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 dot above M can be characterized by a seismic coupling coefficient χ = A E/A T, where A E ∼ dot above M/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 = μD 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, ν0 ∝ inline equation 0/A T V, should scale as ν0 ∝ 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.