These consecutive quakes hit the area of Tulunan, a 2nd-class municipality in the province of Cotabato, and affected several nearby areas in Mindanao. The quakes killed at least 6 people, injured hundreds, and displaced over 8,000 residents. (READ: LOOK: Buildings destroyed by magnitude 6.5 quake that struck parts of Mindanao)
“Central Mindanao (SOCCSKSARGEN Region), which includes Cotabato, is one of the seismically active regions in the country because of the presence of several active faults in the area,” explained Philippine Institute of Volcanology and Seismology (Phivolcs).
The active faults that affect Cotabato are M’lang fault, Makilala-Malungon fault, North Columbio fault, South Columbio fault, and the western extension of the Mindanao fault (Cotabato-Sindangan fault).
“Cotabato Trench is also a major source of earthquakes which can affect the region. In addition, there are other nearby local faults and could be sources of small to strong magnitude earthquakes,” Phivolcs added.
There have been over a hundred earthquakes recorded to have occurred in the Philippines since the 1600s. The strongest was recorded on September 20 and 21, 1897, which happened in the Celebes sea area, between the islands of Sulu and Basilan. (READ: MAP: Strongest earthquakes in the Philippines)
Since the 1900s, meanwhile, Mindanao has been hit by at least 35 earthquakes. Here are the 3 classified by Phivolcs as “destructive,” meaning they were felt at intensity 7 or worse. (READ: FALSE: Magnitude 8.0 earthquake next to hit Mindanao – NDRRMC)
Moro Gulf earthquake
On August 17, 1976, a powerful 8.1 magnitude earthquake rocked Moro Gulf, which is off the coast of Mindanao Island and is part of the Celebes Sea. The tremors were particularly felt in Cotabato City and in the cities and towns surrounding the gulf.
However, what made this incident particularly devastating was the tsunami it caused about two to 5 minutes after the earthquake. Among the cities that bore the brunt of the biggest waves, which reached as high as 9 meters, were Pagadian City, Cotabato City, Zamboanga City, and Lebak in Sultan Kudarat. (READ: Midnight killer: The 1976 Moro Gulf tsunami)
Called by Phivolcs the “largest tsunamigenic earthquake to have occurred in Mindanao in the last two decades,” the Moro Gulf earthquake killed about 8,000 people, including those missing and were never found.
Earthquakes in Agusan del Sur
In June 1999, a magnitude 5.1 earthquake hit Bayugan, a 5th-class city in Agusan del Sur. Despite its moderate magnitude, the intensity of the quake was recorded at level 7, damaging several commercial establishments, roads, schools, and other infrastructure. Two days later, another magnitude 5.0 earthquake hit the province.
Phivolcs explained that what made the quakes in Agusan del Sur particularly damaging was the “possible site amplification due to thick sedimentary fill in the area and the poor construction prevalent in the locality of Bayugan.” Liquefaction is also seen as a possible cause of the destruction.
“Liquefaction is the process by which the ground, which is usually sandy in nature, turns into or starts behaving like a liquid and therefore loses its strength,” explained National Institute of Geological Sciences' Mario Aurelio during a Rappler Talk about the Luzon earthquake in August 2019.
The Palimbang earthquake in Sultan Kudarat is the most recent destructive earthquake to have hit Mindanao, which occurred on March 6, 2002. Eight people died in this incident.
According to Phivolcs, records show that the earthquake injured 41 others and affected 7,684 families in the provinces of Sultan Kudarat, Sarangani, North Cotabato and South Cotabato.
It also damaged roads, bridges, schools, commercial establishments, and other infrastructure, with the total damage cost amounting to P4.175 million. – Rappler.com