余震群の走時を使つて上部地殻の速度勾配とコンラッド面を求める方法

抄録

A new method is presented to study a fine crustal structure, and has been successfully applied in the eastern Chugoku and western Kinki districts. Using aftershocks of the 1983 Tottori-Ken Chubu earthquake (M6.2) whose epicenters are situated at almost the same place with various focal depths, the velocity gradient in the upper crust (granitic layer) and the position of the Conrad discontinuity have been determined. In this method, sufficient information is given by travel times observed at only one station. Using the relation between travel times of direct P waves and focal depths, the velocity gradient has been determined to be 0.00±0.01sec^-1 in the depth range 4-13km, thus the P-wave velocity in the upper crust is almost constant in the concerned districts.
The waves reflected from the Conrad discontinuity (P_XP) are easily found by this method, because the travel-time difference between direct P and P_XP waves is more sensitive to a focal depth than to an epicentral distance. The Conrad discontinuity in the central Tottori prefecture is about 16km in depth and this interface is inclined toward southeast by east with a dip angle of -4 degrees. Since almost all recent earthquakes in the Tottori district are shallower than 14km in depth, we conclude that shallow earthquakes in this district have occurred in the upper crust above the Conrad discontinuity.