Experts comment on an earthquake off Sumatra in Indonesia which has affected the region around the city of Padang.

For comments on the Samoan earthquake/tsunami, click here.

A diagram is available which shows the difference between the 2004 Sumatran earthquake which led to the giant tsunami and the latest Sumatran earthquake which did not. The diagram was prepared by Environmental Systems and Services. Please credit www.esands.com if you use the diagram.

The US Geological Survey have produced a summary poster of this event.

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Kevin McCue is President of the Australian Earthquake Engineering Society (Canberra based).

“There is no causal link that we know of between this morning’s Sumatran earthquake and yesterdays Samoan earthquake although both are on (different) boundaries of the Australian Plate. However, there is a strong link between today’s Sumatran earthquake and the December 2004 great earthquake on an adjacent segment of the same plate boundary. It is certainly within a fault length of the 2004 earthquake, the distance within which stresses have been affected by reworking of the faulted boundary.

Earthquakes aren’t all the same. Sumatra has earthquakes with different characteristics resulting in different effects. This morning’s earthquake initiated about 80 km below the surface so the faulting could have extended to the surface but by then the displacement on the outer ends of the fault rupture at the surface would have been so small that a damaging tsunami would not have been generated.

The mechanism of this and the 2004 great earthquake were also different, this morning’s event was a thrust type earthquake, one side of the fault thrust up and over the other on an inclined surface. The 2004 event was a tsunamigenic earthquake with vertical and horizontal nodal planes. The Sumatran Fault which is mostly onshore is another type, a strike-slip fault where one side moves horizontally relative to the opposite side. Such earthquakes, even great earthquakes under the sea, do not generate damaging tsunamis because there is no vertical displacement of the seafloor.

The Samoan earthquake was caused by normal faulting, one block sliding under gravity down the inclined side of the other block. This occurred at the collision zone between the Pacific and Australian Plates, the Pacific Plate forced to bend under the Australian Plate. The bending caused brittle failure in the 10km thick oceanic crust of the Pacific Plate along a 300km long break. This is an unusually long break and resulting earthquake for a normal fault. The tsunami resulted from the disruption of the seafloor.

The Samoan earthquake was far enough out not to cause damage onshore but close enough to be strongly felt. Samoans have now been made aware of the disastrous possible consequences of a large local earthquake as were Sumatrans in 2004 and north coast Papuan New Guineans in 1997. How long before we can get this simple message to all inhabitants of and tourists to the SW Pacific islands and SE Asia? If you are near the sea and feel a large earthquake then immediately make for a spot at least 10m above the high water mark and wait there for several hours. The earthquake shaking is the best tsunami warming you will get, no good waiting for the phone to ring or a warning to be issued on the radio - too late. If a great undersea earthquake occurs in Alaska, South America or Japan then the latter will provide a useful warning as the shaking will not be felt in PNG, Samoa or Indonesia and the tsunami may take many hours to cross the ocean, even at 700km/hr.”

Associate Professor Dale Dominey-Howes is Co-Director of the Australian Tsunami Research Centre and Natural Hazards Research Laboratory at the University of New South Wales (www.nhrl.unsw.edu.au)

On the Sumatran and Samoan earthquakes:

“These two events are not ‘physically’ connected - not cause and effect. They are just coincidentially close in time so this looks strange.”

UK Science Media Centre Round-up

Dr David Rothery, planetary scientist at the Open University, UK 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.”