Samoa Earthquake and Tsunami of September 29, 2009

Home , Alao, Asili, Niuatoputapu, Poutasi, Tafahi, Upolu

EERI Special Earthquake Report — January 2010

Learning from Earthquakes

Multiple reconnaissance teams Jennifer Donahue (Geosyntec Con- and 75 km east of Tonga’s Niua traveled to Samoa in October and sultants) and Michael J. Olsen (Or- Group. The earthquake occurred November 2009, and contributed egon State University). in a region of high seismicity, with to the preparation of this report. 14 large earthquakes in the region A separate team was comprised of These included EERI member and since the early 1900s. This is the EERI member Lori Dengler (Hum- structural engineer Steven Bal- most significant earthquake on the boldt State University), Kate Long dridge, president of Baldridge & northern bend of the Tonga trench (California Emergency Management Associates Structural Engineering, since 1917 (Okal et al., 2004). The Agency), Jeff Brandt (California De- and an International Tsunami Sur- earthquake did not result from sub- partment of Fish and Game), Heather vey Team, headed by EERI mem- duction of the Pacific plate into the Lazrus (University of Oklahoma), and ber Hermann Fritz of Georgia Tech, Tonga trench (Figure 1), but rather Lesley Ewing (California Coastal that covered the entire Samoan from normal faulting expressing a Commission). This team traveled to archipelago, including the islands lateral tear in the plate as it slides Samoa in late October with a team of Upolu, Savai’i, Manono, Tutuila, past the northern bend of the plate from the American Society of Civil Aunu’u, Ofu and Olosega. Other boundary. Such events are known Engineers. members of this team were Costas elsewhere (Grovers and Wortel, Synolakis and Jose Borrero (Uni- The research, publication and distri- 2005), but their recurrence is even versity of Southern California), bution of this report were funded by more poorly understood than that for Emile Okal (Northwestern Univer- the EERI Learning from Earthquakes great subduction earthquakes. sity), Robert Weiss and Patrick Project, under grant # CMMI-0758529 The ensuing tsunami killed nine peo- Lynett (Texas A&M University), from the U.S. National Science Foun- ple in Tonga, 149 in the independent Vasily Titov (NOAA), Bruce Jaffe dation. country of Samoa, and 34 in Ameri- (USGS), Spyros Foteinis (Techni- can Samoa. It was the deadliest tsu- cal University of Crete), and I-Chi Introduction nami in the Samoa region in living Chan and Philip L.-F. Liu (Cornell On September 29, 2009, at 17:48:10 history. The damage in Samoa University). A survey team from UTC (local time: UTC-11), an M 8.1 alone exceeded $150 million. A tsu- Geo-engineering Extreme Events w earthquake struck about 200 km south nami warning issued by the Pacific Reconnaissance (GEER) included of the main Samoan Islands chain Tsunami Warning Center 16 minutes after the earthquake was too late for many, since the tsunami arrived in 11-15 minutes at some of the hard- est hit villages. Fortunately, many Samoans were aware of tsunamis and knew to get to high ground after an earthquake, behavior attributed to education and evacuation exercises initiated throughout the South Pacific over the past decade. In- deed, evacuation exercises had been conducted in Samoa in the preceding year, and many schools in American Samoa practiced monthly evacuation drills.

Tsunami Field Surveys Eyewitnesses described between one and four main waves, with an initial recession interpreted as a leading depression N-wave (Tade- Figure 1. Tectonic setting of the Samoa Islands region (U.S. Geological palli and Synolakis, 1994). Survey Earthquake Information Center).

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Figure 4. 3D laser scanner setup in Tula, American Samoa.

The harbor geometry at Pago Pago amplified the tsunami from just a few meters at the entrance to 8 m at the head, causing inundation and damage more than 500 m in- Figure 2. Maximum tsunami runup exceeding 14m at Lepa on Upolu Island, land up the Vaipito River. Similar Samoa. inundation distances were observed along the river at Leone in south- east Tutuila. An eyewitness described the flooding at Pago Pago as much more violent than during the 1960 Chilean tsunami. In contrast to Tutuila, which had significant tsunami impact on both north and south coasts, destruction on Samoa’s Islands was confined to the southern coasts. This effect, as well as the location of the highest runup, can be explained by tsunami directivity from the source region that focused wave energy towards the western tip of Tutuila and SE Upolu, and radiated less energy elsewhere. On Upolu, runup reached 14 m at Lepa (Figure 2), while on Savai’i maximum runup exceeded 8 m at Nuu. At nearby Taga, 6 m runup and 200 m of inundation left a boulder deposit field 100 m Figure 3. Tsunami boulder field, wash-out damage and water tank impact at inland (Figure 3). Taga on Savai’i Island, Samoa. LIDAR The measured flow depths and run- on the central north coast, and 9 m The use of LIDAR (Light Detection up heights indicate extreme and at Tula in the east. and Ranging) was important to the significant variation on all main is- Pago Pago on the central south coast investigation for multiple reasons. lands. On Tutuila, maximum runup represents an unfortunate example of First, it provided a quick way to exceeded 17 m at Poloa, near the a town and harbor ideal for protection obtain valuable information before western tip. Runup decreased against storm waves, but vulnerable clean-up was done and vegeta- somewhat along Tutuila from west to tsunami (Fritz and Kalligeris, 2008). tion grew back. Second, although to east, reaching 12 m at Fagasa

