CRUSTAL STRUCTURE AND REACTIVATION PROCESSES ALONG LONG-LIVED INTRAPLATE FAULTS: INSIGHTS FROM THE 2017 MW6.5 BOTSWANA EARTHQUAKE

The 2017 Mw6.5 Botswana Earthquake and aftershocks define a segmented right-stepping cluster that coincides with a distinct >150 km-long NW-SE-striking magnetic-low anomaly lineament. This lineament has been interpreted to represent a crustal-scale fluid-channeling normal fault (named Moiyabana Fault, MF) that reactivated a Precambrian thrust fault within the Limpopo Belt. To better understand reactivation processes along long-lived intraplate faults such as the MF, we integrate aeromagnetic data with relocated earthquakes and crustal-scale magnetotelluric (MT) images of the epicentral area. Our MT transects consists of two 60 km-long profiles, one across the 2017 Mw6.5 epicenter and another across the southern segment of the fault. Our filtered aeromagnetic maps show that the MF consists of a central segment (MFC) of moderate magnetization, northern (MFN) and southern (MFS) segments of low magnetization in which the MFS shows the least magnetization. This variation in the magnetic signatures of the fault zone suggests that the fault segments are decoupled, supported by the clustering of the earthquakes largely within the MFC, extending partially into the MFN. If the observed decoupling signifies a variation in the activity of crustal fluids along the fault zone, we suggest that seismogenic fault reactivation in the MFC is driven by an episodic north-directed pressurized fluid pulse into a low-permeability, locked fault zone. Whereas, the MFS is possibly accommodating reactivation by aseismic slip driven by the fluids. Our MT data shows three geoelectric layers beneath the epicentral area, consisting of (1) shallow, 6-8 km -thick layer-1 of generally high resistivity (44-331 Ohm m), (2) conductive 8 km-thick layer-2 (7-25 Ohm m), and (3) basal layer-3 of moderate resistivity (37-53 Ohm m) that extends down to >44 km depths and exhibits the least electrical heterogeneity. Overall, we interpret that layers 1 and 2 constitute the Limpopo Thrust Wedge, overlying a less conductive and relatively less electrically-heterogeneous Kaapvaal Craton. The MT images of the crustal structure of the epicentral area suggest that the Limpopo Thrust Wedge is ~4km thicker beneath MFC than in the MFS, further supporting the proposition that the MFS has been exposed to deep crustal fluids for extended periods than the MFC.