Tsunami-Battered Sumatra Ripe for More Disasters

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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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