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Tsunami Deposits and Earthquake Recurrence Along the Nankai

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Tsunami deposits and earthquake recurrence along the Nankai, Suruga and Sagami Troughs OSAMU FUJIWARA1 and JUNKO KOMATSUBARA1 1: Active Fault Research Center, AIST, C7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan ([email protected]) INTRODUCTION Histories of great (M~8) subduction-zone earthquakes inferred from tsunami deposits span 3000 years for the Nankai and Suruga Troughs and nearly 10,000 years for the Sagami Trough. The inferred histories contain variable recurrence intervals. The shortest recurrence intervals, 100-200 years for the Nankai Trough and 150-300 years for the Sagami Trough, are similar to those known from written earthquake and tsunami records in the last 1300 years. Longer intervals inferred from the tsunami deposits probably reflect variability in rupture mode, incompleteness of geologic records, and insufficient research. 130E 140E GEOLOGICAL SETTING The Nankai, Suruga and Sagami Troughs comprising the northeastern subduction margin of the Philippine Sea plate are 40N seismically active areas in the world. The Philippine Sea plate converges with overriding plates at a rate of 49-42 mm/year along the Nankai Trough and 27 mm/year around the Sagami Trough (Seno et al., 1993, 1994). Several source areas (segments) 35N Japan are located along these troughs, each of which are 100 - 150 km long along the trough axis and potentially excites M8-class Trench earthquake accompanied by large tsunami and coastal uplift or subsidence. Earthquakes and tsunamis occurred from these areas have repeatedly damaged the Pacific coast of Japan, including Tokyo Nankai Trough and Osaka metropolitan areas. Trench Trench Izu-Ogasawara Izu-Ogasawara Fig. 1 Tectonic Setting TSUNAMI DEPOSITS FROM THE COAST OF NANKAI AND SAGAMI TROUGHS Japanese written records evidence that each trough has considerably regular recurrence intervals of 100 to 200 Fig. 3 Tokyo Nagoya years, but their rupture modes have a wide variability. For instance, the AD1707 Hoei Earthquake (M 8.4-8.6) Kyoto Osaka was a multi-segment earthquake that had a rupture zone over 600 km long extending from off central Japan to B E D Sagami C Suruga Trough off Shikoku. The AD1605 Keicho Earthquake was probably a “tsunami earthquake.” Kii Trough Tonankai Shikoku Island Peninsula Understanding of this variability of subduction-zone earthquakes needs the analyses of long-term history of A earthquakes and tsunamis by using geological and geo-morphological evidences, such as tsunami deposits and Kyushu Nankai Island marine terraces. Tsunami deposits found in the last decade have provided new insights to paleo-seismological Rupture zone Tsunami deposit locality study of the Nankai, Suruga and Sagami Troughs. Historical and prehistorical tsunami deposits back to the middle Nankai Trough Holocene in age have been reported from at least 15 sites along these troughs, including lakes, lagoons, outcrops, 100 km and excavated ruins. Most of those deposits are recognized as unusual sedimentary layers composed of coarser grains in muddy or peaty sediments in both cores and outcrops. Fig. 2 Location Map HISTORICAL TSUNAMI DEPOSITS FROM COASTAL MARSH SRL-4 Four tsunamis, the AD 1498 Meio, AD1605 Keicho, AD1707 Hoei, AD1854 Ansei, and eight storm surges has been recored in Japanese history during the last 500 years around the study site (Loc. G in Fig. 3). 1854 Ansei Four tsunami deposits and one storm surge deposit were identified based on detailed facies Earthquake analyses and 14C dating from the core samples excavated in a back marsh using a geo-slicer, 3 1707 Hoei 6.0-m-long, 0.35-m-wide, and 0.05- to 0.1-m-deep wedge-shaped stainless steel case. These Earthquake event deposits are mainly composed of stratified sand layers and intercalated in humic mud. 1680or1699 Storm event Fig. 7 50 km Tokyo 1605 Keicho Earthquake Miura Boso M Peninsula N Peninsula 2 Lake Hamanako Kanto 1498 Meio L O Tokai Earthquake Izu H I J Penin- F G K sula 1361 Shohei Earthquake ? Elevation (m) Fig. 3 Location Map of Tokai Area 1 14C ages compiled from five cores Sea (South) (with 2σ age range) 8 m SRK-04 SRL-3b Back Marsh 2000 1500 1000AD 4 Sand Dune Historical storm surge Historical tsunami 0 SRL-1 2 3a 4 0 0 100 200 300 m Beach Fig. 4 Topographic Section of Loc.G in Fig. 3 Fig. 6 Tsunami and Storm Deposits Fig. 5 Excavation Using “Long-Geoslicer” in Core SRL-4 at Loc.G Nankai - Suruga Trough Sagami Trough A B C D E F G H I J K L M N O AD cal.BP 2000 3 tsunamis 0 RECORD OF TSUNAMI DEPOSITS Temporal and spatial distribution of the tsunami or psibble tsunami deposits are shown in Fig 8. 5 tsunamis We classify the tsunami deposits into five groups according to the types and nature of evidence 1000 1000 for the identification, i.e accuracy of tsunami deposits. Group I includes the tsunami deposits confidently correlate to historical tsunami events, based on historical records or eye witnesses, 2 tsunamis though they are still very rare. The deposits from the AD 1703 Genroku and AD 1923 Taisho Kanto Earthquakes occurred around the Sagami Trough are representatives of this group. 0 2000 Group II includes tsunami deposits correlated to historical earthquakes based on depositional ages 2 tsunamis (mainly by 14C ages) only. Many of the tsunami deposits reported from the coast of Nankai and Suruga Troughs, such as the AD 1498, 1605, 1707 and 1854 tsunamis, belongs to this group. KgP Group III and IV are generally assigned to prehistorical deposits. Group III has some sedimentary 1000 3000 features indicating tsunami origin. The deposits reported from the Holocene shallow bay sequence in the east Japan have unique sedimentary structures indicating that they were deposited from repeated tsunami waves with large wavelengths and wave period. Group IV includes the tsunami deposits which correspond with tectonic movements suggested by 2000 4000 marine terraces and emerged beach ridges. Group V is assigned to probable or possible tsunami deposits which are not classified into group I to IV. 6 tsunamis 3000 5000 Details of these results were reviewed in Komatsubara et al., (2006) and Komatsubara and Fujiwara (in press). poorly-sorted mica-rich sand from hinterland 4000 6000 ebb stagnant flood 5000 7000 10 cm K-Ah well-sorted mature sand from beach 6000 Earthquake evidence 8000 from archeological site Fig. 7 Inner Structure of the AD1605 Tsunami Deposit (Sangawa, 2004) Rupture zone of historical earthquake Marker tephra 7000 Fig. 8 Temporal and Spatial Distribution of Tsunami Deposit 9000 REFERENCES Group of tsunami deposits Komatsubara, J. and Fujiwara, O. ; Overview of Holocene tsunami deposits V IV III II I Age range along the Nankai, Suruga, and Sagami Troughs, southwest Japan. PAGEOPH (in press). Unknown error 8000 Komatsubara, J., Fujiwara, O. and Kamataki, T. (2006) Tsunami deposits along the Nankai, No error 10000 Suruga and Sagami Troughs, Historical Earthquakes, 21, 93-109, in Japanese .
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