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At 4:19 AM (local time) on August 02, 1968 an earthquake with an intensity of VIII in the Rossi-Forel Intensity Scale rocked the town of Casiguran, Aurora. This was considered the most severe and destructive earthquake experienced in the Philippines during the last 20 years. Two hundred seventy (270) persons were killed and 261 were injured as a result of the earthquake. A six-storey building in Binondo, (Ruby Tower) Manila collapsed instantly during the quake while several major buildings near Binondo and Escolta area in Manila sustained varying levels of structural damages. The cost of property damage was several million dollars. Extensive landslides and large fissures were observed in the mountainous part of the epicentral area. Tsunami was also observed and recorded as far as observation in tide gauge station in Japan.

Date of Event August 02, 1968
Origin Time 4:19 am (20:19 GMT)
Epicenter 16.3 N Latitude 122.11 E Longitude or approximately
Magnitude Ms: 7.3 Mb: 5.9 (ISC)
Depth approximately 31 km from the surface.

Intensity Report:

Intensity VIII Casiguran, Quezon
Intensity VII Manila and Palanan
Intensity VI Baler, Quezon City, Tuguegarao, Aparri, Baguio, Dagupan, Iba, Cabanatuan, Alabat,
Intensity V Tarlac, Ambulong, Infanta, Jomalig
Intensity IV Legaspi, Lucena, Calapan, Aurora, Laoag, Catarman, Virac
Intensity III Romblon, Vigan
Note: Intensity scale used in these observations was the Adapted Rossi-Forel Earthquake Intensity Scale of I-IX.

Casualties:

Place
No. of persons killed
No. of persons injured
Manila (Ruby Tower)
268
260
Manila (Sta. Ana Tenement House)
-
1
Aurora Sub Province
1
-
Guagua, Pampanga
1
-
Total
270
261

Summary of Damages:

Damage to Particular Buildings in Manila

The severe damage area was concentrated in a relatively small part of Greater Manila. This part of Manila lies in the mouth of Pasig River (a major river system in Metro Manila) and includes the deepest and most recent alluvial deposits in the city.

Ruby Tower

The Ruby Tower was a large six-storey building containing 38 commercial units in its first two floors and 76 residential units in its upper four floors. Most of the building collapsed except for a part of the northern end of first and second floors (Photo 1 & 2), killing 268 persons and injuring 260 of the occupants. The upper floors fell southwards while the southern end of the roof moved about 30 feet or 9.15 meters south and 10 feet or3.05 meters east (Photo 3). The lower floors appeared to fall close to their plan position (Photo 4).

Philippine Bar Association Building

The Philippine bar Association (PBA) was a medium rise six-storey commercial building with offices and club rooms (Photo 5). Most of the first storey columns suffered total collapsed or very severe damage and shortening (Photo 6, 7, 8 & 9). The intensity of damage increased towards the southeast end of the building. Exterior columns suffered greater damage and shortening the nearby interior columns. This resulted in very a severe deformation of some interior beams and part of the floor at each storey level.

Aloha Theater

The Aloha Theater is a large eight-storey building that suffered severe damage near its southern end. The damage was initiated by the collapsed of a few very short columns towards the southern end of the fourth floor. It spread out and severely damaging the southern end of the building (Photo 10 & 11).

Tuason Building

Tuason Building is a medium sized six-storey building which suffered severe damage or very close to collapse (Photo 12 & 13). The columns along the southern side wrecked or severely damage (Photo 14).

Other buildings that suffered considerable structural and nNon-structural damage (Manila)

Trinity Building (Photo 15 & 16)
Diamond Tower (Photo 17 & 18)
Liwayway Hotel (Photo 19 & 20)
National Library (Photo 21)
Old Philippine National Bank
Boie Building
Araneta and Tuason Building (Photo 22)
Development Bank of the Philippines
Phoenix Building
La Tondeña Building
New Philippine National Bank (Photo 23)
Overseas Terminal (Photo 24)
Far Eastern University Arts and Sciences Building

Landslides

Landslides occurred in several places on the steep slopes of surrounding mountains near the epicentral area. Landslides produced by the main shock were mostly on the slopes of mountains north of the town of Casiguran, while those that accompanied the big aftershocks were observed on mountains both to the north and to the west. The largest landslide took place on the cliff at Dinajawan Point facing Casiguran Bay (Photo 25) while another landslide was observed in Manglad River, a tributary of Cagayan River (Photo 26). Manglad River traverses behind a cornfield and beside this, the transported unconsolidated sediments produced a small hill (Photo 27).

