tsunamis - PT Asuransi MAIPARK Indonesia

WASPADA
No. 16, August 2011
News On Earth Disasters
TSUNAMIS
Thucydides was a Greek philosopher who was
known as the “father of science”. Whatever he
said was always accompanied by a scientific
analysis rather than the common culture of the
Greek people, associating everything to the power
of the Greek Gods. One of his famous stories is
“a history of the war of the Peloponnese.” In this
book, Thucydides recorded the natural phenomena which destroyed the Bay City of Maliakos
and Euboic. It was the huge sea wave in the summer of 426 BC. He concluded that the destructive
wave must be generated by something else, and it
was an earthquake at the bottom of the sea.
Now, the huge wave is called the ‘tsunami’, derived from the Japanese vocabulary which is literally means harbor wave, referring to the disasters that often occur in the port cities in Japan. In
addition to the earthquakes, such as hyphothised
by Thucydides, a tsunami can also originate from
underwater landslides, volcanic eruptions in the
ocean, or a meteor falling in the ocean. These
events can make the vertical displacement of the
seafloor just in the blinking of an eye, this becomes the starting point of the tsunami theory.
The tsunami wave can propagate in all directions.
The large energy is contained in the height and
speed of the waves. In deep oceans, these waves
propagate at the speed of 500-1000 km per hour
with the height only about 1 meter. This configuration changes as it approaches the coast, the
speed of tsunami wave decreases to 30 km/hour,
but its height increases until ten meters instead.
The Mechanics of Tsunami Formation
At one time, the tsunami regarded as a tidal wave,
but because the tsunamis are not caused by the
gravitational force of the Moon and the Sun,
hence the ‘tidal wave’ term is no longer used.
Tsunami is a height wave that is usually created
by the large-scale movement of the seafloor. Although the ‘body of tsunami’ is barely visible in
the sea, but when it reaches the shoreline, the tsu-
nami waves can cause severe damage.
Tsunamis wave can be triggered by vertical changes in sea floor that move a large
amounts of water. Most of tsunami events
were generated by an earthquake that occurred in the seafloor, when the large amount
of water was driven by the increasing or a
decreasing of hundreds or even thousands
square kilometers of seabed. Landslides
(which often follow the large earthquakes),
volcanic eruptions in the sea, and the meteors can also disrupt the balance of water to
generate the tsunamis.
The tsunami wave can travel thousands kilometers across the ocean without losing
its energy. For example, the tsunami that
occurred in 1920 in Chile, South America,
caused a serious damage in Japan, 14.500
kilometers (9.000 miles) far away. Hawaii,
in the middle of Pacific Ocean, is very vulnerable to the tsunamis because it’s located
in the middle of ‘tsunamis traffic’.
The tsunami was preceded by an earthquake
at the bottom of the sea, where the seabed
oscillates up and down due to pressure from
below (the earthquake). Vertical movements
of the Earth’s crust can be very large and lift
the sea water above the normal sea level.
Thus, the main factor of the tsunami is a vertical deformation of the seabed. The height
of water above sea level is encouraged into
potential energy. Then, the potential energy
is transferred into kinetic energy, which directly affects the speed of tsunami wave.
When the tsunami took place, the speed of
tsunami wave does not only depend on the
kinetic energy of the earthquakes, but also
from the surrounding conditions. The speed
of the tsunami is varied, which is directly
correlated with the depth of the ocean, in
the deep ocean tsunami wave move faster.
At some points in the Pacific Ocean, which
are reaching 4000 meters depth, the tsunami
Contents
TSUNAMIS
Publisher
PT. Asuransi MAIPARK Indonesia
Advisor
Frans Y. Sahusilawane
Bisma Subrata
Editorial Board
Mudaham T. Zen
Andriansyah
Fiza Wira Atmaja
Ruben Damanik
M. Haikal Sedayo
Hengki Eko Putra
Bintoro Wisnu
Jyesta Amaranggana
Editorial Address
Corporate Secretary
PT. Asuransi MAIPARK Indonesia
Setiabudi Atrium Building, 4th
Floor, Suite 408
Jl. HR. Rasuna Said Kav. 62,
Jakarta 12920
Tel.: (021) 521 0803
Fax : (021) 521 0738
E-mail: [email protected]
Website: www.maipark.com
waves can travel more than 700 km/h, with wavelengths
greater than 100 km.
