庄内平野東縁活断層系松山断層における反射法地震探査

抄録

The active fault system along the eastern margin of the Shonai plain is a reverse active fault system that extends NS for 40-km long along the western flank of the Dewa hills, northwestern Yamagata prefecture. The subsurface geometry of this fault system is poorly understood particularly in the southern part. To reveal the subsurface geometry of this fault system, shallow high-resolution seismic reflection profiling was performed across the Matsuyama fault. The common mid-point reflection data were acquired using a mini-vibrator truck and 180- channel digital telemetry recording system along a 6-km-long seismic line. The obtained seismic section demonstrates that the Matsuyama fault is a 35° dipping reverse fault, underlain by a subparallel blind thrust in its footwall. Eastward thinning of the Quaternary Shonai Group above the upper tipline of the blind thrust indicates that they were deposited concurrent with fold growth and fault activity. Based on the known rate of vertical displacement of the southern part of Matsuyama fault, the slip rate of the Matsuyama fault during Holocene is calculated as 1.1 mm/yr.