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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1. Abstract The Cretaceous experienced several Oceanic Anoxic Events (or OAEs). Anoxia in these events is indicated by deposits of black shales, enriched in organic matter (OM) compared to the layers below and above, strong carbon isotope perturbations, often with a negative excursion at the onset of the OAEs followed by a positive excursion, and concentration of redox-sensitive trace-elements (RSTE) (Baudin & Riquier 2014). Considered to be the earliest Cretaceous OAE (Baudin & Riquier, 2014), the Faraoni level is a short event first defined in the late Hauterivian sections of the Umbria-Marche Apennines (Cecca et al. 1994). It presents black shales enriched in OM with high concentrations of RSTE but lacks an important positive δ13C excursion (Baudin & Riquier, 2014). This event follows the “Weissert” event, a ca. 2.3 million year carbon isotope perturbation event taking place during the late Valanginian-early Hauterivian (Sprovieri et al. 2006). This latter event is not considered to be an OAE, as anoxia indicators such as RSTE high concentrations or OM-rich layers are not observed at least in the western Tethys (Westermann et al. 2010). In order to link those two seemingly opposite events, sections of Late Valanginian to Early Barremian age were studied in the Umbria-Marche Apennines, Italy. Lesser magnitude black shale preceding the Faraoni level were identified. They were correlated in two sections using magnetostratigraphy (Fig. 1). Rock-Eval and palynofacies analyses reveal that they are part of a longer-term trend of increased organic matter preservation and burial. In the black shales this is hinted by a progressive increase of total organic carbon (TOC) content, of the hydrogen index (HI), and by increasingly better preserved amorphous organic matter (AOM) towards the Faraoni level (Fig.1). This increase starts in the upper part of the M5n magnetochron. This is coeval with an increase in mercury concentration interpreted to be due to volcanic activity that was measured among others in the Bosso section (Charbonnier et al., 2018). Palaeoenvironmental differences between the Bosso and Frontone sections is shown by differences in palynomorphs and in organic matter preservation, and by the presence of slumps found in Frontone only. Figure 1 : synthetic log of the Bosso and Frontone sections, with magnetostratigraphy and Rock Eval 6 results (TOC and HI) 2. References Baudin, F. & Riquier, L., 2014. The Late Hauterivian Faraoni ‘Oceanic Anoxic Event’: An Update. Bulletin de La Société Géologique de France, 185, 6, 359‑77. Cecca, F., Marini, A., Pallini, G., Baudin, F., & Begouen, V., 1994. A guide level of the uppermost Hauterivian (Lower Cretaceous) in the pelagic succession of Umbria Marches Apennines (Central Italy): the Faraoni level, Rivista Italiana di Paleontologia e Stratigrafia, 99, 4. Sprovieri, M., Coccioni, R., Lirer, F., Pelosi, N. & Lozar F., 2006. Orbital Tuning of a Lower Cretaceous Composite Record (Maiolica Formation, Central Italy). Paleoceanography, 21, 4. Westermann, S., Föllmi, K.B., Adatte, T., Matera, V., Schnyder, J., Fleitmann, D., Fiet, N., Ploch, I. & Duchamp-Alphonse S., 2010. The Valanginian δ13C Excursion May Not Be an Expression of a Global Oceanic Anoxic Event. Earth and Planetary Science Letters, 290, 1‑2, 118‑31. Charbonnier, G., Godet, A., Bodin, S., Adatte, T. & Föllmi, K. B. 2018. Mercury anomalies, volcanic pulses, and drowning episodes along the northern Tethyan margin during the latest Hauterivian-earliest Aptian. Palaeogeography. Palaeoclimatoly. Palaeoecology.
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