Study on lower crustal structure of Northeastren Japan Arc based on waveforms by quarry explosions.

1998 Fiscal Year Final Research Report Summary

• Project/Area Number: 09640503
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: 固体地球物理学
• Research Institution: Yamagata University
• Principal Investigator: HASEMI Akiko Yamagata Univ., Faculty of Science, Professor, 理学部, 教授 (10134149)
• Co-Investigator(Kenkyū-buntansha): HORIUCHI Shigeki National Research Inst.for Earth Science and Disaster Prevention, Solid Earth Sc, 防災科学技術研究所・地圏地球科学科術研究部, 部長
• Project Period (FY): 1997 – 1998
• Keywords: lower crust / artifitial source / Tohoku District / reflectors

Research Abstract

We collected waveforms which were generated by explosions at quarries and recorded by earthquake observation stations of Tohoku University for the period amount of 4 years. After NMO correction, waves with the common mid point (middle point between a station and a quarry) were gathered. By displaying these 'CMP gather', we checked whether there were reflectors beneath the mid point. Reflectors were found around the Moho depth near the coast of the Pacific Ocean, at depths of 20-30km in the region east of the volcanic front and in the mid crust west of the volcanic front.
In Oct., 1997, seismic explosion experiments was carried out along the E-W line connecting Kamaishi and Iwaki in the Tohoku District, We set a N-S observation line of about 20km long in the northern part of Yamagata Prefecture and recorded explosions detonated on the E-W line. 33 stations were put on this N-S line. Clear later phases which were thought to be reflected in the middle part of crust (at depth of about 15km ) were found from shots at Hanamaki (93-103km epicentral distance to the N-S line ) , Sawauchi (80-93km) and Iwaki (62-80km). Amplitudes of later phases from Iwaki were very large and more than 5 times of the first motion. Waves from Omagari (54-71km), however, revealed no distinct later phases. Records obtained by Tohoku University stations near the N-S line showed the similar characteristics. By analyzing amplitudes of first motion, it was found that amplitude decayed when waves passed through the shallow part of the crust along the Japan Sea coast in the south Akita Prefecture. Decay of the first motion may make later phases from Iwaki very prominent.