On February 23rd, a magnitude 7.1 earthquake occurred beneath Sabah, Malaysia, at 12:57 AM local time (February 22nd UTC). Uncommonly deep, this earthquake originated approximately 620 kilometers below the Earth’s surface, well beyond the typical 30-kilometer range for most seismic events. This unusual depth meant that the earthquake was not likely to generate dangerous shaking and was felt primarily as a minor tremor over a broad area.
The U.S. Geological Survey (USGS) indicated that the maximum intensity of shaking was estimated at level V, which is classified as moderate and typically insufficient to cause damage. Reports noted that residents in areas as distant as Singapore, Thailand, and even parts of the Philippines felt the quake, with some reports coming from as far as 1,500 kilometers away.
Deep-focus earthquakes, like this one, have historically played a key role in advancing our understanding of plate tectonics. These seismic events often occur at depths of 100 to 700 kilometers within subduction zones, where tectonic plates collide. As oceanic crust subducts into the mantle, it carries water and cold rock layers, leading to earthquakes that can trace paths of these interactions. However, the occurrence of earthquakes at such extreme depths, such as this recent event, is rare and not fully understood.
Geological studies show that the upper mantle largely consists of the mineral olivine, which undergoes significant structural changes as it is subjected to increasing pressure and temperature. When olivine reaches depths of about 410 kilometers, it transforms into a denser structure known as wadsleyite, further reverting to ringwoodite at 520 kilometers, and finally becoming perovskite and ferropericlase at around 660 kilometers. The current earthquake appears to have occurred near this maximum depth threshold, where the conditions for traditional brittle deformation are unsuitable, suggesting a different process behind these deep quakes.
Researchers have identified other deep-focus earthquakes in Southeast Asia, often linked to the complex tectonic setting surrounding the region. The subduction zones encircling the area, including the Sunda and Halmahera slabs, have been known to produce earthquakes, but this recent event has taken place in an area previously uncharted for such seismic activity. Seismic tomography, an imaging technique using seismic waves, may assist in mapping these deep structures and studying their properties.
Though deep earthquakes like the one in Sabah can be alarming, they typically do not pose a societal threat. Most earthquakes in the region occur at shallower depths, where they have historically caused more damage, such as the deadly M6.0 earthquake in Sabah in 2015. The recent deep quake serves as an intriguing reminder of the dynamic processes occurring within the Earth’s interior, highlighting the significance of ongoing research in understanding our planet’s geology and earthquake potential.