A new mechanical perspective on a shallow megathrust near-trench slip from the high-resolution fault model of the 2011 Tohoku-Oki earthquake

Abstract

The 2011 Tohoku-Oki earthquake generated a surprisingly large near-trench slip, and earth scientists have devoted significant attention to understanding why. Some studies proposed special rupture mechanisms, such as extensive dynamic frictional weakening; others simulated this near-trench slip behavior without supposing the extensive dynamic weakening. However, we have not reached a decisive conclusion for this question due to limited spatial near-trench slip resolution. Hence, in this study we use new tsunami data recorded just above the large slip area in addition to offshore and onshore geodetic data to improve the spatial resolution of stress release in the Tohoku-Oki earthquake and quantitatively examine the mechanical state of the plate interface. A maximum slip of 53 m reaching the trench and an insignificant stress drop (< 3 MPa) at the shallowest portion of the fault were estimated. Based on our modeling results and the past experimental studies, it is suggested that friction at the shallow near-trench portion should be inherently low both before and during the earthquake. This result provides perspectives on the shallow slip behavior along the plate boundary, in which the strain energy accumulation at the deep portion of the fault accounts for the anomalous large shallow slip, but shallow mechanical coupling does not. A large shallow slip has been considered as a result of the release of sufficiently large strain energy at the shallow portion of the plate interface, but we suggest that shallow slips similar to that during the 2011 Tohoku-Oki earthquake may occur in any subduction zones where the energy sufficiently accumulates only in the deeper portion.