2:05 PM - 2:30 PM
[SSS12-02] Recent activity of the Unzen Fault Group (southwestern parts) in Shimabara Bay, Kyushu, southwest Japan.
★Invited Papers
Keywords:Unzen Fault Group, offshore active-fault survey, offshore drilling survey, Late Pleistocene, Holocene, average slip rate
We conducted offshore active-fault survey in Shimabara Bay to reveal activity of the Unzen Fault Group since the Late Pleistocene. In advance of our study, AIST et al. (2010) reported results of offshore active-fault survey including MCS surveys and piston coring. Although they successfully revealed distribution and characteristics of the fault group in the bay, recent activity of the fault remained still indistinct. In response to this, we planned to obtain 35–40 m long sedimentary cores to determine stratigraphy and chronology of the Shimabara Bay and precisely evaluate recent activity of the fault.
As a part of the site surveys of offshore drilling, we analyzed existing MCS records which obtained by AIST et al. (2010) to generate S/N improved seismic sections. Based on the improved sections, we determined 2 drilling sites and obtained a 40-m-long sedimentary core at UTO1 (46.7 m BSL), and a 35-m-long sedimentary core at UTO2 (33.1 m BSL) on the downthrown and upthrown side of the fault, respectively.
Based on sedimentary analysis including CT scan, radiocarbon dating and tephra identification, we classified the sediment into unit A, B and C in descending order. Unit A is correlated with postglacial transgressive and high-stand sequence. The radiocarbon dates from the unit1 shows 1.42–16.57 cal kBP (UTO1) and 3.17–18.23 cal kBP (UTO2). Unit B is correlated with low-stand sequence. At UTO1, Unit B is subdivided into subunit B1 and B2 in descending order. Subunit B1 would be formed during the Last Glacial Maximum. Subunit B2 includes radiocarbon date of 22.49 cal kBP and AT tephra (29–30 ka). At UTO2, unit B includes blocks of pumice correlated with Aso-4 (85–90 ka), however, those blocks are interpreted as reworking. Unit C is correlated with sediment of the Last interglacial. This unit is obtained at UTO2 only, and includes radiocarbon date of 44.64 cal kBP and a tephra layer correlated with Aso-4/3 group.
We composed geological section from the improved seismic section of existing MCS and the stratigraphy based on sedimentary core analysis. We can recognize that unit A and B are displaced by the faults. Top surface of unit B is apparently displaced vertically by 45 m. This offset should be mostly result of the fault activity, because we don’t find any evidence related with of pre-existing fault scarps such as lateral erosion by fluvial systems. Assuming the age of the horizon (top surface of unit B) as 18 ka, averaged vertical slip rate of the fault is figured out as 2.5 m/ky. Our result is concordant with the long-term slip rate (2.4 m/ky since 500 ka) report by AIST et al. (2010) .
Acknowledgement
This study is a part of MEXT “Research Project for advanced and efficient Long-term Evaluation of Active Faults”.
Reference
AIST et al. (2010) : Northern part and Southeastern part of Unzen fault Group (offshore). Survey report of “Supplementary Research Project of Active Faults in Coastal Area for Long-term Evaluation”.
As a part of the site surveys of offshore drilling, we analyzed existing MCS records which obtained by AIST et al. (2010) to generate S/N improved seismic sections. Based on the improved sections, we determined 2 drilling sites and obtained a 40-m-long sedimentary core at UTO1 (46.7 m BSL), and a 35-m-long sedimentary core at UTO2 (33.1 m BSL) on the downthrown and upthrown side of the fault, respectively.
Based on sedimentary analysis including CT scan, radiocarbon dating and tephra identification, we classified the sediment into unit A, B and C in descending order. Unit A is correlated with postglacial transgressive and high-stand sequence. The radiocarbon dates from the unit1 shows 1.42–16.57 cal kBP (UTO1) and 3.17–18.23 cal kBP (UTO2). Unit B is correlated with low-stand sequence. At UTO1, Unit B is subdivided into subunit B1 and B2 in descending order. Subunit B1 would be formed during the Last Glacial Maximum. Subunit B2 includes radiocarbon date of 22.49 cal kBP and AT tephra (29–30 ka). At UTO2, unit B includes blocks of pumice correlated with Aso-4 (85–90 ka), however, those blocks are interpreted as reworking. Unit C is correlated with sediment of the Last interglacial. This unit is obtained at UTO2 only, and includes radiocarbon date of 44.64 cal kBP and a tephra layer correlated with Aso-4/3 group.
We composed geological section from the improved seismic section of existing MCS and the stratigraphy based on sedimentary core analysis. We can recognize that unit A and B are displaced by the faults. Top surface of unit B is apparently displaced vertically by 45 m. This offset should be mostly result of the fault activity, because we don’t find any evidence related with of pre-existing fault scarps such as lateral erosion by fluvial systems. Assuming the age of the horizon (top surface of unit B) as 18 ka, averaged vertical slip rate of the fault is figured out as 2.5 m/ky. Our result is concordant with the long-term slip rate (2.4 m/ky since 500 ka) report by AIST et al. (2010) .
Acknowledgement
This study is a part of MEXT “Research Project for advanced and efficient Long-term Evaluation of Active Faults”.
Reference
AIST et al. (2010) : Northern part and Southeastern part of Unzen fault Group (offshore). Survey report of “Supplementary Research Project of Active Faults in Coastal Area for Long-term Evaluation”.