines, that sometimes are cut off at distances less than 15°. Travel times have been calculated on various velocity assumptions so as to agree with the empirically determined travel times. For the uppermost mantle the velocity has been taken either to be constant or slightly increasing down to a depth sufficiently great for the phases to be recorded, though with small amplitudes, at least to distances of 15°, or a low velocity layer has been inserted that cuts off the time-curve at smaller distances. The bending branch of the time-curve from about 15° onwards can be produced by a slow, gradual increase of velocity downwards from about 100 km depth, or by a somewhat faster increase causing a reversal of the time-curve and a cusp at about 15°, or else by an abrupt increase of velocity and velocity gradient at a depth somewhat greater than 200 km. In this last case reflections and refractions emerge at distances smaller than 15°.

Amplitude-yield scaling for underground nuclear explosions

1973 ◽  
Vol 63 (2) ◽  
pp. 477-500 ◽  
Author(s):  
D. L. Springer ◽  
W. J. Hannon
Keyword(s):  
Rayleigh Wave ◽  
Statistical Tests ◽  
Test Site ◽  
Mean Value ◽  
P Wave ◽  
Source Population ◽  
Yield Data ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions

abstract About 60 sets of seismic amplitude-yield data were examined using standard regression techniques to determine slopes of amplitude-yield scaling relations for explosions in water-saturated tuffs and rhyolites. Both P-wave amplitudes and Rayleigh-wave amplitudes were studied at selected stations located at regional and teleseismic distances. The source population included only those underground nuclear explosions fired near or below the level of the static water table at Pahute Mesa, Nevada Test Site, and covered about three orders of magnitude in yield. Statistical tests applied to the slope parameter (b) indicate that the slopes at regional and teleseismic distances are different. An estimated mean value of b for P-wave amplitude/period (A/T) was slightly greater than 0.6 for regional distances but was almost 1.0 for teleseismic distances. The estimated mean value of b for Rayleigh-wave A/T data was about 1.1. At a given distance the slopes seem to be independent of the yield range considered for both P-waves and Rayleigh-waves.

High-resolution teleseismic body-wave tomography with a 3D initial crustal model for crust-to-upper mantle images in highly heterogeneous media.

2020 ◽  
Author(s):  
Adeline Clutier ◽  
Stéphanie Gautier ◽  
Christel Tiberi
Keyword(s):  
Upper Mantle ◽  
Heterogeneous Media ◽  
Body Wave ◽  
A Priori ◽  
Velocity Model ◽  
Low Velocity ◽  
Local Tomography ◽  
Velocity Models ◽  
The North ◽  
3D Velocity Model

<p>Local and teleseismic body wave inversions are two approaches commonly used to obtain 3D Earth velocity models for shallow and mantle scale, respectively. However, each method used separately is poorly resolved at the mantle/crust boundary while imaging that interface is important to understand the geodynamic processes (e.g. magmatic underplating, mantle delamination, crustal thinning or thickening) occurring at this depth. In order to develop a high-resolved final velocity model, the two approaches were combined. First, an irregular grid was settled, with a higher density of nodes at crustal scale (from 0 to 40 km) and an increasing node step when approaching the limits of the model. Then, an a priori 3D crustal velocity model (from an independent local tomography) was inserted within the 1D IASP91 lithospheric one. Finally, the teleseismic tomographic inversion was carried out at crust-to-upper mantle scale using this new mixed initial model and teleseismic data. We applied the method on a real case that includes both tectonic and magmatic processes, the North Tanzanian Divergence (NTD). Synthetic tests showed that we had no resolution between 0 and 35 km. However, a fine crustal grid with the 3D local model helps to better constrain ray paths, limiting the artefacts and smearing from the mantle to the crust, enhancing details, sharpening the velocity anomalies and modifying the geometry of anomalies at depth (> 150 km). Following these tests, we propose then a final scheme in which we include the a priori crustal 3D velocity model in the finer crustal grid, and we prevent the inversion from modifying it. This insertion of strong crustal constraints in teleseismic inversion provides sharper spatial resolution at both crustal and mantle scales, including areas with poor ray coverage, beneath the NTD region. Our strategy allows to counteract the degradation of the results in areas with low velocity zones (such as rift and hotspot), where the seismic rays go around these anomalies.</p>

scholarly journals A study of the Northwestern Pacific upper mantle

1965 ◽  
Vol 55 (5) ◽  
pp. 925-939
Author(s):  
Daniel A. Walker
Keyword(s):  
Upper Mantle ◽  
Fundamental Problem ◽  
Body Wave ◽  
Lower Boundary ◽  
P Wave ◽  
Northwestern Pacific ◽  
Travel Times ◽  
Low Velocity ◽  
S Waves ◽  
Wave Channel

abstract A fundamental problem of earthquake seismology is the occurrence of the upper mantle low-velocity channel. This study is intended to examine its existence in the upper mantle below the Northwestern Pacific on the basis of body-wave arrivals at a bottom-mounted hydrophone near Wake Island. A comparison of the observed travel times and the Jeffreys-Bullen travel times shows an extreme anomaly in the 21- to 33-degree range for both P and S waves. Assumed linear paths suggest a P-wave-channel upper boundary between 165 km and 185 km, and a lower boundary between 290 km and 542 km. Travel times for P and S waves indicate that the velocities in the channel remain constant at 8.1 km/sec and 4.65 km/sec respectively.

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