千葉県東方沖地震の発生機構について  [in Japanese] On the mechanism of the East-Off-Chiba earthquake  [in Japanese]

A severe earthquake of magnitude 6.7 occurred about 47 km in dep thand about 20 km east of Mobara City, Chiba Prefecture in December 17, 1987, and the after shock area extended northward about 20 km in length near to Naruto. The Kazusa Group is distributed more than 1 km in thickness in the epicentral area. It strikes NNE-SSW in general, and dips gently toward west, so it is correlated to the lowest formation of the group. The isopachous map of each formation shows the general trend of NNE-SSW and westward polarity of sedimentation. The fault plane deduced from the P-wave radiation pattern are N-S in general, so the general trend of geologic structure and isopachous maps must be controlled by the deep structure of the crust and upper mantle. Small deflection from the general trend of geology might be due to the e'chelon structure. The after-shock area and fault plane reflect the deep structure. The recent crustal movement of the BOSO peninsula shows the upheaval movement, so the compressional state of stress is expected under the surrounding area of the peninsula. The maximum compression axis which is nearly horizontal and NE-SW indirection might be due to such stress state near the hypocenter.