1964 Volume 17 Issue 4 Pages 201-209
From the observed data of seismic waves from Siunzi, Annaka and Kawazu explosions and a natural earthquake occurred near Miyake-jima listed in Part I of this paper, crustal structure was derived for the profile along the longitudinal line of 139°E. Although the available data obtained are not enough to draw precise picture on crustal structure, the following is proposed as a plausible one.
The superficial layer varies from 0 to 3.5km in thickness and from 1.61 to 2.83km/sec in P wave velocity. The first layer with P wave velocity of 6.00km/sec has 12km thickess under the Siunzi shot point and its lower boundary has downward inclination towards south with an angle of 1°29′. The second layer has P wave velocity of 6.82km/sec, which is observed in Japan for the first time. The boundary surface of this and the third layer, so-called Mohorovicic discontinuity, was estimated for various values of P wave velocity in the upper mantle. The depth of this surface is about 40km near Siunzi shot point and becomes deeper towards south, reaches its maximum of about 50km at around 150km away from the shot point, and then becomes shallower to about 25km near Miyake-jima.
