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 = AE/AT, where AE ∼ /V is an effective seismic area and AT ∝ L3/2V−1/2 is the area above an isotherm Tref. A global set of 65 RTFs with a combined length of 16,410 km is well described by a linear scaling relation (1) AE ∝ AT, which yields χ = 0.15 ± 0.05 for Tref = 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 N0 and the upper cutoff moment MC = μDCAC depend on AT. Data for the largest events are consistent with a self‐similar slip scaling, DC ∝ AC1/2, and a square root areal scaling (2) AC ∝ AT1/2. If relations 1 and 2 apply, then moment balance requires that the dimensionless seismic productivity, ν0 ∝ 0/ATV, should scale as ν0 ∝ AT−1/4, which we confirm using small events. Hence the frequencies of both small and large earthquakes adjust with AT 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 AC ≤ AE. 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.