Mode of seismic moment release at transform faults

Seismic moments of oceanic transform fault (TF) events being the largest ones at various TFs and covering a wide range of spreading rates, are obtained by the moment tensor and the scalar moment inversion methods for the P wave first motion amplitudes on WWSSN long period records. The seismic moment of the largest event at each TF shows a negative correlation with the spreading rate and a positive correlation with the elapse time, during which the ocean floor newly created at the ridge passes over ridge offset. The seismic moment has a poor correlation with geometrical parameters such as TF length and cumulative moment expected along the TFs. In the next step, cumulative seismic moments are deduced for three TFs, using Rayleigh wave excitations relative to those of the largest events, to infer seismic moments of small TF events. Ratios between the cumulative moments and the moments expected by simple plate kinematics amount to about 5.6% at the Eltanin fracture zone (spreading rate is 9 cm/yr), 29% at the Tasman fracture zone (7 cm/yr) and 35% at the George V fracture zone (7 cm/yr). It is clear that the faster the spreading rate is, the larger is the amount of TF movement released aseismically. This leads to the conclusion that hot mantle materials are supplied by fast moving convection currents and that the mode of the seismic moment release at TFs is principally governed by the thermal state in the uppermost mantle beneath the spreading center.