Complex Crater Collapse: A Comparison of the Block and Melosh Acoustic Fluidization Models of Transient Target Weakening.

The collapse of large impact craters requires a temporary reduction in the resistance to shear deformation of the target rocks. One explanation for such weakening is acoustic fluidization, where impact-generated pressure fluctuations temporarily and locally relieve overburden pressure facilitating slip. A model of acoustic fluidization widely used in numerical impact simulations is the Block model. Simulations employing the Block model have successfully reproduced large-scale crater morphometry and structural deformation but fail to predict localized weakening in the rim area and require unrealistically long pressure fluctuation decay times. Here, we modify the iSALE shock physics code to implement an alternative model of acoustic fluidization, which we call the Melosh model, that accounts for regeneration and scattering of acoustic vibrations not considered by the Block model. The Melosh model of acoustic fluidization is shown to be an effective model of dynamic weakening, differing from the Block model in the style of crater collapse and peak ring formation that it promotes. While the Block model facilitates complex crater collapse by weakening rocks deep beneath the crater, the Melosh model results in shallower and more localized weakening. Inclusion of acoustic energy regeneration in the Melosh model reconciles required acoustic energy dissipation rates with those typically derived from crustal seismic wave propagation analysis.