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Fault Model of the 12Th Century Southwestern Hokkaido Earthquake

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Fault Model of the 12Th Century Southwestern Hokkaido Earthquake Ioki et al. Earth, Planets and Space (2019) 71:54 https://doi.org/10.1186/s40623-019-1034-6 FULL PAPER Open Access Fault model of the 12th century southwestern Hokkaido earthquake estimated from tsunami deposit distributions Kei Ioki1* , Yuichiro Tanioka2, Gentaro Kawakami3, Yoshihiro Kase3, Kenji Nishina3, Wataru Hirose3, Kei’ichi Hayashi3 and Ryo Takahashi3 Abstract Tsunami deposits were collected along the coast of southwestern Hokkaido and Okushiri Island, northern Japan. The distribution of these deposits suggested that large earthquakes and tsunamis have repeatedly occurred of south- western Hokkaido. Along the southern coast of Okushiri Island, fve tsunami sand/gravel layers have been deposited during the last 3000 years. The latest was deposited by the 1741 Oshima–Oshima landslide tsunami and the second by the 12th century tsunami. The later tsunami was probably generated by a large earthquake because submarine seismo-turbidites with similar age exist in the region and a large inland landslide had occurred in Okushiri Island in approximately the 12th century. The ages of paleo-tsunami events prior to the 12th century are 1.5–1.6, 2.4–2.6, 2.8–3.1 ka. In this study, a fault model of the 12th century earthquake was estimated by comparing tsunami deposit distributions and calculated tsunami inundation areas at fve sites in Okushiri Island and Hiyama region. Fault model F17, a submarine active fault in the Japan Sea near Oshima–Oshima, is a probable source for this tsunami. Numerical simulation of the tsunami was performed based on fault model F17; we modifed the fault parameters (length and slip amount) from the original model to explain tsunami deposit distributions. A shorter length of 104 km and a larger slip amount of 18 m were appropriate for the fault model on the basis of parametric studies. The seismic moment of 20 10 2 the earthquake was calculated to be 9.95 10 Nm (Mw 7.9) assuming a rigidity of 3.43 10 N/m . The estimated fault model is located between the focal regions× of the 1993 Hokkaido Nansei-oki earthquake× and the 1983 Japan Sea earthquake. Keywords: Tsunami deposit, Tsunami inundation, Great earthquake, Hokkaido, Japan Sea Introduction 1998). Distribution of active faults in Japan Sea is also Earthquakes with magnitude larger than M w 7.5 have studied (Okamura 2019). repeatedly occurred from of northern Hokkaido to Te 1993 Hokkaido Nansei-oki earthquake (hereaf- Tohoku region in the Japan Sea (Katsumata et al. 2004). ter the 1993 earthquake) occurred near Okushiri Island, Large tsunamis have been generated by these earth- southwestern Hokkaido. A large tsunami was generated quakes and damaged the coastal areas in Hokkaido and by the 1993 earthquake (Shuto 1995), and more than Tohoku region (Watanabe 1998). Earthquake occur- 200 people were killed by the tsunami (Watanabe 1998). rence and tsunami generation are related to active faults Tsunami heights of more than 20 m were recorded along along the eastern margin of the Japan Sea (Okamura et al. the western coast of Okushiri Island (Tsuji et al. 1993; Kawakami et al. 2017a). Tsunami deposits of the 1993 earthquake are widely distributed along the west coast of Oshima Peninsula, southwestern Hokkaido (Nishimura *Correspondence: [email protected] 1 and Miyaji 1995). In a previous study, a source area of Geological Survey of Japan, National Institute of Advanced Industrial 2 Science and Technology, 1-1-1 Higashi, Tsukuba 305-8567, Japan 150 × 70 km was estimated by means of an inverse tsu- Full list of author information is available at the end of the article nami travel time diagram for the 1993 earthquake (Hatori © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ioki et al. Earth, Planets and Space (2019) 71:54 Page 2 of 9 1994). A fault model of the 1993 earthquake was esti- 2016; Kawakami et al. 2017b) (Fig. 1). Distributions of mated using seismic wave, tsunami, and geodetic data these deposits suggest that large earthquakes and tsu- (Satake and Tanioka 1995; Tanioka et al. 1995). A seis- namis have repeatedly occurred of southwestern Hok- 20 mic moment of 4.9 × 10 Nm (Mw 7.7) was also calcu- kaido. Tese deposits are distributed farther inland lated from the estimated slip distribution of the 1993 and at higher elevations than the tsunami inundation earthquake. area of the 1993 Hokkaido Nansei-oki earthquake. Te Te 1983 Japan Sea earthquake (hereafter the 1983 age of the widely distribution tsunami deposits is older earthquake) also occurred of the Tohoku