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[MIS16-P04] Characteristics of grain composition of the 2011 Tohoku-oki tsunami deposit at Hitota Bay, Iwate pref
Keywords:Tohoku Earthquake in the Pacific Ocean, Tsunami deposit, Sand grain mineral composition
The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) occurred on March 11, 2011, and the tsunami generated by the earthquake caused great damage. In this presentation, we tried to identified the origin of the tsunami deposits by grain composition in Hirota Bay. The provenance of Hirota Bay different between east and west side. Early Cretaceous granite is distributed on the east side (Hirota Peninsula) and Paleozoic sedimentary rock on the west side (Karakuwa Peninsula). Kesen River flow into from the back of the bay and lime stone, dacite, granite and mud stone are distributed in the watershed. The beach sand samples were collected from Hirota Peninsula, Karakuwa Peninsula, and Takata matsubara beach and the core samples corrected in 2013 and 2014. Samples were divided into five size sands using sieves.Over 200 counts of sands were identified by stereomicroscope for three size sands (coarse sand, medium sand, and fine sand).
The grain composition of the beach sand were mainly granite fragments, quartz, and feldspar. The granite fragments were the most abundant at all sites and grain sizes. Sedimentary rocks were identified only on Takata matsubara beach and Karakuwa Peninsula, but not at Hirota Peninsula.
Core samples were 13HV10 (depth: 18.0 m, total length: 141.0 cm) collected in 2013 and 14HV6 (depth: 28.5 m, total length: 110 cm) collected in 2014. Both cores are divided into a sandy layer (Layer 1) and a muddy layer (Layer 2). Yokoyama et al. (2021) estimated that Layer 1 were the the 2011 Tohoku-oki tsunami deposits and Layer 2 were pre-tsunami sediments in this bay.
The Layer 1 thickness of the 13HV10 core was about 48 cm from the top. The grain composition was observed at 10 cm intervals. In the result, the main components were granite-fragments, mica, and quartz, and the ratio of granite-fragments and mica was approximately 60-80 % in all layers and grain sizes. The ratio of granite fragments to mica in Layer 1 is highly variable, while the variation in the ratio in Layer 2 is relatively small.
The Layer 1 thickness of the 14HV6 core was about 25 cm from the top, and pre-event deposit identified in the 103-105 cm of Layer 2. The grain composition was observed at 15 cm intervals. In this core, biogenic-fragments which was not observed in 13HV10 core was identified. The main grain components were mica, granite-fragments, and quartz. The pre-event deposit in Layer 2 composition was similar to the Layer 1. The event layer is supposed to have characteristic composition.
These results indicate that the grain composition of the Hirota Bay sediments is mainly composed of granite-fragments and mica. The ratio of granite-fragments and mica were found to be different between the tsunami deposit and the normal sediments in the bay. The composition of pre-event deposit was similar to the tsunami deposit layer. Therefore, the grain composition may provide useful information for the identification of the event deposit.
[References cited] Yokoyama et al. (2021) Journal of Sedimentology, 79(2), 47-69.
The grain composition of the beach sand were mainly granite fragments, quartz, and feldspar. The granite fragments were the most abundant at all sites and grain sizes. Sedimentary rocks were identified only on Takata matsubara beach and Karakuwa Peninsula, but not at Hirota Peninsula.
Core samples were 13HV10 (depth: 18.0 m, total length: 141.0 cm) collected in 2013 and 14HV6 (depth: 28.5 m, total length: 110 cm) collected in 2014. Both cores are divided into a sandy layer (Layer 1) and a muddy layer (Layer 2). Yokoyama et al. (2021) estimated that Layer 1 were the the 2011 Tohoku-oki tsunami deposits and Layer 2 were pre-tsunami sediments in this bay.
The Layer 1 thickness of the 13HV10 core was about 48 cm from the top. The grain composition was observed at 10 cm intervals. In the result, the main components were granite-fragments, mica, and quartz, and the ratio of granite-fragments and mica was approximately 60-80 % in all layers and grain sizes. The ratio of granite fragments to mica in Layer 1 is highly variable, while the variation in the ratio in Layer 2 is relatively small.
The Layer 1 thickness of the 14HV6 core was about 25 cm from the top, and pre-event deposit identified in the 103-105 cm of Layer 2. The grain composition was observed at 15 cm intervals. In this core, biogenic-fragments which was not observed in 13HV10 core was identified. The main grain components were mica, granite-fragments, and quartz. The pre-event deposit in Layer 2 composition was similar to the Layer 1. The event layer is supposed to have characteristic composition.
These results indicate that the grain composition of the Hirota Bay sediments is mainly composed of granite-fragments and mica. The ratio of granite-fragments and mica were found to be different between the tsunami deposit and the normal sediments in the bay. The composition of pre-event deposit was similar to the tsunami deposit layer. Therefore, the grain composition may provide useful information for the identification of the event deposit.
[References cited] Yokoyama et al. (2021) Journal of Sedimentology, 79(2), 47-69.