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).
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.
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 RegionThe 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.
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.
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)
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)http://www.drgeorgepc.com/Tsunami1976Phillipines.html