Tsunami Risk Reduction Measures Phase 2

November 2009

Cover pictures;

Initial water displacements (m) for the Seismicity of the study region for 1963- three northernmost Sunda Arc scenarios 2006, with symbols differentiating the of magnitude M 8.55, 8.53 and 8.60 magnitudes. respectively, as well as the M 8.86 Burma fault scenario.

Merged tsunami hazard Merged tsunami hazard Merged tsunami hazard map for Sri Lanka. map for the Philippines map for Eastern Indonesia

The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the CCOP Technical Secretariat concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

Note: The conclusions and recommendations of this publication have not been specifically endorsed by, or reflect the views of the organizations which have supported the production of this project, both financially and with content.

© Coordinating Committee for Geosciences Programmes in East and Southeast Asia, 2009

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 3

Project

Project: Tsunami Risk Reduction Measures phase 2 Document No.: 20061179-00-227-R Document title: Tsunami Risk evaluations for Indonesia Date: 6 November 2009

Client

Client: CCOP Technical Secretariat Client’s contact person: Niran Chaimanee Contract reference: Contract between CCOP and NGI of 17. April 2008

For NGI

Project manager: Kjell Karlsrud

Prepared by: Bjørn Kalsnes Finn Løvholt, Sylfest Glimsdal, Daniela Kühn, Hilmar Bungum, Helge Smebye Reviewed by: Carl Bonnevie Harbitz

Summary

This report presents tsunami hazard analyses dedicated to the coastlines of eastern Indonesia. Available seismic and tsunami catalogues indicate that there is a high level of seismicity the last 100 years and a large number of historical tsunamis within the last 300-400 years. It is clear that the main areas of tsunami generation are located close to the major fault zones. The database shows that a large number of tsunamis have occurred in eastern Indonesia, and that the sources are distributed to many regions. Even so, most other efforts on tsunami simulations and tsunami warnings nowadays are focusing on western Sumatra and Southern Java.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 4

In agreement with the Center for Volcanology and Geological Hazard Mitigation in Indonesia (CVGHM), the hazard was studied within four regions of eastern Indonesia (Bali/Flores, the Banda Sea, North Sulawesi, and Irian Jaya). Tsunami hazard maps for earthquake generated tsunamis are developed by applying numerical simulations for a total of eleven ‘credible worst case scenarios’. In addition, one landslide generated tsunami scenario for Ceram Island is conducted for demonstration purposes.

Return periods for the individual earthquake scenarios are not quantified. Lower bound return periods for the earthquake magnitudes comparable to the scenarios are found within some of the study regions, with the shortest periods 30-40 years obtained from the regional seismicity. However, the scenario return periods are believed to be a few times longer than the lower bound. The tectonic convergence rates support higher return periods of 100-1000 years for the scenario magnitudes in question. It is noted that the scenarios investigated are considered worst case with respect to both strength and location, and it is assumed that they will contribute to a large part of the total risk compared to smaller scenarios.

Figure 1: Merged regional tsunami hazard map for parts of eastern Indonesia. High population density is visualised using the global GRUMP dataset, with dark colours indicating high density.

A tsunami hazard map obtained by combining the results for all the different study regions is shown in Figure 1. The map clearly shows that large parts of eastern Indonesia may be subject to tsunami maximum water level of several meters, and in some areas more than 10 m. It is emphasized that this region is an extremely complex region tectonics wise, and that a great amount of work remains before a

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 5 more solid understanding of tsunami generation and its underlying sources can be achieved. It should also be kept in mind that the present scenarios do not give a full coverage of all the possible ‘worst case scenarios’ that could occur in this region, even though the ones contributing most to the hazard are included. It is important to note also that tsunamis generated by landslides and volcanoes are not included in the hazard map, as well as earthquake sources in areas like Southwest Sulawesi, Halmahera, the southern part of the Banda Sea, as well as transpacific events. Therefore, if any such additional sources are considered to be as likely as the ones included, additional calculations should be performed, and added to the merged hazard map in the future. It is emphasized that only regional inundation maps are produced. The need for additional local inundation analyses should be assessed for exposed locations. The present study is limited in size and depth and the results should therefore be considered as preliminary. However, even a larger study of this kind would be uncertain, due to the fact that all such studies are based on prediction of future events. Even so, much knowledge of societal importance is now available from the present study and should be put into action.

