|
THE
EARTHQUAKE AND TSUNAMI OF AUGUST 19, 1977 IN THE LESSER SUNDA
ISLANDS OF INDONESIA
George Pararas-Carayannis
(Excerpts from
surveys undertaken by the International Tsunami Information Center
(ITIC) in Indonesia and Australia in August 1977, from ITIC Progress
Reports for 1976-1977 prepared for the 1978 Sixth Session of
ITSU in Manila, Philippines, for 1977-1979 prepared for the 1979
Seventh Session of ITSU in Lima, Peru, and supplemented by more
recently obtained information. Technical information on the August
19th tsunami was compiled and published as ITIC Tsunami Report
77-12)
  Introduction
On August 19, 1977
a great earthquake occurred just south of the Lesser Sunda Islands
(NusaTenggara region), west of Sumba Island in Indonesia. This
was the largest earthquake along the Java Trench in several decades.
The Lesser Sundas include Bali, Lombok, Sumbawa, Sumba, Flores,
Timor, Alor, and the adjacent smaller islands. A major tsunami
was generated which was particularly damaging along the coasts
of Sumba, Sumbawa, Lombok and Bali.
Earthquake
Origin Time, Magnitude, Epicenter and Focal Depth
The earthquake's occurred
at 06:08:52.2 UTC. The Pacific Tsunami Warning Center's preliminary
epicenter location was given as 10.5 S, 118.8 W., about 170 kilometers
SSW of Pradapare, Sumba Island (West Nusa Tenggara). The initial
Richter magnitude was estimated at 7.7. The epicenter was subsequently
revised to 11.09S, 118.46E. and the earthquake's Moment Magnitude
was recalculated (Mw- 8.2, Ms - 8.1, ML - 8.9) recently. The
quake's maximum intensity was estimated to be VIII in the immediate
area, and Mo - 24*10*20 Nm.
 Felt Reports
The quake was very
widely felt and forcing people in Perth, Australia, more than
2000 Km southward, to flee from office buildings.
Death Toll
and Damages
The earthquake caused
damage to office and school buildings, a mosque and a market
in Sumbawa and Bima. The quake and the tsunami caused 107 deaths
in the entire Nusa Tenggara region. 54 more people were reported
missing and presumed dead. A total of 440 houses were destroyed
or heavily damaged, 467 boats were missing or damaged and 5 school
buildings collapsed.
 Tectonic Setting of the Region
The Indonesian region
is one of the most seismically active zones on earth. It is an
island-arc structure of about 17,000 islands spread out along
a belt of intense volcanic and seismic activity. The major tectonic
features, which characterize the region, include a deep oceanic
trench - the great Sunda/Java Trench - on the Indian Ocean side,
a geanticline belt, an extensive volcanic inner arc, and several
marginal basins. The volcanic arc has about 400 volcanoes with
more than 100 being active. The best known of the volcanoes is
Krakatau in the Sunda Strait, between the islands of Java and
Sumatra. The 1883 explosion and collapse of this volcano generated
an enormous tsunami that killed close to 37,00 people.
Subduction of the India and Australian plates beneath the Sunda
and Java sub plates of the Eurasian tectonic block occurs along
the entire length of the great Sunda/Java Trench. The effects
of plate movements are not confined to the boundary region but
extend to the subducted plate itself, resulting in extensive
faulting, uplift or subsidence, offshore and on the islands.
For example, oblique convergence and subduction along the tectonic
boundary in the offshore area of western Sumatra has produced
the great Semangko strike-slip lateral fault on that island.
Similar oblique subduction
occurs along southern Java and the Lesser Sunda Islands as the
Australia plate slides in a north-northeastward direction beneath
the Sunda plate, at about 60 mm/yr. The rate of subduction along
the Sumba and Sumbawa segment of the East Java Trench is estimated
to be about 50 mm/yr. Near New Guinea the subduction rate increases
to as much as 107 mm/yr.
Regional significant,
major and great Earthquakes from 1900- 2003 in Southeast Asia,
Indonesia and the Philippines.
THE
TSUNAMI OF AUGUST 19, 1977
The earthquake of
August 19, 1977 in the vicinity of the Lesser Sunda Islands generated
a major destructive tsunami. Waves of up to 30 meters height
were reported on adjacent Indonesian coasts, and as much as 8
meters in Australia.
