Link to USGS home page
125 years of science for America 1879-2004
Sound Waves Monthly Newsletter - Coastal Science and Research News from Across the USGS
Home || Sections: Spotlight on Sandy | Fieldwork | Research | Outreach | Meetings | Awards | Staff & Center News | Publications || Archives

 
Fieldwork

Astonishing Wave Heights Among the Findings of an International Tsunami Survey Team on Sumatra


in this issue:
 next story

An International Tsunami Survey Team (ITST) studying the effects of the December 26 tsunami on Indonesia's island of Sumatra documented wave heights of 20 to 30 m (65 to 100 ft) at the island's northwest end and found evidence suggesting that wave heights may have ranged from 15 to 30 m (50 to 100 ft) along a 100-km (60-mi) stretch of the northwest coast. These wave heights are higher than those predicted by computer models made soon after the earthquake that triggered the tsunami. "Groundtruthing" the models, which are used to forecast tsunamis for early-warning systems and long-term planning efforts, was one of the main goals of the scientific survey.

Map showing Sumatra and the areas urveyed by the International Tsunami Survey Team. Survey team members.
Above Left: Black rectangles show areas surveyed by the International Tsunami Survey Team in Sumatra. [larger version]

Above Right: Survey team members. Standing, left to right: Budi Waluyo, Takanobu Kamataki, Yuichi Nishimura (leaning over, with cap in hand), Sindhu Nugroho, Rahmat Triyono (wearing red cap), Raphael Paris, Yudhicara, Franck Lavigne, Inyoman Sukanta, Yuichiro Tanioka, team leader Yoshinobu Tsuji, Hideo Matsutomi, and Guy Gelfenbaum. Kneeling in front, left to right: Tsutomu Sakakiyama, Yoshikane Murakami, and Andrew Moore. Alphonso was part of the team in Sumatra but is not pictured. Members of the team who contributed from Japan are Masafumi Matsuyama and Yuichi Namegaya. [larger version]

The survey was conducted from January 20 to 29 in the province of Aceh, which lies only 100 km (60 mi) from the epicenter of the earthquake and sustained what many consider the worst tsunami damage of all affected areas. About a third of the 320,000 residents of Aceh's capital, Banda Aceh, are dead or missing, accounting for much of Indonesia's toll of more than125,000 dead and 90,000 missing.

Led by Yoshinobu Tsuji of the University of Tokyo's Earthquake Research Institute, the survey team consisted of nine scientists from Japan, six from Indonesia, two from France, and two from the United States. The U.S. scientists were Andy Moore of Kent State University and Guy Gelfenbaum of the U.S. Geological Survey (USGS).

The team collected information about wave heights at the beach and inland, inundation distance (how far inland the water reached), runup elevation (the water's height relative to mean sea level at its farthest reach inland), flow directions, erosion, sediment deposition, and coastal subsidence.

illustration of some of the tsunami characteristics typically measured by survey teams including: tsunami hight, inundation distance, and run-up elevation
Above: Some of the tsunami characteristics typically measured by survey teams (see Initial Findings on Tsunami Sand Deposits, Damage, and Inundation in Sri Lanka).

The team gathered some of its information from eyewitness accounts. Though not always reliable (commonly, eyewitnesses are running for their lives as they observe the tsunami), the eyewitness accounts collected in Sumatra provided several consistent pieces of information:

  • In Banda Aceh, the city closest to the earthquake epicenter, the tsunami arrived about 15 to 20 minutes after the earthquake was felt.
  • Many buildings in Banda Aceh withstood the earthquake shaking but were destroyed by the tsunami waves.
  • Likely because of the area's low elevation, the first tsunami wave that struck Banda Aceh did not have time to recede before the arrival of the next wave: the second tsunami wave rode over the first, and the third rode over the second.

