10:45 AM - 12:15 PM
[SSS13-P10] Fault tips and paleostress of the Yunodake Fault in Iwaki City, Fukushima Prefecture
Keywords:active fault, Yunodake Fault, fault tip, composite planar fabric, stress inversion analysis, Fukushima-ken Hamadori earthquake
On April 11, 2011, the Fukushima-ken Hamadori earthquake (4.11 Hamadori earthquake) occurred with an epicenter in Iwaki City, Fukushima Prefecture. This earthquake moved the northern half of the western trace of the Itozawa Fault and its extension (Shionohira Fault) and the Yunodake Fault as normal faults dipping to the west. It attracted a great deal of attention because the faults moved as a normal fault type in the stress field of east-west compression prevailing in the forearc region of northeastern Japan. Since then, many studies about the relationship between the 4.11 Hamadori earthquake and the 2011 off the Pacific coast of Tohoku earthquake, and stress fields before and after the 4.11 Hamadori earthquake have been conducted (for example, Kato et al., 2011; Yoshida et al., 2012; Imanishi et al., 2012). On the other hand, the movement directions and stresses of the Yunodake Fault in the long time scale from the formation of the fault to the 4.11 Hamadori earthquake have not been elucidated. In order to elucidate the motion directions and stress states experienced by the Yunodake Fault in the past, we are proceeding with reconnaissance, outcrop investigation, and paleostress analysis around the fault.
The northwestern tip of the surface rupture of the Yunodake Earthquake Fault is located in Kanzawa, Tono-machi, Iwaki City (Toda and Tsutsumi, 2013). As a result of the geological reconnaissance, the northwestern extension of the Yunodake Fault extends to the Iritonodamkita outcrop located in a tributary of the Iritono River. On the west side of this outcrop, the Yunodake Fault splits into several faults, which eventually become small cracks and disappear. This fault group is mainly classified into three systems, E-W system, WNW-ESE system, and NE-SW system, and all fault planes dip at medium to high angles. As a result of estimating the sense of shear from the composite planar fabric developed in the fracture zone, the sinistral sense is dominant, but some of the NE-SW faults include a prominent dextral sense. These fault systems are geometrically similar to the tip features of a sinistral fault composed of horsetail splay faults and antithetic faults (Kim et al., 2004). In addition, a total of 14 fault slip data obtained from these faults were analyzed by the multiple inverse method (Yamaji, 2000), which is one of stress inversion analysis. As a result, two significant stress states, Stress A and Stress B, were detected. The Stress A consists of σ1 dipping to the SW at a medium angle, σ3 dipping to the NNW at a medium angle, and the stress ratio (Φ = σ2–σ3/σ1–σ3) is 0.4. The Stress B consists of σ1 dipping to the ENE at a low angle, σ3 dipping to the west at a high angle, and the stress ratio is 0.8. When Stresses A and B were applied to the fault plane with E-W striking and a high angle south dipping observed at the Iritonodamukita outcrop where the branching to the northwest begins, sinistral-reverse fault movement is estimated. From the above, it is suggested that the Yunodake Fault has a history of experiencing a sinistral-reverse fault displacement. In addition to the outcrops located in the non-active zone of the 4.11 Hamadori earthquake, including the Iritonodamukita outcrop, we will also report on the research status of the outcrops located in the active zone of the Nakanokita outcrop and the Kanzawa outcrop.
The northwestern tip of the surface rupture of the Yunodake Earthquake Fault is located in Kanzawa, Tono-machi, Iwaki City (Toda and Tsutsumi, 2013). As a result of the geological reconnaissance, the northwestern extension of the Yunodake Fault extends to the Iritonodamkita outcrop located in a tributary of the Iritono River. On the west side of this outcrop, the Yunodake Fault splits into several faults, which eventually become small cracks and disappear. This fault group is mainly classified into three systems, E-W system, WNW-ESE system, and NE-SW system, and all fault planes dip at medium to high angles. As a result of estimating the sense of shear from the composite planar fabric developed in the fracture zone, the sinistral sense is dominant, but some of the NE-SW faults include a prominent dextral sense. These fault systems are geometrically similar to the tip features of a sinistral fault composed of horsetail splay faults and antithetic faults (Kim et al., 2004). In addition, a total of 14 fault slip data obtained from these faults were analyzed by the multiple inverse method (Yamaji, 2000), which is one of stress inversion analysis. As a result, two significant stress states, Stress A and Stress B, were detected. The Stress A consists of σ1 dipping to the SW at a medium angle, σ3 dipping to the NNW at a medium angle, and the stress ratio (Φ = σ2–σ3/σ1–σ3) is 0.4. The Stress B consists of σ1 dipping to the ENE at a low angle, σ3 dipping to the west at a high angle, and the stress ratio is 0.8. When Stresses A and B were applied to the fault plane with E-W striking and a high angle south dipping observed at the Iritonodamukita outcrop where the branching to the northwest begins, sinistral-reverse fault movement is estimated. From the above, it is suggested that the Yunodake Fault has a history of experiencing a sinistral-reverse fault displacement. In addition to the outcrops located in the non-active zone of the 4.11 Hamadori earthquake, including the Iritonodamukita outcrop, we will also report on the research status of the outcrops located in the active zone of the Nakanokita outcrop and the Kanzawa outcrop.