Matsushima Bay As an Early Holocene Coastal Mega-Landslide, Northeast Japan
Total Page：16
File Type：pdf, Size：1020Kb
Matsushima Bay as an Early Holocene coastal mega-landslide, Northeast Japan Shuichi Hasegawa (Kagawa University, Japan) Timihiro Sawada (Sawa Soft Science, Japan) Ranjan Kumar Dahal (Kagawa University, Japan and Tribhuvan University, Nepal) Atsuko Nonomura (Kagawa University, Japan) Minoru Yamanaka (Kagawa University, Japan) Abstract. Matsushima, a group of island at Matsushima Bay visited both Matsushima and Kisakata. He composed three in Miyagi Prefecture, northeast Japan, is one of the three haiku poems for Kisakata, but he could not express his famous scenic spots of Japan. It is composed of more than excitement in a haiku poem for Matsushima. 200 islands in Matsushima Bay and the islands just out into Matsushima and Matsushima Bay have long been the sea. Topographically Matsushima Bay suddenly breaks considered as a typical submerged coast, but they are inferred the gently concaved coastline from Sendai Bay to Ishinomaki to have been formed by a coastal mega-landslide in middle Bay. Matsushima and Matsushima Bay have been considered Holocene age from geological and topographical inferences. as a typical submerged coast, but they are inferred to have been formed by a coastal mega-landslide in middle Holocene age from geological and topographical evidences. Keywords. Mega-landslide, Holocene, Jomon transgression, Active fault, topography 1. Introduction Mega-landslides due to volcanic activities and earthquakes have caused severe damage to the surrounding areas. Sector collapse of volcanoes is one of the most destructive landslides. Debris avalanche deposits from a sector collapse generally form strange topography punctuated by hundreds of small hills, ridges and closed depressions. The 1792 Mayuyama sector collapse of Unzen volcano in Kyushu, southern Japan, caused debris avalanche which flowed through ancient Shimabara City and entered the sea. As a result, a giant tsunami was generated (Inokuchi, 2006) and more than 15,000 people were killed by the landslide and tsunami. The debris avalanches which entered the sea are Fig. 1 Locality of Matsushima Bay (modified after characterized hundreds of many small islands and sunken Editorial Group for Computer Graphics, Geology of rocks in Shimabara Bay. The locality is now called as Japanese Islands, 2002) Tsukumo-jima and it means 99 islands in Japanese. Usually, with the help of topography and geology, pre-historic sector 2. Topography and geology of Matsushima collapses are recognized. Another example of sector collapses 2.1 Topography is found in Kisakata, western Tohoku, northern Japan and it is Matsushima is composed of more than 200 islands. also characterized by a hundred small islands (Tsukumo-jima) Matsushima Bay is a semicircular in shape, with 10 km in scattered in the sea. They are debris avalanche deposits from EN-SW direction and 5 km in NW-SE direction (Fig. 2). Mt. Chokai volcano occurred about 2,600 years ago (Inokuchi, Along the southeastern margin of Matsushima Bay, a number 2006). of islands are concentrated from Miyato Island to Mega-landslides in costal area sometimes generate Shichigahama Peninsula. These islands and peninsula, named mega-tsunamis. In 1958, a rockslide at Lituya Bay in Alaska, as Matsushima Islands, are located between the beach ridges caused by an earthquake of magnitude 7.7, produced a along Sendai and Ishinomaki Bays. It looks like they mega-tsunami measuring 524 m high (Miller, 1960). In 1963, suddenly interrupt the gently concaved coastline from Sendai mega-landslide at Vajont Dam reservoir in Italy generated a Bay to Ishinomaki Bay (Fig. 2). The area of Matsushima 250 m high mega-tsunami and killed almost 2,000 people Islands is roughly equal to those of Matsushima Bay. (Kiersch, 1964). Although mega-Landslides are rare 1:25,000 scale topographic map of coastal areas phenomena, evidence for prehistoric mega-landslides has “Matsushima” (Geographical Survey Institute of Japan, 1982) been found both from land and underwater. and 1:25,000 scale land condition map of coastal areas Matsushima in eastern Tohoku is one of the three famous “Matsushima” (Geographical Survey Institute of Japan, 1984) scenic spots of Japan. It is composed of more than 200 islands have mapped out that Matsushima Bay is flat and less than in Matsushima Bay, which faces the Pacific Ocean (Fig. 1). 4m in depth and a number of sunken rocks are distributed in Matsushima often praised as eastern Matsushima and western and to the south of Matsushima Islands (Fig.3). Kisakata which faces the Japan Sea. Matsuo Basho, the most Summit level of Matsushima Islands are usually about famous haiku poet of Edo period in Japan, praised 50m high. On the contrary, inland Matsushima Hill is about Matsushima as the most beautiful landscape in Japan. He 100m high and is about 50m higher than Matsushima Islands. 