In contrast to the tsunami caused by the earthquake, the tsunami from landslides and volcanic eruptions in the ocean
quickly disappears and rarely affect the beach which is located far from the source. However, this kind of tsunami
still has the same destructive force: in 1883, the tsunami
caused by the eruption of Krakatoa killed more than 36.000
people on nearby islands of Java and Sumatra.
Tsunami-genic earthquake
An earthquake is the release of energy in the earth which
occurs very suddenly. The shallower the earthquake, the
bigger opportunity for the faulting to change the surface
morphology. If the focus of the earthquake is deep, the
faulting can be classified as blind faults, it means the faulting only occurs below the surface and does not reach the
upper surface.
Figure 1 Vertical movements of the Earth’s crust lift the sea water above the
So, in order to generate a tsunami, the earthquake must: (1) normal sea level (Photo: http://www.clinicalcorrelations.org)
be centered at the bottom of the sea, (2) has a shallow focus, and (3) has significant vertical throw (indicated by the amount of energy
that is released). Statistically, the earthquake that causes tsunami is the earthquake that centered in the ocean with the depth < 45 km
with a magnitude > 6.5 Mw (Satyana, 2007).
Figure 3. shows the Indonesian earthquake source zones. The red line indicates the subduction and thrusting zone, which has vertical
faulting characteristics. The orange line indicates the strike slip faults, with horizontal faulting characteristics. So, the subduction
and thrusting zone in the ocean can generate the tsunami-genic earthquakes.
Figure 2 Sketch of shallow and deep earthquake
The Earthquake history in Indonesia: the first and largest
The record of Indonesia tsunami has started in 1608, when Mount Gamalama in Ternate Island erupted and caused a tsunami which
killed 10-50 people. Since then, there were 227 tsunami events recorded all over Indonesia. 23 of them were caused by volcanic
eruptions, just like the Gamalama’s tsunami, and 204 other events were caused by the earthquakes.
So far, Aceh tsunami in 2004 was recorded as the most deadly tsunami in Indonesia. It is estimated that more than 230 thousand
people were killed in this event. The majority of the victims, 167.736 people, are residents of the Province of Nanggroe Aceh Darussalam, Indonesia, followed by Sri Lanka (35.322 people), India (18.045) and Thailand (8.212). The tsunami wave traveled more than
5,000 km to the west coast of Africa and has killed several people in Kenya and South Africa.
Yes, the Aceh tsunami was the most deadly tsunami in Indonesia, but it was certainly not the biggest one. The height of Aceh tsunami waves reached about 10 meters, which is still low if compared to the height of the waves from the Krakatoa eruption in 1883,
which was reported to reach 40 meters. The Krakatoa’s tsunami killed 36.000 people in west coast of Banten and in the south coast
of Lampung Province. The body of Mount Krakatoa that collapsed into the sea caused a huge tsunami waves which reach Anyer just
in one hour, and hit Jakarta Bay in 2.5 hour after the eruption.
Figure 3: Indonesian Earthquake source zone map. The subduction and thrust indicated by the red lines. The position of the earthquake source zone
in the ocean increases the possibility of tsunamis, every time it releases energy.
The last two decades of Earthquake History
in Indonesia
Only within two decades, there were 25 tsunami events in Indonesia. It’s not surprising due to the condition of subduction and
thrusting zone that extends in most parts of Indonesia, from the
West Coast of Aceh to the South Coast of East Java and in the
eastern part of Indonesia and Sulawesi. The large earthquakes
(>7) which followed by tsunamis are: Banyuwangi 1994 (Mw
7.8), Aceh, 2004 (Mw 9.3), Nias 2005 (Mw 8.6), Pangandaran
2006 (Mw 7.7), Bengkulu 2007 (Mw 7.8), Padang 2009 (Mw
7.8), Tasikmalaya 2009 (Mw 7.3), and the recent one is the
Mentawai tsunami in October 2010 (Mw 7.7).
Many scientist claims that this area has not released all of its
energy yet. The three main areas are: (1) the area around the
Siberut Island, (2) Sunda Strait region, and (3) the South Coast
of Java, from Cilacap until Pacitan. So, preparing for the worst
scenario, we can assume that the Mentawai’s tsunami might
not be the last big tsunamis during these two decades.
