Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 8, August 1994

Merapi (Indonesia)  Two new broad-band seismometers detect
      long-period pulses and tremor

central Java, Indonesia (7.54xS, 110.44xE)
All times are local (= GMT + 7 hours)

Two STS2 broad-band seismometers were deployed on 27-29 July by
collaborators of the Geophysical Laboratory of the GMU and Martin
Beisser of GFZ-Potsdam.  The researchers investigated signal
coherency at different points on the volcano to find suitable sites
for a multi-station seismic array that will make permanent records
at a 50-Hz sampling rate.  The researchers measured Merapi
seismicity at a base station located at Klathakan (1.8 km WNW of
the summit between 1,200-1,300 m elevation), the site of a seismic
station for the last 8 years.  The second station was mobile and
GPS-equipped; however, for the following comparisons and discussion
the mobile site remained 400 m N of the base station.

Figure 5 shows amplitude data for 3-components of volcanic shock
from the mobile station.  The volcanic shock event that began at
1750 and 37 seconds on 27 July is here termed Event A.  Figure 6
shows the arriving signals and allows for a visual comparison of
the coherency in the vertical component (top 2 plots), and two
orthogonal horizontal components (lower 4 plots).  From visual
inspection, the best coherency appeared in the vertical-component
data.  Some other types of events received appeared to show less
coherency between the two sites.

Figure 7 shows 3 components of a previously undetected tremor type,
a tremor preceded by or superimposed on a long-period pulse.  On
the record, the interval of greatest short-period amplitude came
after the pulse's maximum.  Examples of this kind of tremor were
seen 3 times in 12 hours.  Whether these events are common on
Merapi and elsewhere still remains uncertain.

Merapi, one of the most active volcanoes in Indonesia, sits
immediately N of the large city of Yogyakarta; at least 50,000
people live adjacent to its SW slope.  The stratovolcano has an
exposed, summit lava dome, the source of abundant glowing blocks
that continue to tumble down its SW slope.  In historical time,
instability of the growing dome has led to nuees ardentes that have
caused many fatalities, disasters described in many popular books
on volcanology.

Information Contacts: Martin Beisser,
GeoForschungs-Zentrum-Potsdam, Telegrafenberg A31, H117, 14473
Potsdam, Germany (Email:; Arnold Brodscholl,
Geophysics Laboratory, Gadjah Mada University (GMU), FMIPA-UGM,
Sekip Unit III, P.O.  Box BLS 21, Yogyakarta 55281, Indonesia