Samoa Earthquake and Tsunami of September 29, 2009
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EERI Special Earthquake Report — January 2010
Learning from Earthquakes
Samoa Earthquake and Tsunami of September 29, 2009
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 Mw8.1 alone exceeded $150 million. A tsu-
Geo-engineering Extreme Events
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 exer-
cises 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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EERI Special Earthquake Report — January 2010
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 de-
scribed the flooding at Pago Pago
as much more violent than during
the 1960 Chilean tsunami.
In contrast to Tutuila, which had sig-
nificant 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 tsuna-
mi 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 ex-
ceeded 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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EERI Special Earthquake Report — January 2010
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. incoming and return waves. Most
A Leica ScanStation 2 laser scanner
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 dations, scour depths did not occur
Widespread damage was seen
sheer cliffs show trim lines up to 17 m. to an extent that contributed to
throughout Samoa and American
Poloa is located on the western side structural failure. Scouring was
Samoa. The village of Lepa, Samoa
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EERI Special Earthquake Report — January 2010
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 ob-
served (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 quantifica-
tion of the erosion, but by combin-
ing LIDAR data from areas of ero-
sion with an approximation of previ-
ous 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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EERI Special Earthquake Report — January 2010
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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EERI Special Earthquake Report — January 2010
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 in-
formal 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 oc-
curred 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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EERI Special Earthquake Report — January 2010
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.
ple were aware of tsunamis. In combination was feeling the earth-
quake and seeing the water with- • Many people drove.
American Samoa, September
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 tsuna-
mi. Although aware, many peo-
ple 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 pro-
vided 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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EERI Special Earthquake Report — January 2010
• 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-
lage pride may sustain community The GEER team also wishes to
his bullhorn and ran through the
efforts. Consider the one-year acknowledge the support of David
village notifying people to evacuate.
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 be-
cause 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.
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