Notes From the Field...
USGS Scientists in Samoa and American Samoa Studying Impacts of Recent Tsunami, October-November 2009
Articles and Reports
“The limit of inundation of the September 29, 2009, tsunami on Tutuila, American Samoa”
“Surprises from the Deadly September 29, 2009, Samoa Tsunami”
“USGS Scientists Respond to Deadly Samoa Tsunami”
“USGS Scientist in American Samoa Helps Calm Fresh Tsunami Fears”
Reports from Mark Buckley, Pago Pago, American Samoa
-- The above reports are from Mark Buckley, while he was in Pago Pago, American Samoa
October 29, 2009
Read selected observations made by tsunami survivors and responders, October 9-17, 2009
These observations were derived from interviews conducted by Walter Dudley, University of Hawaii (Hilo), who accompanied the USGS Team to American Samoa and Samoa (Upolu).
October 24, 2009
A newspaper article in Samoa News discusses Guy Gelfenbaum's presentation at the American Samoa Community College Lecture Hall, Thursday, Oct. 22: "Tsunami scientist presents findings of Sept. 29 wave." Download a copy of Gelfenbaum's talk as a PDF (5.6 MB) file.
Update: October 23, 2009
October 23, 2009: Report from Bruce Jaffe, October 23, 9:30 a.m., Santa Cruz, CA
Doug Fenner, an ecologist with the Department of Marine and Water Resources in Samoa, gave rave reviews of Guy Gelfenbaum's talk at American Samoa Community College yesterday. Guy and Steve Watt are on their way home from American Samoa and just landed at Honolulu. Bruce Richmond and Mark Buckley are on their way home from Samoa and will land in Honolulu at 10:00 a.m. this morning.
A newspaper article, published on Oct. 22 by the Samoa News, gave information about Gelfenbaum's presentation and an interview with a USGS earthquake scientist
October 22, 2009: Report from Bruce Richmond, Apia, Samoa
A draft report (16.9 MB pdf) of our activities will be combined with the other study team reports and submitted to the Samoa (Independent) Government. We were in the field until yesterday evening so the results section is just a few examples of the data collected. Overall a very good study and a true ITST.
Upcoming talk by Guy Gelfenbaum, in American Samoa
Time: 3:00 pm, Thursday, October 22, 2009
Place: American Samoa Community College Lecture Hall on the main campus
Title: The Samoa Tsunami of September 29, 2009: Preliminary Field Data on Tsunami Inundation in American Samoa
Presenter: Dr. Guy Gelfenbaum, Oceanographer, U.S. Geological Survey, Menlo Park, CA, USA
The tsunami of September 29, 2009 caused considerable damage and a number of fatalities on the islands of American Samoa. Scientists from the U.S. Geological Survey, as well as others from the US, Japan, and elsewhere have been on the island to measure tsunami wave heights, inundation distances, and collect other data that will be used to help understand the impact of the tsunami. Scientists measured tsunami runups (the elevation at the location of the furthest inland inundation) of about 12 m (38 feet) and inundation distances of 250 m (about 800 feet). The presenter will show the types of data collected and describe how the data are used to improve the understanding of tsunami hazards in American Samoa and elsewhere around the world.
Dr. Gelfenbaum is an oceanographer with the USGS in Menlo Park, CA, USA. He has been with the USGS for over 20 years studying coastal hazards on US coasts in Florida, Oregon, and Washington. He has been investigating tsunami hazards for 12 years and has been to Papua New Guinea (1998), Peru (2001), and Sumatra (2005) after tsunamis impacted those countries. Dr. Gelfenbaum also models tsunami inundation and sediment transport to help improve tsunami hazards assessments.
October 21, 2009: Report from Bruce Richmond
Significantly, today the geology team took a field trip to Savaiʻi Island.
The team took the 6am ferry at Mulifanua Wharf and drove west from Salelologa Wharf.
The first place where there was some evidence of tsunami inundation was Satuapitea, where the wave was reported to have reached the house over the road (eye witness account). All has been cleaned up, but salt-burned trees and grass still bear evidence of the tsunami inundation.
In Puleia, the tsunami reached the cliff on the other side of the road. Salt-burned coconut trees, fallen vegetation indicating direction of flow (three directions), and some debris are evidence of the tsunami inundation. Runup was at least 3.2 m above mean sea level (road level), with 2.6 m flow depth at the coconut trees, and the maximum tsunami inundation was 60 m from the shore.
