Wave patterns in a shear zone undergoing flash heating in earthquake-like discrete element models

SUMMARY: Building upon our previous rheological analysis (A. Taboada & M. Renouf 2023), we investigate wave patterns generated by dynamic rupture within a granular shear zone under submelting and flash-heating temperatures using discrete element modelling: a semi-periodic numerical system with cohesive micrometric discs cut by a pre-existing fracture and confined between wave-reflective parallel walls. This configuration enables three distinct wave phenomena: (1) resonant oscillations of fault blocks comparable to normal modes of fault-bounded systems, (2) propagating guided elastic waves within the granular shear zone and (3) shear-induced inertial vibrations of fault blocks with periods determined by wall mass and system stiffness. We examine rupture nucleation and subsequent shear-zone growth by applying a seismic slip rate of 1 m s(Formula presented) to the bottom boundary. Nucleation initiates spontaneously at asperity zones, generating asymmetric bilateral ruptures with supershear speeds in the shear direction and sub-Rayleigh speeds in the opposite direction; importantly, these nucleation mechanisms remain independent of adjacent rock mass. Subsequent shear-zone growth is strongly influenced by top wall mass, which drives coupled simple harmonic motion in the vertical direction and shear-induced inertial vibrations in the horizontal direction, manifesting as out-of-phase volumetric strain and normal stress waveforms that lengthen as the shear zone thickens and elastic modulus degrades. We contrast two models representing adjacent rock volume: the low-mass model simulating multistrand fault zones and the high-mass model representing isolated fault strands. The granular shear zone’s intrinsic roughness excites guided waves undergoing total internal reflection with shorter natural periods than the top wall’s harmonic motion, which trigger vibrational shear events (VSEs) correlating with friction drops. These VSEs display contrasting behaviour depending on adjacent rock mass: In the low-mass model, VSEs exhibit quasi-periodic behaviour tightly coupled to volumetric strain cycles; in the high-mass model, major VSEs display weak quasi-periodicity with intermittent distribution occurring predominantly during expansion phases. Our primary finding is that fault rupture dynamics are governed by mass-dependent interactions between guided waves and shear-induced vibrations: while rupture nucleation at asperity scale is mass-independent, the subsequent evolution of wave patterns and friction is strongly controlled by adjacent rock mass. Ultimately, dynamic shearing emerges as an oscillatory, non-smooth process modulated by frictional strength fluctuations, demonstrating that fault zone architecture is a critical control on seismic energy radiation and rupture dynamics.