Investigation on volcanic activity and sector collapse inducing the

Sunda Strait tsunami in Indonesia

Iguchi, M. (PI in Japanese side) (DPRI, Kyoto Univ.) Nakamichi, H., Yamada, M. (DPRI, Kyoto Univ.) Kaneko, T. (ERI, Univ. Tokyo), Inoue, H., Nagai, M., S. Nakada (NIED)

Kasbani (PI in Indonesian side) (CVGHM) Gunawan, H., Kristianto, Triastuty, H., Karyono, Mulia, I., Kartadinata, N., Prambada, O., Haerani, N., Basuki, A., Hendratno, K., Prayoga, A.S. (CVGHM)

Presenter: Maeno, F. (ERI, Univ. Tokyo)

Project operation

Topic Operation Location

1 Elucidation of growth and collapse processes of Anak Krakatau volcano based on geological and geomorphological analyses

Geological survey Krakatau Islands

2 Elucidation of the process where small scale eruption activities resulted in collapse of the mountains

Examination of seismicity in long-term CVGHM

3 Elucidation of the collapse process of the mountains

Analysis broadband seismogram CVGHM

4 Evaluation of possibility of future collapse

Operation fixed-wing UAV POS

Installation microphone CVGHM

Topic 1: Elucidation of growth and collapse processes of Anak Krakatau volcano based on geological and geomorphological analyses

Japan Fukashi Maeno (ERI)

Takayuki Kaneko (ERI) Masashi Nagai (NIED) Study area:

Krakatau Islands

Indonesia Nugraha Kartadinata (CVGHM) Oktory Prambada (CVGHM)

Objective: Clarify the cause and process of the flank collapse of Anak Krakatau volcano, based on geological and geomorphological data of the volcano.

Geology group

©Digital Globe

11 June 2005

Anak Krakatau Volcano before collapse

Size: ~2.1 km EW ~2.3 km NS Height: > 300 m a.s.l. Lava flows and a central cone

Anak Krakatau Volcano before collapse

All photos were taken in September 2014

Size: ~2.1 km EW and ~2.3 km NS Height: >300 m a.s.l. Lava flows and a central cone

Accumulation of pyroclasts from Strombolian and Vulcanian eruptions

Geological background

Sigurdsson et al. (1991)

Before 1883 After 1883

A young volcanic island growing since 1927

Sudradjat (1982)

Sudradjat (1982)

Migration of vent to SW

Geological background At least 3 small-scale collapses in the past

Sutawidjaja (2002) Sudradjat (1982)

Sudradjat (1982)

19 Dec 2018 22 Dec 2018 25 Dec 2018 27 Dec 2018

28 Dec 2018 31 Dec 2018 2 Jan 2019 3 Jan 2019

24 Dec 2018

30 Dec 2018 2 Jan 2019

Planet Lab Planet Lab

ALOS-2 JAXA Sentinel-Hub Sentinel-Hub Sentinel-Hub Sentinel-Hub

Sentinel-Hub Sentinel-Hub Sentinel-Hub Sentinel-Hub

Evolution of Anak Krakatau (SAR, Optical)

Collapse ⬇

Growth

3.9 µm

11 µm

Activity in 2019: Himawari-8 data (Thermal anomaly) Variation of thermal anomaly in 2018 (only night)

è The eruption began ~13:50 UTC, 22 December 2018. This time is 30-60 min before tsunami arrival around Sunda Strait (~14:30-15:00 UTC). Length of a side is 200 km.

⬆ ︎Continuous thermal anomaly has been detected since the middle of June. No specific higher anomaly toward the eruption.

13:50

13:40

14:00

14:10

14:20

13:30

3.9 µm

11 µm

January December

30-40 min before tsunami arrival

Tsun

ami

Erup

tion

The last eruption before the event was the 13:51 21 Dec.

