Earthquake scaling relations for mid‐ocean ridge transform faults

Abstract

A mid‐ocean ridge transform fault (RTF) of length L , slip rate V , and moment release rate can be characterized by a seismic coupling coefficient χ = A E / A T , where A E ∼ / V is an effective seismic area and A T ∝ L 3/2 V −1/2 is the area above an isotherm T ref . A global set of 65 RTFs with a combined length of 16,410 km is well described by a linear scaling relation (1) A E ∝ A T , which yields χ = 0.15 ± 0.05 for T ref = 600°C. Therefore about 85% of the slip above the 600°C isotherm must be accommodated by subseismic mechanisms, and this slip partitioning does not depend systematically on either V or L . RTF seismicity can be fit by a truncated Gutenberg‐Richter distribution with a slope β = 2/3 in which the cumulative number of events N 0 and the upper cutoff moment M C = μ D C A C depend on A T . Data for the largest events are consistent with a self‐similar slip scaling, D C ∝ A C 1/2 , and a square root areal scaling (2) A C ∝ A T 1/2 . If relations 1 and 2 apply, then moment balance requires that the dimensionless seismic productivity, ν 0 ∝ 0 / A T V , should scale as ν 0 ∝ A T −1/4 , which we confirm using small events. Hence the frequencies of both small and large earthquakes adjust with A T to maintain constant coupling. RTF scaling relations appear to violate the single‐mode hypothesis, which states that a fault patch is either fully seismic or fully aseismic and thus implies A C ≤ A E . The heterogeneities in the stress distribution and fault structure responsible for relation 2 may arise from a thermally regulated, dynamic balance between the growth and coalescence of fault segments within a rapidly evolving fault zone.