Acoustic Fluidization of Crushed Rock behind the Shock Front by Damage-induced Secondary Seismic Radiation

Secondary seismic radiation is generated by rock fracture in the "damage zone" of an underground nuclear source and by frictional sliding on preexisting fractures in the "non-linear elastic zone" at larger distances from the shot point. This radiation is important for two reasons: (1) Johnson and Sammis [2000] have shown that it can make an important contribution to the far-field seismic signal, and (2) Sammis [1998] has argued that it can also significantly weaken the granulated rock behind the shock front in the damage zone. This weakening is due to a mechanism called acoustic fluidization originally proposed by Melosh [1979] to explain long-run-out landslides, the fluid morphology of extraterrestrial impact craters, and the low coefficient of effective friction inferred for the San Andreas fault. Such weakening has been shown to be necessary if computer models are to simulate the pulse-broadening observed in the near-field of nuclear explosions in hard rock [Rimer et al., 1987; Rimer et al., 1998]. This paper shows how the secondary radiation calculations of Johnson and Sammis [2000] can be used to calculate the intensity of the high frequency acoustic field behind the shock front and, thereby, to quantitatively assess the strength reduction due to acoustic fluidization.