Tsunami Risk Reduction Measures Phase 2

Transcription

1 Tsunami Risk Reduction Measures Phase 2 November 2009

2 Cover pictures; Initial water displacements (m) for the three northernmost Sunda Arc scenarios of magnitude M 8.55, 8.53 and 8.60 respectively, as well as the M 8.86 Burma fault scenario. Seismicity of the study region for , with symbols differentiating the magnitudes. Merged tsunami hazard map for Sri Lanka. Merged tsunami hazard map for the Philippines Merged tsunami hazard 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

3 Project Project: Tsunami Risk Reduction Measures phase 2 Document No.: R Document title: Evaluation tsunami hazard in Sri Lanka 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: Prepared by: Reviewed by: Kjell Karlsrud Finn Løvholt, Sylfest Glimsdal, Daniela Kühn, Hilmar Bungum, Bjørn Kalsnes, Bjørn Vidar Vangelsten Carl Bonnevie Harbitz Summary This report presents tsunami hazard analyses for the coastline of Sri Lanka. The tsunami threat towards Sri Lanka originates from a range of potential megathrust earthquakes extending from the Burma Fault in the North, to the Sumatra Trench in the South. The return periods associated with the earthquakes ranges from years. A rough regional evaluation based on numerical simulations concludes that maximum water levels of 3-7 m along the East and South coast of Sri Lanka may occur. Even though these large water levels constitutes a threat, a propagation time of 2 hours or more suggests that a well functioning warning system combined with evacuation drills and public awareness could prove effective. Today, the tsunami hazard is lowered somewhat due to the release of stress in the Sumatra-Andaman section of the Sunda Arc after the 2004 Great

4 Summary (cont.) Document No.: R Page: 4 Sumatra earthquake; however the tsunami risk due to earthquakes along the same segment will increase. Elements required performing risk analyses are also highlighted in this report. The elements are also included in a model for tsunami vulnerability and risk at a local level, which is detailed in one of the Appendices of the main project report, NGI (2009).

5 Contents Document No.: R Page: 5 1 Introduction 6 2 Definitions 7 3 Historical tsunamis along the Western Sunda Arc and Burma 10 4 Tsunamigenic earthquake scenarios with return periods 11 5 Tsunami simulations and hazard evaluations for Sri Lanka 12 6 Required input for risk evaluations 14 7 Acknowledgements 14 8 References 14 Review and reference page

6 Document No.: R Page: 6 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. In Sri Lanka, the National Disaster Management Center is the cooperating partner. This report presents tsunami hazard analyses dedicated to the coastline of Sri Lanka (a map is given in Figure 1). Elements required performing risk analyses are also highlighted. In the complete project report (NGI, 2009), findings for all the four countries and more elaborate details of the analysis relevant for Sri Lanka are given. For this purpose, NGI (2009) is extensively cited herein. It is emphasised that this report considers only potential tsunamis of seismic origin, hence tsunamis generated by landslides and volcanoes are omitted. Moreover, it is stressed that a scenario based approach rather than a full probabilistic method is applied in this report. Some of the scenarios described here are based on the Tsunami Phase I risk study of possible future tsunamis towards Thailand (NGI, 2006; Løvholt et al., 2006). However, the current hazard study towards Sri Lanka includes a revised and extended list of large scale scenarios due to the more exposed location of Sri Lanka compared to western Thailand.

7 Document No.: R Page: 7 Figure 1: Map of 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 2. 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 earthquakes, several meters for magnitude 7-9 earthquakes.

8 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 2). 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 mm on a standard Wood-Anderson seismometer located 100 km 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 2). 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 2). Surface elevation Here, defined as the water elevation relative to the mean sea (can be negative or positive). See Figure 2 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.: R Page: 8

9 Figure 2: Definition sketch for tsunami parameters. Document No.: R Page: 9

10 Document No.: R Page: 10 3 Historical tsunamis along the Western Sunda Arc and Burma Figure 3: Source location and year of occurrence for recorded earthquake generated historical tsunamis along the western Sunda Arc and Burma with source information. Yellow markers indicate non seismic or unknown sources, red markers indicate seismic sources. Large stars magnitudes M 8; 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.

11 Document No.: R Page: 11 Figure 3 shows historical tsunami events along the western Sunda Arc and Burma. The historical events reveal 5 megathrust events with magnitude larger than 8.5 along Sumatra. Along the Sunda Arc north of Sumatra, reported magnitudes do not exceed 7.9 (the 1881 earthquake), but the northward rupture of the 2004 Indian Ocean tsunami must be kept in mind. Finally, in 1762, a large earthquake of proposed upper limit of magnitude 8.8 possibly generated a tsunami (Cummins 2007) along the Arakan Fault near Burma and Bangladesh. The existence of large historical earthquakes from southern Sumatra north to Bangladesh illustrates the potential for these fault zones to generate destructive tsunamis over transoceanic distances. 4 Tsunamigenic earthquake scenarios with return periods The seabed displacements due to various earthquake scenarios that may generate tsunamis posing a threat to the coastlines of Sri Lanka are shown in Figure 4. For details on the computation of the seabed displacements, see NGI (2009). For the north-western Sunda Arc and Burma the parameterization essentially comes from an earlier study (Løvholt et al., 2006) where it was concluded that it most likely will take more than 400 years for another event of similar size (>M9) to occur again (the return period is likely to be even longer, see also Jankaew et al., 2008), and that scenario events of the order of M 8.5 may have return periods of the order of 200 years (but occurring less regularly than the megathrust events). It is added that, the northernmost source in Figure 4 complies with the one given by Cummins (2007). Cummins (2007) also estimates a return period of 500 years for a megathrust earthquake along the Burma fault. It is noted that the possibility of another megathrust earthquake of similar magnitude as the 2004 Indian Ocean extending from Sumatra to the Andaman Islands is highly unlikely today (Løvholt et al., 2006; NGI, 2006). However, megathrust earthquakes of magnitudes equal to or larger than 9 may occur along Sumatra, and significant impact of such events towards southern Sri Lanka cannot be excluded. For the south-western part of the Sunda Arc (off western Sumatra), two megathrust scenario earthquakes of magnitudes larger than 9 are defined, as shown in Figure 4. Sieh (2006) reports return periods of years for megathrust events in this region.

