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Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk
Quaternary International xxx (2017) 1e11
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Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk Akihisa Kitamura a, b, *, Takafumi Imai a, Yosuke Miyairi c, Yusuke Yokoyama c, Yasufumi Iryu d a
Institute of Geosciences, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan Center for Integrated Research and Education of Natural Hazards, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan Atmosphere and Ocean Research Institute, University of Tokyo, Chiba, 277-8564, Japan d Institute of Geology and Paleontology Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2017 Received in revised form 21 April 2017 Accepted 24 May 2017 Available online xxx
Over the last two decades, two giant tsunamis generated by earthquakes of approximately Mw 9 (the 2004 SumatraeAndaman and 2011 Tohoku earthquakes), and a large storm surge associated with the 2013 Super Typhoon Haiyan, have caused catastrophic damage to infrastructure, property, and industry in many areas of the western Pacific. If we are to improve coastal hazard assessment for the Pacific coast of the southwestern Japanese mainland, a reconstruction of the history of tsunamis and storm surges during the late Holocene is required. This study surveyed coastal boulders to determine whether such events have previously affected the islands of Niijima, Shikinejima, Kouzushima, and Miyakejima that lie offshore from the Tokyo Metropolitan Area. Coastal boulders on Kouzushima and Miyakejima were found with marine organisms attached. Radiocarbon dating of these organisms indicates that the boulders were emplaced during the periods AD 1694eModern and Modern, respectively. The boulder on Kouzushima (13.3 ton at 1.4 m ground elevation) was transported by historical tsunamis or severe storm surges, whereas the boulder on Miyakejima (33.4 ton at 7.1 m ground elevation) was probably transported by a storm surge associated with Typhoon 7920 in 1979. An emerged marine sessile assemblage on Miyakejima (2.31e3.06 m above mean sea-level (amsl)) was dated to ca. 3900e3500 years BP. This high relative sea-level can be explained by a mid-Holocene highstand and uplift associated with volcanic activity. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Coastal boulders Emerged marine sessile assemblage 14 C dates Holocene Tsunami Storm surge Kouzushima Miyakejima Izu island
1. Introduction Over the last two decades, coastal areas in the western Pacific have experienced two devastating tsunamis, which were generated by the approximately Mw 9 2004 SumatraeAndaman and 2011 Tohoku earthquakes, as well as a major storm surge associated with the 2013 Super Typhoon Haiyan. Following the Tohoku Earthquake in particular, coastal hazard assessments have become increasingly important for the Pacific coast of the southwestern Japanese
* Corresponding author. Institute of Geosciences, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan. E-mail addresses: [email protected] (A. Kitamura), t_imai_ [email protected] (T. Imai), [email protected] (Y. Miyairi), yokoyama@ aori.u-tokyo.ac.jp (Y. Yokoyama), [email protected] (Y. Iryu).
mainland. The Tohoku Earthquake occurred off the Pacific coast of northeastern Japan, and was the largest earthquake (Mw 9.0) ever recorded in Japan. It generated a mega-tsunami with a run-up height of 10e40 m in the coastal areas of Iwate, Miyagi, and northern Fukushima prefectures, and resulted in ~19,000 deaths. Based on lessons learned from this disaster, the Central Disaster Management Council (CDMC, 2011), has defined two types of tsunamis (i.e., Level 1 and Level 2 tsunamis), which could be generated in the Suruga and Nankai troughs, where the Philippine Sea Plate is subducting beneath the Eurasian Plate. Level 1 tsunamis, with wave heights of 5e10 m in coastal areas, have occurred every 100e150 years in the region since the 684 Hakuho Earthquake (e.g., Ando, 1975; Ishibashi and Satake, 1998; Watanabe, 1998; Sangawa, 2001, Fig. 1). Most of these tsunami events were caused by earthquakes of around Mw 8. Level 2 tsunamis are considered to be the
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Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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Fig. 1. (a) General view of the Suruga and Nankai troughs and the study area. Distribution of tsunami height and tsunami run-up (Tokyo Metropolitan Government Disaster Prevention, 2013) and ground survey areas in Niijima (b), Shikinejima (b), Kouzushima, (c) and Miyakejima (d), showing tide-gauge station, and GPS observation. Black lines along the coast are surveyed areas.
largest possible, and are caused by the largest conceivable earthquakes (Mw 9.1) that could occur along the megathrusts of the Nankai Trough (CDMC, 2012). Such events are infrequent but have the potential to cause widespread damage. CDMC (2011) noted that “… when conducting earthquake and tsunami hazard assumptions in the future, the largest-possible mega earthquakes and tsunamis should be considered from every possible angle. … in order to verify the occurrence of mega tsunamis over a time scale of several thousand years it is vital that further enhancement be made not only to seismological research but also to the comprehensive geological, archaeological and historical research, including research into tsunami deposits on land and the ocean floor, geological research into coastal terraces, and research into biological fossils etc.”.
The government of Japan (2012) presented eleven model cases of Level 2 tsunami wave heights. The wave heights of Level 2 tsunamis for model cases 1, 6, and 8 were higher than for the other cases when applied to the Tokyo Metropolitan Area, and the Kanagawa and Shizuoka prefectures, of central Japan. Fig. 1 shows predicted tsunami source area and wave heights of case 1. It is important to note that this tsunami model is not based on historical or geological evidence, and that it far surpasses the largest known historical event (e.g., Goto et al., 2014). CDMC (2011, 2012) initiated studies of tsunami deposits and boulders along coastal areas of Shizuoka Prefecture to determine whether Level 2 tsunamis had previously occurred over time-scales of several thousand years (Abe and Shirai, 2013; Fujiwara et al.,
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
A. Kitamura et al. / Quaternary International xxx (2017) 1e11
2013; Kitamura et al., 2013, 2014; Kitamura and Kobayashi, 2014a, b; Kitamura, 2016; Sawai et al., 2016). As Kitamura (2016) found no geological evidence of a Level 2 tsunamis in the coastal area of Shizuoka Prefecture over the past 4000 years, it seems reasonable to conclude that no such events occurred in the Tokyo Metropolitan Area either. To confirm this inference, we examined boulders deposited along the coasts of Niijima, Shikinejima, Kouzushima, and Miyakejima islands, which lie offshore from the Tokyo Metropolitan Area, for datable biogenic remains. According to the Tokyo Metropolitan Government Disaster Prevention report (2013), the maximum Level 2 tsunami wave heights in these four areas were estimated to be 31, 28, 25 and 18 m, respectively. This study also provides information on ancient storm surges, because coastal boulders are also transported by storm surges (e.g., Etienne and Paris, 2010; Goto et al., 2011; Kennedy et al., 2017). Understanding storm-surge hazards has become increasingly important in mid-latitude areas since Super Typhoon Haiyan which made landfall in the Philippines in November 2013 and was one of
