A few minutes after the last stroke of midnight on August 17, 1976, a violent earthquake occurred in the island of Mindanao spawning a tsunami that devastated more than 700 kms of coastline bordering Moro Gulf in the North Celebes Sea. This offshore event generated by Cotabato trench, a less prominent trench system in the Philippines, was the largest tsunamigenic earthquake to have occurred in Mindanao in the last two decades. It was an earthquake that resulted in massive destruction of properties and great loss of lives. The tsunami generated contributed immensely to the devastation. The cities and provinces of Cotabato took the brunt of the earthquake while the tsunami generated cast its doom on the provinces bordering Moro Gulf especially on the shores of Pagadian City. According to surveys during the event, the tsunami was responsible for 85% of deaths, 65% of injuries and 95% of those missing. After the sea spent its fury and rolled back to its natural flow, thousands of people were left dead, others homeless or missing and millions of pesos lost with the damages of properties. Properties lost not only include establishments for residential and commercial use, but also bancas that, as a whole, represents the livelihood of hundreds of families.
|Date of Event||17 August 1976|
|Time||12:11 A.M. (Local)|
|Epicenter||06.3° N, 124.0° E|
Analysis of seismic records for August 1976 prior to August 17 of the same year showed that there were six events recorded that had epicenters in the same area as the main shock and could be considered as foreshocks of the Moro Gulf earthquake. Also, about a month before that, two quakes were reportedly felt in Zamboanga City that also had epicenters near the area of the main shock. These two events were not recorded at the PAGASA Observatory in Quezon City nor in any of its field stations. This brings to eight the total number of foreshocks, three of which are felt events with intensities ranging from I to IV. (Stratta et.al., 1977)
There were approximately forty (40) aftershocks that were plotted using available data from the seismic network of PAGASA. But it was reported that more aftershocks were felt and recorded locally most of which were felt in the area with Rossi-Forel intensities of up to Intensity VI. Aftershocks in Cotabato City were monitored by the Commission on Volcanology (now Philippine Institute of Volcanology and Seismology) and during the span of time that the aftershocks were monitored, an average of about 140 aftershocks per day were recorded. Monitoring started on the 18th of August 1976 using a three component Hosaka seismograph and a single-component Kinemetrics seismometer.
DAMAGE TO BUILDINGS
COTABATO CHINESE SCHOOL GYMNASIUM
A reinforced concrete and wood structure built in 1962. Its walls fell outward during the earthquake and the roof fell in.
This is a two-story building (designed for three stories) with reinforced concrete frame built in late 1973. The building suffered little damage on some of its walls. A pile foundation had been used.
The campus includes several buildings but only a partially collapsed five-story structure was investigated. The building was built in 1962. It had reinforced concrete frame with reinforced concrete slabs at the second level and at the exterior walkways at the third, fourth and fifth levels. The fifth story was constructed completely of wood and the roof had GI sheeting. The building was reportedly designed for three stories with the fourth and fifth floor added later with no strengthening of the lower stories.
NOTRE DAME UNIVERSITY
This University is located on Notre Dame Avenue approximately 1.5 km southeast of the downtown area. The site has wet and soft marshy ground. Ground water appeared to be very near the surface as ponds were evident throughout the site.
THE ADMINISTRATION BUILDING
A rectangular three-story structure with reinforced concrete columns and girders and concrete floors built in 1960s. Damage to the building was light. There was a 3 m concrete panel at the entrance that was heavily cracked and damage to the frame was minor.
THE AUDITORIUM AND SCIENCE BUILDING
This is a 48 x 30 m auditorium crossed at its entrance by a three-story 51 x 12 m science wing. It had reinforced concrete frame with masonry infills built in 1969. The roofs of the auditorium and science wing had the same elevation. After the earthquake, a fire broke out in the science wing. Its first and second story columns sustained heavy damage. The long span beams also have heavy cracks. After the fire had burned for several hours, the science wing collapsed. The auditorium suffered heavy fire damage. The roof trusses in the stage area sagged heavily due to the intense heat. Large areas of the trusses dropped simultaneously. The infilled walls did not suffer structural damage but the entrance of the auditorium was destroyed when the science wing collapsed.
THE NEW RESIDENCE HALL
This is a rectangular, three-story structure nearing completion and unoccupied at the time of the earthquake. It consists of concrete exterior columns, thin concrete exterior walls, timber interior columns and floor systems, and plywood interior partitions. Damage to the building was light. There were cracks at the floor line, some columns were damaged at the sill line, interior partitions were torn apart, some ceiling panels fell and considerable cracks of the ground floor slab.
THE TECHNICAL SCHOOL
A two-story building built in 1965. It had a concrete frame with a concrete two-way slab floor. This building was linked to an adjacent building by a common wood canopy. Damage to the structure was moderate. The first story columns and fins were damaged at the head and sill levels and the canopy collapsed at its end bay.
HOTELS and RESTAURANTS
A six-story reinforced concrete frame and wall building. There was no damage except for a little working on the floor joints of the south wall of the building.
