No. 10, January 2009

Contents: The 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 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 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 .

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, 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, 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 The earthquake of August 7, 2008 caused quite a lot of earthquakes in 2006. The Bantul 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 the damages and destructions in the 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 . 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 , 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. Typical damages to wooden-elevated houses are The cause of the poor construction: (Fig. 6): Further observation by the survey team has revealed the 1. Collapse of walls cause of the poor quality of the residential buildings in 2. Failure of roof structures the rural area of Dompu, which are (Fig. 7): 3. Slippage of the main columns 4. Cracks and broken columns A. Houses are built without columns or with columns 5. Totally collapsed building but with insufficient reinforcement • Buildings without columns, constructed using bricks which is bonded using mortar (mixing of cement and sand). Most of the buildings use poor mortar; small amounts of cement mixed with too much sand. • Buildings using columns but with insufficient reinforcement, for example, using only one or two steel bars, or even worse using bamboo as reinforcement. • Reinforcement of column using steel bar with small diameter, mostly 6 to 8 millimetres; below the minimum requirement of 10 millimetres.

Figure 5. Typical damages to landed-house

Fig 7.Building without column and column with improper reinforcement

Fig. 6. Typical damage to wooden-elevated houses

B. Poor quality of mortar. Light roof structure helps the houses to be earthquake Interview with local people revealed that the mortar resistant! used to build their houses is made by mix cements Figure 9 is a house in Calabai village, one of the most and sands with ratio of 1 : 7 to 1 : 10, far below the damaged areas. The constructions of the houses are of recommended ratio of 1 : 3 to 1 : 4. Poor mortar is substandard; the reinforcement of the columns uses weak and has low bonding capacity (Figure 8). bamboo, and the quality of mortar is very poor. However, this house is considered resistant to the Sumbawa Earthquake, no damage were seen on its upper structure (which usually caused by heavy roofing). The only minor damage found was a crack on one side of the wall. This was a proof that light roof structure helped the house (even though it is of substandard quality) increases its earthquake resistance.

Fig. 8. Example of a poor quality mortar taken from a house Economic and Insured Losses in Calabai village According to the Technical Department of MAIPARK,

the insured value in Lombok, Sumbawa (Dompu and

Bima area) totally IDR 1,595,837,161,238.27. C. Heavy roof structure MAIPARK’s share amount is IDR 195,545,283,487.77. In Dompu, the number of houses with heavy roof tiles Furthermore, MAIPARK data reveal that the commercial and light zinc plate roof are more or less equal. Even sector occupied 54.69%. though the local people have known that light roof

structures have better earthquake resistance, but for As a whole, the accumulation of earthquake risks is many reasons people are reluctant to use this type of mainly in area (Lombok) which reaches 179 roof. One of the reasons is that roof tile is more risk with an insured value of 100% which amounts to comfortable and also cheaper (they can even make it IDR 1,203,158,338,788.27 of which MAIPARK share themselves using clay dig from own yard). amounts to IDR 119,125,150,644.72.

Conclusions and Recommendations Most of the damaged buildings and dwelling houses were due to unsuited engineering, poor masonry, and the use of poor quality of building materials. In many cases, there was a tendency to build houses with too heavy roofings.

Obviously, there is also a fact that the damages from previous earthquake(s) were not repaired properly.

In most cases, heavy cracks occurred at the joints (poor engineering/architecture and poor masonry). What the Insurance Underwriters must be aware of is the subsurface structure in a certain region, in which wave Fig. 9. A Substandard two storeys house with light roof structure using z inc plates amplification plays a more important role in the distribution of the destruction. Keep in mind the case of

Bantul and Gunung Kidul in Yogyakarta Area in 2006.

Awareness to threat of earthquakes have to be intensified,

and in commensurate to that the insurance literacy has to

be increased. This is for the sake of the society itself and

also for the insurance industry as well.