 Tsunami Survey in Indonesia
With support from
UNESCO- Intergovernmental Oceanographic Commission (IOC) and
from the U.S. National Oceanic and Atmospheric Administration
(NOAA), and with permission of Indonesian authorities the ITIC
team - joined with Indonesian investigators headed by Dr. Susanto,
Institute of Meteorology and Geophysics of the Department of
Communications - conducted a survey of coastal areas on the islands
of Sumba, Sumbawa, Lombok and Bali.
The team surveyed
tsunami damage and run up and got first hand accounts from local
residents. In addition, air reconnaissance and photography was
made along inaccessible portions of the south coast of Sumba
and Sumbawa Islands.
Agencies of the Indonesian
Army, Search and Rescue, the Disaster Relief Coordination Team,
the Provincial Government and Medical Teams had already visited
accessible coastal communities to bring aid, and to assess damage
and casualties. These records were supplemented by radio reports
from isolated communities. A search and rescue team of 19, including
a surgeon and 2 nurses, had parachuted into Lunyuk, where more
than 100 casualties had occurred. The Ministers of Communications
and of Economic Affairs visited severely damage areas to develop
Government relief and recovery programs.
 Effects
of the Tsunami in the Lesser Sunda Islands of Indonesia
Coastal roads do not
exist along the shores of the Lesser Sumba Islands fronting the
Java Trench, but with the exception of Sumbawa some communities
can be reached by road from the interior (generally though the
south coasts are rugged, except for Bali, and villages are small
and scattered). In consequence no major ports were in the seriously
struck areas and casualties thus were relatively low. Incomplete
reports indicated that more than 180 people were killed and 3,900
were left homeless. Personal losses include homes, fishing boats,
and gear.
Sumba Island
No tide records of
the tsunami were available for most areas in Indonesia. However,
the accounts given of the tsunami by observers at several locations
have reasonable consistency. The tsunami arrived on the Indonesian
coast about an hour or two after high tide and commenced with
a recession that frequently left an additional 100-200 meters
of beach exposed. Three large waves followed at intervals of
perhaps 5 minutes or less, the first being the highest and most
destructive with a maximum run-up height of 15 meters. The waves
penetrated about 500 meters inland in some coastal valleys, killed
almost 200 people and left 3,900 homeless.
 A
few observers noted a continued sea disturbance afterwards, but
this was not generally recognized. An unusual feature was that
between the time of the quake and the tsunami arrival residents
in Sumbawa and Lombok communities heard up to 3 explosive sounds,
over a period estimated from a few seconds to a minute or more.
These were described variously as sounding like bombs, aircraft
breaking the sound barrier, or thunder. The sound in each case
came more or less from the direction of the earthquake's epicenter
offshore. Almost every community reported the water turning black,
and some claimed also that it increased in temperature and bad
a bad odor.
Sumbawa Island
Bali - In Kuta, Bali one person was
killed and 5 houses collapsed, 26 boats were broken or missing.
Lompok - On Lombok, 20 persons were killed,
115 houses damaged, 132 boats missing or broken.
Sumbawa - On Sumbawa, 81 people were killed,
53 people missing, more than 1000 people lost their properties,
63 houses, one school building, one mosque collapsed and the
other was cracked. However, on an inaccessible section of Sumbawa
Island, a preliminary study indicates that the waves reached
at least 15 meters above high tide, and penetrated about 500
meters inland in some valleys.
In Kuta - Bali one person was killed and 5
houses collapsed, 26 boats were broken or missing.
 Tsunami Survey in Australia
Following the survey
of the tsunami in Indonesia, the ITIC survey was extended to
Perth in Western Australia. Although the northwest coast was
not visited (because of heavy flight bookings), tide records
from gauging stations were obtained through the Departments of
Public Works and of Meteorology, as well as first hand accounts
and reports of people who witnessed the tsunami. Consulting engineering
firms working with mining and offshore oil drilling firms also
extended much help and cooperation in the collection of data.
Tsunami
Effects in Australia
Reports indicate that
on the Australian coast, as in Indonesia three major waves came
in - with the first being the largest. The disturbance continued
for a number of hours. A wave height of 2 meters was reported
at Dampier, 2 to 4 meters at Port Sampson, and 6 meters at Cape
Leveque. The tide was falling and, at most places, it was near
low and this reduced the tsunami impact. There was apparently
no loss of life in Australia, though it is reported that at least
one person was swept into the sea by the waves.