Survey-team members Franck Lavigne (white cap, second from right) and Alphonso (far right) collect eyewitness accounts from residents in Banda Aceh. A mosque is left standing amid the rubble in Banda Aceh.
Above Left: Survey-team members Franck Lavigne (white cap, second from right) and Alphonso (far right) collect eyewitness accounts from residents in Banda Aceh. Photograph by Guy Gelfenbaum, USGS. [larger version]

Above Right: A mosque is left standing amid the rubble in Banda Aceh. Several mosques survived and may have been saved by the open ground floor that is part of their design. The tsunami waves reached the middle of the second floor. Photograph by Guy Gelfenbaum, USGS. [larger version]

Being just landward of the subduction zone where the tsunami-generating earthquake occurred, northwestern Sumatra was struck by a "near field" tsunami. In contrast, areas across the ocean from the earthquake epicenter, such as Sri Lanka, were struck by "far field" tsunamis. The rapid arrival of the tsunami in near-field locations—just 15 to 20 minutes in Banda Aceh—means that a tsunami early-warning system, now in the planning stages for the Indian Ocean, should be accompanied by tsunami education and long-term emergency and land-use planning efforts for the most effective mitigation of tsunami hazards.

Because the tsunami washed out many roads and bridges, the scientists had to hike long distances to reach some field areas, and on several occasions used makeshift rafts constructed from barrels and boards to cross rivers. Despite such complications, they were able to collect much data, which will be used to improve both the scientific understanding of tsunamis and the computer models used to predict tsunami effects.

To measure wave heights, the scientists looked for water stains on buildings and broken branches and debris in trees (where buildings and trees were left standing), then used laser rangefinders to calculate the heights. The west-facing coastlines were struck by the highest waves, some more than 30 m high. Waves that wrapped around the island to hit the north-facing coastline of Banda Aceh were lower, about 10 m high, but the area's low-lying land allowed those waves to penetrate far inland. Inundation distances in the province were so large that they were most easily measured from satellite images, where sediment deposited by the waves and vegetation killed by the saltwater are clearly visible. One such image shows that the waves that struck the villages of Lampuuk and Lho Nga on the west coast met the waves that struck Banda Aceh from the north.

Graphs of tsunami heights measured by the survey team in Banda Aceh (north coast) and along the west coast from Lampuuk to Leupeung. satellite image of northwestern Aceh province
Above Left: Graphs of tsunami heights measured by the survey team in Banda Aceh (north coast) and along the west coast from Lampuuk to Leupeung. [larger version]

Above Right: Brown tones of tsunami deposits and vegetation killed by saltwater show the extent of the tsunami's inundation in this satellite image of northwestern Aceh province. Arrows indicate tsunami flow directions. Note that tsunami waves flowing in from the north met waves from the west in one area. Satellite image acquired using Space Imaging's IKONOS satellite and processed by the Centre for Remote Imaging, Sensing and Processing (CRISP), National University of Singapore. [larger version]

Steel beam bent by debris carried in the powerful waves that struck Lho Nga on Sumatra's west coast. stranded the coal barge and tugboat shown in these photographs taken south of Lho Nga
Above Left: Steel beam bent by debris carried in the powerful waves that struck Lho Nga on Sumatra's west coast. Photograph by Guy Gelfenbaum, USGS. [larger version]

Above Right: The powerful tsunami waves snapped off trees, swept buildings from their foundations, and stranded the coal barge and tugboat shown in these photographs taken south of Lho Nga. The line along the cliff where thick green vegetation (above) meets brown rock and rubble (below) marks the elevation reached by tsunami waves here: 31 m (102 ft) above sea level. Wide view southward; closeup view northward. Photographs by Guy Gelfenbaum, USGS. [larger version]

Items broken and bent by the tsunami waves, as well as sedimentary structures in the tsunami deposits, were used to determine flow directions. The scientists found that the large tsunami waves flowed around natural barriers, flooding low-lying areas behind them.

The researchers surveyed beach profiles to document erosion (common near the coast) and deposition (common inland) by the tsunami. Sand eroded from beaches probably provided much of the sand that was deposited inland. The survey team dug trenches in the tsunami deposits to measure their thickness and to examine other characteristics that can shed light on how high the waves were and how fast the water was flowing. Data from the sediment deposits will not only tell scientists about the recent tsunami but also help them recognize and interpret the deposits of ancient tsunamis, which, in turn, will help them better understand an area's tsunami history and its likely tsunami risk (see "USGS Scientists Study Sediment Deposited by 2004 Indian Ocean Tsunami" in Sound Waves, February 2005).

