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Jyesta Amaranggana

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Contents:

• Earthquake Occurrences in

the Northern Part of

Indonesia 2008-2009

Fig. 1. A Schematic Map showing the geographic

location of the Gorontalo Earthquake (1),

the Manokuari Earthquake (2), and the Talaud

Earthquake (3) (Not to Scale)

Earthquake Occurrences in the Northern Part of Indonesia

2008-2009

General Introduction

Until very recently earthquakes occurrences have been reported mainly from the

Sumatra Fore Arc Region, and from Sumbawa (the Lesser Sunda Islands Region,

NTB and NTT). Then “the Dragon” seems to move North eastward to Gorontalo,

Manokwari at the Birds head of Papua, and in Talaud Island, Northern Minahasa

(See Fig. 1).

Eventhough not much has been

reported, Minahasa the Eastern tip of

Sulawesi’s North Arm has been

regarded as a very seismic country. But

large destructive earthquakes were rare.

On the other hand the Northern part of

Papua has been frequented by rather

larger earthquakes in the past. This no

surprise, the large Sorong Fault skirted

along the North coast of Papua and

continues on to the West, close to

Sulawesi’s Northeast Arm.

Waspada in its earlier edition, Waspada

No. 7, March 2008 made a comparison

in earthquake occurrences between East

and West Indonesia.

The Gorontalo EQ.

A moderate-to large earthquake of

magnitude 7.3 to the Richter scale

struck the center part of Sulawesi’s North Arm (Fig. 2, 3a-c, 4) on November 17, 2008. Its

epicenter lies on the north coast and its focal depth was reported at 30 km. In fact this

earthquake is situated at the edge of the Sulawesi Sea. Nonetheless this earthquake is

known as the Gorontalo earthquake since this city suffered the heaviest economic loss. The

exact location is reported to be: 122.10 East and 1.29

0 North (Table 1 and 2). The quake

caused excessive destruction. Refer to BNPB’s report, 1,418 houses suffered heavy damage

and 892 houses suffered minor damage. BNPB also reported 4 people perished in this

disaster.

JULY 2009, NO. 11

Table 1. Gorontalo Earthquake Source Parameter

Magnitude

Location

Focal Depth

7.3 Richter Scale

122.1° East

1.29° North

30 km

Table 2. Losses

Dwelling Houses Economic Losses

2,310 units IDR: 418.6 billion

The Manokwari Earthquake

The Manokwari earthquake occurred on Sunday, January

4, 2009, exactly at 04:43:54 AM, and its location is

132.7830E, and 0.510 S. Its focal depth is reported at 35

km, and its magnitude is 7.6 on the Richter scale (Fig. 5).

2,146 houses were heavily damaged and 4,084 suffered

intermediate and minor damage due to Manokwari quake. Even though the destruction was quite large, death toll

reported was “only” four persons.

Fig. 2. A tectonic map of the Eastern part Indonesia taken

from Hamilton’s map (1989) with the Neogene Suture

Region (top center).

Fig. 3a Earthquake distribution in Indonesia 1964 to 2008

Fig. 3b. Magnitude distribution for earthquake occurrences

in the Western part of Indonesia for the period of 1964-2008.

Fig. 3c. Magnitude distribution for earthquake occurrences

in the Eastern part of Indonesia for the period of 1964-

2008.

Further to large destruction of houses in rural area, this

earthquake also damaged number of multi-storey

buildings in urban area which mainly occupied as office

building and hotel.

Table 3. Manokwari EQ Source Parameters

Magnitude

Location

Focal Depth

Epicentral Region

Time of Origin

7.6 on the Richter scale

0.510°S and 132.783°E 35 km

Near the North coast of

Papua Indonesia (Fig. 5)

14:15:56 or 10:15:56 PM

Table 4. Exposure and Estimated Economic Losses

Manokwari (exp.)

Kabupaten Sorong (exp.)

Kabupaten Bintuni (exp.)

Total Estimated Economic

Loss

IDR 0.9 Trill.

(value of buildings)

IDR 3.6 Trill.

(value of buildings)

IDR 0.3 Trill.

(value of buildings)

IDR 312,9 Billion

Eventhough the magnitude is given up as 7.3 on the

Richter scale, this earthquake has been classified as a

very powerful one. Its focal depth is 35 km, but major

ground shaking at the surface has been recorded and mentioned by the USGS (January 4, 2009) and by some

eye witnesses.

The Talaud Earthquake

On February 11, 2009 at 17:34:50 UTC a strong

earthquake hit Talaud Islands (North Minahasa), Its

geographic location: 3.902˚N, and 126.400˚E. Its focal

depth was reported to be 20 km. Seismologist recorded Its magnitude of 7.2 on the Richter scale. Refer to BNPB

report 645 houses suffered heavy damage and 775 houses

suffered intermediate and light damage. Number of public facilities including schools and hospitals were also

reported suffered damages.

A month later, on Monday, March 16, 2009 at 14:15:56

UTC or 10:15:56 PM local time, an earthquake with a

magnitude of 6.3 on the Richter scale occurred in a very

Fig. 4. Earthquake occurrences in the Suture Zone in

Indonesia (see Fig.2) with fault plane solutions.