In Gataivai, ripped-up grass is still to be seen on both sides of the road.
At Nuu Black Sand Beach, the team saw fallen palm trees and salt-burned vegetation. There is hardly any sand on the beach itself. However, as the team had not been at this beach before the tsunami, they do not know whether there was sand over the boulders that are now exposed. The maximum inundation was 44 m, with a runup of 5.8 m. Pebbles on sand 'pedestals' of c. 4 cm height suggest recent erosion during the tsunami. Boulders have been plucked off the cliff edge and washed several metres inshore. Sand in the forest area between the road and the beach might have been deposited due to a number of processes, not necessarily the tsunami.
The team also went to the Alofaaga Blowholes, but there was no evidence of any inundation, nor recent shore platform abrasion.
There was no evidence of any tsunami inundation in Siutu, nor Satuiatua. The team then drove to Fogatuli and then turned back.
In summary, there was some evidence of tsunami inundation in a few locations on the south coast of Savaii Island, but no damage. The tsunami seems to have had only minor impact and effect on Savaiʻi Island.
October 20, 2009: Report from Bruce Richmond
As already mentioned, there are now a moderately large number of geologists still working in the field. As such, they divided in to two separate groups in order to be able to complete more field survey work.
The first geology sub-team spent the day working in the area of Satitoa in the east. The team conducted more detailed field surveys at Satitoa in order to gain a more sophisticated understanding of the lateral extent of sediments deposited by the tsunami.
This took some time since much sediment in close to the settlements has been cleared away as part of the recovery process which is understandable.
A sedimentological transect was completed along the tsunami flow direction, the transect extended 450 m inland indicating that at this location the tsunami inundated at least 450 metres from the shore. Five shallow trenches were excavated at 100 m intervals along the transect. Tsunami sediment varied from 10 cm to 1 cm thickness at the inland limit of inundation.
Sediment samples were collected from each trench at 1 cm vertical intervals. The samples will be analysed for microfossil content and grainsize. This information will be used to increase our understanding of tsunami sediment transport and aid in identification and interpretation of prehistoric tsunami deposits. In addition to the surveying of the tsunami sand deposits, boulders transported from the seawall by the tsunami were located. Their dimensions and orientations of were carefully recorded. Coastal boulder deposits have been used to identify past tsunamis and infer tsunami flow velocities and flow depths and can be correlated with building damage levels. A micro-topographic survey was carried out at Satitoa using RTK-GPS. Profiles were taken from the shoreline up to the limit of inland inundation.
The second geology team commenced its ongoing geological survey at the Coconut Beach Resort and travelled west, returning to Apia via the north coast.
Key sites investigated:
A suitable site to the west of the village was identified for trenching, but time constraints prohibited this being carried out today. We did carry out a ground survey of the indicators of runup which included a line of dead Halimeda micro-algae coral near the maximum extent of inundation. Seaward of this line was a zone of soil rip-up - when the tsunami waves first came onshore here they eroded the coastline - and only started to deposit material inland on the rip-up line. This line was represented by a line of riped-up blocks of soil that were subsequently deposited some 2-10 metres landward. This type of evidence is a classic indicator of tsunami inundation.
The reconnaissance continued back around the coast to Apia from this point. We had insufficient time to stop for further work, but noted variable degrees of inundation and damage to the far west.
October 19, 2009: Report from Bruce Richmond
The geology sub team comprised six international scientists and one representative from the MNRE. Today, a 70 m topo-sedimentological transect was completed using an RTKGPS in Vaovai on the South Coast of Upolu where flow depth was measured at 3.8 m on the seafront and 2.7 m in the mangrove. Flow directions were also recorded. A microtopographical survey of the coastal erosional zone associated with the deposited sand sheet was also undertaken where we collected more than 2100 waypoints. A sand layer up to 20 cm thick composed largely of coarse sand with the upper layer consisting of alternating coarse and thin laminations was described and sampled. The stratigraphy of this layer is comparable to the stratigraphy of the Satitoa deposit.
Five short cores were collected up to 50 cm in length. These largely focus on capturing recent and modern extreme events (tsunami or cyclone) deposits. One of the cores however, contained deposits in the lower portion of the core that may represent a previous extreme event.