Models predict that the type of earthquake that caused the tsunami—a megathrust—will raise the sea floor above the fault rupture and cause subsidence near the coast. So, the team was not surprised to find evidence that coastal land had subsided in Sumatra. Trees with roots and lower trunks submerged in seawater indicate that coastal land subsided 1 to 2 m (3 to 6 ft) in some areas. Japanese team member Yuichiro Tanioka and his Indonesian graduate student Yudhicara resurveyed parts of Banda Aceh for which older elevation maps were available and discovered that the land there had subsided by 28 to 57 cm (about 1 to 2 ft).

Cross-sectional view of tsunami deposits at the village of Lampuuk. Guy Gelfenbaum measures coastal subsidence along Sumatra's northwest coast on the basis of trees whose roots and lower trunks are now submerged in saltwater.
Above Left: Cross-sectional view of tsunami deposits at the village of Lampuuk. A 73-cm thickness of sand was deposited over soil (dark material in bottom center), in beds indicating multiple pulses of sand deposition. The lowermost bed, 45 cm of sand grading upward from coarse to fine, settled from suspension in a single wave and can be used to characterize the wave's height and velocity. Photograph by Guy Gelfenbaum, USGS. [larger version]

Above Right: Guy Gelfenbaum measures coastal subsidence along Sumatra's northwest coast on the basis of trees whose roots and lower trunks are now submerged in saltwater. The land here subsided 1 to 2 m during the earthquake; the treetops were snapped off by the tsunami. [larger version]

The scientists' efforts were focused mainly around the very northwest end of Sumatra, but they also collected data about 100 km (60 mi) to the south, at Kreung Sabe. Wave heights of 15 m (50 ft) at that site suggest that the tsunami waves may have been unusually high, 15 to 30 m, along the entire 100-km stretch of coast from Kreung Sabe to the northwest tip of the island. USGS scientists hope to return to Sumatra in April to test this hypothesis by measuring wave heights at intermediate points along the coastline and to collect additional data, such as nearshore bathymetry and sediment-deposit profiles.

Erosion of beach sand is evident in this before-and-after pair of satellite images of the coast at Lampuuk.
Above: Erosion of beach sand is evident in this before-and-after pair of satellite images of the coast at Lampuuk. White structure near the center of the images is a mosque that survived the tsunami. Satellite image acquired using Space Imaging's IKONOS satellite and processed by the Centre for Remote Imaging, Sensing and Processing (CRISP), National University of Singapore.


Related Sound Waves Stories
USGS Geologist Invited to Map Tsunami Impacts in the Maldives
March 2005
USGS Scientists Study Sediment Deposited by 2004 Indian Ocean Tsunami
February 2005
Indian Ocean Earthquake Triggers Deadly Tsunami
Dec. 2004 / Jan. 2005
Could It Happen Here?
Dec. 2004 / Jan. 2005
Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record
October 2002

Related Web Sites
The December 26, 2004 Indian Ocean Tsunami: Initial Findings on Tsunami Sand Deposits, Damage, and Inundation in Sri Lanka
U.S. Geological Survey (USGS)
Tsunami Generation from the 2004 M=9.0 Sumatra Earthquake
U.S. Geological Survey (USGS)
Tsunamis and Earthquakes
U.S. Geological Survey (USGS)
Preliminary Analysis of Sedimentary Deposits from the 1998 Papua New Guinea (PNG) Tsunami
U.S. Geological Survey (USGS)
Preliminary Analysis of Sedimentary Deposits from the June 23, 2001 Peru Tsunami
U.S. Geological Survey (USGS)

in this issue:
 next story

 

Mailing List:


print this issue print this issue

in this issue: Fieldwork cover story:
Tsunami Survey Finds Evidence of Astonishing Wave Heights

Flood-Response Teams Document Effects of Hurricane Charley

Geologist Invited to Map Tsunami Impacts in the Maldives

Mapping Oyster Beds In Apalachicola Bay

USGS Research Vessel Helps Secure Super Bowl

Outreach Tsunami News Conference

Tsunami Web Sites

Staff & Center News Smith Director of National Wetlands Research Center

Stewart Receives Gulf Guardian Award

Publications March Publications List


FirstGov.gov U. S. Department of the Interior | U.S. Geological Survey
Sound Waves Monthly Newsletter

email Feedback | USGS privacy statement | Disclaimer | Accessibility

This page is http://soundwaves.usgs.gov/2005/03/index.html
Updated April 15, 2014 @ 01:53 PM (JSS)