THE
EARTHQUAKE AND TSUNAMI OF
AUGUST 16, 1976, IN THE PHILIPPINE ISLANDS - The Moro Gulf Tsunami
George Pararas-Carayannis
(Excerpts
from a survey undertaken in the Philippines in August and September
1976 and from recent reports).
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 Introduction
On August 16, 1976,
a devastating earthquake on the Cotabato Trench caused destruction
on the island of Mindanao - the southernmost and largest of the
Phillipine Islands.
The destructive tsunami
that was generated in the Gulf of Moro and in the Celebes Sea
killed about 8,000 people in coastal communities in North and
South Zamboanga, North and South Lanao, North Cotabato, Maguindanao
and Sultan Kudarat (Mindanao), and in the neighboring Sulu Islands.
This was the worst
earthquake and tsunami disaster in the history of the Phillilines.
Also, the Indonesian Hydrographic Office reported that unusual
wave activity affected the islands of Sulawesi (Celebes Island)
and Borneo.
Earthquake
Origin Time, Magnitude, Epicenter, Focal Depth and Aftershocks
The earthquake occurred
at 16:10 UTC of August 16, 1976 (local date August 17, 1976).
The epicenter was in the Celebes Sea between the islands of Mindanao
and Borneo. The Pacific Tsunami Warning Center's preliminary
magnitude was given as 8.0 on the Richter scale and as 7.9 by
other sources. There were many aftershocks following the main
earthquake. A major aftershock on August 17 (local date) had
a magnitude of 6.8. It was proceeded by at least fifteen smaller
aftershocks.
Earthquake
Intensities
The quake was widely
felt throughout the region. PAGASA in the Phillipines gave the
following Modified Mercalli Scale Intensities for Mindanao and
the islands to the north.
Intensity VII - Cotabato
City; Jolo-Sulu; Zamboanga City
Intensity VI - Basilan
City; Pagadian City; Dipolog City; Malaybalay-Bukidnon
Intensity V - Cagayan
de Oro City; Davao City; General Santos City
Intensity IV - Dumaguete
City; Hinatuan Surigao del Sur; Tagbilaran-Bohol; Cebu City;
Surigao-Surigao del Norte
Intensity II - Roxas
City; Iloilo City; Tacloban City; Legaspi City; Palo-Leyte; Catbalogan-Samar
Summary of s field
reports on main shock ground motions (source: PAGASA)
Tsunami
Watch Issued
The Pacific Tsunami
Warning Center (PTWC) in Honolulu issued a Tsunami Watch for
the Pacific and queried tide gauge stations in Okinawa, Yap and
Malakal. Based on negative reports from these stations, the watch
was cancelled. Unfortunately, minutes after the earthquake, a
large local tsunami struck the region. There was no time to issue
a local warning.
Children
playing among the ruins at Cotabato (Photo: G. Pararas-Carayannis)
Death Toll
and Damages
The earthquake occurred
at night when offices and schools in Cotabato, Zamboanga and
other cities were unoccupied - thus the loss of life was reduced.
Although the quake had a large magnitude, surprisingly, it produced
little ground deformation on land areas. However, there was extensive
earthquake damage to buildings, bridges and roads in Mindanao
and particularly at the city of Cotabato. The majority of buildings
failed because of poor construction or inadequate foundations.
A number of such buildings had been constructed on alluvial deposits
with no adequate pile support. Evidence of ground liquefaction
was found in many areas where mud bubbles had reached the surface.
About 8,000 people
lost their lives. Ninety percent of all deaths were the result
of the tsunami.
Bridge
destroyed by the earthquake in Cotabato (Photo: G. Pararas-Carayannis)
Building
destroyed by the earthquake in Cotabato (Photo: G. Pararas-Carayannis)
Detail
of structural building failure caused by the earthquake in Cotabato
(Photo: G. Pararas-Carayannis)
Tectonic
Setting of the Region
The tectonics of the
Celebes Sea-Sulu Sea region, between the Philippine Islands and
the southern Philippine trench on the northeast and Borneo on
the southwest, are complex. The region is characterized by deep
basins and bold submarine ridges which have resulted from intensive,
large-scale faulting of strike-slip, thrust, and block types
accompanied by extensive volcanism (Krauss 1966). There are a
total of 22 active volcanoes in the Phillipines.