A two-story building constructed in 1968. It was a combination of reinforced concrete and wood. The building collapsed completely. (Go to Amicus Building for additional information.)
IMPERIAL HOTEL #1
Imperial Hotel #1, Imperial Hotel #2 and Rita Theatre are situated close together. Imperial Hotel #1 and Rita Theatre drifted to the west and pushed against Imperial Hotel #2. Imperial Hotel #1 is a four-story reinforced concrete building with masonry infills built in 1963. The building experienced a 38 cm permanent offset in the first story and the rear portion of the building collapsed.
IMPERIAL HOTEL #2
This is a six-story building with reinforced concrete frame built in 1967. There was superficial damage to the building that consisted of cracks in a column, its infill panels and part of the slab grade caused by the impact force from the fronts of Rita Theater and Imperial Hotel #1. The impact also caused shear failure of the second story column.
A three-story building with reinforced concrete frame with masonry infilled panels built in 1970. The first story suffered a permanent offset to the south after the quake and its columns sustained heavy damage. On the east side of the building, the panels were pushed out and window infills in its mezzanine floor buckled outward.
NEW SOCIETY HOTEL
A four-story reinforced concrete frame building with shear walls and reinforced concrete slabs for its floors and roof. It was built in 1968. It is located within 30 m of Rio Grande and its elevation is 2 m below street level. The columns were founded on woodpiles with reinforced concrete pile caps. The pile cap and water table were nearly coincidental. Proximity of the river and the high water table would suggest a very strong ground shaking but there had been factors that ruled out this possibility and instead, flaws in the structure were considered as the principal causes of failure. The building collapsed as the building twisted in a counterclockwise motion; the northwest corner of the second floor dropped down to the street; and the opposite southeast corner suffered torsional failure of the corner pilaster and out-of-plane shearing of the adjacent walls. The frame and walls above the first story was practically undamaged.
A four-story structure with reinforced concrete frame built in 1965. The building collapsed completely. (Go to Amicus Building for additional information.)
A five-story building that suffered collapse of the first floor. The collapse of the building must have been slow because the portion above the second floor remained intact.
This is a large structure to the rear of Sultan Hotel. When the hotel collapsed, it caused severe structural damage to the theater complex. It was hard to determine whether the collapse of the hotel caused failure to the theater or merely contributed to an already damaged structure.
A reinforced concrete and wood building built in 1966. The reinforced concrete portion of the building collapsed causing failure of the wood trusses of the roof.
This was a two-story 12 m tall reinforced concrete frame building in front. Its rear portion was a combined reinforced concrete frame with masonry infills and wood and serves as the auditorium. The auditorium roof has two elevations. The front frame drifted to the west along with Imperial Hotel #1. The auditorium frame and its infilled east wall were knocked over by Imperial Hotel #1 and the roof on this part of the structure collapsed. Further to the rear, the roof did not collapsed because the roof elevation was lower. (Go to Imperial Hotel #1 for additional information.)
IMMACULATE CONCEPTION CHURCH
This church was located across the street from Tison Building. Its grounds were very soft and the church was obviously not built on piles. The church tower settled by about 15 cm.
TAMONTAKA CATHOLIC CHURCH
A structure made of unreinforced brick walls with interior timber columns and wooden roof said to have been built by the Spaniards around 1872. It was built on soft marshy soil. Before the earthquake, the building already had some structural cracks that could be either due to a previous earthquake or a differential settlement. The church suffered severe damage.
The Amicus Building, Sagittarius Hotel and D’Max Restaurant formed a complex of three adjacent buildigs that collapsed.
A two-story reinforced concrete building built in 1965. The first story of the build drifted about 60 cm to the west.
COTABATO AUTO SUPPLY
A three-story building built in 1968 with reinforced concrete frame, concrete floor and masonry infilled exterior walls. Its partitions were made of timber and plywood. The first two floors were used for auto parts sales and storage and the third floor served as the owner’s living quarters.
The first story collapsed gradually, according to the proprietor, with the upper stories coming to rest approximately 3 m west of its original location. The rest of the building only sustained minor cracks. Storage shelves in the second floor were still standing after the quake.
A one-story concrete lean-to behind this building also collapsed.
COTABATO FIRE and POLICE STATION
The building settled out of plumb toward the river during the earthquake. Otherwise, there was no structural failure noted on the building itself.
FIRST GIFT AND BOOKSTORE
A four-story building built around 1968 to 69 with reinforced concrete frame resting on a timber pile foundation. This was also known as the Yap building after its owner.
The first floor of this building collapsed during the initial earthquake tremor and fire broke out within the structure. Five to six hours later, the structure collapsed completely. It was noted that this structure leaned into an adjacent three-story building knocking it into a third building, the City Evangelical Church. Damage to the church was light.
LCT HARDWARE AND AUTO SUPPLY
A two-story reinforced concrete structure with wood trusses and GI sheet roof. The first story collapsed toward the west during the earthquake.
