The relationship of source parameters of oceanic transform earthquakes to plate velocity and transform length

Open Access
Publisher Website
Google Scholar
Abstract
The source parameters of large earthquakes on oceanic transform faults are closely related to the thermal and mechanical properties of oceanic lithosphere. Several characteristics of these earthquakes (including magnitude, moment, and apparent stress ηare synthesized according to local plate velocityV, ridge‐ridge offsetL, and average fault widthWestimated by Brune's method. Several relationships result: (1) the maximum momentM0on a transform decreases withV, (2) maximumM0appears to increase withLforL< 400 km and may decrease for greater offsets, (3) ηdoes not clearly depend on eitherVorL, (4) the maximum estimatedW(V) decreases withV, (5) the minimum estimatedW(L) increases withL, and (6) the largest earthquakes on long transforms occur near the transform center. Most of these relationships are consistent with the hypothesis that seismic failure occurs only at temperatures below a fixed value. An inversion of slip rate and magnitude data by transform supports this hypothesis and gives an estimate for the nominal temperature of the boundary separating stick slip and stable sliding. Though the actual thermal structure around oceanic transforms is not known, the idealized spreading plate models used in the inversion give a temperature range for the brittle to ductile boundary of 75°–150°C. If possible uncertainties in the thermal structure are allowed for, a range of 50°–300°C provides a conservative bound on the true limiting temperature. These temperature ranges are consistent with focal depths of transform earthquakes and with laboratory measurements of fault slip in rocks of compositions that are representative candidates for the material being faulted in oceanic transforms.