Contents Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 6

1 Introduction 7 2 Definitions 8 3 Historic background and definitions of study areas 10 4 Results for each study region 13 4.1 Bali and Flores Sea region – Nusa Tenggara 13 4.2 Banda Sea and the Moluccas 18 4.3 Northern Sulawesi 23 4.4 Irian Jaya 27 4.5 South West Mindanao – Cotabato Trench 30 4.6 Demonstration of the modelling of landslide generated tsunamis – a case study for Ceram Island 33 5 Merged hazard maps and input to regional hazard assessment 37 5.1 Use of the merged hazard map – applications for local hazard mapping 39 6 Acknowledgements 42 7 References 42

Review and reference page

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 7

1 Introduction

The project “Tsunami Risk Assessment and mitigation in S&SE Asia – Phase 2” has been financed by The Norwegian Ministry of Foreign Affairs (MFA). The Coordinating Committee for Geoscience Programmes in East and Southeast Asia (CCOP), through their Technical Secretariat in Bangkok, acted as the project responsible institution towards MFA. The Norwegian Geotechnical Institute (NGI) had the role as Technical Executing Organisation (TEO). NGI subcontracted NORSAR for performing the seismological analyses required. The project was contracted in 2008 with four Asian countries: Indonesia, the Philippines, Vietnam, and Sri Lanka. The main goals of the project have been to reduce the tsunami risk in South and Southeast Asia by:

• Enhanced assessment of tsunami hazard and recommendations of risk mitigation measures in specified regions • Enhanced capacities of hazard assessment and risk reduction for regional, national, and local institutions

The detailed scope of work (SoW) for the invited countries the Philippines, Indonesia, Vietnam, and Sri Lanka varied according to the needs defined from previous tsunami hazard assessments and the capabilities of the individual countries. The SoW’s were agreed in project meetings with the countries in the early phase of the project. A map of the study area is shown in Figure 2.

This report presents tsunami hazard analyses dedicated to large parts of eastern Indonesia. In the complete project report (NGI, 2009) findings for all the four countries and more elaborate details of the analyses relevant for eastern Indonesia are given. For this purpose, NGI (2009) is extensively cited herein.

Historical tsunamis and earthquakes were investigated using available catalogues. A regional tsunami database was established, indicating that there is a high level of seismicity the last 100 years and a large number of historical tsunamis within the last 300-400 years. It is clear that the main areas of tsunami generation are located close to the major fault zones. The database shows that a large number of tsunamis have occurred in eastern Indonesia and the Philippines, and that the sources are distributed over many regions. Even so, most other efforts on tsunami simulations and tsunami warnings are nowadays focusing on the western part of the Sunda Arc, from the Andaman Islands to southern Java.

It is emphasised that the hazard evaluations in this report considers only potential tsunamis of seismic origin. Modelling of tsunamis generated by landslides is included for demonstration purposes only. Moreover, it is stressed that a scenario based approach rather than a full probabilistic method is applied in this report.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 8

Figure 2: Map of the study area including Indonesia, the Philippines, Vietnam, and Sri Lanka.

2 Definitions

Below, some definitions of technical key terms used in this text are given to help the reader. As far as possible, compatibility with the UNESCO-IOC tsunami glossary (UNESCO-IOC, 2006) is endeavoured. In addition, a brief definition sketch defining the parameters related to the tsunami inundation process is given in Figure 3. • Fault - A fracture or a zone of fractures along which displacement has occurred parallel to the fracture. Earthquakes are caused by a sudden rupture along a fault or fault system; the ruptured area may be up to several thousand square kilometers. Relative movements across a fault may typically be tens of centimeters for magnitude 6.0-6.5 earthquakes, several meters for magnitude 7-9 earthquakes. • Flow depth – Water elevation above land during inundation. • Hazard - Probability that a particular danger (threat) occurs within a given period of time. Here, the tsunami hazard is the maximum water level associated with a scenario return period. • Inundation distance – Maximum horizontal penetration of the tsunami from the shoreline (see Figure 3). • Magnitude - A measure of earthquake size at its source. Magnitude was defined by C. Richter in 1935 as: “The logarithm to the trace amplitude in 0.001 mm on a standard Wood-Anderson seismometer located 100 km

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 9

from the epicenter” The Wood-Anderson instrument measures the responses in the period range near 1 sec. Other magnitude scales have later been devised based on the responses measured in other period ranges, and on maximum amplitudes of specific wave forms. In this report, we mostly refer to the moment magnitude (with abbreviation Mw). The moment magnitude is based on the seismic moment computed directly from source parameters or from long period components in the earthquake record. Symbol M is also used for this magnitude. • Maximum water level – Here, defined as the largest water elevation above the still water level (see Figure 3). • Probability - A measure of the degree of certainty. This measure has a value between zero (impossibility) and 1.0 (certainty). It is an estimate of the likelihood of the magnitude of the uncertain quantity, or the likelihood of the occurrence of the uncertain future event. • Return period - Average time period between events of a given size in a particular region, cycle time. • Risk - Measure of the probability and severity of an adverse effect to life, health, property, or the environment. Quantitatively, Risk = Hazard × Potential Worth of Loss. This can be also expressed as “Probability of an adverse event times the consequences if the event occurs”. • Run-up height – Water level above the still water level at the inundation limit (see Figure 3). • Surface elevation – Here, defined as the water elevation relative to the mean sea (can be negative or positive). See Figure 3 for a definition sketch. • Threat - The natural phenomenon that could lead to damage, described in terms of its geometry, mechanical and other characteristics. The danger can be an existing one (such as a creeping slope) or a potential one (such as a tsunami). The characterization of a danger or threat does not include any forecasting. Here, the tsunami threat is mostly reported as the maximum water level. • Trench - Topographic depressions of the sea floor. • Vulnerability - (1) The degree of loss to a given element at risk, or set of such elements, resulting from an event of a given magnitude or intensity, usually expressed on a scale from 0 (no loss) to 1 (total loss). (2) Degree of damage caused by various levels of loading.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 10

