SINGAPORE: A moderate-sized earthquake of magnitude 5.5 shook the southern coast of Bali on Mar 22.
Naturally, it caused consternation to those who felt it, but did very little damage. By now, it is yesterday’s news. But there is a very important lesson to be learnt from this quake.
Bali does not often have earthquakes, and so it remains unknown to many that lurking quietly beneath this tourist island of Indonesia is a slumbering tectonic dragon.
This dragon comes in the form of a subducting slab – a gigantic slab of oceanic lithosphere (also known as the Australian plate) that slopes northward beneath Java.
A cross-section of the subduction zone underneath Java. (Source: Earth Observatory of Singapore/Yves Rene Descatoire & Jamie McCaughey)
Occasionally, this subduction zone produces earthquakes, like the magnitude 5.5 quake that struck Bali. As the plate forces its way down into the mantle of the earth, it deforms, and sometimes that deformation leads to a small, sudden snap within the plate.
And that is what happened on Mar 22.
In fact, thanks to seismology, we can determine the exact location of the snap that resulted in the quake.
Initial reports by the United States Geological Survey, based on analysis of seismogrammes from around the world, stated that it occurred about 118 kilometres beneath southern Bali.
Seismologists at the Earth Observatory of Singapore (EOS) later refined that measurement to a more accurate depth of about 106 kilometres.
That is approximately how far beneath the resort communities of southern Bali the slab of oceanic crust coming off of the Australian plate is. Just for reference, that’s about 20 km deeper than the north-to-south span of the island.
It would be fair to say that all this is of little more than academic interest. Except for the fact that other parts of the descending slab can produce giant, deadly earthquakes and resulting tsunamis.
Shallower parts of the descending slab, south of Bali, are closer to the surface of the Earth and to the sea floor. Those parts of the subducting Australian plate are so cold that the overlying plate sticks to it as it slowly descends.
No one yet knows how frequently, but perhaps every few hundred years, the friction that holds the two cold plates together yields and a great slippage occurs.
That sudden slip beneath the seafloor creates a great earthquake and tsunami.
In the entirety of recorded history, such a great earthquake and tsunami has never struck Bali.
One might thus be tempted to breathe a sigh of relief. But that was also the prevailing sentiment in Aceh, in the years and decades prior to the Boxing Day earthquake and tsunami that killed a nearly quarter million people there in 2004.
Since that earthquake and extraordinarily devastating tsunami, scientists from EOS have learned that its predecessors were more than 500 years in the past and not noted in the ancient chronicles of the traders plying the Indian Ocean at the time.
The orange star indicates the location of the epicentre of the Bali quake that took place on Mar 22. Since the 1960s, there has been very little earthquake activity, represented by dots, near Bali island. (Source: Earth Observatory of Singapore/Wang Yu)
Although no one has yet proven it, we think that the great subduction zone offshore and under southern Java and Bali could well generate a magnitude 8.5 to 9.0 quake, or perhaps a series of them.
So the recent quake that mildly shook Bali is an important reminder to us that this “sleeping tectonic dragon” lying beneath will most likely some day produce one or several large earthquakes and tsunamis that could affect the southern coasts of Java and Bali.
Professor Kerry Sieh is Director of the Earth Observatory of Singapore.