region in the than the 1741 Oshima–Oshima tsunami (Satake 2007) Japan Sea. A large tsunami was generated by the earth- and has been estimated to be approximately 900 years quake (Shuto et al. 1995), and more than 100 people were old. Te source of this tsunami is considered to be near killed by the tsunami in Hokkaido, Aomori, and Akita that of the 1741 Oshima–Oshima tsunami (Kawakami Prefectures in the Tohoku region (Watanabe 1998). Tsu- et al. 2017a; Ioki et al. 2019). Te 12th century tsunami nami heights of more than 10 m were recorded along has a high possibility to be generated by an earthquake the coast of Akita Prefecture in the Tohoku region (Hata because of the existence of a seismogenic turbidite bed et al. 1995; Kawakami et al. 2017a). A tsunami source area (Nakajima and Kanai 2000) and a large subaerial land- 2 of 140 × 90 km was estimated by means of an inverse slide following the earthquake (Amemiya et al. 1998; refraction diagram for the 1983 earthquake (Hatori Kawakami et al. 2017b) dated at approximately the 12th 1983). A mechanism of the 1983 earthquake was esti- century and the existence of submarine active faults in mated from surface waves and tsunamis (Satake 1985). A the Japan Sea (Okamura et al. 2005). Te other reason 20 seismic moment of the 1983 earthquake of 4.2 × 10 Nm of that the 12th century tsunami has been generated by (Mw 7.7) was also estimated from tsunami waveform an earthquake is tsunamis have been repeatedly gen- inversion (Satake 1989). erated fve times over 3000 years based on the tsunami Tsunami deposits were collected along the coast of deposit survey (Kawakami et al. 2017b). Fault models of the Japan Sea in Hokkaido by recent studies (Kase et al. expected earthquakes in this region have been reported 43˚ 45˚ Hokkaido 00 0 −1 Hokkaido Okushiri 1993 Is. (b) 00− 2 30 0 − 0 42˚ 0 (a) Hiyama 1983 (c) 40˚ (d) −1 Tohoku 000 (e) 0 −100 Japan − 2 Japan 41˚ Sea 000 35˚ 0 0 −10 Aomori1000 Pacific Ocean Akita −2 30˚ 40˚ 000 130˚ 135˚ 140˚ 145˚ 150˚ 139˚ 140˚ 141˚ Fig. 1 The left fgure is a map of Japan with a depth contour interval of 2000 m. Sources of the 1993 Hokkaido Nansei-oki earthquake and 1983 Japan Sea earthquake were shown in ellipses. The right fgure is focused graphic of southwestern Hokkaido and northern Tohoku with a depth contour interval of 500 m. Tsunami deposit distributions are shown in pink circles; (a) Kaitorima on Okushiri Island, (b) Taisei in Setana town, (c) Himekawa in Otobe town, (d) Kaminokuni, and (e) Ishizaki in Kaminokuni town Ioki et al. Earth, Planets and Space (2019) 71:54 Page 3 of 9 by the Investigative Commission Related to Great Earth- Okushiri Island. Te recurrence interval of tsunami is quakes in the Japan Sea (2014) and the Hokkaido Disaster 200–1100 years (500 years on average). Te tsunami Management Council (2017). In this study, we estimated deposit ages were estimated using tephrochronology a fault model of the 12th century earthquake to explain and the accelerator mass spectrometry 14C age of peat. the tsunami deposit distributions (Fig. 2). Comparing the Tsunami sand sheets from the 12th century have been estimated fault model and the source models of the 1993 found between the Komagatake-d (Ko-d, 1640 AD; Tokui Hokkaido Nansei-oki and 1983 Japan Sea earthquakes, 1989) tephra layer and Baitoushan-Tomakomai (B-Tm, the rupture processes of earthquakes in the Japan Sea of ca 1 ka; Yin et al. 2012) tephra layer in Okushiri Island. Hokkaido, Japan, were discussed. Tsunami deposits from the 12th century tsunami have also been found along the Okushiri and Hiyama coasts Previous studies (Kawakami et al. 2017b). Te 12th century tsunami has Tsunami deposits no historical record because the written history for the Recently, tsunami deposits have been found at Okush- region was started in the late 17th century in southwest- iri Island and southwestern Hokkaido along the Japan ern Hokkaido. Sea coast (Kase et al. 2016; Kawakami et al. 2017b). Based on Geological Survey of Hokkaido (2015), Kase Along the southern coast of Okushiri Island, fve tsu- et al. (2016), and Kawakami et al. (2017b), tsunami nami sand/gravel layers have been deposited during deposit distributions are summarized in the follow- the last 3000 years. Te latest layer was deposited by ing. At Kaitorima on Okushiri Island [(a) in Figs. 1 and the 1741 Oshima–Oshima tsunami, and the second 3], tsunami gravels were found at a sea clif section on a layer was deposited by the 12th century tsunami. Te Holocene marine terrace at the present elevation of 8 m. ages of paleo-tsunami events prior to the 12th century Along the Wasabiyachigawa River lowland on Okushiri are 1.5–1.6, 2.4–2.6, 2.8–3.1 ka.
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