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Exploring the potential of Lake Hamana (Shizuoka Prefecture, Japan) to hold a long and reliable sedimentary record of paleo-earthquakes and -tsunami along the Nankai-Suruga Trough.
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Coastal Lake Hamana is located near the convergent tectonic boundary of the Nankai-Suruga Trough, along which the Philippine Sea slab subducts underneath the Eurasian Plate, giving rise to repeated tsunamigenic megathrust earthquakes (Mw≥8). A good understanding of the earthquake- and tsunami-triggering mechanisms in terms of rupture mode and recurrence pattern in time and space, is crucial in order to better estimate the complexity of seismic risks for the densely populated Enshu-nada coast. Based on existing historical data of paleoseismicity (last ~1300 years), the easternmost segment (Tōkai segment) of the Nankai-Suruga Trough appears to exhibit a seismic gap and is expected to rupture in the near future, causing the next “Tōkai earthquake”. Studying the sedimentary infill of Lake Hamana may help to fine-tune hazard assessment in the area of interest. Thanks to its extensive accommodation space, the Hamana lake basin is considered to be a good archive of past “big wave” events. Fieldwork (Oct.-Nov. 2014) comprised a reflection-seismic survey for imaging the lake’s stratigraphic features, based on which favourable locations for gravity coring were selected. A systematic sampling of bottom sediments from different sites enables us to evaluate vertical as well as lateral changes in depositional environment, including event deposits generated by tsunamis and tropical storms (i.e. typhoons). An important part of the study is dedicated to qualitatively distinguish sedimentary facies of storm deposits from the ones generated by tsunamis, since this is an essential step in correctly assessing future hazards. For identification of marine tsunami incursions, a set of sedimentological, geophysical, geochemical and micropaleontological analyses are applied on the core sediments in a multi-proxy approach. Radionuclide dating provides the necessary timeframe and information on prevailing sedimentation rates. Sites bearing the potential of recording complete and long event histories will be sampled with long cores.
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On how to extract the paleotsunami history from a coastal lake record
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Coastal lake records can be successfully used for reconstructing continuous histories of tsunamigenic megathrust earthquakes. Here, we apply a wide range of methods on one Japanese and two Chilean coastal lakes and show that the selection of coring locations benefits significantly from accompanying geophysical survey data. High-resolution seismic profiles display several strong reflectors in all three lake basins, allowing us to map the depth and extent of coarse-grained tsunami deposits. Side scan sonar imagery enables us to reconstruct past tsunami inundation pathways.
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The QuakeRecNankai project: Palaeoseismic data for improved seismic hazard assessment along the Nankai Trough, Japan
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The QuakeRecNankai project: reconstructing past earthquakes and tsunamis along the Nankai Trough, south central Japan
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The Nankai-Suruga subduction zone faces the densely populated and highly industrialised coastline of south central Japan. The largest possible class of earthquake on the subduction interface could exceed magnitude 9, with tsunami travel times to the closest shorelines of less than 30 minutes. In this presentation, we review geological evidence for past earthquakes and tsunamis in this region and introduce the QuakeRecNankai project, a Belgian, Japanese and German collaboration that aims to reconstruct past seismic shaking and tsunami occurrence from Lake Hamana and the Fuji Five Lakes at the eastern end of the Nankai Trough.
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Geological evidence for historical and older earthquakes and tsunamis along the Nankai Trough, Japan
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In the wake of the devastating 2011 Tōhoku earthquake and tsunami, the Central Disaster Management Council of the Japanese Cabinet Office issued new guidance for assessing seismic hazards in Japan. Before 2011, seismic hazard assessment relied on source models developed from knowledge of a small number of well-documented historical earthquakes. Less well-known historical earthquakes, including the AD 869 Jōgan Sanriku earthquake, were largely disregarded as their seismic intensities or tsunami heights could not be reconciled with the chosen seismic sources. Following the unexpectedly large size of the Tōhoku earthquake, the Cabinet Office advocated renewed investigation of earthquake and tsunami occurrence over historical and longer timescales, with a particular focus on defining the largest possible magnitudes. The new guidelines pay close attention to the Nankai Trough, the subduction zone where the Philippine Sea Plate dives beneath the Eurasian Plate. The Nankai Trough faces the densely populated and highly industrialised coastline of south central Japan and harbours a widely-known seismic gap along its eastern Tōkai segment. A full-length rupture of the Nankai Trough, including the Tōkai segment, could produce an earthquake with a magnitude approaching that of the 2011 event, with tsunami travel times to the closest shorelines of less than 30 minutes. We review geological evidence for historical and older earthquakes and tsunamis along the Nankai Trough. This evidence comes from a wide variety of sources, including uplifted marine terraces, subsided marshes, liquefaction features, turbidites and tsunami deposits in coastal lakes and lowlands. Examining papers published before and after 2011, we investigate the impact of the new Cabinet Office guidelines on attempts to understand the magnitude and recurrence of these events. Additionally, we introduce the Belgian Science Policy Office funded QuakeRecNankai project, a collaboration aiming to supplement existing records by generating a long time series of earthquake and tsunami recurrence from sites at the eastern end of the Nankai Trough. The project uses a diverse range of geophysical, sedimentological, geochemical and microfossil approaches to investigate records of Holocene paleotsunamis in and around Lake Hamana and records of seismic shaking from the Fuji Five Lakes.
