[S-SS13] Active faults and paleoseismology

[SSS13-P04] Relationship between faults and ground characteristics estimated by microtremor observations around Horinouchi of Hakuba Village

*Nobuyuki Nakajima¹, Mamoru Koarai², Shigeki Senna³ (1.Graduate School of Science and Engineering, Ibaraki University, 2.Earth Science course, College of Science, Ibaraki University, 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:the 2014 Northern Nagano Earthquake, Hakuba Village, microtremor observation, Kamishiro Fault

The November 22, 2014, 2014 Nagano Prefecture Northern Earthquake (M6.7) measured an intensity of just under 5 on the Japanese seismic scale in Hakuba Village, Nagano Prefecture, with damage concentrated in the Horinouchi and Mikkaichiba of Hakuba Village, about 5 km from the epicenter. Active faults such as the Kamishiro Fault are distributed around the Horinouchi , and a surface earthquake fault appeared during the 2014 Nagano Prefecture Northern Earthquake. Differences in fault dip angles (Nakano et al., 2015) and the possibility of terraces of landslide origin (Suzuki et al., 2015) have been pointed out as factors that contributed to the concentration of damage in the Horinouchi and Mikkaichiba s. Tomita et al. (2018) pointed out that in the damage within the Horinouchi , the AVS30 (average S-wave velocity to a depth of 30 m) was smaller at the points where the destroyed houses were concentrated, and that a soft layer of landslide origin may have been thickly accumulated. In addition, since there was little major damage between the main fault and its back thrust, the authors concluded that the influence of the fault was not significant in the difference in the degree of damage to houses. Nakajima et al. (2022a) conducted microtremor observation around the Miyanohara Fault in Tsunan Town, Niigata Prefecture, and found an inversion layer where S-wave velocities temporarily slowed down around the fault, suggesting the possibility of the existence of a fracture zone. The possibility of the existence of fracture zones was also pointed out. Considering such cases in other regions, the authors believe that more detailed microtremor observation will clarify the relationship between the fault and damage to houses, as well as the causes of the concentration of damage in the Horinouchi and Mikkaichiba s.

In this study, microtremor observation were conducted in the Horinouchi of Hakuba Village to investigate the relationship between the fault and earthquake damage. In addition to the 21 points in Tomita et al. (2018), and 25 newly observed points were added for a total of 46 points.

Compared to Tsunan Town in Niigata Prefecture and Sakae Village and Iiyama City in Nagano Prefecture (Nakajima et al., 2022b), the Horinouchi was found to be more prone to shaking due to the smaller AVS30 and deeper base depth (mainly the depth at which S-wave velocity exceeds 300 m/s) (Fig1). The large variation in AVS30 and base depth within the community indicates that the ground is locally poor. In particular, Horinouchi and around the Kamishiro fault is particularly poor, and the results are like or worse than those around the Miyanohara fault in Tsunan Town, Niigata Prefecture.

Differences in AVS30 and the degree of damage within the Horinouchi can be attributed to differences in the depositional environment. The west side of the fault is caused by Old Lake Kamishiro, while the east side of the fault has poor ground due to the sedimentary environment caused by a landslide, and the damage between the faults is concentrated directly above the fault, suggesting that the fracture zone has a large influence.

An inversion layer of S-wave velocity was observed around the fault, indicating the possibility of the existence of a fracture zone. The difference in elevation of the ground surface is approximately 5 m, whereas the difference in elevation of the base depth is 15 m, indicating that the fault displacement is cumulative(fig2). This result suggests that the Kamishiro Fault is repeatedly active.