841060 Evidence of tectonic release from underground nuclear explosions in long-period P waves

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.

Download Full-text

The effects of tectonic release on short-period P waves from NTS explosions

Bulletin of the Seismological Society of America ◽

10.1785/bssa0740030819 ◽

1984 ◽

Vol 74 (3) ◽

pp. 819-842

Author(s):

Thorne Lay ◽

Terry C. Wallace ◽

Don V. Helmberger

Keyword(s):

Frequency Component ◽

Temporal Variations ◽

P Wave ◽

High Frequency Component ◽

Stress Regime ◽

P Waves ◽

Underground Nuclear Explosions ◽

Long Period ◽

Short Period ◽

Amplitude Pattern

Abstract The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

Download Full-text

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa05406a1981 ◽

1964 ◽

Vol 54 (6A) ◽

pp. 1981-1996 ◽

Cited By ~ 5

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.

Download Full-text

Amplitudes and energies of primary seismic waves near the hardhat, haymaker, and shoal nuclear explosions

Bulletin of the Seismological Society of America ◽

10.1785/bssa0560030643 ◽

1966 ◽

Vol 56 (3) ◽

pp. 643-653 ◽

Cited By ~ 1

Author(s):

Lynn D. Trembly ◽

Joseph W. Berg

Keyword(s):

Theoretical Model ◽

Seismic Waves ◽

Theoretical Data ◽

Source Model ◽

Similar Data ◽

Underground Nuclear Explosions ◽

Nuclear Explosions ◽

Long Period ◽

Radiation Fields ◽

Amplitude Data

abstract Records of near-source (0.3 to 20 km) primary seismic waves generated by the Hardhat, Haymaker, and Shoal underground nuclear explosions were analyzed in terms of displacement amplitude and energy variations with distance. The observed data were compared to similar data from a theoretical source model to determine the adequacy of the theoretical model. There was evidence that a long-period displacement field existed near the explosions as predicted by the theoretical source. Scatter in the observed amplitude data made it difficult to distinguish between the long-period and the radiation fields. However, the variation of total energy of the observed primary seismic waves with distance showed the presence of the long-period field. The comparison of observed and theoretical data indicates that a theoretical elastic source model approximated the observed sources.

Download Full-text

Amplitude-yield scaling for underground nuclear explosions

Bulletin of the Seismological Society of America ◽

10.1785/bssa0630020477 ◽

1973 ◽

Vol 63 (2) ◽

pp. 477-500 ◽

Cited By ~ 2

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.

Download Full-text