The Sumbawa Earthquake of August 7, 2008
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No. 10, January 2009 Contents: The Sumbawa Earthquake of August 7, 2008 • The Sumbawa Earthquake of August 7, 2008 Introduction A shallow earthquake of magnitude 6.6 on the Richter scale struck the island of Sumbawa on August 7, 2008 at 05:41:01, West Indonesian Time. Its focal depth was reported by Badan Meteorologi, Klimatologi dan Geofisika (BMKG) as to be 10 km. The epicentre location was 8.16°S and 117.74°E, right at the tip of Moyo Island (Fig. 1 and see Table 1). No death casualties have been reported, but damages to dwelling houses were considerable. Published by PT. Asuransi MAIPARK Indonesia PT. Asuransi MAIPARK immediately dispatched a survey team consisting of Board of Advisors two geophysicists, one civil engineer Frans Y. Sahusilawane and one claim expert. Reports on the Bisma Subrata damages inflicted by this earthquake Board of Editors are mostly based on MAIPARK’s Survey Report. Mudaham T. Zen Andriansyah Fiza Wira Atmaja General Geology of Sumbawa Ruben Damanik The northern parts of Nusa Tenggara M. Haikal Sedayo Barat Islands are mostly covered by Heddy Agus Pritasa Quaternary (and recently active) Bintoro Wisnu andesitic volcanism associated with Jyesta Amaranggana the East-West Sunda-Banda Arc through Java and Bali; the Southern Address parts, however, reveal igneous rocks Fig. 1. Index map of Sumbawa Island and the MARKETING DEPARTMENT formed by tertiary island arc epicentre location of the August, 2008 earthquake PT. Asuransi MAIPARK Indonesia volcanism. with respect to Java Island and Bali Setiabudi Atrium Building, Fl. 4, Jl. HR. Rasuna Said Kav. 62, From satellite images (Landsat), air photographs-analysis, ground-truth observations, the Jakarta 12920 island of Sumbawa is being dissected by many faults and fracture patterns: East-West trending and also NWW-SEE trending faults which bounded Saleh Bay in the Western part Tel.: (021) 521 0803 Fax : (021) 521 0738 of Sumbawa. In the South-Western part in particular the region is dissected by North-South E-mail: [email protected] trending faults, Northeast-Southwest trending faults and a more or less East-West trending Website: www.maipark.com faults. The island of Sumbawa, like all of the islands of Nusa Tenggara Barat, and the islands of East Nusa Tenggara are bounded by the Sunda Subduction Zone in the South which continues on to the East into the Banda Subduction which coincides with a deep trench; to the North the islands are bounded by the Flores Thrust. There are speculations among Indonesian geologists that this Flores Thrust continues on to the Eastern part of East Java. Table 1. Source Parameter of the 2008 Sumbawa Earthquake Time of Origin Lat Long Focal Depth Magnitude Source 05:41:01,0 WIB 8,16°LS 117,74°BT 10 km 6,6 BMKG 05:41:01,6 WIB 8,03°LS 117,50°BT 14 km 5,7 USGS 05:41:04,7 WIB 8,15°LS 117,70°BT 30 km 6,1 GFZ-Potsdam As far as Waspada is concerned, we consider the Flores thrust to terminate north of the island of Bali until more geophysical and geological evidences can be shown by marine geophysical exploration and observation. The Flores Thrust as well as the subduction zone in the South are earthquake source zones. The subduction angle in this region is very steep, up to 62°, dipping to the North. The Western most islands of Bali, Lombok and Sumbawa carry the young structural and volcanic belts of Java Fig. 2. Simplified geologic map of Sumbawa Island. After eastward along strike. The Northern parts of these islands SUDRAJAT(1975) and BARBERI et al. (1987) contain many young and active volcanoes, whereas the rocks older than the late Miocene occur only near the Southern coast (Van Bemmelen (1949), Hamilton (1979), Sudrajat (1975), Barberi et al (1987), and Zen (1985). The Structural Pattern of Sumbawa Island Sumbawa Island consists of three parts: (1) the Western part bounded by the Western shore of Saleh Bay, (2) the Eastern part bounded by the Eastern shore of Saleh Bay and the waters of Sape Bay in the East, (3) in between a narrow axis connects the two parts. Most young volcanic Fig. 3a. Schematic north-south profile AB across Sumbawa cones are located in the Northern part of West Sumbawa. based on residual gravity anomalies The big active volcano of Tambora dominates the Landscape of the Sanggar Peninsula (see Fig. 2). The island is dissected by North-South trending faults, WNW-ESE and NW-SE trending faults, and double NS- SW trending faults in West Sumbawa, Saleh Bay, Bima Bay as well as the Waworada Bay are structurally controlled. Next to that the whole island is dissected by fractures (See Fig. 3a-3b). Fig. 3b. Schematic north-south profile CD across Sumbawa based on residual gravity anomalies Economic geology: Batu Hijau deposit mineralization The only economically significant mineralization