Engineering for the Threat of Natural Disasters

Effects of the 2004 Sumatra-Andaman Earthquake and Indian Ocean Tsunami in Aceh Province

This eyewitness assessment of damage from the earthquake and tsunami yields lessons for engineers.

On December 26, 2004, at 07:58:50 local time, a powerful earthquake, moment magnitude (MW) 9.2, occurred in the Indian Ocean. The Sumatra-Andaman earthquake was one of the three largest earthquakes ever recorded. The fault rupture propagated 1,300 to 1,600 kilometers northwest for about 10 minutes along the boundary between the Indo-Australian plate and the Eurasian plate, from northwestern Sumatra to the Nicobar Islands and to the Andaman Islands. The hypocenter, the point where the fault rupture origin-ated, was 10 kilometers deep. The faulting spread updip and downdip from 18 to 25 meters on a low-angle thrust fault plane dipping about 10 degrees northeast. The Indo-Australian plate moved northeast relative to the Eurasian plate. Several excellent papers have been written on the tectonics of the earthquake (e.g., Lay et al., 2005), and the seismological, geologic, and geodetic aspects have been comprehensively described by Kanamori (2006) and Hudnut (2006).

The resulting tsunami affected 12 nations around the Indian Ocean, with Indonesia suffering the greatest damage. In Aceh, the northern province of Sumatra, the United Nations (UN) Field Office reported approximately 131,000 people confirmed dead and 37,000 missing. With more than 80,000 houses sustaining major damage or collapse, the UN estimated that more than 500,000 people were displaced from their homes in Sumatra alone. In addition to the massive damage to housing, utilities, roads, and bridges, the disaster significantly disrupted the social fabric and government of the affected communities.

Shaking Damage
The epicenter of the earthquake was about 250 kilometers off the west coast of Aceh Province. Strong to violent shaking in Aceh Province reportedly lasted five to six minutes. Banda Aceh was the only major city that experienced earthquake-shaking damage. One- to two-story, traditional, concrete-frame and wood-frame buildings survived well and were largely undamaged by the strong ground shaking. However, because the earthquake occurred a significant distance offshore, the resulting long-period ground motions caused serious damage to, or the collapse of, buildings more than three stories high.

Fault Deformation
A compounding problem was tectonic subsidence resulting in 20 to 100 centimeters of down-warping of the Earth’s crust beneath the Aceh region. The subsidence extends for at least 280 kilometers along the entire northwestern Aceh coast (Figure 1—see PDF version for figures). This submergence thwarted rescue efforts and has hindered the restoration of roads, bridges, and utility distribution systems.

Tsunami Damage
The fault rupture uplifted the ocean floor, releasing the most destructive series of tsunami waves in recorded history. The waves spread throughout the Indian Ocean, causing damage in the coastal communities of 12 countries. By far, the most damaging effects were sustained by Aceh Province, where three devastating waves struck the western shore within about 30 minutes. The tsunami waves ranged from 4 to 39 meters high and destroyed more than 250 coastal communities.

In the low-lying areas of western coastal Sumatra, including the city of Banda Aceh, the tsunami waves extended inland as far as 5 kilometers, affecting a large portion of the population of 300,000. The western part of the city has nearly flat topography traversed by rivers and drainage channels. In these areas, the maximum wave-flow height was 4 to 8 meters. In hilly areas south of Banda Aceh, the wave-flow height was significantly greater, due to the topography.

Residential neighborhoods and fishing villages in coastal areas were entirely devastated, and houses were swept inland or out to sea. The traditional construction that had resisted shaking damage could not resist the tsunami forces and most were obliterated. Figure 2 shows what was left of most houses—mostly the concrete floor slabs. The tsunami waves left extensive piles of timber and the remains of buildings.

Most well designed and well constructed buildings and industrial facilities that had withstood the earthquake shaking also withstood the tsunami waves and suffered only minor damage. For example, the La Farge Cement Plant (Figure 3), a well designed and well constructed steel-frame series of industrial structures about 20 kilometers southwest of Banda Aceh, did not experience structural damage from the strong shaking and was not damaged by the tsunami waves, which, as documented by stadia-rod, reached a wave-flow height of 38.9 meters nearby. Several one- and two-story administrative buildings and machine shops were smashed by waves carrying nearly empty large oil-storage tanks. The impact of the waves caused non-structural damage to some of the buildings. For example, metal siding was stripped from the steel-frame buildings up to the height of the waves (Figure 4).

Figure 5 shows a typical mosque south of Banda Aceh, which was impacted by 5-meter-high tsunami waves. Inspection revealed that the quality of construction and of the concrete in most mosques was excellent. Most have steel-reinforced concrete frames as load-resisting systems, along with domes and open arches that allowed tsunami waves to traverse the space without causing serious damage.

The low-lying topography of Banda Aceh and surrounding areas and the height of the water resulted in debris being swept in and out by the three successive destructive tsunami waves. This caused large, heavy projectiles, such as cars, trucks, and fishing boats, to be swept in and out, each time impacting previ-ously undamaged facilities. Many small buildings were structurally damaged by tsunami waves carrying floating debris.

A large number of fishing boats were docked at the coastal and river locations that traverse the city. Fishing boats were torn from their moorings and cast inland during the tsunami. One boat that was permanently docked on the second story of a house (Figure 6) saved 52 people, who were able to climb through the roof-hatch and take shelter there; inside, they found a stranded security person in the captain’s quarters.

