JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG68] Environment formed and maintained by active faults

convener:Naoji Koizumi(School of Environmental Science,the University of Shiga Prefecture), Makoto Yamano(Earthquake Research Institute, the University of Tokyo), Takafumi Kasaya(Japan Agency for Marine-Earth Science and Technology), Hideki Hamamoto(Center for Environmental Science in Saitama)

[SCG68-04] SBP survey around the north-western area of the Biwa lake

*Takafumi Kasaya1, Naoji Koizumi2 (1.Japan Agency for Marine-Earth Science and Technology, 2.University of Shiga Prefecture)

Keywords:Lake bottom seepage, Sub-bottom profiler, Displacement structure

Around the north-wenstern area of the Biwa lake off Takashima city, there is a flat depression at the depth of 90-100 meters. Many lake bottom seepage points on the depression area have been pointed out, and the disribution of these points are lined in a north-south direction. The contribution of ground water seepage from the lake bottom is estimated to be about 10 to 20 %, however their actual condition is not clear.
To study the relationsips between the sedimentary structure and lake bottom seepage, we couducted the sedimentary structure survey using sub-bottom profiler loaded on the R/V Hassaka of The University of Shiga Prefecture. Moreover, fish finder was also used for detection of acoustic anomaly of water column related with the seepage on the lake bottom.
Three SBP survey lines were carried out in east-west direction in the depression area. Penetration depth of this survey was about 15 m. Some strong reflectors were detected in all survey lines. Deepest strongest reflector is presented at the depth of 12-15 m, and this corresponds to the Kikai-Akahoya (K-Ah) tephra (Iuchi et al., 1987). The sedimentary structure is not clear below this reflector.
We expected some displacement or fault structure relating to the water seepage on the lake bottom because some displacement structure had been found in the coastal area of the western area of the Biwa lake. However, we could not detect any clear displacement and fault structure within the penetration depth around the deep depresion area. Therefore, we traced the seepage points from north to south to detect the water column anomaly using fish finder. As a result, we found a clear water column anomlay around the 50 m hight mound at the southern area of the depression and we carried out the SBP survey around this water column anomaly area. The sedimentary structures are almost the same to the previous survey, however a displacemnet structure is detected near the water column anomaly area.