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.

SCATTERING OF SHEAR WAVES BY SPHERICAL OBSTACLES

Geophysics ◽  
1959 ◽  
Vol 24 (2) ◽  
pp. 209-219 ◽  
Author(s):  
Leon Knopoff
Keyword(s):  
Diffraction Pattern ◽  
Poisson’S Ratio ◽  
Wave Length ◽  
Shear Waves ◽  
Scattered Wave ◽  
Phase Shifts ◽  
The Other ◽  
Poisson's Ratio ◽  
S Waves

The problem of the scattering of plane S waves by a perfectly rigid, infinitely dense sphere is formulated. Calculations are made for the case in which the medium outside the sphere has a Poisson’s ratio of [Formula: see text]. The range of sizes of obstacles used in the calculations includes radii very small compared with the wave length and radii comparable to the wave length. The scattered wave motions include a P mode and two S modes. One of the S modes has a formal correspondence to the SH mode of plane seismology; the other corresponds to the SV mode. At large distances from the obstacle the scattered P and S fields are computed together with the phase shifts in time occurring in all the components. For small obstacles, the scattered azimuthal S component is circularly symmetric; the scattered meridional S component diffraction pattern is generally elongated in the direction of propagation; the scattered P component is generally broadside to the direction of propagation.

2-D crustal Poisson’s ratio from seismic travel time inversion in Changbaishan Tianchi volcanic region

2005 ◽  
Vol 18 (3) ◽  
pp. 345-353 ◽  
Author(s):  
Zhi Liu ◽  
Xian-kang Zhang ◽  
Fu-yun Wang ◽  
Yong-hong Duan ◽  
Xiao-ling Lai
Keyword(s):  
Travel Time ◽  
Poisson’S Ratio ◽  
Poisson's Ratio ◽  
Time Inversion ◽  
Volcanic Region ◽  
Travel Time Inversion

Shallow near-surface effects

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. T221-T231 ◽  
Author(s):  
Christine E. Krohn ◽  
Thomas J. Murray
Keyword(s):  
Time Delay ◽  
Velocity Gradient ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
P Waves ◽  
Large Computer ◽  
Near Surface ◽  
S Waves ◽  
High Attenuation ◽  
Pulse Broadening

The top 6 m of the near surface has a surprisingly large effect on the behavior of P- and S-waves. For unconsolidated sediments, the P-wave velocity gradient and attenuation can be quite large. Computer modeling should include these properties to accurately reproduce seismic effects of the near surface. We have used reverse VSP data and computer simulations to demonstrate the following effects for upgoing P-waves. Near the surface, we have observed a large time delay, indicating low velocity ([Formula: see text]), and considerable pulse broadening, indicating high attenuation ([Formula: see text]). Consequently, shallowly buried geophones have greater high-frequency bandwidth compared with surface geophones. In addition, there is a large velocity gradient in the shallow near surface (factor of 10 in 5 m), resulting in the rotation of P-waves to the vertical with progressively smaller amplitudes recorded on horizontal phones. Finally, we have found little indication of a reflection or ghost from the surface, although downgoing reflections have been observed from interfaces within the near surface. In comparison, the following have been observed for upgoing S-waves: There is a small increase in the time delay or pulse broadening near the surface, indicating a smaller velocity gradient and less change in attenuation. In addition, the surface reflection coefficient is nearly one with a prominent surface ghost.

Seismic wave velocity investigation at The Geysers‐Clear Lake geothermal field, California

Geophysics ◽  
1982 ◽  
Vol 47 (5) ◽  
pp. 819-824 ◽  
Author(s):  
Harsh K. Gupta ◽  
Ronald W. Ward ◽  
Tzeu‐Lie Lin
Keyword(s):  
Wave Velocity ◽  
Poisson’S Ratio ◽  
Poisson Ratio ◽  
Volcanic Field ◽  
P Wave ◽  
Poisson's Ratio ◽  
Clear Lake ◽  
Seismic Wave Velocity ◽  
S Waves ◽  
The Geysers

Analysis of P‐ and S‐waves from shallow microearthquakes in the vicinity of The Geysers geothermal area, California, recorded by a dense, telemetered seismic array operated by the U.S. Geological Survey (USGS) shows that these phases are easily recognized and traced on record sections to distances of 80 km. Regional average velocities for the upper crust are estimated to be [Formula: see text] and [Formula: see text] for P‐ and S‐waves, respectively. Poisson’s ratio is estimated at 23 locations using Wadati diagrams and is found to vary from 0.13 to 0.32. In general, the Poisson’s ratio is found to be lower at the locations close to the steam production zones at The Geysers and Clear Lake volcanic field to the northeast. The low Poisson ratio corresponds to a decrease in P‐wave velocity in areas of high heat flow. The decrease may be caused by fracturing of the rock and saturation with gas or steam.

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.

Upper mantle structure in the Basin and Range province, western North America, from the apparent velocities of S waves

1969 ◽  
Vol 59 (4) ◽  
pp. 1653-1665
Author(s):  
Robert L. Kovach ◽  
Russell Robinson
Keyword(s):  
North America ◽  
Wave Velocity ◽  
Velocity Gradient ◽  
Upper Mantle ◽  
Shear Velocity ◽  
Basin And Range ◽  
P Wave ◽  
Western North America ◽  
Basin And Range Province ◽  
S Waves

Abstract The variation of shear velocity with depth in the upper mantle for the Basin and Range province of western North America has been studied with direct measurements of dT/dΔ for S waves in the distance range 14° < Δ < 40°. Three orthogonal components of digital data were used and onset times were determined using the product of the horizontal radial and vertical components of motion and particle motion diagrams. A linear LRSM array in Arizona was used for the measurement of dT/dΔ. An S-wave velocity distribution is derived, compatible with P-wave velocity models for the same region. The derived model consists of a thin lid zone of shear velocity 4.5 km/sec overlying a low-velocity zone and a change in velocity gradient at a depth of 160 km. Two regions of high-velocity gradient are located at depths beginning at 360 km and 620 km.

On the mechanics of the intrusion of sills

1969 ◽  
Vol 6 (6) ◽  
pp. 1415-1419 ◽  
Author(s):  
P. E. Gretener
Keyword(s):  
Principal Stress ◽  
Upper Mantle ◽  
Poisson’S Ratio ◽  
Vertical Stress ◽  
Poisson's Ratio ◽  
Stress Axis ◽  
Principal Stresses ◽  
State Of Stress ◽  
As Stress

Diabase sills contain material originating from the base of the crust or the upper mantle. As a result they must be fed by dike- or plug-like bodies. The formation of a sill thus represents a major reorientation of the form of the intrusion. Tabular intrusive bodies tend to orient themselves perpendicular to the least compressive principal stress axis as shown by E. M. Anderson. It is suggested that diabase sills form under sedimentary strata in which the two horizontal principal stresses exceed the vertical stress (Sx > Sy > Sz). Such strata act as stress barriers and prevent further ascent of the magma, In order for this situation to occur the sediments must be in compression in the x-direction and confined in the y-direction. The parameter of importance to produce the above state of stress is the effective Poisson's ratio.

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