Abstract
A seismic reflection survey with CMP method was conducted west of Mt. Tsukuba, northeast of the Kanto plain. Results of seismic reflection survey are (1) sedimental layers are extensively distributed down to 400m in depth and continuous reflectors are observed at 60m, 140m and 270m in depth, (2) sedimental layers are horizontal and their interval velocities range between 1.7km/s and 2.4km/s, (3) multiple reflections caused in sedimental layers are seen later 0.5s in two way time. From the results of this seismic reflection survey and previous works, the underground geology in the survey area is interpreted (1) weathered top of the granitic pre-Neogene basement is expected because reflectors probably corresponding to the basement top is not so distinct, (2) reflectors from the basement top are interpreted and traced based on calculated interval velocities, (3) vertical gaps in the interpreted basement top may be the east branch of the Karasuyama-Sugaonuma fault, (4) continuous reflectors in sedimental layeres correlate to gravel and sand beds of Pliocene and Pleistocene and (5) the basement has not been active since Pliocene because sedimental layeres are flat and horizontal and they are not deformed by faulting of the basement.
