Damages of infrastructures in the disaster of 26 Dec 2004 earthquake and tsunami in Aceh-Indonesia.

Rahardjo, Paulus P.

The Sumatera earthquake and tsunami disaster of Dec 26, 2004 was tremendous. The destruction and death were over whelming, causing all nations to be awakened by the fact that this is the largest tsunami disaster in the century. This paper describes the seismotectonic of Sumatera and some aspects on the damages from geotechnical concern.

INTRODUCTION

Indonesia is located in the mid of tectonic plate collision of Indo-Australia, Eurasia and Pacific Plates where subduction zone was formed and generates active volcanoes and active faults resulting earthquakes and volcanic eruptions. As the results, Indonesia is prone to geologic hazard which are quite disastrous and destructive.

In the 26 Dec 2004 Sumatera earthquake, more than 280,000 people were swept to death, many infrastructures and buildings were flattened or washed out to sea. The destruction was the most severe in Indonesia, mainly in the city of Banda Aceh and Meulaboh. This paper is intended to describe the seismotectonics of Sumatera which caused the earthquake and to discuss the geotechnical aspects on the damages.

SEISMOTECTONICS AND SEISMICITY OF SUMATERA

The west coast of Sumatera and Aceh is geologically instable. One of the key to understand the crustal deformation of this area is by observation of the mechanism of Sumatera fault system, which stretches out from Aceh to Sunda strait. Sumatera island is part of the Sunda Arc (Hamiltaon, 1979) and the seismicity of Sumatera is characterized as Western Sunda Arc (Puspito and Shimazaki, 1995). The tectonic pattern of Sumatera is controlled by two activities elemets, i.e. (1) subduction zone in the Sumatera-Java trench and (2) shear zone in the Sumatera-Java trench and (2) shear zone along Sumatera island.

As shown in Fig. 1, Sumatera is like other islands of Indonesia, representing and island arc where the formation was related to the activity of subduction zone. In Sumatera, the subduction is oblique (not perpendicular) with direction of approximately N 20[degrees] E, and the impact of this movement cause faulting of the west side of Sumatera island (Fitch, 1972, Beck, !983; Huchon and Le Pichon, 1984). This fault is almost parallel to Sumatera island and according to Tjia (1977) this fault consist of a few segments.

[FIGURE 1 OMITTED]

The India plate is part of the huge Indo-Australian Plate which underlies the Indian Ocean and the Bay of Bengal and is drifting north-east at a rate of about 60 mm per year. The India plates meets the Burma Plate (part of the Eurasian plate) at the Sunda Trench off Sumatera and has been slipping beneath the Burma Plate for million of years. The existence of small islands in the west of Sumatera has close relationship with the Sumatera fault. Mentawai islands and some other islands including Batu, Siberut, Nias and Simeuleu were formed through this mechanism and the formation take place in thousands or even millions of years. Due to collisions between ocean plates and the continental plate, these islands are moving up by 2-3 cm/year. At plate boundaries, the thin oceanic plates slip under continental plates known as subduction. The epicenter of the earthquakes in this area, mostly occur just below the islands such that occurred in Nias last March 2005.

As understood from the seismotectonic of Sumatera, the seismicity of Sumatera is controlled by the subduction zone. Figure 2 shows the epicenters of earthquake magnitude M > 5.0 since 1975. The distribution of earthquakes in Sumatera is dominated by shallow and medium depth earthquake (depth = 0 - 300 km). The depth of earthquakes foci in the subduction zone is maximum 100 km in the Andaman islands to about 300 km south of Sumatera, this means that the subduction period in the north is more recent compared to the south. The earthquakes along the shear zones of Sumatera are generally less in magnitude, however the ground motion are capable of damaging the human settlement because the foci are shallow (<30 km) such as Liwa (1994) and Kerinci (1995) earthquakes, both with M = 7.0

[FIGURE 2 OMITTED]

THE DISASTER OF 26 DEC 2004 EARTHQUAKE AND TSUNAMI

At 00.58:53 GMT and 07.58:53 local time, the slow build up of pressure caused by the continuous grinding of the two plates resulted in the giant earthquake. About 1200 km of the fault line slipped 15 m along the subduction zone. The huge rupture is known as a "megathrust" and produced an earthquake that lasted for more than four minutes and measured 9.0 on the Richter Scale, making it one of the most violent record. In addition to the sideways movement between the plates, the seabed is estimated to have risen by several meters as a result of the quake, thereby displacing millions of tones of water above it (Tibballs, 2005).

