Love-Wave Phase-Velocity Estimation from Array-Based Rotational Motion Microtremor

2020 ◽  
Author(s):  
Kunikazu Yoshida ◽  
Hirotoshi Uebayashi
Keyword(s):  
Phase Velocity ◽  
Rayleigh Waves ◽  
Love Wave ◽  
Velocity Estimation ◽  
Love Waves ◽  
Phase Velocities ◽  
Wave Phase ◽  
Wave Phase Velocity ◽  
Spatial Derivatives ◽  
Rotational Motions

ABSTRACT The most popular array-based microtremor survey methods estimate velocity structures from the phase velocities of Rayleigh waves. Using the phase velocity of Love waves improves the resolution of inverted velocity models. In this study, we present a method to estimate the phase velocity of Love waves using rotational array data derived from the horizontal component of microtremors observed using an ordinal nested triangular array. We obtained discretized spatial derivatives from a first-order Taylor series expansion to calculate rotational motions from observed array seismograms. Rotational motions were obtained from a triangular subarray consisting of three receivers using discretized spatial derivatives. Four rotational-motion time histories were calculated from different triangular subarrays in the nested triangular arrays. Phase velocities were estimated from the array of the four rotational motions. We applied the proposed Love-wave phase-velocity estimation technique to observed array microtremor data obtained using a nested triangular array with radii of 25 and 50 m located at the Institute for Integrated Radiation and Nuclear Science, Kyoto University. The phase velocities of rotational and vertical motions were estimated from the observed data, and results showed that the former were smaller than those of the latter. The observed phase velocities obtained from vertical and rotational components agreed well with the theoretical Rayleigh- and Love-wave phase velocities calculated from the velocity structure model derived from nearby PS logs. To show the ability of the rotation to obtain Love wave, we estimated apparent phase velocities from north–south or east–west components. The apparent velocities resulted in between the theoretical velocities of Rayleigh and Love waves. This result indicates that the calculated rotation effectively derived the Love waves from a combination of Love and Rayleigh waves.

Surface-Wave Tomography of Eastern and Central Tibet from Two-Plane-Wave Inversion: Rayleigh-Wave and Love-Wave Phase Velocity Maps

2020 ◽  
Vol 110 (3) ◽  
pp. 1359-1371
Author(s):  
Lun Li ◽  
Yuanyuan V. Fu
Keyword(s):  
Surface Wave ◽  
Tibetan Plateau ◽  
Phase Velocity ◽  
Rayleigh Wave ◽  
Love Wave ◽  
The Tibetan Plateau ◽  
Surface Wave Tomography ◽  
Wave Phase ◽  
Wave Phase Velocity ◽  
Wave Tomography

ABSTRACT An understanding of mantle dynamics occurring beneath the Tibetan plateau requires a detailed image of its seismic velocity and anisotropic structure. Surface waves at long periods (>50  s) could provide such critical information. Though Rayleigh-wave phase velocity maps have been constructed in the Tibetan regions using ambient-noise tomography (ANT) and regional earthquake surface-wave tomography, Love-wave phase velocity maps, especially those at longer periods (>50  s), are rare. In this study, two-plane-wave teleseismic surface-wave tomography is applied to develop 2D Rayleigh-wave and Love-wave phase velocity maps at periods between 20 and 143 s across eastern and central Tibet and its surroundings using four temporary broadband seismic experiments. These phase velocity maps share similar patterns and show high consistency with those previously obtained from ANT at overlapping periods (20–50 s), whereas our phase velocity maps carry useful information at longer periods (50–143 s). Prominent slow velocity is imaged at periods of 20–143 s beneath the interior of the Tibetan plateau (i.e., the Songpan–Ganzi terrane, the Qiangtang terrane, and the Lhasa terrane), implying the existence of thick Tibetan crust along with warm and weak Tibetan lithosphere. In contrast, the dispersal of fast velocity anomalies coincides with mechanically strong, cold tectonic blocks, such as the Sichuan basin and the Qaidam basin. These phase velocity maps could be used to construct 3D shear-wave velocity and radial seismic anisotropy models of the crust and upper mantle down to 250 km across the eastern and central Tibetan plateau.

