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[MIS16-10] Recurrence intervals between geologically recorded tsunamis in the Tonankai region are not constant
Keywords:tsunami deposit, Nankai Trough, recurrence interval, Tonankai region
To examine recurrence intervals of tsunamis occurred along the Nankai Trough, we collected sediment core samples from two coastal wetlands in Toba and Shima cities, Mie Prefecture and dated them in detail. Both study sites are lower than 1.5 m in elevation, which is lower than the inundation heights in the regions by the Showa Tonankai (1944 CE), Ansei Tokai (1854 CE), and Hoei (1707 CE) earthquake tsunamis, while flooding by the Isewan Typhoon (1959 CE) which is the biggest typhoon ever recorded in the region was not observed at the study sites.
Eighteen and eight sand layers were found in the sediment core samples from Toba City and Shima City, respectively. All of the sand layers in the Shima core and 8 of the 18 sand layers in the Toba core contained remains of marine organisms such as benthic foraminifers and shell hush of bivalves and gastropods. We are currently examining all sand layers in the Toba core for existence of marine diatoms. At present, no remains of marine organisms have been found in some of the sand layers in the Toba core. However, since there are no rivers flowing into the study site and the site was not inundated even by the Isewan Typhoon, we presume that the sand layers, including those without remains of marine organisms, were most likely deposited by tsunamis.
We used OxCal to calculate the ages of the sand layers from the ages of the upper and lower sediments of the sand layers. In both study sites, ages of the youngest three sand layers overlaps with the Meio (1498 CE), Eicho (1096 CE), and Hakuho (684 CE) earthquake tsunamis, while no sand layers were correlated to the Shohei (1361 CE) or Nin'na (887 CE) earthquake tsunamis. Moreover, we examined the recurrence intervals of the events that deposited the sand layers using the difference function of OxCal, and found that the recurrence intervals of the events varied between 100–800 years at the two study sites.
The fact that the recurrence intervals of stratigraphically recorded events at the two sites varied suggests that tsunami recurrence intervals along the Nankai Trough are not "constant between 90 and 150 years," as currently believed. In fact, the intervals of tsunamis that are definitively recorded in historical documents as caused damages in the Tonankai region also varied between 90 and 400 years. Moreover, the fact that no sand layers were found at the two sites that overlap in age with the Shohei (1361 CE) and Nin'na (887 CE) earthquake tsunamis is also consistent with the fact that no historical documents that record inundation and damage in the Tonankai region by the two tsunamis were found. In future studies, it is very important to examine data in an unbiased way.
Eighteen and eight sand layers were found in the sediment core samples from Toba City and Shima City, respectively. All of the sand layers in the Shima core and 8 of the 18 sand layers in the Toba core contained remains of marine organisms such as benthic foraminifers and shell hush of bivalves and gastropods. We are currently examining all sand layers in the Toba core for existence of marine diatoms. At present, no remains of marine organisms have been found in some of the sand layers in the Toba core. However, since there are no rivers flowing into the study site and the site was not inundated even by the Isewan Typhoon, we presume that the sand layers, including those without remains of marine organisms, were most likely deposited by tsunamis.
We used OxCal to calculate the ages of the sand layers from the ages of the upper and lower sediments of the sand layers. In both study sites, ages of the youngest three sand layers overlaps with the Meio (1498 CE), Eicho (1096 CE), and Hakuho (684 CE) earthquake tsunamis, while no sand layers were correlated to the Shohei (1361 CE) or Nin'na (887 CE) earthquake tsunamis. Moreover, we examined the recurrence intervals of the events that deposited the sand layers using the difference function of OxCal, and found that the recurrence intervals of the events varied between 100–800 years at the two study sites.
The fact that the recurrence intervals of stratigraphically recorded events at the two sites varied suggests that tsunami recurrence intervals along the Nankai Trough are not "constant between 90 and 150 years," as currently believed. In fact, the intervals of tsunamis that are definitively recorded in historical documents as caused damages in the Tonankai region also varied between 90 and 400 years. Moreover, the fact that no sand layers were found at the two sites that overlap in age with the Shohei (1361 CE) and Nin'na (887 CE) earthquake tsunamis is also consistent with the fact that no historical documents that record inundation and damage in the Tonankai region by the two tsunamis were found. In future studies, it is very important to examine data in an unbiased way.