Low Dissipation of Earthquake Energy Where a Fault Follows Pre-Existing Weaknesses: Field and Microstructural Observations of Malawi’s Bilila-Mtakataka Fault

Abstract

During earthquakes on low (<1–2 km) displacement faults in isotropic crust, more earthquake energy is consumed by fracturing and gouge formation than in ruptures along more mature faults. To investigate how pre-existing weaknesses affect earthquake energy dissipation along low displacement faults, we studied fault rocks from the 110 km long, 0.4–1.2 km displacement, Bilila-Mtakataka Fault (BMF), Malawi. Where the BMF is parallel to surface metamorphic fabrics, macroscale fractures define a narrow (5–20 m wide) damage zone relative to where the BMF is foliation-oblique (20–80 m), and to faults with comparable displacement in isotropic crust (∼40–120 m). Enhanced microfracturing and widespread gouge formation, typically reported from comparable-displacement faults, are not observed. Therefore, minimal evidence for earthquake energy dissipation into the BMF’s surrounding wall rock exists, despite geomorphic evidence for MW 7.5–8 earthquakes. We attribute this finding to differences in earthquake energy partitioning along incipient faults in isotropic and anisotropic crust.