Abstract
Abstract A two-axis tiltmeter installed at the summit of Semeru volcano, Indonesia, reveals two different modes of deformation, short-term and longer-term. The short-term variations are inflations precursory to the occurrence of summit explosions. The time intervals between the explosions range from several minutes to several tens of minutes, and the maximum precursory tilt change observed is about 0.1–0.2 μrad. This change in tilt is interpreted to reflect a shallow pressurization source, just beneath the active crater. The longer-term tilt changes are displayed over a period of several days to weeks, and a steady inflation during a period of nearly 2 weeks preceded the occurrence of pyroclastic flows on December 22, 2005. Both short-term and longer-term changes are confined to the radial tilt component, pointing toward the active crater and conduit. To explain both of these tilt changes we propose relatively shallow conduit pressurization, although at somewhat deeper levels in the case of longer-term deformation. The causative mechanisms involve gas exsolution, microlite crystallization, and rheological stiffening, which results in greater flow resistance and dynamic pressure in the upper conduit, thus causing the edifice tilts observed. The gas pore pressure in the magma builds until the threshold required to trigger explosive fragmentation is reached.
Highlights
Semeru volcano is an andesitic stratovolcano located inEast Java (Fig. 1)
As the tiltmeter is situated near the active crater and is highly sensitive, disturbances caused by explosions and local volcanic earthquakes caused large offsets or spikes in the tilt record; to aid data interpretation, these
The time intervals between explosions range from several minutes to several tens of minutes, and the precursory tilt changes in each episode are 0.1–0.2 μrad
Summary
The peak’s summit rises 3676 m above sea level, and its crater is about 300 m across. This volcano is the highest active volcano on the island of Java. Frequent explosions at the summit crater of Jonggring Seloko have been occurring continuously since 1941 (Kusumadinata, 1979), and during this period, frequent small vulcaniantype explosions have occurred, producing explosive plumes rising 400–1000 m above the summit. Lava flows, lava dome extrusions and pyroclastic flows have been observed (Kusumadinata, 1979). In 2005, the frequency of the explosions averaged about 100 per day (Fig. 2)
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