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.

Upper mantle P-wave velocity structure in the Japan region from travel-time studies of deep earthquakes using a new analytical method

1971 ◽  
Vol 61 (6) ◽  
pp. 1549-1570
Author(s):  
K. L. Kaila ◽  
V. G. Krishna ◽  
Hari Narain
Keyword(s):  
Wave Velocity ◽  
Calibration Curve ◽  
Velocity Gradient ◽  
Upper Mantle ◽  
Inflection Point ◽  
Velocity Structure ◽  
P Wave ◽  
Central Japan ◽  
P Wave Velocity ◽  
P Wave Velocity Structure

abstract The upper mantle P-wave velocity structure of the region of Japan has been studied in great detail from travel times of 107 earthquakes with focal depths varying from 40 to 600 km, using a new analytical method given by Kaila (1969). In southwestern Japan the P-wave velocity is found to be 7.88 km/sec at a 40-km depth, and it remains almost constant to a depth of 255 km. For northeastern Japan the velocity, determined as 7.88 km/sec at 40 km, increases linearly with moderate gradient to 8.14 km/sec at a 175-km depth. On the other hand, for central Japan the P-velocity is found to be 7.92 km/sec at a 40-km depth, and increases linearly with a high-velocity gradient to 8.33 km/sec at a depth of 180 km. Then, there is a slight decrease in the velocity gradient, but the velocity still increases linearly to 8.52 km/sec at a depth of 365 km. At this transition depth, there is a first-order velocity discontinuity—the velocity increasing from 8.52 to 9.10 km/sec. Below this depth, velocity again increases linearly from 9.10 to 9.99 km/sec at a depth of 600 km. Probable causes for these lateral-velocity inhomogeneities in the upper mantle of the Japan region are discussed. Graphs have been drawn to show the variation with depth of Δ*, the epicentral distance to the inflection point, (Δ2 − Δ1), ptrue = ∂T/∂Δ and ap = (T − pΔ) at the inflection point, the latter acting as a calibration curve for earthquake focal-depth determination in Japan. Using this calibration curve, focal depths are redetermined for all of the earthquakes under study for central Japan, and the same are compared with the values reported in the International Seismological Summaries.

Constraints on the composition of the crust and uppermost mantle in northwestern Canada: Vp/Vs variations along Lithoprobe's SNorCLE transect

2005 ◽  
Vol 42 (6) ◽  
pp. 1205-1222 ◽  
Author(s):  
Gabriela Fernández-Viejo ◽  
Ron M Clowes ◽  
J Kim Welford
Keyword(s):  
Wave Velocity ◽  
Upper Mantle ◽  
Laboratory Data ◽  
High Ratio ◽  
P Wave ◽  
S Wave ◽  
Uppermost Mantle ◽  
Crust And Upper Mantle ◽  
Slave Craton ◽  
Crustal Composition

Shear-wave seismic data recorded along four profiles during the SNoRE 97 (1997 Slave – Northern Cordillera Refraction Experiment) refraction – wide-angle reflection experiment in northwestern Canada are analyzed to provide S-wave velocity (Vs) models. These are combined with previous P-wave velocity (Vp) models to produce cross sections of the ratio Vp/Vs for the crust and upper mantle. The Vp/Vs values are related to rock types through comparisons with published laboratory data. The Slave craton has low Vp/Vs values of 1.68–1.72, indicating a predominantly silicic crustal composition. Higher values (1.78) for the Great Bear and eastern Hottah domains of the Wopmay orogen imply a more mafic than average crustal composition. In the western Hottah and Fort Simpson arc, values of Vp/Vs drop to ∼1.69. These low values continue westward for 700 km into the Foreland and Omineca belts of the Cordillera, providing support for the interpretation from coincident seismic reflection studies that much of the crust from east of the Cordilleran deformation front to the Stikinia terrane of the Intermontane Belt consists of quartzose metasedimentary rocks. Stikinia shows values of 1.78–1.73, consistent with its derivation as a volcanic arc terrane. Upper mantle velocity and ratio values beneath the Slave craton indicate an ultramafic peridotitic composition. In the Wopmay orogen, the presence of low Vp/Vs ratios beneath the Hottah – Fort Simpson transition indicates the presence of pyroxenite in the upper mantle. Across the northern Cordillera, low Vp values and a moderate-to-high ratio in the uppermost mantle are consistent with the region's high heat flow and the possible presence of partial melt.

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