6:15 PM - 7:30 PM
[MIS25-P13] Tsunami origin sediments observed from the Ohtsuchi bay, Iwate Pref., Japan.
Keywords:tsunami sediment, Ohtsuchi bay
The recent 2011 Tohoku tsunami strongly affected the coastal area of the Pacific coast of Tohoku. Tokai University and JAMSTEC team investigated the Tohoku coastal area as a part of Tohoku Ecosystem-Associated Marine Sciences (TEAMS).
We researched using acoustic equipments (Multi narrow echo sounder : MNB and Sub bottom profiler : SBP), and bottom sampler.
SBP data was seen signature reflecting (20-50cm down from seabed), and able to estimate the reflecting surface to depth of approximately 20 m at Ohtsuchi bay. The most strong reflector (R1) near the sea-bottom (20-70cm in depth) have rich in unevenness characters and have large lateral change around the shallow area (5-15m). However, R1 changes flat and smooth reflector to the offing (over -30m in depth).
Characteristic of columnar core sample at Ohtsuchi bay consists by 2 (shallow area) to 3 (deep area) sedimentogical units, such as upper sandy unit (30-110cm), middle reddish brown silt unit (60-130) and lower sandy unit. This coarse sand and gravel layer from Unit-1 eroded out the underlying reddish brown silt (Unit-2) zone. Geological facies (Unit-1) of the cores from the Ohtsuchi correspond with reflector from SBP such as R-1. It is estimated that the upper sandy sediment unit with grading structure above the erosion layer, which observed Ohtsuchi bay, is assumed to be a layer of sediment gravity flow caused by the tsunami activity.
We researched using acoustic equipments (Multi narrow echo sounder : MNB and Sub bottom profiler : SBP), and bottom sampler.
SBP data was seen signature reflecting (20-50cm down from seabed), and able to estimate the reflecting surface to depth of approximately 20 m at Ohtsuchi bay. The most strong reflector (R1) near the sea-bottom (20-70cm in depth) have rich in unevenness characters and have large lateral change around the shallow area (5-15m). However, R1 changes flat and smooth reflector to the offing (over -30m in depth).
Characteristic of columnar core sample at Ohtsuchi bay consists by 2 (shallow area) to 3 (deep area) sedimentogical units, such as upper sandy unit (30-110cm), middle reddish brown silt unit (60-130) and lower sandy unit. This coarse sand and gravel layer from Unit-1 eroded out the underlying reddish brown silt (Unit-2) zone. Geological facies (Unit-1) of the cores from the Ohtsuchi correspond with reflector from SBP such as R-1. It is estimated that the upper sandy sediment unit with grading structure above the erosion layer, which observed Ohtsuchi bay, is assumed to be a layer of sediment gravity flow caused by the tsunami activity.