1973 Ragay Gulf earthquake explained

Type:Strike-Slip
1973 Ragay Gulf earthquake
Casualties:15 dead,[1] ~100 injured
Aftershocks:5.4 [2]
Intensity:
Damages:$2 million
Affected:Quezon, and Camarines Sur, Philippines
Fault:Philippine Fault
Location:13.372°N 122.787°W
Depth:33 km (20.5 mi)
Magnitude:7.4
Local-Time:16:30:51 PST
Local-Date:March 17, 1973
Anss-Url:usp00000z7
Timestamp:1973-03-17 08:30:51
Isc-Event:762981

The 1973 Ragay Gulf earthquake occurred at around 16:30 local time (UTC +8). It measured 7.4 and had a maximum intensity of IX (Violent) on the Modified Mercalli intensity scale. The Philippine Institute of Volcanology and Seismology assigned a maximum intensity of VIII (Very Destructive) on the PHIVOLCS Earthquake Intensity Scale.[3] [4] [5] [6] It killed 14 people, injured 100 others and caused an estimated $2 million in damage.[7]

Earthquake

The epicenter was located around 17 km east northeast of San Andres in 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 corresponding to strike-slip.

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.[8] [9] 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 (PNR) terminal in the municipality of Calauag. Near eastern bank of the Calauag River, multiple mudboils were spotted.[10]

Damage

Calauag

According to PHIVOLCS, in Calauag, Quezon, 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.[11]

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.[12]

Transportation

The earthquake caused damage 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.

See also

Notes and References

  1. Web site: National Geophysical Data Center . Significant Earthquake Information . ngdc.noaa.gov . NGDC . 5 September 2021 . 10.7289/V5TD9V7K . 1973.
  2. Web site: M 5.4 - 27 km NNE of Brgy. Bachaw Norte, Kalibo, Philippines. USGS–ANSS . 31 August 2021.
  3. Web site: M 7.0 - 13 km ENE of San Andres, Philippines. live. 2021-08-30. earthquake.usgs.gov. https://web.archive.org/web/20210831100328/https://earthquake.usgs.gov/earthquakes/eventpage/usp00000z7/executive . 2021-08-31 .
  4. Web site: 1973 March 17 Ms7.0 Ragay Gulf Earthquake.
  5. Web site: NCEI Global Historical Hazard Database. 2021-09-06. www.ngdc.noaa.gov. en.
  6. Web site: ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009) . 2022-09-12 . www.isc.ac.uk.
  7. Web site: Today in Earthquake History. 2021-08-31. earthquake.usgs.gov.
  8. Web site: Coseismic Displacement and Recurrence Interval of the 1973 Ragay Gulf Earthquake, Southern Luzon, Philippines. 2021-08-31. ResearchGate. en.
  9. 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. 10.20965/jdr.2015.p0091. free.
  10. Web site: Ragay Gulf earthquakes other geological features. live. https://web.archive.org/web/20210831100337/https://www.phivolcs.dost.gov.ph/index.php/17-earthquake/38-1973-march-17-ragay-gulf-earthquake . 2021-08-31 .
  11. Web site: Sabillo. Kristine Angeli. 2013-10-15. Deadliest earthquakes that shook the Philippines. 2021-08-31. INQUIRER.net. en.
  12. Web site: NCEI Global Historical Hazard Database. 2021-08-31. www.ngdc.noaa.gov. en.