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Low dissipation of earthquake energy along faults that follow pre-existing weaknesses: field and microstructural observations of Malawi’s Bilila-Mtakataka Fault
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  • Jack Nicholas Williams,
  • Ake Fagereng,
  • Luke N J Wedmore,
  • Juliet Biggs,
  • Hassan Mdala,
  • Felix Mphepo,
  • Michael Hodge
Jack Nicholas Williams
Cardiff University

Corresponding Author:[email protected]

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Ake Fagereng
Cardiff University
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Luke N J Wedmore
University of Bristol
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Juliet Biggs
University of Bristol, UK
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Hassan Mdala
Geological Survey of Malawi
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Felix Mphepo
Geological Survey of Malawi
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Michael Hodge
Cardiff University
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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.