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We investigate a temporal change in a stress parameter of earthquakes that occurred in the 1989 swarms off the eastern Izu Peninsula in Japan. We use the energy index (EI), which have been monitored in deep South African gold mines for predicting major events, as an estimate of apparent stress that is proportional to the ratio of seismic energy (E) to seismic moment (Mo). EI measures an excess or shortage in E with respect to the empirical relationship between E and Mo. We check that EI is almost proportional to the apparent stress for the ranges of Mo and frequency for our analysis, although E is underestimated due to the artifact of limited frequency band of monitoring. The largest events (M = 5.2 and 5.5) took place off the tip of a vertical crack, which opened associated with magma intrusion. While the opening continued to load the source areas of the M 5.2 and 5.5, we find significant decrease in EI prior to the events. Based on the experimental result that the stress decreases when yielding takes place prior to final failure, we interpret this observation as yielding in the vicinity of the large earthquake hypocenters, following a rapid increase in stress caused by magma intrusion.

Stress change prior to the major events in the 1989 earthquake swarm off the eastern Izu Peninsula, Japan