Fig. 4

VLPs and long-lasting oscillations at Kawah Ijen. a Vertical displacement and b spectra corresponding to the red window in (a). c Zoom on the first very-long-period (VLP) event and d spectra of the red window in (c). Instrument responses are deconvolved after detrending and high-pass filtering the data above 0.001 Hz. The dominant period of VLP signals is 8 s with superimposed low-amplitude shorter oscillations [down to 1 s period, subplot (d)]. These long-lasting oscillations have distinct dominant periods at 17 s, 27 s and 50 s [subplot (b)]