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印度尼西亚爪哇岛的默拉皮火山是一座安山岩质成层火山,它呈现长期的喷发活动。在喷发活动期间,一个内生的火山穹丘连续地生长。粘性熔岩丘形成不稳定的块体。这些块体或者崩塌或者变成火山碎屑流。在火山没有呈现任何地表活动的时段内,火山内部的过压缓慢地增加。火山的不稳定期从一个爆发阶段开始,其间前一个熔岩丘部分被毁。在1990～1995年之间我们观测到了不同时间常数的地磁变化,其中包括1990年8月26日的一次气柱喷射以及1992年1月20日和1994年11月22日的2次喷发。与其他类型的活火山相比,本工作中所观测到的火山磁变化是很小的,至多为几个nT。为了区分与总体活动有关的变化及与每一个非休眠阶段相关的变化,必须把这6年时段内不同时间常数的变化分离开。在消除了台网内每个台站磁场的长趋势变化之后,所有地磁差值问的明显相关性便显示出来。中期变化呈现出2轮活动性,它们与导致1992年和1994年喷发的地下火山体系内的应力场演变相吻合。1995年5月开始了一轮新的活动。在确定的每轮活动期内,快速的火山磁信号都与应力场变化有很好的相关性(1991年5月,1991年9月,1993年2月和1993年12月……)。某些火山磁变化是短期前兆信号,如1992年喷发前3个月内幅度达1.3nT的下降变化。地磁资料、地震活动性和地表现象间的对比表明,与默拉皮火山各轮活动期有关的中期火山磁变化是由压磁效应引起的。
Mount Merapi in Java, Indonesia is an andesite stratiform volcano that exhibits long-term eruptions. During the eruption, an endogenous volcano dome grows continuously. Viscous lava mounds form unstable blocks. These blocks either collapse or become pyroclastic flows. Overvoltages within the volcano slowly increased during periods when the volcano did not exhibit any surface activity. Volcanic instability begins with an eruption phase, during which part of the former lava is partially destroyed. From 1990 to 1995, we observed geomagnetic changes with different time constants, including one gas column injection on August 26, 1990 and two eruptions on January 20, 1992 and November 22, 1994. Compared with other types of active volcanoes, volcanic changes observed in this work are small, up to a few nT. In order to distinguish between changes related to the overall activity and those associated with each non-dormant phase, it is necessary to separate the changes of different time constants over the six-year period. After eliminating the long-term changes in the magnetic field at each station in the network, the obvious correlation of all the geomagnetic differences is displayed. The mid-term changes show two rounds of activity that are consistent with the evolution of the stress field in the subterranean volcanic system that led to the eruptions of 1992 and 1994. A new round of activities started in May 1995. Rapid volcanic magnetic signals correlate well with stress field changes over a defined period of activity (May 1991, September 1991, February 1993 and December 1993 ...). Some volcanic magnetic changes are short-term precursors, such as a drop of 1.3 nT in the three months preceding the eruption in 1992. Comparisons of geomagnetic data, seismicity and surface features show that mid-term volcanic magnetic changes associated with the active phases of Mount Merapi are caused by the piezomagnetic effect.