During the last two decades in Eastern Indonesia, tsunamis hit
Flores in 1992 (Mw 7.8), Halmahera, 1994 (Mw 7.0), Timor
Sea 1994 (Mw 6.5), Biak 1996 (Mw 8.2), West Papua, 2002
(Mw 7.7), Alor Islands 2004 (Mw 7.5) and Seram island, 2004
and 2006 (6.7 Mw, Mw 6.7).
Official agencies record that the Flores tsunami killed 1.169
people, whereas the mass media reported that the number of
victims reach 2.500 people. This is the biggest deadly tsunami
in eastern Indonesia. The tsunami waves arrived 5 minutes
after the earthquake. The local people reported that the wave
arrived at least 3 times and the second wave was the highest.
The Flores tsunami derived from the earthquake in the Flores Thrust, which stretchs from Flores to Bali Sea. The Flores
thrust activity in north of Nusa Tenggara Island to Bali can be
a serious threat. History records that the earthquakes and tsunamis occurred in 1815, 1817, 1857 and 1917.
Tabel 1. Eight most deadly tsunamis in Indonesia
Figure 4: The colored polygons indicate the faulting areas in each earthquake event. Siberut Island, Sunda Strait region and the south coast of
Java identified as the un-released energy zone. (The map from US. Geological Survey)
Figure 5: The map of Bali, Nusa Tenggara and Timor Island. The Flores
thrust (red line) in the north of these islands is the source of earthquakes and tsunamis in Flores 1992. (The topographic map from NOAA)
Insurance perspective
The comparison between insurance
losses and economic losses in the two
recent incidents of earthquake-tsunami
disaster are very small, it shows that
insurance penetration is very low.
Whereas, 78% of the loss is personal
and residential property which is the
target of disaster insurance market.
Overview of disasters loss data in Indonesia showed that the national insurance industry has not been well
contributed in the risk management of
the earthquake and tsunami disasters.
Insurance penetration is the major problem that needs to be solved as soon as
possible. The mitigation programs that
increase the people’s knowledge and
awareness to the risk need to be done
on the insurance industry. On the other
hand, the Government should play a
proactive role to encourage people to
insure their property. Indonesian government could follow the Japans insurance scheme that adds an insurance
premium on tax bill.
Paleotsunamis
The tsunami waves arrive with the
speed just like a jet plane, transport
material in the form of sand, plants,
even biotas from the deep-sea, and depositing them in the beach area. On the
beach, tsunamis wipe out everything
on its path, and inundating the coastal
areas.
Based on these characteristics, scientists can track tsunami events in the
past, when there is no record about the
parameters of the tsunamis and earthquakes from seismographs. Through
the tsunami’s traces left in the coastal
area, the geologists can estimate when
and how big the tsunami is. Paleotsunami is the name of the branch of geology that focuses on this study. Generally, the traces of past tsunamis have
remained in the coastal area in the
form of deposits material that brought
and dumped a shore during the event.
From the information of Newcomb
and McCan (1987), it is known that the
earthquakes which followed by tsunamis happened three times: in 1840 and
1859 that hit Bantul, Gunungkidul,
Wonogiri and Pacitan, and in 1921 that
swept Pangandaran, Cilacap, Kebumen, Purworejo, Bantul and Gunung
Kidul (Figure 6) . Unfortunately, there
are no further detailed information
about the earthquake source parameters.
Tabel 2:
Year The catalogue of tsunami events in Indonesia during the last two decades.
