P波初動の重ね合わせから推定される東北日本の起震歪力

抄録

An attempt is made to estimate regional earthquake generating stresses in the northeastern part of Japan. It is known from previous studies that mechanisms of earthquakes occurring in a small region are similar to one another, if the division of seismic region is appropriate. In virtue of this fact, we superpose all the data of initial motions of P-waves obtained from earthquakes that occurred in a subdivided region, and project them on the same focal sphere. The distributions of P-wave initial motions thus obtained are analyzed to determine the fault types, that is, normal faulting or reverse faulting.
The results obtained are summarized as follows:
1) Shallow earthquakes which occurred off the coast of Sanriku (west of 143.5°E and north of 38°N) are characterized by reverse faulting, while earthquakes in the ocean-side (east of 143.5°E and north of 38°N) are characterized by normal faulting.
2) Shallow earthquakes which occurred off the coast of Fukushima prefecture are characterized by reverse faulting, and no normal faulting can be found.
3) It is difficult to explain the initial motion pattern of P-waves from earthquakes beneath the trench (depth; 60-100km) by a normal-fault-mechanism.
4) Pressure axis is nearly parallel to the dip of the seismic plane (down dip compression) deeper than 110km in the north-eastern part of Japan.
5) Tension axis is approximately parallel to the dip of the seismic plane (down dip extension) for depths between 100km and about 60km.
6) No serious contradiction is found between the mechanism solution derived from the superposed data of many small earthquakes in a divided region and that from the data of a large earthquake in the same region.