scholarly journals Deep process and velocity of P-waves in the upper mantle of the transition zones of the Atlantic type

2017 ◽  
Vol 13 (3) ◽  
pp. 62-72
Author(s):  
V.V. GORDIENKO ◽  
L.Ya. GORDIENKO
Keyword(s):  
Upper Mantle ◽  
P Waves ◽  
Transition Zones ◽  
Deep Process

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.

scholarly journals Relative amplitudes of P and S waves as a mantle reconnaissance tool

1969 ◽  
Vol 59 (3) ◽  
pp. 1189-1200
Author(s):  
John R. McGinley ◽  
Don L. Anderson
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Poisson’S Ratio ◽  
Basin And Range ◽  
The Other ◽  
Poisson's Ratio ◽  
P Waves ◽  
S Waves ◽  
High Attenuation ◽  
Short Period

abstract The unified magnitude, the ratio of the amiplitudes of S to P waves, and travel-time residuals were compiled from published data for the five Seismological Observatories, TFO, UBO, BMO, WMO and CBO. Using one of the stations as a reference, a relative measure of the above quantities was calculated for each of the other stations for each of a number of earthquakes. The stations in the Basin and Range Province are consistent with a markedly higher attentuation of P waves and a high attenuation of S relative to P when compared to the other stations. This latter observation indicates a high Poisson's ratio in the mantle under the Basin and Range. The delay times to these stations are also consistent with the high Poisson's ratio and with a low-velocity upper mantle. The ratio of the amplitudes of long-period S waves to short-period P waves varies by a factor of 4 among these stations. BMO, in eastern Oregon, has a high S/P amplitude ratio compared to other stations and a travel-time residual that is comparable to the observatories in the mid-continent. This may be another example of a seismic “window” into the upper mantle that is generated by underthrusting of the oceanic lithosphere.

scholarly journals VELOCITY MODEL OF THE UPPER MANTLE OF THE FLANKING PLATEAUS OF MID-OCEANIC RIDGES

2020 ◽  
Vol 16 (4) ◽  
pp. 19-31
Author(s):  
V.V. GORDIENKO ◽  
L.Ya. GORDIENKO
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Seismic Waves ◽  
Bottom Topography ◽  
Velocity Model ◽  
Transfer Scheme ◽  
Transition Zones ◽  
Velocity Section ◽  
Ocean Ridges ◽  
Using Data

A new element is included in the study of velocity sections of the upper mantle of regions of continents, oceans, and transition zones with different endogenous regimes (according to the advection-polymorphic hypothesis — APH). This is the flanking plateaus (FP) of the mid-ocean ridges (MOR). It is assumed that these regions underwent the process of oceanization in the Mesozoic along with other parts of the oceans. In the Neogene MORs were formed. Significant parts of the basins were engulfed in modern activation, including magmatism. Between these parts of the oceans, relatively narrow strips (200—300 km wide) have survived, which some authors refer to as flanking plateaus. They are located at the edges of the MOR. FP did not experience young activization. This is indicated by the features of the bottom topography, magnetic, gravitational and thermal fields, and a velocity section of the upper horizons of the mantle. An element of checking the nature of the FP can be the construction of a velocity section of the mantle beneath these regions. According to the APH, it should differ from the neighboring ones by the increased velocity of seismic waves in the upper about 200 km. The experimental data for such work turned out to be extremely small. It was possible to build only one travel-time, using data on the southern part of the Atlantic Ocean. Insignificant information was also attracted on the southern part of the East Pacific Rise and the Mid-Indian Ridge. The travel-time corresponds to the velocity section, which completely coincides with the forecast. The latter was calculated according to the heat and mass transfer scheme in the APH version and the thermal model of the mantle. The velocity section of the FP mantle does not contain indications of a partial melting layer. Consequently, there should be no manifestations of young magmatism in FP. Verification showed that in most of the studied fragments of MOR this is true.

Modeling teleseismic P-wave propagation in the upper mantle using a parabolic approximation

1993 ◽  
Vol 83 (3) ◽  
pp. 756-779
Author(s):  
M. G. Bostock ◽  
J. C. VanDecar ◽  
R. K. Snieder
Keyword(s):  
Wave Equation ◽  
Upper Mantle ◽  
P Wave ◽  
Heterogeneous Structure ◽  
Parabolic Approximation ◽  
Initial Portion ◽  
P Waves ◽  
Low Frequencies ◽  
Reference Velocity ◽  
The Time Domain

Abstract Teleseismic waves propagating in the upper mantle are subject to considerable distortion due to the effects of laterally heterogeneous structure. The magnitude and scale of velocity contrasts representative of features such as subducted slabs may be such that wave diffraction becomes an important process. In this case forward modeling methods based on high-frequency asymptotic approximations to the wave equation will not accurately describe the wavefield. A method is introduced to model the propagation of teleseismic P waves in a laterally heterogeneous upper mantle that accounts for distortion of the initial portion of the wavefield including the effects of multipathing and frequency-dependent diffraction. The method is based on a parabolic approximation to the wave equation that is solved in the time domain on a finite-difference grid which mimics the expected pattern of energy flow in a reference velocity field. Numerical examples for a simple two-dimensional subducting slab model demonstrate the application of the method and illustrate the effects of multipathing and diffraction which dominate waveform distortion at high and low frequencies, respectively.

Crustal characteristics from short-period P waves

1968 ◽  
Vol 58 (5) ◽  
pp. 1681-1700
Author(s):  
R. M. Ellis ◽  
P. W. Basham
Keyword(s):  
Upper Mantle ◽  
Matrix Theory ◽  
Spectral Ratio ◽  
Matrix Formulation ◽  
P Waves ◽  
Crust And Upper Mantle ◽  
Travel Path ◽  
Short Period ◽  
Teleseismic Events

Abstract Thirty-four teleseismic events, recorded on the deep horizontal sediments of central Alberta, using one fixed and one movable station, have been analyzed as a test of the Haskell matrix formulation applied to short period P waves. Only limited agreement is obtained between averaged experimental vertical-horizontal spectral ratio curves and those calculated theoretically using known layer thicknesses and velocities. Scattering in the crust and upper mantle is indicated by large transverse amplitudes including distict phases and by lower coherency for smaller epicentral distances where the travel path is confined to the crust and upper mantle. Anomalous SV/P ratios are believed to contribute to the difficulties. A study of 20 events in the azimuth range 285° to 310° indicates an apparent azimuth approximately 18° more northerly than the true azimuth. Localized dips of approximately 15° on the crustal boundaries are required to explain this deviation. It is concluded that this region for which the sediments are horizontally layered does not fulfill the requirements of the Haskell matrix theory due to scattering and anomalous PS conversions in the crust and upper mantle.

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.

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