Low dissipation of earthquake energy along faults that follow
pre-existing weaknesses: field and microstructural observations of
Malawi’s Bilila-Mtakataka Fault
Abstract
Fracturing and gouge formation absorb ≥50% of earthquake energy on low
displacement (<1-2 km) faults in isotropic crust. To assess
how these processes absorb earthquake energy in anisotropic crust, we
performed field and microstructural investigations on the 110 km long,
0.4-1.2 km displacement, Bilila-Mtakataka Fault (BMF), Malawi. Where the
fault is parallel to surface metamorphic fabrics, macroscale fractures
define a 5-20 m wide damage zone. This is narrow relative to where the
BMF is foliation-oblique (20-80 m), and to faults with comparable
displacement in isotropic crust (~40-120 m). There is
minimal evidence for cataclasis and microfracturing along the BMF;
therefore, despite its 110 km length and geomorphic evidence for
MW 7-8 earthquakes, widespread fault zone fracturing has
not occurred. We attribute lack of damage to fault growth along shallow
and deep-seated pre-existing weaknesses. This conclusion implies that
earthquake energy dissipates differently along incipient faults in
isotropic and anisotropic crust.