10:45 AM - 12:15 PM
[SSS13-P02] Evaluation of active structures crossing sea coast by using of the last interglacial marine mud as a deformation reference -a case study around the Ise Bay, central Honshu –
Keywords:reference, active fault, last Interglacial era, marine mud , Ise Bay, Atsuta Formation
1. Introduction
Geological Survey of Japan, AIST have carried out a fundamental research program on evaluation of activity of active structures crossing sea and land boundaries since 2008. We got some knowledge on these subjects around the Ise Bay area.
2. Features of the last interglacial deposits around the Ise Bay
Tectonically subsiding inner-bay (Ise Bay), uplifting terraces and large sediments supplying rivers (such as the Kiso three rivers) develop around the Ise Bay. The river mouth terraces and deposits (the Atsuta Formation and its same time formations) formed in the last Interglacial era are widely distributed in conjunction with such circumstances. They consist of marine mud layer (lower part of the Atsuta Formation) and deltaic sand layer (upper part of the Atsuta Formation). The upper part of the Atsuta Formation conformably overlies on the lower one, and the boundary horizon of these layers (the upper surface of lower part of the Atsuta Formation) can be widely traced from center of the bay to terrace area. The On-Pm-1 tephra (ca. 100ka: MIS 5c; Machida and Arai, 2003) is contained in the basal part of the upper part of the sandy Atsuta Formation from underground of the Nobi Plain to off the coast of Tsu City, and it helps to determine the age of sediments in boreholes.
3. Evaluation of active structures by using of the upper surface of the lower part of the Atsuta Formation as a reference
The upper surface of the lower part of the Atsuta Formation is thought to be a boundary surface between the pro-deltaic marine mud layer which deposited in the bottom of the inner-bay and delta-front sandy sediments which deposited on the bay without time interval with pro-deltaic marine mud. So, this surface is thought to be a primarily sequential and almost flat surface. The maximum water depth of the Ise Bay is about 40 m at the present (water depth of delta-front is generally 5 to 25 m), and the On-Pm-1 yields just above that surface at least from the Nobi Plain to off the coast of Tsu City. So, it would have difference of 20 m (40 m in maximum) in elevation and maximum difference of ca. 25,000 years in this surface at the beginning of formation. Many geotechnical drilling data have gathered in this area, it can be discuss on the continuity and long-term average deformation rate of active structures by using the sonic prospecting data and drilling data.
The elevation distribution of this surface shows followings. 1) The western end of the submarine Shirako-Noma fault is located near the western coast of the Ise Bay (It does not extend to terrestrial area.) 2) The submarine Yokkaichikou (Yokkaichi port) fault continues to the terrestrial Aguragawa Anticline, of which western end reaches at the Kuwana Fault. 3) Vertical displacement of this surface of the Isewan (Ise Bay) fault is 70 m, and its mean displacement rate is 0.5 to 0.7 m / 1000 yrs. This rate is much larger than previous studies.
4. Application to other areas and future subjects
Sea level during the last Interglacial maximum is estimated to be ca. +5 m (Machida et al., 2003). The upper surfaces of the last Interglacial marine mud under the river mouth terraces of middle terrace are discovered in many areas in Japanese archipelago. So, it will be possible to trace continuously the upper surfaces of the last Interglacial marine mud from land area to inner-bay to where large sediment supplying rivers inflow, and to evaluate continuity and activity of active faults. Paleo water depth indicators such as ostracods and mollusks will help to estimate paleo depth at sedimentation, they will make more definitely clear the amount of vertical deformation. We hope to make clear the continuity and activity of active structures of coastal areas by determine the age of the upper surfaces of the last glacial marine mud by using of the (crypto-) tephra and photoluminescence dating.
Reference
Machida, H. and Arai, F., (2003) Atlas of tephra in and around Japan. Tokyo University Press.
Machida, H. et al. eds. (2003) Daiyonkigaku (Quaternary Research). Asakura shoten.
Geological Survey of Japan, AIST have carried out a fundamental research program on evaluation of activity of active structures crossing sea and land boundaries since 2008. We got some knowledge on these subjects around the Ise Bay area.
2. Features of the last interglacial deposits around the Ise Bay
Tectonically subsiding inner-bay (Ise Bay), uplifting terraces and large sediments supplying rivers (such as the Kiso three rivers) develop around the Ise Bay. The river mouth terraces and deposits (the Atsuta Formation and its same time formations) formed in the last Interglacial era are widely distributed in conjunction with such circumstances. They consist of marine mud layer (lower part of the Atsuta Formation) and deltaic sand layer (upper part of the Atsuta Formation). The upper part of the Atsuta Formation conformably overlies on the lower one, and the boundary horizon of these layers (the upper surface of lower part of the Atsuta Formation) can be widely traced from center of the bay to terrace area. The On-Pm-1 tephra (ca. 100ka: MIS 5c; Machida and Arai, 2003) is contained in the basal part of the upper part of the sandy Atsuta Formation from underground of the Nobi Plain to off the coast of Tsu City, and it helps to determine the age of sediments in boreholes.
3. Evaluation of active structures by using of the upper surface of the lower part of the Atsuta Formation as a reference
The upper surface of the lower part of the Atsuta Formation is thought to be a boundary surface between the pro-deltaic marine mud layer which deposited in the bottom of the inner-bay and delta-front sandy sediments which deposited on the bay without time interval with pro-deltaic marine mud. So, this surface is thought to be a primarily sequential and almost flat surface. The maximum water depth of the Ise Bay is about 40 m at the present (water depth of delta-front is generally 5 to 25 m), and the On-Pm-1 yields just above that surface at least from the Nobi Plain to off the coast of Tsu City. So, it would have difference of 20 m (40 m in maximum) in elevation and maximum difference of ca. 25,000 years in this surface at the beginning of formation. Many geotechnical drilling data have gathered in this area, it can be discuss on the continuity and long-term average deformation rate of active structures by using the sonic prospecting data and drilling data.
The elevation distribution of this surface shows followings. 1) The western end of the submarine Shirako-Noma fault is located near the western coast of the Ise Bay (It does not extend to terrestrial area.) 2) The submarine Yokkaichikou (Yokkaichi port) fault continues to the terrestrial Aguragawa Anticline, of which western end reaches at the Kuwana Fault. 3) Vertical displacement of this surface of the Isewan (Ise Bay) fault is 70 m, and its mean displacement rate is 0.5 to 0.7 m / 1000 yrs. This rate is much larger than previous studies.
4. Application to other areas and future subjects
Sea level during the last Interglacial maximum is estimated to be ca. +5 m (Machida et al., 2003). The upper surfaces of the last Interglacial marine mud under the river mouth terraces of middle terrace are discovered in many areas in Japanese archipelago. So, it will be possible to trace continuously the upper surfaces of the last Interglacial marine mud from land area to inner-bay to where large sediment supplying rivers inflow, and to evaluate continuity and activity of active faults. Paleo water depth indicators such as ostracods and mollusks will help to estimate paleo depth at sedimentation, they will make more definitely clear the amount of vertical deformation. We hope to make clear the continuity and activity of active structures of coastal areas by determine the age of the upper surfaces of the last glacial marine mud by using of the (crypto-) tephra and photoluminescence dating.
Reference
Machida, H. and Arai, F., (2003) Atlas of tephra in and around Japan. Tokyo University Press.
Machida, H. et al. eds. (2003) Daiyonkigaku (Quaternary Research). Asakura shoten.