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the strongest tropical cyclones of the satellite era (Kennedy et al., 2017). Moreover, many recent studies have suggested that tropical cyclone intensities will increase and that their latitudes of maximum intensity will shift polewards as the climate warming (Balaguru et al., 2016; Sobel et al., 2016). We also analyzed emerged marine sessile assemblages, because pre-instrumental data of vertical crustal movements are necessary to calibrate the elevation during the deposition of boulders. Lack of calibration can lead to under- or over-estimation of coastal hazards. Thus, our geological study contributes to assessing coastal hazards in and around the study areas. 2. Study areas Determining the timing of boulder transportation is essential, if we are to accurately estimate the magnitude of past coastal inundations. This is because coastal inundation magnitude is controlled by temporal changes in relative sea-level and the
Fig. 2. (a) Mean relative sea-level changes since 1964, as measured by tide gauges (Coastal Movements Data Center). Tide gauge stations are shown in Fig. 1. (b) Relative vertical displacement at GPS stations during the period 1 January 1998 to 18 April 2015 at Kouzushima and Miyakejima. These data are from time-series data of the F3 solution (Nakagawa et al., 2009), based on permanent GPS data provided by the Geospatial Information Authority of Japan. GPS stations are shown in Fig. 1.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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geometry of the coast. The coastal areas of Niijima, Shikinejima, Kouzushima, and Miyakejima islands were surveyed for boulders. Only two coastal boulders, with calcareous marine biogenic remains attached, were found on Kouzushima and Miyakejima. This study therefore deals with the boulders on these two islands. To our knowledge, there are no reports disturbance of these boulders by local people. Kouzushima and Miyakejima are situated on the IzueMariana Arc (Fig. 1), and are composed mainly of Quaternary volcanic rocks (Isshiki, 1960, 1984, 1987) that are predominantly rhyolitic and basaltic, respectively (Isshiki, 1960; Taniguchi, 1977; Tsukui et al., 2001). There are no reports of historical tsunamis on either Kouzushima or Miyakejima (Watanabe, 1998). However, the 1923 Taisho Kanto (Mw 7.9) and 1703 Genroku Kanto (Mw 8.4) earthquakes occurred along the megathrust in the Sagami Trough, and generated tsunamis with wave heights of 10e12 m at Izu Oshima, which lies about 70 km north of Kouzushima (Watanabe, 1998). The 2011 Tohoku Earthquake produced tsunami waves of 0.85 m on both Kouzushima and Miyakejima (Japan Meteorological Agency (JMA), undated; Shibata et al., 2012). Approximately 80 typhoons have approached within 150 km of Kouzushima and Miyakejima since 1951 (JMA undated; Kitamoto,
undated). Severe typhoons such as Typhoon 7920 (see below) have often inundated the Izu islands (JMA, undated). The coastlines in the study areas are characterized by wavedominated and microtidal regimes, with a maximum tidal range of 1.6 m during spring tides. The mean sea level (msl) in the study areas is based on tidal records from Miyakejima and Izu Oshima. Tide-gauge data (Japan Coast Guard, Hydrographic and Oceanographic Department, undated) and Global Positioning System (GPS) data indicate ~0.9 m of uplift off west Kouzushima Island during the period 1990e2008 (Fig. 2). This uplift is related to the pressure source located in the northeastern part of Kouzushima (Kimata et al., 2000) and dike intrusion associated with the AD 2000 eruption of Miyakejima (Murakami et al., 2001; Nishimura et al., 2001; Yamaoka et al., 2005). GPS data collected since March 1997 revealed that eastern Miyakejima experienced subsidence of about 0.5 m in 2000 (Fig. 2) due to dike intrusion associated with the eruption of Miyakejima (Murakami et al., 2001; Nishimura et al., 2001; Yamaoka et al., 2005). Imai et al. (2017) reported that an emerged marine sessile assemblage occurs at an elevation of 4.4e6.9 m above mean sea level (amsl) on Kouzushima Island, and that the 14C ages range from AD 613 to AD 1165.
Fig. 3. (a) Map of Kouzushima (Digital Map 25,000 Kouzushima) showing the coastal boulder location, tide-gauge station, and GPS observation. (b) Detailed map showing the location of coastal boulder (open circle).
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
A. Kitamura et al. / Quaternary International xxx (2017) 1e11
3. Methods This study identified two coastal boulders that carried calcareous marine biogenic remains on Kouzushima and Miyakejima (Figs. 3e6; Table 1), hereinafter referred to as K1 and M1, respectively. We found many well-rounded boulders close to M1 and measured eleven of them along a transect. They are referred to
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M2M12 (Fig. 7; Table 1). We also measured boulder distances from the shoreline or cliff edge, and boulder elevations. Boulder volumes and masses were calculated from the a, b, and c axes. Boulder weights were estimated by measuring the density of small specimens of the same rock types as the boulders. As noted above, data are available regarding vertical crustal motion over the past 1000 years on Kouzushima (Imai et al., 2017).
Fig. 4. (a) Map of Miyakejima (Digital Map 25,000 Miyakejima) showing the coastal boulder location, emerged marine sessile assemblage, tide-gauge station, and GPS observations. (b) Detailed map showing the location of the coastal boulder (open circle). (c) Detailed map showing the location of the emerged marine sessile assemblage (open circle).
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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Table 1 Measured boulders. Sample number Kouzushima K1 Miyakejima M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12
Coordinates (Lat/Long)
Ground Elev. (m)
Top lev. (m)
Dimension (m)
Weight (tons)
Distance from the edge (m)
Age
34.1313 139.0744
1.4
3.7
2.90 2.05 1.92
13.30
10.7
AD 1694Modern
7.1 8.4 8.6 8.6 8.7 9.0 9.2 9.2 9.3 9.3 9.4 9.4
9.7 9.2 9.2 9.5 9.3 9.4 10.5 10.1 10.1 10.0 10.4 10.1
4.90 1.06 0.70 1.10 1.02 1.10 1.46 0.75 0.70 0.86 1.15 0.90
33.41 0.84 0.31 1.14 0.67 1.00 1.00 0.32 0.28 0.45 0.74 0.34
1.5 2.5 7.7 8.2 10.8 16.3 22.0 23.0 23.8 24.7 26.0 27.0
Modern e e e e e e e e e e e
34.0351 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353
139.2842 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843
2.64 0.82 0.60 0.94 0.77 0.91 1.06 0.63 0.70 0.63 0.97 0.66
1.80 0.72 0.55 0.82 0.63 0.74 0.48 0.50 0.42 0.62 0.49 0.42
Fig. 5. Boulder (K1) located in northwest Sawajiri Bay, Kouzushima. (a), (b) Photographs of boulder. (c) Photograph of bivalve. (d) Close-up photograph of bivalve. (e) Sketch profiles at this site.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
A. Kitamura et al. / Quaternary International xxx (2017) 1e11 Table 2 Results of
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14
C dating of samples from Kouzushima and Miyakejima.
Coordinates (Lat/Long)
Height (m)
Materials
Conventional
34.035120 139.284240 34.035120 139.284240 34.13139 139 07 446 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175
8.15 8.18 3.60 2.58 2.58 2.31 3.06 3.06 3.06
Barnacle Barnacle Bivalve Favia sp. Favia sp. Serpulidae sp. Septifer exvisus Lithophaga sp. Lithophaga sp.
