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:

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