11:00 AM - 1:00 PM
[HDS11-P02] Geomorphic events and the ages of buried trees unearthed in river valleys in the Minami Shinshu region of Nagano Prefecture
Keywords:buried wood, slope failure, triggers, radiocarbon dating, oxygen isotope ratio dendrochronology
Buried dead trees due to geomorphic events such as landslides or slope collapses induced by ancient earthquakes or heavy rains, are sometimes unearthed in later years. In order to reconstruct geomorphic events based on the dating of buried trees, we need to start by relating the events that occurred around the estimated year, taking into account the occurrence of the samples and the intrinsic limitations of the age-dating methods applied. If the age is determined on a yearly basis by methods that has become particularly sophisticated in recent years such as oxygen isotope ratio dendrochronology, it is necessary to specify the direct cause of the death of the tree, for example, whether it was the main shock of a huge earthquake, its foreshock or aftershock, or sediment runoff from a loosened slope. In the case of a small number of samples, it is impossible to judge the inclusion of individual trees that died due to other factors than a specific geomorphic event because of the restrictions imposed by the preservation and collection conditions (e.g., presence or absence of outermost annual rings). We can therefore improve the certainty of the age estimation of the event if a large number of samples are employed for age determination to detect the signal of the simultaneous death of trees by the event. Furthermore, the distribution of trees that died simultaneously can also be used to assess the extent of the impact of the event.
In this report, we collect and rearrange the age data for the following cases in the Minami Shinshu region of Nagano Prefecture, where many buried trees have been discovered since the late 1900s (e.g., Teraoka et al., 2006, Iida City Museum). By constructing a database including new measurements, we re-examine the temporal correlation between tree death and the occurrence of geomorphic events.
(1) Buried forests along the Toyama River: The buried forests that emerged along the Toyama River from Oshima to Kizawa areas in Iida City as the riverbed declined have been discussed in relation to earthquakes that occurred around the 8th century (e.g., Teraoka et al., 2006, Iida City Museum).
(2) A dammed lake of the Ikeguchi River: Ikeguchi River confluences with the Toyama River. Geomorphic events have been discussed based on surveys of the landforms and sediments of the dammed lake along Ikeguchi River, which was formed at the same time as the buried forest along the Toyama River (Muramatsu et al., 2009, Geological Society of Japan Conference Abstracts, 330-330).
(3) Mt. Yahazu collapse: About 10 buried trees found about 1,200 m upstream of the Mt. Yahazu collapse at the northern foot of Mt. Yahazu along the Toyama River are thought to have been buried in a dammed lake formed by this collapse (Southern Alps Geopark Eco Park Site Card, 2015).
(4) The Kazakoshiyama Fault: Buried trees were collected from collapsed sediments buried in a small valley along the Kazakoshiyama Fault in order to study the activity of the Ina Valley Fault Zone (Okumura et al., 1998, Summaries of Researches using AMS at Nagoya University, 9, 64-75).
(5) Streams of Yunohora and Kuwabatazawa, Achi: Based on the analysis of buried trees found in the collapsed sediments along the streams in Achi Village, the relationship between geomorphic events and the Tensho Earthquake is discussed (Matsushima, 2000, Historical Earthquakes, 16, 53-58).
(6) Kawaji-Hanagosho: A buried forest of about 17,000 years ago was found at a quarry site in Kawaji-Hanagosho, Iida City (Matsushima et al., 1998, Iida City Museum Research Bulletin 8, 107-118).
In this report, we collect and rearrange the age data for the following cases in the Minami Shinshu region of Nagano Prefecture, where many buried trees have been discovered since the late 1900s (e.g., Teraoka et al., 2006, Iida City Museum). By constructing a database including new measurements, we re-examine the temporal correlation between tree death and the occurrence of geomorphic events.
(1) Buried forests along the Toyama River: The buried forests that emerged along the Toyama River from Oshima to Kizawa areas in Iida City as the riverbed declined have been discussed in relation to earthquakes that occurred around the 8th century (e.g., Teraoka et al., 2006, Iida City Museum).
(2) A dammed lake of the Ikeguchi River: Ikeguchi River confluences with the Toyama River. Geomorphic events have been discussed based on surveys of the landforms and sediments of the dammed lake along Ikeguchi River, which was formed at the same time as the buried forest along the Toyama River (Muramatsu et al., 2009, Geological Society of Japan Conference Abstracts, 330-330).
(3) Mt. Yahazu collapse: About 10 buried trees found about 1,200 m upstream of the Mt. Yahazu collapse at the northern foot of Mt. Yahazu along the Toyama River are thought to have been buried in a dammed lake formed by this collapse (Southern Alps Geopark Eco Park Site Card, 2015).
(4) The Kazakoshiyama Fault: Buried trees were collected from collapsed sediments buried in a small valley along the Kazakoshiyama Fault in order to study the activity of the Ina Valley Fault Zone (Okumura et al., 1998, Summaries of Researches using AMS at Nagoya University, 9, 64-75).
(5) Streams of Yunohora and Kuwabatazawa, Achi: Based on the analysis of buried trees found in the collapsed sediments along the streams in Achi Village, the relationship between geomorphic events and the Tensho Earthquake is discussed (Matsushima, 2000, Historical Earthquakes, 16, 53-58).
(6) Kawaji-Hanagosho: A buried forest of about 17,000 years ago was found at a quarry site in Kawaji-Hanagosho, Iida City (Matsushima et al., 1998, Iida City Museum Research Bulletin 8, 107-118).