A MINOR 3.5-magnitude earthquake that occurred off the coast of Batu Pahat, Johor, on Saturday is likely part of a broader tectonic adjustment process within the Earth's crust, according to a senior geologist
Dr Norasiah Sulaiman, Senior Lecturer in Structural Geology and Tectonics at Universiti Kebangsaan Malaysia (UKM), explained that Peninsular Malaysia sits atop the Sunda Plate, which is considered geologically stable and far removed from any major plate boundaries.
“As such, most earthquakes in this region are ‘intraplate’ in nature – caused by regional tectonic stress, particularly from interactions in the Sumatra-Andaman region, which is then transferred into the crust of the Peninsula, reactivating ancient fault lines,” she told Harian Metro.
Referring to the recent seismic activity in Johor, Dr Norasiah said the ongoing series of tremors observed since August is best described as an “earthquake swarm”.
“The strongest event recorded thus far was a magnitude 4.1 in Segamat on 24 August 2025. However, it still falls within the weak to moderate category and is not significant enough to be considered a classical mainshock,” she said.
Commenting specifically on the Batu Pahat quake, Dr Norasiah said the tremor occurred approximately 25 kilometres offshore but remains within the region's intraplate seismic zone.
“This situation indicates that the tremors in Johor are part of a tectonic adjustment process, where the Earth’s crust responds to ongoing regional stress,” she explained.
“This process releases energy progressively along ancient fault zones, producing the series of minor earthquakes we’ve been experiencing since August.”
She noted that Johor is underlain by several ancient faults, including the northwest-southeast-oriented Mersing-Endau Fault Zone and a near north-south trending lineament identified through morphotectonic mapping.
“These systems are typical of fault zones with multiphase histories and segmented geometries,” she said, adding that both systems could be reactivated locally under modern tectonic stress conditions.
“In the case of Batu Pahat, the earthquake’s epicentre was located offshore, away from the main fault traces on land, but still under the influence of intraplate faults.
“However, the ‘active’ status of any major fault must be verified through field evidence and instrumental data. Therefore, further studies are essential to confirm the true potential of these faults in Johor.”
Dr Norasiah added that although seismic risk in Johor remains low due to the relatively small magnitudes, recurring ground vibrations may pose long-term concerns for older buildings and coastal infrastructure. - September 28, 2025
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