Research ArticleGEOPHYSICS

The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption

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Science Advances  15 Jan 2020:
Vol. 6, no. 3, eaaz1377
DOI: 10.1126/sciadv.aaz1377
  • Fig. 1 Affected area, tsunami amplitude, and arrival times at tidal gauge stations from the 22 December 2018 Sunda Strait tsunami.

    Cyan zones indicate areas affected by the tsunami, adapted from the map of Tsunami Selat Sunda created by Badan Nasional Penanggulangan Bencana dated 14 January 2019. Zero-to-peak wave heights (H) and arrival times (hh:mm UTC on 22 December 2018) are from Joint Research Centre emergency reporting released by 24 December 2018. The origin time of the source is estimated to be around 13:57 UTC, as discussed in the text. The insert map locates the Sunda Strait and Anak Krakatau in Indonesia.

  • Fig. 2 Regional broadband three-component ground motions from the Indonesian (IA) and GSN (AU.XMI, MS.BTDF) networks.

    The map shows the source location (red star) at Anak Krakatau and the regional broadband seismic network, with stations to the northwest labeled in blue and stations to the southeast labeled in purple. The lower figures show vertical (left), north-south (center), and east-west (right) components of ground displacement in the frequency band of 0.01 to 5.0 Hz. For each station, distance (Δ) and azimuth (ϕ) from the source (star) are listed in parenthesis.

  • Fig. 3 Sentinel-1A images showing the changes of Anak Krakatau from 10 to 28 December 2018.

    (A) SAR image acquired on 10 December 2018. (B) SAR image acquired on 22 December 2018, ~8 hours after the landslide generated tsunami. Possible flank failure occurred in the southern (outlined by dashed red line) and western (difference between red outlines and white outline to the west of Anak Krakatau). (C) SAR image acquired on 28 December 2018, including further modification of the edifice by large eruptions on 23 and 24 December. (D) Interpretation of the evolving geomorphology of Anak Krakatau superimposed on the SAR image on 22 December. Colored lines in (D) indicate the changes in island surface area with the color coding used in (A) to (C). The looking directions of these images are similar, around 44°.

  • Fig. 4 Regional waveform inversion solutions for the double-couple and single-force sources.

    (A) Best double couple from the W-phase moment-tensor inversion of regional waveforms in the period band of 40 to 200 s. Corresponding waveform comparisons are shown in fig. S2. W-phase inversion for the period band of 30 to 83.3 s is shown in fig. S3. (B) Single-force source representation, showing azimuth and force strength with dip of 0.1° downward from the horizontal indicated by the negative sign, from inversion of regional waveforms in the period band of 30 to 83.3 s. Corresponding waveform comparisons are shown in fig. S4. For each solution, the number of traces used is indicated along with the source geometry and centroid location information. (C) Comparisons of centroid locations of the W-phase moment tensor solution from (A), single-force solution from (B), and the moment tensor solution from GEOFON (GFZ), along with the epicenter locations from GFZ using regional IA network and LDEO using global long-period surface waves. The magenta star shows an approximate location of Anak Krakatau, and dashed magenta arcs indicate distances of 10, 20, and 30 km from Anak Krakatau.

  • Fig. 5 Stacked force-time history of the landslide and constraint on the dip angle.

    (A) Red and blue curves are linear averages of force-time histories obtained by deconvolving tangential (T comp.) and vertical (Z comp.) broadband data shown in (C) and (D), respectively. The force geometry is a single force with dip −12° (dipping upward) and azimuth 42°, used to calculate Green’s functions for each station for the deconvolutions. Red dashed lines indicate the approximate time window of the force-time history. (B) Variation of peak force amplitudes for the tangential (red) and vertical (blue) components inferred from the first peak of the stacked deconvolutions as a function of the force dip angle. The force azimuth is held fixed at 42°. A dip of −12° gives the optimal agreement between the force estimates and is the preferred case. (C) and (D) show the observed (black) and reconstituted (magenta) waveform comparisons of broadband displacements in the period band of 8 of 125 s for tangential and vertical components, respectively. The right panels show the deconvolved force-time history for each trace. Inferred peak-force values (in 1011 N) are listed for each trace.

  • Fig. 6 Time sequence of geological activities on Mt. Anak Krakatau.

    A nonlinear timeline is shown, extending from the first emergence of Anak Krakatau above sea level in 1929 through its construction of a ~300-m-high edifice by 22 December 2018 and its subsequent landslide and eruption resulting in large mass loss by 28 December 2018. Key times and intervals of processes related to the buildup and follow-on to the landslide along with key observation times from seismic network and Sentinel satellite are noted. The interval in which infrared satellite imagery of eruptive perturbation of the atmosphere occurred is indicated by the horizontal red line.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaaz1377/DC1

    Fig. S1. F-net ground displacements in Japan for time windows of 370 to 2000 s after 13:55:48.7 on 22 December 2018 (UTC).

    Fig. S2. Regional W-phase waveform fits for the moment-tensor source in Fig. 4A.

    Fig. S3. Moment-tensor inversion using regional waveforms in the passband 30 to 83 s.

    Fig. S4. Waveform fits for the single-force source model in Fig. 4B.

    Fig. S5. Bootstrap results for 1,000,000 single-force regional waveform inversions.

    Fig. S6. Stacked force-time history for variable dip angles.

    Fig. S7. Estimated slide volume as functions of average basal friction and average detachment dip angle.

    Fig. S8. Comparison of key estimates of the 2018 Anak Krakatau landslide with other landslides.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. F-net ground displacements in Japan for time windows of 370 to 2000 s after 13:55:48.7 on 22 December 2018 (UTC).
    • Fig. S2. Regional W-phase waveform fits for the moment-tensor source in Fig. 4A.
    • Fig. S3. Moment-tensor inversion using regional waveforms in the passband 30 to 83 s.
    • Fig. S4. Waveform fits for the single-force source model in Fig. 4B.
    • Fig. S5. Bootstrap results for 1,000,000 single-force regional waveform inversions.
    • Fig. S6. Stacked force-time history for variable dip angles.
    • Fig. S7. Estimated slide volume as functions of average basal friction and average detachment dip angle.
    • Fig. S8. Comparison of key estimates of the 2018 Anak Krakatau landslide with other landslides.

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