Monday, March 8, 2010

Two Years Later, New Rumblings Over Origins of Sichuan Quake

Science 5 March 2010:
Two Years Later, New Rumblings Over Origins of Sichuan Quake

Richard A. Kerr and Richard Stone
BEIJING—When experts suggested that the disastrous 2008 Wenchuan earthquake might have been triggered by the reservoir behind the Zipingpu Dam, establishment scientists in China remained largely silent (Science, 16 January 2009, p. 322). Now they've weighed in, ruling out reservoir triggering. But
many earth scientists don't buy their arguments.

No large quake had ruptured the Beichuan-Yinxiu fault in southwestern China's Sichuan Province in at least a millennium or two. Then engineers built Zipingpu Dam on the Min River just 500 meters from the fault and in late September 2005 began filling it with upward of 900 million tons of water. Two-and-a-half years later, the magnitude-7.9 Wenchuan earthquake got started 5 kilometers from the reservoir.

In the January issue of International Water Power and Dam Construction, three dam engineers at the China Institute of Water Resources and Hydropower Research here argue that the Zipingpu-Wenchuan situation was so unlike that of the four largest known reservoir-triggered earthquakes—all in the magnitude-6 range—that there could not have been a connection between reservoir and quake. The authors, led by structural engineer Chen Houqun, who has co-authored China's design code for building earthquake resistance into dams, contend that the timing was mere coincidence.

Figure 1

Trickle-down theory. Did filling the Zipingpu Dam trigger the magnitude-7.9 Wenchuan earthquake? Chen Houqun (inset) says no. CREDITS: WINGCHINA; (INSET) R. STONE/SCIENCE
[Larger version of this image]
Unlike the four big triggered quakes, the authors point out, the Wenchuan quake occurred on a thrust fault: One block of crust is pushed up the inclined fault over the other block. According to the authors, the reservoir's weight lies on a "relatively stable region" of the footwall, or underlying block.
In addition, the 300-kilometer Wenchuan rupture began 18.8 kilometers beneath the surface, according to new data from a team led by geophysicist Liu Qiyuan of the China Earthquake Administration's Institute of Geology in Beijing. The reservoir's water pressure could not have driven water that far down through cracks and pores, Chen argues. Such water infiltration over months or years is thought to weaken a fault by pushing its sides apart.

Finally, Chen and his colleagues note, in the four cases, filling a dam's reservoir led to an increasing number of temblors until a large earthquake struck. But a seismic-monitoring network around Zipingpu reservoir established 13 months before impoundment began showed only "normal variation" of seismic activity between impoundment and the quake, the group writes. "All of these factors rule out triggering," says Chen.

Martin Wieland, chair of the International Commission on Large Dams' committee on seismic aspects of dam design, says the paper makes a persuasive case. And Liu adds that the Wenchuan quake's focal depth by itself discounts a link to Zipingpu. But neither categorically rejects a role for Zipingpu. There are too many uncertainties, says Wieland, an earthquake engineer at Poyry Energy in Zurich, Switzerland.

Many seismologists say the Water Power authors overstate their case. "I don't think [triggering] has been put to rest yet," says seismologist Arthur McGarr of the U.S. Geological Survey (USGS) in Menlo Park, California, who co-authored a review of reservoir-triggered seismicity (RTS) in 2002. The problem, many seismologists say, lies in comparing Wenchuan to the four big RTS earthquakes. "Wenchuan doesn't fit the pattern? What pattern?" asks seismologist Ross Stein of USGS Menlo Park. "The [four] examples are all over the map as to how seismicity has responded to dam impoundment. It shows just how little we know about this process." Although RTS does tend to be shallow, McGarr says, the high Aswan Dam caused a magnitude-5.3 event in 1981 at a depth of 18 kilometers, similar to Wenchuan's depth. And a magnitude-4.5 RTS quake in Tajikistan struck in a region of thrusting—as did Wenchuan.

Last October, hydrogeologist Shemin Ge of the University of Colorado, Boulder, and colleagues presented modeling evidence in Geophysical Research Letters that water infiltration from Zipingpu "potentially hastened the occurrence of the Wenchuan earthquake by tens to hundreds of years." Other researchers see Zipingpu's fingerprints all over the seismic data. One of the most vocal advocates for a dam-quake link, geologist Fan Xiao of the Sichuan Bureau of Geology and Mineral Resources in Chengdu (Science, 8 May 2009, p. 714), says swarms of small quakes struck the region near Zipingpu 3 months before the Wenchuan earthquake. "The fact is, there were obvious foreshocks," Fan says. Chen says he welcomes "scientific debate"—but he is sticking to his guns.

What researchers still want almost 2 years after the earthquake is wide dissemination of the raw data from the Zipingpu monitoring. Until such data sets become commonplace, says geophysicist Evelyn Roeloffs of USGS in Vancouver, Washington, "it's always going to be this kind of story."

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