Abstract
The major controls on the localization of deep crustal intraplate earthquakes remain enigmatic due to their deep hypocentral depths and rarity of coseismic surface ruptures. Here, we investigate the 3-D crustal density structure of the 2017 Mw 6.5 Botswana earthquake epicentral region, a strong lower-crustal (∼24–29 km) event which is suspected to have reactivated a Precambrian structure via normal faulting. We perform a 3-D inversion of the gravity data using published geological constraints, then integrate the resulting density model with aftershock hypocenter distribution and use as constraints in a 3-D thermo-mechanical geodynamic model implemented in ASPECT. Our results reveal crustal blocks of density anomalies, with the aftershocks clustering along a prominent NW-trending, NE-dipping density contrast separating a high-density (>2708 kg/m3) footwall and lower-density (2670–2700 kg/m3) hanging wall blocks. Additionally, a secondary density contrast boundary in the hanging wall coincides with a splay of aftershock clusters at depth. Our observations suggest that the 2017 Mw6.5 Botswana earthquake nucleated near a fault intersection in the lower crust and is associated with brittle normal faulting reactivation of a long-lived basement fault that follows a prominent deep-reaching density contrast boundary. Further, as demonstrated by geodynamic modeling results, we argue that in regions of low tectonic loading rates, where stress perturbations are high enough, faulted crustal-scale density contrast boundaries are preferential concentrators of strain that may localize intraplate earthquakes.
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