Localization of large intraplate earthquakes along faulted density-contrast boundaries: Insights from the 2017 Mw6.5 Botswana earthquake

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.