Earthquake Mudslides Emit Greenhouse Gas

Mudslides that followed the May 2008 Wenchuan, China earthquake, ranked by the US Geological Survey as the 11th deadliest earthquake ever recorded, may cause a carbon-dioxide release in upcoming decades equivalent to 2 percent of current annual global carbon emissions from fossil fuel combustion, a new study shows.

Mudslides wipe away plants and topsoil, depleting terrain of nutrients for plant regrowth and burying swaths of vegetation. Buried vegetable matter decomposes and releases carbon dioxide and other gases to the atmosphere.

The expected carbon dioxide release from the mudslides following the Wenchuan earthquake is similar to that caused by Hurricane Katrina's plant damage, report Diandong Ren, of the University of Texas at Austin, and his colleagues, who used a computer model to predict the ecosystem impacts of the mudslides.

What's more, the vegetation destruction will lead to a loss of nitrogen from the quake-devastated region's ecosystem twice as large as the loss of that nutrient from California ecosystems because of the October 2007 wildfires there, Ren says. And, as the biomass buried by the China quake rots, 14 percent of the nitrogen will be spewed into the atmosphere as nitrous oxide, a pollutant typically released from agricultural operations, automobiles, and other sources.

The team will publish its findings in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

Although landscapes devastated by the Chinese earthquake may re-green soon, the recovery will be cosmetic, says Ren. "From above, the area will look green in a few years, because grass grows back quickly, but the soil nutrients recover very slowly, and other kinds of plants won't grow," he says.

The magnitude-7.9 Wenchuan quake was followed by many aftershocks in the Sichuan Basin, an area that, because of its geological features – deep valleys enclosed by high mountains with steep slopes – is already prone to landslides. May is also the rainy season in Sichuan, and the combination of aftershocks and major precipitation events in the days following the earthquake caused severe mudslides. The avalanches killed thousands, destroyed roads and blocked rivers and access to relief, and shredded water and power stations, among other facilities. To predict ecosystem impacts of the mudslides, Ren and his collaborators applied a comprehensive computer model of landslides that incorporates several physical parameters, such as soil mechanics, root mechanical reinforcement (the root's grip of the dirt, which mitigates erosion), and precipitation.

Ren's model also shows that the primary mudslides following the earthquake removed large areas of nutrient-rich topsoil, leaving behind deep scars in the land that will take decades to recover, preventing the re-growth of vegetation.

The researchers write in their paper that, although being able to predict the location and timing of a mudslide is essential to mitigate its impacts, current mudslide models are not accurate enough.

"Previous approaches, which are mainly based on statistical approaches and empirical measures, have no predictive ability of where mudslides are going to happen," Ren says. His model, he claims, could be applied to forecast under what circumstances a landslide would occur at a specific location. He points out this would be particularly useful for places such as Southern California, where global warming predictions call for an increase in the frequency of these events.
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The Next Great Earthquake

The 2004 Sumatra-Andaman earthquake and resulting tsunami are now infamous for the damage they caused, but at the time many scientists believed this area was unlikely tocreate a quake of such magnitude.

In the March 23 issue of the journal Science, a geophysicist from Rensselaer Polytechnic Institute urges the public and policy makers to consider all subduction-type tectonic boundaries to be "locked, loaded, and dangerous."

"Seismologists have long tried to determine which subduction boundaries are more likely than others to break," says Robert McCaffrey, professor ofearth and environmental sciences at Rensselaer. "Yet, the great earthquake of 2004 ruptured a segment that was thought to be among the least likely to go."

On Dec. 26, 2004, the earth beneath the Indian Ocean buckled and ruptured, unleashing one of the largest earthquakes in recorded history. Shockwaves from the magnitude 9.2 (M9) quake created a wall of rushing water that devastated communities up to 1,000 miles away.

M9 earthquakes typically occur at a specific type of tectonic boundary called a subduction zone, where one plate is gently slipping underneath another plate, which causes friction, cracking, and lifting of the plates. An M9 earthquake can be created by only 20 meters of slip between two converging plates -- less then the length of an 18-wheeler truck -- but its effects can be global in their impact.

Slips of this length only occur every 200 to 1,000 years or more at aparticular boundary, leaving no reliable historic records to track their frequency, McCaffrey notes. Complete records are only available going back 100 years. Scientists had widely accepted that the age and speed of the subducting plate is important in creating M9 earthquakes, based primarily on support from this 100-year historical record.

But this narrow understanding put the Sumatran subduction zone in avery low risk category. McCaffrey suggests that such limited records are incapable of mapping a trend in geological events that could be several centuries or more apart.

Geologists also focused on the temperature of subduction zones, indicating that temperature at the plate convergence region plays a strong role in the strength of a resulting earthquake. These thermal considerations place the Andaman subduction zone in the high-magnitude class, but one pitfall with this type of classification is that it characterizes some subduction zones as being incapable of producing an M9.

"[The day of the quake], Earth gave us a stark reminder of the important difference between improbability and impossibility," McCaffrey says. "Our understanding of where and when the next great earthquake will happen is in its infancy at best. We have not had enough time to decipher M9 behavior."

In creating new public policy, McCaffrey urges officials to consider all subduction zones as lethal. "Several are near densely populated landareas, and the potential impacts of shaking and tsunamis cannot be overstated," he says.

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