Sequential dome-collapse nuees ardentes analyzed from broadband seismic data, Merapi Volcano, Indonesia

During the sequential dome collapse of Merapi Volcano on 22 November 1994, a broadband seismic station on the western slope was the only operational seismic equipment that provided continuous on-scale recording of the event. According to visual and seismic observations, the collapse activity lasted about 10 h. We divide the activity into two phases: (I) a period with fluctuating but generally increasing seismic activity associated with 24 moderate to large dome-collapse nuees ardentes, lasting about 40 min and culminating with the largest event at 10:54 tall times reported as local time): and (II) activity from 11.42 until 20:00 described by scattered clusters of individual rockfalls and 20 nuees ardentes. The broadband data were evaluated using the assumption that avalanches with the same source areas and descent paths exhibit a linear relation between source volume and recorded seismic-amplitude envelope area. A result of the analysis is the determination of the volume of selected individual events. From the field surveys, the total volume of the collapsed dome lava is 2.6 Mm(3). Discounting the volumetric influence of rockfalls, the average size of the 44 nuees ardentes is therefore about 60,000 m(3). The largest collapse event at 10:54 is estimated to involve 260,000 m3, based on an analysis of the seismicity. The remaining 23 phase I events averaged 60,000 m(3), with the total volume of all phase I events accounting for 63% of the unstable dome. The 20 phase II events comprised 37% of the total volume and averaged 47,000 m(3). The methods described here can be gut to practical use in real-time monitoring situations. Broadband data were essential in this study primarily because of the wide dynamic range. (C) 2000 Elsevier Science B.V. All rights reserved.