What caused the Magnitude 7.8 Mindanao quake?
The 7.8-magnitude earthquake that struck several areas in Mindanao on Monday, June 8, was caused by movement in the Cotabato Trench, according to PHIVOLCS-DOST.
Speaking with the press Monday morning, Winchelle Ian Sevilla, chief of PHIVOLCS-DOST's Seismological Observation and Earthquake Prediction Division, said movement in the trench also caused the destructive Moro Gulf earthquake in 1976. Per the National Centers for Environmental Information, the 1976 quake and its resulting tsunami killed over 8,000 persons and injured 10,000, leaving 90,000 homeless in Mindanao.
"Aktibo 'yung Cotabato Trench. Marami tayong nare-record na earthquakes, maliliit. In fact, last January hanggang May, nagkaroon ng tinatawag nating seismic swarms. Naramdaman ng mga kababayan natin sa Sultan Kudarat," Sevilla said.
"May mga pagkakataon din na nagko-cause din [ang trench] ng malalakas na lindol na nagdudulot ng pagkasira at ganun din ang tsunami," he added, referring to a 6.8-magnitude quake in 2002.
In an interview with ANC on the same day, Paulo Sawi, PHIVOLCS science research analyst, confirmed the powerful quake had a depth of 33 km, which the agency considers shallow. An earthquake with a shallow depth will bring more intense shaking on the ground. If an earthquake is deep, the seismic waves grow weaker as they travel to the surface, causing less shaking.
According to Sevilla, an earthquake's intensity reflects the strength of the ground vibrations people feel. The power of the intensity depends on a person's proximity to the quake's epicenter.
"Usually, kung malapit kayo sa epicenter ng lindol, mas malakas 'yung ground shaking na mararamdaman niyo. Mas mataas 'yung intensity na mare-record doon. Habang papalayo kayo, papahina 'yung paggalaw, so lumiliit 'yung intensity," Sevilla said.
Sevilla added that if a person is standing on soft ground, even if they are far away from the epicenter, they may still feel strong ground vibrations.
The quake was felt in different parts of Mindanao at 7:37 a.m. on June 8, the first day of school for public schools and numerous private schools nationwide. PHIVOLCS-DOST initially traced the epicenter to General Santos City, which had a 7-magnitude quake. In an update an hour later, the agency confirmed the epicenter was located in Maasim, Sarangani, with 7.8 magnitude.
As of 10 a.m. on Monday, Sawi reported a total of 138 aftershocks on record. He added that PHIVOLCS is expecting "thousands of aftershocks" more to take place, with the highest magnitude possibly at 6.7.
