Study on Microearthquake Sequences in the Eastern Chugoku and Northern Kinki Districts, Southwest Japan (II)

Abstract

Microearthquake activities are considered to be caused by fractures in the crust due to stress concentration and inhomogeneous nature of the medium. The fractured regions are separately distributed along the seismically active zones or active-fault systems. The activity of each fractured region may have its own properties; the life time or duration time of activity, the size and shape of fracture and their changes according to time, temporal variation of focal mechanism and other seismological characteristics.
In the previous papers [TSUKUDA & NAKAO (1976); TSUKUDA (1977)], following results were obtained: The size of each fractured region is found to be within several hundred meters in linear dimension; Adjacent fractured regions have often nearly common active periods showing the successive migration of activity from region to region.
This paper is a continuation of the above studies. Hypocenter locations and determinations of focal mechanism have been made by using the data obtained through the telemetering observation system of the Tottori Microearthquake Observatory during the period from April 24, 1976 to March 31, 1977. Such waves as reflected at the Conrad discontinuity are used for calibration of hypocenter location.
The Microearthquake sequence which originated in the vicinity of the eastern part of the estimated fault accompanying the Tottori earthquake of 1943 (M7.4) is one of noteworthy materials for our study. Its source region deduced from hypocenter location is found to have been expanded southeastward, nearly perpendicular to the strike of the fault of the old large earthquake, in the last stage of the activity. The focal mechanism shows a temporal variation. This may indicate a zigzag propagation of rupture.
In addition to the above example, the present paper deals with a lot of sequential earthquakes which originated in the following regions: in the vicinity of the Shikano fault, Tottori Prefecture; around the eastern end of the Yamasaki fault, Hyogo Prefecture; and around Wachi, Kyoto Prefecture, and Sasayama, Hyogo Prefecture.