What caused the Johor quake and is S'pore safe from such tremors?

An earthquake with a magnitude of 4.1 struck Johor on Aug 24, with tremors felt across several states, including Negeri Sembilan and Melaka.

- Chin Hui Shan

The epicentre was located near Segamat, about 180km from Singapore, said the Malaysian Meteorological Department. No casualties have been reported so far.

The quake was unusual as it did not take place at the boundaries of tectonic plates, where most seismic and volcanic activity usually happens.

Tectonic plates are large sections of rock that make up the Earth's crust and upper mantle. All the world's oceans and lands sit on these plates, which are constantly moving slowly, a few centimetres a year.

Instead, the Johor quake was likely an "intraplate" one that happened within one tectonic plate, said National University of Singapore senior lecturer of geography Muhammad Nawaz.

"The Johor tremor was likely caused by stress release along a minor fault within the Sunda Plate," he said.

The Sunda Plate is a tectonic plate located in South-east Asia. It comprises Cambodia, Malaysia, Singapore, Vietnam, Indonesia and parts of the South China Sea and Andaman Sea.

The Straits Times explores the science of earthquakes and whether Singapore is at risk from intraplate quakes.

WHAT CAUSES EARTHQUAKES?

Large earthquakes often happen along the edges of tectonic plates, said geologist Aron Meltzner, an assistant professor at NTU's Asian School of the Environment. "This is why those regions are so dangerous," he said.

During a process called subduction where one plate moves beneath another, immense pressure builds up and this often leads to powerful earthquakes.

Take, for example, the Pacific Ring of Fire, a horseshoe-shaped series of seismic fault lines around the Pacific Ocean.