Talk Seeks Geohazard Awareness

Special Paper Set in Perth

Even the bravest of souls are known to fear Mother Nature’s wrath, which can be mighty scary, unpredictable and often fatal.

In the case of potentially devastating events like slow-moving hurricanes, there’s time to prepare to ride it out and take your chances -- or else “get out of Dodge.”

At other times, the forces of nature come together quickly and quietly to wreak catastrophic damage in seemingly no time at all.

Such was the case on Dec. 26, 2004, when an earthquake registering 9.3 on the Richter scale occurred far beneath the Indian Ocean close to the island of Simeuleu off the northern tip of Sumatra. The earthquake was one of the largest ever recorded, according to Ian Norton, tectonic plate modeling specialist at ExxonMobil.

In typical fashion, the resulting tsunamis initially caused water to suddenly recede from land -- a brief, almost momentary warning of what would ensue that was recognized by a mere handful of the populace at best. As innocent locals and tourists alike reveled in the novelty of grabbing exposed fish, the seaward water movement suddenly reversed, and the ocean came rushing over the land with deadly ferocity.

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Even the bravest of souls are known to fear Mother Nature’s wrath, which can be mighty scary, unpredictable and often fatal.

In the case of potentially devastating events like slow-moving hurricanes, there’s time to prepare to ride it out and take your chances -- or else “get out of Dodge.”

At other times, the forces of nature come together quickly and quietly to wreak catastrophic damage in seemingly no time at all.

Such was the case on Dec. 26, 2004, when an earthquake registering 9.3 on the Richter scale occurred far beneath the Indian Ocean close to the island of Simeuleu off the northern tip of Sumatra. The earthquake was one of the largest ever recorded, according to Ian Norton, tectonic plate modeling specialist at ExxonMobil.

In typical fashion, the resulting tsunamis initially caused water to suddenly recede from land -- a brief, almost momentary warning of what would ensue that was recognized by a mere handful of the populace at best. As innocent locals and tourists alike reveled in the novelty of grabbing exposed fish, the seaward water movement suddenly reversed, and the ocean came rushing over the land with deadly ferocity.

The giant earthquake was sited in the collision zone where Indian oceanic crust is subducting, or sliding beneath Asia. In all subduction zones, movement is accomplished by stress that builds up along the dipping contact between the plates, Norton said, and sudden release of stress when the rocks break, triggering an earthquake.

He noted this is graphically illustrated in the 2004 event by the sudden emergence of some old rice paddies from out of the sea on the Simeuleu coast. They are probably from the 19th century or earlier and subsided until they were flooded and re-emerged after the quake.

‘Enormous’ Scope

There are several unique features about this Boxing Day quake.

For starters, there’s the enormous size of the rupture zone

Seismologists have developed a scale other than Richter that is more useful for large events, which is called the seismic moment scale, Norton said. It’s directly related to the rupture and the amount of slip on the fault.

“One way to understand the size of this quake is to compare with other earthquakes,” Norton said. “This was such a large event that this seismic moment actually equals the sum of all seismic moments of all the earth’s earthquakes for the last 11 years.

“Using calculations based on this scale, the rupture area of the Boxing Day event can be estimated as 500 by 250 km with 10 meters of displacement (slip),” he noted. “It was this huge displacement over such a large area that produced the destructive tsunamis.”

Another exceptional feature of this earthquake is its unique tectonic position.

The initial rupture close to the northern end of Simeuleu propagated quickly northward, traversing nearly 1,500 km to the plate boundary offshore Myanmar. This represents rupture of essentially the entire boundary of the Burma plate, which is a small plate allowing plate motion between India and Asia. The Burma plate is detached from Asia along the Sagaing fault, which is a north-south strike-slip fault, according to Norton.

“Aftershocks of the main event show faulting within Asia, in the Burma plate and also within the subducting India plate,” Norton said. “Some seismicity indicates the boundary between the Indian and Australian plates may run through this area as well.

“This would mean that the epicenter is at a very unusual location at the junction between four plates,” Norton noted. “This location may have implications for recurrence time of such an event.”

A Valuable Lesson

Hundreds of aftershocks followed the initial catastrophe on Boxing Day. Some of these were from reactivation of the fault that initially ruptured, while others reflect fault activity on neighboring faults that were stirred up by the “big one.” In fact, Norton said the whole subduction zone between Sumatra and Myanmar has been reactivated by this event.

In case you’re wondering about the more-recent Indonesian earthquake of this past spring, which hit 6.3 on the Richter scale, Norton said it had no relation to the big episode in 2004.

It should come as no surprise this expert has a warning for beach lovers everywhere.

“If you’re on a beach anywhere in the world -- who would have expected tsunamis in Somalia? -- and you see the sea recede, don’t wander down and pick up fish. Run like hell to higher ground!”