Decoupling approximation of P- and S-wave phase velocities in orthorhombic media

Geophysics ◽  
2021 ◽  
pp. 1-74
Author(s):  
Bowen Li ◽  
Alexey Stovas
Keyword(s):  
Phase Velocity ◽  
P Wave ◽  
Wave System ◽  
S Wave ◽  
Phase Velocities ◽  
Wave Phase ◽  
Algebraic Expressions ◽  
Wave Phase Velocity ◽  
Independent Parameters ◽  
Velocity Approximation

Characterizing the kinematics of seismic waves in elastic orthorhombic media involves nine independent parameters. All wave modes, P-, S1-, and S2-waves, are intrinsically coupled. Since the P-wave propagation in orthorhombic media is weakly dependent on the three S-wave velocity parameters, they are set to zero under the acoustic assumption. The number of parameters required for the corresponding acoustic wave equation is thus reduced from nine to six, which is very practical for the inversion algorithm. However, the acoustic wavefields generated by the finite-difference scheme suffer from two types of S-wave artifacts, which may result in noticeable numerical dispersion and even instability issues. Avoiding such artifacts requires a class of spectral methods based on the low-rank decomposition. To implement a six-parameter pure P-wave approximation in orthorhombic media, we develop a novel phase velocity approximation approach from the perspective of decoupling P- and S-waves. In the exact P-wave phase velocity expression, we find that the two algebraic expressions related to the S1- and S2-wave phase velocities play a negligible role. After replacing these two algebraic expressions with the designed constant and variable respectively, the exact P-wave phase velocity expression is greatly simplified and naturally decoupled from the characteristic equation. Similarly, the number of required parameters is reduced from nine to six. We also derive an approximate S-wave phase velocity equation, which supports the coupled S1- and S2-waves and involves nine independent parameters. Error analyses based on several orthorhombic models confirm the reasonable and stable accuracy performance of the proposed phase velocity approximation. We further derive the approximate dispersion relations for the P-wave and the S-wave system in orthorhombic media. Numerical experiments demonstrate that the corresponding P- and S-wavefields are free of artifacts and exhibit good accuracy and stability.

Wave propagation and subsurface velocity structure at the Virgo gravitational wave detector (Italy)

2020 ◽  
Author(s):  
Gilberto Saccorotti ◽  
Sonja Gaviano ◽  
Carlo Giunchi ◽  
Irene Fiori ◽  
Soumen Koley ◽  
...  
Keyword(s):  
Wave Propagation ◽  
Phase Velocity ◽  
Rayleigh Wave ◽  
Gravitational Wave ◽  
Rayleigh Waves ◽  
Velocity Structure ◽  
Band Frequency ◽  
Frequency Bands ◽  
Phase Velocities ◽  
Wave Phase