Epicenters
of the August 16, 1976 and of the March 6, 2002 earthquakes and
proximity to the Cotabato Trench (Source:PHIVOCS)
There are several
fault zones in the region that are capable of producing major
earthquakes and destructive local tsunamis. The two major fault
zones that are more dangerous are the Sulu Trench in the Sulu
Sea and the Cotabato Trench. The Cotabato Trench is a region
of subduction that crosses the Celebes Sea and the Moro Gulf
in Southern Mindanao.
Deep-focus earthquakes
occur along the NNE axis of the Celebes sea basin into the southern
Philippines. Shallow-focus earthquakes occur between this axis
and the southwestern side of the Philippine trench.
Seismicity
of the Region
The Moro Gulf earthquake
of August 16, 1976 occurred near the Cotabato trench - a region
of subduction.
According to the PHIVOLCS
historical catalog of earthquakes for the last 100 years, this
region of the southern Phillipines is characterized by moderate
to high seismicity. Most of the earthquakes that occur along
the Cotabato trench are shallow, although very deep events also
occur.
Distribution
of earthquake hypocenters along a section of the Cotabato Trench
subduction boundary (After B. Bautista 1996, PHIVOLCS)
The diagram by B.
Bautista (1996) shows the concentration of earthquake hypocenters
along a section of the tectonic boundary defined by the Cotabato
Trench.
Recent
Earthquakes in the Same Region
The March 6, 2002
Earthquake in Southern Mindanao - The most recent earthquake along the Cotabato
Trench region of subduction occurred on March 6, 2002, at 05:15
am (local time). According to PHIVOLCS, the epicenter was at
6.1 N; 124.0 E, about 81 km SW of Isulan, Sultan Kudarat - in
the same general area as that of August 16, 1976. Its focal depth
was 15 km. According to Pacific Tsunami Warning Center (PTWC)
the quake had a surface magnitude of 6.8. The U.S. Geological
Survey assigned a moment magnitude (Mw) of 7.2 and a body wave
magnitude (mB) of 6.3.
According to the Phillipine Office of Civil Defense (OCD), the
earthquake caused several fatalities, numerous injuries and widespread
damage in the provinces of Sultan Kudarat, Sarangani, North Cotabato
and South Cotabato.
THE
TSUNAMI OF
AUGUST 16, 1976, IN THE EASTERN AND WESTERN MORO GULF AND THE
SULU ISLANDS
Minutes after the
earthquake, a destructive tsunami swept through coastal fishing
villages and was responsible for great loss of life and damage.
This was the most devastating tsunami disaster in recent times
in the Phillipines.
The first of the destructive
tsunami waves reached some coastal areas in the Moro Gulf within
five minutes after the earthquake. Hardest hit were the Moslem
Communities in the Moro Gulf where a number of residences are
either close to the coastline, or living in houses sitting on
stilts in the water.
Destructive waves
of up to five meters in height struck North Cotabato, Sulu Islands,
Zamboanga del Norte, Zamboanga del Sur, Lanao del Norte, Lanao
del Sur, Maguindanao and Sultan Kudarat. About 8,000 people were
killed.
 Earthquake and Tsunami
Survey
In response to this
tsunami disaster, Dr. George Pararas-Carayannis (then Director
of ITIC) and Mr. Sydney Wigen (Associate Director of ITIC) flew
to the Phillipines to conduct a survey. In Manila they met with
the administrator of PAGASA, Dr. Roman L. Kintanar and Commodore
Jayma Prsbitero, Director of the Coast and Geodetic Survey and
staff members. A tentative plan for the survey of the stricken
area was drawn, with the support of these two agencies.
Dr. George
Pararas-Carayannis (then Director of ITIC) and Dr. Robert Wallace
of the U.S. Geological Survey getting ready to board military
helicopters for the survey of the Moro Gulf in Mindanao.
