[G04-3] Earth rotation and geodynamics III

[G04-3-06] Vertical velocity profile and possible velocity changes in SW Japan from GNSS data over the last 20 years

^○Yutaro Iwasa, Kosuke Heki (Hokkaido University, Hokkaido, Japan)

Although GNSS (Global Navigation Satellite System) observations can measure 3-D crustal movement, past studies have focused on horizontal components, i.e. the vertical component has not been well utilized. This is due mainly to the lower signal-to-noise ratio of the vertical components. However, vertical movements are not much influenced by plate motions, and are more straightforward in their interpretation. In Japan, Aoki and Scholz (2003) analyzed vertical crustal movements in the Japanese Islands for 1996-1999. Here we present 20 years of vertical crustal movement data in 1996-2016 from the Japanese dense GNSS array GEONET. Using such long-period data, we analyzed the interplate coupling in the Nankai Trough, Southwest Japan, and estimated the coupling of individual segments of the interface between the Philippine Sea Plate and the Amurian Plate.
Such interplate coupling, in the Japan Trench, is reported to have gradually weakened over 7-8 years before the 2011 Tohoku-Oki earthquake. Classical studies of viscous flow contribution in subduction zones have also suggested that crustal deformation rate may change within an earthquake cycle. In the Nankai Trough, the next inter-plate earthquake is anticipated to occur within the coming years/decades. With the data spanning ~20 years, we could study temporal changes of the vertical velocities. Here we modeled them using quadratic functions of time, and discuss the significance of the quadratic terms. We compared the linear trend and the quadratic components of stations with various distances from the trench, and examined if the quadratic terms show real crustal movements or just a leakage from the movement of the reference point.