5:15 PM - 6:45 PM
[MIS20-P07] Estimation of the prehistoric tsunami sizes using tsunami boulder in the Sakishima Islands
Keywords:Tsunami boulder, Prehistoric tsunami, Numerical simulation, Sakishima Islands
In the Sakishima Islands, large tsunamis occurred in AD1771 (Meiwa tsunami; Event Ⅰ), 620-920 years ago (Event II), and 1670-1250 years ago (Event III) (Ando et al., 2018). The damage caused by the 1771 Meiwa tsunami are well documented in historical descriptions. The tsunami source models were numerically estimated using historical records as constraints (e,g., Imamura et al., 2001; Nakamura, 2009; Miyazawa et al. 2012), while we recently updated a 1771 Meiwa tsunami source model using the latest dataset of historical descriptions and bathymetry.
There is no historical description of the large tsunamis before the 1771 Meiwa tsunami (i.e., Events II and III). Therefore, geological records are the sole tools to estimate the size of the tsunamis before 1771. In the Sakishima Islands, not only sandy tsunami deposits but also many tsunami boulders are reported (Goto et al., 2010). For example, the "Tsunami-Ufuishi" (hereinafter TU boulder) is a tsunami boulder in Ishigaki Island and its moving history by prehistoric tsunamis is well known (Kawana et al., 1987; Sato et al., 2013: Hisamatsu et al., 2014). Specifically, the TU boulder has moved twice from the reef edge to its present location (10 m above sea level and 100 m from the coastline) possibly by two prehistoric tsunamis (Events Ⅱ and Ⅲ).
Hisamatsu et al. (2014) estimated the sizes of these two prehistoric tsunamis, which can reproduce the moving history of TU boulder, using the boulder transport model (Imamura et al., 2008). However, their work can be updated by using the latest tsunami source model as well as the geological and archaeological knowledge. In this study, we re-estimated the size of the prehistoric tsunamis (Events Ⅱ and Ⅲ) based on the latest Meiwa tsunami model. The results show that the fault slip of at least one of two events (Events Ⅱ or Ⅲ) should be larger than that of the Meiwa tsunami. In addition, tsunami deposits reported in Shiraho-Saone Cave, Ishigaki Island can be correlated with Event Ⅲ and the elevation of the cave mouth is about 40 m (Okinawa Prefectural Archeological Center, 2017). Since the Meiwa tsunami source model cannot calculate tsunami inundation to this elevation, we estimated that Event Ⅲ, not Event Ⅱ, could be larger than the Meiwa tsunami.
There is no historical description of the large tsunamis before the 1771 Meiwa tsunami (i.e., Events II and III). Therefore, geological records are the sole tools to estimate the size of the tsunamis before 1771. In the Sakishima Islands, not only sandy tsunami deposits but also many tsunami boulders are reported (Goto et al., 2010). For example, the "Tsunami-Ufuishi" (hereinafter TU boulder) is a tsunami boulder in Ishigaki Island and its moving history by prehistoric tsunamis is well known (Kawana et al., 1987; Sato et al., 2013: Hisamatsu et al., 2014). Specifically, the TU boulder has moved twice from the reef edge to its present location (10 m above sea level and 100 m from the coastline) possibly by two prehistoric tsunamis (Events Ⅱ and Ⅲ).
Hisamatsu et al. (2014) estimated the sizes of these two prehistoric tsunamis, which can reproduce the moving history of TU boulder, using the boulder transport model (Imamura et al., 2008). However, their work can be updated by using the latest tsunami source model as well as the geological and archaeological knowledge. In this study, we re-estimated the size of the prehistoric tsunamis (Events Ⅱ and Ⅲ) based on the latest Meiwa tsunami model. The results show that the fault slip of at least one of two events (Events Ⅱ or Ⅲ) should be larger than that of the Meiwa tsunami. In addition, tsunami deposits reported in Shiraho-Saone Cave, Ishigaki Island can be correlated with Event Ⅲ and the elevation of the cave mouth is about 40 m (Okinawa Prefectural Archeological Center, 2017). Since the Meiwa tsunami source model cannot calculate tsunami inundation to this elevation, we estimated that Event Ⅲ, not Event Ⅱ, could be larger than the Meiwa tsunami.