The megathrust earthquake that struck near Indonesia on 26 December 2004 at 0h58′53″ UTC (+7h for Thailand local time) was likely the 3rd largest earthquake ever recorded [Stein and Okal, 2005]. From its epicenter, located 80 km west of the coast of northern Sumatra (at approximately 95°51′W, 3°25′N), the earthquake proceeded approximately northward, rupturing 1200–1300 km of the Andaman-Sunda trench in about 8–10 min [Ammon et al., 2005; Lay et al., 2005] (Figure 1). Liberating enormous energy, corresponding to a Mw ≃ 9.3 moment magnitude, the earthquake triggered a tsunami that was one of the most devastating natural disasters ever witnessed in modern history, causing more than 292,000 fatalities in 12 countries bordering the Indian Ocean basin (T. Kawata et al., The December 26, 2004 earthquake tsunami disaster of Indian Ocean. Research Group on The December 26, 2004 Earthquake Tsunami Disaster of Indian Ocean, 2006, http://www.drs.dpri.kyoto-u.ac.jp/sumatra/index-e.html#casualty) (hereinafter referred to as Kawata et al., online report, 2006). The largest tsunami runups, over 30 m, occurred south of Banda Aceh, Sumatra, whose shore is closest to the epicenter, only about 10 minutes away in terms of tsunami propagation time (Figure 1). This area suffered the majority of fatalities (almost 230,000 dead or missing) and the most intense and widespread destruction during the 12/26/04 event (T. Kawata et al., Comprehensive analysis of the damage and its impact on coastal zones by the 2004 Indian Ocean tsunami disaster, 2005, Disaster Prevention Research Institute, http://www.tsunami.civil.tohoku.ac.jp/sumatra2004/report.htm, 2005) (hereinafter referred to as Kawata et al., online report, 2005). The next most heavily impacted area was the coast of Thailand, although it is located on the other side of Sumatra, not in direct line of the epicenter. It took the tsunami 1h45′ to 2h to reach this location [Tsuji et al., 2006]. Thousands of fatalities occurred in Thailand even though, on this east side of the fault, the first tsunami wave to arrive was a large depression wave that caused a significant withdrawal of the ocean at many locations, a crucial sign of tsunami arrival that often was not correctly read.
 All of the six Thai provinces that border the Andaman coast (Ranong, Phang Nga (Khao Lak area), Phuket, Krabi, Trang, and Satun; Figures 2–5) have exposed coastlines that were severely damaged by the tsunami. Among these, the province of Phang Nga suffered the most fatalities, accounting for 71% of the 8,500 people reported dead or missing in Thailand [Bagai et al., 2005; Kawata et al., online report, 2006] and widespread coastal destruction. Throughout this province, most of the fishing villages and their associated ecological environment were completely destroyed; many cultural landmarks suffered partial or total destruction. The largest tsunami runups (11 to 14 m) and destruction in Phang Nga province were observed near Khao Lak [Tsuji et al., 2006] (see also A. Siripongse, Investigation and risk evaluation on tsunami disaster and suggestions on monitoring and prevention of tsunami, in A First Report Under the Project: Investigation for Reclamation of Natural Resources and Environment by Chulalongkorn University, submitted to the Ministry of Natural Resources and Environment, Thailand, 2005) (hereinafter referred to as Siripongse, submitted manuscript, 2005) on a 20 km stretch of shoreline that includes several popular beaches and resorts (Bahn Khao Lak, Nang Thong, Bang Niang, Pa Ka Rang, and Pak Tawib; from south to north in Figure 3, in the Khao Lak area). Damage to tourist resorts, residential areas, and commercial buildings was widespread. A number of pictures and personal video recordings made in this area show