Nonlinear wavefield characteristics of seismic translation and rotation in small-strain deformation from moment tensor simulations

Abstract

Seismic rotational motions recorded in near-field and strong-motion observations show discrepancies with theoretical predictions derived from linear elastodynamic theory. To investigate potential nonlinear contributions, this study incorporates geometric nonlinear effects into wave propagation theory using the Green-Lagrange strain tensor. A staggered-grid finite-difference method is used to simulate six-component (translational and rotational) wavefields generated by three basic moment-Tensor source types: isotropic (ISO), double-couple (DC), and compensated linear vector dipole (CLVD). The results demonstrate that nonlinear effects, as a secondary source, have a universal intensity pattern adhering to the physics of wave motion. However, the efficiency of exciting these effects and the modulation of wavefield attributes (such as P-wave polarization) depend strongly on source type. Simulations show that rotational components exhibit higher sensitivity to nonlinear effects driven by S-waves. Reference simulations of two moderate-To-strong earthquakes suggest that surface waves are the primary carriers of nonlinear features.