Contents:

• The West Sumatra and the

West Java Earthquake of

September 30, 2009 and

September 2, 2009

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

Hengki Eko Putra

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

No. 13, February 2010

The West Sumatra and the West Java Earthquake

of September 30, 2009 and September 2, 2009

Abstract

Like lightning on a clear blue sky, two destructive earthquakes hit West Java Province and West Sumatra Province in Indonesia on September 2, 2009 and September 30, 2009. The West Java Earthquake, more popularity nicknamed the Tasikmalaya earthquake, had a magnitude 7.0 (Richter scale), whereas the West Sumatra Earthquake, nicknamed the Padang Earthquake, had a magnitude of 7.6 on the Richter scale. Their focal depths are respectively 46 km and 81 km both were in the Indian Ocean. Economic loss and Death toll. Compared to the Tasikmalaya earthquake the economic loss as well as the death toll of the Padang earthquake was by far much higher. The economic loss inflicted by the Tasikmalaya earthquake was Rp. 6.9 Trillion (±US$ 700 Million) whereas the economic loss inflicted by the Padang earthquake reached Rp. 23 Trillion (US$ 2.3 Billion); the death toll of the Tasikmalaya earthquake counted 81 people, whereas the death toll of the Padang earthquake reached more than 1.100 people. Distribution of the destructive regions. The destructive effects of the two earthquakes differed in their geographical distribution. The damaged regions in West Java were more randomly distributed compared to the destructed regions in West Sumatra which were relatively confined to the city of Padang and a narrow strip along the coastal plain between the cities of Padang and Pariaman. The basement of the city of Padang as well as the narrow zone between Padang and Pariaman consisted of quaternary water rich and soft unconsolidated sediments mixed with sand. However, there is a great difference between the destructions in Padang and in the narrow zone between Padang and Pariaman. In the city of Padang most destroyed structures were commercial buildings, whereas in the narrow strip between Padang and Pariaman the destroyed items were mostly dwelling houses. Soft geological basement and faulty structures. The destructions in Padang City were due to the soft geological subsurface and buildings supported by weak lower structures carrying very heavy upper parts. Most of the destroyed Minang buildings showed much heavier on the upper structures (the graceful horn-like roofings) carried by very weak foundations and weak pillars. It is purely an engineering problem, a problem of earthquake engineering found a problem of civil engineering. The Padang earthquake was also typified by numerous landslides burying entire villages including their population. Most of the landslides occurred in hilly countries. The hills consisted of loose volcanic sandy tuffs produced by the Maninjau volcano during its gigantic outburst leading towards the formation of the volcano tectonic depression of Maninjau (Westerveld, 1953; Zen, 1974). 115.000 houses were completely destroyed and 135.000 houses were heavily damaged.

Significance of the Padang Earthquake. Until Today, Indonesia’s experience with earthquakes have been confined to destruction of simple dwelling houses in the rural areas. The destruction in Padang City gave the Indonesian government and its people the first experience of an earthquake which hit a city with several storied buildings (no skyscrapers yet!). It showed us two things:

I. Building standards and building codes have not been

properly applied. There has been very “vague” and

“weak” building inspections, and no sanctions

whatsoever.

During the Bengkulu earthquake in 2007, many buildings

in Padang City showed cracks. But these have been

“waved” away by saying: “Oh…we are lucky, only cracks

occurred. Nothing serious”. There have been very little

serious efforts to refurbish damaged buildings or dwelling

houses more properly.

The Padang Earthquake of 2009 teached us a valuable

thing! But … did it really teach us? These are:

The death toll in the Tasikmalaya earthquake was only 81,

most were due to a rather big landslide in the Cianjur

district.

The number of the destroyed dwelling houses in West

Java was as follows:

- 45.690 dwelling houses were heavily damaged

- 94.862 dwelling houses were moderately damaged

- 119.374 dwelling houses were lightly damaged

The damaged houses were spread over 15 districts or Kabupatens; distributed very randomly over the whole province. One thing can be noted of the West Java, or the Tasikmalaya earthquake. It was felt by many people in Jakarta, Bandung, and some other places in Central Java.

II. Seismic micro zonation followed by a land use policy

(spatial planning) is absolutely necessary. In fact, this has

never been done in Indonesia. The price we paid and had

to pay in the future would be very high.

Introduction

The geological structures between Java and Sumatra are completely different from each other. The island of Sumatra is dissected over its entire west coast from NWW to SSE by the 1650 km long right lateral transcurrent fault zone. Java on the otherhand does not have such a large and long transcurrent fault like the great Sumatra Fault. Both islands are bounded on its ocean sides by subduction zones. Underneath Java the oceanic crust is subducted frontally, underneath Sumatra the oceanic crust is subducted obliquely under the island. Three structural elements influenced the seismicity of Sumatra

Island, these are:

1) The oblique Sumatra subduction

2) The Mentawai Thrust between Sumatra Island and the

Subduction zone

3) The Great Sumatra Fault Zone which dissects the entire

Barisan Mountains from Banda Aceh in the North to the

Semangka Bay in the South. In fact this fault zone continues

on SW of Sumatra until it dissects the Java trough in the

Indian Ocean South of Sukabumi. The Great Sumatra Fault

does not consist of one single fault; it is a fault zone.