Figure 3: Definition sketch for tsunami parameters.

3 Historic background and definitions of study areas

Figure 4 shows source locations and source type for historical tsunamis recorded throughout the last 400 years of recorded history obtained by combining different tsunami and earthquake databases (more details are given in NGI, 2009). The database shows that a large number of tsunamis have occurred in eastern Indonesia, and that the sources are distributed to many regions. Even so, most efforts on tsunami simulations and tsunami warnings nowadays are focusing on western Sumatra. Naturally, the results shown in Figure 4 should be used with caution, as there are large uncertainties related to these data. Most of the tsunamis are reported to be purely due to tectonic events, whereas the number of volcanic/landslide generated tsunamis are also significant although they are clearly fewer. The non-seismic sources tend to be confined to specific source regions.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 11

Figure 4: Source location and year of occurrence for recorded earthquake generated historical tsunamis in eastern Indonesia with source information. Yellow markers indicate non seismic or unknown sources, red markers indicate seismic sources. Large stars magnitudes M ≥ 8.5; small stars 8.5 > M ≥ 8.0; squares 8.0 > M ≥ 7.5; circles M < 7.5; asterisk, no magnitude reported. Upward-pointing triangles indicate volcanoes or combinations of volcanoes and other sources. Downward-pointing triangles indicate landslides or landslides and earthquakes. Diamonds indicate unknown sources.

In agreement with CVGHM, four study regions (Bali/Flores, the Banda Sea, North Sulawesi, and Irian Jaya) for the eastern part of Indonesia were defined as shown with red squares in Figure 5. The figure illustrates very clearly the high level of seismicity recorded since 1963, showing that earthquakes above magnitude 7.0 are not at all rare. At the same time Figure 5 also depicts transform faults and trenches. Comparing Figure 4 and Figure 5 it is clear that the main areas of tsunami generation are located close to the major fault zones.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 12

Figure 5: Seismicity in eastern Indonesia 1963-2006, with symbols differentiating the magnitudes. The largest red stars are for earthquakes above magnitude 8, the orange magnitudes 7.5-8, whereas the blue and cyan colored stars depict the smaller events. The black lines are transform faults and the green lines are trenches (subduction zones).

The tectonic situation in eastern Indonesia can be briefly summarized as follows (Kreemer et al., 2000): • Along the Java trench west of the Sumba Island the subduction involves about 150 Ma old oceanic lithosphere. • East of Sumba, the Australian continent collides with the Banda Arc and New Guinea. • The Flores and Wetar back arc thrusts are more active seismically than the subduction thrust fault near Timor trough (McCaffrey, 1988), suggesting a jump in the locus of convergence from Java trench to back arc east of 118°. • New Guinea accommodates an oblique convergence (at an angle of about 60°) between the Pacific and Australian plates, including also strain partitioning. • The Molucca sea collision zone and the North Sulawesi trench features also high seismic activity associated with subduction.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 13

4 Results for each study region

Results from tsunami simulations for each of the four study regions (Bali/Flores, Banda Sea and the Moluccas, Northern Sulawesi, Irian Jaya) are briefly summarized below. The selection of the various earthquake scenarios are performed by combining information of historical tsunami records and seismicity. Below, the historical events from the regional tsunami database developed within this project as well as seismic focal mechanisms are shown for the various regions. Table 1 shows the colour legend for the figures. The analyses of the seismic data leading to the scenarios are elaborated by NGI (2009) that also includes the 3D visualization of the seismicity. For all the study regions in question, the lower bound return period for earthquakes with the same magnitude as the scenario earthquakes are given, however the individual scenario return periods are not quantified. These are in most cases expected to be a few times longer than the lower bound. Tectonically derived return periods for this region range from 100 – 1000 years. It is stressed that the range of the return periods is certainly small enough to warrant implementation of mitigation and precautionary measures.

