September 30, 2009

expert reaction to events in Sumatra and Samoa

Within a day of each other, a large earthquake struck the Indonesian island of Sumatra while a tsunami triggered another large earthquake which struck the Pacific islands of Samoa and Tonga. The two events are unrelated.

On the Sumatran earthquake:

Dr David Rothery, planetary scientist at the Open University, said:

“The earthquake offshore of Sumatra was powerful but fortunately did not trigger a damaging tsunami, perhaps because it happened about 50 miles (80 km) down. The nearby rupture that caused the Indian Ocean tsunami on 26 December 2004 was much shallower. Both were caused by the same process – the floor of the Indian Ocean is being dragged down northeastwards below Indonesia. The motion is not smooth, but goes in fits and starts. When a sudden jerk happens, that’s what causes an earthquake, and sometimes a tsunami if the epicentre is below the sea but not too deep.

“Unfortunately however the ground-shaking in Padang was enough to collapse the roofs of many buildings. Probably if buildings, notably hospitals, had been better constructed fewer people would have died. Often it is schools – built at public expense but on the cheap – that fall down. Maybe that is a story that will emerge from Padang too, though because the quake was after 5pm local time maybe the schools were empty by then.”

On the Samoan tsunami:

Prof Bill McGuire, Professor of Geophysical & Climate Hazards and Director of the Aon Benfield UCL Hazard Research Centre, said:

“This most recent tsunami disaster shows that there are lessons we yet need to learn from 2004. Most critically, populations living close to faults capable of producing earthquakes that trigger tsunamis must be taught to self-evacuate when the ground shakes or the sea recedes. Waiting for a warning from a central monitoring station could mean the difference between life and death.”

On both events and their relatedness:

Dr David Booth, Seismologist at the British Geological Survey, said:

“The occurrence of two large earthquakes in Sumatra and the Pacific on the same day, and to a lesser extent, a magnitude 5.9 in Bolivia and a later M6.6 in Sumatra, has tested earthquake and tsunami preparedness and prompted the question: could the largest earthquake have triggered the others?

“The M8.0 Samoa earthquake was large, offshore and caused a depression of the seabed which caused a tsunami wave to form. The tsunami wave built up in size as it came ashore, generating a wall of water up to 20feet high travelling faster than a man can run. It is not hard to see how the momentum of the water and the debris carried with it causes complete devastation and much loss of life. It is possible to warn of an impending tsunami as it travels much more slowly over the Earth than the seismic waves, and the Pacific Tsunami Warning centre was quick in issuing a warning. A sudden low tide and the shaking caused by the earthquake are also warnings to those on the shoreline. Fortunately most of the inhabitants received warning in these different ways, and reacted to it immediately. However advance warning of the tsunami could only be a few tens of minutes at most, due to the closeness of the earthquake, and not all were able to reach safety in time.

“The M7.6 Sumatra earthquake occurred on the same fault system which caused the M9.2 2004 Boxing Day earthquake and tsunami, and being smaller, closer to land and much deeper did not cause a significant tsunami. Since it was close to Padang, a major city, much destruction and loss of life has been caused. A much smaller M5.9 also occurred in Bolivia on the same day, and now a M6.6 has occurred close to the M7.6 Sumatra, one day later.

“Could the large Samoa earthquake have triggered the others? At present, there is little evidence to suggest that, certainly regarding the Bolivia earthquake. Firstly the earthquakes occur on completely different fault systems, so that stress is not transmitted along the same fault segment, as is happening after the 2004 Sumatra earthquake. The new M7.6 Sumatra earthquake is more likely to be linked to the 2004 earthquake than the Samoa earthquake. The nearby and later M6.6 earthquake is not exactly on that fault line but it may have been triggered by the M7.6 by transfer of stress across the fault system.

“Some scientists believe that seismic waves from distant very large earthquakes can weaken the fault in the region of an impending earthquake. This may be possible though fluid movement near a rupture point, but the evidence so far is very small. The events happening on the same day could easily be a coincidence – 1-2 M8+, 18 M7+, 150 M6+ and 800 M5+ earthquakes happen in a year on average and one of each could happen on the same day.

“In any case, proof of a relationship between the earthquakes would not bring us much closer to determining the exact time of an impending large earthquake. In the short term it is better to concentrate on improving buildings in earthquake prone areas and the reaction times to a tsunami warning.”