that, after the initial ocean withdrawal, a large bore appeared, maybe reaching up to 8 m in height, and propagated as an almost straight line front approaching the Khao Lak beach and causing large runup. The second most impacted area in Thailand was the island of Phi Phi, which is located in Krabi province, 80 km east of the southern tip of Phuket (Figure 5; 98.8°E, 7.8°N). Phi Phi island suffered 15% of the fatalities reported in Thailand, when up to 6 m waves submerged a highly populated, narrow and low-lying sand isthmus (∼100–1,000 m wide and 2–2.5 m elevation), connecting two mountainous headlands between Tonsai bay (south coast of Phi Phi island) and Lohdalum bay (north of Phi Phi island). Eyewitnesses reported that waves hit the sand isthmus from both bays, first from the north side of the island, and a few minutes later from the south side; this was confirmed by personal pictures and video recordings (SEATOS, Sumatra earthquake and tsunami offshore survey, Cruise Report, 2005, http://www.oce.uri.edu/seatos/report.html) (hereinafter referred to as SEATOS, online report, 2005). Finally, Phuket Island was the third region of Thailand to be severely impacted by the tsunami, although it was much less heavily devastated than the Khao Lak area, and only locally, suffering 5% of the total fatalities in Thailand. A 5.5- to 6-m-high wave hit the western coast of the island, causing large runups (up to 10 m; Figure 3) and major damage, particularly at Kamala and Patong beaches. This resulted in 9% of the fatalities suffered in Thailand, with Kamala beach experiencing the most significant loss of life on the island. Destruction was widespread in Patong Beach, where not a single property escaped damage and eyewitnesses reported at least a 2-m-high surge that lasted for well over an hour, following the initial withdrawal.
 To better understand the large runups and destruction observed in coastal Thailand, and in view of the likelihood of similar future events occurring in the region (large earthquakes with Mw = 7.8–9.0 have occurred in 1797, 1833, 1861, 1881, 1907 and 1941 along this plate boundary [Lay et al., 2005]), in this study, we perform detailed numerical simulations of tsunami runup and impact along the coast of Thailand for the 12/26/04 event. In earlier work [Grilli et al., 2007], using a state-of-the-art Boussinesq model of tsunami generation, propagation, and runup, we had iteratively calibrated and validated a tsunami source for this event by comparing tsunami predictions with observations made at tide gauges in the Indian Ocean and the Andaman Sea, and JASON-1 satellite altimeter data measured in deep water. Here further model simulations are performed with a much finer regional grid defined over a smaller geographic area, using highly resolved bathymetric and topographic data in coastal Thailand. Specifically, the objectives of this study are to simulate: (1) runups over the whole Andaman coast of Thailand, where most post-tsunami field observations were made [Tsuji et al., 2006; Choi et al., 2006; Kawata et al., online report, 2005; Siripongse, submitted manuscript, 2005]; and (2) the sequence of events, at locations where these are available from eyewitness reports [e.g., Papadopoulos et al., 2006]. We will show that our simulation is robust, in the sense that it explains most of the observed features of the tsunami along the Andaman Coast of Thailand, without these having been used to calibrate the tsunami source. Once these objectives are reached, we will use our validated synoptic predictions of tsunami impact in Thailand to globally analyze the event, including in areas where no observations were made. We will thus assess which areas may be safe or most likely vulnerable to future tsunamis in the region.