Month Date M 1992 12 1994 1 1994 6 1994 Area 12 7.8 FLORES SEA Latitude Longitude Victim -­‐8.480 121.896 1.169 1.015 127.733 2 7.8 JAVA, INDONESIA -­‐10.477 112.835 250 6 3 6.6 JAVA, INDONESIA -­‐10.362 112.892 1994 6 4 6.5 TIMOR SEA -­‐10.777 113.366 1994 10 -­‐1.258 127.980 1 1995 5 14 6.9 TIMOR SEA -­‐8.378 125.127 11 1996 1 1 7.9 SULAWESI 0.729 119.931 9 1996 2 17 8.2 IRIAN JAYA -­‐0.891 136.952 110 1998 11 29 7.7 TALIABU ISLAND -­‐2.071 124.891 2000 5 4 7.6 SULAWESI -­‐1.105 123.573 2002 10 10 7.6 IRIAN JAYA -­‐1.757 134.297 2004 1 28 6.7 SERAM ISLAND -­‐3.120 127.400 2004 11 11 7.5 KEPULAUAN ALOR -­‐8.152 124.868 2004 12 26 9.3 SUMATRA 3.295 95.982 227.898 2005 3 28 8.7 NIAS 2.085 97.108 10 2005 4 10 6.7 KEPULAUAN MENTAWAI -­‐1.644 99.607 2006 3 14 6.7 SERAM ISLAND -­‐3.595 127.214 4 2006 7 17 7.7 JAVA -­‐9.254 107.411 690 2007 9 12 8.4 SUMATRA -­‐4.438 101.367 2008 2 25 6.5 SUMATRA -­‐2.486 99.972 2008 11 16 7.3 SULAWESI 1.271 122.091 21 7 HALMAHERA 8 6.8 HALMAHERA Figure6: Three tsunami record data in the south coast of Java.
Tsunami in Pangandaran 1921, likely has repeated by the tsunami
in last 2007. While Bantul, Gunung Kidul, Wonogiri and Pacitan
never hit by the tsunamis as they were happened in 1840 and 1859
(The figures and notes of the tsunami are taken from Newcomb and
McCan, 1987)
In order to analyize the raw data from Newcomb and McCan, the Research and Development team of PT. Asuransi
MAIPARK Indonesia conducted a paleotsunami study in
Wonosari and Pacitan. The main target of this survey is to
estimate the wave height and coverage from the tsunami that
occurred nearly a century ago.
In addition, by studying the morphology of the areas through
satellite images and ground observation, the survey team discussed the disaster mitigation in the areas concerned. These
tsunamis which occurred nearly a century ago, could happen
again in the future. So the concrete actions from the government institutions in tsunami mitigation has to be done right
away and be planned as accurate as possible.
From field observations, the team has found the layers of sand
which are assumed to come from the tsunami that occurred a
hundred years ago. On Sepanjang Beach and Baron Beach,
the team found a layer that indicates one tsunami event. The
layer was found at a depth of 1.8 m (Sepanjang Beach) and
1.7 m (Baron Beach). Meanwhile, in Pacitan, the survey team
found the layers of tsunami deposit at a depth of 0.6 m.
Figure 7: The Satellite image of the south coast of Wonosari. The red stars
are the points where MAIPARKs team held the Paleotsunami excavation; Krakal
Beach (KRK 01 and 02), Baron Beach (BRN 01) and Sepanjang Beach (SPJ 01 and
02). (This Satellite image is from Google Earth, 2010).
South Coast of Java
The seismicity levels in the South Coast of Java are dominated by the subduction activities of Indian Ocean - Australia
plate that subducted to the Eurasian plate. The subduction
earthquakes that followed by the tsunami in the last 2 decades are Banyuwangi Earthquake (1994) and Pangandaran
Earthquake (2006).
Banyuwangi tsunami occurred in June 3, 1994, the earthquake epicenter was about 200 km from the nearest coastline,
to the Indian Ocean. The tsunami caused serious damage and
numerous victims in the coastal areas. Desa Rajekwesi, Pancer and Lampón, Banyuwangi suffered the greatest damage.
The tsunami wave also reached Desa Soka, Tabanan, Bali.
There were no fatalities in this village, but the run-up off the
tsunami waves reached 5 meters from the coastline washed
several fishing boats.
An Mw 7.7 earthquake in the Indian Ocean triggered a tsunami that arrived on the coast line about an hour after the earth-
Figure 8: Pacitan Bay. Pacitan city, located in the coastal area, is very
vulnerable to the tsunami waves. MAIPARKs team held the Paleotsunami excavation on PCT 01, 600 meters from coast line. ). (This Satellite image is from
the Google Earth, 2010).
quake. Reportedly, the area around Pangandaran is the most devastated by the 3 metres tsunami waves. In Cilacap, at least 157 people
dead and 104 missing, while in Kebumen, 10 died and 33 missing. In
Yogyakarta, six people were confirmed dead - two victims in Drini
Beach, Gunung Kidul and one person in Parangtritis, Bantul.
