Simulations of Tsunami Triggered by the 1883 Krakatau Volcanic Eruption:Implications for Tsunami Hazard in the South China Sea

NH41A-1698

YEN JOE TAN1, 2 JIAN LIN1

1Woods Hole Oceanographic Institution, Woods Hole, MA, USA 2Lafayette College, Easton, PA, USA, tany@lafayette.edu

Conclusions

1. Source region model setup

Abstract

Acknowledgements

5. Eruption deposit (ring) vs. collapse (hole)

Manila TrenchPhilippines Trench

7. Tsunami hazard in the South China Sea

4. South China Sea vs. Indian Ocean

6. Two-stage eruption process

3. Linear vs. non-linear simulations

2. Modeling results

Fig. 2. GEBCO 30” bathymetric map of source region used for tsunami modeling. Grid lines represent model resolution.

Fig. 4. Donut-shape “hole and ring” initial sea surface displacement used in modeling. “Hole” represents caldera that is 6 km in diameter and 270 m deep. “Ring” represents eruption deposit from the event. This was constructed based on estimated volume of deposition and change in surrounding bathymetry after the 1883 eruption event (Verbeek, 1884; Sigurdsson et al., 1883).

104.5º 105º 105.5º 106º 106.5º

-6.5º

-6º

-5.5º

Sumatra Island, IndonesiaSumatra Island, IndonesiaSouth China Sea

Bathymetry (km)Bathymetry (km)

-1.1-1.1 0.10.1-2.3-2.3

Java Island Java Island

Indian OceanIndian Ocean

KrakatauKrakatau

South China Sea

AustraliaAustralia

IndonesiaIndonesia

Sri LankaSri Lanka

IndiaIndia

Fig. 3. Sites of historical records around the source region. (1) Merak; (2) Anjer; (3) N. Watcher Island; (4) Java-Batavia; and (5) Tyringin.

Fig. 1. Map of modeling region (google.map.com). Figure to the left shows the Krakatau Island before and after the 1883 eruption.

Fig. 5. Time series of modeled wave propagation in the source region in the first 60 minutes after the eruption. This shows that the tsunami waves entered the Indian Ocean earlier than the South China Sea (SCS), and the maximum amplitude over the Indian Ocean region is slightly greater than that in the SCS.

Table 1. Compiled comparison of reported (Symons, 1888) and calculated travel time from origin and wave height.

Arrival time from origin (hr)

LinearNon-linear

0

10

20

30Observed max. amp.

0 2 4

0

10

0 2 4

0

1

2

0 2 4

Observed arr. time

Location 1 Location 2 Location 3

Ampl

itude

(m)

Fig. 6. Comparison of the computed tsunami waves between models using linear vs. non-linear approximations for locations #1-3 shows that waves generated from linear approximation are less stable. A linear approximation is unsuitable for our study as the source region is very shallow relative to the tsunami wave height.

0 1 2 3 4 5

0

1

2

Arrival time from origin (hr)

Ampli

tude (

m)

Indian OceanSouth China Sea

Fig. 8. Calculated tsunami time series at A and A’. The shallowness and narrowness of the entrace pathway into the SCS could have slowed and limited the tsunami energy transfer into the SCS relative to the Indian Ocean.

Fig. 7. A-A’ profile shows two points of equal distance from the tsunami origin, with A located in the Indian Ocean and A’ located in the South China Sea.

-5.5º

-6º

-6.5º

105º 106º

South China Sea

Indian Ocean

Fig. 9. Computed tsunami waves at locations #1-4 using the “hole and ring”, “hole only”, and “ring only” model. The “hole only” model consistently produces smaller wave amplitude. This suggests that the ring of eruption deposit plays a larger role in generating the large wave amplitude observed during the tsunami event.

Fig. 10. Comparison of computed tsunami waves at location #1 for a single event and a two-stage eruption process with different separation time. The single event has the same source energy as the combined energy of the two-stage eruption while the two-stage eruptions have two events of the same energy. We can see that there is interference between the tsunami waves generated by two separate events, which could result in a second wave of higher amplitude despite a second eruption of the same magnitude. The constructive interference effect is negligible for a two-stage eruption with two or more hours of separation.