2. Overview of the Sumatra Fault Tectonics
 The relative motion between the Indian and Sunda Plates is on the order of 4 cm per year in direction 20°N while, between the Australian and Sunda plates, it is on the order of 5 cm per year in direction 8°N [Socquet et al., 2006] (Figure 1). The 26 December 2004 Mw ≃ 9.3 megathrust earthquake [Stein and Okal, 2005] was a consequence of strain accumulated in the Indian/Sunda junction, some of which had not experienced a large earthquake for the past 150 years or so. Recent large events in the region include Mw ∼ 8.4 in 1797, Mw ∼ 9 in 1833, and Mw ∼ 8.5 in 1861, for the Australian/Sunda boundary, and weaker Mw ∼ 7.9 events for the Indian/Sunda boundary in 1881 and 1941 [Lay et al., 2005]. This unbalanced partition of past earthquake magnitudes and recurrence times between the two plate boundaries indicates that larger strains had accumulated in the Indian/Sunda boundary prior to the 26 December 2004 event, and explains both the epicenter location at the junction between the subducting Indian and Australian plates and the overriding Eurasian plate (Burma and Sunda subplates) and the northward rupture propagation, where most of the aftershocks were recorded along a ∼1300 km arc of the Andaman trench [Lay et al., 2005]. The 28 March 2005 Mw = 8.7 event was a second large megathrust earthquake that occurred farther south, liberating additional strain on another stretch of the Australian/Sunda boundary and generating a small tsunami, locally causing a 4 m runup near the Islands of Nias. Finally, more recently, on 17 July 2006, a Mw = 7.7 earthquake occurred off southwest Java, liberating some more strain even further south along the same plate boundary and causing a devastating tsunami along 150 km of Java's coastline.
3. The 26 December 2004 Earthquake and Tsunami Events
 Before witnessing this event, scientists analyzed tsunamis generated by small-scale seismic ruptures as instantaneously triggered. This was a fairly good approximation because, for small rupture propagation times, the delay between tsunami time of triggering by coseismic bottom motion, and actual fault rupture was generally negligible as compared to travel time to the nearest coasts (e.g., the 16 November 1999 Vanuatu earthquake and tsunami [Ioualalen et al., 2006]).
 The large size of the ruptured area of the 26 December 2004 event, however, raised many questions regarding the relationships between rupture speed and tsunami modes and timing of triggering by coseismic bottom motion. As far as past large-scale earthquakes and derived tsunamis, none were sufficiently well observed (through seismic and hydrographic networks) to initiate a comprehensive study of these relationships. The 26 December 2004 event is a milestone in this respect, because of its widespread observation with a sufficiently comprehensive and dense network to initiate such studies.
3.1. Summary of Earthquake Mechanism
 The earthquake occurred at 0h58′53″ UTC off the northern coast of Sumatra, Indonesia, at 95°51′, 3°25′ (Figure 1). The earthquake was measured in great detail over the Indian Ocean basin, using seismographs and GPS stations. Seismic inversion models [Ammon et al., 2005; Bilham et al., 2005; Lay et al., 2005] indicate that, for about 500 s, the rupture propagated approximately northward from the epicenter, along 1,200–1,300 km of the Andaman-Sunda trench (with an average rupture speed of 2.5–3 km/s), causing up to ∼6 m of bottom subsidence and ∼10 m of uplift over a region 100–150 km wide across the subduction area. According to Bilham , up to 10 m uplift and subsidence were generated by the earthquake elastic rebound, offshore of Banda Aceh (northern tip of Sumatra). Seismic inversion and GPS records further indicate that fault slip was not homogeneous along the ruptured area varying between 15 and 25 m, with a gradual decrease northward from the epicenter [Vigny et al., 2005]. (See Grilli et al.  for a more detailed overview of rupture and bottom processes.)
3.2. Tsunami Observations
 Many real time observations of the tsunami were made in the Indian Ocean, perhaps so extensively for the first time owing to recent progress in observational techniques. Thus data are available from many tide gauges [Merrifield et al., 2005; Nagarajan et al., 2006] (also Royal Thai Navy, http://www.navy.mi.th/hydro/tsunami.htm, 2005) (hereinafter referred to as Royal Thai Navy, online report, 2005), a few satellite altimeters [e.g., Gower, 2005; Smith et al., 2005], and a satellite Multi-angle Imaging Spectro-Radiometer (MISR) [e.g., Garay and Diner, 2007]. The very large extent of the ruptured area and large associated tsunami that was generated also contributed to their easier detection over a large domain. Beside these instrument records, numerous post-tsunami field surveys were made over the whole Indian Ocean basin [e.g., Tsuji et al., 2006; Choi et al., 2006; Kawata et al., online report, 2005; Siripongse, submitted manuscript, 2005]. This large amount of nonseismic data has helped better characterize the earthquake through constraints provided by the associated tsunami, such as arrival time of successive waves at tide gauges and along satellite transects.