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Progress in paleoearthquake and paleotsunami research along the Nankai Trough following the 2011 Tōhoku earthquake
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In the wake of the devastating 2011 Tōhoku earthquake and tsunami, the Central Disaster Management Council of the Japanese Cabinet Office issued new guidance for assessing seismic hazards in Japan. Before 2011, seismic hazard assessment relied on source models developed from knowledge of a small number of well-documented historical earthquakes. Less well-known historical earthquakes, including the AD 869 Jōgan Sanriku earthquake, were largely disregarded as their seismic intensities or tsunami heights could not be reconciled with the chosen seismic sources. Following the unexpectedly large size of the Tōhoku earthquake, the Cabinet Office advocated renewed investigation of earthquake and tsunami occurrence over historical and longer timescales, with a particular focus on defining the largest possible magnitudes. The new guidelines pay close attention to the Nankai Trough, the subduction zone where the Philippine Sea Plate dives beneath the Eurasian Plate. The Nankai Trough faces the densely populated and highly industrialised coastline of south central Japan and harbours a widely-known seismic gap along its eastern Tōkai segment. A full-length rupture of the Nankai Trough, including the Tōkai segment, could produce an earthquake with a magnitude approaching that of the 2011 event, with tsunami travel times to the closest shorelines of less than 30 minutes. Here, we review geological evidence for past earthquakes and tsunamis along the Nankai Trough. This evidence comes from a wide variety of sources, including uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. Examining papers published before and after 2011, we investigate the impact of the new Cabinet Office guidelines on attempts to understand the magnitude and recurrence of these events. We summarise current knowledge of the largest paleoearthquakes and paleotsunamis and make recommendations for further investigations of this highly critical subduction zone.
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Current status of palaeoseismic research along the Nankai Trough, Japan
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In the wake of the devastating 2011 Tōhoku earthquake and tsunami, the Central Disaster Management Council of the Japanese Cabinet Office issued new guidance for assessing seismic hazards in Japan. Following the unexpectedly large magnitude of the earthquake and the size of the tsunami, the Cabinet Office advocated renewed investigation of earthquake and tsunami occurrence timescales exceeding the historical period, with a particular focus on defining the largest possible magnitudes. The new guidelines pay close attention to the Nankai Trough, the subduction zone where the Philippine Sea Plate dives beneath the Eurasian Plate. The Nankai Trough faces the densely populated and highly industrialised coastline of south central Japan. Here, we review geological evidence for past earthquakes and tsunamis along this subduction zone. This evidence comes from a wide variety of sources, including uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. More than 70 sites yield evidence, however the number of events recorded at each site varies depending on site-specific evidence creation and preservation thresholds. The longest record exceeds ten thousand years, however the majority are less than four thousand years long. Our compilation suggests that earthquakes in AD 684 and 1361 were similar predecessors of the AD 1707 earthquake, widely regarded as the largest event of the historical period. The more limited distribution of evidence for other historical earthquakes highlights the variability in rupture mode that characterises the Nankai Trough. The palaeoseismic catalogue is, however, limited due to issues over alternative hypotheses for proposed palaeoseismic evidence, poor chronological control and sampling approached insufficient to address the recurrence of the largest past earthquakes and tsunamis. We highlight recent advances and propose future directions for Nankai Trough palaeoseismology.