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Figure 5. Near Lepa, Samoa. The high water line was Figure 6. Debris piled up at more than 12 m elevation at measured at 13.6 meters above sea level here. Poloa, American Samoa.

Figure 7. Scouring to a depth of 0.65 m underneath a Figure 8. In Lepa, Samoa, scour caused the collapse of large multi-family dwelling in Leone, American Samoa. part of a village meetinghouse. no previous data were available, (Figure 5) was completely destroyed. of Tutuila Island and has a north- LIDAR provided high-resolution From the beachhead inland, there western exposure. At the time of data allowing for much more accu- is a small rise in topography, then a this report, all remaining residents rate damage quantification than can ravine, then a steep hillside. Most have abandoned Poloa and have be had from traditional methods. of the devastation occurred in the moved into the surrounding hills. Finally, it allowed for models to be ravine, which was littered with debris, Scour: Scour around foundations available for scientists to study with- houses and cars. Almost 90% of was visible at many locations (Fig- out having to travel to the site. the structures in this area were ure 7). It was caused by both the destroyed. A Leica ScanStation 2 laser scanner incoming and return waves. Most was used by the GEER team for the Of all villages on the island of Ameri- scour was observed around the survey. 360o panoramic overview can Samoa, Poloa sustained the most foundations of homes, but some scans were completed for each damage (Figure 6). All structures was also observed around utility setup (Figure 4). were destroyed except for the church, poles. which sustained heavy damage to For most of the buildings, even General Tsunami Damage doors, windows and furniture, but those with relatively shallow foun- only minor structural damage. The Widespread damage was seen dations, scour depths did not occur sheer cliffs show trim lines up to 17 m. throughout Samoa and American to an extent that contributed to Poloa is located on the western side Samoa. The village of Lepa, Samoa structural failure. Scouring was

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most prevalent in the villages Lepa, Alao, Leone, Asili, Poloa, and Tula. The GEER team used both the traditional means of measuring tape and LIDAR to document scour (Figure 8). Erosion: Substantial erosional damage from the tsunami was observed (Figure 9), and additional erosion and landslides are anticipat- ed with future precipitation because of the vegetation destroyed by the tsunami. Unfortunately, reference data are not available for overall quantification of the erosion, but by combin- ing LIDAR data from areas of erosion with an approximation of previous topography, it is reasonable to estimate overall quantification of the erosion. Figure 9. Wave-induced bluff erosion observed in Aufaga, Samoa.

Figure 10 (a) ▲On Niuatoputapu Island’s north tip: the entire forest was overwhelmed by the tsunami, with stripped and uprooted trees as well as coral boulders. (b) ▼The scars on the bark of the tree — at the far right (c) ▼Tafahi Island looking north from the maximum 22 m in (a) — indicate 9.4 m flow depth above terrain, 6 m runup, with broken branches in the foreground and the above sea level and 200 m from the beach, with scour of destroyed forest along the beach. more than 2 m at the roots.