1599 ± 20 1708 ± 22 615 ± 27 5292 ± 35 5274 ± 32 4743 ± 31 3932 ± 31 4069 ± 26 3880 ± 38
In contrast, no study has examined vertical crustal motion for the pre-instrumental period on Miyakejima. Thus, this study examined emerged marine sessile organisms as well as the fossils of corals and molluscs collected at one site on Miyakejima (Fig. 3). Before the biogenic remains were identified, each sample was cleaned with a micro-knife and organic matter was removed using hydrochloric acid. The coral samples were analyzed using X-ray
14
C age (yr BP)
Calibrated age (2s) (cal yr)
Calibrated age (2s) (cal BP)
Modern Modern Cal AD 1694 - Modern Cal BC 3749e3401 Cal BC 3701e3380 Cal BC 3094e2697 Cal BC 2008e1642 Cal BC 2197e1818 Cal BC 1951e1570
Modern Modern 256 - Modern 5698e5350 5650e5329 5043e4646 3957e3591 4146e3767 3900e3519
diffraction (XRD) to check for diagenetic alteration. Accelerator Mass Spectrometry (AMS) radiocarbon dating of the samples was performed at the University of Tokyo, Japan (Yokoyama et al., 2016). The radiocarbon ages were calibrated onto a calendar timescale using the program OxCal 4.2.4 (Bronk Ramsey, 2009), based on comparisons with Marine 13 data (Reimer et al., 2013), after applying the local correction of DR ¼ 109 ± 60 (Yoneda et al., 2000)
Fig. 6. Photographs of boulder (M1) at the Ako Port locality, Miyakejima. (a), (b), (c) Photographs of the cliff-top boulder. (d), (e) Detailed photographs showing the occurrence of barnacles on boulder. (f) Sketch profiles at this site.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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for Shimoda, Shizuoka Prefecture. 4. Results 4.1. Kouzushima The Kouzushima coastal boulder (K1) was located 10.7 m from the shoreline and 1.43 m amsl (ground elevation) in the northwest area of Sawajiri Bay (Fig. 3). K1 was well rounded and ellipsoidal in shape, 2.90 2.05 1.92 m in size, composed of rhyolite (density 2.23 g/cm3), and had an estimated mass of 13.3 tons (Fig. 5). A wellpreserved fragment of mollusc was attached (Fig. 5c and d), but was not identified. The elevation of the specimen was 3.6 m amsl and its 14 C age was determined to be in the range AD 1694eModern (Table 2).
have been caused by acid rains associated with the release of SO2 gas during the 2000 eruption of Miyakejima (Nakada et al., 2004). The 14C ages of the specimens were Modern (Table 2). Boulders M2M12 were distributed 2.5e27.0 m from the cliff edge at ground elevations of 8.4e9.4 m amsl (Fig. 7; Table 1). They were well-rounded and ellipsoidal in shape. Their estimated masses ranged from 0.23 to 1.14 tons, and decreased landwards (Table 1). The emerged marine sessile assemblage was located at Anouzaki, on east Miyakejima (Fig. 8). XRD analysis indicated that two in situ colonial zooxanthellate scleractinian corals of Favia sp. (2.58 amsl; Fig. 8a, b) were 100% aragonite, showing that they were not diagenetically altered. Individual examples of Septifer exvisus, Serpulidae sp., and Lithophaga sp. were observed at 2.31e3.06 m amsl on coastal bedrock (Fig. 8c, d), and their 14C ages fell within the range 5698e3519 cal. BP (Fig. 9; Table 2).
4.2. Miyakejima 5. Discussion The Miyakejima boulder (M1), with barnacles attached, was found in an area of low cliffs south of Ako Port (Fig. 3b). M1 was located 1.5 m from the cliff edge at an elevation of 7.1 m amsl. M1 was composed of angular basaltic rock (density 2.57 g/cm3), was 4.90 2.64 1.80 m in size, and had an estimated mass of 33.41 tons (Fig. 6). As barnacle surfaces had been affected by dissolution (Fig. 6d and e), the species could not be identified. Dissolution could
5.1. Coastal boulders The 14C dating indicates that boulders K1 (Kouzushima) and M1 (Miyakejima) were deposited at AD 1694eModern and modern (i.e., after AD 1950), respectively (Table 2). We infer that the deposition of K1 and M1 was not related to Level 2 tsunami events,
Fig. 7. Photographs of well-rounded boulders close to M1 at the Ako port locality, Miyakejima. (a) Photographs of the boulders. (b) Close-up photograph of boulders.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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Fig. 8. Photographs of Anou-zaki. (a) Close-up of the exposure. (b) Detailed photograph of Favia sp. (sample M2). (c) Close-up of the exposure. (d) Detailed photograph of Septifer exvisus (sample M3).
Fig. 9. Elevations and ages of emerged sessile assemblages at Anou-zaki, Miyakejima.
but was caused by either storm surges or historical tsunamis. As noted above, there is no evidence of historical tsunamis on Kouzushima (Watanabe, 1998). However, in Izu Oshima, which is located 55 km north of Kouzushima, the AD 1703 Genroku Kanto (Mw 8.4), AD 1854 Ansei-Tokai (Mw 8.4) and AD 1923 Taisho Kanto (Mw 7.9) earthquakes produced tsunami waves of 10, 3, and 12 m, respectively (Watanabe, 1998). However, west Kouzushima Island
experienced 0.9 m of uplift during the period 1990e2008. If one of these historical tsunamis transported K1, then we can estimate that the boulder's pre-uplift ground and top elevations were 0.5 m and 2.8 m, respectively. Recently, Kennedy et al. (2017) presented the masses of coastal boulders transported by known storm waves as a function of ground elevation. Based on this relationship, the emplacement of K1 (13.3 tons) can be explained by a storm surge.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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It is certain that the cliff-top Miyakejima boulder M1 was emplaced after 1950. The western area of Miyakejima has experienced up to 0.8 m of subsidence since the 2000 volcanic eruption (Fig. 4b), so the ground elevation at the time of boulder emplacement would have been 7.9 m amsl. As no large tsunamis have been reported along the Miyakejima coast since 1950, a storm surge must have deposited M1 and probably M2eM12 as well. Etienne and Paris (2010) reported boulders of up to 70 tons in mass with a possible storm-surge origin in Iceland, where tsunami impacts have not been recorded historically. The possible stormsurge origin of cliff-top boulders in the Okinawan islands, Japan, has also been proposed (Goto et al., 2011). In addition, the relationship between boulder mass and ground elevation (Kennedy et al., 2017) further supports the suggestion that boulder M1 on Miyakejima (33.4 tons) was deposited at 7.1 m amsl by a storm surge. An example of a storm capable of transporting boulders of this size is Typhoon 7920, which passed Japan in 1979 with a minimum atmospheric pressure of 870 hPa and a maximum wind speed of around 50 m/s (JMA, undated). At Irozaki, on the southern Izu Peninsula, the significant wave height and significant wave period of Typhoon 7920 reached 8.24 m and 12.9 s, respectively. It is likely that this extreme storm surge transported boulders M2eM12 on Miyakejima and K1 on Kouzushima. About 5,000 peoples live on Miyakejima and Kouzushima islands, but about 74,000 visit every year. There are active volcanos in these islands. In the 20 century, Miyakejima Volcano erupted at AD 1940, 1962, 1983 and 2000 (Tsukui et al., 2001; Yamaoka et al., 2005). The last eruption caused complete evacuation of Miyakejima island for four years. As noted above, the climate warming enhances intensities of tropical cyclone and poleward shift of their latitudes of maximum intensity (Balaguru et al., 2016; Sobel et al., 2016). This raises the possibility that severe storm surge would occur at volcanic activity. Our results is also important to assessing the complex disaster in Miyakejima island.
