The Horonobe Underground Research Laboratory (URL) is assumed to be located in an aseismic region and earthquakes are rarely occur. An earthquake with a moment magnitude of 4 occurred in at 5:28 (JST) in early morning of June 20, 2018. The strong motions induced by this earthquake were recorded by the accelerometers installed in the URL as well as the Kik-Net and K-Net strong motions networks operated by the National Research Institute for Earth Science and Disaster Prevention of Japan (NIED). The authors explain the results of analyses carried out on the ground amplification and frequency characteristics of the acceleration records at the URL and those of the Kik-net strong motion station and the structural effect of the URL on the ground amplification and frequency characteristics. Furthermore, the implications of the results obtained from this study in practice and the safety of the nuclear waste disposal at depth are discussed.

1 Introduction

In order to cover the general geological environment in Japan, two URLs, one for sedimentary rock and another for crystalline rock have been planned, one is the Horonobe Underground Research Laboratory the other is the Mizunami Underground Research Laboratory (JNC, 2001; 2002). One purpose of this plan is to confirm the technical reliability of the geological disposal methods for high-level radioactive waste, as indicated by the Second Progress Report (JNC, 2000).

The site of the URL project for sedimentary rock is located at Horonobe, in the northern part of Hokkaido, north of Japan. The geology consists of Tertiary and Quaternary sedimentary rocks. Conceptual design for the Horonobe URL at present is as follow:

- Two 500 m access shafts and one Ventilation Shaft; and

- Experiment levels, at 140 m, 250 m, 350 m and 500 m depths.

In 2012, excavation of the Ventilation Shaft and East Access Shaft reached 350 m depth, and the experiment gallery at 350 m depth was excavated between 2012 and 2014. In 2012, excavation of the West Access Shaft started, and reached 350 m depth in 2013. Countermeasures against rock bursts and large volume/high-pressure inflows of water or inflammable gas are important issues to be addressed during excavation of shafts and research galleries. Figure 1 shows the layout of the Horonobe URL.

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