单选题
The massive subduction zone earthquake in Japan caused a significant level of soil "liquefaction" that has surprised researchers with its widespread severity, a new analysis 1
"We"ve seen localized examples of soil liquefaction as extreme as this before, but the distance and 2 of damage in Japan were 3 severe," said Scott Ashford, a professor of geotechnical engineering at Oregon State University. "Entire structures were tilted and sinking into the sediments," Ashford said. "The shifts in soil destroyed water, drain and gas pipelines, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as 4 as four feet."
Some degree of soil liquefaction is common in almost any major earthquake. It"s a phenomenon in which soils soaked with water, 5 recent sediments or sand, can lose much of their 6 and flow during an earthquake. This can allow structures to shift or sink or collapse.
But most earthquakes are much shorter than the recent event in Japan, Ashford said. The length of the Japanese earthquake, as much as five minutes, may force researchers to 7 the extent of liquefaction damage possibly occurring in situations such as this.
"With such a long-lasting earthquake, we saw 8 structures that might have been okay after 30 seconds just continued to sink and tilt as the shaking continued for several more minutes," he said. "And it was clear that younger sediments, and especially areas built on recently filled ground, are much more 9 "
The data provided by analyzing the Japanese earthquake, researchers said, should make it possible to improve the understanding of this soil phenomenon and better prepare 10 it in the future. Ashford said it was critical for the team to collect the information 1 , before damage was removed in the recovery efforts.
"There"s no doubt that we"ll learn things from what happened in Japan that will help us to reduce risks in other similar 12 ," Ashford said. "Future construction in some places may make more use of techniques known to reduce liquefaction, such as better compaction to make soils dense, or use of reinforcing stone columns."
Ashford pointed out that northern California have younger soils vulnerable to liquefaction—on the coast, near river deposits or in areas with filled ground. The "young" sediments, in geologic terms, may be those 13 with in the past 10,000 years or more. In Oregon, for instance, that describes much of downtown Portland, the Portland International Airport and other cities.
Anything near a river and old flood plains is a suspect, and the Oregon Department of Transportation has already concluded that 1, 100 bridges in the state are at risk from an earthquake. Fewer than 15 percent of them have been reinforced to 14 collapse. Japan has 15 tremendous losses in the March 11 earthquake, but Japanese construction standards helped prevent many buildings from collapse—even as they tilted and sank into the grourd.