Cycle of Interplate Earthquake Along the Sagami Trough, Deduced from Tectonic Geomorphology

A tectonic geomorphological study is one of the best methods of evaluating the timing and the crustal deformation of pre-historic earthquake. To reveal the cycle of interplate earthquakes along the Sagami Trough, I investigatedthe emergedshoreline topography andthe fossilizedsessile assemblages in the Miura Peninsula andthe Boso Peninsula. The distribution pattern of coseismic vertical displacement during the 1703 Genroku Kanto Earthquake inferredfrom the height distribution of the paleo-shoreline suggests that the fault source model consists of a dual fault system of the Fault A andB. Fault A is also the source of the 1923 Taisho Kanto Earthquake. The geometry and ages of the emerged shoreline topography divided into several levels indicate that the characteristic earthquake generatedfrom Fault A has occurredabout every 400 years. One of several events, it is accompaniedwith a slip of Fault B, which has a recurrence interval of 2000-2700 years

Reconstruction of the 869 Jogan Earthquake, the predecessor of the 2011 Tohoku earthquake, by geological evidence combined with tsunami simulation

International Symposium on Backwards Problem in Geotechnical Engineering and Monitoring of Geo-Construction, Green Hall, Kensetsu-Koryu-kan, 2011/07/14-15We reconstructed the tsunami inundation area of the 869 Jogan earthquake by geological evidence such as tsunami deposit, and inferred fault source model by tsunami simulation. From analysis of nearly 400 geological core samples obtained from the Sendai and Ishinomaki Plains, it is inferred that the 869 Jogan tsunami inundated to 3-4 km further inland from shoreline and was generated from subduction megathrust fault which has 200 km long and 100 km width off Miyagi and Fukushima. Magnitude and source of the Jogan tsunami is very similar to the 2011 Tohoku earthquake

Geological evidence for Holocene earthquakes and tsunamis along the Nankai-Suruga Trough, Japan

The Nankai-Suruga Trough, lying immediately south of Japan's densely populated and highly industrialised southern coastline, generates devastating great earthquakes (magnitude > 8). Intense shaking, crustal deformation and tsunami generation accompany these ruptures. Forecasting the hazards associated with future earthquakes along this >700 km long fault requires a comprehensive understanding of past fault behaviour. While the region benefits from a long and detailed historical record, palaeoseismology has the potential to provide a longer-term perspective and additional insights. Here, we summarise the current state of knowledge regarding geological evidence for past earthquakes and tsunamis, incorporating literature originally published in both Japanese and English. This evidence comes from a wide variety of sources, including uplifted marine terraces and biota, marine and lacustrine turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. We enhance available results with new age modelling approaches. While publications describe proposed evidence from > 70 sites, only a limited number provide compelling, well-dated evidence. The best available records allow us to map the most likely rupture zones of eleven earthquakes occurring during the historical period. Our spatiotemporal compilation suggests the AD 1707 earthquake ruptured almost the full length of the subduction zone and that earthquakes in AD 1361 and 684 were predecessors of similar magnitude. Intervening earthquakes were of lesser magnitude, highlighting variability in rupture mode. Recurrence intervals for ruptures of the a single seismic segment range from less than 100 to more than 450 years during the historical period. Over longer timescales, palaeoseismic evidence suggests intervals ranging from 100 to 700 years. However, these figures reflect thresholds of evidence creation and preservation as well as genuine recurrence intervals. At present, we have not identified any geological data that support the occurrence earthquakes of larger magnitude than that experienced in AD 1707; however, few published studies seek to establish the relative magnitudes of different earthquake and tsunami events. Alongside the paucity of research designed to quantify the magnitude of past earthquakes, we emphasise a number of other issues, including alternative hypotheses for proposed palaeoseismic evidence, the lack of robust chronological frameworks and insufficient appreciation of changing thresholds of evidence creation and preservation over time. These key issues must be addressed by future research.QuakeRecNanka

Geological evidence for Holocene earthquakes and tsunamis along the Nankai-Suruga Trough, Japan

The Nankai-Suruga Trough, lying immediately south of Japan’s densely populated and highly industrialised southern coastline, generates devastating great earthquakes (magnitude > 8). Intense shaking, crustal deformation and tsunami generation accompany these ruptures. Forecasting the hazards associated with future earthquakes along this >700 km long fault requires a comprehensive understanding of past fault behaviour. While the region benefits from a long and detailed historical record, palaeoseismology has the potential to provide a longer-term perspective and additional insights. Here, we summarise the current state of knowledge regarding geological evidence for past earthquakes and tsunamis, incorporating literature originally published in both Japanese and English. This evidence comes from a wide variety of sources, including uplifted marine terraces and biota, marine and lacustrine turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. We enhance available results with new age modelling approaches. While publications describe proposed evidence from > 70 sites, only a limited number provide compelling, well-dated evidence. The best available records allow us to map the most likely rupture zones of eleven earthquakes occurring during the historical period. Our spatiotemporal compilation suggests the AD 1707 earthquake ruptured almost the full length of the subduction zone and that earthquakes in AD 1361 and 684 were predecessors of similar magnitude. Intervening earthquakes were of lesser magnitude, highlighting variability in rupture mode. Recurrence intervals for ruptures of the a single seismic segment range from less than 100 to more than 450 years during the historical period. Over longer timescales, palaeoseismic evidence suggests intervals ranging from 100 to 700 years. However, these figures reflect thresholds of evidence creation and preservation as well as genuine recurrence intervals. At present, we have not identified any geological data that support the occurrence earthquakes of larger magnitude than that experienced in AD 1707; however, few published studies seek to establish the relative magnitudes of different earthquake and tsunami events. Alongside the paucity of research designed to quantify the magnitude of past earthquakes, we emphasise a number of other issues, including alternative hypotheses for proposed palaeoseismic evidence, the lack of robust chronological frameworks and insufficient appreciation of changing thresholds of evidence creation and preservation over time. These key issues must be addressed by future research.QuakeRecNankai projec

