MANILA, Philippines – Just a minute and a half before the hour of three in the afternoon of June 11, 2024 (Manila Time), an offshore area about a hundred kilometers off the eastern coast of Puerto Princesa, capital of Palawan, was struck by an earthquake, an extremely rare event given that Palawan is believed, at least by geologists, to be sitting on a stable continental platform. 

We learn from high school Earth Science that the spherical earth is made up of three main layers: the core in the innermost section, the mantle in the middle and the crust at and near the surface. The theory of Plate Tectonics further says that the upper part of the mantle and the crust (that compose the lithosphere) are broken into pieces called tectonic plates. There are two main types of plates: continental and oceanic. Mainly by virtue of the differences in their composition, continental plates are more buoyant (lighter) than oceanic plates. 

Generally, all the tectonic plates are moving around and about resulting from the dynamism of the earth. As this happens, the plates interact at their boundaries. When plates move away from each other, they are said to “spread” apart. Spreading normally results to the formation of big oceans, such as the Atlantic, Pacific and Indian Oceans. 

When plates come close to each other, they may either “collide” when both have the same buoyancy, or “subduct” (one plate diving beneath the other) when one is less buoyant than the other. Collisions are what build tall mountains such as the Himalayan Range and the European Alps, while subductions produce volcanic chains such as those that comprise archipelagoes like the Philippines, Indonesia, Japan and New Zealand. 

When plates interact at their boundaries through spreading, collision and subduction, earthquakes are produced. Within immobile plates, earthquakes are not expected. 

Tectonics and Philippine earthquakes

Some tectonic plates are more mobile than others. Most immobile plates are composed of the lighter continental crust, such as Africa, North America and Antartica. Oceanic plates, such as the Pacific, Indian and Atlantic plates are more mobile, with the Pacific Plate moving at the fastest at an average speed of about 10 centimeters per year (note: the average rate at which finger nails grow is about three and a half [3.5] centimeters per year). 

In the Philippines, Palawan sits on a relatively immobile continental plate. Within this plate, earthquakes are not expected to occur. However, at and close to the boundary with the Philippine volcanic arc such as in Mindoro and Panay, the northern Palawan islands of Busuanga, Coron and Cullion can experience earthquakes every so often (see diagram). 

The epicenter of the recent Palawan earthquake plots in an area within the supposedly immobile section of the continental crust called the northwest Sulu Sea Basin, far from any active plate boundary. The earthquake was thus not expected here (see diagram below).

Rare but not zero

So why did the earthquake happen? 

Although continental Palawan is barely moving, it is being pushed by the mobile sections of the Philippine archipelago from the east, where the tectonic plate that contains such islands as Mindoro, Panay, Negros and the oceanic southeast Sulu Sea Basin is sliding at an average rate of about three and half (3.5) centimeters per year. Over a long period of time, this seemingly slow push eventually causes the buildup of stress that when the continental crust is not anymore able to resist it, a section of the crust needs to break/rupture in order to release the accumulated stress. Think of slowly pressing a balloon with both palms until it bursts – the balloon is stable at the initial application of pressure, but bursts at the point where it can no longer bear the pressurized air inside. 

A naturally occurring earthquake is produced by an active fault. At the epicenter of the recent Palawan earthquake, existing maps do not indicate the presence of any active fault. However, seismic profiles which are images of the crust below the surface, indicate the presence of several potential faults that may have caused the earthquake. 

Seismic profiles are usually obtained for the petroleum industry because they provide information necessary to establish the subsurface structure that may contain the petroleum deposit. Faults play an important role in the formation of petroleum deposits. But when such faults become active and pierce through the surface, they allow oil to escape from the reservoir and spill out to the surface, diminishing the commercial value of the resource eventually. 

At this point, earthquake scientists and petroleum explorationists, both specialized fields of the geological profession, are discussing in an attempt to identify the culprit fault, but also to understand any significant implications of the earthquake to the petroleum industry, considering that the northwest Sulu Sea Basin is among the areas being explored for petroleum resources. 

The Structural Code of the Philippines, used as guide by engineers to build earthquake-resilient infrastructure, classifies Palawan as Zone 2 – lowest vulnerability to earthquakes. 

Despite the recent magnitude 5.1 earthquake, Palawan still remains the most tectonically stable land in the Philippines. Like a father who serves as the “haligi ng tahanan,” Palawan is the tectonic anchor of the rest of the Philippine islands. – Rappler.com

The author is a professor at the National Institute of Geological Sciences-University of the Philippines (UP-NIGS) who studies earthquakes, among his other research interests. His former students made the diagram. John Dale “JD” Dianala, DPhil came back to teach at UP NIGS after recently obtaining his doctorate from Oxford University, the first ever Filipino UP-NIGS alumnus to do so. Dainty Rabang recently left UP-NIGS as a teacher, to now serve at the Palawan Provincial Disaster and Risk Reduction Management Office.