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Observations in Tonga’s Niua Group A follow-up expedition from 23 to 28 November surveyed the three main islands of Tonga’s northern-most Niua group. The tsunami impact on this group surprisingly eclipsed the Samoan observations in all as- pects, with maximum runup of 22 m on both the east and west coasts of Tafahi Island as well as flow depth of 15 m and inundation of 1 km on Niuatoputapu Island’s east coast. Tafahi represents tsunami impact on a volcanic island characterized Figure 11. The damage to the roof of the fale provides evidence of the height by steep hill slopes (typically 1V:5H) of the tsunami. The concrete foundation and columns sustained minor damage. and fringing reefs within 100 m of the shoreline. up the principal, who had already Many of the low-lying villages have Niuatoputapu’s flat coastal topogra- evacuated. An additional Tongan churches near and facing the phy and near-shore fringing reefs victim at Hihifo returned to his house ocean. The typical construction is allowed for massive inland penetra- to close a shop between tsunami a combination of concrete frames tion of the tsunami waves along the waves. and concrete masonry (CMU) infill. south and east coasts. While for- Hence only the keeper of the Palm There were several examples of the ests may provide some tsunami Tree Island Resort on Hunganga tsunami entering churches through attenuation at flow depths below Island, which was totally submerged the front door and flowing out the 5 m, the forests on the south coast by tsunami waves, was an unprevent- windows along the sides. While were completely overwhelmed by able fatality in the Kingdom of Tonga. doors, windows, and interior furni- local flow depths of up to 10 m ture were damaged, the structures above ground (Figure 10). Structural Damage did not appear to have any dis- Fortunately, the coral reefs and ti- tress from the hydrostatic loading. Many masonry buildings and rein- However, the contents of the build- dal flats extending between 1 and forced concrete columnar open struc- 2 km offshore reduced the tsunami ings were either washed from the tures known as fales appeared to buildings or strewn about within the impact along the north shore for withstand the forces of the tsunami villages such as Hihifo. structures, resulting in significant inundation with only minor structural financial loss (Figure 12). Some Seven of the nine victims on Niua- frame damage. Most of the columnar foundations failed due to scouring, toputapu were loaded onto a truck fales are almost completely open, but most foundations were found in caught by the tsunami while head- allowing the tsunami to flow through acceptable conditions. ing back to the high school to pick them (Figure 11).

Figure 12. Damage around and within the church in the village of Poloa, American Samoa.

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Figure 13 (a). While the windows, doors, and contents of Figure 13 (b). This CMU residence suffered collapse. this CMU residence were destroyed by the tsunami, the The remaining debris provides evidence that most of the structure was relatively intact. CMU cells were not reinforced or grouted.

While most CMU buildings with- as vehicles, shipping containers and agement, environmental science, stood the tsunami, those that were boats. anthropology, emergency manage- poorly constructed did not fare well The structural damage that was ob- ment, and mitigation. The team (Figure 13). Wood-framed and poor- served appeared to be primarily from focused on identifying the fac- ly reinforced masonry buildings in the tsunami effects and not from tors that influenced the impacts of most cases were torn completely strong ground motion during the the tsunami, and looked at how from their foundations (Figures 14 earthquake. coastal land use planning and man- and 15). agement, emergency planning and Several columnar structures with Community Impacts response, and culture, education weak reinforcement were severely and awareness of tsunami hazards The interdisciplinary team that visit- affected outcomes. damaged. There was evidence of ed Samoa and American Samoa in damage created by the impact late October had expertise in coast- Human Behavior: All three EERI forces of large floating debris such al and port engineering, coastal man- teams talked with eyewitnesses to the tsunami. Most people were aware of tsunami hazards and had heard that earthquake ground shaking was a natural warning, but many reported evacuating only after watching others do so or once they saw the water withdraw. A number of communities used informal community notification sys- tems such as church and school bells (Figure 16). Several victims perished during the evacuation while inside cars; this may be attributed to conflicting official statements issued in Samoa on the use of vehicles during a tsunami evacuation. Factors that reduced impacts: • Time of day: The tsunami occurred early enough in the day Figure 14. The lower floor of this two-story wood frame residence was that few were at work or on the destroyed by the tsunami. road, but late enough so that

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Figure 15. This single-story wood frame residence collapsed completely.

everyone was awake. Many • Village structure provided shelter Figure 16. Church bell at Poutasi, people were outside just getting and high ground: There were no Samoa. The bells serve to notify ready to go to work or school tent cities of evacuees. All villages community members of church and in good positions to see the include both high and low ground times and other events, and were water drawdown. and the family ties assured that no also used in some communities by • Little earthquake damage: one was isolated or left alone. pastors and others to warn people Shaking damage was minimal Factors that exacerbated impacts: about the tsunami. and did not disrupt roads or • A very large near-source tsunami structures. with first wave arrivals before any Tsunami hazard zone signs had • First significant wave a drawdown official warning could be issued. been posted in American Sam- oa but there were no visual sig- • Availability of high ground: In • Most people required at least two nals as to how high was high American Samoa, almost all vul- indications that they were at risk enough. There were no marked nerable coastal areas were in before they evacuated; very few evacuation routes with direction close proximity to high ground. people responded to the ground and distance to tsunami safe • Tsunami awareness: Most peo- shaking alone. The most common areas and shelters. combination was feeling the earth- ple were aware of tsunamis. In • Many people drove. American Samoa, September quake and seeing the water with- was emergency preparedness draw. Other additional indicators • Lack of awareness that there month, and a number of projects included hearing a bell or siren, or could be more than one wave. targeted tsunamis funded the alerting of neighbors. • Embarrassment: young people through the TsunamiReady pro- • Confusion about where to go. were aware that ground shak- gram, including showing videos of the 2004 Indian Ocean tsunami. Although aware, many people did not expect it was some- thing that could happen in Sam- oa. Some schools practiced regular tsunami evacuation drills. • Altruism: Many people put them- selves in harm’s way to save others. • Rapid cleanup. • Strong 2-3 story buildings provided vertical evacuation. Figure 17. Poutasi, Samoa. Inland lagoons and water bodies that wrap be- hind coastal communities exacerbate their vulnerability by cutting off evacu- • Coastal protection structures ation. Other land forms and built structures, such as steep terrain and pig generally performed well. styes, had the same effect.

