5:15 PM - 7:15 PM
[SSS14-P11] Continuation of the Tsu-oki Flexure toward the Kumozu Lowland, western coast of the Ise Bay, central Japan
Keywords:Tsu-oki Flexure, submarine active fault, borehole data, Eastern margin Fault Zone of the Nunobiki Mauntains, Kumozu Lowland
In the Ise Plain, located in western coast of the Ise Bay, there are numerous active faults belonging to such as the Yoro-Kuwana-Yokkaichi and the Nunobiki Mountains eastern margin Fault Zones, as well as the Suzuka-oki and the Shirako-Noma Faults (Headquarters for Earthquake Research Promotion [HERP] 2005; Suzuki et al., 2010). In the central part of the plain, the Tsu-oki flexure has been identified around 3 km offshore of Tsu City (Iwabuchi et al., 2000), and it was active during the Holocene period has been pointed out (Okamura et al., 2013). HERP (2005) interpreted the Tsu-oki flexure as a part of the eastern part of the Nunobiki Mountains eastern margin fault zone. Okamura et al. (2013) suggested that the southern end of the Tsu-oki flexure located in the sea area, around 34°42′ N, 136°33′ E, however pointed out the possibility that the flexure may be continuous its southward because of other active structures found southeast of the flexure.
The GSJ has been carrying out the "Investigations of Geology and Active Faults in the Coastal Zone of Ise and Mikawa Bay". We have also been investigating the Quaternary subsurface geology and active structures in the area. Especially, we intensively collected borehole logs were at around the central part of the Ise Plain, which located between coastal plain of the Ano, Shitomo and Iwata Rivers and the Kumozu Lowland. Based on analysis of borehole logs, we investigated distribution of the Alluvial basement, top and bottom of the marine mud during the Last Interglacial period, and basement rocks and estimated stratigraphic deformation.
In coastal zone of the study area, there are a number of Pleistocene buried terraces beneath the alluvial lowland. These terraces are composed of muddy sediments with shell fossils (N values 5 to 20), and are covered by sand and gravel sediments with a thickness of 5 to 20 m. The stratigraphy and sedimental facies indicate that this muddy sediments are likely to be marine sediments. The muddy sediments around Shirako, Suzuka City characterized by abundant Lagerstroemia pollen fossil, and intepreted as marine sediments during the Last Interglacial period (MIS5e) (Sato et al., 2020). This muddy sediments is possibly corresponding to those of the study area. Around the north part of the study area, it is easy to trace the top of the muddy sediments with good lateral continuity at an elevation of -30 to -32 m. Alothough this tends to gradually deepen towards seaward, we recognized that this surface decreases sharply by about 4.6 to 8.8 m in a range of 300 to 500 m on the left side of the Kumozu River. In this area, the basement of the muddy layer also decreased sharply as same as the top, and which suggest that this displacement is likely to be caused by fault. In addition, the base of the Alluvium decreases by several meters on the southern of the Tsu-oki flexure. These suggest that the Tsu-oki flexure may extend to the land area of the Kumozu Lowland. The total length of the Tsu-oki flexure is estimated to be ~12 km, which is ~5 km longer than that indicated by Okamura et al. (2013).
Assuming that the muddy sediments were deposited during MIS5e (~125ka), mean vertical displacement rate is calculated to be 0.04-0.07 m/ky. This is smaller than that in the sea area confirmed by sonic surveys (0.12 m/ky, Okamura et al., 2013). In addition, this is slightly smaller than those rates, 0.03-0.14 m/ky, estimated from faults distributed south of the Takachaya Fault beloinging to the eastern part of the Nunobiki Mountains Eastern Margin Fault Zone (Mie Prefecture, 2000).
It is noted that no obvious displacement of the Alluvium was identified. Sediment supply to around the present coastal area occurred after ca. 5-6 ka (Kawase, 2003; Funabiki et al., 2010), which is newer than the last seismic event of the Tsu-oki flexure (6.2-6.8 ka). Therefore, it might be difficult to detect fault displacement of the flexure from alluvium sediment around the coastal area.
References
Funabiki, A. et al. (2010) Quaternary Research (Daiyonki-Kenkyu), 49, 201-218.
