Fig. 3

Ten-hour seismograms at Kawah Ijen. a Vertical velocity (station DAM). b Horizontal velocity (station DAM). c Vertical displacement (station DAM). d Horizontal displacement (station DAM). e Horizontal displacement (station POS). For a and b, instrument responses are deconvolved after detrending and high-pass filtering the data above 0.001 Hz. The resulting ground velocity is further integrated, detrended and band-passed between 1000 and 250 s to produce c–e. The dominant period of VLP signals is 8 s with superimposed low-amplitude shorter oscillations (down to 1 s period) while the HF bursts show broadband spectra with oscillations between 1 and 20 Hz. The dominant period of the ULP tilt signals in (d) are oscillating between 12 and 18 min. Note that the vertical component in (c) is about 20 times smaller than horizontal components in (d) and can be barely distinguished from the background noise