Electric Power
Most well designed and well constructed electric power plants in Aceh Province did not experience structural damage from the earthquake or tsunami. The electric generating facilities experienced light damage to the generating capacity and no damage to the transmission network. However, there was substantial damage to the distribution network in the affected area. Most above-ground distribution systems were seriously damaged or destroyed by the tsunami. Damage to the power supply was concentrated in western Aceh Province, along low-lying areas in Banda Aceh and toward the south along the west coast to just beyond Meulaboh. The main damage was to the power distribution networks (small substations and hollow-core distribution poles). About 170,000 customers were affected by loss of power in Banda Aceh and along the low-lying coastal plain to Meulaboh.

Indonesia’s public electric supply is provided by PT PLN, the state-owned electric company. Banda Aceh’s electric power comes from the Aceh regional electric grid and, in central Banda Aceh, the Luengbata diesel-generation plant (50-megawatt, 11 units), which reported damage only to some generation transformers. An 11-megawatt diesel-generating station, mounted on a barge offshore, was swept inland more than 3 kilo-meters from the harbor in Banda Aceh by tsunami wave action. Although the power plant was undamaged, it left a path of destruction of houses and commercial buildings as it charged inland. PT PLN plant operators informed us that neither the intense shaking nor the 3 kilometer transport of the barge-station was the reason the plant was not operating; the main problem was lack of demand. PT PLN reported that electric power was restored to most emergency-response customers in Banda Aceh within three days and to the remaining customers within about two weeks.

PT PLN reported that the electric system generally was not affected by earthquake shaking, except for the newly built headquarters building, which was more than three stories high and had to be abandoned. The tsunami did not affect the 150-kV substation or the inland diesel-generating power stations. A small (1 megawatt) diesel-powered plant was destroyed at Calang, directly on the coast about halfway between Banda Aceh and Meulaboh. The Meulaboh Lamno diesel plant did not experience significant damage. The 150-kV transmission line and associated substations transmitting power from power plants to the east functioned normally during and after the earthquake and tsunami. In fact, the electric power in western Aceh Province did not shut down. Some PT PLN emergency-response workers were electrocuted when they attempted to restore electricity to emergency facilities because they had assumed the tsunami had tripped the power supply.

Gas and Liquid Fuel Facilities
The state-owned Pertamina petroleum company suffered substantial damage to fuel depots, where storage facilities were damaged and some fuel was lost, mostly on the west coast of Aceh Province, particularly in Banda Aceh and south to Meulaboh. The deep-water port at Kreung Raya, the petroleum storage and distribution facility, lost half of its above-ground piping and 3 of 12 liquid fuel (diesel, high-octane gas, oil, and kerosene) storage tanks. None of the tanks was anchored to its foundations, and the three that were swept away by tsunami waves were only partially full. The nine full storage tanks were not affected. As with the electric system, most above-ground distribution systems were seriously damaged or destroyed by the tsunami.

Roads and Bridges
Roads and bridges were devastated by the force of the tsunami waves. Many bridges were swept off their supports, and connecting earth embankments were significantly scoured, disabling the transportation network for hundreds of kilometers along the west coast of Aceh Province. Hundreds of bridges were picked up and swept inland by the tsunami waves, some more than a kilometer. The extensive damage to bridges severely constrained rescue and relief efforts, as the bridges had been vital links to population centers in the region. Many of the bridges on the coastal road to Meulaboh were destroyed and washed away, and sections of the road disappeared, which isolated many small communities. Survivors could be reached only by boat or helicopter. In addition, the destruction of the bridges resulted in the disruption of the electric distribution system at bridge crossings.

Although earlier reconnaisances reported no evidence of liquefaction, earthquakes of this magnitude and duration commonly cause liquefaction in coastal areas. During a reconnaissance by helicopter, we observed extensive liquefaction in near-shore beach deposits for at least 150 kilometers along the Aceh coast, from south of Meulaboh to north of Calang. Figure 7 shows massive earthquake-induced sand-blows, with craters scoured by tsunami wave action. These liquefaction effects may have been the deciding factor in the destruction of the PT PNL 1-megawatt power plant on the coast at Calang.

Although routinely constructed houses and buildings may have been able to survive the earthquake shaking, tsunami waves devastated almost all of them. Most well designed and well constructed utility and industrial facilities had sufficient capacity to withstand both the earthquake and the tsunami. Partially full storage tanks, bridges, and other light structures that were not anchored to their foundations were not able to resist tsunami forces. Tectonic subsidence and liquefaction were significant contributors to the devastation.

This paper is based on a team (Lloyd Cluff, George Plafker, and Stuart Nishenko) reconnaissance, sponsored by Pacific Gas and Electric Company, to Aceh Province, Indonesia, in May 2005, almost five months after the December 26, 2004, earthquake and tsunami. The purpose of the investigation was to assess the performance of gas and electric systems and related industrial infrastructure. Our reconnaissance focused on northern Aceh Province, where the earthquake and tsunami effects were most severe.

Hudnut, K.W. 2006. Geologic and geodetic aspects of the December 2004 great Sumatra Andaman earthquake and 2005 Nias-Simeulue earthquake: 2004 great Sumatra earthquakes and Indian Ocean tsunamis of December 2, 2004, and March 28, 2005. Earthquake Spectra 22(S3): S13–S42.
Kanamori, H. 2006. Seismological aspects of the December 2004 great Sumatra-Andaman earthquake, 2004 great Sumatra earthquakes and Indian Ocean tsunamis of December 26, 2004, and March 28, 2005. Earthquake Spectra 22 (S3): S1–S12.
Lay, T., S. Das, D. Helmberger, G. Ichinose, J. Polet, and D. Wald. 2005. Rupture process of the 2004 Sumatra-Andaman earthquake. Science 308(5725): 1133–1139.

About the Author: Lloyd S. Cluff is director of geo-sciences and earthquake risk management, Pacific Gas and Electric Company, and an NAE member.