 Thus, in our earlier work, we used many hydrographic data sets, including amplitude, timing, periodicity and sequence, of tsunami waves measured by various instruments, to iteratively develop and calibrate parameters of a multisegment coseismic tsunami source for the 26 December 2004 event [Grilli et al., 2007] (Figure 1). The two main data sets used in this calibration are detailed below.
 The first data set consists of digital tide gauge or point surface elevation records. Most of these are tide gauges that are part of the Global Sea Level Observing system (GLOSS) network, monitored by the Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM). Tide gauges that were used in the source calibration are located in Hannimaadhoo, Male and Gan (Maldives), Colombo (Sri Lanka), Diego Garcia (British Territory) and Cocos Island (Australia). UHSLC provides digital tide residuals, which can be directly compared with the simulated time series. A discussion of the tsunami signal detected by the tide gauges, including arrival times and sequences of tsunami waves, was given by Merrifield et al. . Additional tide gauges operated in Thailand were used, particularly that at Taphao-Noi (Royal Thai Navy, online report, 2005). Finally, a depth sounding record made a mile off Nai Harn Bay near the southwestern end of Phuket Island, onboard the yacht Mercator, in 12 m of water, was used that showed the arrival of three main waves over a duration of 35′.
 The second data set is the sea level anomaly detected by JASON-1's satellite altimeter, which happened to cut across the evolving tsunami wave pattern in a north-south direction approximately 2 hours after the earthquake, during cycle 109 of pass 129 [Gower, 2005; Smith et al., 2005]. Grilli et al.  calculated the sea level anomaly over a diagonal transect in the Indian Ocean by subtracting measurements made during the earlier cycle 108 from cycle 109; they corrected for the travel speed of the satellite in their comparison with model results. Phenomena other than the tsunami may affect sea surface anomaly and cause errors, such as the internal and wind-forced variability of the ocean but, at relatively low latitudes such as here, the dominant timescales derived from basin-wide eddies are much larger than the period between two satellite cycles (around 10 days). Still, the obtained signal was noisy, maybe because of the relatively small Bay of Bengal basin, which may locally yield higher variability; thus the discrepancy between two cycles can be on the order of 20%, with or without a tsunami signal. Nevertheless, considering its magnitude (up to 1.20 m from peak to trough), the tsunami signal can be clearly identified in the records.
 A third data set, used here but not in the tsunami source calibration, consists in the runup values measured during post-tsunami field surveys made along the Andaman coast of Thailand [Tsuji et al., 2006; Choi et al., 2006; Siripongse, submitted manuscript, 2005]. Mostly densely populated areas, however, were surveyed, such as resort beaches in Khao Lak, Phuket, and Phi Phi island. Again it is one purpose of this work, through model simulations, to provide a synoptic and complete picture of tsunami impact in Thailand, including at locations where measurements were not made, and try to identify regions vulnerable to future tsunamis, independent of the density of the population. Such information would help in future regional development plans that might be considered.
 Grilli et al.  performed model simulations using a 1′ × 1′ grid (and 1.2 s time step), in a computational domain covering the entire Bay of Bengal, the Andaman Sea, and part of the Southern Indian Ocean (from 72° to 102°E in longitude and from 13°S to 23.5°N in latitude). They simulated runups only at key locations (Banda Aceh in Indonesia, Khao Lak in Thailand), where the tsunami was most destructive, and favorably compared these with observations. In the present work, we use a finer 0.25′ grid (with a 0.5 s time step), starting west of the northern tip of Sumatra and covering the Andaman sea up to the northern coast of Thailand, i.e., from 91° to 101°E in longitude and from 3.6°N to 12°N in latitude (Figure 5).
