ANOMALOUS CRUSTAL ACTIVITY IN THE IZU PENINSULA, CENTRAL HONSHU

Abstract

Earthquake prediction research on the recent anomalous crustal activity in the Izu Peninsula is summarized. Swarm activity of shallow microearthquakes in the eastern Izu Peninsula began in August 1975 and became more active in October. The epicenters clustered near Togasa-yama at first but spread over other places in the eastern part of and east off the peninsula in early 1976. Releveling carried out in January-April 1976 has disclosed crustal uplift of 15cm centered at Hiekawatoge, several kilometers north of Togasa-yama. The uplift area, more than 30km in diameter, covered the eastern half of the peninsula. Leveling, gravimetric, and tidal data showed that this uplift had developed only during the preceding 1-1.5 year period. Rapid changes in length of base-lines were also detected by repeated geodimeter measurements. These phenomena were noted by the Coordinating Committee for Earthquake Prediction and observations were intensified. The information about the crustal activity was made public by the Committee in May 1976. On August 18, the Kawazu earthquake (M=5.4) occurred in the southern part of the uplift area. Short-term precursors except for foreshocks were not so clear in spite of the intensified observations.