Electro-Optical Measurement of Horizontal Strains Accumulating in the Swarm Earthquake Area (3).

There have been repeated Geodimeter surveys in the Matsushiro and its adjacent areas since October, 1965. Their purpose is to study accumulation of horizontal strains at various stages of the seismic activity. The present paper will deal mainly with the strain events observed from September of this year, since the data for the earlier period were reported in the previous papers. Strains still actively accumulated in Matsushiro during the period September to October. The base-lines of Sorobeku, Nishiterao and Zozan changed their length for +26, +22 and -3cm, resulting in the total variation after the first survey as much as +116, +72 and -21 cm, respectively. In the remaining period (October-November), however, the authors observed no active accumulation of strains. It is notable that the base-lines of Sorobeku and Nishiterao recorded contraction in contrast with their former sense of deformation, extension. Similar kinds of information, suggesting the recent reversal of the strain activity mode, has been derived from levelings and tiltmeter observations, too. Strain accumulations are less active in Wakaho, Nakano, and Omachi, in that order. Discussion is developed briefly on several notable aspects of strain events in these areas