A geologist's perspective

A geologist's perspective
The view from the peak of Mt. Kota Kinabalu.

Earthquake tremors are the result of a rapid release of energy where seismic waves move in all directions from the source, known as a focus.

A point on the earth's surface above the focus is called the earthquake centre. For example, the centre of the Ranau earthquake is near the peak of Mount Kinabalu. This means the focus of the quake was underneath its centre; in this case, it was 10km below the centre. This is what caused the massive landslides.

Tremors usually occur along faults, or longitudinal cracks in the earth that have bent and snapped back in what is called an elastic rebound. Imagine taking a plastic ruler and bending it. After a certain point it will break and release all the energy put into it from our effort to bend it.

When an earthquake occurs, the rocks in the earth are behaving in a similar fashion. Then come the aftershocks, which are smaller tremors that take place while the earth's crust settles.

So far, 33 aftershocks have been recorded by the Malaysian Meteorological Department of Friday's earthquake.

Seismic waves refer to the energy released after an earthquake during the elastic rebound. There are three types of seismic waves - primary, secondary shear and surface waves.

Primary waves are the fastest and move forward and backward like a lurching car. These can make people feel dizzy, as surface waves also move from left to right. All three types of waves can be felt almost at once if one is close to the centre of an earthquake.

About 80 per cent of earthquakes occur in the Pacific Rim due to a collision of tectonic plates in the earth's crust. The rest occur outside those regions in areas with active fault lines.

The Ranau earthquake was caused by the presence of active fault lines. This is because Sabah is still receiving compression forces from the interaction of three main tectonic plates. Sabah is located on the south- eastern Eurasian Plate, which is bordered by the Philippine Plate and the Pacific Plate.

The Philippine and Pacific plates move westward at a rate of about 10cm a year, colliding with the Eurasian Plate.

Additionally, the southern part of the Australian Plate is moving north at a rate of 7cm per year, and this plate boundary is the most active and unstable.

Although Sabah is 1,000km away from the collision of the plates, it still receives more compression force than Sarawak and Peninsular Malaysia as it is relatively close to the Philippines and Sulawesi in Indonesia.

Also, what occurred in Ranau was not linked to volcanism, which is possible only in Tawau and Semporna, where an explosion occurred several hundred years ago.

Volcano craters are still visible in Tawau. Mount Kinabalu will not erupt, as the igneous intrusion that formed the mountain is caused by the compression of the three plates mentioned previously.

This can also explain why the water at the Poring Hot Springs turned murky for a few hours, as the earthquake appears to have disrupted a clay deposit that interrupts the fault gap that heats up the rainwater which enters the earth. The water flowed more smoothly and had access to more heat.

Strong tremors in Ranau will continue to occur whenever the stored energy exceeds the earth's ability to store it.

This happens on a regular basis. In the case of Ranau, it happens at an approximate rate of every 25 years, with the first recorded in 1966 at a magnitude of 5.3 on the Richter scale. This was followed by a 5.2-magnitude earthquake in 1991.


The writer is a geologist with Universiti Malaysia Sabah.


This article was first published on June 10, 2015.
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