Tsunami
Generating Sources in Indonesia
Destructive tsunamis
in Indonesia can be generated along the entire coasts of Western
Sumatra, Southern Java and Bali, the Nusa Tenggara islands (Lesser
Sundas), the Banda Sea and Southeast Sulawesi, the Molluccas,
North and Southeast Sulawesi, Irian Jaya and the Makassar Strait.
With the exception of the tsunami generated by the 1883 explosion
of the Krakatau volcano (which produced a wave of more than 100
feet in height) the highest tsunami ever recorded in Indonesia
was caused by an earthquake near Flores Island on December 12,
1992. This tsunami had waves of almost 26 meters in height.
There are two major
seismic source areas in Indonesia that are capable of producing
major and great earthquakes. These two source areas have generated
destructive tsunamis in the past and can be expected to generate
more in the future.
The first of the source
areas lies along the Sunda Arc - which extends approximately
5,600 km from the Northern Andaman Islands in the northwest to
the Banda Arc in the east. The Sunda Arc consists of three primary
segments: the Sumatra segment, the Sunda Strait Segment and the
Java Segment. Most of the major and great earthquakes occur along
the fore-arc region of Western Sumatra and Southern Java. These
segments are capable of producing large, shallow tsunamigenic
earthquakes with maximum Richter magnitudes of up to 8 (up to
9 or even more in Moment Magnitude if more than one segment breaks).
Also, large deeper earthquakes with focal depths in excess of
70 km can occur, but these do not generate tsunamis. Tsunamis
generated along Western Sumatra can be destructive throughout
the Indian Ocean. Tsunamis generated along Southern Java and
the Lesser Sundas can have destructive effects in Indonesia and
Australia.
The second source area of major earthquakes is along the shallow
crustal faults that generally parallel the Sunda Arc (east-west),
with some transform surface faulting that marks north-south trending
boundaries of landmasses. Earthquakes with maximum magnitudes
of 7.0 and focal depths of less then 70 km can occur along this
second source area. Peak ground accelerations of surface seismic
waves can range from 0.15 g (gravitational acceleration) to as
much as 0.35 g or more. These earthquakes can also generate destructive
tsunamis but their destructive effect is more localized.
REFERENCES
- Additional Sources of Background Information
Hamilton, W., 1979,
Tectonics of the
Indonesian region:
U.S. Geological Survey Prof. Paper 1078.
Pararas-Carayannis,
G., 1977. International
Tsunami Information Center A Progress Report For 1974-1976. International Coordination
Group for the Tsunami Warning System in the Pacific, Vina Del
Mar, Chile, 1977.
Pararas-Carayannis,
G., 1977, Indonesian
Earthquake and Tsunami of August 19, 1977, Intern. Tsunami Information Center Report,
Abstracted article in Tsunami Newsletter, Vol. X, No. 3, September,
1977.
Pararas-Carayannis,
G. 1977. International
Tsunami Information Center, Tsunami Report 77-12 , Tsunami Reports for 1977.
Pararas-Carayannis,
G. The Great Tsunami
of August 26, 1883 from the Explosion of the Krakatau Volcano
("Krakatoa") in Indonesia
http://drgeorgepc.com/Tsunami1883Krakatoa.html
Pararas-Carayannis,
G. 2003, Near and
Far-Field Effects of Tsunamis Generated by the Paroxysmal Eruptions,
Explosions, Caldera Collapses and Slope Failures of the Krakatau
Volcano in Indonesia, on August 26-27, 1883, Presentation for the International Seminar/Workshop
on Tsunami "In Memoriam 120 Years Of Krakatau Eruption _
Tsunami And Lesson Learned From Large Tsunami" August 26th
_ 29th 2003, Jakarta and Anyer, Indonesia. Also published in
the Journal of Tsunami Hazards, Volume 21, Number 4. 2003
http://drgeorgepc.com/Tsunami1883Krakatau.html
Return to
 
Links to other
Pages
Now available
from Amazon, Barnes and Noble and other major bookstores. A signed
by the author copy can be also ordered by contacting directly
by email Aston
Forbes Press.
Other
Miscellaneous Non-technical Writings
(©) Copyright
1963-2007 George Pararas-Carayannis / all rights reserved / Information
on this site is for viewing and personal information only - protected
by copyright. Any unauthorized use or reproduction of material
from this site without written permission is prohibited.
|