Ground Ruptures

In the epicentral area, around the town of Casiguran, cracks that were parallel to the nearest rivers were observed. Surface soil in this part is mostly loose deltaic sand. The length of the fissures varies from 10 to 20 meters but in some areas, it reached a length of 400 to 500 meters. The space between the cracks varies from 5 to 20 meters Fissures on the road from Casiguran to Barrio Tabas produced a 0.5 meters crack and the surface subsidence varied to as much as two meters (Photo 28). This road is approximately 8 meters from the Casiguran River at the top of a steep bank approximately 2.5 meters high. Photo 29, shows another fissure on a logging road, 30 meters away from and parallel to river bank in Casiguran area.

References:

Osome, S., Osawa, Y., Skinner, I., Yoshima, and Y., PHILIPPINES: Luzon Earthquake of August 02, 1968, UNESCO, Serial No. 977/BMS. RD/SCE. NR, Paris, January 1969.

Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE). Series in Seismology, Volume IV (Philippines), 1985

During the second field survey to the epicentral area, a 3.4 meters offset of the shoreline in Barrio Cabong,
Guinayangan was observed. Ground displacement was laso left lateral.

Other Geologic Features:

The strong shaking of the ground during the Ragay Gulf Earthquake caused two areas along the Calauag-Guinayangan municipal road between kms 236-238 to subside. One of the resulting depressions was 225 meters long while the other was 95 meters long. The longer depression was 2 kms. NW from the first.

A fissure, 15 cms. Wide with 2 unknown lenth, lies along the foothills some 200 meter nw of the PNR terminal in Calauag. Its orientation is N80W. In Lopez, two fissures were observed along Lopez-Jaena St. These may be due to settlement of the bank of Talolong River.

Close to the eastern bank of the Calauag River in Barrio Sumulong and Mabini, several sand boils were found. Mudboils are formed when water- laden sediments are subjected to compressional forces thereby causing the water and fine sands and muds to be injected into the air through fissures or to just upwellm towards the surface.

References:

Southeat Asia Association of Seismology and Earthquake Engineering; Volume IV, Philippines, 1985

Journal of the Geological Society of the Philippines; Volume XXVIII, June, 1974

Summarized by: Erlinda Q. Amin

At 8:18 P.M. of 17 August 1983, an earthquake with a magnitude of 5.3 (Ml) on the Richter Scale and an intensity of VII on the Rossi-Forel Scale hit the province of Ilocos Norte. The tremor was perceptible over a distance of 400 kilometers from the epicenter. This was the most sever earthquake in North-western Luzon in 52 years and probably the second largest earthquake event to hit Laoag city and it's immediate vicinity in historical times. This earthquake has caused death of 16 people and injuries of forty seven persons (PDE).
Date of Event August 17, 1983
Origin Time 8:17 pm (12:17 GMT)
Epicenter 18.231 N Latitude 120.860 E Longitude or approximately 30 aerial kilometers east-northeast of Laoag City.
Magnitude 6.5 Ms ( 5.3 Ml on the Richter Scale)
Depth approximately 42 km from the surface.

Intensity Report:

Intensity VII Laoag City, Pasuquin- Ilocos Norte
Intensity VI Vigan-Ilocos Norte
Intensity V Aparri-Cagayan, Santa-locos Sur
Intensity IV Tuguegarao-Cagayan, Baguio City
Intensity III Dagupan City, Callao-Cagayan, Manila

Note: Intensity scale used in these observation was the Rossi-Forel Earthquake Intensity Scale.

