3 major earthquakes in 11 days: Are the earthquakes related?

Just when Cebuanos began to subside due to a decline in the strength and number of daily aftershocks from the magnitude 6.9 earthquake that struck north of Cebu in the Visayas Islands on September 30, 2025, pent-up tectonic stresses were released in two major tremors of magnitude targeting southeastern Mindanao this time – magnitude 7.4 and magnitude 6.9 respectively within just 10 hours of each occurring at the same time. almost. place.

Mindanao earthquakes struck offshore east of Manai City in Davao Oriental on the same day, October 10, 2025 – a magnitude of 7.4 at 9:43 a.m., and a magnitude of 6.9 at 7:12 p.m.

Volume versus intensity

Magnitude refers to the absolute amount of energy released by a fault rupture that triggers an earthquake. For large earthquake events, energy is measured in terms of momenta, a logarithmic scale that means that a magnitude 6.0 earthquake, for example, has 100 times greater ground motion than a magnitude 4.0 earthquake (102).

On the other hand, intensity is a measure of the strength of an earthquake which usually translates to ground shaking. Based on the effects of ground shaking on people, objects, structures and the environment, a scale is determined from weakest to strongest. Different countries use different severity scales.

The Philippine Institute of Volcanology and Seismology (Phivolcs) created an Earthquake Intensity Scale (PEIS) for use in the country. This scale classifies intensity I as the weakest (perceivable to people under favorable conditions; finely balanced objects are disturbed slightly; still water in containers oscillates slowly) and intensity floor; Many trees are felled, broken and uprooted.

Intensity can be measured by calculating the strength of ground shaking, which can be determined by measuring the distance a point on the ground moves as a function of the square of time during an earthquake. This amount of ground shaking, in metric units of centimeters per second squared (cm/s2), is often expressed as a measure of the gravitational acceleration constant (c = 980 cm/s2). A ground vibration rate greater than 1.0 g will cause an object to be thrown against the force of gravity.

This ground acceleration value is among the parameters entered into equations that engineers use to design earthquake-resistant buildings. It is a function of several factors, including the type of subsurface material on which the buildings are built, as well as the distance of the site to the earthquake source.

In general, the weaker the subsurface such as in coastal areas, riverbanks and floodplains, and the closer the location to the epicenter, the greater the ground speed during an earthquake. It is the intense vibration of the ground that causes buildings that were not designed properly to fall and be damaged.

Focal depth, central distance and error type explain the difference

The energy released by the 7.4 magnitude Manai earthquake is about 11 times greater than the 6.9 magnitude Cebu earthquake. However, the damage to infrastructure resulting from the less powerful Cebu earthquake was much more severe than that caused by the higher magnitude Manai earthquake.

Reports of damage within a day of the 7.4 magnitude earthquake from netizens include shattering concrete walls and glass panels of commercial establishments, government offices and residential buildings from Tagum in Davao del Norte to Davao City in Davao del Sur. The Office of Civil Defense (OCD) reported massive and widespread landslides in the mountainous parts of Manai town in Davao Oriental, causing severe damage to the city's general hospital.

But none of the reports in Mindanao so far have matched the severity of the damage to buildings in Bogo City, north of Cebu, after the September 6.9 magnitude earthquake, which included the complete failure of vertical columns that led to the collapse of entire buildings.

The focal depth of the Manai earthquake is more than 50 km, while the depth of the Cebu earthquake is only 5 km. The epicenter of the Manai earthquake is also located at least twice (>40 km) offshore from the Cebu earthquake epicenter distance (~20 km) to the Bogo coast.

The Cebu earthquake, caused by a shallow strike-slip fault (where one part of the ground moves laterally relative to another), produced a shallow rupture on the ground. On the other hand, the Manai earthquake, which was caused by a deep reverse fault (where one part of the ground moves up relative to another), did not produce a superficial rupture on the ground.

Even with a much weaker magnitude of 6.9, more than 10 times less powerful than the Davao Oriental earthquake's 7.4, the Cebu earthquake's shallow focal depth, closer distance to the epicenter, and nature of the slide made the difference in causing more severe damage to infrastructure. This is supported by the highest intensity felt and effectively recorded for both earthquakes reported by Phivolcs, namely Intensity V for Manay but Intensity VII for Cebu.

Is there a relationship between Cebu and Davao Oriental earthquakes?

To test whether earthquakes that occur sequentially at different locations are related to each other, seismologists use a technique called Coulomb stress transfer (CST) modeling.

This technique revolves around the principle that when a crack moves, it releases pressure. This released stress does not disappear but simply moves to locations close to the fault that has ruptured. In this new location, pressure then builds up.

When enough stress builds up in this new location, any fault located in the positively oriented region may lead to failure. When this happens, the next earthquake is triggered.

CST testing on the Cebu and Davao earthquakes indicates that the two are not related to each other, mainly because of the distance between them and that the faults that generated them are not physically connected. It appears that the magnitude 6.9 is not strong enough to transfer stress as far as the fault that generated the magnitude 7.4 East Davao earthquake.

On the other hand, the two East Davao earthquakes appear to be related to each other. CST modeling indicates that the focus of the magnitude 6.9 earthquake is on the edge of stress release, but closer to areas of stress accumulation.

This suggests that the 7.4 magnitude event may have triggered a 6.9 magnitude earthquake. Both are caused by faults associated with the subduction of the Philippine Sea plate (when one tectonic plate slides under another) beneath the Philippine Moving Belt along the Philippine Trench. – Rappler.com

Coulomb stress transfer modeling was performed by Abigail Manahan and Sandra Catugas, both graduate students at the National Institute of Geological Sciences-University of the Philippines (UP NIGS) under the guidance of the author, who heads the Structural Geology and Tectonics Laboratory at the Geological Institute. Darwin Regier, an exploration geologist in Tagum, Davao del Norte, contributed information on damage to infrastructure in Davao del Norte, Davao del Sur, and Davao Oriental.

Mario A. Aurelio, Ph.D., is a structural geologist and professor at the National Institute of Geological Sciences-University of the Philippines (UP NIGS) who includes earthquake studies among his research interests. He is a recipient of the 2025 Distinguished Alumni Award for Conservation in Earth Sciences Education, presented by the University of the Philippines Alumni Association

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