5.2. Vertical displacement at Miyakejima As the results of 14C dating indicate that M1 was emplaced after AD 1950, data from the emerged marine sessile assemblage were not used to calibrate the ground elevation of M1. Nonetheless, these data can be applied to evaluate pre-historical extreme high-energy wave deposits. GPS station Miyakejima 3 near Anou-zaki experienced 0.5 m of subsidence during the 2000 eruption of Miyakejima Volcano (Fig. 4b). Septifer exvisus (sample M4-1, 3.06 m amsl) and Lithophaga sp. (sample M4-2, 3.06 m amsl) inhabit the intertidal zone (þ0.8 to 0.8 m amsl) to depths of 20 m (Okutani, 2000). Assuming that these specimens inhabited the upper limit of this vertical range (i.e., 0.8 m amsl), then subsidence of 0.5 m since 2000 indicates that, at the time of their active growth (3900e3500 years BP), sealevel would have been 2.76 m higher than at present. The Japanese archipelago is located far from former ice sheets (so called “far-field” regions). The mid-Holocene sea-level highstand (HHS) has been recorded at many locations in Japan (e.g., Ota et al., 1990). The HHS was caused by sea level equivalent ice melting in Antarctica (Nakada and Lambeck, 1987; Fleming et al., 1998). Yokoyama et al. (2012) showed that the sea-level reached a maximum HHS of 2 m above the present-day sea level around 4000 years ago on the Shimokita Peninsula, northern Japan. The remaining 0.76 m can be attributed to local uplift, possibly caused by volcanic activity after the emergence of the marine sessile assemblage.
6. Conclusions Two coastal boulders with calcareous biogenic remains attached were found on Kouzushima and Miyakejima islands. Radiocarbon dating of the attached marine organisms showed that the boulders were emplaced during the periods AD 1694eModern and Modern, respectively. The Kouzushima boulder was transported by historical tsunamis or severe storm surges, whereas the Miyakejima boulder was probably transported by a severe storm surge caused by Typhoon 7920 in 1979. In summary, we found no geological evidence of Level 2 tsunamis in the coastal areas studied, which supports Kitamura's (2016) conclusion that Level 2 tsunamis might not have occurred in the Tokyo Metropolitan Area or in Kanagawa and Shizuoka prefectures over the past 4000 years. An emerged marine sessile assemblage was discovered at one Miyakejima site. Based on the distribution of species and crustal movement records, the sea-level around 3900e3500 years BP is estimated to have been 2.76 m amsl. This high relative sea-level can be explained by a midHolocene highstand combined with uplift associated with volcanic activity. Acknowledgements We thank Dr. Kenji Harada for support on discussion. Sampling of specimens was conducted under permit from the Agency for Cultural Affairs, Kanto Regional the Ministry of the Environment. We would like to thank anonymous reviewers, for their thoughtful input into this manuscript. This study was funded by Grants-in-Aid (26287126, 17H02972) awarded by the Japan Society for Promotion of Science. References Abe, T., Shirai, M., 2013. Event deposits correlated with a historical tsunami in the Edo period, on the coastal lowland of the Atsumi Peninsula, central Japan. Quat. Res. (Daiyonki-Kenkyu) 52, 33e42. Ando, M., 1975. Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan. Tectonophysics 27, 119e140. Balaguru, K., Foltz, G.R., Leung, L.R., Emanuel, K.A., 2016. Global warming-induced upper-ocean freshening and the intensification of super typhoons. Nat. Commun. 7, 13670. http://dx.doi.org/10.1038/ncomms13670. Bronk, R.C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51, 337e360. Cabinet Office, Government of Japan, 2012. Report of Investigative Commission of Large Earthquake at Nankai Trough (Second Report)dTsunami Fault Model, p. 103 (in Japanese). http://www.bousai.go.jp/jishin/nankai/nankaitrough_info. html. Accessed 26 January 2017. Central Disaster Management Council, 2011. Report of the committee for technical investigation on countermeasures for earthquakes and tsunamis based on the lessons learned from the “2011 off the Pacific coast of Tohoku earthquake”. http://www.bousai.go.jp/kaigirep/chousakai/tohokukyokun/pdf/Report.pdf. Accessed 26 January 2017. Central Disaster Management Council, 2012. Final report - toward the reconstruction for sound and unwavering Japan. http://www.bousai.go.jp/kaigirep/ chuobou/suishinkaigi/english/pdf/Final%20Report.pdf. Accessed 26 January 2017. Coastal Movements Data Center (accessed on 3 December 2016) Table of annual mean sea level along the Japanese coast. http://cais.gsi.go.jp/cmdc/center/ annual.html#9. Etienne, S., Paris, R., 2010. Boulder accumulations related to storms on the south coast of the Reykjanes Peninsula (Iceland). Geomorphology 114 (1), 55e70. Fleming, K., Johnston, P., Zwartz, D., Yokoyama, Y., Lambeck, K., Chappell, J., 1998. Refining the eustatic sea-level curve since the Last Glacial Maximum using farand intermediate-field sites. Earth Planet. Sci. Lett. 163 (1), 327e342. http:// dx.doi.org/10.1016/S0012-821X(98)00198-8. Fujiwara, O., Sato, Y., Ono, E., 2013. Researches on tsunami deposits using sediment cores: 3.4 ka tsunami deposit in the Rokken-gawa Lowland near Lake Hamana, Pacific coast of Central Japan. J. Geogr. (Chigaku Zasshi) 122, 308e322 (in Japanese with English abstract). Goto, K., Miyagi, K., Kawana, T., Takahashi, J., Imamura, F., 2011. Emplacement and movement of boulders by known storm wavesdfield evidence from the Okinawa Islands, Japan. Mar. Geol. 283 (1), 66e78. Goto, K., Fujino, S., Sugawara, D., Nishimura, Y., 2014. The current situation of tsunami geology under new policies for disaster countermeasures in Japan. Episodes 37, 258e264. Imai, T., Kitamura, A., Miyairi, Y., Yokoyama, Y., Tokuda, Y., 2017. Late Holocene
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Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk Akihisa Kitamura a, b, *, Takafumi Imai a, Yosuke Miyairi c, Yusuke Yokoyama c, Yasufumi Iryu d a
Institute of Geosciences, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan Center for Integrated Research and Education of Natural Hazards, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan Atmosphere and Ocean Research Institute, University of Tokyo, Chiba, 277-8564, Japan d Institute of Geology and Paleontology Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2017 Received in revised form 21 April 2017 Accepted 24 May 2017 Available online xxx
Over the last two decades, two giant tsunamis generated by earthquakes of approximately Mw 9 (the 2004 SumatraeAndaman and 2011 Tohoku earthquakes), and a large storm surge associated with the 2013 Super Typhoon Haiyan, have caused catastrophic damage to infrastructure, property, and industry in many areas of the western Pacific. If we are to improve coastal hazard assessment for the Pacific coast of the southwestern Japanese mainland, a reconstruction of the history of tsunamis and storm surges during the late Holocene is required. This study surveyed coastal boulders to determine whether such events have previously affected the islands of Niijima, Shikinejima, Kouzushima, and Miyakejima that lie offshore from the Tokyo Metropolitan Area. Coastal boulders on Kouzushima and Miyakejima were found with marine organisms attached. Radiocarbon dating of these organisms indicates that the boulders were emplaced during the periods AD 1694eModern and Modern, respectively. The boulder on Kouzushima (13.3 ton at 1.4 m ground elevation) was transported by historical tsunamis or severe storm surges, whereas the boulder on Miyakejima (33.4 ton at 7.1 m ground elevation) was probably transported by a storm surge associated with Typhoon 7920 in 1979. An emerged marine sessile assemblage on Miyakejima (2.31e3.06 m above mean sea-level (amsl)) was dated to ca. 3900e3500 years BP. This high relative sea-level can be explained by a mid-Holocene highstand and uplift associated with volcanic activity. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Coastal boulders Emerged marine sessile assemblage 14 C dates Holocene Tsunami Storm surge Kouzushima Miyakejima Izu island
