10:45 〜 12:15
[SSS08-P01] Azimuthal anisotropy of the crust and upper mantle beneath Botswana: Seismotectonic and geodynamic implications.
キーワード：Anisotropy, Ambient noise, Seimotectonics, Southern Africa
Botswana is thought to host the terminus of the southwesterly branch of the East African Rift System in the country's north and SE regions (e.g. Scholz et al., 1976; Modisi et al., 2000; Bufford et al. 2012). Despite observation of a low-velocity region connecting the two regions (Fadel et al., 2020; Fadel et al., 2018), the regional stress relationship between the two regions remains elusive, and efforts to reconcile the source(s) of the observed seismicity have been mostly inconclusive. In this study, we explore the relationship between the two seismogenic regions by studying the azimuthal anisotropy induced on Rayleigh waves phase velocity measurements using the two-receivers method of Kaestle et al. (2016). Measurements at 8 periods between 15s and 50s are considered in order to analyze the variation of the anisotropy with depths up to about 80km and enable a reconstruction of the regional stress state. Results from anisotropic eikonal tomographic inversion (Lin et al., 2009; Kaestle et al., 2022) indicate fast directions that are parallel to the respective seismogenic fault systems in the crust, suggesting crustal fabrics related to the respective stresses. The anisotropic fast axes change direction systematically with depth into the upper-most mantle in diverging near-rotational patterns, whose axis is a zone of lower Rayleigh wave phase velocity connecting the two seismogenic regions. We propose a new geodynamic model to explain the observed anisotropy as well as a mechanism for rift initiation and consequent seismicity generation.