Asthenospheric Viscosity and Stress Diffusion: A Mechanism to Explain Correlated Earthquakes and Surface Deformations In Ne Japan

SUMMARY A significant correlation is found, in both space and time, between the intraplate (land) and interplate (sea, thrust zone only) earthquakes in Tohoku, NE Japan that has persisted since the times of reliably reported events in AD 1600. the correlation peaks at a land‐lead of about 36 yr with an average correlation distance of 200km, with the implication of an average strain migration rate of 5.6 km yr ‐1 . the correlation is highly significant (> 99 per cent), both from formal statistics and from tests of random shuffles of the data. Additional analysis of the data, as a point process, confirms the results of the correlation analysis. the sharpness of the correlation peak, when compared to the individual times of occurrence of the land and sea events suggests a trigger mechanism. To explain the correlation, the general model ofsubduction‐rupture‐rebound is extended to include additional features; the buckling of the land plate from the force of the subducting slab, and the viscoelastic coupling of the plate to the underlying asthenosphere. A buckle produces a high‐stress region in the continental plate where earthquakes are more prone to occur, thus producing the spatial correlation in the data. This may also explain the preferred location on land for the smaller modern‐day seismic events in NE Japan. the viscoelastic coupling controls the interaction between the land and sea events, resulting in the temporal correlation in the data. Because of viscosity, the model equations are diffuse‐like with strain pulses as solutions; thus from the inferred strain migration rate it is possible to estimate asthenospheric viscosity (η=7 × 10 18 Pa s) using this model. A large land shock generates a strain pulse that affects the locked fault at the thrust zone several decades later. As the continental plate tends to pull away from the subducting slab, the frictional force arising from the overburden pressure is reduced, thus unlocking the fault and triggering a sea earthquake. The viscoelastic model is also used to explain surface deformations measured by triangulation surveys in Japan in 1904 and 1964. Horizontal displacements, which we believe are surface manifestations of the strain pulse from the large 1896 Riku‐U land shock ( M = 7.5) in NE Japan, are fit well by the model and provide a viscosity estimate η= 13 × 10 18 Pa s.