Seismotectonics of Botswana: New insights from seismic velocity and anisotropy structure of the upper lithosphere

概要

The April 2017 Mw 6.5 earthquake of central Botswana, an intraplate region, was a
potential disaster that raised worrying questions on disaster preparedness in the region.
While understanding intraplate earthquakes such as this, remains an elusive challenge,
detailed knowledge of the seismic velocity structure, tectonics and geodynamics of the
region is important in understanding the potential dangers and implementing disaster
mitigation strategies. Botswana, whose location straddles the southern edge and
northwestern part of Congo craton and Kalahari craton, respectively, has seen limited
seismological studies compared to other parts of the world. However, as a result of recent
improvements in seismic station coverage, it is now known that the crust beneath the
Okavango rift Zone (ORZ) is thinner than the surrounding regions (e.g. Leseane et al.,
2015; Yu et al., 2015b; Fadel et al., 2018), and that this thinner crust extends into central
Botswana (Fadel et al., 2018; 2020). It is within this region that two main zones of
seismicity are observed, and despite a reported low velocity anomaly linking the two
zones (Fadel et al., 2020), their relationship in terms of tectonics and geodynamics
remains unknown. In this thesis, this relationship is explored by investigating the velocity
and anisotropy structure of the crust and uppermost mantle, which are important
indicators for crustal modification processes and regional stress state, leading to clues on
the associated seismotectonics in the region. The investigations are divided into three
sub-studies focusing on (i) intracrustal shear wave velocity (Vs) structure, (ii) dipping and
anisotropic shallow structures and (iii) azimuthal anisotropy of the crust and uppermost
mantle. ...