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

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.