Fast Displacement Rate of the Subducting Pacific Plate After the 2011 Tohoku-oki Earthquake Measured by GPS/Acoustic Surveys

Abstract:

The 2011 Tohoku-oki earthquake was a huge interplate earthquake, which posed various issues against our previous perceptions on the subduction process. One of the unprecedented ideas is the acceleration of the subducting plate after the earthquake suggested by Heki and Mitsui (2013). They expect that temporal loss of interplate coupling following a huge interplate earthquake will cause acceleration in subducting rate; Onshore GPS observations after the 2003 Tokachi-oki earthquake also support this hypothesis. However, this is an indirect evidence of the acceleration, and hence direct geodetic evidence is needed to examine this hypothesis. Here, we have challenged to directly detect the subducting rate of the Pacific plate using GPS/Acoustic technique. We show the displacement rate of the site on the Pacific plate, where four times of surveys have been conducted for 1.5 years during Sept., 2012 – Mar., 2014. In each survey, we determine the precise locations of the center of the transponder array. The displacement rate is estimated from the linear regression of time-series of the array positions at four campaigns. This shows clear northwestward movement amount to 17.2±4.3 cm/yr relative to the North American plate. This rate is roughly twice as fast as the global steady motion of 8.3 cm/yr given in MORVAL. However, the observed fast movement can be interpreted not only as the acceleration of the subduction but also as the effect of viscoelastic responses that relax the coseismic elastic deformation. At this moment, we cannot quantitatively distinguish the two interpretations above. According to Heki and Mitsui (2013), the acceleration propagates southern Kuril area. This region would show low effect of viscoelastic relaxation. So, if seafloor observation in this region is conducted, it may reveal the presence or absence of the acceleration. In this presentation, we will show forthcoming seafloor observation data planned to be obtained on this September and discuss on this issue in detail.