Table 1: Colour legends used for subsequent figures. The abbreviation m.w.l. indicates maximum water level.

Tsunami surface elevation and Earthquake magnitudes run-up

The scenario earthquake magnitudes represent the upper range of expected, but still realistic future events, and represent so called ‘credible worst case scenarios’. The initial seabed displacement is computed by a standard dislocation model (Okada, 1985) and the tsunami propagation is modeled using the dispersive GloBouss model. Rough estimates of the maximum water level are computed using the method of amplification factors. Details on the computational methods are described by NGI (2009).

4.1 Bali and Flores Sea region – Nusa Tenggara

The seismicity of the Bali/Flores region (Figure 6) is well organized along the subduction zone with an increased seismicity in the deepest parts. The prominent tectonic features of the Bali/Flores region are the Java Trench as well as the Flores and Wetar thrust. For the tsunami modelling, two shallow fault planes belonging

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 14 to the Flores and Wetar thrust as well as two deep fault planes at 200 km depth related to the Java trench subduction zone are chosen, all fault planes situated north of Sumbawa Island and Flores Island, respectively.

The earthquake scenarios defined for this region are shown in Figure 7 (initial water displacements). Based on the regional seismicity and the historical events shown in Figure 6, it was found that M 7.8 earthquake represents ‘credible worst case scenarios’ for the Flores thrust, whereas a potential for larger magnitudes (8.4) were found for the deep Java Trench. However, due to the large depth of the Java Trench scenarios, their tsunami potential is quite limited. The lower bound return period of a magnitude 7.8 earthquake within this study region shown in Figure 5 is approximately 40 years, and is obtained from the regional seismicity. However, the return period for an individual scenario at a specific location is believed to be a few times longer.

Figure 6: Upper panel: historical tsunami maximum run-up heights / maximum water levels colour-coded. Lower panel: map view of fault planes (red rectangles: shallow fault planes, black rectangles: deep fault planes; green lines: trenches) and CMT solutions colour-coded according to magnitude. For legend see Table 1.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 15

Figure 7: Upper panel, initial water displacements (m) for the Mw 7.8 scenarios for the Bali and Flores Sea region. Lower panel, initial water displacements (m) for the Mw 8.4 scenarios for the Indian Ocean-Australian plate subducted at the Java trench. Note the difference in colour scale.

Figure 8 shows the simulated maximum surface elevations for the two magnitude 7.8 scenarios shown in Figure 7. Estimated maximum water level in regions relatively close to the sources using the method of amplification factors (NGI, 2009) are shown in Figure 9. The magnitude 7.8 scenarios may cause maximum water level of more than 5 m most notably along the islands of Lombok, Sumbawa, and Flores, but also with more widespread effects in the order of 2-5 m maximum water level. Finally, it is noted that tsunami simulations of the two deep magnitude 8.4 scenarios shown in Figure 7 gave relatively small wave heights of less than 1 m; we therefore omit details of the magnitude 8.4 simulations.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 16

Figure 8: Maximum simulated surface elevation and directivity for the Mw 7.8 Bali Sea scenario (upper panel), and the Mw 7.8 Flores Sea scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 17

Figure 9: Estimated maximum water level (m) using the method of amplification factors for the Mw 7.8 Bali Sea scenario (upper panel), and the Mw 7.8 Flores Sea scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 18

4.2 Banda Sea and the Moluccas

The seismicity in the Banda Sea region (Figure 10) is complicated and needs to be broken into smaller units in order to be delineated and understood. The Banda Sea region is characterized by two different subduction systems: the first is the Java trench – Timor trough – Aru trough system and the second the Ceram trough subduction zone. The relationship between both is undetermined. Earthquake focal mechanisms (Harvard CMT solutions) are variable within the whole region, and the distribution of seismicity in the region is very complex. Okal and Reymond (2003) computed the focal mechanism of the great Banda Sea earthquake on the 1st February 1938, which took place on the extreme eastern boundary of the Weber Basin and is one of the 10 largest magnitudes ever computed (Kanamori, 1977).

Based on the tsunami catalogues and regional seismicity, the possibility of magnitude 8.1-8.2 earthquake sources cannot be excluded. The lower bound return period for a randomly located magnitude 8.1-8.2 earthquake within the Banda Sea study region in Figure 5 is approximately 50-70 years. It is noted that the return period for the individual scenario is expected to be a few times longer. For a more complete discussion, see NGI (2009). Proposed scenarios for the tsunami simulations are shown in Figure 11 (initial water displacements), and consist of a magnitude 8.2 scenario north of Buru Island (at the Ceram trough subduction zone), a second magnitude 8.0 scenario situated on the thrust fault southwest of Ambon, as well as a composite magnitude 8.1 scenario of three fault planes located at the Weber Basin.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 19

Figure 10: Upper panel: historical maximum run-up heights / water levels colour- coded. Lower panel: map view of fault planes (red rectangles: shallow fault planes; green lines: trenches) and CMT solutions colour-coded according to magnitude. For legend see Table 1.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 20

Figure 11: Scenarios for the Banda Sea, initial water displacements (m).

