Published by PT. Asuransi MAIPARK Indonesia

Board of Advisors Frans Y. Sahusilawane

Bisma Subrata

Board of Editors Mudaham T. Zen

Andriansyah

Fiza Wira Atmaja

Ruben Damanik

M. Haikal Sedayo

Heddy Agus Pritasa

Bintoro Wisnu

Jyesta Amaranggana

Address MARKETING DEPARTMENT

PT. Asuransi MAIPARK Indonesia

Setiabudi Atrium Building, Fl. 4,

Jl. HR. Rasuna Said Kav. 62,

Jakarta 12920

Tel.: (021) 521 0803

Fax : (021) 521 0738

E-mail: maipark@maipark.com

Website: www.maipark.com

Contents:

• The Sumbawa Earthquake of

August 7, 2008

Fig. 1. Index map of Sumbawa Island and the

epicentre location of the August, 2008 earthquake

with respect to Java Island and Bali

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.

PT. Asuransi MAIPARK immediately

dispatched a survey team consisting of

two geophysicists, one civil engineer

and one claim expert. Reports on the

damages inflicted by this earthquake

are mostly based on MAIPARK’s

Survey Report.

General Geology of Sumbawa

The northern parts of Nusa Tenggara

Barat Islands are mostly covered by

Quaternary (and recently active)

andesitic volcanism associated with

the East-West Sunda-Banda Arc

through Java and Bali; the Southern

parts, however, reveal igneous rocks

formed by tertiary island arc

volcanism.

From satellite images (Landsat), air photographs-analysis, ground-truth observations, the

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

of Sumbawa. In the South-Western part in particular the region is dissected by North-South

trending faults, Northeast-Southwest trending faults and a more or less East-West trending

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.

No. 10, January 2009

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

eastward along strike. The Northern parts of these islands

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

cones are located in the Northern part of West Sumbawa.

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

Fig. 3a. Schematic north-south profile AB across Sumbawa

based on residual gravity anomalies

Fig. 2. Simplified geologic map of Sumbawa Island. After

SUDRAJAT(1975) and BARBERI et al. (1987)

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

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

southwest of the South-western tip of Sumbawa. This

earthquake was accompanied by a tsunami, 28 people

perished by the tsunami, 79 people were wounded. The

shocks were also felt in Lombok.

The damage from this earthquake was wide spread. 295

buildings (mosques, churches and temples) were heavily

damaged, 197 schools and educational institutions, 116

business infrastructures, 73 government buildings and

3,977 dwelling houses were damaged.

There are other minor earthquakes in other islands. In

Sumbawa itself there was an earthquake which occurred

on November 26, 2007. In total 504 dwelling houses were

damaged, 2 people died and 146 were wounded.

The earthquake of August 7, 2008 caused quite a lot of

damage in many towns and villages such as Pekat,

Nangamiro, Kadindi and Tambora. Some villages on

Moyo Island also were devastated. In total, 1.500

dwelling houses were damaged.

Some of the damaged houses were already damaged

during the earthquake of 2007. They were not properly

renovated or restored.

The intensity distribution of the earthquake is depicted in

Fig. 4a whereas the attenuation curve (Intensity-Distance)

is depicted in Fig. 4b.

Moyo Island itself, which is the closest area to the

epicentre, is relatively less damaged than the other places,

even though the intensity in Moyo was at VII (MMI).

This is due to the fact that the foundation of Moyo island

consists of coral reefs, whereas many areas in the Sanggar

Peninsula (where Mt. Tambora is located) their

amplification of the seismic waves were less, most part of

the Sanggar Peninsula is covered by loose pyroclastic

product of the 1815 eruption of Mt. Tambora. Most

damaging to the building and houses are the transverse or

secondary waves.

This resembles the situation of the Yogyakarta

earthquakes in 2006. The Bantul regency is covered by

water saturated river deposits and loose volcanic products

from Mt. Merapi in the East. The adjacent regency of

Gunung Kidul is built of Tertiary Limestone and

crystalline limestone. Compared to Bantul Regency the

damages and destructions in the Gunung Kidul Regency

were minor.

Damages

The Sumbawa earthquake caused damages to almost

1.500 residential buildings, which 90% of the damaged

building were located in Kabupaten Dompu. Pekat,

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.

Typical Damages

Landed house suffered most damage due to Sumbawa

earthquake. The typical damages are (Figure 5):

Fig. 4b. A simple attenuation curve (Distance-Intensity) of

the August 7, 2008 earthquake. This curve is derived from

the Fig. 4a

Fig. 4a. Isoseist map of the August 7, 2008 earthquake in

the Sanggar Peninsula

1. Small to large cracks of walls

2. Fractures of the joints

3. Damages to roof structure (falling of roof tiles to

failure of the roof structure)

4. Damages to building structures

5. Totally collapsed building

Many wooden-elevated houses were also inflicted by the

quake, particularly in Moyo Island. This type of house is

known for its quake resistance, and has performed well in

Dompu Island during the Sumbawa quake. But in Moyo

Island, where 70% of the buildings are wooden-elevated

houses, most of the buildings suffered damages, although

rather light. This is caused by the age of the buildings

which are mostly old and also due to poor quality of the

woods. Typical damages to wooden-elevated houses are

(Fig. 6):

1. Collapse of walls

2. Failure of roof structures

3. Slippage of the main columns

4. Cracks and broken columns

5. Totally collapsed building

The cause of the so many damages

A 6.6 Richter scale earthquake is classified as an

intermediate quake. But as the epicentre of the Sumbawa

earthquake is very shallow and close to the residential

area, it is not a surprising fact that the quake caused a lot

of damages.

There is a very interesting thing that in some damaged

areas there were few better-constructed buildings which

suffered only minor damage while other buildings (with

poorer construction) in its surroundings suffered major

damages. A conclusion we get from this is poor

construction is the main cause of the so many damages

caused by the quake

The cause of the poor construction:

Further observation by the survey team has revealed the

cause of the poor quality of the residential buildings in

the rural area of Dompu, which are (Fig. 7):

A. Houses are built without columns or with columns

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.

Fig 7.Building without column and column with improper

reinforcement

Fig. 6. Typical damage to wooden-elevated houses

Figure 5. Typical damages to landed-house

B. Poor quality of mortar.

Interview with local people revealed that the mortar

used to build their houses is made by mix cements

and sands with ratio of 1 : 7 to 1 : 10, far below the

recommended ratio of 1 : 3 to 1 : 4. Poor mortar is

weak and has low bonding capacity (Figure 8).

C. Heavy roof structure

In Dompu, the number of houses with heavy roof tiles

and light zinc plate roof are more or less equal. Even

though the local people have known that light roof

structures have better earthquake resistance, but for

many reasons people are reluctant to use this type of

roof. One of the reasons is that roof tile is more

comfortable and also cheaper (they can even make it

themselves using clay dig from own yard).

Light roof structure helps the houses to be earthquake

resistant!

Figure 9 is a house in Calabai village, one of the most

damaged areas. The constructions of the houses are of

substandard; the reinforcement of the columns uses

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.

Economic and Insured Losses

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.

MAIPARK’s share amount is IDR 195,545,283,487.77.

Furthermore, MAIPARK data reveal that the commercial

sector occupied 54.69%.

As a whole, the accumulation of earthquake risks is

mainly in Mataram area (Lombok) which reaches 179

risk with an insured value of 100% which amounts to

IDR 1,203,158,338,788.27 of which MAIPARK share

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

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.

Fig. 9. A Substandard two storeys house with light roof

structure using z inc plates

Fig. 8. Example of a poor quality mortar taken from a house

in Calabai village