1. Introduction Over the last two decades, coastal areas in the western Pacific have experienced two devastating tsunamis, which were generated by the approximately Mw 9 2004 SumatraeAndaman and 2011 Tohoku earthquakes, as well as a major storm surge associated with the 2013 Super Typhoon Haiyan. Following the Tohoku Earthquake in particular, coastal hazard assessments have become increasingly important for the Pacific coast of the southwestern Japanese
* Corresponding author. Institute of Geosciences, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka, 422-8529, Japan. E-mail addresses: [email protected] (A. Kitamura), t_imai_ [email protected] (T. Imai), [email protected] (Y. Miyairi), yokoyama@ aori.u-tokyo.ac.jp (Y. Yokoyama), [email protected] (Y. Iryu).
mainland. The Tohoku Earthquake occurred off the Pacific coast of northeastern Japan, and was the largest earthquake (Mw 9.0) ever recorded in Japan. It generated a mega-tsunami with a run-up height of 10e40 m in the coastal areas of Iwate, Miyagi, and northern Fukushima prefectures, and resulted in ~19,000 deaths. Based on lessons learned from this disaster, the Central Disaster Management Council (CDMC, 2011), has defined two types of tsunamis (i.e., Level 1 and Level 2 tsunamis), which could be generated in the Suruga and Nankai troughs, where the Philippine Sea Plate is subducting beneath the Eurasian Plate. Level 1 tsunamis, with wave heights of 5e10 m in coastal areas, have occurred every 100e150 years in the region since the 684 Hakuho Earthquake (e.g., Ando, 1975; Ishibashi and Satake, 1998; Watanabe, 1998; Sangawa, 2001, Fig. 1). Most of these tsunami events were caused by earthquakes of around Mw 8. Level 2 tsunamis are considered to be the
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Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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Fig. 1. (a) General view of the Suruga and Nankai troughs and the study area. Distribution of tsunami height and tsunami run-up (Tokyo Metropolitan Government Disaster Prevention, 2013) and ground survey areas in Niijima (b), Shikinejima (b), Kouzushima, (c) and Miyakejima (d), showing tide-gauge station, and GPS observation. Black lines along the coast are surveyed areas.
largest possible, and are caused by the largest conceivable earthquakes (Mw 9.1) that could occur along the megathrusts of the Nankai Trough (CDMC, 2012). Such events are infrequent but have the potential to cause widespread damage. CDMC (2011) noted that “… when conducting earthquake and tsunami hazard assumptions in the future, the largest-possible mega earthquakes and tsunamis should be considered from every possible angle. … in order to verify the occurrence of mega tsunamis over a time scale of several thousand years it is vital that further enhancement be made not only to seismological research but also to the comprehensive geological, archaeological and historical research, including research into tsunami deposits on land and the ocean floor, geological research into coastal terraces, and research into biological fossils etc.”.
The government of Japan (2012) presented eleven model cases of Level 2 tsunami wave heights. The wave heights of Level 2 tsunamis for model cases 1, 6, and 8 were higher than for the other cases when applied to the Tokyo Metropolitan Area, and the Kanagawa and Shizuoka prefectures, of central Japan. Fig. 1 shows predicted tsunami source area and wave heights of case 1. It is important to note that this tsunami model is not based on historical or geological evidence, and that it far surpasses the largest known historical event (e.g., Goto et al., 2014). CDMC (2011, 2012) initiated studies of tsunami deposits and boulders along coastal areas of Shizuoka Prefecture to determine whether Level 2 tsunamis had previously occurred over time-scales of several thousand years (Abe and Shirai, 2013; Fujiwara et al.,
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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2013; Kitamura et al., 2013, 2014; Kitamura and Kobayashi, 2014a, b; Kitamura, 2016; Sawai et al., 2016). As Kitamura (2016) found no geological evidence of a Level 2 tsunamis in the coastal area of Shizuoka Prefecture over the past 4000 years, it seems reasonable to conclude that no such events occurred in the Tokyo Metropolitan Area either. To confirm this inference, we examined boulders deposited along the coasts of Niijima, Shikinejima, Kouzushima, and Miyakejima islands, which lie offshore from the Tokyo Metropolitan Area, for datable biogenic remains. According to the Tokyo Metropolitan Government Disaster Prevention report (2013), the maximum Level 2 tsunami wave heights in these four areas were estimated to be 31, 28, 25 and 18 m, respectively. This study also provides information on ancient storm surges, because coastal boulders are also transported by storm surges (e.g., Etienne and Paris, 2010; Goto et al., 2011; Kennedy et al., 2017). Understanding storm-surge hazards has become increasingly important in mid-latitude areas since Super Typhoon Haiyan which made landfall in the Philippines in November 2013 and was one of
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the strongest tropical cyclones of the satellite era (Kennedy et al., 2017). Moreover, many recent studies have suggested that tropical cyclone intensities will increase and that their latitudes of maximum intensity will shift polewards as the climate warming (Balaguru et al., 2016; Sobel et al., 2016). We also analyzed emerged marine sessile assemblages, because pre-instrumental data of vertical crustal movements are necessary to calibrate the elevation during the deposition of boulders. Lack of calibration can lead to under- or over-estimation of coastal hazards. Thus, our geological study contributes to assessing coastal hazards in and around the study areas. 2. Study areas Determining the timing of boulder transportation is essential, if we are to accurately estimate the magnitude of past coastal inundations. This is because coastal inundation magnitude is controlled by temporal changes in relative sea-level and the
Fig. 2. (a) Mean relative sea-level changes since 1964, as measured by tide gauges (Coastal Movements Data Center). Tide gauge stations are shown in Fig. 1. (b) Relative vertical displacement at GPS stations during the period 1 January 1998 to 18 April 2015 at Kouzushima and Miyakejima. These data are from time-series data of the F3 solution (Nakagawa et al., 2009), based on permanent GPS data provided by the Geospatial Information Authority of Japan. GPS stations are shown in Fig. 1.
Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040
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geometry of the coast. The coastal areas of Niijima, Shikinejima, Kouzushima, and Miyakejima islands were surveyed for boulders. Only two coastal boulders, with calcareous marine biogenic remains attached, were found on Kouzushima and Miyakejima. This study therefore deals with the boulders on these two islands. To our knowledge, there are no reports disturbance of these boulders by local people. Kouzushima and Miyakejima are situated on the IzueMariana Arc (Fig. 1), and are composed mainly of Quaternary volcanic rocks (Isshiki, 1960, 1984, 1987) that are predominantly rhyolitic and basaltic, respectively (Isshiki, 1960; Taniguchi, 1977; Tsukui et al., 2001). There are no reports of historical tsunamis on either Kouzushima or Miyakejima (Watanabe, 1998). However, the 1923 Taisho Kanto (Mw 7.9) and 1703 Genroku Kanto (Mw 8.4) earthquakes occurred along the megathrust in the Sagami Trough, and generated tsunamis with wave heights of 10e12 m at Izu Oshima, which lies about 70 km north of Kouzushima (Watanabe, 1998). The 2011 Tohoku Earthquake produced tsunami waves of 0.85 m on both Kouzushima and Miyakejima (Japan Meteorological Agency (JMA), undated; Shibata et al., 2012). Approximately 80 typhoons have approached within 150 km of Kouzushima and Miyakejima since 1951 (JMA undated; Kitamoto,
undated). Severe typhoons such as Typhoon 7920 (see below) have often inundated the Izu islands (JMA, undated). The coastlines in the study areas are characterized by wavedominated and microtidal regimes, with a maximum tidal range of 1.6 m during spring tides. The mean sea level (msl) in the study areas is based on tidal records from Miyakejima and Izu Oshima. Tide-gauge data (Japan Coast Guard, Hydrographic and Oceanographic Department, undated) and Global Positioning System (GPS) data indicate ~0.9 m of uplift off west Kouzushima Island during the period 1990e2008 (Fig. 2). This uplift is related to the pressure source located in the northeastern part of Kouzushima (Kimata et al., 2000) and dike intrusion associated with the AD 2000 eruption of Miyakejima (Murakami et al., 2001; Nishimura et al., 2001; Yamaoka et al., 2005). GPS data collected since March 1997 revealed that eastern Miyakejima experienced subsidence of about 0.5 m in 2000 (Fig. 2) due to dike intrusion associated with the eruption of Miyakejima (Murakami et al., 2001; Nishimura et al., 2001; Yamaoka et al., 2005). Imai et al. (2017) reported that an emerged marine sessile assemblage occurs at an elevation of 4.4e6.9 m above mean sea level (amsl) on Kouzushima Island, and that the 14C ages range from AD 613 to AD 1165.
Fig. 3. (a) Map of Kouzushima (Digital Map 25,000 Kouzushima) showing the coastal boulder location, tide-gauge station, and GPS observation. (b) Detailed map showing the location of coastal boulder (open circle).
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A. Kitamura et al. / Quaternary International xxx (2017) 1e11
3. Methods This study identified two coastal boulders that carried calcareous marine biogenic remains on Kouzushima and Miyakejima (Figs. 3e6; Table 1), hereinafter referred to as K1 and M1, respectively. We found many well-rounded boulders close to M1 and measured eleven of them along a transect. They are referred to
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M2M12 (Fig. 7; Table 1). We also measured boulder distances from the shoreline or cliff edge, and boulder elevations. Boulder volumes and masses were calculated from the a, b, and c axes. Boulder weights were estimated by measuring the density of small specimens of the same rock types as the boulders. As noted above, data are available regarding vertical crustal motion over the past 1000 years on Kouzushima (Imai et al., 2017).
Fig. 4. (a) Map of Miyakejima (Digital Map 25,000 Miyakejima) showing the coastal boulder location, emerged marine sessile assemblage, tide-gauge station, and GPS observations. (b) Detailed map showing the location of the coastal boulder (open circle). (c) Detailed map showing the location of the emerged marine sessile assemblage (open circle).
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Table 1 Measured boulders. Sample number Kouzushima K1 Miyakejima M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12
Coordinates (Lat/Long)
Ground Elev. (m)
Top lev. (m)
Dimension (m)
Weight (tons)
Distance from the edge (m)
Age
34.1313 139.0744
1.4
3.7
2.90 2.05 1.92
13.30
10.7
AD 1694Modern
7.1 8.4 8.6 8.6 8.7 9.0 9.2 9.2 9.3 9.3 9.4 9.4
9.7 9.2 9.2 9.5 9.3 9.4 10.5 10.1 10.1 10.0 10.4 10.1
4.90 1.06 0.70 1.10 1.02 1.10 1.46 0.75 0.70 0.86 1.15 0.90
33.41 0.84 0.31 1.14 0.67 1.00 1.00 0.32 0.28 0.45 0.74 0.34
1.5 2.5 7.7 8.2 10.8 16.3 22.0 23.0 23.8 24.7 26.0 27.0
Modern e e e e e e e e e e e
34.0351 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353 34.0353
139.2842 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843 139.2843
2.64 0.82 0.60 0.94 0.77 0.91 1.06 0.63 0.70 0.63 0.97 0.66
1.80 0.72 0.55 0.82 0.63 0.74 0.48 0.50 0.42 0.62 0.49 0.42
Fig. 5. Boulder (K1) located in northwest Sawajiri Bay, Kouzushima. (a), (b) Photographs of boulder. (c) Photograph of bivalve. (d) Close-up photograph of bivalve. (e) Sketch profiles at this site.
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A. Kitamura et al. / Quaternary International xxx (2017) 1e11 Table 2 Results of
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14
C dating of samples from Kouzushima and Miyakejima.
Coordinates (Lat/Long)
Height (m)
Materials
Conventional
34.035120 139.284240 34.035120 139.284240 34.13139 139 07 446 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175 34.063947 139.333175
8.15 8.18 3.60 2.58 2.58 2.31 3.06 3.06 3.06
Barnacle Barnacle Bivalve Favia sp. Favia sp. Serpulidae sp. Septifer exvisus Lithophaga sp. Lithophaga sp.
