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

1995年10月至1997年2月，我们在东京以南约150公里的三宅火山（Miyake-jima）上连续测量了8个测站的地磁合力强度和2个测站的自电位（SP）。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东西向转向南北向。1996年6月，河曲恢复正常的东西向水流。在三宅岛，总强度增加了5 nT或更多。SP的变化也观察到根据曲折。这是第一个在一个孤岛上的多个台站同时观测到与弯曲洋流有关的电场和磁场变化的例子。运动感应的电场和磁场被证明是最严重的噪声的自然起源的目的，观看三宅岛火山活动的电磁（EM）观测。但只有当黑潮主流绕岛流动时，其影响才占主导地位。我们得到了三宅岛各测点的短周期地磁扰动的传递函数，作为三宅岛下电阻率结构的信息来源。岛的影响，我。e.由于陆地-海洋电导率对比，海洋中感应电流的畸变产生的局部磁场相当弱。这意味着在岛的下面很浅的深度处存在一个非常导电的层。本文根据1983年三宅岛火山喷发前后的电磁观测资料，结合其它地球物理和地质资料，提出了三宅岛火山可能的喷发机制。本研究针对可能在下个世纪初爆发的三宅岛火山，给出了电磁观测系统的优化设计及其可靠性评估。

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.