Abstract
The deep crustal structure is studied by using waves converted at the Mohorovicic and Conrad discontinuities. The data are earthquakes recorded by a short period seismometer with high sensitivity for frequencies ranging from 1 to 200Hz for a period of about three months from 1974 to 1975 at Tsukuba Seismological Observatory of the Earthquake Research Institute. The S-P times are about 5 to 10 seconds. The magnitudes are 1.2 to 3.4. The epicenters are located in southwestern Ibaraki, off the coast of northern Ibaraki, near the Tochigi-Ibaraki border and around Chiba city. The focal depths are about 45 to 79km.
Crustal thickness is estimated to be about 23 to 26km at four sites around Tsukuba. A Conrad discontinuity is also found at two sites in the same district. Its depth is about 9km and 20km, respectively. It is shown that the converted waves are a very useful tool for determining the deep crustal structure.
