Seismic Refraction Study in the Kitakami Region, Northern Hounshu, Japan.

An extensive seismic refraction experiment with the use of explosive sources was conducted in the Kitakami region, northern Honshu, Japan, on November 1, 1990. The experiment area is divided into two geological units by the Hayachine Tectonic Belt (HTB). The southern terrane consists of pre-Silurian basements and Silurian-lower Cretaceous marine sediments, while the northern one is characterized by a Jurassic accretionary complex. Both of the units were intruded by Cretaceous granitic rocks.An almost N-S seismic refraction profile of 194-km length was extended from Kuji City, Iwate Prefecture to Ishinomaki City, Miyagi Prefecture, on which 4 shots with a charge size of 450-700 kg were fired. The generated seismic signals were recorded at 179 stations, from which a precise crustal structure was determined. The uppermost crust is covered with a very thin (0.5-1 km) surface layer with a velocity of 3.1-5.4 km/s. Our results show a remarkable structural difference between the northern and southern Kitakami terranes. A velocity of the uppermost crust is 5.90 km/s in the northern part of the profile while 6.05-6.15 km/s in the southern part. This lateral velocity change occurs just beneath the HTB. A mid-crustal interface corresponding to well-recorded wide-angle reflections (PiP phase) shows an abrupt southward depth decrease from 25 to 20 km beneath the HTB. The reflections from the Moho boundary (PmP phase) were well observed in all of the record sections. The Moho depth determined from the PmP phase also decreases southward from 34 to 32 km. The velocity of the lower crust is 6.8-7.0 km/s. The refracted waves from the uppermost mantle (Pn phase) were observed with weak amplitudes at offsets greater than 160 km. These data suggest that the Pn velocity in the experiment area is less than 7.7 km/s.The VP/VS ratio within the crust was determined from S-waves recognized in the vertical component seismograms. The VP/VS ratio in the upper crust also shows a significant lateral change at the HTB, namely 1.72-1.73 in the northern part whereas 1.75-1.76 in the southern part. In the lower crust, the VP/VS ratio is 1.73-1.76. In the present experiment, weak refracted waves from the upper mantle (Sn phase) were observed at offsets greater than 170 km, from which the Sn velocity was estimated to be 4.3 km/s.