CHARACTERISTICS OF GEOMAGNETIC TOTAL INTENSITY VARIATIONS ON MIYAKE-JIMA VOLCANO AND ITS DEEP RESISTIVITY STRUCTURE

自1995年10月至1997年2月，我们在东京以南约150公里的玄武岩火山三宅岛上连续测量了8个地磁总力强度和2个地磁自电位。1995年11月下旬，西北太平洋最主要的海流黑潮在三宅岛附近由西向东转向南向北。曲流在1996年6月恢复到正常的西向东流。在三宅岛，总强度增加了5nT或更多。根据曲径也观察到SP的变化。这是第一次在一个孤岛上的许多站点同时观测到与蜿蜒的洋流有关的电场和磁场变化。运动感应电场和磁场是电磁观测三宅岛火山活动时最严重的自然噪声源。然而，只有当黑潮的主流流经岛屿时，它的影响才占主导地位。得到了各磁强计测点的短周期地磁扰动传递函数，作为宫古岛地下电阻率结构的信息来源。岛屿效应，即由于海陆电导率对比造成海洋中感应电流畸变而产生的局地磁场，是相当微弱的。这意味着在岛屿下面的浅层深处存在着一个非常导电的层。一项更定量的模型研究正在进行中。根据1983年三宅岛火山喷发前后的电磁观测资料，结合其他地球物理和地质资料，提出了三宅岛火山喷发的可能机制。本文对拟于下个世纪初喷发的三宅岛火山电磁监测系统进行了优化设计和可靠性评估。

We conducted continuous measurements of geomagnetic total force intensity at 8 stations and self electric potential (SP) at 2 stations on Miyake-jima island, a basaltic volcano about 150 km south of Tokyo, since October 1995 till February 1997. In late November of 1995, Kuroshio, the most dominant ocean current in the northwestern Pacific, changed its flow direction from west-east to south-north near around Miyake-jima island. The meanders recovered to normal west-east flow in June 1996. On Miyake-jima island, the total intensity increased by 5nT or more. SP variations were also observed in accordance with the meanders. This is the first example that electric and magnetic field variations associated with the meandering ocean flow were simultaneously observed at many stations on an isolated island. The motionally-induced electric and magnetic fields turned out to be the most serious noise of natural origin for the purpose of watching for the activity of Miyake-jima volcano by electromagnetic (EM) observations. However, its effect is dominant only when the main stream of Kuroshio current flows around the island.We obtained the transfer functions of short-period geomagnetic disturbances at each magnetometer site as sources of information of the resistivity structure beneath Miyakejima island. The island effect, i. e. local magnetic field produced by distortion of induced electric currents in the ocean due to the land-sea conductivity contrast is rather weak. This implies that there exists a very conducting layr at a shallow depth beneath the island. A more quantitaive modeling study is now in progress.We presented a possible eruption mechanism of Miyake-jima volcano based on EM observations during and after the 1983 eruption, combined with other geophysical and geological data. The present study gives the optimum design as well as its reliability assessment of the EM watching system for Miyake-jima volcano, which would most probably erupt early in the next century.