Seismic effects of the Caloris basin impact, Mercury

Abstract

Striking geological features on Mercury's surface have been linked to tectonic disruption associated with the Caloris impact and have the potential to provide information on the interior structure of Mercury. The unusual disrupted terrain located directly at the antipode of the 1500-km-diameter Caloris basin could have plausibly formed as a consequence of focused seismic waves generated by the massive impact event. In this paper, we revisit the antipodal seismic focusing effects of the Caloris impact by developing physically consistent structure models for Mercury and parameterized seismic source models for the Caloris impact. If the focused seismic body waves caused the disrupted terrain, then the amplitudes of the waves and the areal extent of surface disruptions could be used for estimating the seismic energy imparted by the impact. In this study, we show that effects of direct body waves are small relative to those of the focused guided waves. Two types of guided waves are generated by the Caloris impact. One is the conventional Rayleigh wave generated by the impact. The second is the mantle guided waves trapped between the core and the free surface. Mantle guided waves, not mentioned in previous studies, may have played an important role in the creation of the disrupted terrain. We find that the early core state has only moderate effects on the antipodal response to the Caloris impact. The fact that the zone of predicted disruption for both fluid and solid core cases is smaller than the observed region of chaotic terrain suggests either that the antipodal response to the Caloris impact may have been modulated by the shallow structure of Mercury, or that the energy imparted by the impact was larger than those used in this study.