Figure 12 shows the simulated maximum surface elevations for the Banda Sea and the Moluccas scenarios. Estimated maximum water level in regions relatively close to the sources using the method of amplification factors are shown in Figure 13. The figures clearly show that all scenarios give large local wave heights, but that the far-field effects are only moderate. The upper panel in Figure 13 shows that the magnitude 8.2 Banda Sea scenario gives the largest maximum water level on Obi and Sula islands north of the source, with a maximum of 17 m. The mid panel in Figure 13 shows that the magnitude 8.0 Banda Sea scenario gives maximum water level above 12 m at Ambon Island and Western Ceram, with a maximum maximum water level of 17 m. At more distant locations this scenario gives 5-10 m maximum water level for many islands in the far field. The lower panel in Figure 13 shows that the magnitude 8.1 Banda Sea scenario generates maximum water level up to 10 m southeast off Ceram, and 7-8 m along south western Ceram. As for the magnitude 8.0 scenario, the 8.1 scenario may give rise to maximum water level waves up to 5 m in many islands located in the far-field.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 21

Figure 12: Maximum simulated surface elevation and directivity for the Mw 8.2 Banda Sea scenario (upper panel), Mw 8.0 Banda Sea scenario (mid panel), and the Mw 8.1 Banda Sea scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 22

Figure 13: Estimated maximum water levels (m) using the method of amplification factors: Mw 8.2 Banda Sea scenario (upper panel), Mw 8.0 Banda Sea scenario (mid panel), Mw 8.1 Banda Sea scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 23

4.3 Northern Sulawesi

The seismicity (Figure 14) in North Sulawesi region is too complex to be easily summarized. A number of crustal blocks take part in a rotational plate motion. Scenarios are defined as dip-slip earthquakes located along the Minahassa Trench subduction zone north of Sulawesi. Two scenario locations north of the Minahassa trench are shown in Figure 14 and Figure 15. Since the seismicity is low at shallow depths northwest of the Gorontalo region, the faultplane is buried here, whereas the fault plane north of Minahassa peninsula is situated at the surface. Proposed scenarios are shown in Figure 15 (initial water displacements). Both scenarios are pure dip slip events along the North Sulawesi Trench.

It was found that magnitude 7.9 earthquakes represent ‘credible worst case scenarios’ for this area. The lower bound return period for a magnitude 7.9 earthquake within the North Sulawesi study region in Figure 5 is approximately 30 years, but again we stress that the return period of the individual scenarios is believed to be a few times longer. For a more complete discussion, see NGI (2009).

Figure 14: Upper panel: historical tsunami run-up heights / water levels colour- coded. Lower panel: map view of fault planes (red rectangles: shallow fault planes; green lines: trenches, black lines: transform faults) and CMT solutions colour-coded according to magnitude. For legend see Table 1.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 24

Figure 15: Mw 7.9 scenarios in the North Sulawesi Trench, initial water displacements (m).

Figure 16 shows the simulated maximum surface elevations for the two magnitude 7.9 scenarios. Estimated maximum water level in the Celebes Sea using the method of amplification factors are shown in Figure 17. For both scenarios, large maximum water level heights of more than 10 m are found along the northern coastline of Sulawesi, with a maximum of 18 m in the northern Gorontalo region. Moreover, significant maximum water level of 2-7 m is found in the far-field even as far north as in Malaysia and the Philippines.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 25

Figure 16: Maximum simulated surface elevation and directivity for the eastern Mw 7.9 Sulawesi scenario (upper panel) and the western Mw 7.9 Sulawesi scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 26

Figure 17: Estimated maximum water level (m) using the method of amplification factors for the eastern Mw 7.9 Sulawesi scenario (upper panel) and the western Mw 7.9 Sulawesi scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 27

4.4 Irian Jaya

For the Irian Jaya region the situation is different from three other study regions in that the seismicity there is mostly crustal (shallow depth hypocenters), even though the region is known for both its high seismicity and its frequent tsunamis. New Guinea accommodates the highly oblique convergence between the Pacific and the Australian Plate. Based on the regional seismicity and the historical events in Figure 18 (initial water displacements), it was found that magnitude 8.5-8.6 earthquakes represent ‘credible worst case scenarios’ for this area. We propose three scenarios at the New Guinea trench as shown in Figure 19 . The lower bound return period for randomly located earthquakes of this magnitude within the Irian Jaya study region in Figure 5 is approximately 150 years. It is noted that the return period for an individual scenario at a specific location is believed to be a few times longer. For a more complete discussion, see NGI (2009).