108 2.2 Geology of Matsushima Matsushima Islands mainly consist of the Miocene Matsushimawan Group which strike NW-SE direction (Ishii et al., 1982; Ishii et al., 1983). Similarly Matsushima Hill is mainly composed of the the Miocene Matsushimawan Group which strike NW-SE direction. The eastern extension of the active Nagamachi-Rifu Fault Zone, which is trending NE-SW, is inferred in Matsushima Hill, but fault topography is not recognized. The Matsushimawan Group in Matsushima and Matsushima Hill forms similar gentle anticlines and synclines trending from northwest to southeast. The difference of geologic structure of Matsushima Islands from Matsushima Hill is that strike faults are distinct in the Matsushimawan Group in Matsushima Islands (Ishii et al., 1982; Ishii et al., 1983). Fig. 2 Topography of Matsushima (Illustrated by Matsushima Bay is underlain mainly by Holocene marine Kashimir3D), inferred beach between Sendai Bay to clay sediments which directly cover the Miocene bedrocks Ishinomaki Bay is also shown in the map (Ishii et al., 1983). The Pleistocene sediments have not been reported. 3. Mega-landslide as the origin of Matsushima 3.1 Topographical restoration Topography of Matsushima Bay and Matsushima Islands indicate that Matsushima Bay is the source area of a mega-landslide and Matsushima Islands are slide masses. This interpretation indicates that previously, Matsushima Islands including Shichigahama Peninsula were situated on the area of Matsushima Bay and beaches and sand ridges continue from Sendai Bay to Ishinomaki Bay. This restoration suggests that hills which had been located on the area of Matsushima Bay slid about 5km toward southeast direction and have decrease there elevation about 50 m. The sunken rocks also indicate the subsequent secondary slides and third Fig. 3 Topography of Matsushima mega-slide (Modified slides (Fig.3) after Geographical Survey Institute of Japan (1984) Fig. 4 Geology of Matsushima mega-landslide (modified after Ishii et al.(1982) and Ishii et al.(1983) ) 109 3.2 Geological restoration The topographical restoration coincides with the geological restoration by lithology, successions and geologic structures (Fig. 4). This restoration suggests that hills which had been located on the area of Matsushima Bay slid toward southeast direction parallel to the strike of bedrocks without separated into small fragments. Some of strike-slip faults distributed in Matsushima Islands are inferred to have formed by sliding and the sliding hills might be separated into several large masses. The sliding surface is not exposed, but the Fig. 6 History of Satomaha shell mound in Miyato Island restoration indicates that the sliding surface is almost (modified after Historical Museum of Jomon Village horizontal (Fig.5). Okumatsushima (2008) ) 4. Cause and result of mega-landslide 4.1 Tigger of mega-landslide As Matsushima is not a volcano, a strong earthquake from nearby active fault was the most probable trigger of mega-landslide. Matsushima area has suffered the 1978 Mitagikenoki subduction earthquake (M7.4) and the 2003 Miyagikenhokubu earthquake (M6.4), but no large-scale deep-seated landslide occurred. Recent earthquake-induced large-scale deep-seated landslides support big earthquake caused by nearby active fault is the most probable trigger. Matsushima Hill is located at the eastern extension of the active Nagamachi-Rifu fault zone whose estimated magnitude is from7.0 to 7.5. Moreover the fault extension coincides with the scar area of the mega-landslide (Fig. 7). Therefore the Nagamachi-Rifu fault is the most probable trigger. The last Fig. 5 Schematic profiles of Matsushima mega-slide faulting is estimated after about 16,000 yrs. B.P. (Section lines are shown in Fig. 4.) (Headquarters of Earthquake Research Promotion, 2002). 3.3 Age of Matsushima mega-landslide Matsushima Bay is underlain mainly by Holocene marine clay sediments which directly cover the Miocene bedrocks (Ishii et al., 1983). This indicates that beginning of deposition of marine clay in Matsushima Bay was just after the occurrence of mega-landslide. As the marine clay attains 20 m at Shiogama, the age of the sediments can be traced back to Early to Middle Holocene. Drilling core of Matsushima Bay is essential to determine the exact time of beginning of sedimentation. In Matsushima area, shell mounds of Jomon age (early to late Holocene) are found on the hills of Matsushima Island (see Fig. 1). A carbon-14 dating from Daigigakoi shell mound located at the highland (about 40m in height) of Shichigahama Peninsula is 5680±120yrsB.P. (Ito, 2002). At Satohama shell mounds in Miyato Island, early Jomon settlements had started from highlands and had descended elevation during Jomon age (Fig. 6). Interesting fact is that the percentage of shells on rocky coasts was high at early Jomon age and shells in sandy and silty beaches has gradually increased as dating up to latest Jomon age (Fig. 5). These histories of the shell mounds indicate that the early Jomon people had to start settling down on safe but inconvenient highlands. At that time, rocky coasts were widely distributed in Matsushima Islands. Probably early Jomon people had tried to prevent disasters caused by mega-landslide. Theses geological and archeological information indicates that the mega-landslide occurred about 6000 yrs B.P.