Contribution of Viscoelastic Stress to the Synchronization of Earthquake Cycles on Oceanic Transform Faults

Earthquake clustering can be promoted by local, regional, and remote triggering. The interaction between faults by static and dynamic stress transfer has received much attention. However, the role of quasi-static stress interaction mediated by viscoelastic flow is still poorly understood. Here, we investigate whether the tight synchronization of moment-magnitude 6 earthquakes every about 6 years on distant asperities in the Gofar-Discovery fault system of the East Pacific Rise may be caused by mechanical coupling within the lithosphere-asthenosphere system. We build a three-dimensional numerical model of seismic cycles in the framework of rate- and state-dependent friction with a brittle layer overlaying a viscoelastic mantle with nonlinear rheology to simulate earthquake cycles on separate asperities. The brittle section of the West Gofar fault consists of two frictionally unstable 20 km-long by 5 km-wide asperities separated by a velocity-strengthening barrier, consistent with seismic observations, allowing stress transfer by afterslip and viscoelastic relaxation. We find that viscoelastic stress transfer can promote the synchronization of earthquakes. Even if the asperities are separated by as far as 30 km, synchronization is still possible for a viscosity of the underlying mantle of 10(17) Pa s, which can be attained by dislocation creep or transient creep during the postseismic period. Considering the similarities in tectonic and structural settings, viscoelastic stress transfer and earthquake synchronization may also occur at 15'20 (Mid-Atlantic Ridge), George V (Southeast Indian Ridge), Menard and Heezen transform fault (Pacific-Antarctic Ridge).