Image from Ikonos satellite shows the impact of the tsunami
in Pangandaran Beach in July 19, 2006. The most dramatic
evidence in this picture is the border of tsunami’s run up which
showed fragments of building materials, most likely from the
building in coastline that devastated by tsunami waves.
Seismic Gap
In the last two decades, western and eastern part of south coast
of Java was have been hit by the tsunami waves. The middle
part of south coast of Java also had the same potential to be affected by the tsunami. The history of earthquakes in the South
coast of Java from Newcomb and McCan’s note indicate that
this area was last hit by the tsunami in 1840 and 1859. From
the seismicity point of view, this area is in the process of accumulating seismic energy, indicated from the low b-value.
Tsunami Mitigation
MAIPARK research found the indications of the tsunamis in
South Coast of Java, definitely, the tsunamis could happen
Figure 9: The deposit of tsunami at Sepanjang Beach. The irregular path of
the sand layer shows that the tsunami came up in a series of waves. The
thickness of the sand deposit reaches 30 cm.
Figure 11: The tsunami deposit at Baron Beach in 0.6 m depth. The thickness of the layer at Figure 10: The tsunami deposit at Baron Beach in 1.7
m depth. The thickness of the layer is about 20 cm.
least 25 cm.
Figure 12: Earthquakes in South of Java
again in the future. But not all regions in the South coast of
Java are highly tsunami prone, only some areas have a ‘trap’
morphology which are considered dangerous when tsunamis
occur. The tsunami waves will hit this area and the population
are trapped between the walls of limestone hills, if no mitigation program is conducted.
Therefore, the Government is urged to start the tsunami mitigation program. The tsunami mitigation program must have an
Figure 13: Housing and cottage on the coast in Rajakwesi that washed away by the tsunami (Photo: Maramai
and Tinti, The 3 June 1994 Java Tsunami: A Post-Event
Survey of the Coastal Effects)
early warning system. This should not always be interpreted as to
be very modern system with sophisticated and automated system. If
we do not have a modern system, it can also be done through simple
devices! Such as, a simple installation of observation towers along
the coast, as practiced by European countries during World War II.
The watcher focus to observe the behavior of coastal waves, and
when he sees the sign of the tsunami (a sudden receding of water
from the coastline for example) he must send the emergency sig-
nal to the surrounding communities immediately. It also can be
done with the simple equipment, like the kentongan bamboo.
Every time the kentongan bamboo was alarmed, the people
must run to the safer place. This technique has a weakness that
can only be done in daytime, but it is much better than modern
early warning system equipment which we don’t have.
In the other hand, the morphology in some places has advantages. The limestone hills with the height up to 14 meters can be
a safe place to escape when the tsunami wave came. But, these
hills must be well facilitated, so in that time of emergency, people only need a few minutes to reach this place. The easy and
safe evacuating routes must be prepared for everyone, even the
elder, women and children.
From satellite images observation, areas with hills of limestone
often found in the eastern part of Java Island, extending from
the Gunung Kidul to Banyuwangi. In contrast, from Parangtritis to the western part of South Coast of Java, coastal areas tend
to have flat morphology. Figure 18 is examples of the bay areas
in the South Coast of Java that “advantaged” by the presence
of limestone hills in the vicinity. This limestone hill could be a
good place for evacuation in an emergency tsunami.
To take those advantages, the tsunami drill or simulation must
be held regularly and continuously, every 6 months for example. The residents should be prepared for emergencies, so they
will not only understand the risk of tsunamis, but also be well
trained when disaster occurs. So, the panic that occurs when the
disaster came can be reduced. This role must be taken by the
Local Disaster Management Agency (BPBD), in coordination
and direction of the National Disaster Management Agency
(BNPB). This is not an impossible thing to be done, but a necessary thing to do in all those areas.
MAIPARK take the initiative to conduct the tsunami drill.
Baron Beach at Kabupaten Gunung Kidul was chosen as a pilot project area. Construction of tsunami evacuation route and
counseling session to coastal people will precede the tsunami
drill program. Local Disaster Management Agency, Public
Work Agency and Tourism Agency along with government of
Kabupaten Gunung Kidul will involve in the program.