Within just 15 minutes after the earthquake, the big wave struck Aceh Province, 248 km north-east of epicenter, 15 minutes later, the wave hit Andaman and Nicobar Island and then Malaysia. About one and a half hour, Thailand was swept and then after two hours it attacked Srilanka and the south east coast of India. Next Maldives and incredibly, seven hours later the tsunami arrived in East Africa, 4500 km from epicenter with sufficient destructive energy.

Based on the extent of main shocks and after shock, the rupture area can be determined, and this has very close relationship with the magnitude of the earthquake. Aceh earthquake of 26 Dec 2004 is one of the largest in the last 50 years after Chilie earthquake 1960 (Mw = 9.5, Mo = 20x1029 dyne cm) and Alaska earthquake of 1964 (Mw = 9.2, Mo = 8 x1029 dyne cm). The three earthquakes as mentioned above caused damaging tsunami (Fig. 3).

[FIGURE 3 OMITTED]

DAMAGES CAUSED BY THE TSUNAMIS IN ACEH

The damages by earthquake and tsunami in Nangroe Aceh Darussalam were due to ground vibration and wave energy of tsunami which intensely ruin infrastructures and buildings in the city. Since the earthquake epicenter is a relatively far from the city, the tsunami effect is more significant. The area affected by wave run up was estimated 10,000 km2 as shown on Figure 4. The damages were more in Banda Aceh and Meulaboh because the elevation of the cities are mostly less than 20 m from sea level. The appearance of the affected area were drastically changed due to destruction and washing out after tsunami. The green appearance changed into brown (soil) or black (mud). The earthquake has caused disconnection and mal-function of communication facilities and electricity. Other life lines such as roads and bridges, airports and harbors were practically washed out or filled with black muddy material. At the airport the runway were flooded by post tsunami event and in the harbor, all of the sea traffic signs were gone. The jettys were full of debris and water channel can not be identified.

[FIGURE 4 OMITTED]

The damages after tsunami will continue due to sea water flooding that damaged also city drainage system, causing innundation in the city. Water innundated low areas and the soils were saturated. When the rains come, this area can no longer be able to absorb water and flooding could easily occur. Due to the damages of buildings and infrastructures, these facilities need to be evaluated for future use and properness. This will take time before they can be reutilised. The roads damages in NAD may be more than 280 km, the worst being Aceh-Meulaboh (223.5 km) where 10 bridges collapsed.

The damages in Western area of NAD are the most serious. It is interesting to view the damages in terms of geotechnical aspects. Most western and northern part of Banda Aceh and North Sumatera mainly consist of loose fine sand sediments. The geotechnical concern on this type of material is the problem of liquecfaction and wash out material. As shown in the Figure 5, the collapse of building and bridges are due to the tsunami energy but liquefaction may occur prior to the arrival of the wave.

[FIGURE 5 OMITTED]

Banda Aceh is located in sedimentary area with V shape. The actual run up could reach more than 10 km although most of the damages are in the area of about 3 km from the coast line. The wave entered Banda Aceh through low land and two channels, one is from the east of the city. The drainage canal provided waterways for incoming water so that tsunami flooding could reach far into the city.

In fact the channel that was constructed in 1981, was meant for flood control at the city of Banda Aceh due to surplus of water from Krueng Aceh Lama. This channel brought the water into the heart of the city. Although the grand mosque was located 3 km away from the shoreline, when the water came in secondary channel, water turbulance were created causing even more damages. The damages of Banda Aceh were tragic as shown from sattelite photograph.

SUMMARY

* Knowledge on geology and seismotectonics of Sumatera is very useful to understand the mechanism of earthquake and tsunami in Aceh.

* Coastal morphology and the soil condition may have significant effect on the damages by tsunami. Since the soil condition in Aceh is predominantly loose sandy sediments, the material may have been liquefied and washed out easily upon wave return to the sea and hence light buildings or infrastructures can not survive.

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PAULUS P. RAHARDJO

Departement of Civil Engineering, Parahyangan Catholic University Bandung, Indonesia