4. Tsunami Simulations
 Numerical simulations of tsunami coastal impact require three components: (1) a source, reflecting the known geology and seismology of the event; (2) ocean bathymetry and coastal topography, and (3) a tsunami propagation and runup model, representing the relevant physics.
 Here we simulate tsunami propagation and inundation with FUNWAVE, a Boussinesq water wave model developed at the University of Delaware [Wei and Kirby, 1995; Wei et al., 1995
Case study: tsunami
On Sunday 26 December 2004, a magnitude 9 earthquake occurred off the West Coast of Northern Sumatra in the Indian Ocean. This caused the Indian Ocean tsunami that affected 13 countries and killed approximately 230,000 people.
This tsunami was particularly devastating because:
The earthquake which caused the tsunami was magnitude 9.
The epicentre [epicentre: The point on the Earth's surface directly above the focus of an earthquake. ] was very close to some densely populated coastal communities, eg Indonesia. They had little or no warning. The only sign came just before the tsunami struck when the waterline suddenly retreated, exposing hundreds of metres of beach and seabed.
There was no Indian Ocean tsunami warning system in place. This could have saved more people in other countries further away from the epicentre.
Many of the countries surrounding the Indian Ocean are LEDCs [LEDCs: A less economically developed country (LEDC). This type of country is less wealthy or has lower standards of health and education than many other countries.] so they could not afford to spend much on preparation and prevention.
In some coastal areas, mangrove forests [mangrove forests: Tropical evergreen trees which help protect coastal zones.] had been removed to make way for tourist developments [tourist development: Things that are built for holiday makers to use.] and therefore there was less natural protection.
Social impacts of the tsunami (effects on people)
230 000 deaths.
1.7 million homeless.
5-6 million needing emergency aid, eg food and water.
Threat of disease from mixing of fresh water, sewage and salt water.
1,500 villages destroyed in northern Sumatra.
Economic impacts of the tsunami (effects on money and jobs)
Fishing industry devastated – boats, nets and equipment destroyed. An estimated 60% of Sri Lanka’s fishing fleet destroyed.
Reconstruction cost billions of dollars.
Loss of earnings from tourism - foreign visitors to Phuket dropped 80% in 2005.
Communications damaged, eg roads, bridges and rail networks.
Environmental impacts of the tsunami
Farm land ruined by salt water.
8 million litres of oil escaped from oil plants in Indonesia.
Mangrove forests along the coast were destroyed.
Coral reefs [coral reefs: Underwater structures found in warm seas. ] and coastal wetlands damaged.
Responses to the tsunami
Non-Governmental Organisations (NGOs) and local authorities typically have immediate and secondary responses to devastation of this kind.
Search and rescue.
Emergency food and water.
Re-establishing infrastructure [infrastructure: The basic structures needed for an area to function, for example roads and communications. ] and communications.
Re-building and improving infrastructure and housing.
Providing jobs and supporting small businesses.
Giving advice and technical assistance.
Responses to the 2004 Indian Ocean tsunami can also be divided into short and long term:
In many areas local communities were cut off and had to help themselves.
The authorities ordered quick burial or burning of the dead to avoid the spread of disease [disease: Illness affecting plants and animals.] .
Food aid was provided to millions of people, eg from the World Food Programme.
$7 billion (just under £4.5billion) of aid was promised by foreign governments – but there were complaints that not all money pledged was given.
The British public gave £330 million through charities, eg the average Actionaid donation was £84 – their best ever response.
Reconstruction [reconstruction: The rebuilding of an area after damage has been caused, eg following an earthquake.] is still taking place.
International scale: an Indian Ocean tsunami warning system has now been set up.
Local scale: some small-scale sustainable [sustainable: When something is able to keep going over time without harming people or the environment.] development projects have been set up by charities to aid recovery and help local people help themselves to rebuild and set up small businesses.