Besides, it is segmented into 12-13 segments arranged en

echelon wise (Fig. 1).

Fig 1. A sketch map of Sumatra Island showing the

subduction zone, the Mentawai fault zone, and the

segmented Sumatra fault zone.

The Source Parameter of the fore mentioned earthquakes is shown in Table 1.

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The Padang Earthquake

On September 30, at 17:16 an earthquake of magnitude Mw=7.6 on the Richter scale struck West Sumatra. The epicenter lies 87 km NW of Pariaman in the Indian Ocean, whereas its focal depth was 71 km (see source parameter Table 1). It was followed by a quite strong aftershock of magnitude 6.2 located at Sungai Penuh in Jambi Province, with a focal depth of 110 km. It is very interesting to note that according to the official joint report of BNPB, Bappenas, the Provincial and district/city government (2009) this Sungai Penuh earthquake was labeled as an aftershock of the Padang earthquake. The R&D Team of Maipark mentioned that the Padang earthquake was a subduction earthquake, whereas the Sungai Penuh earthquake was related to the Sumatra Transcurrent Fault movement. The difference in depth, the distance between the two epicenters and the different initial motions or focal mechanism could lead to a slight different interpretation.

The Destructive Effects

The most interesting phenomenon is namely that the earthquake intensity were quite high; VI-VII MMI around the city of Padang and V to VI MMI in the surrounding districts, like in the Kabupaten of Padang Pariaman (Fig.2). The results of the Padang earthquake were tremendous. A large number of one and two storey houses/buildings collapsed completely. The destruction could be traced in Padang City itself, and many one-storey houses in Padang Pariaman, Agam, Solok, Pasaman, and Pariaman, accompanied by landslides in many places. In total, 1.117 people died, 1.214 people were seriously injured, and 1.688 people were moderately injured (BNPB, October 15, 2009). Many perished and buried under landslides. The total economic loss was reported at Rp. 23 Trillion (if considered from the standpoint of reconstructing and rebuilding effort). This is according to the official joint report of BNPB, Bappenas, and the Provincial governments of West Sumatra, 2009.

Fig 2. The intensities of the West Sumatra earthquake

One thing must be noted though. The people of West Sumatra did not lose themselves too long in sorrow and sadness. Within less than a week the people in the rural areas started rebuilding their houses with whatever material they manage to collect from the ruins of their houses. The people whose dwelling houses were destroyed were not evacuated and settled into tents. They built their own tents next to the ruins of their houses and started rebuilding and reconstructing with whatever material they could find. The only thing which were requested in many cases are larger tents (for 30-40 people) to be used as emergency class rooms. Schools started immediately.

Analysis

Looking at the geological map and studying the geological history of West Sumatra including the subsurface geology of Padang City, one can clearly see that Padang City is located on a very soft water saturated quaternary alluvial deposits mixed with volcanic pumicious sand, a produced during the formation of the volcano-tectonic depression of Maninjau (Westerveld, 1953; Zen, 1974). Due to the presence of sand layers liquefaction was reported to occur in the zone between Padang City and Pariaman. Some of the collapsed building in Padang City could be due to liquefaction (Fig. 3).

Fig. 3. A completely collapsed commercial building in Padang.

This could be the result of liquefaction

Liquefaction is a natural process. When loose or medium-dense saturated cohesion less materials are subjected to earthquake vibrations; the tendency to compact is accompanied by an increase in water pressure in the soil resulting in movement of water from the voids. Water is thus to flow upwards to the ground surface where it emerges in the form of mud spots or sand boils. Under such conditions, saturated cohesion less material loses most of all its shear strength and deforms like liquid. Such a condition will result in dramatic damages to buildings. In order to assess the risk of soil liquefaction it is necessary to estimate the possible acceleration at the depth of the particular soil layer. The destruction is confined mainly to two areas, these are: 1) Padang City itself, and 2) a narrow zone between Padang City and Pariaman, plus 3) scattered widely over several districts in West Sumatra like Padang Pariaman, Agam, Solok, Pasaman Barat, Pesisir Selatan, Tanah Datar and Mentawai (BNPB et al, October 2009). However the heaviest destructions were confined to Padang City and the narrow zone between Padang City and Pariaman. In Padang City itself the destroyed buildings were commercial buildings, government offices and dwelling houses as well. But most numerous were the commercial buildings whereas outside Padang City, in particular in the narrow zone between Padang and Pariaman, the destroyed units were mainly dwelling houses (Fig. 4a, 4b). Most of the commercial buildings and government offices in Padang City showed very weak lower structures. Some had very very weak and too small supporting pillars (Fig. 5). The upper structures were always big and heavy, including the so called

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“rukos”; these are buildings of which the ground floor functions as a shop and the first or second floor function as living quarters.