Boulder morphometry and orientation were also measured in this area. Two types of boulders have been distinguished: basaltic ones (30 measurements) and coral ones (40 measurements). They are distributed in different areas relative to the coastal road: basaltic boulders represent wash-over of the incoming wave, whereas coral boulders were mobilized by the backwash waves moving rubble material from the roadside filling. These data will be helpful in estimating tsunami flow velocity.
It appears that topographic lows lying behind the coastal road have endured more severe erosion due to an acceleration of the incoming flow. Then, following this erosion phase, these areas have acted as a trap for sediments. Flow depths and orientation were also measured in Poutasi.
October 19, 2009: Report from Bruce Jaffe (now back in Santa Cruz, CA office)
Just got off the phone with Guy Gelfenbaum, who is in American Samoa. He and Steve Watt (USGS) are going to Tula on the east side of the island today to map inundation and look for tsunami deposits. Tula was hit hard by the tsunami even though it is on the opposite of the island from where the tsunami originated. The platform that American Samoa is on bent the waves around to hit the east coast. Coastally trapped waves, called edge waves, may also have contributed to the large runups there. Maximum runups were 5-8 meters (16-26 feet) in Tula.
October 17, 2009: Report from Guy Gelfenbaum
We [Guy Gelfenbaum, Steve Watt, and Alex Apotsos] went to the north coast with Brian Peck [USDA] on Wednesday and Thursday and collected a lot of data in two bays. Both are completely uninhabited and had significant inundation and runup. We got great bathymetry, topography on the reef flat at low tide, topography on land, wave heights, runup, inundation distance, and even some sand deposits at one of the bays (up to 15 cm thick with multiple layers). We also measured 16 coral boulders on the beach that had recently alive coral. All had been transported by the tsunami. The largest was 2.5 m A-axis.
We are scheduled to head out again next week (Tuesday) with Brian to the section of the north coast that had the highest predicted runups (from NOAAs simulations).
October 17, 2009: Report from Bruce Richmond
Work again focused in the area of Satitoa. Another sizeable deposit was identified that is certainly of tsunami origin, further confirmed by the fact that the deposit was found on top of the concrete base of a destroyed fale. The deposit consists of a sand sheet that overlies a dark soil that is distinctly different from the tsunami derived deposit with a very clear contact. A boulder field is also associated with this sand deposit.
Over a 225 metre section, ten trenches were put in to examine the nature of the deposit. Each of these trenches was logged and the sand layers were sampled at 1cm intervals in order to obtain any minor structures. A small number of short cores were also collected. The tsunami derived layer has distinct structure and a number of sub-layers identified. The layer is between 7-10cm thick and varies little over the entirety of the 225m section. Sub-layers were less distinct closer to the road and became more distinct as one moved away (possibly due to clean-up operations). At the base of the sand layer is a layer of coarser sand composed of shell fragments and other carbonate material. The upper portion of the layer is finer in nature, but, depending on location, contains a number of separate laminations. These consist of alternating coarser and finer layers some containing basaltic material.
Fifty boulders, emplaced by the tsunami, were measured as part of the analysis of the deposit. Size and orientation of the boulders was measured. The source of the boulders is the seawall constructed along the coastal strip. A full topo-profile was also undertaken to tie in the deposit with local topography.
October 16, 2009: Report from Bruce Richmond
Several sedimentological transects were made using DGPS in Siumu on the South Coast of Upolu where inundations of 40 to 150 m, run-up of up to 5 m and flow directions (N10) were observed. Samples have been collected from the reef edge to the most inner inundation line. The area of sedimentation located within the inner part of the reef flat was mapped. Coral tables freshly uprooted and transported by the tsunami waves were encountered in the western part of the reef flat and their location recorded.
At the Sinalei Reef Resort where severe damage to tourist infrastructure was observed (i.e. total destruction of seaside restaurant, nautical activities centre, premium beach bungalows), morphological impacts of the tsunami on the volcanic shore platforms were investigated and measured. We noted that several boulders were transported toward the beach and are good indicators of the incoming flow direction. The resort owners, when asked, told us about their grandmother telling them about the tsunami of 1917. She was living in Apia and saw the water receding and the fish on the beach. She ran uphill and didn't see the wave.
