1976 Moro Gulf earthquake

File:Lebak Tsunami.jpg]]

Several fault zones in the region are capable of producing major earthquakes and destructive local tsunamis. The two major fault zones that are most dangerous are the Sulu Trench in the Sulu Sea and the Cotabato Trench, a region of subduction that crosses the Celebes Sea and the Moro Gulf in Southern Mindanao. According to the PHIVOLCS historical catalog of earthquakes for the last 100 years, this region of the southern Philippines is characterized by moderate to high seismicity. The most recent earthquake along the Cotabato Trench region of subduction being the March 6, 2002, earthquake in Southern Mindanao.{{Citation needed|date=March 2022}}

The earthquake occurred in an region where typical seismicity originates at {{cvt|500|km}} depth on the Wadati–Benioff zones associated with subduction along the Sangihe and Mindanao islands. The Philippines is characterised by several complex and active subduction zones. The most geologically dominant in the region is the Philippine Trench located east of Mindanao. It dips to the west as represented by the Wadati–Benioff zone which can be detected to at least {{cvt|200|km}}.{{cite journal |last1=Stewart |first1=Gordon S. |last2=Cohn |first2=Stephen N. |title=The 1976 August 16, Mindanao, Philippine earthquake (Ms = 7.8) – evidence for a subduction zone south of Mindanao |journal=Geophysical Journal International |date=1979 |volume=57 |issue=1 |pages=51–65 |doi=10.1111/j.1365-246X.1979.tb03771.x |doi-access=free |bibcode=1979GeoJ...57...51S}}{{cite journal |last1=Beck |first1=Susan L. |authorlink1=Susan L. Beck |last2=Ruff |first2=Larry. J. |title=The rupture process of the 1976 Mindanao Earthquake |journal=Journal of Geophysical Research: Solid Earth |date=1985 |volume=90 |issue=B8 |pages=6773–6782 |doi=10.1029/JB090iB08p06773 |bibcode=1985JGR....90.6773B}}

The mainshock had a shallow focal depth and represented thrust faulting along a plane dipping northeast at a shallow angle. The shock in the North Celebes Sea was associated with the smaller Cotabato Trench; a north–south bathymetric feature along the island's coast where it meets the Moro Gulf. Geologists have interpreted the earthquake as evidence for east–northeast-dipping subduction beneath Mindanao. The feature is also thought to be geologically young due to the absence of an active volcanic arc. In 1918, a major earthquake and tsunami ruptured a segment of the subduction zone south of the 1976 earthquake; both earthquakes ruptured a combined {{cvt|300|km}} of the trench. The rupture propagated northward from the epicenter along the Cotabato Trench, evident from the aftershock zone. However, the strongest aftershock, measuring {{M|s|link=y}} 6.8, was the result of strike-slip faulting and occurred away from the aftershock zone.

File:PhilippineMobileBelt007.jpg in southern Mindanao and the Philippine Mobile Belt.]]

{{Location map+|Mindanao|float=right|width=270|alt=Earthquakes in Mindanao|caption=Near the Cotabato Trench, two of the largest 20th century Philippine earthquakes: the 1918 Celebes Sea earthquake (8.3 M_w) and the 1976 Moro Gulf earthquake (8.0 M_w). Also included is the 2002 Mindanao earthquake (7.5 M_w). The Moro Gulf, part of the Celebes Sea, is labeled for context.|places=

{{Location map~|Mindanao|lat=6.50|long=123.00|position=left|label=

{{Location map~|Mindanao|lat=6.29|long=124.09|position=left|label=

{{Location map~|Mindanao|lat=5.97|long=124.38|position=left|label=

{{Location map~|Mindanao|lat=6.03|long=124.25|position=right|label=

}}

The initial earthquake was widespread and was felt as far as the central Philippine islands of the Visayas. A massive tsunami devastated 700 kilometers of coastline bordering the Moro Gulf in the North Celebes Sea, resulting in destruction and death in the coastal communities of the Sulu Archipelago, southern Mindanao particularly the provinces of Sultan Kudarat and Sarangani (formerly part of South Cotabato), and in the Zamboanga Peninsula including Zamboanga City and Pagadian City.{{Citation needed|date=March 2022}}

