Change of deep subduction seismicity after a large megathrust earthquake

Abstract

Subduction zones are home to the world’s largest and deepest earthquakes. Recently, large-scale interactions between shallow (0-60 km) and intermediate (80-150km) seismicity have been evidenced before megathrust earthquakes along with large-scale changes in surface motion. Large-scale deformation transients following major earthquakes have also been observed possibly due to a change in slab pull or to a bending/unbending of the plates, which suggests the existence of interactions between the deep and shallow parts of the slab. In this study, we analyze the spatio-temporal variations of the Japanese seismicity before and after the 2011/3/11 Mw9.0 Tohoku-oki earthquake. We observe that the rate of the intermediate to deep (150-450km) seismicity underwent a deceleration of 55% south of the rupture zone and an acceleration of 30% north of it, coherent with the GPS surface displacements. This shows for the very first time how a megathrust earthquake can affect the stress state of the slab over a 2500 km lateral-range and a large depth range, demonstrating that earthquakes interact at a much greater scale than the surrounding rupture zone usually considered. Our vision of subduction zones must consequently be renewed as a whole, from shallow to large depths.