Gravity derived crustal thickness model of Botswana: Its implication for the Mw 6.5 April 3, 2017, Botswana earthquake

Abstract

Botswana experienced a M w 6.5 earthquake on 3rd April 2017, the second largest earthquake event in Botswana's recorded history. This earthquake occurred within the Limpopo-Shashe Belt, ~350 km southeast of the seismically active Okavango Rift Zone. The region has no historical record of large magnitude earthquakes or active faults. The occurrence of this earthquake was unexpected and underscores our limited understanding of the crustal configuration of Botswana and highlight that neotectonic activity is not only confined to the Okavango Rift Zone. To address this knowledge gap, we applied a regularized inversion algorithm to the Bouguer gravity data to construct a high-resolution crustal thickness map of Botswana. The produced crustal thickness map shows a thinner crust (35-40 km) underlying the Okavango Rift Zone and sedimentary basins, whereas thicker crust (41-46 km) underlies the cratonic regions and orogenic belts. Our results also show localized zone of relatively thinner crust (~40 km), one of which is located along the edge of the Kaapvaal Craton within the M W 6.5 Botswana earthquake region. Based on our result, we propose a mechanism of the Botswana Earthquake that integrates crustal thickness information with elevated heat flow as the result of the thermal fluid from East African Rift System, and extensional forces predicted by the local stress regime. The epicentral region is therefore suggested to be a possible area of tectonic reactivation, which is caused by multiple factors that could lead to future intraplate earthquakes in this region.

Crustal and Upper Mantle Imaging of Botswana Using Magnetotelluric Method

“…Also, Moorkamp et al (2019) suggest that the 6.5 Mw earthquake reactivated existing fault from the deformation process of the collision between Kaapvaal and Zimbabwe cratonic blocks. However, other studies suggest that ascending fluids or melt from the EARS extends to central Botswana through the weak lithospheric zones and played a role in triggering the earthquake (Chisenga et al, 2020b;Fadel et al, 2020;Paulssen et al, 2022).…”

“…According to Fadel et al (2020), the ascending fluids or melt from the EARS into the region below central Botswana may be the cause of the 6.5 Mw earthquake. Similarly, Chisenga et al (2020b) modelled the crustal thickness of the crust beneath Botswana using gravity data. From their results, the crust beneath the epicentre of the 6.5 Mw earthquake in central Botswana is thinner with an approximate thickness of 40 km compared to 43 and 46 km thicknesses in the adjacent Kaapvaal Craton and central part of Limpopo Belt, respectively.…”

“…However, more recent seismological studies in the area argue that the Maltahohe microcraton exist as a separate structure from the Kaapvaal Craton, which is evident from the observed different Vp/Vs ratios from receiver functions and 3D shear wave velocity model (Fadel et al, 2018;Fadel et al, 2020). Similarly, Chisenga et al (2020b), in a study using gravity and aeromagnetic data supported the existence of the Maltahohe microcraton. However, the study argues that the location of the Maltahohe microcraton is likely south of the region suggested by Fadel et al (2020), Fadel et al (2018).…”

“…In the last 2 decades, different novel country-wide geophysical data have been compiled and processed in Botswana including gravity, magnetics, seismic, and MT data. Country-wide gravity and magnetic data provided some of the earliest understanding of the geological provinces and tectonics of Botswana due to the obscuring of the Precambrian geology by thick overburden formed from Kalahari group sediment (Hutchins and Reeves, 1980;Reeves and Hutchins, 1982;Chisenga et al, 2020b; Figure 1D). Also, several seismological studies have been done to understand the tectonics of Botswana.…”

Crustal Thickness Variations and Tectonic Settings in the Southwest Cameroon Inferred from Gravity and Topography Data

“…Thus, one of the motivating objectives is to make a joint analysis of the gravity and the topography signal which is essential in understanding the dynamic and role of the deep Earth's crust in controlling tectonic processes and surface deformation especially across the CC edge. This is particularly relevant in areas dominated by subduction fault that may give rise to large variations of stress in plate interiors [19,20].…”