The maximum height of the waves reached 9 meters at Lebak on the isla; 4.3 meters at Alicia; 3 meters at Resa Bay on the eastern coast of Basilan; and at the islands of Jolo and Sacol.{{Cite book |last1=Department |first1=Commerce |last2=Dunbar |first2=Paula K. |title=Pacific Tsunami Warning System: A Half Century of Protecting the Pacific, 1965–2015 |date=April 24, 2015 |pages=46–47 |url=https://books.google.com/books?id=Hb9w5gzoN1QC&pg=PA47 |access-date=August 22, 2017 |publisher=Government Printing Office |isbn=978-0-9962579-0-9}} At least 5,000 people died during the earthquake and tsunami, with thousands more remaining missing.[http://www.ringsurf.com/online/2412-tsunamis_through_history.html History of Tsunami Devastation] Some reports say that as many as 8,000 people lost their lives in total, with ninety percent of all deaths the result of the following tsunami.

Initially over 8,000 people were officially counted as killed or missing, 10,000 injured, and 90,000 homeless, making it one of the most devastating disasters in the history of the Philippine Islands.{{cite book |title=Catalog of Tsunamis in the Pacific 1969–1982 |url=https://books.google.com/books?id=umZ--INFYEEC&pg=PA103 |author=Staff of the Academy of Sciences of the USS |publisher=Diane Pub |isbn=978-0-7881393-1-4 |pages=103, 104 |date=1997}} After the initial earthquake the people were unaware of the need to move to higher ground; when the tsunami hit it sucked most of the victims out to sea. Based on the investigation on the affected region it was confirmed that the waves reached up to {{convert|4|to|5|m}} when they hit the areas. There were reports of weak tsunami activity as far as Japan.

In Zamboanga City, 14 buildings were partially damaged. The city was spared from serious damage of the tsunami triggered by this earthquake because the Basilan Island and the Santa Cruz Islands served as a buffer and deflected waves.{{cite web |url=http://www.phivolcs.dost.gov.ph/html/update_SOEPD/2012_Earthquake_Bulletins/Destructive_Earthquake/1976MoroGulfEQ/index-moro.html |title=Moro Gulf Earthquake – 17 August 1976 |publisher=Philippine Institute of Volcanology and Seismology |access-date=October 15, 2013 |archive-date=October 15, 2013 |archive-url=https://web.archive.org/web/20131015062653/http://www.phivolcs.dost.gov.ph/html/update_SOEPD/2012_Earthquake_Bulletins/Destructive_Earthquake/1976MoroGulfEQ/index-moro.html |url-status=dead}}

• List of earthquakes in 1976
• List of earthquakes in the Philippines

{{reflist|refs=

{{cite book |title=International Handbook of Earthquake & Engineering Seismology |series=Part A, Volume 81A |chapter=Global seismicity: 1900–1999 |first1=E. R. |last1=Engdahl |first2=A. |last2=Vallaseñor |year=2002 |chapter-url=https://earthquake.usgs.gov/data/centennial/centennial.pdf |publisher=Academic Press |edition=First |isbn=978-0124406520 |page=683}}

}}

• {{cite book |last1=Eadillo |first1=Victor |last2=Astilla |first2=Zinnia |title=Moro Gulf Tsunami of 17 August 1978 |date=1978 |publisher=Special Committee on Tsunami Warning System, National Committee on Marine Sciences, National Science Development Board |location=Quezon City |url=https://www.mindanews.com/wp-content/uploads/2019/08/MoroGulf_1976complete.pdf |access-date=10 September 2024}}

• [http://www.phivolcs.dost.gov.ph Philippines Institute of Volcanology and Seismology ] {{Webarchive|url=https://web.archive.org/web/20100129060134/http://www.phivolcs.dost.gov.ph/ |date=January 29, 2010 }}
• [http://www.drgeorgepc.com/Tsunami1976Phillipines.html The Earthquake and Tsunami of August 16, 1976, in the Philippines – The Moro Gulf Tsunami"] – George Pararas-Carayannis
• {{EQ-isc-link|709878}}

{{Earthquakes in 1976}}

{{Earthquakes in the Philippines}}

{{DEFAULTSORT:Moro Gulf Earthquake}}

1976 Moro

Category:1976 in the Philippines

Category:August 1976 in Asia

Category:1976 earthquakes

Moro

Category:History of Zamboanga del Sur

Category:History of Maguindanao del Norte

Category:History of Maguindanao del Sur

Category:1976 disasters in the Philippines