Title: Seismic radiation from regions sustaining brittle rock damage

Abstract:

Analytical results indicate that dynamic changes of elastic moduli in source volumes produce “damage-related-radiation” associated with tensorial products of the changes of elastic moduli and the elastic strain components at the source. Decreasing elastic moduli (as produced by brittle deformation of low-porosity rocks and explosions) increase the radiation to the surrounding medium, while increasing moduli (which may be produced during the formation of compaction bands in porous rocks) decrease the radiation to the bulk. A tensorial decomposition analysis indicates that the damage-related-radiation has a large isotropic component. While the total radiation may still be dominated by shear, the dynamic dilation associated with the isotropic radiation can significantly reduce the heat generation near the rupture front and produce a pulse-type rupture and rock fragmentation. Numerical simulations confirm these expectations. The damage-related-radiation involves small length scales (proportional to the rupture width) and is therefore difficult to observe in the far field. However, analyses of earthquake waveforms using seismic data recorded close to earthquake ruptures reveal elevated P-wave radiation and appreciable explosive isotropic source terms (e.g., 5-15% of the total event potencies). Geological studies of deeply exhumed faults in dry granulite rocks show evidence of pulverization and delayed heating in earthquake rupture zones consistent with strong dynamic dilation at the rupture fronts. The high potential importance of damage-related-radiation to the local physics of earthquake ruptures motivates additional theoretical, experimental, and observational research.