In Mulivai, at the forest fringe, we collected a short core (14 cm in length) of sediment in a marsh and we measured salinity of water. No evidence of sand deposition is present but ponding water was still brackish (3-7.4‰) despite the heavy rain of yesterday confirming that tsunami waves have reached this area although the vegetation show no signs of salt burning. This confirms that geochemical analysis can provide valuable evidence of past tsunamis within the coastal geological record.
Further west, evidence of inundations (based upon the deposition of and damage to vegetation) were observed and mapped in Lotofaga and Lefaga but no signs of beach erosion or building damage were noted.
October 15, 2009: Report from Bruce Richmond
Initial work focused on the wharf that has been constructed near the village of Satitoa. Evidence was found that the tsunami had damaged the infrastructure present at the wharf. A large set of concrete steps have been broken away from the end of the wharf, moved across the wharf and lodged beneath metal girders. Scrape marks on the concrete that makes up the wharf indicate that the steps were not fully in suspension whilst being moved and also indicate flow direction. We noted these impacts on structures as they were very clear (even though this team is not focused on building impacts).
The remainder of the day's work focused largely in the area of Satitoa. Here a sizeable sand sheet was identified that is almost certainly of tsunami origin. This sand sheet overlies a hard compacted sand that is dark in colour and is distinctly different from the tsunami deposit. There is a very clear boundary between these two sediment layers.
Seven trenches, located at varying intervals from the coastal strip in a landward direction, were dug to identify the extent of the tsunami derived deposit and any variations in sediment thickness. These trenches revealed that the layer varies in thickness moving away from the coastline. Closest to the road that runs near to the coastline, the layer is between one and four centimetres thick. The layer then thickens up to maximum and ten centimetres and then thins again back to 1-2 cm thickness.
The tsunami sediment layer has clear structure with a number of sub-layers identified. At the base is a layer of coarser material that sometimes contains volcanic pebbles and larger shell material. The upper portion of the layer is overall finer in nature but, depending on location, contains up to six separate layers, with clear variations in sediment character (coarser and finer layers).
The nature of the material that underlies the tsunami deposit (compacted material) suggests the tsunami initially stripped off any sediment that may have previously existed above this compacted sand and then proceeded to deposit the sand sheet. The sub-layers present in the sand sheet suggest the possibility of multiple deposition events (that is, several waves!). Further work in the coming days will continue work on this sand sheet and involve exploration of the southern and south-western coast, looking to identify other sand sheets.
October 14, 2009: Report from Bruce Richmond
A team of geologists worked together to undertake a rapid reconnaissance of the east and south coasts to identify locations that preserve a record of the landscape changes and geological impact and record of the tsunami. Specifically, the team intends to measure and record how the landscape has changed onshore as a result of the tsunami. The geology team had a very good day identifying sites that meet this need. They identified locations where the tsunami eroded sediments at the surface as well as places where it deposited sediment. Such observations are very helpful for understanding past frequencies of tsunamis.
October 12, 2009: Report from Bruce Richmond
Bruce Jaffe left Pago Pago on Sunday evening (10/11) after spending a week with the International Tsunami Survey Team (ITST) collecting data on an island-wide basis of tsunami runup, inundation, flow depth and direction, and wave heights. In addition they made critical observations on tsunami impacts to coastal infrastructure and natural environments. Bruce established numerous contacts with local and federal partners.
Bruce Richmond and Mark Buckley (USGS/WCMG) and Walter Dudley (UH Hilo) arrived in Pago Pago on Thurs 10/08 and have undertaken detailed studies on morphological change, sediment transport, and tsunami flow characteristics for areas particularly impacted by the tsunami. Richmond, Buckley, and Dudley have been joined by Brian McAdoo of Vassar and will leave for Apia, Samoa on Tues 10/13 to join the ITST which will begin surveys in Independent Samoa on 10/14.
Guy Gelfenbaum, Steve Watt, and Alex Apotsos (USGS/WCMG) arrived in Pago Pago on the evening of Oct. 11 and began conducting detailed topographic mapping and deposit characteristic measurements on Oct 12.
We have enjoyed fantastic support and interest from the village chiefs and inhabitants where we have conducted our surveys. In many cases the villagers have lost their homes and possessions, yet are willing to help us with our studies.
October 7, 2009: International Tsunami Survey Team
Reported by Bruce Jaffe, USGS WCMG:
More work remains to be done. Tomorrow I will go by small boat to hard to access sites on the north side of the main island. Other team members are going to Ofu and Tau today. The USGS team studying boulder and sand transport during the tsunami and the buffering effects of coral reefs starts arriving on Thursday, October 8, 2009.