Fig. 4a. A damaged Super Market in Padang. The lower

part of the building was sandwiched between the first floor

and the upper part of the building.

Fig. 4b. One of the thousands of destroyed dwelling houses

in Padang City

Fig. 5. Big buildings supported by small pillars without

proper foundations

Landslides in hilly countries like in Tandike, other places in Agam Districts and Padang-Pariaman. One of the worst took place in a hilly country which consists of sand and pumice tuffs described before took place Tigo in Tandike sub-district. Very few survived; it was said that around 300 people were buried alive in Tigo (Fig. 6).

Fig. 6. A landslide debris in Tigo where 300 people

were believed to be buried during the Padang

earthquake

Creativity of the Sungai Penuh People (Jambi)

A rather big aftershock (BNPB, 2009) occurred at Sungai Penuh (Jambi) with a magnitude 6.6 (USGS). This shock was felt very strongly at Sungai Penuh in Jambi (+/- 46 km away) had a magnitude of M=6.6 (USGS and occurred at 08:52:29 on October 1, 2009). The earthquake zone was the Sumatra Fault Zone. 2 people died due to the collapse of their houses. In total only 635 dwelling houses were slight to heavily damaged (medium). Compared to Padang, this is rather insignificant, but not very surprising. The houses in Sungai Penuh are better built. The people at Sungai Penuh are really very creative. Right now, without the suggestions from anybody else they started on their own to build houses with earthquake resistant design. They realized that they are in the Sumatra Fault Zone. Therefore they build their houses cautiously and very selective in using their building materials (in selecting the woods used). It turned out that this has been their tradition. This they experienced from the quake of 1995 which hit them severely. The magnitude of that particular earthquake was M=6.8 (USGS) with a focal depth of 33 km. During that earthquake 84 people died; 558 people were heavily injured and 1303 people lightly injured. Those death casualties and injured people were due to the collapse of 7.137 houses. After that they built stronger houses and their death casualties could be reduced to 2 deaths and 29 people were lightly injured in October 2009 (Table 2). Table 2. Source Parameter of the Padang and Sungai Penuh

earthquakes in 1955 and 2009

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The West Java Earthquake The West Java earthquake, popularly known as the Tasikmalaya earthquake occurred at 14:55:01 West Indonesian Time, on September 2, 2009 with a magnitude 7.0 on the Richter scale. This earthquake was felt in Jakarta, Bandung and several on the places in Central and East Java. For further details on the Source Parameter, see Table 1. This is a very interesting case. Since 1930 West Java had never really experienced a devastating earthquake. There was indeed a small one offshore Pangandaran with a magnitude of M= 7.7 triggering a small tsunami which caused some damages at Pangandaran. There was another earthquake which also occured around Tasikmalaya and Garut which caused some damage in the 70s.

The Seismotectonics of the Western Part of Java

In contrast to West Sumatra, West Java does not have a large transcurrent fault like the Great Sumatra Fault. The Western Part of Java is dominated by local faults, like the Baribis Fault zone which cuts across West Java from the border of Central Java, trending northwest. It reaches the area between Subang and Purwakarta (Fig. 7a-7b). The Cimandiri Fault Zone starts from Pelabuhan Ratu in the South and trends NEE reaching the area near Cirebon. One small branch went eastward at the North of Bandung, known as the Lembang Fault, North of Lembang, cutting the foot of Mt. Tangkuban Perahu. MAIPARK’s R&D department does not believe that the Lembang fault is an extension of the Cimandiri fault. The Lembang fault is a local fault connected to the volcano – tectonic structure of Mt. Tangkupan Perahu.

Fig. 7a. A general schematic seismo-tectonic map of West

Java showing the position of the Cimandiri fault. The

Baribis Fault, and the Lembang Fault. This fault pattern

influences also the distribution of damages of dwelling

houses during the West Java (2009) earthquake.

The Cimandiri Fault as well as the Lembang Fault are active, however there were no historical records on a real activity triggering a significant earthquake. The South Coast of Western Java is seismically very active. See Fig. 7a-7b the south coast of Western Java (West Java Province-Banten Province and South Lampung) is seismically active. In fact, Zen et al 2009 already defined this particular region a seismic gap (Zen et al, 2009). This region deserves a special attention, especially it is very close to the Sunda Strait where Anak Krakatau is located (Zen, 2009).

