Asthenospheric viscosity inferred from correlated land–sea earthquakes in north-east Japan

Abstract

The viscosity of the Earth's mantle has been estimated from studies of post-glacial rebound1,2, post-seismic deformations of the ground following large earthquakes3,4, and aftershock sequences5–7. Here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction-induced seismicity between the intraplate (land) and interplate (sea) earthquakes in north-east Japan. The correlation persists since the time of reliably reported earthquakes in AD 1600; land events precede sea events by ∼36 yr, with a mean distance between land–sea pairs of ∼200 km. Because of the visocoelastic coupling of the lithosphere to the asthenosphere, a plausible mechanism to explain the correlation is stress migration, governed by the viscosity of the asthenosphere. Large land shocks generate diffuse-like stress pulses which sweep past, and unlock, the thrust fault in the subduction zone, thus triggering the sea events. The correlation time and distance provide a measure of the speed of diffusion (5.6 km yr−1) and hence an estimate of the viscosity (7 × 1018 Pa s).