Earthquake scaling relations for mid-ocean ridge transform faults
Boettcher, Margaret S.
Jordan, T. H.
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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.
Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B12302, doi:10.1029/2004JB003110.
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