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• Develop a protocol to assess the Thanks go to Don Vargo, Marie vulnerability of coastal villages, Can-Kau, Sharon Fanolua and including exposure to tsunamis, Agnes Vargo of American Samoa evacuation procedures, access to Community College for assistance evacuation sites on high ground or with logistics, translation, and shar- inland away from the coast, land ing their insights; Tisa Fa’amuli, use (shoreline protection features, Candy Mann, Eti Sapolu, and Me vegetation), barriers to evacua- Sapolu for lodging and logistical tion such as rivers and mangrove assistance, translation and discus- swamps, and other hazards in the sion; Joe Toland of FEMA for shar- area such as landslides, and hur- ing his geospatial images; Laura ricanes Kong and Brian Yanagi of the • Chronicle the recovery process International Tsunami Informa- over time in terms of rebuilding, tion Center for logistical help and use of devastated areas, institu- coordination with other Interna- tionalizing tsunami warning, evac- tional Tsunami Survey Teams; Jan uation procedures, education and Steffan of UNESCO for facilitating outreach, role of village cultural the work in Samoa; Ausitalia of the structures (matai, faife’au, etc.), Ministry of Natural Resources and and memorials of the event. the Environment for the Samoan Figure 18. The mayor of Amenave Government for providing an inter- had attended a workshop for village • Build on traditional village struc- preter and guidance, and all of the mayors on tsunami hazards. When ture to promote resiliency. Incor- people who shared their stories. he felt the earthquake, he grabbed porating tsunami safety into vil- The GEER team also wishes to his bullhorn and ran through the lage pride may sustain community acknowledge the support of David village notifying people to evacuate. efforts. Consider the one-year anniversary of the tsunami as an Evans and Associates, Inc. for opportunity for villages to develop providing the 3D laser scanner and ing was a sign of an impending evacuation routes and practice equipment on very short notice; tsunami but were worried that drills. Jim Griffis and Marcus Reedy, who other people would think they made the necessary arrangements; were silly if they evacuated. • Examine informal warning sys- and Yumei Wang, Harry Yeh, the tems such as bells, private sirens, entire GEER steering committee, • Infrastructure that could not re- and bull horns (Figure 18), and sist uplift forces. Bridges were and Bruce Jaffe of the U.S. Geolog- develop criteria to support their ical Survey for their administrative, particularly vulnerable to buoy- use for near-source tsunami ancy forces. technical and logistical support. events. They also extend thanks to Leica • Barriers to evacuation. A number Geosytems for providing licenses of communities had difficulty Acknowledgements for the Cyclone software. with access to high ground because of rivers or mangrove Steven Baldridge’s participation in swamps that isolated coastal the field investigation was funded by References areas, landslides, fences, and in the EERI Learning from Earthquakes one case, pig styes (Figure 17). Program sponsored by the National Fritz, H. M. and N. Kalligeris, Science Foundation. The Interna- 2008. Geophys. Res. Lett., 35, tional Tsunami Survey Team was sup- L01607. Recommendations ported by an NSF Rapid Response Govers, R. and M.J.R. Wortel, • Develop credible tsunami/multi- Research award. A grant from the 2005. Earth Plan. Sci. Letts., v. hazard maps for all low-lying NSF-sponsored Geo-engineering 236, pp. 505-523. communities based on likely Extreme Events Reconnaissance sources, both nearby and else- Association provided core support Okal, E.A. et al., 2004. Geophysical where in the Pacific, delineating for the GEER participants. Financial J. Int. 157, 164-174. evacuation zones, routes, and support for the team that traveled to Tadepalli, S. and C.E. Synolakis, safe areas. Signs can indicate Samoa in late October was provided 1994. Proceedings: Mathemati- the hazard zones once they are by EERI, the California State Lands cal and Physical Sciences, 445 defined, and educational efforts Commission, ASCE, and donations to (1923): 99-112. can be developed. the Humboldt State University Spon- sored Program Foundation.