So far, an extremely successful trip.
I should mention that I also was able to help with the response to the tsunami warning from the Vanuatu event this afternoon. The people were in a state of panic and many of the roads were nearly grid-locked as people tried to get to their homes. I went to the command center, told them that the event did not likely generate a tsunami that would be large in Samoa, and led them through the data I used to come to that conclusion. (I also called a radio station to [persuade] people [to] stop the panic, and to listen to the emergency management for more information as it came in.) The warning was called off about 1 hr before the tsunami, which I later found out NOAA/PMEL modeling predicted to be about 6 cm in Pago Pago.
The ITST led by Herman Fritz has completed runup, flow depth, flow direction, and inundation measurements on Tutuila, American Samoa. Below is a map showing preliminary maximum runup heights. These are not tide correct and the scale of the figure precludes showing all of the details of the observed spatial variation in runup. Locations of some of the higher runups are shown.
We would like to thank Gerard Fryer for his leadership and Lauren Wetzell, John Goeke, Don Vargo, and many others in the American Samoa scientific community for their help with this survey.
We would also like to thank the people of American Samoa who welcomed the team into their villages to collect this critical data.
We hope that this data, and data collected by the Japanese team now in American Samoa and subsequent teams, will decrease loss in future tsunamis in American Samoa and elsewhere.
--Bruce Jaffe for the entire ITST
October 6, 2009: Post-Tsunami Survey
The USGS post-tsunami investigation team has begun arriving on American Samoa with deployment planned through Oct. 18. Bruce Jaffe has arrived and joined one of five survey teams currently measuring runup, inundation distance, and flow depth. He reports flow depths of 5-6 m and runups of 10-12 m on the northwest side of the island. Five additional USGS scientists (Bruce Richmond, Mark Buckley, Guy Gelfenbaum, Steve Watt, and Alex Apotsos) are scheduled to deploy starting Oct. 8. They will be accompanied by Walter Dudley, executive director of the Tsunami Museum in Hilo HI and a filmmaker, who is working with USGS to document the work of the survey teams and interview survivors to develop future preparedness instructional materials. The USGS team will also focus on sedimentary deposits and near-shore bathymetric measurements. Two of our scientists (Gelfenbaum and Richmond) may go on to join an international team heading to the Country of Samoa (formerly Western Samoa) on or about Oct. 15.
September 30, 2009
On September 29, 2009, a magnitude-8.0 submarine earthquake occurred at 6:48a.m. Samoa Standard Time approximately 190 km (120 mi) south of Samoa and triggered a tsunami that caused more than 100 deaths and widespread damage in Samoa, American Samoa, and Tonga. Observers reported four tsunami waves that ranged from approximately 1.5 to 6 m high and reached as far as 1.5 km inland.
A rapid-response team of USGS scientists is traveling to American Samoa to collect data that will be quickly degraded or destroyed by recovery activity and natural processes. USGS oceanographer Bruce Jaffe arrived in Pago Pago, on the island of Tutuila, American Samoa, on October 4 and was joined later in the week by Bruce Richmond, Mark Buckley, Guy Gelfenbaum, Steve Watt, and Alex Apotsos. Oceanographer Walter Dudley of the University of Hawai‘i, Hilo, will work with the USGS team. The team will collect time-sensitive data to help them determine the height of tsunami waves at various sites and the distances the waves traveled inland. They will study the transport of sediment and other debris, look for and measure evidence of subsidence and uplift caused by the earthquake, document erosion caused by the tsunami waves, and make other observations critical to the better understanding of tsunami impacts and processes.
According to observers (as of 30 September), the first waves reached coastlines soon after the earthquake, giving residents little time to flee to higher ground; one survivor from a village in Samoa estimated just 5 to 10 minutes between the end of the shaking and the arrival of the first wave. In some cases, residents who felt the earthquake did not flee because earthquakes they had felt before had not been followed by a tsunami. The death toll was 191 on October 4 and expected to rise. The USGS team will coordinate planning with other Federal and local agencies to ensure that scientific activities do not interfere with rescue and recovery activities.
One or two members of the USGS American Samoa response team will join an international rapid-response effort to map tsunami impacts in Samoa, tentatively scheduled to begin October 15, 2009, in Apia, Upolu, Samoa.
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