0

10

20

30

0 1 2 3 4

0

4

8

Hole and ringHole onlyRing onlyObserved max. amp.Observed arr. time

Arrival time from origin (hr)

Ampli

tude (

m)

Location 1

Location 2

0

1

2 Location 3

0 2 4 6

0

1

2Location 4

Single event One hour separation

Six hours separationTwo hours separation

0 4 8 122 6 10

2

0

-2

0 4 8 122 6 10Arrival time from origin (hr)

Amplit

ude (

m)

2

0

-2

2

0

-2

-4

4

2

0

-2

80˚ 90˚ 100˚ 110˚ 120˚ 130˚ 140˚

-10˚

0˚

10˚

20˚

30˚

Sunda Trench

Manila Trench

Krakatau

Philippine Trench

SubmarinelandslidesSubmarinelandslides

Ryukyu Trench

Manila Trench

Ryukyu Trench

Fig. 11. For countries inside of the SCS, tsunami risk from the Sunda, Java, Ryukyu, and southern Philippines trenches as well as the Krakatau volcanic chains is limited by the narrow and shallow entrance pathways. Therefore, the primary hazard for large tsunami in the SCS could come from tsunami sources inside the SCS, including earthquakes from the Manila and northern Philippines trenches and potential submarine landslides along continental margins inside the SCS.

The 1883 Krakatau eruption in Indonesia was one of the largest recorded volcanic eruptions in recent history. The volcanic eruptions and the associated tsunamis claimed about 36, 000 lives and recorded run-up heights more than 30 m along the coastal regions in the Sunda Strait between the Indian Ocean and the South China Sea. In this study, we investigate the source mechanisms of this tsunami using the non-linear approximation approach of the Cornell Multi-grid Coupled Tsunami Model (COMCOT). Model results reveal that a donut-shape “hole and ring” initial condition for the tsunami source is able to explain the key characteristics of the observed tsunami. A “hole” of about 6 km in diameter and 270 m in depth corresponds to the collapse of the Krakatau volcano on August 27, 1883, while a “ring” corresponds to the eruption deposit from the event. We found that the shallowness and narrowness of the entrance pathway of the Sunda Strait limited the northward transfer of the tsunami energy from the source region into the South China Sea. Instead, the topographic and bathymetric characteristics favored the southward transfer of the energy into the Indian Ocean. This might explain why Sri Lanka and India suffered casualties from this event, while areas inside the South China Sea, such as Singapore, did not record significant tsunami signals. Modeling results further suggest that eruption deposit from the event is more capable in generating large wave amplitudes compared to depression from caldera collapse.

1. A “hole and ring” model is able to explain the observed tsunami propagation characteristics at the five observation sites near the Sunda Strait.

2. Eruption deposit from the event (“ring”) is more capable in generating large wave amplitudes compared to depression (“hole”) from volcanic collapse.

3. The shallowness and narrowness of the entrance pathway of the Sunda Strait might have limited the propagation of tsunami energy into the South China Sea compared to the more open path of tsunami propagation into the Indian Ocean.

4. Non-linear approximation is more suitable for the modeling in our study due to the shallowness of the source region relative to the generated tsunami wave amplitude.

5. There is a possibility of interference between the tsunami waves generated by multiple eruption events and the interference effect depends on the time separation between the consecutive eruptions and the distance from the source.

We express gratitude to the Woods Hole Oceanographic Institution (WHOI) Summer Student Fellowship Program for financial support, as well as to AGU and WHOI for travel grants.

3

LocationReported

travel traveltime fromorigin (hr)

1. Merak, Indonesia 0.75 0.7 35 - 41 4.92. Anjer, Indonesia 0.6 0.5 > 10 63. N. Watcher Island, Indonesia 1.5 1.9 2.4 0.94. Java-Batavia, Indonesia 2.5 2.7 1.8 0.65. Tyringin, Indonesia 0.5 0.4 15 66. Vlakke Hoek, Indonesia unknown 0.6 15 67. Port Blair, Andaman islands 5.5 4.6 0.2 0.018. Galle, Sri Lanka 6.5 4.7 unknown 0.059. Negombo, Sri Lanka 6.5 5.2 unknown 0.0210. Arugam Bay, Sri Lanka 5 5.6 unknown 0.0911. Madras, India unknown 5.7 0.56 0.0612. Negapatam, India unknown 5.8 unknown 0.0713. Cossack, W. Australia 5.25 5.4 1.5 0.114. Geraldton, W. Australia 9 4.9 1.8 0.3

Calculatedtravel traveltime fromorigin (hr)

Reportedwave height

(m)

Calculatedmaximum wave

amplitude(m)

A

A’

15 m0-15 15 m0-15

t = 60 mint = 50 mint = 40 min

t = 30 mint = 20 mint = 10 min

t = 60 mint = 50 mint = 40 min

t = 30 mint = 20 mint = 10 min

-5.9º

-6º

-6.1º

-6.2º

105.2º 105.3º 105.4º 105.5º 105.6º

Initial sea surface displacement (m)

0 150-150

Hole

Ring

-5.5º

-6º

-6.5º

105º 106º

South China Sea

Indian Ocean

Tsunami source

Obs. points

0 25 50 km0 25 50 km

142

5

0 50 km250 50 km25

Expected arrival timefrom 1st stage eruptionExpected arrival timefrom 2nd stage eruption