Induced earthquakes in the Izu peninsula by the Izu-Hanto-Oki earthquake of 1974, Japan

Recently small earthquakes in the Izu Peninsula, central Japan, occurred in a region where differential strain, or shear strain on the nodal planes, may have been enhanced by the Izu-Hanto-oki earthquake of 1974 (M = 6.9 after JMA). It is suggested that the seismic ctivity was induced by the redistribution of strain accompanying the Izu-Hanto-oki earthquake. The activity from August, 1975, may have also been affected by an abnormal uplift in the northeastern part of the peninsula. Based on plausible models, the uplift caused the accumulation of differential strain in the focal region of the subsequent earthquakes. Quantitatively, this change of crustal strain was of the order of 10⁻⁶; it is ten times as much as the average annual accumulation. Consequently, the sudden or rapid change of strain was likely to have played an essential role in the subsequent seismic activity. This effect could be one of the factors which trigger a shallow intra-plate earthquake.