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Tsunami-Battered Sumatra Ripe for More Disasters

John Roach
for National Geographic News
January 7, 2005
 
The force of the magnitude 9 earthquake that struck northern Sumatra
on December 26, 2004, may have caught much of the world by surprise.
But scientists say the region has a violent geologic past and is ripe
for more cataclysmic earthquakes and volcanic eruptions in the
millennia to come.

The Indonesian island sits in an area of the Indian Ocean where several large chunks of Earth's crust, known as tectonic plates, collide. Tectonic plates can slip past, beneath, and over the top of each other. In the Sumatra region, the Indian and Australian plates are slowly creeping alongside and—in a process called subduction—diving beneath, the Burma plate, part of the larger Eurasian plate.


The plates meet at the Sunda trench, a subduction zone that runs 3,400 miles (5,500 kilometers) from Myanmar (Burma) south past Sumatra and Java and east toward Australia. The trench runs roughly parallel to the west coast of Sumatra about 125 miles (200 kilometers) offshore.

In addition, the Sumatra fault spans the entire length of the island. Akin to California's San Andreas Fault, the Sumatra fault is a strike-slip fault, where two sections of rock pass horizontally by each other in opposite directions, like the pieces of slide rule.

Earthquakes occur along both the Sunda trench and the Sumatra fault.

"So Sumatra gets at least a double [earthquake] whammy, and then there's the volcanoes associated with all subduction zones," said Rob McCaffrey, a geophysicist at the Rensselaer Polytechnic Institute in Troy, New York. McCaffrey has studied the Sumatra tectonic system since the 1980s.

Indeed, McCaffrey and other scientists say Sumatra sits in the midst of one of the world's most geologically active regions. Earthquakes of greater than magnitude 8 struck Sumatra in 1797, 1833, and 1861. Quakes of more than magnitude 7 rocked nearby islands in 1881, 1935, 2000, and 2002.

"But I don't think anyone was expecting this segment of the subduction zone to produce a [magnitude] 9 [earthquake]," McCaffrey said.

Subduction Zone

The region of contact between plates in a subduction zone is called an earthquake fault, or a megathrust. The Sunda trench is considered a megathrust fault.

At the Sunda trench near Sumatra, the Indian and Australian tectonic plates creep north-northeast at 2.4 inches (61 millimeters) each year as they slip past and beneath the Burma plate. But the process is not smooth, explains James Dewey of the U.S. Geological Survey's National Earthquake Information Center in Golden, Colorado.

The plates push against each other, building up pressure "until the stress reaches a critical level, and that's when it starts rupturing," Dewey said. The ruptures, which are felt as earthquakes, do not occur along the entire length of the subduction zone, but rather in patches.

Several research groups have used data from the global seismic network to estimate that the rupture that occurred on December 26 propagated from the epicenter west of the Indonesian province of Aceh to the north along the Sunda trench for about 745 miles (1,200 kilometers). The majority of the rupturing concentrated on the first 250 miles (400 kilometers) of the trench.

Ken Hudnut is a geophysicist with the U.S. Geological Survey's Earthquake Hazards Assessment Project in Pasadena, California. He said great earthquakes along patches of fault in this subduction zone come as no surprise.

"There are many prior great events farther to the southeast," Hudnut said. "[But] this event happened in the northernmost part, which had no known early historic great events of magnitude 8 or greater, and to my knowledge its prehistoric activity is not known."

Kerry Sieh, a geologist at the California Institute of Technology in Pasadena, has led extensive research on the Sumatran tectonic system. In a media statement he noted that, along the Sumatran portion of the trench, major earthquakes, such as the magnitude 8.7 earthquake of 1833, occur about once every two centuries.

The December 26 Aceh earthquake ruptured only the northern portion of the Sumatran section of the megathrust. That makes Sieh worried. It has been more than a century since the last major earthquake along the southern portion of the trench, which is generally more active.

"Other parts within this section of this fault should be considered dangerous over the next few decades," Sieh wrote.

Volcanoes

Where there are subduction zones on Earth, there are also volcanoes. Sumatra is no exception. Several major volcanic eruptions have occurred on the island.

Scientists believe the Toba explosion 75,000 years ago sparked the last ice age. The 1883 eruption of Krakatau (Krakatoa), the volcano and island between Java and Sumatra, killed more than 36,000 people, many from a large tsunami generated by the explosion.

According to Chris Newhall, a volcanologist with the U.S. Geological Survey in Seattle, Washington, much of the magma, or molten rock, formed beneath Sumatra tends to be of a very high silica content. This makes the magma viscous, or thick. The magma is also less dense than the rock it melts from.

"As a result, it tends to rise toward the surface but not easily erupt. It stalls en route and will tend to form large subsurface ponds," he said.

If fresh magma does not erupt immediately but rather pools below the surface, the upper layer of the pool hardens, forming a carapace like a turtle shell, Newhall said. The carapace and viscosity of magma below the shell prevent gasses such as water vapor and carbon dioxide from escaping, allowing the pool to grow more volatile.

Eventually, an intrusion of more magma may trigger an eruption.

Earthquakes can trigger such intrusions by either opening up or squeezing the crust, allowing magma to move. There is no report to date that the recent Sumatra earthquake has triggered any volcanic activity, Newhall said.

"If the next Sumatran eruption is from one of these gas-charged large bodies of magma, it could be big and explosive," Newhall said. "If it's from a volcano that erupts more frequently, then the eruption will probably be small, especially in comparison to the [recent] earthquake and tsunami."

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