Subsurface structure and faulting of the Median Tectonic Line, southwest Japan inferred from GPS velocity field

Abstract

The Median Tectonic Line (MTL) is the longest arc-parallel fault system in southwest Japan whose right-lateral strike-slip is related to oblique subduction of the Philippine Sea plate (PH). We constructed a dense Global Positioning System network along a 200 km-long traverse line across the MTL in 1998 to estimate deep fault structure and slip distribution. Horizontal velocities were determined at 65 sites through campaign measurements and show crustal shortening in the direction of the plate convergence. Using multi-rectangular segments and depthdependent coupling at the plate interface, we calculate and remove elastic deformation caused by the PH subduction. The residual velocity field shows right-lateral strike-slip block motion of about 5 mm/yr across the MTL, consistent with geological estimates. However, the block boundary does not coincide with the surface trace of the MTL, being displaced 20-30 km to the north. The residual velocity field is reproduced by a model with a 35-45• northwarddipping fault plane, full locking of the upper portion to a depth of 15 km, and steady slip of 5 mm/yr below. GPS results are supported by imaging of an inclined fault plane revealed by seismic profiling and currently low activity of shallow earthquakes.

Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan

“…1). The oblique subduction of Philippine Sea Plate (Seno et al 1993) induces a shear stress in the vicinity of the MTL and separates the fore-arc sliver from the crust of the arc at depth (Kamata and Kodama 1994;Miyazaki and Heki 2001;Tabei et al 2002). Along the western extension of the MTL in Kyushu, the shear stress is found to be partly released by deformation (Nishimura and Hashimoto 2006;Wallace et al 2009;Matsumoto et al 2015).…”

Present-day deformation across the southwest Japan arc: Oblique subduction of the Philippine Sea plate and lateral slip of the Nankai forearc

“…We model the residual velocity field in Fig. 4(a) according to the same procedures employed by Tabei et al (2002). First, we assume that the fault plane of the MTL dips to the north at a shallow dip angle based on the seismic reflection survey results of Ito et al (1996).…”

“…Figure 3(a) shows horizontal velocities at 23 sites together with those at 42 stations of the nationwide continuous array based on campaign-mode analyses. They calculated elastic deformations caused by plate subduction using a multisegment plate interface (Sagiya and Thatcher, Tabei et al, 2002). An open rectangle embedded in the figure represents the area of seismic reflection study by Ito et al (1996).…”

“…In the following, we will test the possibility by using different kinds of data. Tabei et al (2002) have conducted GPS campaign measurements along a 200 km-long traverse line across the MTL since 1998. Figure 3(a) shows horizontal velocities at 23 sites together with those at 42 stations of the nationwide continuous array based on campaign-mode analyses.…”

“…Adding velocity data from dense GPS campaign measurements across the MTL, Tabei et al (2002) showed that the forearc movement could be modeled by the steady aseismic slip at the depth of the MTL. McCaffrey (1996) showed that about half of modern subduction zones in the world have mobile forearc blocks, and the above studies indicate the Nankai Trough is one of them.…”