1599 ± 20 1708 ± 22 615 ± 27 5292 ± 35 5274 ± 32 4743 ± 31 3932 ± 31 4069 ± 26 3880 ± 38
In contrast, no study has examined vertical crustal motion for the pre-instrumental period on Miyakejima. Thus, this study examined emerged marine sessile organisms as well as the fossils of corals and molluscs collected at one site on Miyakejima (Fig. 3). Before the biogenic remains were identified, each sample was cleaned with a micro-knife and organic matter was removed using hydrochloric acid. The coral samples were analyzed using X-ray
14
C age (yr BP)
Calibrated age (2s) (cal yr)
Calibrated age (2s) (cal BP)
Modern Modern Cal AD 1694 - Modern Cal BC 3749e3401 Cal BC 3701e3380 Cal BC 3094e2697 Cal BC 2008e1642 Cal BC 2197e1818 Cal BC 1951e1570
Modern Modern 256 - Modern 5698e5350 5650e5329 5043e4646 3957e3591 4146e3767 3900e3519
diffraction (XRD) to check for diagenetic alteration. Accelerator Mass Spectrometry (AMS) radiocarbon dating of the samples was performed at the University of Tokyo, Japan (Yokoyama et al., 2016). The radiocarbon ages were calibrated onto a calendar timescale using the program OxCal 4.2.4 (Bronk Ramsey, 2009), based on comparisons with Marine 13 data (Reimer et al., 2013), after applying the local correction of DR ¼ 109 ± 60 (Yoneda et al., 2000)
Fig. 6. Photographs of boulder (M1) at the Ako Port locality, Miyakejima. (a), (b), (c) Photographs of the cliff-top boulder. (d), (e) Detailed photographs showing the occurrence of barnacles on boulder. (f) Sketch profiles at this site.
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for Shimoda, Shizuoka Prefecture. 4. Results 4.1. Kouzushima The Kouzushima coastal boulder (K1) was located 10.7 m from the shoreline and 1.43 m amsl (ground elevation) in the northwest area of Sawajiri Bay (Fig. 3). K1 was well rounded and ellipsoidal in shape, 2.90 2.05 1.92 m in size, composed of rhyolite (density 2.23 g/cm3), and had an estimated mass of 13.3 tons (Fig. 5). A wellpreserved fragment of mollusc was attached (Fig. 5c and d), but was not identified. The elevation of the specimen was 3.6 m amsl and its 14 C age was determined to be in the range AD 1694eModern (Table 2).
have been caused by acid rains associated with the release of SO2 gas during the 2000 eruption of Miyakejima (Nakada et al., 2004). The 14C ages of the specimens were Modern (Table 2). Boulders M2M12 were distributed 2.5e27.0 m from the cliff edge at ground elevations of 8.4e9.4 m amsl (Fig. 7; Table 1). They were well-rounded and ellipsoidal in shape. Their estimated masses ranged from 0.23 to 1.14 tons, and decreased landwards (Table 1). The emerged marine sessile assemblage was located at Anouzaki, on east Miyakejima (Fig. 8). XRD analysis indicated that two in situ colonial zooxanthellate scleractinian corals of Favia sp. (2.58 amsl; Fig. 8a, b) were 100% aragonite, showing that they were not diagenetically altered. Individual examples of Septifer exvisus, Serpulidae sp., and Lithophaga sp. were observed at 2.31e3.06 m amsl on coastal bedrock (Fig. 8c, d), and their 14C ages fell within the range 5698e3519 cal. BP (Fig. 9; Table 2).
4.2. Miyakejima 5. Discussion The Miyakejima boulder (M1), with barnacles attached, was found in an area of low cliffs south of Ako Port (Fig. 3b). M1 was located 1.5 m from the cliff edge at an elevation of 7.1 m amsl. M1 was composed of angular basaltic rock (density 2.57 g/cm3), was 4.90 2.64 1.80 m in size, and had an estimated mass of 33.41 tons (Fig. 6). As barnacle surfaces had been affected by dissolution (Fig. 6d and e), the species could not be identified. Dissolution could
5.1. Coastal boulders The 14C dating indicates that boulders K1 (Kouzushima) and M1 (Miyakejima) were deposited at AD 1694eModern and modern (i.e., after AD 1950), respectively (Table 2). We infer that the deposition of K1 and M1 was not related to Level 2 tsunami events,
Fig. 7. Photographs of well-rounded boulders close to M1 at the Ako port locality, Miyakejima. (a) Photographs of the boulders. (b) Close-up photograph of boulders.
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A. Kitamura et al. / Quaternary International xxx (2017) 1e11
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Fig. 8. Photographs of Anou-zaki. (a) Close-up of the exposure. (b) Detailed photograph of Favia sp. (sample M2). (c) Close-up of the exposure. (d) Detailed photograph of Septifer exvisus (sample M3).
Fig. 9. Elevations and ages of emerged sessile assemblages at Anou-zaki, Miyakejima.
but was caused by either storm surges or historical tsunamis. As noted above, there is no evidence of historical tsunamis on Kouzushima (Watanabe, 1998). However, in Izu Oshima, which is located 55 km north of Kouzushima, the AD 1703 Genroku Kanto (Mw 8.4), AD 1854 Ansei-Tokai (Mw 8.4) and AD 1923 Taisho Kanto (Mw 7.9) earthquakes produced tsunami waves of 10, 3, and 12 m, respectively (Watanabe, 1998). However, west Kouzushima Island
experienced 0.9 m of uplift during the period 1990e2008. If one of these historical tsunamis transported K1, then we can estimate that the boulder's pre-uplift ground and top elevations were 0.5 m and 2.8 m, respectively. Recently, Kennedy et al. (2017) presented the masses of coastal boulders transported by known storm waves as a function of ground elevation. Based on this relationship, the emplacement of K1 (13.3 tons) can be explained by a storm surge.
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A. Kitamura et al. / Quaternary International xxx (2017) 1e11
It is certain that the cliff-top Miyakejima boulder M1 was emplaced after 1950. The western area of Miyakejima has experienced up to 0.8 m of subsidence since the 2000 volcanic eruption (Fig. 4b), so the ground elevation at the time of boulder emplacement would have been 7.9 m amsl. As no large tsunamis have been reported along the Miyakejima coast since 1950, a storm surge must have deposited M1 and probably M2eM12 as well. Etienne and Paris (2010) reported boulders of up to 70 tons in mass with a possible storm-surge origin in Iceland, where tsunami impacts have not been recorded historically. The possible stormsurge origin of cliff-top boulders in the Okinawan islands, Japan, has also been proposed (Goto et al., 2011). In addition, the relationship between boulder mass and ground elevation (Kennedy et al., 2017) further supports the suggestion that boulder M1 on Miyakejima (33.4 tons) was deposited at 7.1 m amsl by a storm surge. An example of a storm capable of transporting boulders of this size is Typhoon 7920, which passed Japan in 1979 with a minimum atmospheric pressure of 870 hPa and a maximum wind speed of around 50 m/s (JMA, undated). At Irozaki, on the southern Izu Peninsula, the significant wave height and significant wave period of Typhoon 7920 reached 8.24 m and 12.9 s, respectively. It is likely that this extreme storm surge transported boulders M2eM12 on Miyakejima and K1 on Kouzushima. About 5,000 peoples live on Miyakejima and Kouzushima islands, but about 74,000 visit every year. There are active volcanos in these islands. In the 20 century, Miyakejima Volcano erupted at AD 1940, 1962, 1983 and 2000 (Tsukui et al., 2001; Yamaoka et al., 2005). The last eruption caused complete evacuation of Miyakejima island for four years. As noted above, the climate warming enhances intensities of tropical cyclone and poleward shift of their latitudes of maximum intensity (Balaguru et al., 2016; Sobel et al., 2016). This raises the possibility that severe storm surge would occur at volcanic activity. Our results is also important to assessing the complex disaster in Miyakejima island.
