VOLCANOMAGNETIC SIGNALS ASSOCIATED WITH THE QUASI-CONTINUOUS ACTIVITYOF THE ANDESITIC MERAPI VOLCANO, INDONESIA - 1990-1495

Merapi volcano in Java island (Indonesia) is an andesitic stratovolcan o which presents lone periods of effusive activity during which an end ogeneous dome is continuously growing. The viscous lava dome gives ris e to unstable blocks which collapse or turn into pyroclastic Flows. Wh en the volcano does nor exhibit any surface activity, the overpressure within the volcano slowly increases. Depending on the quietness durat ion, the unrest of the volcano can start with an explosive phase durin g which the former dome is partly destroyed. Magnetic variations of di fferent time constant are observed during the 1990-1995 period which i ncludes one gas plume emission on August 26, 1990 and two eruptions on January 20, 1992 and on November 22, 1994. Compared with Other types of active volcanoes, the observed volcanomagnetic variations are very small, at the most a few nanoteslas (nT). To discriminate the variatio ns associated with the global activity from the signals correlated wit h each unrest phase. one has to dissociate the different time constant variations over the six-year time span. When long-term trends are rem oved from the magnetic field in each station of the network. an outsta nding correlation between all the magnetic differences is emphasised. The midterm variations point out 2 cycles of activity which fit the st ress field evolution within the edifice lending to the 1992 and 1994 e ruptions. A new cycle has started in May 1995, In every identified cyc le, rapid volcanomagnetic signals are well associated with stress fiel d changes (May 1991, September 1991, February 1993, December 1993,...) . Some of the volcanomagnetic variations are short-term precursory sig nals as the three months decrease, up to 1.3 nT. preceding the 1992 er uption. The comparison between magnetic data. seismicity and surface p henomena implies that the midterm volcanomagnetic variations associate d with the cycles of Merapi activity ale of piezomagnetic origin. (C) 1998 Elsevier Science B.V.