The program that will directly involving the community from
all ages, including elementary school age, very expected to be
a valuable “equipment” for the people to deal with the tsunami
disaster.
Figure 14: Damage caused by tsunami wave in Desa Lampon (Photo: Maramai
and Tinti, The 3 June 1994 Java Tsunami: A Post-Event Survey of the Coastal
Effects)
Figure 15: Aerial view of Pangandaran beach after tsunami July 2006 (Photo:
http://earthobservatory.nasa.gov).
Conclussion
1. Not all areas indicated in the ‘tsunami map’ of Java pu-
2.
3.
blished by BNPB and BMKG are tsunami prone. In contradiction with the North coast, the South coast of Java in
numerous place are naturally protected by limestone cliffs
which consist of coral reefs, rise perpendicular from the
Figure 16. Historical Earthquake reported in Newcomb and McCan (1987). 1840
sea, forming sea walls which at many places reach up to and 1859 earthquake is figured as tsunami-earthquake events.
30 meters from sea level. These regions are naturally protected againts tsunami events.
However, not the entire coastline is naturally protected,
at some places there are inlets, bays with lowlands and
pretty beaches. These beaches are frequented by vacationers during weekends and holidays. These beaches are not
protected. There are no early warning system, no guard
posts with guards to supervise the safety of the swimmers
againts drowning or being swept away by rip currents into
the ocean. Beaches in bays like this are numerous, like
Pangandaran, Baron, Krakal, Kukup, Sundak and Pacitan.
If an earthquake happens in the seismic gap area mentioned
17 The b-value map for subduction zone of Southern part of Java. Low
before, these inlets will become death traps for vacationers Figure
value indicated by the blue color is expected to be an accumulating energy
zone. This “seismic gap” would likely to produce a big earthquake.
and for the local population who live there.
Figure 18: The shape of ‘trap’ morphology. The tsunami waves will hit this area and trapped between the walls of limestone hills.
Recommendations
Four things are needed to prevent a catastrophy to occur, these are :
i.
Local population and the vacationers must be ‘educated’ with respect to what tsunamis are, how to save oneself and where to
run.
ii. A training and drill program must be conducted regularly, to train the local population and vacationers. If they are not regularly trained, a panic will occur and could be worse than the tsunami itself.
iii. The lowlands and beaches in the South Coast of Java are usually surrounded by limestone hills or bioherms, or limestone hills
running parallel to the sides of the bays or inlets. They are quite steep. A path or stairs must be provided for women, children
and elder people to go up to hills for safety. These path-ways must be ‘maintained’ constantly.
iv. A traditional or simple early warning system must be set up like a wooden/bamboo watchout tower with a guard watching the
sea continuously.
a.
A tsunami wave can usually be seen approaching the shore from kilometers away.
b.
Very often than not, one can see the water receding far from the shore before the waves come rolling back. Our local wisdom from Simeuleu Island in Sumatra teaches us: ‘Jika melihat laut sekonyong-konyong menyusut jauh dari
bibir pantai, larilah ke tempat-tempat yang lebih tinggi!’ (if you see the sea suddenly receding far away from the
shore, run to the higher ground). With traditional signals, the guard from the water post could alarm the people
to run to safer ground, and still have time to save himself to higher grounds. Of course a traditional and simple
system like that will not be effective at night. Considering that most vacationers, swimmers and surfers will be in
the beach at day time, a device described can still be useful, compare to having nothing at all.
However, setting up such a device needs the supervision from the Local Disaster Management Agency (BPBD) or
any local authority coordinated by BNPB.
Last but not least, those guards must be well paid, so that they will conduct their duty properly. The most difficult
thing is the continuousness and persistent discipline. This is not a one day, week, or month activity, but for always,
as long as Indonesia has beaches which are tsunami prone.
Finally, a thorough survey along the South coast of Java and other islands have to be carried out by BNPB, LIPI,
BPPT, universities, research institutions and by the NGO’s, so that Indonesia will have a more complete information of the shores and beaches. This will certainly take years, but with the aid of satellite images followed by ground
truths observation on the spots, will shorten the time for the assessment of tsunami prone coast line.