Learning from Bengkulu Earthquake : Preliminary Observation on Impacts of the September 12, 2007 Earthquake in Bengkulu, West Sumatra, Indonesia
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Learning from Bengkulu Earthquake : Preliminary observation on Impacts of the September 12, 2007 Earthquake in Bengkulu, West Sumatra, Indonesia By Dwikorita Karnawati 1), Iman Satyarno2) and Subagyo Pramumijoyo3) Introduction Bengkulu earthquake which was tsunamigenic (with 2m run up sea water) occurred on Wednesday of September 12, 2007 at 18.10 with the epicenter depth of 10 km, and magnitude of Mw 8.3. The epicenter of such earthquake was located on S 4.67o and E 101.13o. Several impacts of this earthquake are briefly reported in this article, and those include: a. Tsunami. b. Surface cracks in association with land subsidence and potential landslides. c. Landslides d. Building damaged. e. Liquefaction The impacts a. Tsunami Several houses located at the beach of Serangai District in South part of Mukomuko Regency were found damaged or dragged and the electricity piles were tilted, some of them collapsed, due to the tsunami (Figure 1). Searching from the remnant of mud found at the wall of the houses and from the wooden debris, it was estimated that the run up sea water achieved 2 m height and extended up to 500 m onshore. b. Surface cracks, potential landslide and land subsidence. Ground motion during the earthquake also results in surface cracks, in association with land subsidence and potential landslides. The surface cracks were found mainly on the highway, either perpendicular (Figure 2) or sub parallel to the street. It is interesting that the directions of the crack extensions are coincide with the directions of collapse or tilting of pillars at the building. Most of the land subsidence (especially those found at the highway connecting the City of Bengkulu to Regency of North Bengkulu and Regency of Mukomuko) occurred at the site nearby the valley or slope, and potentially result in landslide (Figure 3). The maximum depth of subsidence exceeded 1 m, and this become a serious obstacle for the transportation across the regencies. c. Landslides. One of large earthquake-induced landslide was found at the road cut of the highway in Penyangka Village, at Air Besi District, the Regency of North Bengkulu. This large landslide consisted of several smaller landslides (each of those having length of crown up to 30 m), and thus the total extension of moving slope exceeded 800 m (Figure 4). Such slope was formed by sandy clay weathered from conglomerate with tuff intercalation. It was apparent that the existing mining activities on the slopes to produce clay bricks increased the vulnerability of such slope to fail as the landslides, and thus become a serious threaten to the safety and usability of the highway. 1. Dr. in Engineering Geology, Faculty of Engineering, Gadjah Mada University, Indonesia 2. Dr. in Structural Engineering, Faculty of Engineering, Gadjah Mada University, Indonesia 3. Dr. in Structural Geology and Tectonics, Faculty of Engineering, Gadjah Mada University, Indonesia Page 1 d. Building damage The epicenter was at the offshore with the distance to the damaged area is relatively far, which is about 130 km away from the City of Bengkulu, or about 275 km away from Regencies of Mukomuko. But still, the earthquake resulted in some damages on the houses and buildings. The most affected areas are situated at the Regency of Mukomuko (mainly at the District of Lubuk Pinang) as well as at the Regency of North Bengkulu (especially at the District of Lais) and the city of Bengkulu. According to Provincial Government report, there were 109 building damages in Bengkulu City and 1462 damaged houses in the Regencies of North Bengkulu and Mukomuko. Most of them are brick masonry houses. From site investigation it can be found that the cause of damage is not merely due to the high ground acceleration that occurred during the earthquake. This can be seen by the fact that not all of the typical houses damaged at the same location. Like in other earthquake region such as the one in Yogyakarta, it can be noted that the damaged houses are also commonly caused by: a. the low materials quality, b. do not follow earthquake resistant house requirements c. built on unstable ground and in adequate foundation design. In general, there are three important materials to be used for making the primary structural elements that will resist the earthquake force in the brick masonry houses. They are mortar, concrete and steel reinforcement. The mortar which is commonly used for bed joints is made from water, cement, and sand. The concrete is made from water, cement, sand and gravel with the skeleton of steel bars used for the reinforced concrete of primary structural elements. The