SEISMIC PRECURSORS OF THE FEBRUARY 10, 1990 ERUPTION OF KELUT VOLCANO, JAVA']JAVA

Kelut volcano, eastern Java, Indonesia erupted on February 10-12, 1990. Seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption. During the pre-eruptive stage, the number of events was eight per day on average and the cumulative seismic energy released was 5.6 X 10(15) erg; these low values are uncommon for an andesitic volcano. Seismic activity mainly consisted of swarms of very shallow volcano-tectonic earthquakes, many of which were part of seismic multiplets. In the last three weeks before the eruption, seismic activity remained at a very low level. This quiescent period was followed, 24 hours prior to the eruption initiation, by an intense swarm of low-energy events which progressively became continuous spasmodic tremor. Most of the data obtained during the pre-eruptive stage are consistent with a process of fracturation of a shallow and low-strength plug blocking the eruptive conduit before the eruption. Several observations suggest that water is an important element in the preeruptive and eruptive phenomena at Kelut volcano. In particular, the monochromatic events observed at Kelut are characterized by one narrow peak in their spectra; the estimations of Q-value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water.