9:15 AM - 9:30 AM
[SSS10-02] Simultaneous rupturing of the Tanna fault and its branch faults in the Kita-Izu fault zone
Keywords:Kita-Izu fault zone, Tanna fault, branch fault, coulomb failure function (delta CFF), 2016 Kumamoto earthquake
The Kita-Izu fault zone, a 32 km-long NS striking sinistral strike-slip fault zone, is located in the northeastern part of the Izu Peninsula. A branch fault group showing NW-SE strike is distributed in the east side of the Tanna and Ukihashi-central faults. At the 1930 Kita-Izu earthquake, which is the latest event of the fault zone, surface ruptures of some branch faults were reported (Matsuda, 1972). This suggests the triggered slip on the branch faults by the Tanna fault activity. Consideration of a simultaneous rupturing of active faults is important for the mitigation of surface rupturing around active faults, the assessment of minor active faults around a high slip-rate active fault, and the development of fault topography. Thus, we clarified the detailed distribution and geometry of active faults in the northeastern part of the Izu Peninsula, based on the geomorphic interpretation using the high-resolution LiDAR DEMs. Then, we conducted drilling survey and tephra analysis and estimated the slip-rates of the branch faults. In addition, we calculated the coulomb failure function (delta CFF) that the Tanna fault gives to the branch fault. Based on the detailed distribution and geometry of active faults, slip-rates of the branch faults and the delta CFF, we discuss the simultaneous rupturing of the Tanna fault and its branch faults.
In this study, we divided branch faults into 3 types based on the geomorphic interpretation. The type 1 is a dextral strike-slip fault (mainly NW striking). The type 2 is a NW striking normal fault forming a graben structure, located in the southeast of the Lake Ashinoko. The type 3 is a mainly NW striking branch fault, showing vertical and little horizontal displacements. From drilling survey and tephra analysis, we identified 5 tephras in the cores, that is, Fuji-Hoei scoria tephra (AD1707 ; Machida and Arai, 2003), Kozushima-Tenjosan tephra (AD838 ; Machida and Arai, 2003), Fuji-Zunasawa scoria tephra (ca.3.0 ka ; Geological Survey of Japan, AIST, 2021), Amagi-Kawagodaira tephra (ca.3.2 ka ; Tani et al., 2013), and Kikai-Akahoya tephra (ca.7.2 ka ; Smith et al., 2013). Using the ages of tephras and accumulated vertical displacement along the branch faults, we estimated the vertical slip-rate of the YG fault (type 2) and the NM fault (type 3) to be 0.1-1.0 mm/yr.
Furthermore, we calculated the coulomb failure function (delta CFF) that the Tanna fault gives to the branch fault, and discussed whether the movement of the Tanna fault promotes or suppresses the activity of the branch fault. The value of delta CFF to the type 1 branch faults is positive, and it is highly possible that the Tanna fault activates the type 1. The value of delta CFF to the type 2 is negative, and it is unlikely that Tanna fault activates the type 2. However, there are similarities between normal faults in the Kuradake area, which ruptured at the 2016 Kumamoto earthquake and that in the Izu Tanna area. Therefore, as in the case of the 2016 Kumamoto earthquake (as described in Fujiwara et al., 2020), it is likely that the type 2 is activated by the dynamic stress change caused by the seismic shaking of the Tanna fault. The value of delta CFF to the type 3 (NM fault : locating in the area sandwiched between two type 1s) is negative, and it is unlikely that the Tanna fault activates the type 3. However, if the Tanna fault and type 1 faults, which are dextral faults, move simultaneously, there is a possibility of the co-movement of the rupturing of the NM fault and them. When the value of dextral displacement of the type 1 faults are 2-3 times of the value obtained from the empirical relationship of Wells and Coppersmith(1994), delta CFF to the type 3 is positive, in that case, it is likely that the Tanna fault activates the type 3.
In conclusion, it is possible that the Tanna fault movement activates branch faults in the Kita-Izu fault zone by various factors, such as delta CFF and dynamic stress change. Moreover, comparing between the Izu and Kumamoto areas, there are similarities between active fault distributions and landform position. This may indicate the relationship between active fault development and geomorphology, geology and stress field.
In this study, we divided branch faults into 3 types based on the geomorphic interpretation. The type 1 is a dextral strike-slip fault (mainly NW striking). The type 2 is a NW striking normal fault forming a graben structure, located in the southeast of the Lake Ashinoko. The type 3 is a mainly NW striking branch fault, showing vertical and little horizontal displacements. From drilling survey and tephra analysis, we identified 5 tephras in the cores, that is, Fuji-Hoei scoria tephra (AD1707 ; Machida and Arai, 2003), Kozushima-Tenjosan tephra (AD838 ; Machida and Arai, 2003), Fuji-Zunasawa scoria tephra (ca.3.0 ka ; Geological Survey of Japan, AIST, 2021), Amagi-Kawagodaira tephra (ca.3.2 ka ; Tani et al., 2013), and Kikai-Akahoya tephra (ca.7.2 ka ; Smith et al., 2013). Using the ages of tephras and accumulated vertical displacement along the branch faults, we estimated the vertical slip-rate of the YG fault (type 2) and the NM fault (type 3) to be 0.1-1.0 mm/yr.
Furthermore, we calculated the coulomb failure function (delta CFF) that the Tanna fault gives to the branch fault, and discussed whether the movement of the Tanna fault promotes or suppresses the activity of the branch fault. The value of delta CFF to the type 1 branch faults is positive, and it is highly possible that the Tanna fault activates the type 1. The value of delta CFF to the type 2 is negative, and it is unlikely that Tanna fault activates the type 2. However, there are similarities between normal faults in the Kuradake area, which ruptured at the 2016 Kumamoto earthquake and that in the Izu Tanna area. Therefore, as in the case of the 2016 Kumamoto earthquake (as described in Fujiwara et al., 2020), it is likely that the type 2 is activated by the dynamic stress change caused by the seismic shaking of the Tanna fault. The value of delta CFF to the type 3 (NM fault : locating in the area sandwiched between two type 1s) is negative, and it is unlikely that the Tanna fault activates the type 3. However, if the Tanna fault and type 1 faults, which are dextral faults, move simultaneously, there is a possibility of the co-movement of the rupturing of the NM fault and them. When the value of dextral displacement of the type 1 faults are 2-3 times of the value obtained from the empirical relationship of Wells and Coppersmith(1994), delta CFF to the type 3 is positive, in that case, it is likely that the Tanna fault activates the type 3.
In conclusion, it is possible that the Tanna fault movement activates branch faults in the Kita-Izu fault zone by various factors, such as delta CFF and dynamic stress change. Moreover, comparing between the Izu and Kumamoto areas, there are similarities between active fault distributions and landform position. This may indicate the relationship between active fault development and geomorphology, geology and stress field.