12 Document No.: R Page: 12 Figure 4: Left panel: Upper panel: Initial water displacements (m) for the three northernmost Sunda Arc scenarios of magnitudes M w 8.55, 8.53, and 8.60, respectively (the two northernmost ones partly overlapping), as well as the magnitude M w 8.86 Burma fault scenario. Right panel: Initial water displacements (m) of the two south-western Sunda Arc scenarios of magnitudes 9.19 and 9.13 respectively (partly overlapping). 5 Tsunami simulations and hazard evaluations for Sri Lanka For the scenarios shown in Figure 4, tsunami simulations were performed in the Bengal Bay, and for the Sumatra scenarios also for the Indian Ocean, using a dispersive wave propagation model described by NGI (2009). For all simulations, results were compiled for different offshore control points along the Sri Lankan coastline. For each control point, the largest surface elevation from all simulations was extracted. The further amplification of the surface elevation to the maximum water level was computed using the method of amplification factors described by NGI (2009). This method is an approximate way of incorporating wave amplification regionally, and may be used to give a rough overall picture of the tsunami hazard. Figure 5 shows the resulting maximum water level evaluated at different locations along the Sri Lankan coastline. The largest waves are mostly originating from earthquake scenarios between Sumatra and Andaman, and are hence associated with return periods of more than 400 years. However, for some south-eastern locations of the Island, the largest waves originate from earthquakes off southern Sumatra, having return periods of roughly years. Clearly, the resulting maximum water levels displayed in Figure 5 show that tsunamis still represent a severe threat, and that Sri Lanka should be prepared for future tsunamis. On the other hand, the potential maximum water levels, particularly in the southern and

13 Document No.: R Page: 13 western part of Sri Lanka, are clearly smaller than the run-up heights and maximum water levels found for the 2004 Indian Ocean tsunami (Goff et al., 2006). It is finally noted that for all locations along the Sri Lankan coastline, the travel time of the tsunami is generally at least two hours. Such large travel times are expected to increase the success rate of a well functioning warning system. On the other hand, a natural warning of sea withdrawal will not take place in Sri Lanka, as the positive wave will arrive first. Figure 5: Maximum water level caused by credible worst case tsunami scenarios along the Sri Lankan coastline. It is stressed that although the control points are located offshore, the water levels represent shore line values. Rough estimates of the return periods range from approximately years. Three of the southern locations are associated with years return periods (maximum water levels of 1.8, 2.5, and 4.7 m); whereas the other locations are associated with years return periods.

14 Document No.: R Page: 14 6 Required input for risk evaluations The previous chapters briefly present the tsunami hazard towards Sri Lanka for large scale events. Risk analyses, however, also include the effects a tsunami event may have on a society, ranging from number of fatalities to economical or environmental consequences of a disaster. This project is primarily focussed on tsunami risk related to loss of human lives (mortality risk). Key elements in such risk analyses (beyond the hazard analyses described above) are elements at risk and vulnerability. In coming studies, the following information is therefore required: Demographic maps along the coastline showing the population density or numbers versus elevation height. Vulnerability factors related to physical elements (arrival time, housing, building types, number of floors, infrastructure, etc.) and social elements (training, age, gender etc.). Existing and planned early warning systems and mitigation measures. Risk management systems such as emergency plans, awareness procedures, stakeholder processes, etc. In the main project report, NGI (2009) presents a methodology for tsunami risk assessment for an example case study in the city of Batangas, the Philippines. This methodology demonstrates some elements that enter the risk assessment procedure. 7 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 Sri Lanka, the Disaster Management Center, DMC. 8 References Cummins, P.R. (2007). The potential for giant tsunamigenic earthquakes in the northern Bay of Bengal. Nature, 449, Goff, J., Liu, P. L.-F., Higman, B., Morton, R., Jaffe, B.E., Fernando, H., Lynett, P., Fritz, H., Synolakis, C., and Fernando, S. (2006). Sri Lanka Field Survey after the December 2004 Indian Ocean Tsunami. Earthquake Spectra, 22, S3, S155 S172. Jankaew, K., Atwater, B.F., Sawai, Y., Choowong, M., Charoentitirat, T., Martin, M.E., and Prendergast, A. (2008). Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand. Nature, 455, , doi: /nature07373.

15 Document No.: R Page: 15 Løvholt, F., Bungum, H., Harbitz, C.B., Glimsdal, S., Lindholm, C.D., and Pedersen, G. (2006). Earthquake related tsunami hazard along the western coast of Thailand. Nat. Hazards Earth Sys. Sci., 6, NGI (2006). Tsunami Risk Reduction Measures with Focus on Land use and Rehabilitation, NGI report , available at: F:\i\41\KOR\Informasjon\Ekstern\ summary report-complete.pdf NGI (2009). Tsunami Risk Reduction Measures phase 2 Main Report. NGI report no R. Sieh, K. (2006). Sumatran Megathrust Earthquakes - From Science to Saving Lives. Philos. Trans. R. Soc. London A 364(1845), UNESCO-IOC (2006). Tsunami Glossary.

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