Earth-Science Reviews 107 (2011) 66–75 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Insights on the 2009 South Pacific tsunami in Samoa and Tonga from field surveys and numerical simulations Hermann M. Fritz a,⁎, Jose C. Borrero b,c, Costas E. Synolakis c,d, Emile A. Okal e, Robert Weiss f, Vasily V. Titov g, Bruce E. Jaffe h, Spyros Foteinis d, Patrick J. Lynett i, I.-Chi Chan j, Philip L.-F. Liu j a School of Civil and Environmental Engineering, Georgia Institute of Technology, Savannah, GA 31407, USA b ASR Limited, Raglan 3225, New Zealand c Department of Civil and Environmental Engineering, Univ. of Southern California, Los Angeles, CA, USA d Department of Environmental Engineering, Technical University of Crete, Chanea 73100, Greece e Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL, USA f Department of Geology and Geophysics, Texas A&M University, College Station, TX, USA g Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA h Pacific Science Center, US Geological Survey, Santa Cruz, CA, USA i Department of Civil Engineering, Texas A&M University, College Station, TX, USA j School of Civil and Environmental Engineering, Cornell University, Ithaca, NY, USA article info abstract Article history: An Mw ≈8.1 earthquake south of the Samoan Islands on 29 September 2009 generated a tsunami that killed Received 16 July 2010 189 people. From 4 to 11 October, an International Tsunami Survey Team surveyed the seven major islands of Accepted 3 March 2011 the Samoan archipelago. The team measured locally focused runup heights of 17 m at Poloa and inundation of Available online 10 March 2011 more than 500 m at Pago Pago. A follow-up expedition from 23 to 28 November surveying the three main islands of Tonga's northernmost Niua group revealed surprising 22 m runup and 1 km inundation. We analyze Keywords: tsunami the extreme tsunami runup and complex impact distribution based on physical and societal observations earthquake combined with numerical modeling. That an outer rise/outer trench slope (OR/OTS) event is responsible for a South Pacific tsunami disaster in the Pacific calls for care in identifying and defining tsunami hazards. Evacuation exercises Samoa conducted in Samoa in the preceding year may have limited the human toll; however, cars were identified as Tonga potential death traps during tsunami evacuations. This event highlights the extreme hazards from near source tsunamis when the earthquake's shaking constitutes the de facto warning, and further underscores the importance of community based education and awareness programs as essential in saving lives. © 2011 Elsevier B.V. All rights reserved. Contents 1. Event overview .............................................................67 2. Post-tsunami reconnaissance .......................................................67 3. Observations in the Samoan archipelago ..................................................67 4. Observations in the Tongan archipelago ..................................................69 5. Tsunami modeling ............................................................69 5.1. Tsunami source models ......................................................69 5.2. Model results: Tutuila .......................................................71 5.3. Model results: Far field......................................................72 6. Conclusions ...............................................................75 Author. Author contributions contributions . ...........................................................75 Acknowledgements ..............................................................75 References ..................................................................75 ⁎ Corresponding author. Tel.: +1 912 966 7947. E-mail address: [email protected] (H.M. Fritz). 0012-8252/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2011.03.004 H.M. Fritz et al. / Earth-Science Reviews 107 (2011) 66–75 67 1. Event overview eyewitnesses according to protocols reviewed by Synolakis and Okal (2005). The recorded combined Samoa and Tonga survey database is On September 29, 2009 at 17:48:10 UTC (6:48:10 local time), an composed of 444 tsunami runup and flow depth measurements (Okal Mw ≈8.1 earthquake struck ~200 km S of the main Samoan Islands et al., 2010). Eyewitnesses described between one and four main chain and ~75 km E of Tonga's Niua Group. This is the most significant waves with an initial recession, interpreted as a leading depression earthquake on the northern bend of the Tonga trench since 1917 (Okal N-wave (Tadepalli and Synolakis, 1994). Several eyewitnesses et al., 2004). This event does not represent simple subduction of the reported evacuating only after watching others do