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Radiometric and luminescence dating of historical tsunamis and storm deposits from Shirasuka, south central Japan
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Differentiating between the deposits left by tsunamis and storms remains a major challenge for the paleoseismology and paleotempestology communities. The coast of Shizuoka Prefecture, south central Japan, is impacted by both typhoons and by tsunamis triggered by great subduction zone earthquakes (M>8) along the adjacent Nankai Trough. Previous investigations along this coastline have reported abruptly emplaced sand layers interbedded between organic marsh deposits (Komatsubara et al., 2008). At Shirasuka, a narrow strip of low-lying land sandwiched between a beach ridge and the 60-70m high riser of the Middle Pleistocene Marine terrace (Tenpakubara terrace) is well-placed to record both types of deposits. At this site, fine-grained organic silt and clay deposition, beginning in the 13th century AD, has been repeatedly interrupted by coarse-grained overwash beach sand layers (Fujiwara et al., 2006). These sand sheets have been variously attributed to historical typhoons and tsunamis (Komatsubara et al., 2008); however, in part due to the plateau in the radiocarbon calibration curve, the precise correlation between sand layers and historical events remains unclear. We seek to resolve this uncertainty by applying both radiometric and luminescence-based dating approaches to the site. By constraining the timing of each sand layer, we seek to conclusively link geological evidence to known historical events and, in doing so, facilitate comparison between the different sedimentary expressions of these natural hazards in the coastal zone. Fujiwara, O., Komatsubara, J., Takada, K., Shishikura, M., Kamataki, T., 2006. Temporal development of a Late Holocene stran plain system in the Shirasuka area along western Shizuoka Prefecture on the Pacific coast of central Japan. Journal of Geography 115, 569-581. Komatsubara, J., Fujiwara, O., Takada, K., Sawai, Y., Aung, T.T., Kamataki, T., 2008. Historical tsunamis and storms recorded in a coastal lowland, Shizuoka Prefecture, along the Pacific coast of Japan. Sedimentology 55, 1703-1716.
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The QuakeRecNankai project: Towards New Geological Evidence of Past Earthquakes and Tsunami Along the Nankai Trough, Japan
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The east coast of Japan is prone to tsunamigenic megathrust earthquakes, as tragically demonstrated in 2011 by the Tōhoku earthquake (Mw 9.0) and tsunami. The Nankai Trough subduction zone, to the southwest of the area affected by the Tōhoku disaster and facing the densely populated and heavily industrialized southern coastline of central and west Japan, is also expected to generate another great megathrust earthquake and tsunami in the near future. Historical records suggest the subduction zone segmented and characterized by a variable rupture mode, involving single- as well as multi-segment ruptures, with immediate implications for the tsunamigenic potential. This renders the collection of sufficiently long records of past earthquakes and tsunami in this region fundamental for adequate hazard and risk assessment. In response to the 2011 Tōhoku earthquake, the Cabinet Office of the Japanese government proposed new guidelines for assessing the risk of similar earthquakes affecting the Nankai subduction zone (Central Disaster Management Council, 2011). These guidelines advocate renewed investigation of earthquake and tsunami occurrence over historical and longer timescales, with a particular focus on defining the largest possible magnitudes. The QuakeRecNankai project contributes to this goal by generating a long and coherent time series of megathrust earthquake and tsunami recurrences along the Nankai Trough subduction zone by integrating all existing evidence with new geological records of paleo-tsunami in the Lake Hamana region and of paleo-earthquakes from selected lakes in the Mount Fuji area. We combine extensive fieldwork in coastal plain areas and lakes with advanced sedimentological and geochemical analyses and innovative dating techniques.
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The last 300 years of sedimentation in the Fuji Five Lakes: the impact of natural disasters with a special focus on earthquakes
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Within the framework of the QuakeRecNankai project, which focuses on the geological record of paleoearthquakes rupturing the Nankai-Suruga subduction zone, we sampled bottom sediments of the Fuji Five Lakes at the foot of Mount Fuji. In October 2014, we acquired 23 short gravity cores from Motosu, Sai, Kawaguchi and Yamanaka Lakes. We focus on investigating the sensitivity of each lake for recording natural hazards (storms, floods, volcanic eruptions, earthquakes). Recent natural hazards impacting the Fuji Five Lakes area are expected to be recorded as rapidly deposited layers within the background hemipelagic sediments. We seek to identify event deposits in the collected short cores based on visual identification combined with high resolution analyses, comprising geophysical and geochemical properties, grain size and X-ray images. We establish an age-depth model by combining radionuclide (14C,137Cs,210Pb) dating with the identification of historical tephra layers, in particular the one deposited during the Hoei eruption (AD 1707) of Mount Fuji. The sedimentary events in each lake are compared to a historical catalogue of natural hazards in the Fuji Five Lakes area, including historical records of megathrust earthquakes rupturing the Nankai subduction zone and other earthquakes occurring along the inland active faults that produced significant shaking (MKS intensity >7) in the Fuji Five Lakes area. The preliminary sedimentary study highlights the high sensitivity of Motosu Lake compared to the very shallow Yamanaka and Kawaguchi Lakes, which we attribute to its specific geomorphology. Motosu Lake is characterized by a deep basin, surrounded by very steep slopes. The generation of earthquake triggered mass transport deposits/turbidites and the preservation of the deposits are favored due to the architecture of the basin. The Fuji lake sensitivity to megathrust events will be the target of a future coring campaign to retrieve a longer geological record.
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