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SH-SV Polarization Anisotropy: Isotropic Interpretation of Experimentally Measured Love and Rayleigh Wave Phase Velocities and Amplitude Attenuations

Session: Sensors, Seismicity and Imaging

Type: Oral

Date: 4/21/2021

Presentation Time: 05:30 PM Pacific

Description: 

The Love-Rayleigh wave discrepancy is a well-known phenomenom in seismology, in which phase velocities computed for a single isotropic model are unable to fit both the observed fundamental mode Love and Rayleigh waves. While it has been shown that isotropic models with relatively thin, high-contrast layers inserted in them might be able to fit both the Love and Rayleigh phase-velocity data, many authors instead resort to body-wave, velocity anisotropy to explain this phenomenon. Here, we attempt to complete and extend the verification of these successful isotropic models, but with realistic, anelastic layers.

The use of very simple structural models with three thin high contrast layers superposed: an "olivine" LVZ at 152 km of depth, a "lower-crustal" LVZ at the Moho depth and a "granitic" LVZ at the Conrad-discontinuity depth resulted in a huge volume of isotropic solutions for the phase velocity data. However, when we added the Love and Rayleigh wave amplitude-attenuation data to restrict our solutions, we found a single successful model for the central U.S.: (1) a preferred (idealized) physical model for the "olivine" LVZ: a building-block model of the crystal aggregate with a 3-D laminate of basalt-melt film separating crystals over a 0.1 m vertical region centered at the 152 km depth, i.e. lubricated inter-crystal interfaces with an average melt fraction of 0.001 %; and, appropriately modified, (2) a preferred, similar idealization for the "lower-crustal" LVZ which is distributed over a 5 km vertical region centered at a depth of 38 km, with the region's average melt fraction below 0.1%.

This completely isotropic mechanism: (1) successfully explains (removes) the apparent "discrepancy" between measured Love and Rayleigh wave phase velocities in the central U.S.; (2) contains a corrected form of the Gutenberg LVZ in the asthenosphere, with a mobile interface replacing the usual, thick, static LVZ layer; it (3) contains a like interface at the Moho; and thus (4) contains plausible bases for the continental portion of drift, or plate motion.

Presenting Author: Gyanendra Gurung

Student Presenter: No

Authors