Historical Background:

Since 1862 upto 1981, (excepting the years 1941 to 1949) fifty-six earthquakes have affected Laoag City. Of these, the strongest was recorded on 19 March, 1931. This earthquake reportedly had an intensity of VII - IX. Prior to the 17 August earthquake, two tremors were recorded on the eleventh and the thirteenth of August 1983. These were believed to be foreshocks of the intensity VII earthquake (Macalincag, T. G., personal communication). The first had an intensity of V and the succeeding one an intensity of II in the Rossi-Forrel Scale.

Summary of Damages:

Damages on buildings:

A number of reinforced concrete buildings either totally crumbled or sustained major structural damage beyond rehabilitation. The failure in most of the damaged buildings can be attributed to shear and compressional waves, thereby producing horizontal and vertical stresses. The most heavily damaged structures in Laoag City are those situated near the Laoag River flood plain and along reclaimed stream channels. These buildings were condemned by the City Engineer's Office. Nearly all the damaged buildings in the area were of reinforced concrete frame. Most of the external walls and internal partitions were of concrete hollow blocks. There are however, some buildings with wood partitions.

List of buildings that totally collapsed, suffered severe structural damage, sustained considerable damage or considerable non-structural damage:

Laoag City The Laoag Cathedral
Sinking Bell Tower
Yabes Juan Department Store Building
Denson Building
Golden Hardware Building
Laeno Building
Avenue Lumber and Hardware
Philippine Veterans Bank Building
Teresita Building
Philippine National Bank Building
Shirley Building
Far East Building
Sunrise Building
Isabel Building
SE Asia building
Julian Building
Pichay's Building
Castro Building
Mique Residence
Gerardo Building
E. Ang Residence
San Nicolas Kaunlaran Building
San Nicolas Church and Bell Tower
Bacarra Bacarra Church and Bell Tower
Sarrat Sta. Monica Church and Bell Tower
Sarrat Municipal Hall
Marcos Guesthouse
Vintar Vintar Church
San Nicolas Academy
Vintar Municipal Hall
Burgos Bojeador Lighthouse

Minor Effects:

Landslides: Several earthquake induced landslides were observed in places where the slopes along road cuts were steep to very steep. This condition had been aggravated by prolonged rainy days, absence of vegetation to hold the soil, moderately weathered and indurated rocks. Areas affected by landslides were the Sarong Valley in Vintar and Patapat Mountains in Pagudpud, both in Ilocos Norte.

Sandboils or Sandblows: Several sandboils were reportedly observed in Barangay Zamboanga, Laoag City; Barangay Puyupuyan, Pasuquin; and Barangay Calayab, Paoay. The diameter of their craters vary from a few centimeters to 2.5 meters. Sandblows or sandboils are the spouting of hydrated sand caused by moderate to severe earthquakes. This connate water that has been entrapped in the interstices of sediments at the time of deposition may have come from either South China Sea or Laoag River.

Differential Settlement: Majority of the bridges in Ilocos Norte had experienced differtial settlement of approach and or abutments. Some of the buildings were also observed to have differential settlement in addition to being out of plumb. Step fractures due to collapse of foundation were observed at Marcos Guesthouse in Sarrat. Magnitude of differential settlement measured range from a few centimeters to approximately 30 centimeters.

Shear Fractures: A tilted road pavement along J. P. Rizal Street, Laoag City was observed after the main tremor. Gaping tension fractures along Vintar-Bacarra Road and along asphalt pavement on the southern approach of Bacarra Bridge were also observed. Gaping Step tension fracture along Vintar Poblacion-Tamdagan road was found. Numerous irregular cracks and small fissures were discovered along seashores, river banks and alluvial fans.

References:

Santiago, N.G and Rillon, E.A ( December 1983): Assessment on the effects of the August 17, 1983 Earthquake in Laoag City: Bureau of Mines and Geo-sciences.

Valenzuela, R.G. and Garcia, L.C. (10 October 1983) Laoag Earthquake of 17 August 1983 SummaryReport: PAGASA.

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
Magnitude 8.1
FORESHOCKS

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)

AFTERSHOCKS

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.

COTABATO CITY

DAMAGE TO BUILDINGS

SCHOOLS

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.

ADMINISTRATION BUILDING
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.

HARVADIAN COLLEGE
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

DAWNS HOTEL
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.

D’MAX RESTAURANT
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.

