Report and preliminary results of R/V SONNE Cruise SO251 - Extreme events Archived in the GEologial Record of JAPAN's subduction margins (EAGER-JAPAN)

Leg A SO251-1, Yokohama - Yokohama, 04.10.2016 - 15.10.2016, Leg B SO251-2, Yokohama - Yokohama, 18.10.2016 - 02.11.201

Geological evidence for historical and older earthquakes and tsunamis along the Nankai Trough, Japan

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.QuakeRecNanka

Paleoearthquakes recorded in the Fuji Five Lakes during the last ca. 6000 years (Fuji Five Lakes, Japan

In Japan, one pioneering lacustrine paleoseismological study was conducted in Lake Biwa in the 1990s. However, despite the high seismicity of Japan, the field of lacustrine paleoseismology did not expand. Paleoseismological studies were more focused on inland trenches and coastal records. The paleoseismological data obtained by that way covers the last ~6000 years but contains hiatuses. Using lacustrine sediment allows us to span the same time period and to have a continuous record. Here, we present the second lacustrine paleoseismological study conducted in Japan. The Fuji Five Lakes are situated close to the triple junction, where the North American Plate, the Eurasian plate and the Philippine Sea Plate meet. Therefore, the region can be impacted by megathrust earthquakes generated along the Nankai-Suruga and the Sagami subduction zones. In addition, intraplate earthquake may affect the Fuji Five Lakes region. In the framework of the QuakeRecNankai project, we investigated two of the Fuji Five Lakes, Lake Motosu and Lake Sai. Here, we present the paleoseismological record of Lake Motosu and Lake Sai over the last 6000 and 2000 years, respectively. The turbidites were identified based on geophysical (magnetic susceptibility, grainsize) and geochemical properties (XRD, XRF) as well as SEM analysis. The turbidites were dated by 210Pb/137Cs, 14C dating and correlated with historical earthquakes. For prehistorical earthquakes (i.e., before the 6th century), they were correlated with geological evidences recorded along the Eastern Honshu coastline (i.e., tsunami deposits, coastal uplift, emerged ridge beaches). Over the last 6000 years, seismo-turbidites occurred with a mean recurrence time of 184±8 years. The near absence of large mass-transport deposits in the last 6000 years suggest that earthquake shaking mostly induced the remobilization of thin veneers of sediments in Lake Motosu. In Lake Sai, the earthquake fingerprint differs from Lake Motosu. Over the last 1200 years, past earthquake shaking induced turbidites, delta collapse and liquefaction (sediment volcanoes).QuakeRecNanka