Volcanic Earthquakes at Merapi (Central Java) During the Lava Dome Building Beginning in October 1986
This study presents results of seismic measurements on Merapi volcano from August 1986 to July 1987. This time period includes the volume increase of the magma dome by about 2 million m3 in October 1986. The seismic data were recorded by three stations which are located within 5 km distance from the summit, (KLT: 1.5 km from the summit. north-west; LBH and KKN: 2.2km and 5 km, south).
With the beginning of the lava doming, the following changes in the seismicity were observed: the number of shocks per hour increased, with an increase in the ratio of recorded shocks between the stations LBH to KLT and KKN to KLT. Before the building of the lava dome, a significant periodicity at 24 and 12 h in the occurrence of the shocks was noted. This phenomenon was greatly reduced with the accumulation of outcoming magma. The diurnal and semidiurnal occurrence possibly follows barometric pressure fluctuations acting on the volcano.
Furthermore, the dominant frequencies in the seismic shocks shifted to lower values. As the first onset of the shocks can not be accurately determined (Minakami B-type), epicentres and source depths were estimated using the maximum amplitude ratios found at the different stations. The locations of the shocks with respect to the lava dome building are discussed.
KeywordsMigration Europe Attenuation Indonesia Body Wave
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