quality of mortar and concrete is determined by the proportion of each material component mentioned above. Among these components, the cost of cement is the most expensive one. Therefore the amount of cement is commonly kept to be lower than the required amount, in order to minimize the total cost. As the amount of cement is lower than required, the quality of mortar and concrete in terms of compressive strength is low. This low quality of mortar and concrete can be easily broken even under earthquake force at quite low acceleration. The second factor that causes the damaged is that the building does not follow the earthquake resistant requirements, related to the existence and dimensions of primary structural elements and the detail connection between elements. An earthquake resistant brick masonry house is required to have primary structural elements such as plinth beams, columns with maximum distance of 3 m, ring beams, and gable beams as shown in Figure 5. More over it is also required that connection between elements is adequately detailed. Column reinforcement is anchoraged into the foundation at least 40 cm, the wall is anchoraged into the column with reinforcing bar with diameter of 8 mm, anchoraged 30 cm length at every 6 brick layers. The roof truss must be anchoraged with adequate bracing between roof truss or roof gable. However, as the cost of steel reinforcing bar is also quite expensive, most houses minimize or omit some primary structural elements to minimize the cost. In some cases the condition is worsen by the lack or in adequate steel reinforcing detail as shown in Figure 6. The last factor is because the houses are built on unstable soil with in adequate design of foundation. This house or building is built on quite soft soil but the foundation design is the same as other houses which are built of common better soil. It was found by Karnawati and Pramumijoyo that most of damaged and collapced houses in Lais District located at North Bengkulu Regency as well as in the city of Bengkulu were built on loose fine sand – sandy clay of aluvium deposits or as the product of highly weathered conglomerate. It is also apparent that many damaged houses or buildings were situated at the slope or nearby the valley, such as the damage of Social and Politic Faculty Building at the area of Bengkulu University (Figure 7). Another quite intensive damaged area was in Lubuk Pinang District at Regency of Mukomuko, Page 2 where most of the houses were built on clay deposits (fluviatil or swamp sediments), mainly which were located at or nearby the river/ valley or slope (Figure 8). Moreover, Pramumijoyo analysed that the direction of tilted pillar and tower was either approximately in parallel with the direction of the pillars and towers to the epicenter, or perpendicular to such direction. For example, at the beach of Bengkulu, there was a tilted monument at the position of S 3o 48’ 48.6”-E 102o 16’ 10.8” which inclined to the direction of N320oE (to the North West). This tilted- direction is about perpendicular to the direction to the epicenter, which was located at the position of S 4.67o and E 101.13o. Moreover, at Kota Agung (at the position of S 3 o 32’ 34.3”-E 102 o 04’ 52”), the central pillar of Babussalam Mosque (Figure 9a) was tilted to the direction of N50 o to the East, which was in parallel with the direction to the epicenter. Meanwhile, at Air Besi Village, in Lais District, at the position of S 3o 32’ 48.1”-E 102 o 05’ 05.5” there was a tower on the side of street (Figure 9b) which was tilted towards the direction of epicenter, i.e. at the direction of 20 o to the SW. At Lais, S 3 o 32’ 01.4”-E 102o 03’ 02”, the village tower also tilted to the direction of N 145 o to the south east, which was about perpendicular to the direction to the epicenter. Additionally, the Elementary school Ketahun-03, at the position of S 3 o 23’ 50.3”-E 101 o 51’ 27.5” was collapsed into direction of N 80 o E to the west, which was also about perpendicular to the epicenter direction. e. Liquefaction One of the liquefactions occurs nearby the Bridge of Pasar Seblat, at Putri Hijau District, Mukomuko Regency. This liquefaction was indicated by the appearance of 8 sets of semi- circular surface cracks in association with fine sand. The extensions of such cracks were about 100 m – 150 m at the river bank nearby the bridge (Figure 10 left). The eyewitness reported that there was water and sand burst coming from the cracks during the earthquake. The sand also came from the two wells nearby the bridge, and the wells now have been fully filled with sand (Figure 10 middle). Accordingly, the foundation of one supporting bridge pillar at one side of river bank now is exposed, because the supporting gabions on the pillar side was collapsed due to this liquefaction.