Enhancing Detection of Small Earthquake Events from the 2017 Mw 6.5 Botswana Earthquake Using Cross-Correlation Back-Projection Analysis

Abstract

On 3 April 2017, an Mw 6.5 earthquake occurred in central Botswana, situated inside the southern African plate. However, in a region with sparse station coverage like Botswana, the earthquake catalog is usually largely incomplete at smaller magnitudes. Here we propose a modified back-projection method using a correlation characteristic function, which represents energy similarity and mitigates interference due to waveform complexity in regional seismic networks, aiming to enhance the detection of small seismic events. The energy-related characteristic function exhibits robust detections for low signal-to-noise ratio data, a capability not achievable by traditional arrival time picking methods. Detection reliability is ensured through a dual search constraint, optimizing effective signal count, considering nearby epicentral station signals, and accounting for seismic events clustering, which involves discarding seismic events from regions with infrequent earthquake occurrences. A total of 365 events (magnitude completeness of ∼ M 2), about four times more than the catalog obtained by other methods, have been detected within seven days after the Botswana mainshock. Most are low signal-to-noise ratio events for that are not localized by traditional methods. The resulting earthquake sequence occurred in at least three NW-SE fault strands between the Kaapvaal Craton and the Limpopo Belt. Our results, when combined with other geophysical data, suggest that the Botswana earthquake mainly results from the difference of intraplate basal dragging forces at various tectonic regions driven by the clockwise plate motions, which would generate extensional strains along the weak edges of rigid blocks.