Fig. 4
From: Complex hazard cascade culminating in the Anak Krakatau sector collapse
Seismic and infrasound recordings of the Anak Krakatau sector collapse. a Locations of stations with regional seismic instruments. Station I06AU is an infrasound station located 1150 km to the SW of Krakatau (beyond the edge of the map). The inset shows the probabilistic moment tensor solution with the best nodal plane striking NW–SE and dipping to the SW and a large compensated linear vector dipole (CLVD) component. The red line indicates the nodal surface of the best double couple component from the full moment tensor solution. b Smoothed envelopes of vertical component 0.4–1 Hz bandpass-filtered seismic records. c Normalized seismic amplitudes at the closest station (CGJI), located 64 km from Anak Krakatau, revealing the occurrence of a high-frequency event (1) 115 s prior to the sector collapse (2). The spectrogram reveals that collapse is a 1–2-minute-long low-frequency signal presumably related to the landslide, followed by ~5 mins of strong emissions at high frequencies. d Upper and middle panels: Infrasonic spectrograms of the best beam and beam power from the seven-element infrasound array at I06AU, where the onset of a strong impulsive signal was registered at 15:01:09. The beam direction and trace velocity, displayed in the lowermost panel, point towards Krakatau (back azimuth 55–56°, compared with Supplementary Figure 8) with a trace velocity of ~60 m s−1. Although the strong impulsive signal (the landslide) spanned a duration of only ~1 min, in the time window shown in the figure, the beam focuses continuously towards Krakatau, indicating continuous energy flow from Krakatau for a much longer time span (see also Supplementary Figure 8). The red mark (labeled Is) shows the theoretical arrival time of the stratospheric phase predicted by infrasound modeling, taking into account the origin time of the seismic event at 13:55:49 at Krakatau