For maximum coverage
of the extensive survey area, the team split. Dr. Pararas-Carayannis
proceeded to Mindanao accompanied by Mr. Antonio Sucra of PAGASA.
Mr. Wigen proceeded to the Sulu Islands. In Zamboanga City, Mindanao,
the ITIC/PAGASA team was joined by Mr. Rolando Valensuela, supervising
seismologist of PAGASA and an assistant. The objective of the
survey was to obtain measurements of the tsunami wave heights,
extent of inundation and gather additional information on the
earthquake and the tsunami and its effects in the region.
 After a preliminary survey of
the coastal areas in the vicinity of Zamboanga City, the survey
party flew with army helicopters to Pagadian City in the state
of Southern Zamboanga. On the second day in Pagadian City, the
ITIC/PAGASA team was joined by Dr. Robert Wallace of the U.S.
Geological Survey. In extending the survey of the area, the team
was given logistical support by the Governor of South Zamboanga,
Mr. Tecsen. The military commander at Cotabato provided army
support and two helicopters. With such support the team surveyed
the villages of Margosatubig, Malangas, Alicia, Tabina, San Pablo,
Labangan and Tukuran. The survey of the eastern part of the Moro
Gulf was extended as far north as Bolon and in the offshore islands
of Sacol, Malanipa and Tictauan. In addition to the ground measurements
an aerial photographic survey was conducted, flying at low altitude
along the coast.
The tsunami
runup height at the entrance of a store in Pagadian City (note
the discoloration on the wall)
In the following two
days the survey extended along the north and eastern part of
Llana Bay, including the area of Karomatan, Malabang, Cotabato
City, Bongo Island, Linek, Kinini, Resa Bay and Port Lebak. Port
Lebak was the southernmost point along the eastern part of the
Moro Gulf that was visited.
Following the investigation
of the north and eastern side of the Moro Gulf the survey party
returned to Zamboanga City to begin inspection of the western
side of the Moro Gulf. Aircraft for this leg of the survey were
made available by the military command in Zamboanga City. The
survey continued in the islands of Basilan and Jolo and some
data was collected. The survey along the eastern portion of the
islands of Basilan and Jolo in the Sulu Island Group was somewhat
hampered by logistical problems and an ongoing civil war. Unable
to proceed further on these islands, the survey was extended
to the north of Zamboanga City and to the offshore islands of
Sacol, Malanipa and Tictsuan.
 The Effects of the
August 16, 1976 Tsunami in the Moro Gulf
Based on the survey,
it was determined that the maximum height of the tsunami waves
in the entire Moro Gulf were in the order of 4.5 meters (14-15
feet), which was considerably less than what had been reported
in the newspapers. Such large waves occurred at Alicia, Pagadian
City, Bongo Island, Lebak, Resa Bay and the east coasts of Basilan
and Jolo Islands.
Pagadian
City -
Pagadian City was
the major city in the area that was struck by both the earthquake
and the tsunami and sustained the greatest number of casualties.
Tsunami
destruction at a Moslem fishing village in Llana Bay (Photo:
G. Pararas-Carayannis)
The survey of Pagadian
City and of the vicinity included measurements of the horizontal
and vertical extent of inundation, observations on the failure
of structures and buildings, collection of information on the
seismic affects, interviews of eyewitnesses and officials, and
observations on the response of the people to the disaster. Statistical
information was gathered on loss of life and property.
State of
South Cotabato - No
casualties were reported from the State of South Cotabato other
than a seiche in Lake Sebu, an inland lake, which according to
reports destroyed 200 homes. Since no death reports were given
for this part of southern Mindanao it was assumed that the waves
in this area were insignificant.
Western
Moro Gulf -
The islands along
the Western Moro Gulf were struck by destructive tsunami waves
that caused many deaths. On the island of Basilan, maximum waves
of up to 4.3 meters (14 ft) killed 56 people. On the island of
Jolo, similar waves were responsible for the death of 89, with
107 more reported as missing (at the time of the survey).
