Abstract

The Kumamoto earthquakes in April 2016 have triggered numerous damages to the Tawarayama Tunnel in Kumamoto Prefecture, Japan. Distribution and characteristics of these seismic damages have been investigated and summarized to assess potential influencing factors. While some of influencing factors have not yet been clearly verified. Recently, it is thought that ground deformation around the underground structures might be one vital potential factor. It may be as an index for revealing the seismic wave propagation direction. With regard to the ground deformation, the ground horizontal and vertical displacements were detected by Structure from Motion (SfM) method. This study aims to explore possibility that the seismic damages to the Tawarayama Tunnel influenced by ground deformation.

1.
Introduction

A moderate-size earthquake on April 14, 2016 (21:26 JST-Japan Standard Time) with a magnitude 6.5 (Mj), at a depth of about 11 km, struck in Kumamoto City of Kumamoto Prefecture in Kyushu Region, Japan (Asian Disaster Reduction Center, 2016) (Fig.1). The fault rupture originated from the northern segment of the Hinagu fault. Subsequently, at 01:25 on April 16, 2016 (JST), a larger earthquake with magnitude 7.3 (Mj) at a depth of about 12 km occurred along the Futagawa fault (Fig.1). Furthermore, many aftershocks and related earthquakes occurred over a relatively long duration and wide area.

Whether it caused ground deformation is one of main problems caused by earthquake sequence (Park et al., 2017). In Japan, inland earthquakes with magnitudes greater than 7 are known to cause ground deformation; however, this rarely occurs for earthquakes with magnitudes below 6.5. For the mainshock of Kumamoto earthquake, at the Kumamoto GEONET station (32.8421N, 130.7648E), 0.75 m horizontal deformation in the ENE direction and 0.20 m downward deformation were recorded, at the Choyo GEONET station (32.8707N, 130.9962E), 0.97 m horizontal deformation in the SW direction and 0.23 m upward deformation were recorded (Goda et al., 2016; Zhang et al., 2018). Displacement measurement for detection of ground deformation is an important task in civil engineering projects. Currently, precise displacement monitoring methods, such as Global Navigation Satellite System(GNSS), Interferometric Synthetic Aperture Radar (InSAR), Differential Interferometry Synthetic Aperture Radar (DInSAR) and Structure from Motion (SfM) in geotechnical engineering are available (Chang and Chao, 2011; Fujiwara et al., 2016; Ozawa et al., 2016; Oda et al., 2016).

The Kumamoto earthquake sequence caused significant damage and disruption to the Kumamoto region, especially in areas near the fault (e.g., Mashiki Town, Nishihara Village, and Minami Aso Village). In the Takamori Line of Kumamoto Prefectural Route 28, Tawarayama Tunnel is located at a distance of about 22.4 km from the epicenter of the mainshock (Mj7.3), as illustrated in Fig.1. When subjected to the 2016 Kumamoto earthquakes, it was damaged severely. Distribution and characteristics of these seismic damages were investigated and summarized to assess potential influencing factors. Seismic damages are categorized into five patterns as follows: lining cracks, spalling and collapse of concrete lining, construction joint damage, pavement damage and groundwater leakage. Main factors influencing the seismic damages involving earthquake parameters, tunnel depth, surrounding rock quality, fault zone and slope at the portal were outlined and analyzed (Zhang et al., 2018).

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