A two-story reinforced concrete building that pancaked.
SOUTH SEAS TRADING
This was a three-story building built in 1967. It had a concrete frame and floor slab. This building completely collapsed.
TAN BO BUILDING
A four-story building constructed around 1971. Its frame was of reinforced concrete while the walls are infilled hollow blocks. The whole structure was built on timber piles. The only damage noted was the cracks on the walls near the stairs.
This was the only building in Cotabato City known to have been designed with seismic considerations. It was built on precast concrete friction piles on good soil. It survived the earthquake with only a slight crack in a concrete block partition.
A large number of warehouses were located at the edge of Rio Grande west of Manday River. They look like they were made of masonry walls, timber trusses, and corrugated GI sheets. They were poorly built. All of the warehouses collapsed.
DAMAGE TO BRIDGES
This is a four-span structural steel bridge over the Rio Grande. Each span is 40 m long. The second span from the south end collapsed into the river during the earthquake. The third span from the south end nearly collapsed and cracks appeared several centimeters below the base of the south abutment.
This bridge spans about 230 m across Tamontaka River approximately 6 kms south-southwest of Cotabato City. The bridge is made up of six spans resting on pile-supported piers. The girders, piers and piles are made of reinforced concrete. The bridge was constructed in three sections. After the earthquake, the center section moved east and west in excess of 38 cm each way evidenced by the broken concrete keepers on each end of the supporting piers. The northern section moved even greater distances. The southern section moved but with lesser distance. There was damage to the railings at the abutments and the expansion joints.
Fourteen buildings in this City of Flowers were partially damaged while twenty-six buildings sustained minor damage. The City Hall bore noticeable cracks along its façade. Ateneo de Zamboanga sustained failures at the sill level of its columns on the fourth floor due probably to poor concreting and column weakening because of water seepage from the GI downpipes embedded in the columns. Zamboanga Agricultural and Engineering College sustained damage to columns due to failure at end moments.
Zamboanga City was spared from the onslaught of the tsunami on account of the strategic geographic location of Basilan and Santa Cruz Islands that served as buffers and deflected the waves that otherwise could have inflicted heavy damage along Zamboanga City’s coastline. Damage in buildings consisted mostly of cracks on its masonry walls and insufficient lateral ties in some columns.
The coastal districts of Santa Lucia, Santiago, San Pablo, San Roque and White Beach Barangay were hardest hit by the tsunami. Almost all of the houses along the coast within 500 meters inland were destroyed. Some houses made of reinforced concrete hollow blocks were able to withstand the force of the waves and also served as protection to other house made of light materials. The approach to the Pagadian City wharf settled down, causing cracks in the slabs of the approach area and in the concrete deck. The five-story reinforced concrete building of Saint Columban College had noticeable cracks in the masonry infilled walls. Shear cracks in two columns were observed at the junction.
DAMAGE DUE TO TSUNAMI
Just after the earthquake stopped, the sea, stirred by the powerful movement of the earthquake, swelled and moved away from the coastline for about three kilometers. About ten minutes later, it roared back to the shore and beyond in three succeeding waves soaring as high as the treetops according to some reports. The sea unloaded its fury on everything near the shore. Houses and properties along the coastal beaches of Lanao del Sur and Pagadian were practically washed out. Bits of houses littered the sea and bodies littered the shore. The casualties and victims of the earthquake and tsunami numbered thousands just in Regions 9 and 12. (Region 9 covers Pagadian City, Zamboanga del Sur, Zamboanga City, Basilan, and Sulu while Region 12 covers the areas of Sultan Kudarat, Maguindanao, Cotabato City, Lanao del Sur and Lanao del Norte.) A tabulation of the victims and casualties in these regions is as follows.
Source: Badillo, V.L. and Astilla, Z.C.: Moro Gulf Tsunami of August 17 1976
*Some of the data in this section was estimated at 6 members per family
The major cause of the great number of casualties during the event could be attributed to the fact that (1) the tremor happened just after midnight when most people were sleeping; (2) a great tsunami was spawned, struck the coasts from different directions and caught the people unaware.
Stratta, James L., et. al.; 1977, EERI Reconnaissance Report Mindanao, Philippines Earthquake August 17, 1976, 106 pp.
Badillo, Victor L. and Astilla, Zinnia C.; 1978, Moro Gulf Tsunami of 17 August 1976, 41 pp.
Stewart, Gordon S. and Cohn, Stephen N.; 1978, The 1976 August 16, Mindanao, Philippine
Earthquake (Ms = 7.8) — evidence for a subduction zone south of Mindanao, 14 pp. Acharya, H.K., 1978, Mindanao Earthquake of August 16, 1976: Preliminary Seismological Assessment : Bulletin of the Seismological Society of America Vol. 68, 1459-1468.
Southeast Asia Association of Seismology and Earthquake Engineering, 1985, Series on Seismology Volume IV Philippines, 489-515.