10:45 AM - 12:15 PM
[SSS13-P14] Uplift rate and subsurface tectonic structure of the southern part of the western marginal fault of the Tsugaru Mountain
Keywords:Western marginal fault of the Tsugaru Mountain, Seismic reflection survey, Volcaniclastic deposit
The southern part of the western marginal fault of the Tsugaru Mountain (hereinafter referred to as Tsugaru Mountain Fault Zone; TMFZ) is a generally north-south trending, east-dipping reverse fault that forms the boundary of the Tsugaru Plain and the Tsugaru Mountains. TMFZ can be divided into northern and southern parts, and geomorphological and structural studies were conducted in the northern part (e.g., Miyauchi et al., 1999). However, no reliable slip rate has not been reported to the fault zone, because the deformation of the topography or a sediment layer as the proxy for estimating the displacement is unclear (Earthquake Research Committee, the Headquarters for Earthquake Research Promotion, 2004).
In the Takadate and Takehana in Kuroishi City and Hongo areas in Aomori City, along the southern part of TMFZ, the flexure scarp can be observed on the lower terrace surface. In this study, the following investigations were conducted to clarify the dip angle and displacement of the fault zone and to reveal the subsurface tectonic structure of the lower side of the fault.
P- and S-wave seismic reflection surveys were conducted to estimate the subsurface structure at depths shallower than 1.5 km and 250 m, respectively, along a seismic line that crosses the fault. One borehole was drilled to the depth of 80 m on the seismic reflection survey line at the downside of the fault to determine the subsurface structure of sediments. Two boreholes, two pits, and outcrop investigations were conducted on the uplift and downsides of the fault to confirm the distribution and deformation of the strata. Based on these results, we estimated the subsurface geometry of the fault and determined the vertical displacement of the topography and some layers.
The results indicate that the main fault in the southern part of TMFZ is an east-dipping reverse fault (east side up), which dips about 60 degrees to the east, and seems to form broad flexural deformation with a west-dipping back-thrust in the shallow part. Based on the vertical displacement and the eruption time of the Towada-Ofudo pyroclastic flow deposit (c. 36 ka; Kudo et al., 2019), the averaged vertical slip rate of the main thrust is estimated to be about 0.3-0.4 mm/yr (< 0.6 mm/yr).
In the Takadate and Takehana in Kuroishi City and Hongo areas in Aomori City, along the southern part of TMFZ, the flexure scarp can be observed on the lower terrace surface. In this study, the following investigations were conducted to clarify the dip angle and displacement of the fault zone and to reveal the subsurface tectonic structure of the lower side of the fault.
P- and S-wave seismic reflection surveys were conducted to estimate the subsurface structure at depths shallower than 1.5 km and 250 m, respectively, along a seismic line that crosses the fault. One borehole was drilled to the depth of 80 m on the seismic reflection survey line at the downside of the fault to determine the subsurface structure of sediments. Two boreholes, two pits, and outcrop investigations were conducted on the uplift and downsides of the fault to confirm the distribution and deformation of the strata. Based on these results, we estimated the subsurface geometry of the fault and determined the vertical displacement of the topography and some layers.
The results indicate that the main fault in the southern part of TMFZ is an east-dipping reverse fault (east side up), which dips about 60 degrees to the east, and seems to form broad flexural deformation with a west-dipping back-thrust in the shallow part. Based on the vertical displacement and the eruption time of the Towada-Ofudo pyroclastic flow deposit (c. 36 ka; Kudo et al., 2019), the averaged vertical slip rate of the main thrust is estimated to be about 0.3-0.4 mm/yr (< 0.6 mm/yr).