INDONESIA: Latest quake may have transferred stress further south, reports Dick Ahlstrom
The risk of another large earthquake off the Indonesian island of Sumatra continues despite the two devastating quakes that have already occurred there. The seismic stresses are likely to have been passed further south along the Sumatran coastline, according to earthquake experts.
There have already been more than two dozen aftershocks since the latest significant tremor that occurred off Sumatra just after 4pm Irish time on Monday. The associated death toll has already passed 1,000.
The very large event, felt as far away as Bangkok and Singapore, caused widespread panic in the region, in particular over fears of a cataclysmic tsunami similar to that triggered by the earthquake off Sumatra on December 26th.
This latest earthquake was not an aftershock from December but was linked to it due to a transfer of stress along a seismic structure known as the Sunda Trench, said Sandy Steacy, a lecturer in the University of Ulster's school of environmental science.
Dr Steacy co-authored a report, with University of Ulster professor John McCloskey, in the journal Nature, published on March 17th, warning that this transfer of stress along the Sunda Trench was likely to lead to yet another large quake. She now believes, however, that this latest event will only have moved the stress, and the possibility of yet another major quake further south.
"We are doing the calculations now," she said yesterday. "Our preliminary results are that [ Monday's] quake would have reduced stress where it occurred but increased stress along the Sunda Trench. Basically it has moved the stress further south."
The earth's apparently solid surface is actually broken up into distinct slabs or "plates" that move across the globe at about the speed that a fingernail grows. Earthquakes and volcanoes occur most frequently at the fault lines where these plates jostle against one another, and the Sunda Trench marks just such a junction.
The rocky crust produces friction that prevents the plates from sliding smoothly past or under one another. After a time the stresses become so large that the plates lurch forward along a fault line in a massive release of energy that sets off the shock waves we know as earthquakes.
The huge Indo-Australian plate that crushes up against the Eurasian plate to form the Himalayan Mountains is also sliding slowly under the much smaller Burma microplate along the Sunda Trench in a process known as subduction.
This structure was responsible for the December quake that passed stress further south along the subduction zone. In turn this stress triggered this latest earthquake and the dozens of aftershocks that followed, according to the scientific adviser to the director of the US Geological Survey, Jim Devine.
"We have some pretty strong indications that the stress adjustments are going to occur along the structure," he said yesterday. This in turn increases the possibility of yet another quake. "I would not be at all surprised if we saw another one along the structure."
Something similar is happening on a major east/west fault line in Turkey, he said. "We have seen this in Turkey along the North Anatolian fault." There have been eight to 10 quakes over the past 75 years, each one occurring adjacent to the previous with the most recent in 1999.
"It has walked down the line. This really worries us because following the last quake the next earthquake will be right at Istanbul," he said.
The hope for Sumatra is that each subsequent quake might be somewhat less severe if indeed a string of tremors does occur along the Trench, something impossible to predict, he said. Monday's event was 15 times weaker than the powerful December quake. "Maybe it is adjusting with less and less destruction as we move down the line."
This latest quake did not trigger a tsunami like the December tremor despite being classed by the US Geological Survey as a "great" quake registering 8.7 on the Richter scale. The December event was known as a "megathrust" quake with a huge piece of seabed shifting forward, displacing billions of tonnes of water to produce a tsunami wave.
This seabed displacement did not seem to occur during Monday's event, Dr Steacy said. "I would guess it just didn't break the [ seabed] surface," she said.