Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

Abstract

Rayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3‐D crustal and upper mantle shear wave velocity model of Botswana. The model shows low crustal velocities associated with the Passarge and Nosop sedimentary basins, whereas the Kaapvaal, Zimbabwe, Maltahohe, and Congo Cratons are recognized by high mantle velocities. The lowest upper mantle shear wave velocity, beneath northeastern Botswana, is associated with the southwestern branch of the East African Rift System. This low‐velocity mantle anomaly appears to be linked to the crust of the Okavango Rift Zone and the location of the 3 April 2017 Mw 6.5 earthquake in central Botswana. We suggest that fluids or melt at the base of the crust from the southward continuation of the East African Rift Zone triggered the intraplate earthquake in an extensional tectonic setting.

Topography of the 410 and 660 km Discontinuities Beneath the Cenozoic Okavango Rift Zone and Adjacent Precambrian Provinces

“…Thick lithosphere beneath the Kalahari Craton is revealed with an average thickness of 242 ± 39 km, which is comparable to values of about 220 km from magnetotelluric (MT) inversions (e.g., Evans et al, 2011; Miensopust et al, 2011; Muller et al, 2009) and ~200 km or greater determined from a recent surface wave tomography study (Fadel et al, 2020). Both the southern Rehoboth Province (241 ± 35 km) and the Magondi Belt (251 ± 41 km) have a lithospheric thickness that is comparable with that of the Kalahari Craton, an observation that is consistent with the suggestion that both areas represent the westward extension of cratonic lithosphere based on craton‐like high‐velocity anomalies (Ortiz et al, 2019) and similar electrical structure (Muller et al, 2009).…”

“…This observation, together with results from other studies using the SAFARI data, including minor (≤ ~5 km) crustal thinning (Fadel et al, 2018; Yu, Liu, Reed, et al, 2015), absence of rifting‐related mantle flow field as revealed by shear wave splitting analysis (Yu, Gao, et al, 2015), and small (~ −1%) velocity anomalies in the upper mantle directly beneath the rift zone compared to other continental rifts (Yu et al, 2017), is inconsistent with the hypothesis that active mantle upwelling plays a major role in rift initiation. Minor mantle velocity anomalies associated with the ORZ are also suggested by recent body (Ortiz et al, 2019) and surface wave (Fadel et al, 2020) seismic tomography studies utilizing the same data set that is used for MTZ discontinuity imaging in this study.…”

“…For instance, strain localization during the initiation of the ORZ is proposed to be promoted by fluid‐assisted lithospheric weakening without asthenospheric involvement based on the shallow Curie point depth values and high heat flow along major rift‐related border faults and the Proterozoic orogenic bounders (Leseane et al, 2015). The recent crustal and upper mantle shear wave velocity model of Botswana from Rayleigh wave inversion (Fadel et al, 2020) indicates that fluids or melt in the lower crust and uppermost mantle beneath the ORZ is connected to the continuation of the southwestern branch of the EARS. A lack of elevated mantle conductivity has been revealed from inversion of MT data (Khoza et al, 2013), and tomography studies (Ortiz et al, 2019; Yu et al, 2017) have suggested that a localized low‐velocity anomaly is constrained in the upper asthenosphere beneath the ORZ without reaching the MTZ.…”

“…The thick lithosphere beneath the Kalahari and Congo cratons is commonly revealed from surface‐wave (e.g., Chevrot & Zhao, 2007; Fadel et al, 2020; Li & Burke, 2006) and body wave tomography (James et al, 2001; Ortiz et al, 2019; Youssof et al, 2015; Yu et al, 2017), RF (Wittlinger & Farra, 2007), and MT studies (e.g., Evans et al, 2011; Khoza et al, 2013; Miensopust et al, 2011; Muller et al, 2009). Most body wave tomography investigations report a much thicker lithosphere (Figure 8) beneath the Kaapvaal and Zimbabwe cratons extending to depths of at least 250 km and locally reaches 300–350 km (e.g., James et al, 2001; Youssof et al, 2015), which is comparable to the average value of 300 km from inversion of RFs (Wittlinger & Farra, 2007).…”

“…A recent body wave tomography reveals a craton-like root beneath southern Rehoboth Province (Ortiz et al, 2019), and whether such a thick lithosphere affects the MTZ is unknown. In this study, we take full advantage of the recent availability of a broadband seismic data set recorded by 39 stations under seismic networks of BX and NR ( Figure 1; Fadel et al, 2018Fadel et al, , 2020 to conduct a systematic investigation of MTZ structure beneath the ORZ and its surrounding regions. The greatly expanded data set enables us to image the MTZ discontinuities beneath a much larger area with a higher spatial resolution.…”