<p>The performances and sensitivity of gravitational wave (GW) detectors are significantly affected by the seismic environment. In particular, the seismic displacements and density fluctuations of the ground due to seismic-wave propagation introduce noise in the detector output signal; this noise is referred to as gravity-gradient noise, or Newtonian Noise (NN). The development of effective strategies for mitigating the effects of NN requires, therefore, a thorough assessment of seismic wavefields and medium properties at and around the GW detector. In this work, we investigate wave propagation and the subsurface velocity structure at the Virgo GW detector (Italy), using data from a temporary, 50-element array of vertical seismometers. In particular, we analyze the recordings from the catastrophic Mw=6.2 earthquake which struck Central Italy on August 24, 2016, and six of the following aftershocks.  The general kinematic properties of the earthquake wavefields are retrieved from the application of a broad-band, frequency-domain beam-forming technique. This method allows measuring the propagation direction and horizontal slowness of the incoming signal; it is applied to short time windows sliding along the array seismograms, using different subarrays whose aperture was selected in order to match different frequency bands. For the Rayleigh-wave arrivals, velocities range between 0.5 km/s and 5 km/s, suggesting the interference of different wave types and/or multiple propagation modes. For those same time intervals, the propagation directions are scattered throughout a wide angular range, indicating marked propagation effects associated with geological and topographical complexities. These results suggest that deterministic methods are not appropriate for estimating Rayleigh waves phase velocities. By assuming that the gradient of the displacement is constant throughout the array, we then attempt the estimation of ground rotations around an axis parallel to the surface (tilt), which is in turn linearly related to the phase velocity of Rayleigh waves. We calculate the ground tilt over subsequent, narrow frequency bands. Individual frequency intervals are investigated using sub-arrays with aperture specifically tailored to the frequency (wavelength) under examination. From the scaled average of the velocity-to-rotation ratios, we obtain estimates of the Rayleigh-wave phase velocities, which finally allow computing a dispersion relationship. Due to their diffusive nature, earthquake coda waves are ideally suited for the application of Aki’s autocorrelation method (SPAC). We use SPAC and a non-linear fitting of correlation functions to derive the dispersion properties of Rayleigh wave for all the 1225 independent inter-station paths. The array-averaged SPAC dispersion is consistent with that inferred from ground rotations, and with previous estimates from seismic noise analysis.  Using both a semi-analytical and perturbational approaches, this averaged dispersion is inverted to obtain a shear wave velocity profile down to ~1000m depth. Finally, we also perform an inversion of the frequency-dependent travel times associated with individual station pairs to obtain 2-D, Rayleigh wave phase velocity maps spanning the 0.5-3Hz frequency interval. </p>

A crust-mantle profile from Mould Bay, Canada, to Tucson, Arizona

1968 ◽  
Vol 58 (6) ◽  
pp. 1821-1831
Author(s):  
A. J. Wickens ◽  
K. Pec
Keyword(s):  
Phase Velocity ◽  
Upper Mantle ◽  
Love Wave ◽  
Great Circle ◽  
Careful Selection ◽  
Precambrian Shield ◽  
Phase Velocities ◽  
Wave Phase ◽  
Adjacent Segments ◽  
Great Circle Path

ABSTRACT Love-wave phase velocities were determined for five adjacent segments of a 5000 kilometer great circle path from Mould Bay, Canada, to Tucson, Arizona. Mean-phase velocity curves were obtained from curves based on reciprocal data, thus minimizing the detrimental effects of non-parallel layering. By careful selection and precise treatment of the data over relatively short distances (800 km), detail hitherto suppressed has been retained. Finally, by using reciprocal seismograms, the effect of sloping interfaces was observed. The crustal and upper mantle models obtained indicate significant differences in structure between different provinces of the Precambrian Shield.

scholarly journals Local Coupling and Conversion of Surface Waves due to Earth’s Rotation. Part 1: Theory

2020 ◽  
Author(s):  
Roel Snieder ◽  
Christoph Sens-Schönfelder
Keyword(s):  
Rayleigh Wave ◽  
Surface Waves ◽  
Coriolis Force ◽  
Rayleigh Waves ◽  
Love Wave ◽  
Mode Conversion ◽  
Love Waves ◽  
Earth’S Rotation ◽  
Earth's Rotation ◽  
Phase Velocities

Summary Earth’s rotation affects wave propagation to first order in the rotation through the Coriolis force. The imprint of rotation on wave motion has been accounted for in normal mode theory. By extending the theory to propagating surface waves we account for the imprint of rotation as a function of propagation distance. We describe the change in phase velocity and polarization, and the mode conversion of surface waves by Earth’s rotation by extending the formalism of Kennett (1984) for surface wave mode conversion due to lateral heterogeneity to include the Coriolis force. The wavenumber of Rayleigh waves is changed by Earth’s rotation and Rayleigh waves acquire a transverse component. The wavenumber of Love waves in not affected by Earth’s rotation, but Love waves acquire a small additional Rayleigh wave polarization. In contrast to different Rayleigh wave modes, different Love wave modes are not coupled by Earth’s rotation. We show that the backscattering of surface waves by Earth’s rotation is weak. The coupling between Rayleigh waves and Love waves is strong when the phase velocities of these modes are close. In that regime of resonant coupling, Earth’s rotation causes the difference between the Rayleigh wave and Love wave phase velocities that are coupled to increase through the process of level-repulsion.

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