Seismic radiation from regions sustaining brittle damage

Dynamic changes of elastic moduli in source volumes are predicted to produce "damagerelated-radiation" associated with products of the changes of elastic moduli and total elastic strain components in the source volume [Ben-Zion and Ampuero, 2009]. Decreasing elastic moduli (as produced by brittle deformation of low-porosity rocks and explosions) increase the radiation to the bulk, while increasing moduli (which may be produced during the formation of compaction bands in porous rocks) decrease the radiation. Order of magnitude estimates show that in cases with high initial shear stress the damage-related contribution to the seismic motion (neglected in standard calculations) can have appreciable amplitude. A decomposition analysis indicates that the damage-relatedradiation is expected to have an appreciable isotropic component. A number of recent studies found evidence for isotropic source terms of earthquakes in geometrically-complex structures and other cases likely involving fresh faulting. Ross and Ben-Zion [2013] observed systematic rotations of double-couple-constrained aftershock mechanisms near the edges of the 1992 Landers earthquake. Synthetic calculations show that the observed rotations can result from neglecting small isotropic components in the derivation of the double-couple mechanisms. Castro and Ben-Zion [2013] observed amplification of P radiation of about 5-10 in the frequency range 1-10 Hz from aftershocks of the 7.2 El Mayor-Cucapah 2010 earthquake, consistent with isotropic source terms produced by rock damage. Kwiatek and Ben-Zion [2013] found anomalously low ratios of S-to-P radiated energy of well-recorded earthquakes in the Mponeng deep gold mine, South Africa, consistent with enhanced-P damage-related radiation. Ross et al. [2014] found statisticallysignificant explosive isotropic components for several Mw > 4.2 events in the complex trifurcation area of the San Jacinto fault zone, corresponding to ~0.4-8% of the total potency/moment of the sources. The results can have important implications for multiple aspects of earthquake physics, discrimination of small explosions from earthquakes and other topics.