scholarly journals Trans-Pacific upper mantle shear velocity structure

2007 ◽  
Vol 112 (B8) ◽  
Author(s):  
Ying Tan ◽  
Don V. Helmberger
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Shear Velocity

Upper mantle shear velocity structure beneath the equatorial East Pacific Rise from array-based teleseismic surface-wave dispersion analysis

2019 ◽  
Author(s):  
Qiushi Zhai ◽  
Huajian Yao ◽  
Zhigang Peng
Keyword(s):  
Phase Velocity ◽  
Rayleigh Wave ◽  
Upper Mantle ◽  
Velocity Dispersion ◽  
Velocity Structure ◽  
Shear Velocity ◽  
Wave Phase ◽  
Phase Velocity Dispersion ◽  
Wave Phase Velocity ◽  
Rayleigh Wave Phase Velocity

Summary The Discovery/Gofar transform faults system is associated with a fast-spreading center on the equatorial East Pacific Rise. Most previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multi-dimensional stacking and tracing to determine the average fundamental mode Rayleigh wave phase-velocity dispersion curve (period band 20–100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh wave phase-velocity dispersion (period band 2–25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2–100 s) and the first-higher mode (period band 3–7 s) Rayleigh wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and 200-km depth of the final model is about 4.18 km/s. There exists an ∼5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km/s. Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15–60 km depth) is 4.03–4.23 km/s (∼10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5–5 per cent) mainly occurs in the shallow upper mantle zone beneath this young (0–2 Myr) oceanic region. In the deeper portion (60–200 km depth), the Vs of a weak LVZ is 4.15–4.27 km/s (∼5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary (LAB) with ∼15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper mantle lithosphere and asthenosphere in this active ridge-transform fault region.

scholarly journals Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography

2017 ◽  
Vol 458 ◽  
pp. 405-417 ◽  
Author(s):  
Martin J. Pratt ◽  
Michael E. Wysession ◽  
Ghassan Aleqabi ◽  
Douglas A. Wiens ◽  
Andrew A. Nyblade ◽  
...  
Keyword(s):  
Surface Wave ◽  
Upper Mantle ◽  
Velocity Structure ◽  
Shear Velocity ◽  
Surface Wave Tomography ◽  
Crust And Upper Mantle ◽  
Wave Tomography

The use of leaking modes in seismogram interpretation and in studies of crust-mantle structure

1965 ◽  
Vol 55 (6) ◽  
pp. 989-1021 ◽  
Author(s):  
Sergio S. Su ◽  
James Dorman
Keyword(s):  
Phase Velocity ◽  
Upper Mantle ◽  
Velocity Structure ◽  
Shear Velocity ◽  
Theoretical Treatment ◽  
Mantle Structure ◽  
Additional Information ◽  
Wave Trains ◽  
Compressional Velocity ◽  
High Degree

Abstract Certain normally and inversely dispersed wave trains appearing in the interval between P and S at moderate epicentral distances are identified as specific higher leaking modes. Observed periods of normally dispersed wave trains vary from 7 to 5 sec and from 12 to 9 sec; observed periods of inversely dispersed wave trains increase from about 15 to 35 sec. While the normally dispersed wave trains are generally observed at epicentral distances less than 20°, the inversely dispersed wave trains may be observed at distances up to about 50°. This latter type of wave, with group velocities between 7.0 and 8.5 km/sec, is shown to be controlled by upper mantle structure, and thus represents a possible tool for upper mantle investigations. These waves differ from waves of the Rayleigh and Love modes in that they are more sensitive to the compressional velocity structure than to the shear velocity structure of the waveguide. PL and shear-coupled PL data which are identified with the fundamental leaking mode were obtained for two paths in South America. Crustal thicknesses for the path between Buenos Aires and Rio determined independently from PL, shear-coupled PL and Rayleigh wave data agree well with one another. An analysis of the method of obtaining phase and group velocity curves consistent with the shear-coupled PL data shows that this method and this type of data together provide a high degree of precision for the determination of phase velocity curves. In the theoretical treatment, a rapid, approximate method, which is basically Haskell's (1962) method, was used to obtain dispersion curves for the leaking modes. The results of this method are virtually identical to those of Gilbert (1964) and those of Oliver and Major (1960). The main advantage of the present method is that it provides additional information on particle motion and on the relative amplitudes of the predicted arrivals as functions of phase velocity and period.

Universal dispersion tables

1969 ◽  
Vol 59 (4) ◽  
pp. 1667-1693
Author(s):  
Don L. Anderson ◽  
Robert L. Kovach
Keyword(s):  
Upper Mantle ◽  
Free Oscillation ◽  
Velocity Structure ◽  
Shear Velocity ◽  
Average Density ◽  
Earth Model ◽  
The Core ◽  
The Earth ◽  
Small Change ◽  
Dominant Parameter

Abstract The effect of a small change in any parameter of a realistic Earth model on the periods of free oscillation is computed for both spheroidal and torsional modes. The normalized partial derivatives, or variational parameters, are given as a function of order number and depth in the Earth. For a given mode it can immediately be seen which parameters and which regions of the Earth are controlling the period of free oscillation. Except for oSo and its overtones the low-order free oscillations are relatively insensitive to properties of the core. The shear velocity of the mantle is the dominant parameter controlling the periods of free oscillation and density can be determined from free oscillation data only if the shear velocity is known very accurately. Once the velocity structure is well known free oscillation data can be used to modify the average density of the upper mantle. The mass and moment of inertia are then the main constraints on how the mass must be redistributed in the lower mantle and core.

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