recognized in Southwest Sumbawa is the Batu Hijau deposit. A drill-inferred resource for Batu Hijau of 334 million tons grading 0.8wt% Cu, and 0.69 gm per ton Au was reported in September, 1992 (van Leeuwen, 1994). Earthquake History of Sumbawa Bali, Lombok and Sumbawa experienced lots of earthquakes in the past. The most devastating one occurred in May 30, 1979; the epicentre was located Fig. 4b. A simple attenuation curve (Distance-Intensity) of the August 7, 2008 earthquake. This curve is derived from southwest of the South-western tip of Sumbawa. This the Fig. 4a earthquake was accompanied by a tsunami, 28 people perished by the tsunami, 79 people were wounded. The shocks were also felt in Lombok. Moyo Island itself, which is the closest area to the epicentre, is relatively less damaged than the other places, The damage from this earthquake was wide spread. 295 even though the intensity in Moyo was at VII (MMI). buildings (mosques, churches and temples) were heavily This is due to the fact that the foundation of Moyo island damaged, 197 schools and educational institutions, 116 consists of coral reefs, whereas many areas in the Sanggar business infrastructures, 73 government buildings and Peninsula (where Mt. Tambora is located) their 3,977 dwelling houses were damaged. amplification of the seismic waves were less, most part of the Sanggar Peninsula is covered by loose pyroclastic There are other minor earthquakes in other islands. In product of the 1815 eruption of Mt. Tambora. Most Sumbawa itself there was an earthquake which occurred damaging to the building and houses are the transverse or on November 26, 2007. In total 504 dwelling houses were secondary waves. damaged, 2 people died and 146 were wounded. This resembles the situation of the Yogyakarta The earthquake of August 7, 2008 caused quite a lot of earthquakes in 2006. The Bantul regency is covered by damage in many towns and villages such as Pekat, water saturated river deposits and loose volcanic products Nangamiro, Kadindi and Tambora. Some villages on from Mt. Merapi in the East. The adjacent regency of Moyo Island also were devastated. In total, 1.500 Gunung Kidul is built of Tertiary Limestone and dwelling houses were damaged. crystalline limestone. Compared to Bantul Regency the damages and destructions in the Gunung Kidul Regency Some of the damaged houses were already damaged were minor. during the earthquake of 2007. They were not properly renovated or restored. Damages The Sumbawa earthquake caused damages to almost The intensity distribution of the earthquake is depicted in 1.500 residential buildings, which 90% of the damaged Fig. 4a whereas the attenuation curve (Intensity-Distance) building were located in Kabupaten Dompu. Pekat, is depicted in Fig. 4b. Calabai, Nangamiro, and Kadindi village in Sumbawa Island, and Labuhan Haji village in Moyo Island were some of the worst affected area. While in the capital city of Dompu regency, the earthquake caused minor damages. Severe damages observed in the city were caused by the previous earthquake in November 2007. There are two types of residential buildings which are commonly used by local people. Rumah pangggung (wooden-elevated house) which is usually used by the original people of Sumbawa and rumah biasa/non- panggung (landed house) which is usually used by transmigrants who are mostly come from Lombok Island. Fig. 4a. Isoseist map of the August 7, 2008 earthquake in the Sanggar Peninsula Typical Damages Landed house suffered most damage due to Sumbawa earthquake. The typical damages are (Figure 5): 1. Small to large cracks of walls The cause of the so many damages 2. Fractures of the joints A 6.6 Richter scale earthquake is classified as an 3. Damages to roof structure (falling of roof tiles to intermediate quake. But as the epicentre of the Sumbawa failure of the roof structure) earthquake is very shallow and close to the residential 4. Damages to building structures area, it is not a surprising fact that the quake caused a lot 5. Totally collapsed building of damages. Many wooden-elevated houses were also inflicted by the There is a very interesting thing that in some damaged quake, particularly in Moyo Island. This type of house is areas there were few better-constructed buildings which known for its quake resistance, and has performed well in suffered only minor damage while other buildings (with Dompu Island during the Sumbawa quake. But in Moyo poorer construction) in its surroundings suffered major Island, where 70% of the buildings are wooden-elevated damages. A conclusion we get from this is poor houses, most of the buildings suffered damages, although construction is the main cause of the so many damages rather light. This is caused by the age of the buildings caused by the quake which are mostly old and also due to poor quality of the woods.