Quake, tsunami among worst ever

The geologic forces that caused the massive earthquake and tsunami in Southeast Asia late last month are so enormous, even an expert like Gomaa Omar says he has a hard time putting them into words.

“These forces are huge,” says Omar, a graduate group chairman in Penn’s Department of Earth and Environmental Science. “They are beyond human comprehension.”

Omar tries anyway: Put into broad context, Omar says, the amount of energy released when the earthquake erupted 30 kilometers below the surface of the Indian Ocean was probably equivalent to the amount of energy that would be released by a Category 5 hurricane—if that hurricane sustained full power over a period of 70 days.

The earthquake, by comparison, released the same massive amount of energy in just a split second—and the suddenness of that release, Omar says, is a big reason why the event has caused so much damage, over such a large area, and also produced a horrific and deadly tsunami.

“When it comes down to it, beside the human tragedy, the bottom line is the geology—the earthquake,” Omar says. “And for those of us in tectonics, this was not a surprise.”

The human tragedy that followed the Indonesian quake and killer tsunami that followed is now well-documented—roughly 226,000 people are confirmed dead and more than 1.2 million have been displaced in South Asia and East Africa. It was among the most destructive natural disasters in recorded history.

The quake, measured at 9.0 on the Richter Scale, was the fourth-largest earthquake to strike the earth since 1900 and is the largest since the massive 1964 Prince William Sound earthquake in Alaska. But the 9.0 rating alone does not tell the full story of the quake’s magnitude—the quake was huge, even in geologic terms.

It also was not altogether unexpected, Omar says.

That’s because the Indonesian quake was centered about 155 miles southeast of Indonesia, smack in the middle of one of the most active earthquake zones on earth. The constant earthquake activity in the area can be blamed on the ongoing battle between two plates of the earth’s crust, the Indian Plate and the Burma Microplate.

These two plates have been slowly butting heads, and causing massive disturbances, for ages. Through a process called subduction, the Indian Plate is literally being pushed underneath the Burma Plate, at a rate of about 2 inches per year.

But while tremendous forces are constantly compelling the Indian Plate to bury itself beneath its neighbor, large-scale movement happens only occasionally.

In fact, the plates can remain “locked” along the fault line for years, even as the gargantuan forces pushing on them build and build. “This is a recipe for disaster,” Omar says.

When the stalemate is finally broken, Omar says release is sudden—and the force is spectacular. “This can be a fraction of a second,” Omar says. “We’re not talking hours. We’re not talking minutes.”

In the case of the Indonesian quake, the forces moved the plates along a fault line that stretched 745 miles. The earth along that fault line was moved by an average of 15 meters.

Even for Omar, who has studied geologic events for years, the numbers are astounding. “That’s just unheard of,” Omar says of the quake’s scope. “That’s tremendous.”

The quake was so enormous, in fact, it also pushed up the seafloor by several meters, displacing an untold volume of seawater and producing the tsunami that has literally reshaped the geography of the Indian Ocean.

“We’re not talking about a spoonful of water. We’re not talking about a swimming pool,” Omar says.“We’re talking about a huge amount of water, and it had so much energy behind it, it just spread in all directions.”

In open ocean, the tsunami waves could have traveled at speeds up to 500 miles per hour, although they would have likely gone unnoticed because the massive depth of the ocean was able to absorb the disturbance. Only as the wave approached land, and the seafloor grew shallower, did the waves begin to rise.

And unlike traditional waves that hit shore and then fall back toward the ocean, the tsunami waves—carrying all the energy released when the Indian and Burma plates struck—continued to push far inland. With such tremendous force pushing them along, they could only continue moving forward—hence the destruction throughout Southeast Asia. “Not all earthquakes cause tsunamis,” Omar says. “But unfortunately, when they happen, they really happen.”

To find out what Penn is doing to help tsunami victims, go to www.upenn.edu/pennnews/tsunami.php.

Originally published on January 27, 2005