An analysis has been made of the rupture mechanism of two earthquakes which occurred in the vicinity of the oblique collision beneath the tip of the descending Philippine Sea plate and the Pacific plate beneath Chiba, Japan. The M=6.5 1956 Chiba earthquake occurred in a dense nest of seismicity at a depth of between 70 and 80km beneath Chiba city, and may represent deformation at the tip of the Philippine Sea plate. The mechanism of the M=5.5 1965 Chiba earthquake indicates that the stress field in the lower seismic plane of the Pacific plate beneath Chiba is undisturbed by the collision.
The M=6.1 1968 Saitama earthquake may be regarded as an interplate event occurring on the upper surface of and near the tip of the Philippine Sea plate. The high stress drop of several hundred bars of this earthquake and of the 1956 earthquake in the Chiba nest may reflect the penetration of the tip of the Philippine Sea plate into the Eurasian and Pacific plates, respectively.