The Philippine seismology agency said earthquakes in Mindanao island are not linked to volcanic activity

One misleading post was published here on Facebook on November 3, 2019, three days after a deadly earthquake rocked Mindanao, the southern Philippines, on October 31. 

The 6.5-magnitude earthquake followed a 6.6-magnitude tremor on October 29, causing more than 20 deaths, destroying buildings and displacing tens of thousands of people, according to this AFP report.

Below is a screenshot of the misleading post:

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Screenshot of Facebook post

The post contains an image of a map of the southern Cotabato province and its surrounding areas that were affected by the October earthquakes.   

The post’s Tagalog-language caption translates to English as: “ACCORDING TO PHIVOLCS: GOOD NEWS MINDANAO pressures of volcanoes are stating to regulate just a few aftershocks maybe a few more days these will stop THANKS LORD IT STOPS SOON HELP AND GUIDE US ALWAYS BECAUSE YOU KNOW BETTER AND HAVE POWER”.

“Phivolcs” refers to the Philippine Institute of Volcanology and Seismology.

The post was shared more than 2,300 times before it was removed. Another Facebook post with a similar claim that did not mention Phivolcs was published here and shared more than 15,000 times.  

The claim is misleading; Phivolcs has said all three earthquakes that hit Mindanao in October 2019 were caused by the movement of the earth’s tectonic plates and were not related to volcanic activities. 

Renato Solidum, the head of Phivolcs, told AFP by text message on November 7, 2019, that statements online linking the recent earthquakes to volcanic activities were “incorrect and did not come from Phivolcs”.

“All the earthquakes that have been recorded in Cotabato area are due to fault movements and not related to any volcanic activity,” he said. 

The United States Geological Survey (USGS) states on its website: "A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake -- or may occur slowly, in the form of creep." 

This Tagalog-language infographic posted by Phivolcs to Twitter on November 3 translates to English as: "The earthquakes that happened in the province of Cotabato and nearby areas on October 16 (M6.3), 29 (M6.6) and 31 (M6.5) and the succeeding smaller earthquakes are due to fault movements (tectonic earthquake), not volcanic activity. 

"However, DOST-PHIVOLCS is closely monitoring 24/7 all active volcanoes near the epicentre of the earthquakes like Matutum and Parker. As well the potentially active volcano Mt. Apo."

Below is a screenshot of the tweet containing the infographic:

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Screenshot of infographic

“These earthquakes are coming from an interrelated fault system,” Leyo Bautista, an associate scientist at Phivolcs told AFP in a phone interview on October 31.

“One fault movement might be triggering the movement of other faults in the area,” she said.

Earthquakes that come with volcanic eruptions are usually not big enough to destroy buildings, according to this USGS page.

The article states in part: “Earthquakes associated with eruptions rarely exceed magnitude 5, and these moderate earthquakes are not big enough to destroy buildings and roads.

“The largest earthquakes at Mount St. Helens in 1980 were magnitude 5, large enough to sway trees and damage buildings, but not destroy them. During the huge eruption of Mount Pinatubo in the Philippines in 1991, dozens of light to moderate earthquakes (magnitude 3 to 5) were felt by several hundred thousand people. Many houses collapsed, but not primarily because of the shaking. Heavy ash from the eruption (made heavier by rain from a hurricane) accumulated on roofs and crushed them.

“Stronger earthquakes sometimes DO occur near volcanoes as a result of tectonic faulting.”

Each of the earthquakes that rocked Mindanao would generate its own aftershocks, Phivolcs' Bautista said.

“That’s why we always say previously damaged structures should be inspected immediately so people don’t go back to them,” she said.