弾性波速度測定に基づく跡津川断層周辺浅部地殻物質の推定

抄録

Shallow crustal materials in the Atotsugawa fault area were inferred based on measured elastic wave velocities of various rocks sampled in this area. The Atotsugawa fault is situated in the Niigata-Kobe Tectonic Zone, where large crustal deformation has been observed by a dense GPS network. Active microseismicity has been observed along the fault. The subsurface structure of this area has been geophysically investigated in search of structural factors in observed tectonic activities. Elastic wave velocities of crustal materials are required to infer constituent materials and physical conditions from geophysical data. Based on previous geological studies, Hida granitic rocks, Hida metamorphic rocks, a Tedori-group sandstone and a Nohi rhyolite were sampled as typical rocks in this area. Measurements of elastic wave velocity were conducted at room temperature under confining pressures up to 180MPa. Tedori-group sandstone and Nohi rhyolite show low compressional wave velocity of 5.6-5.7km/s at 180MPa. Compressional wave velocity is 6.2-6.3km/s in Hida granitic rocks, while 6.0-6.1km/s in metagranites. The higher velocity of Hida granitic rocks is attributed to the abundance of hornblende. Gneisses show a wide range of the compressional wave velocity of 6.0-6.7km/s, reflecting the variation in mineralogy. Samples rich in hornblende or calcite tend to show higher compressional wave velocity. On the other hand, a sample rich in biotite shows lower velocity. The sandstone has a low velocity ratio V_p/V_s around 1.6, while other rocks have V_p/V_s of 1.7-1.8. Considering the previously investigated temperature dependence of the compressional wave velocity, we infer the subsurface lithology around the Atotsugawa fault from the seismic velocity structure. The high velocity body beneath the Atotugawa fault is mostly composed of gneisses. Hida granites and/or metagranites could be minor components. At 30km south from the fault, the Nohi rhyolite and Hida granitic rocks are expected to coexist at the depth of 3km. The high velocity body at 6km is mostly composed of Hida granitic rocks. Considering the continuity of the high velocity body, gneisses could coexist with granitic rocks