A month-long deployment of broadband seismographs at the summit of Merapi volcano (Java, Indonesia) in 1998 has yield a remarkable data set of volcanic earthquakes associated with active andesitic dome growth. These earthquakes, traditionally termed multiphase (MP) events, are usually characterized by 3-4 Hz ground motions on short-period stations. However, data collected within a few hundred meters of the dome showed relatively high-amplitude, distinctive, very-long-period (4 sec) horizontal ground motions superposed on the MP waveforms, with nearly identical waveforms at the same stations over many events. Very-long-period pulses are not observed at broadband stations at greater distance. Dominance of horizontal ground motions, pulse amplitude decay with distance, the close proximity of stations near lava dome, and character of modern broadband seismographs all suggest that these deformational pulses are due to dynamic, near-field tilts caused by lava extrusion processes at the actively growing Merapi dome. A two year-project is proposed to analyze these data using numerical methods to compute synthetic displacement and rotation fields and two conduct a one-month long PASSCAL/RAMP deployment of broadband seismographs, in conjunction with high-data-rate tiltmetry, to collect more data during expected sustained actiity at Merapi. The results of this study will have important implications in interpreting broadband signals associated with volcanic processes, and may lead to fundamentally new understanding of seismic processes associated with active volcanic dome growth.
