[S-SS07] Crustal Structure

[SSS07-02] Deep seismic reflection profiling across the Tsugaru peninsula, northern Honshu, Japan

*Hiroshi Sato^1,2, Tatsuya Ishiyama¹, Hideo Saito³, Mamoru Nakata³, Naoko Kato¹, Susumu Abe³ (1.Earthquake Research Institute, The University of Tokyo, 2.Center for integrated Research and Education of Natural Hazards, Shizuoka University, 3.JGI, Inc.)

Keywords:Seismogenic source fault, deep seismic reflection profiling, Tsugaru Peninsula, 1766 Tsugaru Meiwa Earthquake

A large number of active reverse faults are distributed along the Sea of Japan coast of Norther Honshu. To reveal their deep geometry is significant for better estimates of strong ground motions and tsunami hazards. In 2020, a deep seismic reflection profiling was carried out across the southern part of the Tsugaru peninsula as a part of the “Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan” funded by MEXT. The length of the seismic line is 59 km from the coast of the Aomori bay to the coast of the Sea of Japan across the Tsugaru mountains and Tsugaru plain. Seismic source was four vibroseis trucks and seismic signals were recorded by 5 Hz geophones. As a standard mode, receiver interval was 50 m and shot interval was 100 m. In a higher-resolution mode, we used 25 m receiver interval and 50 m shot interval. Fixed 1887 channel were used for the seismic survey. To obtain deeper P-wave velocity structure, 50 to 100 stationary sweeps were carried out at every 4 km along the seismic line. Seismic data were processed using standard CMP-method and refraction tomography method. Geologic were interpreted using geological maps published by Geological survey, Japan.

The seismic sections revealed the geologic structure down to 6 km and portray an anticlinorium with steeper eastern limb beneath the Tsugaru mountains. P-wave velocity profile shows the thick low velocity layer, suggesting basin inversion processes. The reverse faults bounding the eastern limb of the range was generated as normal faults during the opening of the Sea of Japan. The 1766 Meiwa Tsugaru earthquake (M 7.0) damaged the southern part of the Tsugaru plain. The damaged area is located at the hanging wall of the faults bounding the eastern edge of the Tsugaru mountains. Thus, it is highly probable that this fault was the source fault of the earthquake of 1766.

Along the western margin of the Tsugaru plain, west dipping blind-reverse fault is clearly demonstrated on the seismic profile. On the hangingwall of this fault, stage 5e marine terraces are distributed, suggesting the late Quaternary movements along this fault. Based on the Bougue anomaly and distribution of marine terraces, this subsurface active fault extends 40 km trending NS. From the P-wave velocity structure and pattern of reflectors, this fault also shows the basin inversion.