It is pointed out that the focal distribution in central and eastern Shikoku obtained on the assumption of a structure model of four horizontal layers shows some remarkable characteristics. For example, the uppermost boundary of focal distribution of shallow earthquakes inclines downwards to the south in the Chichibu and Shimanto Belts and is very shallow in the Sanbagawa B. Therefore, in order to investigate the crustal structure in relation to such distributions of earthqakes, seismic waves generated by quarry blasts on Mt. Torigatayama, Kochi Pref. were observed at 48 temporary stations along 4 profiles of the E, NE, N and S directions. Apparent velocities are considerably different for directions of the profiles. This difference is elucidated by inclination of the crustal layers. From the travel time curves obtained, the following crustal structure model was inferred.
P-wave velocities are estimated at 4.8, 5.5 and 6.1km/sec for three layers, respectively. Trend directions are calculated at E21-25°N for these layers. This direction coincides nearly with the tectonic axis of the Shikoku Island and the direction of the Mikabu Tectonic Line in central Shikoku. The first layer with the velocity of 4.8km/sec exists only in the southern region of the Mikabu T. L. and its thickness changes southwards from 0.6 to 3km. The lower interface of the second layer with the velocity of 5.5km/sec decends southwards with steep gradient and its thickness changes from 4 to 12km. This gradient of the upper interface of the 6.1km/sec layer is in harmony with the uppermost distribution of focal depths of shallow earthquakes. Small thickness of the 5.5km/sec layer in the Sanbagawa Metamorphic Belt compared with its surrounding regions may suggest the upheaval of the Belt by recent tectonic activities.
