Dams and Earthquakes

Geologists reflect on last year’s earthquake in Sichuan

(June 5, 2009) When the Sichuan province was rocked by a massive 7.9-magnitude earthquake last year, many scientists and government leaders were caught off guard. Previous studies by geologists stated that while the area—on the surface—appeared to be seismically active, their research showed otherwise.

A recent article by Alexandra Witze in Nature magazine examines why geologists ignored the possibility that the region would be hit by a massive earthquake and what they’re doing about it a year after the devastating event occurred.

Scientists have been performing seismic research in the Sichuan province for decades, particularly the geology of Longmen Shan, Dragon’s Gate Mountains—the center of last year’s quake. According to Witze, “the range [Longmen Shan[ marks the line where the 5,000 metre high Tibetan plateau rams into the low, stable Sichuan plain.” The unique geography of the area produces one of the steepest geographical reliefs in the world, a place where, citing Massachusetts Institute of Technology (MIT) geologist Clark Burchfiel, “over a distance of just 50 kilometres as the crow flies, surface elevation changes by more than 4 kilometres.”

This makes the Longmen Shan an ideal spot for seismic activity. “That kind of topography does not persist without active geological forces at work, continually building the steep mountain belt,” says Witze.

In the 1980s Burchfiel and his colleagues at MIT were convinced that when they started mapping the area, they would find evidence of large ground movement along the ridge. They predicted that they would see something around 10mm a year of ‘shortening’ – where the plateau and the ridge ram into one another and push up the mountain range.

But, according to Witze, “years of walking the faults unearthed no evidence for this amount of shortening in recent geological past.” Instead, the scientists found 1-2 mm of movement per year.

That kind of movement is relatively innocuous to geologists, as seismically active regions typically need more movement between plates to produce regular earthquakes. Witze says that in last year’s earthquake, “there was nearly 5metres [italics added] of slip along the Beichuan fault, the biggest of the faults that ruptured last year.” But based on the measurements from Burchfiel and other geologists, “quakes of that scale should only occur very infrequently [in the area], about every 2,000 to 10,000 years.”

So, while Burchfiel and his colleagues were surprised at how little movement they found when they studied the area in the 80s, they had no choice but to publish their results and move on—believing the region to be relatively safe from major earthquakes.

Since Burchfiel published his research, his findings were supported by other researchers in recent years. Witze says that “researchers measured ground motion in the area using Global Position System receivers and found low rates of slip across the Longmen Shan, confirming the 1-2 millimetres per year suggested by Burchfiel.”

And if the Longmen Shan region was prone to major earthquakes in the past, scientists should have been able to see this in local geology. But because of heavy rains and high erosion in the area, much of that evidence has been washed away. After talking to Chen Zhiliand, a geologist from the Chengdu Institute of Geology and Mineral Resources, Witze says, “most of the recent known quakes along the Beichuan fault have been much smaller than the 2008 quake, including one magnitude-6.2 quake in 1958 and another in 1970.”

Witze adds that, “there is no archaeological evidence that the town of Beichuan itself has ever been destroyed by a quake since it was founded some 1,500 years ago.”

And Leigh Royden, a geophysicist at MIT who has done research in the area told Witze, “I don’t think there was a reason to say there would have been a major quake there.”

So, while it would appear that the region should be ripe for a major earthquake, based on its unique geology, scientists and the organizations that fund them couldn’t justify spending more money on research in an area that seemed seismically quiet. Instead, they began focusing their attention on a more seismically active area to west at the Annign He and Xianshui faults – both of which move up to 10 mm a year.

Witze says the, “China Earthquake Administration has spent most of its monitoring efforts on these active faults, including deploying nearly 300 broadband seismometers—ones that capture a wide range of vibrational frequencies—in the world’s densest array to map the underlying crust.”

Yet, what occurred in last year’s earthquake was different than the what geologists expect when measuring and studying quakes. Traditionally, scientists look at individual fault segments to measure the size of the earthquake. But when multiple, smaller sections connect, they can create an earthquake far more severe than they expect.

That’s exactly what happened in the quake that hit the Sichuan province last year. Witze says, “the Beichuan fault ruptured across several segments totaling 240 kilometres, while a secondary fault to its southeast, the Pengguan fault, broke for 72 kilometres.” Chinese geologists now realize that the segments connect at a deeper level, which allowed the earthquake to be more severe than they would have otherwise expected.

Now scientists are beginning to wonder what the effect last year’s earthquake will have on fault lines across western China. One study says that last year’s quake may have increased the chance of earthquakes in other nearby faults in western China. And another study believes that the chance of another magnitude-7 quake in Sichuan over the next decade is 8-12% higher than it was before last year.

Because of the severity and the uniqueness of last year’s earthquake, scientists are beginning to use the massive amount of data available to reach a more fundamental understanding of the region’s geology. In recent years, the Chinese government went on a spending spree for earthquake research, constructing an array of 300 seismometers in western Sichuan.

Witze says “the array boasts the densest arrangement of seismometers of any large network around the world.” So far, the stations have produced more than seven terabytes of data—providing an in-depth look at last year’s earthquake. Witze reports that nearby stations “are yielding an unprecedented glimpse into the crust of western Sichuan; a major quake has never been captured in such detail by a network like this.” Early data is already suggesting that there is a drastic alteration in geology around 20 kilometres beneath the surface—where brittle matter gives way to soft rock.

The network was originally used to monitor the faults that scientists thought posed the biggest threat of earthquakes—mainly the Anning He and Xianshui faults. But since last year’s earthquake, they have been moved in order to get a more thorough understanding of the Beichuan fault.

Yet with all the new research equipment, one thing the government may not be learning from last year’s quake is how to build in seismically-prone regions. The government says that new homes along the Longmen Shan should be capable of withstanding a magnitude-8 earthquake. But Witze warns that, “in many places, however, reconstruction is taking place so quickly that no one is confident that building codes are being followed.”

And even if the government did construct stronger homes, the ever-present threat of landslides in the region continues to loom over the new developments. Cui Peng, a geomorphologist at the Institute of Mountain Hazards and Environment in Chengdu told Witze that, “more than one-fifth of the people who died in the quake were killed by landslides or mudflows” Some estimates say that at least 50,000—but possibly as many as 100,000—landslides occurred in last year’s quake.

This year there may be more landslides. “The quake destabilized a number of slopes in the area, making them particularly prone to failure after the rain,” Witze warns. But in the meantime houses are popping across the hillside—likely not in government-approved areas.

Whether scientists should have been better prepared for last year’s earthquake is still up for debate. What is certain is that the quake rocked scientists assumptions of geological activity in areas like Sichuan. While that doesn’t bring much relief to the families of the 70,000 people killed in the earthquake, it may help prevent disasters in the years to come.

Brady Yauch, June 5, 2009


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