4:15 PM - 4:30 PM
[SSS11-14] Did the rupture of the Shohei (1361 CE) earthquake extend off the Shizuoka Prefecture?
★Invited Papers
Keywords:Nankai Trough, Shohei earthquake, tsunami deposit
Based on strong seismic shaking over a wide area including the Kii Peninsula and the tsunami documented in Tokushima and Osaka prefectures, it is considered that fault rupture of the 1361 CE Shohei (Ko’an) earthquake occurred at western area (segments A and B) of the Nankai Trough. Although no written documents about tsunami associated with the Shohei earthquake have been found in the Tokai and Tonankai regions (segments C–E), Ishibashi (2004) considered that fault rupture also occurred in the Tonankai area (segments C and D), because seismic shaking were recorded in Kyoto within a few days before the earthquake. This hypothesis is now widely accepted. Thereafter, in support of the hypothesis, sediments that are correlated to the Shohei earthquake tsunami (Garrett et al., 2018; Kitamura et al., 2020) and a bivalve assemblage that is considered to have uplifted and emerged during the earthquake (Kitamura et al., 2018) were reported in Shizuoka Prefecture.
However, Komatsubara et al. (2008) provided an age younger than the Shohei earthquake from the deposit that Garrett et al. (2018) identified as deposit of the Shohei earthquake tsunami. Therefore, even if the bed is a tsunami deposit, it is not reasonable to correlate it to the Shohei earthquake tsunami. Kitamura et al. (2020) interpreted deposits found in sediment core samples collected in Suruga Bay coast as tsunami deposits based on their grain-size similarity to beach sediments. However, it is not evident from the data presented in Kitamura et al. (2020) whether the grain-size composition of the deposits is similar with beach sediments. In addition, among the deposits interpreted as tsunami deposits, there are even some that show little grain-size difference from the upper and lower deposits and are not clearly layered. The age of the bivalves assumed by Kitamura et al. (2018) to have been emerged during the Shohei earthquake indicates the limiting maximum age of the uplift event, rather than the age of the uplift event itself. Thus, overlap in ages of some of the bivalves with the Shohei earthquake does not necessarily indicate that the bivalve assemblages were emerged at the time of the Shohei earthquake. Therefore, it should not be concluded that rupture of the Shohei earthquake extended to the Tokai and Tonankai regions based on these studies in Shizuoka Prefecture.
Results of tsunami deposit survey does not support the hypothesis that the rupture of the Shohei earthquake extended to off Shizuoka Prefecture (segment D) and generated a tsunami in the Tonankai region. Although historical documents tell the damage from seismic shaking (Okuno and Okuno, 2017) in Mie Prefecture, no record about tsunami associated with the Shohei earthquake has been found in the Tonankai region at present. In fact, no tsunami deposits dated to the Shohei earthquake tsunami were found at two study sites in the Shima Peninsula, Mie Prefecture. On the other hand, tsunami deposits that overlap in age with the Meio (1498 CE) and Eicho (1096 CE) earthquake tsunamis which are documented in the Tonankai region were found in both sites. It is necessary to examine the extent of rupture of the Shohei earthquake in unbiased way.
However, Komatsubara et al. (2008) provided an age younger than the Shohei earthquake from the deposit that Garrett et al. (2018) identified as deposit of the Shohei earthquake tsunami. Therefore, even if the bed is a tsunami deposit, it is not reasonable to correlate it to the Shohei earthquake tsunami. Kitamura et al. (2020) interpreted deposits found in sediment core samples collected in Suruga Bay coast as tsunami deposits based on their grain-size similarity to beach sediments. However, it is not evident from the data presented in Kitamura et al. (2020) whether the grain-size composition of the deposits is similar with beach sediments. In addition, among the deposits interpreted as tsunami deposits, there are even some that show little grain-size difference from the upper and lower deposits and are not clearly layered. The age of the bivalves assumed by Kitamura et al. (2018) to have been emerged during the Shohei earthquake indicates the limiting maximum age of the uplift event, rather than the age of the uplift event itself. Thus, overlap in ages of some of the bivalves with the Shohei earthquake does not necessarily indicate that the bivalve assemblages were emerged at the time of the Shohei earthquake. Therefore, it should not be concluded that rupture of the Shohei earthquake extended to the Tokai and Tonankai regions based on these studies in Shizuoka Prefecture.
Results of tsunami deposit survey does not support the hypothesis that the rupture of the Shohei earthquake extended to off Shizuoka Prefecture (segment D) and generated a tsunami in the Tonankai region. Although historical documents tell the damage from seismic shaking (Okuno and Okuno, 2017) in Mie Prefecture, no record about tsunami associated with the Shohei earthquake has been found in the Tonankai region at present. In fact, no tsunami deposits dated to the Shohei earthquake tsunami were found at two study sites in the Shima Peninsula, Mie Prefecture. On the other hand, tsunami deposits that overlap in age with the Meio (1498 CE) and Eicho (1096 CE) earthquake tsunamis which are documented in the Tonankai region were found in both sites. It is necessary to examine the extent of rupture of the Shohei earthquake in unbiased way.