1973 Ragay Gulf earthquake

The 1973 Ragay Gulf earthquake occurred at around 16:30 (UTC +8) recorded as a magnitude 7.0 Ms tremor on the surface wave magnitude scale and with a maximum intensity of IX (Extreme) 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.[3][4][5] It killed 14 people, about 100 were injured and an estimated $2 million in damages.[6]

1973 Ragay Gulf earthquake
UTC time1973-03-17 08:30:51
ISC event762981
USGS-ANSSComCat
Local dateMarch 17, 1973
Local time16:30:51 PST
Magnitude7.0 Ms
Depth33 km (20.5 mi)
Epicenter13.372°N 122.787°E / 13.372; 122.787
FaultPhilippine Fault
TypeStrike-Slip
Areas affectedBicol region
Total damage$2 million
Max. intensityPEIS – VIII (Very Destructive)
IX (Violent)[1]
Aftershocks5.4 Mw[2]
Casualties15 dead,[1] ~100 injured

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 Philippine's largest fault line the Philippine Fault with the focal mechanism most likely a Strike-Slip earthquake.[4]

Surface rupture and fissures

The earthquake produced an onshore surface rupture 30km along the Guinayangan segment of the Philippine Fault. It also left a lateral offset on a beach line approximately 3.2 meters.[7][8]There were also fissures one of them being 15 centimeters in width 2 other fissures were also found though with unknown lengths, lying along the foothills Northwest of the PNR terminal in the municipality Calauag.[4] Close to the eastern bank of the Calauag River, multiple mudboils were spotted.[9]

Damage

Calauag

According to PHIVOLCS: in Calauag, Quezon, where it was worst hit, 270 houses were partially damaged 98 of them were 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.[10][4]

Lopez

In the neighboring town of Lopez, the concrete hollow blocks of the walls of a 5-room PTA building of the Lopez Provincial School collapsed, as well as a residential 3-story concrete building tilted to the north. The facade of the Sto. Rosario Catholic Church of Lopez suffered cracks. In Barrio Hondagua a theater which converted into a restaurant collapsed completely and a Chapel was partially destroyed. The concrete columns of the housings of the conveyer machines of the Philippine Flour Mills buckled down.[11][4]

Transportation

The earthquake caused damages to all forms of transportation linking to and from the Bicol Region. At least four highway bridges on the Manila South Road were reported to have suffered severe damage. A PNR bridge crossing the Calauag River and about 600 meters north of the highway bridge was badly damaged though fortunately it didn't collapse. Another PNR bridge in Morato Tagkawayan detected slight movement. It's ties were observed to have moved to 8 centimeters to the East, and the base plate of it's westerns abutment was moved to 5 centimeters to the south.[4]

Agriculture

The agricultural sector on the epicenter of the quake was mainly on coconuts. 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. Furthermore the roots of some trees were situated in a distance of about a kilometer.[4]

See also

References
  1. National Geophysical Data Center (1973). "Significant Earthquake Information". ngdc.noaa.gov. NGDC. doi:10.7289/V5TD9V7K. Retrieved 5 September 2021.
  2. "M 5.4 - 27 km NNE of Brgy. Bachaw Norte, Kalibo, Philippines". USGS–ANSS. Retrieved 31 August 2021.
  3. "M 7.0 - 13 km ENE of San Andres, Philippines". earthquake.usgs.gov. Retrieved 2021-08-30.
  4. "1973 March 17 Ms7.0 Ragay Gulf Earthquake".
  5. "NCEI Global Historical Hazard Database". www.ngdc.noaa.gov. Retrieved 2021-09-06.
  6. "Today in Earthquake History". earthquake.usgs.gov. Retrieved 2021-08-31.
  7. "(PDF) Coseismic Displacement and Recurrence Interval of the 1973 Ragay Gulf Earthquake, Southern Luzon, Philippines". ResearchGate. Retrieved 2021-08-31.
  8. Yasuda, Hirotake; Bacolcol, Teresito; Daag, Arturo; Bariso, Ericson; Mitiam, Emmanuelle; Marjes, Jaime; Nakata, Takashi (2015). "Geometry and Structure of the Philippine Fault in Ragay Gulf, Southern Luzon". Journal of Disaster Research. 10 (1): 91–98. doi:10.20965/jdr.2015.p0091.
  9. "Ragay Gulf earthquakes other geological features".
  10. Sabillo, Kristine Angeli (2013-10-15). "Deadliest earthquakes that shook the Philippines". INQUIRER.net. Retrieved 2021-08-31.
  11. "NCEI Global Historical Hazard Database". www.ngdc.noaa.gov. Retrieved 2021-08-31.
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