Date of Event: March 17, 1973
Epicenter: 13.41N ; 122.87E
Ms = 7.0
Focal Mechanism: Strike Slip

Summary of Damages:

Buildings and Other Civil Structures

The town worst hit by the earthquake is Calauag, Quezon where 98 houses were totally destroyed and 270 others were partially destroyed. In barrio Sumulong of the same town, 70% of the school buildings were damaged. Most of the partially to completely destroyed houses and buildings were situated along the seashore in the northern section of the town proper. The damaged houses were largely wooden and some were poorly built concrete buildings.

The town of Lopez ranks next to Calauag with respect to the extent of destruction. The place is relatively farther from the causative fault and the epicenter of the mainshock, but soft underlying sediments present in Calauag are similarly found in Lopez.

The concrete hollow block retaining walls of a 5-room PTA building of Lopez Provincial School collapsed on both sides of the building along the N-S direction. A residential 3-story concrete building was severely tilted to the north.

The facade of the Sto. Rosario Catholic Church of Lopez suffered cracks and some parts of the CHB walls on both sides toppled down.

The 1 km. long concrete seawall along the ESE coast of Calauag suffered minor cracks mostly along construction joints. About its mid-section in one of its stairways there was a 10 cms. crack. One section was displaced 5 cms to the north from the other section.

In Barrio Hondagua, Lopez 5 km east of Calauag, some buildings were totally or partially damaged. The CHB wall of one of the classrooms of Hondagua Elementary School toppled down.

The Hondagua Theater which had been converted into a restaurant completely collapsed and the Catholic chapel of the Barrio was partially destroyed. The concrete columns of the housings of the coveyor machines of the Philippine Flour Mills in Hondagua buckled down. There was differential settlement of the ground along fills in the pier such that floorings of some of the buildings became uneven and were cracked.

Transportation / Communication Lines and Underground Pipes

The earthquake wrought damages to roads, railroads and bridges. This hampered travel to and from Bicol Region. At least four highway bridges on the Manila South Road were reported to have suffered damages ranging from a partial to total collapse. The bridge which totally collapsed was the Sumulong highway bridge in Sumulong, Calauag.

A PNR bridge crossing the Calauag River and situated about 600 meters north of the highway bridge was badly damaged although it did not collapse. The rails along the bridge were badly twisted.

A slight movement was detected at the PNR bridge in Morato Tagkawayan. Its ties were observed to have moved to 8 cms. to the east, and base plate of its western abutment was moved 5 cms. to south.

Damages to national and municipal roads were limited to cracking of the concrete slabs along the Manila South Road. Subsidence occurred along the Sumulong-Guinayangan road. Minor cracks were observed along the national highway from Km 217, up to Km 234 in Calauag.

Betweeen the town of Lopez and Calauag the rails of the PNR were reported to have been badly twisted. The major twisting of the raiways however occurred some 300 meters from the southwestern approach of the PNR trestle bridge in Sumulong. This provided a remarkable manifestation of the lateral movement of the ground.

Electric systems, waterwoks systems and telegraph systems in the town of Lopez, Calauag and Guinayangan were severely disrupted.. In Calauag, water main pipes were either fracured or severed. Electric and telegraph lines snapped due to appreciable horizontal movements of the ground. Fires which broke out during the earthquake were immediatel controlled by alert local firefighters.

In the town of Lopez, a concrete electric post broke on its base and toppled down, pinning 5 youngsters who embraced the post to prevent themselves from being thrown off their feet due to the strong shaking of the ground.

Disruption to water system was minimal. In Barrio Hondagua, Lopez a 3’4’’ water pipe was subjected to tensional forces which resulted in the breaking and seperation of the pipes.

Agriculture

The agricultural industry in the epicentral area is based mainly on coconut. The effect of earthquake on this industry was not felt immediately, but after a few months had elapsed. Coconut production registered low because of the shaking down of young nuts during the earthquake.

Furthermore, the roots of some trees situated within a distance of about 1 km. on both sides of the fault traces were cut off as a consequence of the horizontal ground movement, thus affecting their fruit bearing capability.

Concrete fishpond dikes in the epicentral region were cracked although to a minor extent. Mud dikes which were loosely made toppled down while others were fissured.

Geologic Features and Effects

Features and Effects Related to Faulting

The most interesting feature in this earthquake was the remarkable extent of faulting. The farthest observable fault trace from the epicenter is 90 kms. away in the coastal barrio of Sumulong, Calauag.

Ground breakages were seen along the segment of the Philippine Fault, from the western coast of Ragay Gulf to Calauag Bay, a stretch of about 30 kms. The fault traces exhibited moletrack features with ground fissures arranged in enchelon to one another in an E-W trend. From Barrio Cibong towards barrio Sintones in the town of Guinayangan, some 6 kms. northwestward, the traces were observed to have followed a moderate deppression.

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.

Referemces:

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