Surface rupture and characteristics of a fault associated with the 2011 and 2016 earthquakes in the southern Abukuma Mountains, northeastern Japan, triggered by the Tohoku-Oki earthquake

Keitaro Komura,Kotaro Aiyama,Takahiro Nagata,Akihiro Yamada,Hiroshi P Sato,Yasuhira Aoyagi

doi:10.1186/s40623-019-1085-8

Abstract

The 2011 Tohoku-Oki offshore subduction earthquake (Mw 9.0) triggered many normal-type earthquakes inland in northeastern Japan. Among these were two very similar normal-faulting earthquakes in 2011 (Mw 5.8) and 2016 (Mw 5.9), which created surface ruptures along the newly named Mochiyama fault within the southern Abukuma Mountains, northeastern Japan, where no active faults had been previously mapped by interpretation of aerial photographs. We conducted field surveys in this area immediately after both earthquakes, and we performed trench excavations and observations of fault fracture zones after the 2016 event. These activities were complemented by an interferometric synthetic aperture radar analysis that mapped the areas of deformation and locations of surface discontinuities for both events. The combined results document the coseismic behavior of the Mochiyama fault during both events. Subtle tectonic geomorphic features associated with the fault were evident in a lidar digital elevation model of the area, and layered structures of gouge were documented in the field. These lines of evidence indicate repeated activity at shallow crustal levels and the possibility of Quaternary activity. In addition, our trench excavations revealed at least one faulting event before 2011. Our comparison of paleoseismic records on this and two other normal faults in the Abukuma Mountains suggests that great earthquakes in the Japan Trench supercycle of 500–700 years do not consistently trigger ruptures on these faults, and the case of 2011, in which the Tohoku-Oki megathrust earthquake triggered all three faults, is a rare occurrence.

Highlights

The 2011 Tohoku-Oki subduction earthquake (Mw 9.0) triggered many normal-type earthquakes in northeastern Japan, in the hanging wall of the convergent plate boundary along the Japan Trench (e.g., Kato et al 2011; Okada et al 2011; Imanishi et al 2012)
The LOS offset maps of the 2011 and 2016 events (Fig. 2a, b) show nearly identical curvilinear surface displacements extending about 10 km from northwest of Mochiyama past the Shin-Koyama bridge to southeast of Wakaguri
The interferometric synthetic aperture radar (InSAR) analyses showed that both events produced linear surface displacement and areas of deformation in the same locations, indicating the presence of an active normal fault that we have named the Mochiyama fault

Summary

Introduction

The 2011 Tohoku-Oki subduction earthquake (Mw 9.0) triggered many normal-type earthquakes in northeastern Japan, in the hanging wall of the convergent plate boundary along the Japan Trench (e.g., Kato et al 2011; Okada et al 2011; Imanishi et al 2012). The southern Abukuma Mountains, an area ~ 240 km southwest of the Tohoku-Oki epicenter measuring ~ 170 km from north to south and ~ 50 km from east to west, experienced many shallow normal-type earthquakes. The largest of these, the 11 April 2011 Iwaki earthquake (Mw 6.6), produced two subparallel surface ruptures ~ 15-km long and 14-km long (Fig. 1; Mizoguchi et al 2012; Toda and Tsutsumi 2013). A Mw 5.8 event on 19 March 2011 and a Mw 5.9 event on 28 December 2016, struck the same part of the southern Abukuma Mountains (Fig. 1).

Methods

Results

Discussion

Conclusion