so or upon Pacific plate into the Tonga trench, but rather started as a normal observing the first wave. Most drivers did not feel the earthquake faulting rupture expressing a lateral tear in the plate as it slides past the inside their cars. There were also cases of victims killed in cars, Northern bend of the plate boundary (Okal et al., 2010). It then triggered overcome by the wave while stopped in traffic during the evacuation. a smaller, but still substantial episode of interplate thrusting, as modeled This was an unfortunate occurrence possibly attributable to confusion by Li et al., 2009 and later Lay et al., 2010. Such events are known resulting from conflicting official statements recently issued in Samoa elsewhere (Govers and Wortel, 2005), but their recurrence is, if on the use of vehicles during a tsunami evacuation. Indeed, part of anything, even more poorly understood than for great subduction the rationale for Samoa changing from left to right hand drive on 7 earthquakes; five large OR/OTS events versus 59 mega-thrusts have September 2009, just 3 weeks before the tsunami, was justified as a been identified since 1900 (Kirby et al., 2009). That another non- step to reduce greenhouse gases, through the availability of used cars subduction source mechanism is responsible for yet another tsunami from Australia and New Zealand, and to allow more citizens to own disaster in the South Pacific is a call for care in identifying and defining cars and thus escape from the sea shore (WSJ, 2009). Similarly, seven tsunami hazards. of the victims on Niuatoputapu were riding a truck that was The ensuing tsunami killed 9 in Tonga, 146 in Samoa, and 34 in subsequently caught by the tsunami while heading back to the American Samoa, the highest tsunami death toll on US territory since high-school to pick-up the principal who had, in the meantime, 1964. The damage in Samoa alone exceeded $150 million (UN-OCHA). A already evacuated. An additional Tongan victim at Hihifo returned to tsunami warning issued 16 min after the earthquake was too late for his house to close a shop between tsunami waves. Only the keeper of many, as the tsunami arrived within 15 min at some of the hardest hit the Palm Tree Island Resort located across the tidal channel from villages. Fortunately, many Samoans and Tongans knew to get to high Hihifo on Hunganga Island, which was totally submerged by tsunami ground after experiencing an earthquake, a behavior enhanced by waves, was an unpreventable fatality in the Kingdom of Tonga. education and evacuation exercises initiated throughout the South Pacific over the past decade; in Samoa, the latter had taken place in the 3. Observations in the Samoan archipelago preceding year. Fig. 1 shows the measured flow depths and runup heights throughout 2. Post-tsunami reconnaissance the Samoan archipelago. The data exhibit pronounced extrema and significant variation on all main islands. On Tutuila, a maximum runup The surveys we report took place from 4 to 11 October covering spike exceeded 17 m at Poloa, near the western tip, where damage was the entire Samoan archipelago, including the islands of Upolu, Savai'i, extensive and reminiscent of the impact of the 2004 tsunami in many Manono, Tutuila, Aunu'u, Ofu and Olosega, and from 23 to 28 locales in Sri Lanka (Liu et al., 2005). The tsunami wrackline and rafted November focusing on the Niua group (Niuatoputapu, Tafahi and debris at Poloa along a relatively steep transect with a short 100 m Niuafo'ou islands) of the Tongan archipelago. The survey teams distance from shoreline to maximum runup are shown Fig. 2c–e. Runup documented tsunami runup, flow depth and inundation; wave decreased somewhat along Tutuila from west to east reaching 12 m at induced deposition or erosion, structural damage and interviewed Fagasa, on the central north coast and 9 m at Tula in the east. Pago Pago, Fig. 1. Samoan Islands with measured tsunami runup (blue) and tsunami height z+h (red) at survey locations on south and north coasts. Runup and tsunami height z+h are relative to the sea level at time of tsunami arrival with z terrain elevation and h flow depth above terrain. 68 H.M. Fritz et al. / Earth-Science Reviews 107 (2011) 66–75 Fig. 2. Tutuila, American Samoa field observations: (a) Pago Pago: First wave receding with a water level ~1 m below maximum flood levels in a morning eastward view 250 m from the shoreline at location PA in Fig. 3 (photo: Gordon Yamazaki, NOAA); (b) Pago Pago: Vehicle projectile impact 300 m from the shoreline and flow depth measurement at location PB in Fig. 3; (c) Poloa: Trimline in vegetation behind the church marks the maximum runup of 17 m; (d) Poloa: Multiple vehicles piled up within tsunami