Figure 18: Upper panel: historical tsunami run-up heights / water levels colour- coded. Lower panel map view of fault planes (red rectangles: shallow fault planes; green lines: trenches, black lines: transform faults) and CMT solutions colour-coded according to magnitude. For legend see Table 1.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 28

Figure 19: Mw 8.5-8.6 scenarios along the New Guinea Trench. Initial water displacements (m).

Perspective plots showing the simulated maximum surface elevations for the New Guinea scenarios are displayed in

Figure 20. The spatial distribution of the high maximum surface elevation is constrained locally due to the topography in this region. For all three scenarios, large maximum water level heights of up to 10-12 m are found along the northern coastline of Papua facing the New Guinea Trench, using the method of amplification factors (Figure 21).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 29

Figure 20: Maximum simulated surface elevation and radiation pattern for the western Mw 8.6 New Guinea scenario (upper panel), the Mw 8.5 New Guinea scenario (mid panel), and the eastern Mw 8.6 New Guinea scenario (lower panel).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 30

Figure 21: Estimated maximum water level (m) using the method of amplification factors for New Guinea scenarios. Western Mw 8.6 New Guinea scenario (upper panel), the Mw 8.5 New Guinea scenario (mid panel), and the eastern Mw 8.6 New Guinea scenario (lower panel).

4.5 South West Mindanao – Cotabato Trench

Historical tsunamis and seismicity for the South West Mindanao is shown in Figure 22. The Philippine earthquake on the 16th of August 1976 was, although very large, not associated with the dominant tectonic feature of the Philippine trench, but with a less prominent trench system in the Moro Gulf (North Celebes Sea), the Cotabato Trench. Bathymetric data reveal the presence of a trench striking north-south in the region of the earthquake and curving west—north-west

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 31 to east—south-east paralleling the southern Mindanao coastline. Since the 1976 event caused a locally destructive tsunami, this location is chosen for a scenario.

Figure 22: Upper panel: source location of historical tsunamis with maximum run-up / water levels colour-coded. Lower panel: map view of fault planes (red rectangles: shallow fault planes; green lines: trenches, black lines: transform faults) and CMT solutions colour-coded according to magnitude. For legend see Table 1.

The initial surface displacement for a scenario mimicking the 1976 Cotabato earthquake and tsunami are shown in Figure 23. The earthquake parameters, i.e. the shear strength, dip angle, length, width, and slip were first based on the paper by Stewart and Cohn (1979), and next updated after preliminary numerical tsunami simulations and discussions (Okal 2009, pers. comm.). Moreover, Okal (2009, pers. comm.) pointed out that the location and strength of the 1976 source hypocentre was highly uncertain due to the limited quality of the seismic traces.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 32

Figure 23: Mw8.1 Cotabato trench scenario. Initial water displacement (m).

Figure 24 shows the simulated maximum surface elevations and the estimated maximum water elevation using the method of amplification factors (NGI, 2009) for the Cotabato Trench scenario. Maximum-water levels of 3-8 m are found in the Moro Gulf and along the South West Mindanao, which is roughly consistent with reported run-up heights / maximum water levels from field surveys indicating a range of about 3.5-9 m with the highest value in Lebak close to the source (Badillo and Astilla, 1978).

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 33

Figure 24: Upper panel, maximum simulated surface elevation and directivity for the Mw 8.1 Cotabato Trench scenario. Lower panel, estimated maximum water level (m) using the method of amplification factors for the Mw 8.1 Cotabato Trench scenario.

4.6 Demonstration of the modelling of landslide generated tsunamis – a case study for Ceram Island

The 30th September 1899 an earthquake and a subsequent tsunami occurred in Ceram Island (Verbeek, 1901). The earthquake resulted in several slumps/landslides in the area, and it is believed that these resulted in several smaller tsunamis. A tsunami simulation due to a synthetic landslide case offshore the village of Paulohi, Figure 25, is demonstrated here. In Yudhicara (2008) the field observations are described. The motivation for this study is to demonstrate modelling techniques for landslide generated tsunamis, as well as illustrating the required dimensions and velocities. However, the simulations should not be interpreted as a hindcast of the 1899 event. Although major slumps were detected at the shoreline, detailed submarine geomorphological data for the historical landslides is not available. Conducting the necessary seabed investigations for detecting landslides would require large resources.

