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 M can be characterized by a seismic coupling coefficient χ = AE/AT, where AE ∼ M/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 ∝ N0/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.