MELBOURNE HOTEL
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.

SAGUITTARIUS HOTEL
A four-story structure with reinforced concrete frame built in 1965. The building collapsed completely. (Go to Amicus Building for additional information.)

SULTAN HOTEL
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.

THEATERS

COTABATO CINEMA
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.

FRANCEL THEATER
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.

RITA THEATRE
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.)

CHURCHES

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.

OTHERS

AMICUS BUILDING
The Amicus Building, Sagittarius Hotel and D'Max Restaurant formed a complex of three adjacent buildigs that collapsed.

BOSTON BAKERY
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.

MELINEEN BUILDING
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.

TISON BUILDING
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.

WATERFRONT WAREHOUSES
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

QUIRINO BRIDGE
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.

TAMONTAKA BRIDGE
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.

ZAMBOANGA CITY
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.

PAGADIAN CITY
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.

Area
Dead
Missing
Injured
Homeless*
Region 9
1,440
909
7,701
49,848
Region 12
3,351
1,379
2,227
43,534
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.

REFERENCES:

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.

This shallow seated tectonic earthquake with magnitude 6.8, struck the island of Bohol at 3:15 pm, caused panic to general public, damaged several houses and infrastructure and presented several geologic disturbances. Its epicenter was located about 17 kilometers east of Tagbilaran City with a maximum felt intensity of VIII, based on Rossi-Forel Intensity Scale, in the towns of Jagna, Duero and Guindulman all situated on the lower area of the NE quadrant of the island. It was felt at intensity VII in Garcia Hernandez, Loboc, Valencia and Anda, Intensity VI in Tagbilaran City, the rest among the 16 municipalities of Bohol and in the neighboring islands of Cebu and Camiguin. Intensity V was felt over areas of Cagayan de Oro in Mindanao, Dumaguete City in Negros, Intensity IV in the areas of Canlaon in Negros and Cotabato City in Mindanao. Reported felt intensities ranging from I to III was also felt as far as Palo in Leyte and Bislig in Surigao.
Observed geologic phenomena related to this event include ground fissures, landslides, rockfalls, ground subsidence and collapse, sand/mud fountaining and sudden increase on the sea level. Most of the manifestations were particularly observed and experienced by the towns of Jagna, Valencia, Duero, Guindulman and Garcia Hernandez. The force of the incoming waves from the sea caused Alijuan River in Duero to flow inland immediately after the earthquake.

Based on the orientation of the main fracture zones, focal mechanism solution and aftershock distribution, the earthquake may have represented subsurface rupture along segments of the NE-SW Alicia thrust fault. Studies by the Bureau of Mines (1986), however, point to the fact that in most portion of the fault is being overlained by Miocene to recent limestone which does not reflect any deformation suggesting that the fault has been inactive for quite a long time. This would pose a question as to whether the earthquake represented reactivation of an old fault or indicated new fault movement in the island.

Impact and damage documentation revealed that the worstly affected portion of the island was sustained by the eastern and southeastern coastal areas, observed to be mostly underlained by alluvial deposits which have tendency to amplify ground motions generated by an earthquake. Likewise, most of the damaged buildings were either old/poorly-built or lacked the necessary reinforcements to resist strong ground shaking. About 3,000 units of houses, buildings and churches were affected and damaged where a total of 182 were totally collapsed including two historical churches built centuries ago. Some 200,000 sq.m. of fishpond in the town of Guindulman sustained damage due to cracked and collapsed dikes. Mud eruptions on these fishpens contributed to the death of fishes and prawns.

The bridge connecting the towns of Jagna and Duero collapsed. Roads to Anda sustained cracks and fissuring. Landslides and rockfalls blocked some portions of the roads that caused inaccessibility to some areas between Anda and Garcia Hernandez.

Six fatalities were reported and more than 200 were injured in the event. About 46,000 people were displaced by the event and at least 7,000 among them were rendered homeless. Estimated damage to properties is amounting to 154 million pesos.

References:

Umbal, J.V., Masigla L.M., Medrano R. N. and Diolata G.P. - Report of Investigation
on the February 08, 1990 Earthquake in Bohol Province. PHIVOLCS