Continuous observations of crustal deformation have been carried out at Rokko-Tsurukabuto observation station since 1970, in order to study the behavior of crustal deformation in and around the fracture zone of an active fault. The station is located at Tsurukabuto incline of the Rokko tunnel of San'yo Shinkansen in Kobe City, southwestern Japan. An observation tunnel of the station was excavated across Otsuki fault that is typical of active faults with a strike of NE-SW in the Rokko mountain area . The following results were deduced from the observations with extensometers during the period of 1977-1984. The secular strain rates observed were about one order of magnitude greater than those observed at other stations. The largest one was nearly 1 ×10
-5/ year . The direction of the maximum compressive axis of average strain rates outside the fracture zone, on the north of the main fault-gouge layer of the Otsuki fault, was WNW-ESE. This direction was almost coincident with that of horizontal stress obtained through in situstress measurements at the Rokko-Suwayama site. On the other hand, the direction of the maximum compressive axis in the fracture zone, on the south of the main faultgouge layer of the Otsuki fault, was nearly perpendicular to that outside the fracture zone. The absolute values of average strain rates inside and outside the fracture zone decreased 30-70% after the end of 1980. Seismic activity in the region around Osaka Bay became active almost simultaneously. It seems that the strain rate changes observed at the Rokko-Tsurukabuto station reflect the tectonic activity of the crustal block that is bounded by active fault systems such as Arima-Takatsuki Tectonic Line, Median Tectonic Line, active faults in the region from the Ikoma to the Kongo mountains and an active fault system from the Rokko mountains to Awaji Island.
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