Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 2, February 1995 Merapi (Indonesia) Seismic data associated with the 22 November 1994 dome collapse Merapi Java, Indonesia 7.54S 110.44E; summit elev. 2,911 m All times are local (= GMT + 7 hours) A collapse of Merapi's gravitationally unstable summit lava dome took place on 22 November 1994 and VSI volcanologists estimated that it mobilized roughly 13 x 10^6 m^3 of pyroclastic debris (Bulletin v. 19, no. 10). Workers at the GMU Geophysical Laboratory and Martin Beisser of GFZ-Potsdam recorded seismic data during the collapse from their station at Klathakan, 1.8 km WNW of the summit. Their broad-band seismic instrument showed the associated disturbance beginning on 22 November at 1007 and 32 seconds (radial-component data shown on figure 2). So far as the GMU and GFZ workers know, the wide dynamic range of their broad-band instrument preserved the event with a minimal amount of high-amplitude signal "clipping." Also, in their interpretation, the collapse and seismic disturbance began simultaneously. In other words, the initial displacement at the beginning of the seismic record is thought to correspond to the arrival of signals from the inception of the collapse. The collapse-related seismic event lasted for almost an hour (figure 2). The initial signals were set against a moderately quiet background, and maximum amplitude generally increased with time. Highest-amplitude signals were received about 40 minutes after the event began. These largest signals had amplitudes that reached approximately 30 mm/sec, whereas at the beginning of the collapse the maximum amplitudes were only about 0.05 mm/sec. Thus, on the seismic records, amplitudes ultimately grew to 600 times as large as the initial signals. The eruption and collapse also appear in a 200-hour time window showing measured seismic amplitude in specified wavelengths (figure 3). The figure was prepared using signal processing techniques, which for the high frequency (0.1-1.0 Hz) data involved significant averaging of the maximum values (to once an hour). These depictions show that one or two noteworthy seismic disturbances took place at about 150 and 180 hours prior to the collapse (cause unknown). Compared to the other seismic disturbances on these records, the collapse and eruption induced larger amplitude and much more sustained signals. The post-collapse signals were also followed by an interval of at least 10 hours of elevated background (most noticeable in the 1-12 Hz range). Using the available data, the investigators failed to find any clearly related premonitory seismic signals for the collapse. Sufficient collateral data (for example, teleseismic and meteorological data) might help constrain detected collapse and eruption earthquakes, or shed light on the cause of the pre-collapse seismic disturbances. Since our last report (Bulletin v. 19, no. 12), continued dome building occurred at Merapi. On 5 January another collapse brought 1 x 10^6 m^3 of debris downslope. This collapse produced a small pyroclastic flow on the S slope. Information Contacts: Arnold Brodscholl and Kirbani Sri Brotopuspito, Geophysics Laboratory, Gadjah Mada University, FMIPA-UGM, Sekip Unit III, Yogyakarta 55281, Indonesia; Martin Beisser, GeoForschungs-Zentrum-Potsdam, Telegrafenberg A31, H117, 14473 Potsdam, Germany (Email: email@example.com); Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia, Jalan Diponegoro 57, Bandung, Indonesia. Figure 2. Seismic record for the Merapi 22 November dome collapse. The component shown is horizontal, radial to the edifice; amplitude scale is arbitrary. The data were recorded on a data logger connected to a Streckeisen STS2 seismometer (with a 50 Hz sampling rate, a 8.33 mHz to 50 Hz linear response, and a 32-bit analog-to-digital converter). Courtesy of Arnold Brodscholl and Kirbani Brotopuspito. Figure 3. Radial component of the Merapi 22 November dome collapse showing a seismic amplitude (arbitrary scale) versus time for stated wavelength ranges. The inception of the collapse lies at the zero point of the time scale. Courtesy of Arnold Brodscholl and Kirbani Brotopuspito.