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There is a persistent body of literature that suggests low-angle normal faults (LANF) form and slip seismically; if true, the effective friction coefficient is much lower (<0.3) than that found in laboratory tests of rock friction (c. 0.8) and in low-displacement faults that lack well-developed fault cores. This paper summarizes and discusses the mechanisms proposed to explain the low apparent friction of crustal-scale faults with low resolved shear stresses. Emphasis is placed on differentiating static weakening mechanisms, operating at strain rates c. 10−12 s−1–10−15 s−1, from dynamic weakening mechanisms, operating at strain rates >10−1 s−1. Previous published explanations for low fault friction do not appear to meet the key requirements of (i) reducing both static and dynamic frictional strength of LANF and (ii) operating only along crustal-scale faults. Fault rock assemblages in quartzo-feldspathic continental crust reveal that grain size reduction, or comminution, plays a fundamental role in fault zone development. As a fault accrues displacement, a fault core forms that contains granular material. We postulate that dynamic rock fragmentation occurs during the shearing of confined granular material; dynamic fragmentation is a volume-dependent mechanism responsible for reducing the static and dynamic frictional strengths of faults.

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