Fig. 5. Location of the Manokwari Earthquake

Fig. 5a. A residential building (top) and a three-storey

building (below) which are damaged by Manokwari quake.

close location with the February earthquake. The

geographic location was recorded at 3.82˚N, and

126.500˚E with focal depth of 35 km. The epicentral

region was Talaud Islands (North Minahasa) (Fig. 6). This epicenter is located at 2.460 km from Jakarta, 1.335

km from Manila, and 315 km from Manado (capital city

of North Sulawesi Province) (See Table 6 and 7).

Table 6a. Talaud EQ February 11, 2009 Source Parameters

Magnitude Location

Focal Depth

Origin Time and Date

7.2 on the Richter scale

3.902°N, 126.400°E

20 km 17:34:50 UTC; Wednesday,

February 11, 2009

Table 6b. Talaud EQ March 16, 2009 Source Parameters

Magnitude

Location

Focal Depth

Origin Time and Date

6.3 on the Richter scale

3.82°N, 126.500°E

35 km

14:15:56 UTC; Monday,

March 16, 2009

Table 7. Vulnerability

Inventory at Talaud Islands

Inventory at Sangihe Islands

IDR 4.7 Trill.

IDR 3.7 Trill

Some Remarks on the Suture Zone

One of the most interesting features of the tectonics of Northeast Indonesia is the Neogene Suture Zone between

the volcanic arc of Halmahera and the volcanic arc of

Minahasa-Sangihe with a double subduction; one dipping

Eastward beneath Halmahera to a depth of about 250 km,

and the other one dipping Westward beneath the

Minahasa-Sangihe arc to a depth of about 650 km

(Hamilton, 1989). The active volcanoes of both arcs lie

about 100 km above the tops of the respective seismic

zones (see Fig. 7) Further up North, and slightly to the East stretches the Philippine Trench which continues on

to the North, and slightly West of the Minahasa Arc and

trending North-Northwest; there lies the Cotabato Trench.

This complicated double subduction feature produces an

earthquake distribution pattern depicted in Fig. 7.

Underneath Halmahera there is already the zone of the

Sorong Fault (Fig. 7). It is no wonder that this region is

tectonically very active. Fortunately the exposures on

both sides of the Suture Zone are relatively not as high as

in many places in the Western Part of Indonesia such as

the Sumatra Fore Arc Region. An East-West and a NW-SE Profile across the Suture Zone is provided by

Cardwell et al 2001 (Fig. 8, 9). It is therefore imperative

that a close watch should be held on this part of the

world.

Fig. 6a. Location of the Talaud Island earthquake on

February 11, 2009

Fig. 6b. Location of the Talaud Island earthquake on

March 16, 2009

Fig. 7. Tectonics of the Eastern Part of Indonesia

(taken from Hamilton’s map, 1989)

What Underwriters Ought to Know

For the time being the exposures are still very low for the Eastern Part of Indonesia. But the situation will not

remain the same during the coming two decades. The

pace of development will pick up albeit not yet the same

like in the Western Part, but things are changing.

Look at Fig. 10. This map shows the location of gold and copper locations. Oil and gas found at the neck of the

Birds’ head (Bintuni Bay) are very promising. Besides,

the potential for fisheries, mariculture and fresh water

aqua culture, timber and other potential for marine

industry is enormous. The insurance industry should not

be a late comer.

Fig. 8. Major tectonic feature in the Philippine and

Northeast Indonesia Island, modified from Hamilton

(1979)

Fig. 9 (left).

(a) Block diagram illustrating the geometry of the

Molucca Plate being subducted in the Molucca Sea

region South of Mindanao. Latitude and

longitudinal are given for reference and the effect

of the earth’s sphercity is included. The

configuration of the plate is given by a grid of two

intersecting sets of curves. One set of curve lies in

the horizontal plate at 50 km depth intervals. The

other set of curves is oriented down the dip of the

plate and is approximately orthogonal to the first

set.

Fig. 9 (b) Cross section showing the inferred

geometry of the plates in the Northern Molucca

sea at about 50N. The seas are shown in section E,

F, and G: (1) thrusting of Celebes basin beneath

the Sangihe Ridge at the Cotabato Trench, (2)

down-dip compression within the Molucca Plte at

great depths, (3) down-dip compression within the

Molucca Plate at intermediate depths, (4)

underthrusting of the Philippine Basin beneath the

Talaud Ridge at the Philippine Trench.

Fig. 9 (c) Cross section showing the inferred

geometry of the plates in the Southern Molucca

sea across the Halmahera and Sangihe island arc.

(Cardwell et all, 2001).

Fig. 10. Location on the gold and copper deposits and

major prospect in Indonesia. Deposits are those that are

either past producers or for which reserve/resource

estimates are available.

References

1. Cardwell, R. K, B. L. Isack, D. E. Karig, The spatial

distribution of earthquakes, focal mechanism solutions and subducted lithosphere in the

Philippines and North eastern Indonesian Islands,

special edition of geofisika, Indon. Aes. Of

Geophysicist, Bandung, 2001.

2. Carlile, J. C. and AHG Mitchel. Magmatic arcs and

associated gold and copper mineralization in Indonesia, Journal Geochemical Exploration,

Elsevier, Vol. 50-NoS. 1-3, 1994.

3. Hamilton, W., Convergent plate tectonics viewed from the Indonesian Region, Geol. Indonesia, v. 12,

no. 1, Jakarta, 1989.

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