Anisotropy and deformation processes in Southern California from rotational observations

Abstract

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.

Seismic Anisotropy From 6C Ground Motions of Ambient Seismic Noise

“…The purple line-a and line-b represent the fast-axis directions of P waves at around 9 and 16 km, respectively, using the P wave tomography method (Wu et al, 2022). The black arrows show the plate motion directions (Tang et al, 2023a).…”

“…from Tang et al (2023a). The local velocity model at the PFO array can available from Shaw et al (2015).…”

“…The characteristics of rotational ground motions of surface waves in an anisotropic medium are well resolved in the recent study (Tang et al, 2023b) based on the first-order perturbation theory. It indicates that the azimuthdependent surface wave dispersion curve can be directly obtained by using the amplitude ratio of translation to rotation or strain, which allows us to investigate the mantle flow and plate deformations (Tang et al, 2023a). Combining our recently proposed rotational ambient noise theory (Tang & Fang, 2023) and rotational anisotropy theory (Tang et al, 2023b), we can study local seismic anisotropy from azimuth-dependent 6C cross-correlation functions.…”

“…Rotational motion, as part of the wavefield gradient, can be measured directly by rotational seismometers at high frequencies (Bernauer et al, 2021;Lefevre, 2014;Zembaty et al, 2021) and in broadband ranges (Igel et al, 2007(Igel et al, , 2021. Combining three-component translation and three-component rotation, a single 6C seismometer shows great potential in seismological observations, which can investigate the propagation direction of seismic waves (Sollberger et al, 2018;Yuan et al, 2021) and constrain the local Earth's velocity structure (Fang & Tang, 2021;Igel et al, 2007;Keil et al, 2021;Tang & Fang, 2021bTang et al, 2023aTang et al, , 2023bWassermann et al, 2016). The ambient seismic noise, as continuous sources on the Earth, has shown its powerful potential in studying Earth's structure based on conventional translation observations (Aki, 1957;Haney et al, 2012;Lobkis & Weaver, 2001;Shapiro et al, 2005).…”

“…Studying local seismic anisotropy (Noe et al, 2022;Pham et al, 2010;Tang & Fang, 2021a) from rotational ground motions can provide additional constraints on the tectonic evolution of the Earth (Tang et al, 2023a). The characteristics of rotational ground motions of surface waves in an anisotropic medium are well resolved in the recent study (Tang et al, 2023b) based on the first-order perturbation theory.…”