HERP (2005) Evaluation of the Nunobiki Mountains eastern margin fault zone. 27p.
Iwabuchi, H. et al. (2000) Hydrographic Department Research Report, 36, 73-96.
Kawase, K. (2003) Geographical Review of Japan, 76, 211-230.
Mie Prefecture (2000) Annual research reports on the Nunobiki Mountains Eastern Margin Fault Zone.
Okamura, Y. et al. (2013) Annual Report on Active Fault and Paleoearthquake Research, 13, 187-232.
Suzuki, Y. et al. (2010) GSI Technical Report D, 1-No. 540.
The GSJ has been carrying out the "Investigations of Geology and Active Faults in the Coastal Zone of Ise and Mikawa Bay". We have also been investigating the Quaternary subsurface geology and active structures in the area. Especially, we intensively collected borehole logs were at around the central part of the Ise Plain, which located between coastal plain of the Ano, Shitomo and Iwata Rivers and the Kumozu Lowland. Based on analysis of borehole logs, we investigated distribution of the Alluvial basement, top and bottom of the marine mud during the Last Interglacial period, and basement rocks and estimated stratigraphic deformation.
In coastal zone of the study area, there are a number of Pleistocene buried terraces beneath the alluvial lowland. These terraces are composed of muddy sediments with shell fossils (N values 5 to 20), and are covered by sand and gravel sediments with a thickness of 5 to 20 m. The stratigraphy and sedimental facies indicate that this muddy sediments are likely to be marine sediments. The muddy sediments around Shirako, Suzuka City characterized by abundant Lagerstroemia pollen fossil, and intepreted as marine sediments during the Last Interglacial period (MIS5e) (Sato et al., 2020). This muddy sediments is possibly corresponding to those of the study area. Around the north part of the study area, it is easy to trace the top of the muddy sediments with good lateral continuity at an elevation of -30 to -32 m. Alothough this tends to gradually deepen towards seaward, we recognized that this surface decreases sharply by about 4.6 to 8.8 m in a range of 300 to 500 m on the left side of the Kumozu River. In this area, the basement of the muddy layer also decreased sharply as same as the top, and which suggest that this displacement is likely to be caused by fault. In addition, the base of the Alluvium decreases by several meters on the southern of the Tsu-oki flexure. These suggest that the Tsu-oki flexure may extend to the land area of the Kumozu Lowland. The total length of the Tsu-oki flexure is estimated to be ~12 km, which is ~5 km longer than that indicated by Okamura et al. (2013).
Assuming that the muddy sediments were deposited during MIS5e (~125ka), mean vertical displacement rate is calculated to be 0.04-0.07 m/ky. This is smaller than that in the sea area confirmed by sonic surveys (0.12 m/ky, Okamura et al., 2013). In addition, this is slightly smaller than those rates, 0.03-0.14 m/ky, estimated from faults distributed south of the Takachaya Fault beloinging to the eastern part of the Nunobiki Mountains Eastern Margin Fault Zone (Mie Prefecture, 2000).
It is noted that no obvious displacement of the Alluvium was identified. Sediment supply to around the present coastal area occurred after ca. 5-6 ka (Kawase, 2003; Funabiki et al., 2010), which is newer than the last seismic event of the Tsu-oki flexure (6.2-6.8 ka). Therefore, it might be difficult to detect fault displacement of the flexure from alluvium sediment around the coastal area.
References
Funabiki, A. et al. (2010) Quaternary Research (Daiyonki-Kenkyu), 49, 201-218.
HERP (2005) Evaluation of the Nunobiki Mountains eastern margin fault zone. 27p.
Iwabuchi, H. et al. (2000) Hydrographic Department Research Report, 36, 73-96.
Kawase, K. (2003) Geographical Review of Japan, 76, 211-230.
Mie Prefecture (2000) Annual research reports on the Nunobiki Mountains Eastern Margin Fault Zone.
Okamura, Y. et al. (2013) Annual Report on Active Fault and Paleoearthquake Research, 13, 187-232.
Suzuki, Y. et al. (2010) GSI Technical Report D, 1-No. 540.