Aperiodic recurrence of geologically recorded tsunamis during the past 5500 years in eastern Hokkaido, Japan

Along Hokkaido\u27s Pacific coast near the town of Kiritappu, sandy deposits in a muddy lagoon and on a nearby beach-ridge plain provide evidence for 15 tsunamis between 200 and 6000 years ago. Additional sand beds at the lagoon probably represent the historical tsunamis of A.D. 1843 and 1894. We observed the sequences of sandy deposits in continuous slices 2 to 4 m deep. Some of the deposits consist of just a single sand bed, whereas others contain multiple units of sand, muddy sand (or sandy mud), and mud caps including plant detritus. We also found at the lagoon a 17th century tsunami deposit that thickens and thins regardless of elevation or distance inland. We bracketed the ages of most of the inferred tsunamis by radiocarbon dating of detritus, mainly seeds and leaves at the lagoon and charcoal at the beach-ridge plain, from pretsunami and posttsunami beds. Tsunami dates computed from the bracketing ages commonly have uncertainties spanning 2 to 4 centuries. Within these uncertainties, the inferred sequence of 15 prehistoric tsunamis at the lagoon, beginning almost 6000 years ago, can be matched tsunami by tsunami with the inferred history at the beach-ridge plain, 15 km away. The sand sheet extents suggest that most of these tsunamis were larger than any generated at Hokkaido in the last 200 years. The intervals between these inferred outsized tsunamis average nearly 400 years but range widely from about 100 to about 800 years

Sedimentary diversity of the 2011 Tohoku-oki tsunami deposits on the Sendai coastal plain and the northern coast of Fukushima Prefecture, Japan

Abstract This paper documents the sedimentary characteristics of the widespread deposits associated with the 2011 Tohoku-oki tsunami on the lowlands along the Pacific coast of the Sendai and Fukushima regions, northern Japan, and observed tsunami inundation depths. In eight areas of the region, field observation was carried out at a total of 123 locations and sampling at a total of 49 locations. Grain-size analysis and soft X-ray imaging reveal that the tsunami deposits are usually composed of sheetlike sandy beds and generally show landward-thinning and landward-fining trends and a landward increase in mud content, although site-specific distributional patterns are apparent along each transect. These thickness and grain-size patterns indicate a landward decrease in flow capacity. This information on the sedimentology of tsunami deposits and observed inundation depths will assist with the identification of paleo-tsunami deposits in the geological record and provide valuable constraints for mathematical analyses of tsunami hydraulic conditions related to sedimentary characteristics

Marine inundation history during the last 3000 years at Lake Kogare-ike, a coastal lake on the Pacific coast of central Japan

Abstract Sediment cores collected at Lake Kogare-ike, a coastal lake on the Pacific coast of central Japan, record the marine inundation history during the last 3000 years. The sediments consist mainly of organic mud, sand, gravel, inorganic mud, and volcanic ash, and inundation events were recognized as 19 event deposits (E1–E19, from top to bottom) interbedded with the organic mud. Visual observation by naked eyes and X-ray computed tomography (CT) images identified 16 event deposits based on quantitative and qualitative changes in sand contents and changes in the textures and colors of the sediment samples (E1–E3, E5, E6, E8, E9, and E11–E19). The other three event deposits (E4, E7, and E10) were identified only on the CT images as layers with higher radiodensity than the underlying and overlying organic mud layers. The sedimentary features, the spatial bias of the event deposits toward seaward areas, the diatom assemblages, and the frequency of inundation events suggest that 13 (E1–E10 and E12–E14) of the 19 event deposits were formed by tsunamis or extraordinary storms. To constrain the depositional ages of the event deposits, Bayesian age–depth models were constructed based on radiocarbon dating of plant macrofossils and concentrated fossil pollen and the 137Cs profile. The depositional ages of the event deposits indicate that five or possibly six event deposits can be correlated with historical tsunamis along the Nankai Trough: E2, either of E3 or E4, E5, E7, and E9 correspond to the 1707 CE Hoei, the 1605 CE Keicho, the 1498 CE Meio, the 1096 CE Eicho, and the 684 CE Hakuho tsunamis, respectively. E1 was possibly formed by the 1944 CE Showa-Tonankai tsunami, the 1854 Ansei–Tokai tsunami, the 1959 Isewan typhoon, or a combination of two or all three events