2018-12-22

1 year

1 day

Sequence of the 2018 event

Eruption plume

UTC 14:30 14:00 16:00 15:00 15:30

Eruption plume

Erup

tion

inte

nsity

Sustained eruption à

Tsunami arrival at coast

13:30 17:00 16:30

Field survey on 14-16 November 2019

Sertung Panjang (Kecil)

Rakata

Anak Krakatau

Pasauran (Carita) ⇔ Krakatau islands • Anak Krakatau • Panjang island

1212 m

© D︎igital Globe Google Earth

11 June 2005 11 January 2019

Before and after the event

Intermittent small-scale explosions are continuing (Surtseyan-type eruption by magma-water interaction)

Loc. 3

Stratified-cross stratified deposits, suggesting origin of pyroclastic surges, consists of main cone

Bread-crust scoria, indicating a product by magma-water interaction

Future works

n Anak Krakatau has collapsed several times through its eruptive history. n Satellite data suggest that a small eruption began at around 13:50

(UTC) 22 Dec 2018, which is 30-40 minutes before tsunami arrival at coasts along Sunda Strait. Then it was followed by the main sustained eruption.

n We found various types of deposits associated with the 2018 collapse-eruption event at Krakatau islands.

n The eruption started with pyroclastic surge generation. n Distribution, facies, and grain characteristics of deposits will be

important clues to constrain the process of 2018 event.

Summary of geological study

• Grain component and size distribution analyses for deposits. • Geochemical analysis of essential products and comparison

with the past products. • Discuss collapse and eruption processes.

Topic 3: Analysis broadband seismogram Elucidation of the collapse process of Anak Krakatau: Landslide movement history estimated from seismic waveforms Yamada, M., Nakamichi, H. (Kyoto Univ.) Mulia, I., Karyono (CVGHM)

12:00 18:30 19:00 21:03 21:30

Not clear

Plume

Tsunami

CCTV

Erupting

Signal lost Eruption? (official)

CCTV Event Seismic Signal Dec. 22 (WIB)

Time of Eruption

0 50km

MDSI

SBJI

SKJI

BBJI

CGJI

CNJI

LEM

CCTB

Sumatra

Jawa

105.35̊ 105.4̊ 105.45̊

−6.15̊

−6.1̊

−6.05̊

0 1 2 3km

SRTGKRA2KRA3

Broadband Shortperiod

100km

200km

20:52 21:02

Long-period waveforms (> 5 s)

0km 20:54 20:56

Vs~2km/s

Data by CVGHM

Time of Eruption

12:00 13:30 18:30 19:00 20:54 20:56 21:03 21:30

Not clear

Plume

Tsunami

CCTV

Erupting

Amplitude increase

Small collapse Large edifice collapse

Eruption? (official)

CCTV Event Seismic Signal

Signal lost

Dec. 22 (WIB)

Residual Surface of the Inversion

103 ˚ 104 ˚ 105 ˚ 106 ˚ 107 ˚ 108−8̊

−7̊

−6̊

−5̊

−4̊

0 50km

MDSI

SBJI

SKJI

CGJI0.2

0.2

0.20.2

0.20.4

The optimal location is Anak Krakatau.

Force in horizontal plane

A B

B

A

Estimation of the Collapsed mass

Stage1 Stage2

We analyzed the near-source seismic data of the 2018 Anak Krakatau volcano eruption

• Origin time 20:56 (local time) • Collapsed from NE à SW, about 1min, low dip angle • Mass 2.5-5.0 × 1011 kg • Small coefficient of friction

n The volcanic eruption may trigger edifice collapse.

n A warning from seismic signals may be possible if infrastructure is well-prepared.

Summary of seismic waveform analyses

Sequence of the 2018 event

Eruption plume

UTC 14:30 14:00 16:00 15:00 15:30

Eruption plume

Edifice collapse 0.1-0.2 km3

13:56 min

Erup

tion

inte

nsity

Sustained eruption à

Eruption start ~ Edifice collapse à Main eruption start

13:30 17:00 16:30

Tsunami arrival at coast

Summary

Issues: Relationship between collapse and eruption. Detail process and mechanism are still problematic. Implication to volcanoes in Japan Evaluation of collapse potential and mass movement will be important.

We investigate volcanic activity and sector collapse inducing the Sunda Strait tsunami on 22 Dec 2018. n Small-scale eruptions had repeated in the eruption day. Satellite data

shows no large eruption before the event but a significant eruption began at around 13:50 (UTC).

n We found deposits from the collapse/eruption event at Krakatau islands. n The eruption started with pyroclastic surge generation. n Seismic waveform inversion suggest that origin time of collapse was

20:56 (local time), collapse direction NE à SW, and collapse duration about 1 min.

n Collapse volume is estimated to be 2.5-5.0 × 1011 kg (~0.1-0.2 km3). n Collapse occurred with an eruption then followed by a main sustained

eruption.