Anisotropy and deformation processes in Southern California from rotational observations

doi:10.21203/rs.3.rs-3224880/v1

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Anisotropy and deformation processes in Southern California from rotational observations

https://doi.org/10.21203/rs.3.rs-3224880/v1

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published 01 Dec, 2023

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Seismic anisotropy in the upper mantle reveals geodynamic processes and the tectonic evolution of the Earth. The two most powerful methods, surface wave tomography, and shear-wave splitting observations, cannot investigate the deep local anisotropy with good vertical and lateral resolution, resulting in poor constraints on plate deformation processes of the complex plate boundary beneath the Southern California region. Here, we show that the amplitude ratio of translational displacement and rotation makes it possible to retrieve the local anisotropy in the upper mantle. Azimuthal anisotropy in the asthenosphere is well determined and resolved in lateral and vertical directions. The fast axis retrieved from the amplitude observation indicates the local rapid changes in plate deformation and complex pattern of mantle flow, which is compatible with the distributions of horizontal mantle flow illuminated by geodetic measurements, providing new insights on geodynamic processes of the Southern California region.

Seismology

Rotational motion

Upper mantle anisotropy

Mantle flow

figS1.jpg
Distributions of 110 events with magnitude larger than 7.0 (only a few events are between 6.5 and 7.0) from the global earthquakes catalog from November 2014 to November 2022.
figS2.jpg
Relative locations of three Broadband seismic arrays. The red stations are used to output the translation. All stations are from CI network of Southern California. The seismometers contain STS2, CMG3, Trillium and PBB.
figS3.jpg
Normalized waveform comparison between translational acceleration ($m/s^2$) and retrieved rotational velocity ($rad/s$) using the A1 and A3 arrays from the earthquake: M 8.2 - 99 km SE of Perryville in 2021, Alaska.
figS4.jpg
Velocity variation of A1 array from 120s to 260s. Red lines are the best-fit $2\psi$ curves. Blue lines are the best-fit curves when $2\psi$ and $4\psi$ terms are included. Green lines are the best-fit $1\psi$ curves.
figS5.jpg
Velocity variation of A2 array from 120s to 260s.
figS6.jpg
Velocity variation of A3 array from 120s to 260s.
figS7.jpg
The L2 norm of three arrays between observed phase velocity and estimated phase velocity (best-fit curve). Black curve: the isotropic best fit curve. Red curve: the cos($2\psi$)+sin($2\psi$) best fit curve. Blue curve: the cos($2\psi$)+sin($2\psi$)+cos($4\psi$)+sin($4\psi$) best fit curve. Green curve: the cos($\psi$)+sin($\psi$) best fit curve.
figS8.jpg
Fast wave directions and peak-to-peak anisotropy (1$\sigma$) of A1, A2, and A3 arrays.
figS9.jpg
Half-space checkerboard HTI (horizontal transversely isotropic) model with $2%$ anisotropy for P=8.0km/s and S=4.0km/s velocity. The black trangles represent the 9 stations located in a 100km*100km anisotropic sub-model. The red lines indicate that the fast wave axis along the X-axis (azimuth is 0 or 180 degrees). The black lines indicate that the fast wave axis along Y-axis (azimuth is 90 or 270 degrees).
figS10.jpg
Azimuthal anisotropy of Rayleigh wave of checkerboard model at 150s (wavelength is about 550km, anisotropic sub-model is 100km*100km in Figure S9) using the amplitude ratio Equation (1). The vertical solid lines (red and black) represent the theoretical fast directions. The fast directions are in agreement with the theoretical model (red curves: 0 or 180 degrees and black curves: 90 or 270 degrees).
figS11.jpg
Azimuthal anisotropy of Rayleigh wave of checkerboard model at 200s (wavelength is about 740km, anisotropic sub-model is 100km*100km in Figure S9) using the amplitude ratio Equation (1). The vertical solid lines (red and black) represent the theoretical fast directions. The fast directions are in agreement with the theoretical model.
figS12.jpg
Azimuthal anisotropy of Rayleigh wave of checkerboard model at 250s (wavelength is about 920km, anisotropic sub-model is 100km*100km in Figure S9) using the amplitude ratio Equation (1). The vertical solid lines (red and black) represent the theoretical fast directions. The fast directions are in agreement with the theoretical model.

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Journal Publication

published 01 Dec, 2023

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Anisotropy and deformation processes in Southern California from rotational observations

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