Earthquake scaling relations for mid-ocean ridge transform faults

A mid-ocean ridge transform fault (RTF) of length L, slip rate V, and moment release rate (M)over dot can be characterized by a seismic coupling coefficient chi=A(E)/A(T), where A(E)similar to(M)over dot/V is an effective seismic area and A(T)proportional toL(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)proportional toA(T), which yields chi=0.15+/-0.05 for T-ref=600degreesC. Therefore about 85% of the slip above the 600degreesC 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 beta=2/3 in which the cumulative number of events N-0 and the upper cutoff moment M-C=muD(C)A(C) depend on A(T). Data for the largest events are consistent with a self-similar slip scaling, D(C)proportional toA(C)(1/2), and a square root areal scaling (2) A(C)proportional toA(T)(1/2). If relations 1 and 2 apply, then moment balance requires that the dimensionless seismic productivity, nu(0)proportional to(N)over dot(0)/A(T)V, should scale as nu(0)proportional toA(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)less than or equal toA(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.