Thomas R. McGetchin

Global seismic effects of basin-forming impacts

Abstract Models of the thermal evolution of the moon and the terrestrial planets suggest that basin-forming impacts occurred when the planets had partially molten interiors overlain by thickening lithospheres, comparable in thickness to the basin radii. We are investigating the effects of large impacts on planetary surfaces using a Lagrangian computer program which treats shock wave propagation and includes the effects of material strength, elastic-plastic behavior and material failure. In this paper we describe the computer code and some physical details of our numerical techniques, and report the results of several initial calculations. We study the global seismic effects for cratering energies (10 24 and 10 25 J) intermediate between the Copernicus and Imbrium events on the moon, and compare the phenomenologies for assumed solid and molten planetary interiors. The principal results are as follows: 1. (1)|Far-field effects are largely independent of cratering mechanisms (e.g., simulated impact vs. buried explosion). 2. (2)|Antipodal seismic effects are significantly enhanced by focusing and are of substantial magnitude. Vertical ground motion may be on the order of kilometers, and accelerations approach one lunar gravity. 3. (3)|The most violent activity occurs at significant depth beneath the antipode, considerably after the passage of the initial compressive/rarefactive shock wave, and results from complex interations with the free surface. 4. (4)|Seismic effects are decidedly more pronounced for a molten planet than for a solid one. 5. (5)|Tensile failure may occur at depths of tens of kilometers beneath the antipode, and may also occur over the entire surface, although at shallower depths. These results support the suggestion of Schultz and Gault that the unusual terrains antipodal to large planetary basins may have been catastrophically modified by seismicity generated by the basin-forming impacts. We would further suggest that these impacts may in fact have pervasively and repeatedly brecciated the entire lithospheres of the terrestrial planets as these lithospheres formed and thickened.