5:15 PM - 6:30 PM
[SSS10-P06] Tectonic geomorphology and slip rates along the Omi segment of the Nagano Basin Western Margin Fault Zone
Keywords:Omi Fault, active fault, slip rate, Nagano Basin Western Margin Fault Zone
The Omi fault is active fault discovered by Tajikara et al. (2013), and is evaluated by the Headquarters for Earthquake Research Promotion (2015) as the Omi section that occupies the southernmost part of the Nagano Basin Western Margin Fault Zone (NBWMFZ). Although the distribution of the Omi Fault has been mapped by Tajikara et al. (2013), the details of the tectonic landforms, deformation style, slip rate are still unclear. For the purpose of clarifying these points, we conducted more detailed aerial photograph interpretation, observation of tectonic landforms and creation of topographic cross sections by field survey, and sampling and analysis of terrace deposits.
The distribution of fault traces obtained from the detailed topographic interpretation and field observations is roughly consistent with the previous report (Tajikara et al., 2013). The Omi fault consists of NNE-SSW trending right-lateral faults and NW-SE trending left-lateral fault in the north part, NE-SW trending thrust faults and anticline in the middle part, and N-S trending thrust faults and anticline in the south part, respectively.
Geomorphic surfaces of the study area are classified into six fluvial terraces (T1 to T6), alluvial, and higher low-relief surfaces. Alluvial surfaces are well developed along the Omi river and the Tojo river, and are outstandingly widespread along the upper part of the Omi river which is on the subsiding side of the Omi fault. Most of the fluvial terraces are terraced fans formed by small tributaries joining the Omi and Tojo rivers. Fluvial terraces are significantly developed on the right bank of the Omi river and the left bank of the Tojo river, where are on the uplifting side of the faults. In contrast, on the left bank of the Omi river and the right bank of the Tojo river, where are on the subsiding of the faults, fluvial terraces are rarely developed except for the lowest terraces (T6).
We drilled a pit on T5 terrace and a borehole on T3 terrace near Kanori in the middle part of the fault, collected samples of the covering layer of the terrace deposits, conducted analysis tephra in these deposits. Although these results are tentative because detailed tephra analysis has not been completed, AT tephra was detected on the T5 terrace, and tephra likely to be correlated to K-Tz, TKN2826, and Iz-Ny tephra on T3 terrace. Therefore, T5 and T3 terraces are estimated to have formed at about 30 ka and 180 ka, respectively.
In the middle part of the fault, near Kanori and Shimoibori, we measured vertical displacements of the flexure by field surveying and creating topographic cross-sections from the GSI topographic map (Web version). The vertical displacements are about 2.5 m on the T5 terrace and 8.5-8.3 m on the T3 terrace. From these data and the ages of the terraces above, vertical slip rates are estimated to be 0.08 mm/yr for T5, and 0.05 mm/yr for the T3. Because these slip rates are less than 1/10 of these of the main part of the NBWMFZ, activity of the Omi fault is considered to be correspondingly low. As the length of the fault is about 15 km, the unit displacement is estimated to be about 1.5 m. When compared with the displacement of T5 near Kanori, which is 2.5 m (or more if net slip is considered), we estimated two events in the last 30 kyrs, and the interval of these events is very long, about 10-30 kyrs.
Slip rates obtained in this study are not sufficiently reliable because of the remaining uncertainty in the ages of terrace surfaces. Future needed work includes more detailed age estimation of terrace surfaces, evaluation of the entire slip rate of fault zone considering the parallel fault traces and anticlinal deformation, and net slip rate based on subsurface fault geometry. In order to evaluate the probability of earthquake, trench survey also will be needed to reconstruct the activity history.
The distribution of fault traces obtained from the detailed topographic interpretation and field observations is roughly consistent with the previous report (Tajikara et al., 2013). The Omi fault consists of NNE-SSW trending right-lateral faults and NW-SE trending left-lateral fault in the north part, NE-SW trending thrust faults and anticline in the middle part, and N-S trending thrust faults and anticline in the south part, respectively.
Geomorphic surfaces of the study area are classified into six fluvial terraces (T1 to T6), alluvial, and higher low-relief surfaces. Alluvial surfaces are well developed along the Omi river and the Tojo river, and are outstandingly widespread along the upper part of the Omi river which is on the subsiding side of the Omi fault. Most of the fluvial terraces are terraced fans formed by small tributaries joining the Omi and Tojo rivers. Fluvial terraces are significantly developed on the right bank of the Omi river and the left bank of the Tojo river, where are on the uplifting side of the faults. In contrast, on the left bank of the Omi river and the right bank of the Tojo river, where are on the subsiding of the faults, fluvial terraces are rarely developed except for the lowest terraces (T6).
We drilled a pit on T5 terrace and a borehole on T3 terrace near Kanori in the middle part of the fault, collected samples of the covering layer of the terrace deposits, conducted analysis tephra in these deposits. Although these results are tentative because detailed tephra analysis has not been completed, AT tephra was detected on the T5 terrace, and tephra likely to be correlated to K-Tz, TKN2826, and Iz-Ny tephra on T3 terrace. Therefore, T5 and T3 terraces are estimated to have formed at about 30 ka and 180 ka, respectively.
In the middle part of the fault, near Kanori and Shimoibori, we measured vertical displacements of the flexure by field surveying and creating topographic cross-sections from the GSI topographic map (Web version). The vertical displacements are about 2.5 m on the T5 terrace and 8.5-8.3 m on the T3 terrace. From these data and the ages of the terraces above, vertical slip rates are estimated to be 0.08 mm/yr for T5, and 0.05 mm/yr for the T3. Because these slip rates are less than 1/10 of these of the main part of the NBWMFZ, activity of the Omi fault is considered to be correspondingly low. As the length of the fault is about 15 km, the unit displacement is estimated to be about 1.5 m. When compared with the displacement of T5 near Kanori, which is 2.5 m (or more if net slip is considered), we estimated two events in the last 30 kyrs, and the interval of these events is very long, about 10-30 kyrs.
Slip rates obtained in this study are not sufficiently reliable because of the remaining uncertainty in the ages of terrace surfaces. Future needed work includes more detailed age estimation of terrace surfaces, evaluation of the entire slip rate of fault zone considering the parallel fault traces and anticlinal deformation, and net slip rate based on subsurface fault geometry. In order to evaluate the probability of earthquake, trench survey also will be needed to reconstruct the activity history.