Nearly 700km Underground

In seismic science, depth matters as much as magnitude, earthquake geologist Navakanesh M Batmanathan tells Garasi Bernama. 

‍Malaysia’s Meteorological Department (MetMalaysia) reported the earthquake’s depth at approximately 678km, while the United States Geological Survey (USGS) recorded it at about 620 kilometres. Even at the lower estimate, the earthquake was extraordinarily deep.

For context, earthquakes are generally classified as:

“The recent earthquake was very deep. If it was closer to the surface  – that is, less than 30km – we might have seen some damage and even casualties,” explains Batmanathan.

In fact, the recent Sabah earthquake falls within the deepest category possible. Very few earthquakes occur beyond 700km.

In contrast, the earthquake that struck Ranau, Sabah in June of 2015 occurred at a shallow depth of roughly 10km. Because it was close to the surface and near populated areas, the shaking was intense, says Batmanathan, who investigated the geological impacts of events like the 2015 Sabah earthquake.

Despite being smaller in magnitude than Sabah’s Feb 23 event, the Ranau earthquake triggered landslides on Mount Kinabalu, killing 18 hikers and damaging buildings, making it one of Malaysia’s most significant natural disasters in recent decades.

To understand why depth matters, imagine tapping a table from underneath versus striking it directly on the surface. A surface strike produces sharp, immediate vibrations. A deep push from far below may still move the table, but the intensity felt at the top is significantly reduced.

Similarly, when an earthquake occurs hundreds of kilometres below the surface, seismic waves must travel a vast vertical distance before reaching communities. Along the way, much of the destructive energy disperses.

The depth of the recent Sabah earthquake also means that it did not trigger significant seabed displacement, which could have caused a tsunami, says Batmanathan, whose work at Universiti Kebangsaan Malaysia’s Southeast Asia Disaster Prevention Research Institute focuses on mapping active fault lines and analysing vertical land motion related to sea-level rise.

How Magnitude is Measured

Shortly after the earthquake, different agencies reported slightly different magnitudes. MetMalaysia recorded the earthquake at 6.8, while USGS placed it at 7.1.

Such discrepancies are common and do not mean one agency is wrong. Different seismic agencies use varying calculation methods and station networks. 

“Local agencies like MetMalaysia use data from its national seismic network and their monitoring stations are closer to the epicentre,” says Batmanathan. “USGS, on the other hand, does global averaging from hundreds of stations worldwide.”

Because magnitude is calculated using wave amplitude, distance, and modelling assumptions, small differences can arise depending on the dataset used. A variation of 0.2 to 0.3 in magnitude is not unusual in global seismic reporting.

What matters more is that both agencies agree: this was an unusually strong earthquake for Malaysia.

Earthquake magnitude is measured on a logarithmic scale. This means each whole number increase represents a significant jump in energy release.

A magnitude 7.0 earthquake releases about 32 times more energy than a magnitude 6.0. In simple terms, the Feb 23 Sabah earthquake released far more energy at its source than the 2015 Ranau earthquake.

However, magnitude measures the total energy released underground, not the level of destruction at the surface.

Damage depends on several additional factors, including:

In this case, one factor stood out above all others: depth.

Why Sabah Gets Strong Quakes

Sabah’s geological setting is the key to both explaining why earthquakes happen here and why they rarely cause major casualties.

While Peninsular Malaysia sits on the relatively stable Sunda Shelf, Sabah occupies a far more complex geological setting, says Batmanathan.

Sabah lies near what scientists describe as a “triple junction” region – an area influenced by the interaction of three major tectonic plates: the Eurasian Plate, the Indo-Australian Plate and the Philippine Sea Plate.

“This (movement and collision of plates) causes the stress buildup within the earth’s crust. On top of that, we have active fault systems that crisscross the Borneo region, which are reactivated possibly due to present-day boundary forces,” he explains.

In addition to present-day plate interactions, Sabah’s seismic activity is also linked to ancient subduction processes. Subduction occurs when one tectonic plate is forced beneath another, sinking deep into the earth’s mantle.

Even though some of these subduction zones are no longer active at the surface, remnants of the descending slab can continue to generate earthquakes hundreds of kilometres underground.

What are subduction zones?

Imagine the Earth’s surface as a giant puzzle made of slow-moving pieces called tectonic plates.

A subduction zone happens when two of these massive pieces crash into each other, and because one (usually the ocean floor) is heavier and denser than the other (the land), it acts like a "sinker" and dives beneath the lighter plate.

As this heavy plate sinks deep into the Earth's hot interior, it creates a deep-sea trench, gets recycled into molten rock, and builds up immense pressure that eventually escapes as powerful earthquakes or fuels the rise of volcanoes on the surface.

Feb 23’s deep-focus earthquake is likely associated with such ancient tectonic processes rather than surface-level fault movement.

‍Some studies have also explored how gravitational collapse and uplift linked to Mount Kinabalu may influence regional stress patterns, although this remains an area of ongoing research.

Should Malaysians Be Concerned?

Deep-focus earthquakes like the Feb 23 event are generally less likely to cause catastrophic surface damage compared to shallow earthquakes of similar magnitude.

However, Sabah remains one of Malaysia’s more seismically active regions.

The key takeaway is not that Malaysia is immune to earthquake risk, but that not all strong earthquakes carry the same level of threat.

Understanding the science behind these events can help the public interpret magnitude numbers more accurately.Ultimately, staying informed transforms uncertainty into preparedness.

– BERNAMA