Tectonomagnetic Signals Related to the Seismo-Volcanic Activity in the Izu Peninsula.

Abstract

Results of magnetic observations in the eastern Izu Peninsula for the past decade (1980-1990) are summarized. For the period from 1980 to 1983, the geomagnetic total intensity remained constant or slightly increased. Steep increase in the total intensity occurred at three stations (SGH, YSD, and HAT) successively in 1982, when the center of uplift migrated from inland toward east coast. In 1984 a remarkable decrease in the total intensity started on the east-coast upheaval, and lasted until the end of 1986. Since 1987 two stations across the swarm earthquake zone showed synchronous changes with opposite sign, positive on the north (HAT) and negative on the south (ARA). ARA station is the closest to the 1989 sea-floor eruption site, where the total intensity showed rapid increase within one month including the seismo-volcanic crisis in early July. Magnetic changes due to temperature and stress effects of an active magma reservoir are investigated quantitatively. With the aid of knowledge on magnetic subsurface structure (i.e., Curie point depth) beneath around Izu Peninsula, the cause of magnetic changes is discussed. No prominent long-term magnetic change has been observed along with the inland uplift from 1975 to 1982. This is because the magma reservoir lies too deep below the Curie point isotherm. However, a few rapid total intensity changes with positive sign have been observed, which were associated with local episodic upheaval. This suggests another cause of uplift: i.e. intrusion of high-pressure and low-temperature fluid into shallow depth. Remarkable decrease over the east coast bulge during 1984-1986 is caused by thermal demagnetization due to hydrothermals dispatched from magma reservoir. The paired anomaly at HAT and ARA for 1987-1988 is also due to thermal demagnetization, which can be regarded as a long-term precursor to the 1989 sea-floor eruption. The change at ARA during and after the seismo-volcanic crisis in July, 1989, is interpreted as piezomagnetic effect due to shallow pressure source with low temperature.