contents: the sumbawa earthquake of august 7, 2008 · and sands with ratio of 1 : 7 to 1 : 10, far...
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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: [email protected]
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