Tsunami
Generating Area
Based on the distribution
of wave heights, estimates of travel times and the directional
failure of structures, it was concluded that the tsunami generating
area was in the upper part of Moro Gulf, somewhat south of Baganian
Peninsula and that the fault line was primarily underwater and
had an orientation from southeast to northwest - paralleling
the Cotabato Trench. This conclusion is supported by ground deformation
and building failures at both Tabina and Cotabato City and reported
earthquake intensities.
Another
scene of tsunami destruction at a Moslem fishing village in Llana
Bay (Photo: G. Pararas-Carayannis)
Lessons
Learned
Ninety percent of
all the deaths were caused by the tsunami. Hardest hit were Moslem
Communities where the most of the homes were close to the shoreline
or built on piles in the sea. The great number of deaths in these
communities is attributed to lack of awareness of the potential
danger from earthquakes and tsunamis in the region. Although
in some areas a 5 to 15 minute interval passed after the earthquake
and before the arrival of the tsunami waves, the people in the
area did not seek higher ground after the earthquake, but remained
in their homes.
Boats
carried inland by the tsunami at Pagadian City (Photo from helicopter: G. Pararas-Carayannis)
Recent
Tsunami in the Phillipines
Philippines
- Mindoro - 15 November 1994 - An
earthquake with magnitude of 7.1- centered 11 km N22°W of
Baco, Mindoro, near Verde Island - generated a local destructive
tsunami. In oriental Mindoro, the combined effects of the earthquake
and the tsunami killed a total of 78 people, injured 430, damaged
or destroyed 7566 houses in 13 out of 15 municipalities, damaged
roads, destroyed or damaged 24 bridges, and sunk numerous fishing
boats. There was no time to issue a warning. Approximately five
minutes after the tremor, tsunami waves struck along a 40 km
stretch of the northern and eastern shoreline of Mindoro island,
from Puerto Galera up to Pinamalayan. Also affected were Verde
and Baco Islands, north of Mindoro. Waves with a maximum runup
of 8.5 meters occurred at Pulong Malaki (Baco Island). Minor
waves were also reported at Batangas Bay.
Areas hardest hit by the tsunami were in Barangays Malaylay,
Old Baco, Wawa, and Baco Islands. Waves with maximum runup of
6 meters caused the greatest destruction, leaving at least 41
persons dead and destroying fishing boats and 1530 houses. Fortunately,
being well prepared by the Philippine Civil Defense authorities,
most of the inhabitants in the area reacted quickly to the earthquake's
natural warnings. After being awakened from their sleep by the
strong ground motions of the earthquake, they heard a strong
jet like sound of water, first receding then coming back. Knowing
that a tsunami was coming, they evacuated quickly to higher ground
and were thus able to save themselves from the incoming waves.
What also helped was the fact that the tide was at its lowest
level at that time of the night.
Unfortunately, most of the people that died in this area were
children and old people that could not move fast enough to higher
ground. Almost half of the casualties who drowned were children
below 10 years old. The lesson learned was the importance of
educational programs and preparedness, particularly for the young.
Apparently, such programs are now in place in the Philippines
where, in recent years, several tsunami disasters have killed
thousands of people. Without such preparedness the death toll
for this Mindoro tsunami would have been much greater. Nonetheless,
this disaster also indicated the need for these educational programs
to be continuous and intensive, particularly in areas known to
be vulnerable to the tsunami disaster.
REFERENCES
- Additional Sources of Background Information
Krause D. C. 1966.
Tectonics, marine geology, and bathymetry of the Celebes sea-Sulu
sea region GSA Bulletin; August 1966; v. 77; no. 8; p.
813-831
Pararas-Carayannis,
G. 1976. Survey of Philippine Earthquake and Tsunami of August
16. 1976, ITIC Report 1976. "Severe Earthquake and Tsunami
Hit the Philippines, August 16, 1976. Abstracted article, Tsunami
Newsletter , Vol. IX, No. 3, September, 1976.
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. 1978 " International Tsunami Information Center - A Progress
Report For 1976-1978". Sixth Session of the International
Coordination Group for the Tsunami Warning System in the Pacific,
Manila, Philippines, 20-26 Feb. 1978.
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