Issue 147, June 2015
Shedding new light on recent Sabah earthquake
Seismological recordings indicate earthquake was caused by sudden slippage along 10km fault south of Mount Kinabalu, say EOS scientists

Destructive earthquakes in easternmost Malaysia have been very uncommon historically, note scientists from the Earth Observatory of Singapore (EOS) at NTU, Singapore.

In the past century, Sabah has experienced only three earthquakes greater than Friday’s magnitude 6.0. In 1923 and 1976, magnitude 6.3 and 6.2 earthquakes occurred about 100km to the southeast. In 1951, a magnitude 6.1 earthquake occurred about 50km to the north.

Professor Kerry Sieh, Director of EOS, said: "All of us at the Earth Observatory and at NTU extend our deepest condolences to the families and friends of the victims of this unusual natural disaster.

"Unlike Sumatra, Nepal, Taiwan and Japan, which straddle fast-moving tectonic plate boundaries, Sabah is not a place well-known for destructive earthquakes, so Friday’s destructive earthquake came as quite a surprise,” added Prof Sieh, a renowned geologist.

“But this event has drawn our attention to the fact that they do occur there. Seismic recordings from around the world, measurements of ground deformation from orbiting satellites, and analysis of Sabah’s mountainous topography are now helping us to understand what happened and why."

A quick initial exploration of the records of past earthquakes by EOS scientists indicates that within a 300-km radius of the recent epicentre, there have been just four earthquakes equal to or greater than magnitude 6 in the past century, and none larger than 6.3.  In comparison, Indonesia’s Sumatra has experienced in the past 15 years over a hundred earthquakes with magnitudes greater than or equal to 6, with 14 of these having magnitudes between 7 and 9.2.

Seismological recordings indicate that last week’s earthquake was caused by sudden slippage along a fault about 10km in size and centred a little over 10km beneath the Earth’s surface, south of Mount Kinabalu.

During the rupture of the fault, the Earth’s crust around the fault stretched nearly a metre, northwest to southeast. The EOS scientists anticipate that analysis of satellite observations will show that the block northwest of this fault moved down relative to the block southeast of the fault, probably by about 20 cm. These numbers are very small compared to the several metres of shortening and metres of uplift during Nepal’s much larger, magnitude 7.8 earthquake in April.

A cursory inspection of the mountainous topography of this part of Sabah suggests that the fault that ruptured during the earthquake is part of a system of faults that courses nearly 200 km from northeast to southwest across this part of Sabah. The earthquake of 1951 may have occurred on the northern portion of this system of faults.

© Nanyang Technological University Singapore