茨城県地域に見られるペア地震現象

抄録

The off-Ibaraki, central Japan earthquake (M=7.0) of July 23, 1982 was followed by the southern Ibaraki earthquake (M=6.0) of February 27, 1983 with a lag time of seven months. Although those two shocks are separated by 200km, a physical correlation between them is suggested since both the earthquakes are the largest events in the respective areas for the last forty years. Investigating past seismicity, it was found that similar phenomenon, which we name the “paired earthquakes”, repeatedly occurred in the Ibaraki region.
The trench side earthquake of the pair is an interplate earthquake associated with the subducting Pacific plate, while the land side one is interpreted as an earthquake representing mutual motion between the Pacific plate and the Philippine Sea plate at a depth of 60-70km beneath Ibaraki Prefecture. The paired earthquake phenomenon is accounted for by accelerated subduction of the Pacific plate which is expected to occur when one of the plate interfaces is ruptured by the preceding event of an earthquake pair.