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 nuées ardentes, lasting about 40 min and culminating with the largest event at 10:54 (all times reported as local time); and (II) activity from 11:42 until 20:00 described by scattered clusters of individual rockfalls and 20 nuées 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 Mm³. Discounting the volumetric influence of rockfalls, the average size of the 44 nuées ardentes is therefore about 60,000 m³. 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 m³, 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 m³. The methods described here can be put to practical use in real-time monitoring situations. Broadband data were essential in this study primarily because of the wide dynamic range.