深発地震域内の地震波速度

抄録

By the application of the refraction law to the readings of P and S times at Morioka (MOR), Miyako (MIY) and Ofunato (OFU), in northern Honshu, for deep earthquakes that occurred in Hokkaido and the Sea of Okhotsk, the seismic wave velocities in the "deep earthquake zone" (high-V, high-Q zone) are estimated.
The seismic waves of relatively shallow earthquakes occurring in this zone travel from their origins to the above stations over the deep earthquake zone alone. However, waves from origins deeper than a certain depth refract on their way in the "low-V, low-Q region" that exists just beneath the deep earthquake zone (Fig.7). This low-V, low-Q region is also characterized by high-a, as was suggested by Utsu (1969) from the analysis of the values of Vp/Vs. The observed values of Vp/Vs may suggest that the waves passed through the low-V, low-Q region or not.
For the present data, the values of Vp/Vs for earthquakes deeper than about 300 km are somewhat larger than those for earthquakes shallower than that depth (Fig.8). This suggests that the waves from earthquakes deeper than about 300 km may have travelled on the way in the low-V, low-Q region beneath the deep earthquake zone. Then, only those data that are shallower than about 300 km are available for estimating the velocity.
In order to strip off the upp e rmost layer of 50 km thickness, the travel times of P and 5, and values of dT/dA for an origin of 50 km depth are obtained from earthquakes of about 50 km depth which occurred off the east coast of northeastern Honshu (Fig.9). On reference to the observed values of dT/di, the data on the surface of the earth are converted to those at the surface of a depth of 50 km.
In the calculation of the velocity, a layered structure is assumed and the velocity in a layer of depths from 50 km to 100 km, v1, is given properly. In the first place, the average velocities for layers from 100 km to the depths of the earthquakes are estimated from the converted A and T for respective shocks of depths of 114 km and 132 km. Next, by applying Bullen's relation, v=A exp (-kh), to the value of v1 and estimated velocities, v, the value of k for the layer from 50 km to 130 km is estimated, and the velocity at a depth of 115 km is adopted as the average one in the layer from 100 km to 130 km.
By the same procedure, average velocities for layers of depths from 130 km to 160 km and from 160 km to 220 km are estimated from earthquakes occurring in these layers. Finally, Bullen's relation is applied once again to all values of v.
Velocities with depth in the deep earthquake zone, larger by about 2-6% than those by Jeffreys-Bullen or Honda, and Sagisaka and Takehana, were obtained and shown in Fig.12, where the values given of v1 are 8.05 km/sec and 4.63 km/sec for P and S waves, respectively.