Fig. 7b. Sketch map of the Seismo-tectonics of the Western part

of Java

Losses Caused by the Tasikmalaya Earthquake

The areas which were damaged by the Tasikmalaya earthquake are randomly spread (See Fig. 8a, 8b). Compare the devastated areas in the West Java Earthquake and the West Sumatra Earthquake. Most heavily hit were the regions of Tasikmalaya, Garut and Cianjur where a big landslide occurred.

(a) (b)

Fig 8. These figures illustrate in general the difference

in the distribution of damages in West Java (a) and in

West Sumatra (b). In West Java, it is more random, in

West Sumatra, in general, the destructions are confined

to 1) the city of Padang, and 2) in a narrow zone

between Padang and Pariaman.

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Some photographs of the devastated areas are shown in this paper (Fig. 9-13). As can be seen from the photographs most of the damages were dwelling houses. No multi storied buildings or office buildings were reported as destroyed.

Damages and Casualties

81 people were reported dead 1.917 people were heavily and lightly wounded 45.690 houses were heavily damaged 94.862 houses were light-heavily damaged 119.374 houses were lightly damaged Spread over 15 districts (Kabupaten)

Fig 9-13. Devastated dwelling houses in West Java

The Economic Loss of the West Java Earthquake

The total economic loss due to the West Java earthquake was reported to be Rp. 7.9 Trillion (BNPB & Bappenas Report of 2009).

Some additional notes on past earthquakes in West Java

Tasikmalaya earthquakes of November 2 in 1979. An earthquake of magnitude M= 7.7 on the Richter scale (USGS) struck Tasikmalaya on November 2, 1979 (Waspada do not have further details on the source parameters). But we have some information on the effects of the earthquake :

- 1.430 dwelling houses were heavily damaged including a

new mosque.

- 10 people were reported dead.

- The focal depth was reported to be 62 km. (No information

concerning the location of the epicenter).

- One thing is worth noting. The newly built mosque was

heavily damaged; the old one and much smaller one was

still standing nearby undamaged. So, it must be something

wrong with the design and the engineering of the most.

The Pangandaran Earthquake of July 17, 2007 A rather strong earthquake of magnitude M= 7.7on the Richter scale (slightly bigger than the Padang earthquake (7.6) hit Pangandaran, a sea resort on the south coast of West Java, in the district of Ciamis exactly.

- The focal depth was reported to be 34 km. This quake was

accompanied by a small tsunami which swept the small city

of Pagandaran.

- 654 people died (by the tsunami?)

- 1.908 dwelling houses including some commercial

buildings (hotels) were heavily damaged, and 514 were

slightly damaged.

- Pangandaran was a sea resort with many hotels and a

multitude of dwelling houses are built in front of the beach.

That explains why so many commercial buildings and

dwelling houses were heavily damaged.

- Ironically, it was reported officially that a few days before

the earthquake, there was a sign on the beach saying: There

will be no earthquake /tsunami in Pangandaran.

Lesson From Padang and Tasikmalaya Earthquake for

Underwriters

As mentioned in the beginning of this article, Padang earthquake is the first experience for Indonesia a destructive earthquake hit a city and destroyed multi-storey buildings. Insurance penetration in Padang city is quite high. Most of the damaged buildings have their insurance protection. This is also the first experience for insurance industry to deal with earthquake in the middle of a city with huge claim amount. With insurance losses approximately IDR 1 trillion, Padang earthquake is the biggest EQ single event loss in Indonesia at this moment. Lesson from the earthquake:

1. Insurance should carefully monitor their earthquake accumulation especially in the earthquake prone areas.

2. Reinsurance protection should be sufficiently well arranged. If needed, alternative mechanism can be used, Cat Bonds for an instance.

3. Insurances should perform quick and accurate claim handling. Beside to keep its good reputation, this will help fasten the rehabilitation process.

4. Pre-existing damages due to previous earthquake(s) should be carefully assessed for new business. Pre-risk survey is necessary.

5. Protection is the business of insurance. Insurance should not decline earthquake risk, what should be done is proper and better risk management.

References

1. Westerved, J., Eruptions of acid pumice tuffs and related

phenomenon along the great Sumatra Fault-trough system.

Pacific Science Congress, 7th New Zealand. P. 411-438

2. Zen, M. T., Distribution of pyroclastic plateaux in the

Padang Highlands. Bulletin of the Geology Dpt of ITB,

1974.

3. Zen, M. T. Andryansyah, Atmaja, F.W., Sedayo, M.H.,

Damanik, R., Putra, H.E., Krakatau in the Sunda Strait: A

Mega Catastrophy in the Making, “Asean Forum on Natural

Catastrophe Exposures in Asean.” Singapore July 16-17,

2009.

4. Van Bemmelen, Geology of Indonesia, Martinus Nijhoff,

The Haque, 2nd Printing, 1949.

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