Abstract
Space-time distribution in recent seismic activities around Shizuoka City are investigated using data of hypocenters recalculated by the new method which has been adopted by the JMA since 1983. A difference in hypocentral distribution is found between the north-eastern side and the southwestern side of a tectonic line connecting the southern tip of the Izu Peninsula and Shizuoka City. In the northeastern side shocks deeper than 30 km are seldom, as well as shocks whose depth of hypocenter is shallower than 10 km. The difference of hypocentral distribution is clearly recognized in the cross section taken in the direction perpendicular to the tectonic line. It is also noted that very shallow earthquakes are seen to be distributed around the tectonic line. Concerning the temporal change, seismic activities in the northeastern side of the tectonic line resumed after the 1974 Izu-hanto-oki earthquake, while occurrence of shocks had been relatively concentrated near the line before the event. In relation to this fact it should be noted that the seismicity in the Izu Peninsula became active after the 1974 Izu-hanto-oki earthquake. It is very probable that concurrent activation of seismicity in the Izu Peninsula and in the northeastern side of Suruga Bay is not a mere coincidence. It may indicate that in accordance with the activation of crustal movement in the Izu Peninsula the stress in the coupling region between the Eurasian plate and the Philippine sea plate also increased. Further, it is found that the seismicity in the western side of Suruga Bay was more active before the earthquake with a magnitude of 6.1 in 1965 than after the event. This is confirmed by the change of the number of felt earthquakes within 30 km of epicentral distance from the Shizuoka Local Meteorological Observatory. A tendency of concentration and diffusion of seismic activity is observed before and after the 1965 event along the tectonic line, though it is not highly distinct. In the regions around Shizuoka City it is certain that many shocks occurred immediately after the 1944 Tonankai earthquake, the correlation between earthquake occurrence in this region and that in the southern Izu Peninsula is recognized as well. All these characteristics indicate that the area around Shizuoka City is remarkable in being sensitive to the change of stress in the surrounding regions.
