11:00 〜 11:15
[HDS07-08] 縄文時代に発生した地すべりとの比較からわかる，2018年北海道胆振東部地震による地すべり群発の一因
キーワード：地震地すべり、ハロイサイト、火山灰土、古地震学
Previous studies discovered landslide deposits during excavation of the Apollo 1 site, a Jomon period archaeological site in Atsuma-Cho, Hokkaido, Japan, in 2003. We suggested that landslides were induced by an earthquake (Jomon period: 4 ka) on the Ishikari lowland east margin fault zone west of the site.
The 2018 Hokkaido Eastern Ibri Earthquake triggered a series of tephra slides in the mountainous area, which also occurred on the southern slope of the Apollo 1 site in Atsuma Town, 8 km north of the epicenter. We investigated the location and components of the landslides in 2018 and compared them with those of the Jomon event. The main constituents of the landslide-bodies in the 2018 and Jomon event are similar: Tarumae-d pumice fall deposit (Ta-d: 9ka) overlain by black soil, both 2-3m in total thickness. The 2018 slide was accompanied by thin layers of Ta-c (2.5ka) and Ta-b (AD1667) interbedded in black soil. The 2018 tephra slide occurred on most of the back slopes, while the Jomon event occurred only in a limited area of the western slope. However, the 2018 earthquake's mobile material flowed and moved long distances compared to the Jomon event. In the 2018 earthquake, the slip surface formed directly above the Ta-d basement (including talus and redeposited sediments) that was deposited on top of the underlying mudstone and rubble layer on the central to eastern slopes. In contrast, on the western slope, the slip surface was formed on the pumice-bearing volcanic ash soil Vs-En of the Eniwa, a pumice fall deposit (En-a: 19-21 ka) just below the Ta-d basement. The stratigraphic level of the Jomon event is not exposed, but it is highly likely that it is located in Vs-En just below Ta-d considering the event occurred on the western slope.
As described above, the tephra slip of the 2018 earthquake appears larger in scale and activity than that of the Jomon event. Tajika et al. (2016) considered the Jomon event an inland earthquake caused by the activity of the main part of the Ishikari Lowland East Margin Fault Zone (Umaoi Fault) given the coincidence of the timing of activity. The activity in the main part of the Ishikari Lowland East Margin Fault Zone is expected to cause a strong tremor (JMA seismic scale six lower or higher) in most of the Atsuma area(Earthquake Research Committee of the Headquarters for Earthquake Research Promotion, 2004). The intensity of the shaking is estimated to be the same as that of the 2018 earthquake.
One of the main causes for the widespread rupture in the 2018 earthquake is attributed to the widespread distribution of mantle-bedded Ta-d and the presence of weathered halloysite zones in the Ta-d (Chigira et al., 2019; Tajika et al., 2020). The slip surface of the fallen pyroclastic material that generated the tephra layer slip during the earthquake was formed in a layer rich in halloysite (Tanaka, 1992; Chigira, 2018) although the exact mechanism is unclear.
The weathering of pumice is influenced by the chemical composition of the host rock, moisture conditions such as precipitation, and the amount of organic matter. It is also well known that clay mineralization, in particular, requires time. The formation of halloysite takes about 10,000 years after deposition (Okada et al., 1986; Inoue, 1996). In other words, the Ta-d that erupted 9,000 years ago may not have been sufficiently clay mineralized to form halloysite during the Jomon period (4,000 years ago). This is supported by our finding fact that the weathering zone structure of the sliding body of the Jomon event largely formed after the landslide occurred. The weathering of En-a, which erupted about 20,000 years ago, and Spfa-1, which erupted 46,000 years ago, suggest that clay mineralization had already progressed to the point at which halloysite was sufficiently formed by the Jomon period. This is concordant to the limited occurrence of the tephra slip of the Jomon event on the western side. We suggest that the swarm of tephra slide induced by the 2018 earthquake is rooted in the halloysite formation of Ta-d pumice over 9,000 years.
The 2018 Hokkaido Eastern Ibri Earthquake triggered a series of tephra slides in the mountainous area, which also occurred on the southern slope of the Apollo 1 site in Atsuma Town, 8 km north of the epicenter. We investigated the location and components of the landslides in 2018 and compared them with those of the Jomon event. The main constituents of the landslide-bodies in the 2018 and Jomon event are similar: Tarumae-d pumice fall deposit (Ta-d: 9ka) overlain by black soil, both 2-3m in total thickness. The 2018 slide was accompanied by thin layers of Ta-c (2.5ka) and Ta-b (AD1667) interbedded in black soil. The 2018 tephra slide occurred on most of the back slopes, while the Jomon event occurred only in a limited area of the western slope. However, the 2018 earthquake's mobile material flowed and moved long distances compared to the Jomon event. In the 2018 earthquake, the slip surface formed directly above the Ta-d basement (including talus and redeposited sediments) that was deposited on top of the underlying mudstone and rubble layer on the central to eastern slopes. In contrast, on the western slope, the slip surface was formed on the pumice-bearing volcanic ash soil Vs-En of the Eniwa, a pumice fall deposit (En-a: 19-21 ka) just below the Ta-d basement. The stratigraphic level of the Jomon event is not exposed, but it is highly likely that it is located in Vs-En just below Ta-d considering the event occurred on the western slope.
As described above, the tephra slip of the 2018 earthquake appears larger in scale and activity than that of the Jomon event. Tajika et al. (2016) considered the Jomon event an inland earthquake caused by the activity of the main part of the Ishikari Lowland East Margin Fault Zone (Umaoi Fault) given the coincidence of the timing of activity. The activity in the main part of the Ishikari Lowland East Margin Fault Zone is expected to cause a strong tremor (JMA seismic scale six lower or higher) in most of the Atsuma area(Earthquake Research Committee of the Headquarters for Earthquake Research Promotion, 2004). The intensity of the shaking is estimated to be the same as that of the 2018 earthquake.
One of the main causes for the widespread rupture in the 2018 earthquake is attributed to the widespread distribution of mantle-bedded Ta-d and the presence of weathered halloysite zones in the Ta-d (Chigira et al., 2019; Tajika et al., 2020). The slip surface of the fallen pyroclastic material that generated the tephra layer slip during the earthquake was formed in a layer rich in halloysite (Tanaka, 1992; Chigira, 2018) although the exact mechanism is unclear.
The weathering of pumice is influenced by the chemical composition of the host rock, moisture conditions such as precipitation, and the amount of organic matter. It is also well known that clay mineralization, in particular, requires time. The formation of halloysite takes about 10,000 years after deposition (Okada et al., 1986; Inoue, 1996). In other words, the Ta-d that erupted 9,000 years ago may not have been sufficiently clay mineralized to form halloysite during the Jomon period (4,000 years ago). This is supported by our finding fact that the weathering zone structure of the sliding body of the Jomon event largely formed after the landslide occurred. The weathering of En-a, which erupted about 20,000 years ago, and Spfa-1, which erupted 46,000 years ago, suggest that clay mineralization had already progressed to the point at which halloysite was sufficiently formed by the Jomon period. This is concordant to the limited occurrence of the tephra slip of the Jomon event on the western side. We suggest that the swarm of tephra slide induced by the 2018 earthquake is rooted in the halloysite formation of Ta-d pumice over 9,000 years.