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New light on surprise Sabah quakes
Published on: Sunday, March 11, 2018

QUAKE phobia may become a new psychological stress as Sabah folks now wish that the June 5, 2015 violent quake is just a passing strike because it can be so deadly.

In fact, a largely quake-free Sabah had been a historical fact.

Destructive earthquakes in East Malaysia have been very uncommon, according to a study by scientists from the Earth Observatory of Singapore (EOS) at the National Technological University, prompted by the 5.9 magnitude quake that killed 18 and injured 130 in the early morning of that fateful day.

In a June 2015 (Issue 147) article entitled "Shedding new light on recent Sabah earthquake" published in a monthly e-zine for alumni and friends, the surprised scientists noted this:

"In the past century, Sabah has experienced only three earthquakes greater than the June 5, 2015 magnitude 5.9. In 1923, and 1976, magnitude 6.3 and 6.2 earthquakes occurred about 100km to the southeast (that is, in the Lahad Datu area). In 1951, a magnitude 6.1 earthquake about 50km to the north (that is, the Kudat area)."

Sudden slippage 10km deep
Professor Kerry Sieh, Director of EOS, who led the study, described the June 5 killer quake as an "unusual natural disaster".

Seismological recordings indicate the June 5 earthquake was caused by sudden slippage along a fault 10km in size and centred a little over 10km beneath the surface (hypocentre or point of initial rupture underground) south of Mount Kinabalu, the article reported.

'Kinabalu heavily faulted' – American geologists
Now consider the following expert insight:
In another report entitled "Age and emplacement of Mount Kinabalu" by a team of American geologists, David A Swauger, Charles S Hutchinson, Steven C Bergman and John E Graves published in the Geological Society Malaysia Bulletin 44, July 2000, it says this:

"The vicinity of Mt Kinabalu is heavily faulted, and broken beds are common."

Factoring that in, and a 8 March 2018 5.2 magnitude tremor which struck just three years after the violent 2015 quake, Sieh and his team might be wondering whether that "unusual" June 5 disaster may soon become no more a "surprise", after another but much weaker 5.2 magnitude tremor on March 8 that was felt in many places across Sabah.

"Unlike Sumatra, Nepal, Taiwan and Japan, which straddle fast-moving tectonic plate boundaries, Sabah is not a place well-known for destructive earthquakes, so the destructive June 5 (2015) earthquake came as a surprise," Prof Sieh, a renowned geologist, confessed.

June 5 changed outlook
So the long-held opinion of a quake-free Sunda plate bound Borneo and Sabah had changed.

"These events have drawn our attention to the fact that they (quake) do occur there (Sabah)," Prof Sieh conceded.

"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," he said.

A quick initial exploration of the records of past earthquakes by EOS scientists indicates that within 300km radius of the recent epicentre (point of ground level directly above point of rupture underground), there have been just four earthquakes equal to or greater than magnitude 6 in the past century, and none larger than 6.3, Prof Sieh pointed out.

In comparison, Indonesia's Sumatra has experienced in the past 18 years over a hundred earthquakes with magnitudes greater than or equal to six, with 14 of these having magnitudes between seven and 9.2, he added.

What happened on June 5
So, what happened on June 5, 2015? The answer is a sudden rupture of underground rocks 10km deep occurred along a fault about 10km in size 14km west north west of Ranau.

During the rupture of the fault, earth's crust around the fault stretched nearly a metre, northwest to southeast.

Through analysis of satellite observation, the EOS scientists said the block northwest of this fault moved down relative to the block southeast of the fault, by about 20cm.

These numbers, according to the report, are actually very small compared to the several metres of uplift, during Napal's much larger, magnitude 7.8 earthquake on April 2015 which killed 9,000 people and injured 22,000.

But because the June 5 violent quake killed close to 10 Singaporeans, mainly pupils of Katong Primary School, which prompted top notch EOS interest to take a special look at Mt Kinabalu's geological behaviour, we get this interesting insight, however brief.

Big rupture part of 200km fault system
The article concluded in this manner:
"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 200km from northeast to southwest across this part of Sabah," it noted.

"The earthquake of 1951 (which hit Kudat area) may have occurred on the northern portion of this system of faults."

Is Sabah destined for more shake-ups?
So, given the verdict of American geologists who concluded that "the vicinity of Mount Kinabalu is heavily faulted, and broken beds are common", is Sabah destined to see more sudden ruptures from these faults that will shake up the state with rising frequency?

May or may not, because not every crack in the ground is a fault, some geologists assure.

What defines a fault is the movement of the rock on either side of a fault, they say.

If a sudden movement (in the form of a slippage or rupture) occurs because a massive rock locked along an active fault perhaps for hundreds of years suddenly fractures, the abrupt release of a gigantum amount of what they call 'stored elastic strain energy' shakes up the surface of the earth with violent seismic waves.

That's exactly what happened on June 5, 2015, according to the OES scientists.

Given that "the vicinity of Mount Kinabalu is heavily faulted", Sabah can only hope that either sides of these heavily faulted surfaces are mainly clean fractures that can move past each other relatively smoothly and not riddled with locked rocks so that that are waiting to burst into big ruptures.

But rough bumps tend to mark most fault surfaces which make them prone to what geologists call 'stick-slip' behaviour and therefore unleash destructive quakes! - Kan Yaw Chong

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