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 Project Project: Tsunami Risk Reduction Measures phase 2 Document No.: 20061179-00-226-R Document title: Tsunami Risk evaluations for the Philippines 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, Bjørn Vidar Vangelsten, Regula Frauenfelder, Helge Smebye, Gunilla Kaiser Reviewed by: Carl Bonnevie Harbitz Summary This report presents tsunami hazard analyses dedicated to parts of the Philippines. In agreement with the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the tsunami hazard was studied within three regions, the Manila Trench, the South Western Mindanao, and for far field sources from the Northern Sulawesi. 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. Using a scenario based approach; a merged tsunami hazard map is developed for earthquake generated tsunamis for the Manila Trench study area. The map gives more than 10 m maximum water elevation along large sections of western Luzon Island. In addition, a hindcast of the 1976 Moro Gulf Summary (cont.) Document No.: 20061179-00-226-R Date: 2009-11-06 Page: 4 tsunami is conducted, as well as one landslide generated tsunami scenario for Calapan - Mindoro Island, the latter for demonstration purposes. Sources located along Northern Sulawesi may also affect the Southern part of the Philippines. 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, obtaining 30-120 years from the regional seismicity. However, the scenario return periods are believed to be a few times longer than the lower bound. Moreover, the tectonic convergence rates support higher return periods of 100-1000 years for the scenario magnitudes in question. Inundation modelling has been undertaken for the city of Batangas, which constituted the basis for a more detailed study for demonstration of local tsunami vulnerability and risk assessment. The study resulted in a critical facility map, as well as a mortality risk map identifying the most affected areas in the city. For a Mw 8.2 Manila Trench scenario with a 1 m sea level rise (high tide), the analyses indicates a loss of several hundreds of lives. Due to the possibility for a relatively long warning time for the scenario investigated, a proper Early Warning System, including technical installations and a well organized dissemination system, as well as defined evacuation routes, may be effective for the most affected areas. A barrier of 2 m above sea level properly located will most likely reduce the wave impact significantly and hence reduce the consequences in the most exposed areas of Batangas city. Contents Document No.: 20061179-00-226-R Date: 2009-11-06 Page: 5 1 Introduction 6 2 Definitions 7 3 Historic background 9 4 Results from scenario simulations 12 4.1 South West Mindanao – Cotabato Trench 13 4.2 Northern Sulawesi 15 4.3 Manila Trench 20 4.4 Calapan landslide tsunami scenario – Mindoro Island, the Philippines 24 5 Batangas, the Philippines 27 5.1 Inundation model for the southern magnitude 8.2 Manila Trench scenario 27 5.2 Vulnerability and risk assessment 29 6 Mitigation measures 32 7 Acknowledgements 33 8 References 33 Review and reference page Document No.: 20061179-00-226-R Date: 2009-11-06 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. A map of the study area is shown in Figure 1. This report presents selected tsunami analyses for the Philippines. These include tsunami hazard assessment due to earthquakes along the Manila Trench, a brief hindcast of the 1976 Moro Gulf event, demonstration of numerical methods for simulating landslide generated tsunamis, and demonstration of tools for tsunami risk evaluations and vulnerability for the city of Batangas. In the complete project report (NGI, 2009) the findings for all the four countries and more elaborate details of the analysis relevant for the Philippines are given. For this purpose, NGI (2009) is extensively cited herein. 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.: 20061179-00-226-R Date: 2009-11-06 Page: 7 Figure 1: 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 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 6.0-6.5 earthquakes, several meters for magnitude 7-9 earthquakes. • Flow depth – Water elevation above land during inundation (Figure 2). • 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 – Horizontal penetration of the tsunami from the shoreline. • 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 Document No.: 20061179-00-226-R Date: 2009-11-06 Page: 8 0.001 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.