The 1973 Ragay Gulf earthquake occurred at around 16:30 local time (UTC +8). It was measured as magnitude 7.4 M_w on the moment magnitude scale. It had a maximum intensity of IX (''Violent'') on the MMI, according to the National Geophysics Data Center, while according to the Philippine Institute of Volcanology and Seismology, the tremor had an intensity of VIII (''Very Destructive'') on the PHIVOLCS Earthquake Intensity Scale. It killed 14 people, injured 100 others, and caused an estimated $2 million in damages.

Earthquake

The epicenter was located around 17 km east northeast of San Andres on the Ragay Gulf with a depth of 33 km (20.5 mi). It was due to the movement of one of the Philippines' largest fault; the Philippine Fault System with the

focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...

corresponding to

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

Surface rupture and fissures

The earthquake produced an onshore surface rupture 30 km along the Guinayangan segment of the Philippine Fault. It also caused left-lateral offset on a beach line for approximately 3.2 meters. There were also fissures, one of them being 15 centimeters in width. Two more fissures were found though with unknown lengths, along the foothills northwest of the

Philippine National Railways The Philippine National Railways (PNR) ( fil, Pambansang Daang-Bakal ng Pilipinas and es, Ferrocarril Nacional de Filipinas) is a state-owned railway company in the Philippines which operates one commuter rail service between Metro Manila an ...

(PNR) terminal in the municipality of Calauag. Near eastern bank of the Calauag River, multiple mudboils were spotted.

Damage

Calauag

According to PHIVOLCS, in

Calauag, Quezon Calauag, officially the Municipality of Calauag ( tgl, Bayan ng Calauag), is a 1st class municipality in the province of Quezon, Philippines. According to the 2020 census, it has a population of 71,809 people. It is southeast of Manila and e ...

, the worst hit, 270 houses were partially damaged, and 98 completely destroyed; most of which were poorly built or entirely made of wooden materials. In Barrio Sumulong, also part of Calauag, 70% of school buildings were damaged.

Lopez

In the neighboring town of Lopez, concrete hollow blocks of the walls in a 5-room PTA building of the Lopez Provincial School collapsed. A three-storey concrete residential building tilted to the north. The facade of the Rosario Catholic Church of Lopez suffered cracks. In Barrio Hondagua, a theater which was converted into a restaurant collapsed completely and a chapel was partially destroyed. The concrete columns of the housings of the conveyor machines of the Philippine Flour Mills buckled down.

Transportation

The earthquake caused damages to all forms of transportation linking to and from Bicol Region. At least four highway bridges on the Manila South Road suffered severe damage. A PNR bridge crossing the Calauag River, and about 600 meters north of the highway bridge was badly damaged though it did not collapse. Another PNR bridge in Morato Tagkawayan was slightly moved. Its ties moved eight centimeters to the east, and the base plate of its westerns abutment moved five centimeters to the south.

Agriculture

The agricultural near the epicenter of the quake were mainly coconut plantations. The tremor's effects on the industry were not immediately felt, however after a few months, coconut production was on a down low due to young nuts that were shaken by the earthquake.

References

{{Earthquakes in the Philippines 1973 earthquakes 1973 in the Philippines 1973 disasters in the Philippines March 1973 events in Asia Earthquakes in the Philippines History of the Bicol Region History of Camarines Norte History of Camarines Sur History of Quezon