5-8. Study on Tectonoelectromagnetism

 

 Temporal variation of the electromagnetic field implies crustal fluids' motion (via electrokinetic phenomena), stress change (via piezomagentic effect) and temperature change (via thermal magnetic effect). Temporal variation of resistivity structure indicates change especially in volume fraction of crustal fluid and its connectivity. Recently, in virtue of remarkable progress in techniques for monitoring ground motion such as GPS, SAR, absolute gravimeter or wideband seismometer, tiny and/or long period gournd motion, which could not be caught in the past, has been detected in a regional scale. Joint interpretations of temporal electromagnetic variation together with such geodetic and seismic phenomena have a potential to establish new views of the crustal activities, which are associated by the fluid motion and could not be obtained merely by the dynamics.

 We are, therefore, performing continuous electromagnetic field monitoring in the eastern part of the Izu peninsula and Tokai-Kanto districts (Fig.1), cooperating with researchers of several universities and institutions. Total magnetic forces and self potentials are monitored also in the Izu islands. Recent decrease of total force variation intensity is associated with reducing crustal activities in and off the eastern coast of the Izu peninsula. Electromagnetic disturbances in the active periods implied that the seismic swarm activity and the ground uplift were associated with hydrothermal intrusion. At Oishigasawa station, in the north of the Ito city, remarkable total force decrease (-30nT/ 5 years) was detected until the latter half of 1998. Then, for a time, the decrease stopped and again started to slowly decrease from Sep. 1999. In spite of the remarkable variation, we still can not specify its origin. Annual variations are detected at almost all the stations with various amplitudes. Total force mapping just around respective sensor (in 10m x 10m area) revealed that the annual variation could be explained by annual change of magnetization just near the sensor due to annual temperature variation. At Tokai stations, trend of long-period total force variation was changed from the latter half of 2000. We are now examining possible relationship between this and the Tokai slow event or the crustal activies due to Miyakejima eruption and intense seismic swarm activities in the Izu islands. In Miyakejima island, growth of thermal demagnetization which started 2 years before the 2000 eruption, generation of subsurface cavity prior to appearance of the surface caldera were indicated by temporal variation of total magnetic force, which also revealed detailed process of generation, enlargement and deepening of the caldera. Repeated simultaneous occurence of characteristic self potential variation with the tilt steps implied that fluids were intruded from the source under the summit to the surroundig area.

Fig.1 Electric and magnetic stations distributed in the Tokai-Kanto districts. An enlarged map for the eastern part of the Izu peninsula is shown in the top-left. (red circles: proton magnetometers, green triangles: proton & fluxgate magnetometers and electric field measurements, blue square: proton magnetometer and resistivity monitoring)