The landslide is located south of Paulohi and is given a length of 0.5 km, a width 0.5 km, and a thickness 40 m. The landslide is rounded to avoid artificial grid noise, giving a volume of approximately 13 Mm3. The maximum velocity of the slide is 20 m/s with a total run-out of 4 km to the south-south-east. For the

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 34 tsunami generation and propagation we have applied the dispersive GloBouss model (NGI, 2009) with a grid resolution of approximately 100 m (0.001o). The grid was composed by a GEBCO 0.5’ grid merged with local fine grid data (BAKOSURTANAL, 1997). However, due to lack of high-resolution data for the shallowest part of the domain (water depths of about 1-5 m) and topography on land, some irregularities are found at the shoreline.

Some snapshots of the surface elevations after 2 and 5 min (Figure 25), the maximum surface elevation of the slide scenario (Figure 26), and some mariograms (time history of surface elevation) are also extracted (Figure 27). According to the registered wave heights, Paulohi (point 2) was struck by the generated tsunami leading to a run-up height / maximum water level of at least 9 m. At Amahai (point 6) the observed run-up height / maximum water level was 8.3 m. The modelling indicates a run-up at Paulohi of the same order, but the landslide generates smaller waves than observed near Amahai. Note that only surface elevations are extracted in Figure 27 at the locations (2-6), and that inundation simulations are not conducted. For the run-up, a further amplification is expected.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 35

Figure 25: Upper panel, location of the submarine landslide scenario at Paulohi, Ceram Mid and lower panels, snapshots of the surface elevations after 2 and 5 minutes, respectively.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 36

Figure 26: Simulated maximum surface elevation for the landslide scenario at Paulohi, Ceram.

Figure 27: Time series evolution of the simulated landslide generated waves. The locations of the time series are given in Figure 25.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 37

5 Merged hazard maps and input to regional hazard assessment

A rough tsunami hazard map for parts of eastern Indonesia is obtained through a compilation of the maximum water level for the individual earthquake tsunami scenarios described above, and is visualized in two different ways in Figure 28. In addition, results from a tsunami scenario simulation with the earthquake source at the Cotabato Trench, the Philippines, are included (NGI, 2009). As a result, a total of 11 earthquake scenarios are included. It is emphasized that tsunamis originating from the Indian Ocean south of Java and west of Sumatra are not included, and that transpacific tsunamis are also not assessed. The earthquake scenario magnitudes range from Mw7.8 to Mw8.6, and the lower bound return periods for such magnitudes from 30 to 160 years depending on the magnitude and location. As repeatedly stressed above, the individual scenario return periods are believed to be a few times longer, which roughly indicate return periods above 100 years in most cases. This is supported by the tectonic convergence rates that range from 100 - 1000 years.

It is emphasized that this region is an extremely complex region tectonics wise, and that a great amount of work remains before a more solid understanding of tsunami generation and its underlying sources can be achieved. It should be kept in mind also that the present scenarios do not give a full coverage of all the possible ‘worst case scenarios’ that could occur in this region, even though the ones contributing most to the hazard should be included.

It is important to note also that tsunamis generated by landslides and volcanoes are not included in the hazard map, as is the case also for earthquake sources in areas like Southwest Sulawesi, Halmahera, and the southern part of the Banda Sea. Therefore, if any such additional sources are considered to be as likely and powerful as the ones included, additional calculations should be performed, and added to the merged hazard map in the future.

It is emphasized that only regional inundation maps are produced. The need for additional local inundation analyses should be assessed for exposed locations.

The present study is limited in size and depth and the results should therefore be considered as preliminary. However, even a larger study of this kind would be uncertain, due to the fact that all such studies are based on prediction of future events. Even so, much knowledge of societal importance now is available from this study and should be put into action.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 38

Figure 28: Merged tsunami hazard map for the study areas in eastern Indonesia included in this report. Upper panel, 3D visualisation with vertical bars representing the maximum water level. High population density is visualised using the global GRUMP dataset, with dark colours indicating high density. Lower panel, 2D visualisation where the maximum water level is depicted using coloured markers.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 39

5.1 Use of the merged hazard map – applications for local hazard mapping

Due to the vast and complex coastlines of eastern Indonesia, the task of performing local inundation analysis and detailed hazard maps is formidable. Whereas the mapping of some hotspot locations has already been performed by others, there may be other locations that lack such detailed hazard maps. At locations where detailed inundation analysis and hazard maps are lacking, utilization of the merged regional maps for producing a rough local hazard map may be a rational alternative. The use of the merged hazard maps to create inundation maps are exemplified below.

The merged tsunami hazard map in Figure 28 shows only a resampled part of the computed maximum water level data. However, the merged maximum water level data are provided densely, with the distance between each point along the shoreline varying from several hundred meters to less than one hundred meters depending on the shoreline curvature. Examples of the maximum water level data at full resolution are shown in the upper panels in Figure 29 and Figure 30 along the shorelines of the provincial capitals of Ambon and Jayapura.

