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

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. Plain Language Summary: Earthquakes release elastic energy stored in the Earth's crust. Some of this energy radiates to the Earth's surface, where it causes the ground to shake and poses a risk to human life and infrastructure. However, some of this energy is also absorbed by breaking and fragmenting the rocks around the fault. This process is important in faults with small total displacements (<1–2 km), because these faults are still growing by breaking surrounding intact rock. The Bilila‐Mtakataka Fault (BMF) in southern Malawi is 110 km long but has a low displacement (0.4–1.2 km). We show that limited fracturing of the surrounding rock has occurred, despite an inferred history of MW 7.5–8 earthquakes. We propose that this is because earthquakes along the BMF exploited older planes of weakness in the Earth's crust, such as cracks and aligned mineral grains. Our results suggests that these pre‐existing weaknesses can influence how earthquake energy is released. Key Points: The 110 km long, 0.4–1.2 km maximum displacement, Bilila‐Mtakataka Fault (BMF) locally follows shallow crustal weaknessesThe BMF has limited fault gouge and a narrow damage zone compared to a similar displacement fault in intact crustLimited fracturing implies less seismic energy is consumed by fracture and frictional energy when ruptures follow pre‐existing weaknesses [ABSTRACT FROM AUTHOR]