[H-DS06] Tsunami and tsunami forecast

[HDS06-10] Comparison of tsunami inundation areas by the 1662 Hyuga-nada earthquake and 1707 Hoei earthquake in Miyazaki Prefecture

*Kei Ioki¹, Yusuke Yamashita², Yoshihiro Kase³ (1.National Institute of Advanced Industrial Science and Technology, 2.Miyazaki Observatory, Disaster Prevention Research Institute, Kyoto University, 3.Research Institute of Energy, Environment and Geology, Hokkaido Research Organization)

Keywords:great earthquake, tsunami, inundation simulation, tsunami deposit, Hyuga-nada

The Hyuga-nada, located to the east of Kyushu Island, is an active area for shallow slow earthquakes and has occurred M7-class interplate earthquakes with an interval of several decades. The largest earthquake that occurred in the Hyuga-nada in historical records was the 1662 Hyuga-nada Earthquake. The 1662 earthquake generated a tsunami that was estimated to be 4-5 m high in the coastal area of Miyazaki City, causing extensive damage. The 1662 tsunami was much larger than the tsunamis generated by M7-class earthquakes in the Hyuga-nada, making it difficult to explain with a previous source model and necessitating the construction of a new fault model. Based on the findings of the 2011 Tohoku-oki earthquake, we hypothesized that the source area of the 1662 tsunami extended to the shallow slow earthquake zone, which generated a huge tsunami, and we tested this hypothesis. First, we assumed a fault model of the 1662 Hyuga-nada earthquake based on recent seafloor seismic observations of shallow slow earthquakes and the location of the plate boundary. Next, to reconstruct the inundation area of the 1662 tsunami, we surveyed tsunami deposits along the Pacific coast of Miyazaki Prefecture (Nobeoka City to Kushima City). We found event deposits at several sites that may be related to the 1662 tsunami, and identified the event deposit at Komei, Nichinan City, as a tsunami deposit based on its sedimentological characteristics. Numerical simulation of tsunami using the assumed fault model shows that the calculated inundation area can explain the distribution of tsunami deposits at Komei, Nichinan City, and calculated tsunami heights can explain tsunami heights along the coast of Miyazaki Prefecture according to historical records. The magnitude of the earthquake calculated from the fault model is Mw7.9, indicating that the 1662 Hyuga-nada earthquake may have been a M8-class great earthquake. It is also reported that all Livistona chinensis occupied most of the island's area were died by the 1662 tsunami in Aoshima, Miyazaki City. The oldest of the tens of thousands of trees growing is estimated to be about 350 years old. The damage to Aoshima by the 1662 tsunami is consistent with the calculated inundation area based on the fault model. On the other hand, the tsunami damage at Aoshima in Miyazaki City and the distribution of tsunami deposits at Komei in Nichinan City cannot be explained by the inundation area calculated from the fault model of the 1707 Hoei Earthquake based on previous studies. Therefore, it is reasonable to explain these tsunami evidences at Aoshima and Komei using the fault model of the 1662 Hyuga-nada earthquake.