第十五篇 “Liquefaction” Key to Much of Japanese Earthquake Damage

The massive subduction zone1 earthquake in Japan caused a significant level of soil "liquefaction"2 that has surprised researchers with its widespreadseverity, a new analysis shows.

"We've seen localized3 examples of soil liquefaction as extreme as this before, but the distance and extentof damage in Japan were unusually severe," said Scott Ashford, a professor of geotechnical engineering4 at Oregon State University5. "Entire structures were tilted and sinking into the sediments," Ashford said. "The shifts in soil destroyed water, drain and gas pipelines6, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as much as four feet."

Some degree of soil liquefaction7 is common in almost any major earthquake. It's a phenomenon in which soils soaked with water, particularly recent sediments or sand, can lose much of their strengthand 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 reconsider the extent of liquefaction damage possibly occurring in situations such as this8.

"With such a long-lasting earthquake, we saw how 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 recentlyfilled ground, are much more vulnerable."

The data provided by analyzing the Japanese earthquake, researchers said, should make it possible to improve the understanding of this soil phenomenonand better prepare for it in the future. Ashford said it was critical for the team to collect the information quickly,  before  damage was removed in the recovery efforts9.

"There's no doubt that we'll learn things from what happened in Japan10 that11 will help us to reduce risks in other similar events," 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 depositedwithin 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 neara river and old flood plains is a suspect12, 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 prevent collapse. Japan has suffered 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 ground.

译文： “液化“是日本地震破坏的关键

一份分析报告表明，在日本，巨大的俯冲带地震致使土壤液化，其程度之严重、范围之广，令研究者们着实吃了一惊。

 “我们以前也看到过这么严重的小范围的土壤液化现象，但是，日本的破坏程度范围之广是极为严重的。所有结构都发生倾斜并陷到沉淀物中，土壤中的断层摧毁了自来水管、排水和煤气管道，社区的公共和基础设施陷于瘫痪。我们看到有些地区下陷了4英尺”。来自俄勒冈州立大学岩土工程系的Scott Ashford说道。

 几乎所有大地震都会带来某种程度的土壤液化。这种现象是由于地震中浸满了水的土壤，尤其是刚沉淀下来的沉淀物或细砂土在振动作用下突然失去支撑的力量而呈现液态变化。从而导致地面建筑物移动、下沉或倒塌。

“然而，多数地震比起最近日本发生的地震持续时间都短的多”。Ashford说道。日本发生的地震持续了五分钟，这就促使研究者们重新审视类似这次(持续时间长的)地震中液化可能造成的毁坏规模。

 “由于地震持续时间长，我们看到以前的地震持续30秒时地面建筑物仍旧完好无损，如果震动再延续几分钟，建筑物就会继续下沉或倾斜，很明显，那些建在新近填就的地面上的建筑物最易受到伤害。”Ashford讲到。 

研究者对日本地震作出了分析并得出数据，这些数据能大大提高人们了解土壤浸水后的这类现象，以便做好准备对付未来可能发生的同类现象。Ashford认为，重建工作首先要清理废墟，这样地震现场就被破坏了。所以，科学家要赶在重建工作开始之前收集好地震资料。

 “毋庸置疑，我们从日本地震中学到的东西有助于在将来类似的情况中降低风险。一些地方的建筑物会更多应用液化减少技术，如加强土壤的夯实或用石柱加固”。Ashford说。

 Ashford指出，北加利福尼亚州那些沿岸的、距离河口堆积物较近或填充的土壤易于受到液化的侵害。“年轻”的沉淀物这个词在地质学中是指那些年代少于或略超过一万年的沉淀物。比如，在俄勒冈州，它指的是Portland市区的大部分地区、Portland国际机场和其他城市。

凡是在河流和原洪泛区的土壤很有可能是危险的。俄勒冈交通部断定，该州的1100座桥梁如发生地震会有危险。只有不到15％的桥梁得到了加固以防坍塌。日本在3月11日的地震中损失惨重，但是震区的建筑物倾斜、下沉，却没有倒塌，日本的建筑标准功不可没。