In the lower panels in Figure 29 and Figure 30, the process of generating inundation maps from the maximum water level data provided at the shoreline is illustrated using ASTER topographic maps (a similar procedure was performed using SRTM, but is not shown here). However, it must be noted that global elevation data such as ASTER and SRTM are generally hampered with artificial data that may lead to underestimation of the inundation zone. The land elevation may be increased due to buildings, forest, etc., that are smeared out over the pixels of 30 m × 30 m, and may lead to false blocking of the inundation paths. The projection of the merged maximum water levels to create hazard maps locally is therefore dependent on fine grid topographic maps for a proper coverage of the inundated area. On the other hand, in many circumstances the amplification method is conservative, i.e. giving high estimates of the maximum water level. Possible overestimation is due to lack of friction and breaking in the method of amplification factors.

The inundated areas shown in the lower panels in Figure 29 and Figure 30 are obtained by assuming a constant water level elevation to the tsunami. The inundated areas are constrained to connected parts. For Ambon and Jayapura, the maximum inundation distance is typically 1-2 km or less. Encountering areas of large flat flood plains, this technique may lead to artificially large inundation distance. The maximum inundation distance should then be limited through an analytical formula taking into account the bottom friction, or alternatively, detailed inundation analyses should be applied.

Local knowledge of the additional sea level variations due to tides and storm surges, and even sea level rise due to warming, may be added to the shoreline maximum water level to give additional inundation.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 40

Figure 29: Visualisation of an inundation map generated using the regional hazard data for the city of Ambon. Upper panel, maximum water levels using the amplification factors near the shoreline. Lower panel, inundation height using the ASTER elevation data.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 41

Figure 30: Visualisation of an inundation map generated using the regional hazard data for the city of Jayapura. Upper panel, maximum water levels using the amplification factors near the shoreline. Lower panel, inundation height using the ASTER elevation data.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 42

6 Acknowledgements

The NGI and NORSAR staffs in charge of producing this report greatly acknowledge the contributions made from the coordinating committee CCOP under the guidance of Mr. Niran Chaimanee, and from our partner in Indonesia, Center for Volcanology and Geological Hazard Mitigation in Indonesia (CVGHM).

7 References

Badillo, V.L. and Astilla, Z.C. (1978). Moro Gulf tsunami of 17. August 1976. Report to the special committee on tsunami warning system, National Committee on Marine Sciences, National Science Board, the Philippines. BAKOSURTANAL (National Coordinating Agency for Survey and Mapping and Hydro-Oceanography of Indonesian Navy - DISHIDROS) (1997). Map of Coastal Environment of Indonesia, Scale 1 : 50.000, Sheet No. 2612-10, MASOHI, published. Kanamori, H. (1977). The energy release in great earthquakes. J. Geophys. Res., 82, 2981-2987. Kreemer, C., Holt, W.E., Goes, S., and Govers R. (2000). Active deformation in eastern Indonesia and the Philippines from GPS and seismicity data. J. Geophys. Res., 105 (B1), 663-680. McCaffrey, R. (1988). Active tectonics of the eastern Sunda and Banda arcs. J. Geophys. Res., 93 (B12), 15163-15182. NGI (2009). Tsunami Risk Reduction Measures phase 2 – Main Report. NGI report no. 20061179-00-3-R. Okada, Y. (1985). Surface deformation due to shear and tensile faults in a half- space. Bull. Seismic Soc. of Am., 74 (4), 1135-1154. Okal (2009). Epicenter location of the 1976 Moro Gulf tsunami, personal communication. Okal, E.A. and D. Reymond (2003). The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event. Earth Plan. Sci. Let., 216, 1-15. Soloviev, S.L. and Go, C.N. (1984). Catalogue of Tsunamis on the Western Shore of the Pacific Ocean. Nanka Publishing House, Moscow, 1974 (In Russian, see Canadian Translation of Fisheries and Aquatic Sciences No. 5077, KIA OS2, 1984, 439 pp for English translation). Stewart, G.S. and Cohn S.N. (1979). The 1976 August 16, Mindanao, Philippine earthquake (MS = 7.8) – evidence for a subduction zone south of Mindanao. Geophys. J. R. Astr. Soc., 57, 51-65. UNESCO-IOC (2006). Tsunami Glossary.

Document No.: 20061179-00-227-R Date: 2009-11-06 Page: 43

Verbeek, D.R.D.M. (1901). Aard- en zeebeving op Ceram den 30sten September 1899, Kort Verslag. Met Een Blad Teekeningen, Batavia Landsdrukkerij, 1900 (in Dutch, see Soloviev and Go (1984) for English translation). Yudhicara (2008). Study on underwater landslide – it’s implication to tsunami potential along the southern coast of Ceram, Indonesia. Research Proposal. Geological Agency of Indonesia.