5.2. Vertical displacement at Miyakejima As the results of 14C dating indicate that M1 was emplaced after AD 1950, data from the emerged marine sessile assemblage were not used to calibrate the ground elevation of M1. Nonetheless, these data can be applied to evaluate pre-historical extreme high-energy wave deposits. GPS station Miyakejima 3 near Anou-zaki experienced 0.5 m of subsidence during the 2000 eruption of Miyakejima Volcano (Fig. 4b). Septifer exvisus (sample M4-1, 3.06 m amsl) and Lithophaga sp. (sample M4-2, 3.06 m amsl) inhabit the intertidal zone (þ0.8 to 0.8 m amsl) to depths of 20 m (Okutani, 2000). Assuming that these specimens inhabited the upper limit of this vertical range (i.e., 0.8 m amsl), then subsidence of 0.5 m since 2000 indicates that, at the time of their active growth (3900e3500 years BP), sealevel would have been 2.76 m higher than at present. The Japanese archipelago is located far from former ice sheets (so called “far-field” regions). The mid-Holocene sea-level highstand (HHS) has been recorded at many locations in Japan (e.g., Ota et al., 1990). The HHS was caused by sea level equivalent ice melting in Antarctica (Nakada and Lambeck, 1987; Fleming et al., 1998). Yokoyama et al. (2012) showed that the sea-level reached a maximum HHS of 2 m above the present-day sea level around 4000 years ago on the Shimokita Peninsula, northern Japan. The remaining 0.76 m can be attributed to local uplift, possibly caused by volcanic activity after the emergence of the marine sessile assemblage.
6. Conclusions Two coastal boulders with calcareous biogenic remains attached were found on Kouzushima and Miyakejima islands. Radiocarbon dating of the attached marine organisms showed that the boulders were emplaced during the periods AD 1694eModern and Modern, respectively. The Kouzushima boulder was transported by historical tsunamis or severe storm surges, whereas the Miyakejima boulder was probably transported by a severe storm surge caused by Typhoon 7920 in 1979. In summary, we found no geological evidence of Level 2 tsunamis in the coastal areas studied, which supports Kitamura's (2016) conclusion that Level 2 tsunamis might not have occurred in the Tokyo Metropolitan Area or in Kanagawa and Shizuoka prefectures over the past 4000 years. An emerged marine sessile assemblage was discovered at one Miyakejima site. Based on the distribution of species and crustal movement records, the sea-level around 3900e3500 years BP is estimated to have been 2.76 m amsl. This high relative sea-level can be explained by a midHolocene highstand combined with uplift associated with volcanic activity. Acknowledgements We thank Dr. Kenji Harada for support on discussion. Sampling of specimens was conducted under permit from the Agency for Cultural Affairs, Kanto Regional the Ministry of the Environment. We would like to thank anonymous reviewers, for their thoughtful input into this manuscript. This study was funded by Grants-in-Aid (26287126, 17H02972) awarded by the Japan Society for Promotion of Science. References Abe, T., Shirai, M., 2013. Event deposits correlated with a historical tsunami in the Edo period, on the coastal lowland of the Atsumi Peninsula, central Japan. Quat. Res. (Daiyonki-Kenkyu) 52, 33e42. Ando, M., 1975. Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan. Tectonophysics 27, 119e140. Balaguru, K., Foltz, G.R., Leung, L.R., Emanuel, K.A., 2016. Global warming-induced upper-ocean freshening and the intensification of super typhoons. Nat. Commun. 7, 13670. http://dx.doi.org/10.1038/ncomms13670. Bronk, R.C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51, 337e360. Cabinet Office, Government of Japan, 2012. Report of Investigative Commission of Large Earthquake at Nankai Trough (Second Report)dTsunami Fault Model, p. 103 (in Japanese). http://www.bousai.go.jp/jishin/nankai/nankaitrough_info. html. Accessed 26 January 2017. Central Disaster Management Council, 2011. Report of the committee for technical investigation on countermeasures for earthquakes and tsunamis based on the lessons learned from the “2011 off the Pacific coast of Tohoku earthquake”. http://www.bousai.go.jp/kaigirep/chousakai/tohokukyokun/pdf/Report.pdf. Accessed 26 January 2017. Central Disaster Management Council, 2012. Final report - toward the reconstruction for sound and unwavering Japan. http://www.bousai.go.jp/kaigirep/ chuobou/suishinkaigi/english/pdf/Final%20Report.pdf. Accessed 26 January 2017. Coastal Movements Data Center (accessed on 3 December 2016) Table of annual mean sea level along the Japanese coast. http://cais.gsi.go.jp/cmdc/center/ annual.html#9. Etienne, S., Paris, R., 2010. Boulder accumulations related to storms on the south coast of the Reykjanes Peninsula (Iceland). Geomorphology 114 (1), 55e70. Fleming, K., Johnston, P., Zwartz, D., Yokoyama, Y., Lambeck, K., Chappell, J., 1998. Refining the eustatic sea-level curve since the Last Glacial Maximum using farand intermediate-field sites. Earth Planet. Sci. Lett. 163 (1), 327e342. http:// dx.doi.org/10.1016/S0012-821X(98)00198-8. Fujiwara, O., Sato, Y., Ono, E., 2013. Researches on tsunami deposits using sediment cores: 3.4 ka tsunami deposit in the Rokken-gawa Lowland near Lake Hamana, Pacific coast of Central Japan. J. Geogr. (Chigaku Zasshi) 122, 308e322 (in Japanese with English abstract). Goto, K., Miyagi, K., Kawana, T., Takahashi, J., Imamura, F., 2011. Emplacement and movement of boulders by known storm wavesdfield evidence from the Okinawa Islands, Japan. Mar. Geol. 283 (1), 66e78. Goto, K., Fujino, S., Sugawara, D., Nishimura, Y., 2014. The current situation of tsunami geology under new policies for disaster countermeasures in Japan. Episodes 37, 258e264. Imai, T., Kitamura, A., Miyairi, Y., Yokoyama, Y., Tokuda, Y., 2017. Late Holocene
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Please cite this article in press as: Kitamura, A., et al., Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.05.040