Slower Tremors May Be Worse for Skyscrapers
地震慢慢搖 高樓傷更大
By Kenneth Chang

In April, as email alerts ricocheted with the news that a large earthquake had shaken Nepal near its capital, Kathmandu, experts expected a toll comparable to the roughly 85,000 who died in 2005 when a smaller earthquake struck a more sparsely populated area in the Kashmir region.
今年四月，一封警示電郵帶著新聞彈出，一場大地震撼動尼泊爾首都加德滿都。專家估計，死亡人數可能與2005年有8萬5000人死亡的地震相近，那場較小的地震襲擊人口稀疏的喀什米爾。

Over the following days, the number of deaths in Nepal from the magnitude 7.8 earthquake rose to about 9,000.
接下來幾天，尼泊爾這場規模7.8的地震死亡人數增加至9000人左右。

That figure “is actually a small number given the density of the population in the Kathmandu area and the vulnerability of the buildings,” said Jean-Philippe Avouac, a professor of geology at the California Institute of Technology and the University of Cambridge in England, and an author of two scientific papers about the Nepal earthquake. “I think we understand why that is the case.”
加州理工大學及劍橋大學地質學教授，也是最近尼泊爾地震兩篇論文的作者阿夫艾克說，「就加德滿都地區人口稠密和建築物脆弱的程度而言，這實際上是小數字。我想我們了解何以如此。」

The relatively low death toll is owed neither to luck nor the sturdiness of Nepalese construction.
死亡數字相對而言較低，原因既非運氣好，也非尼泊爾建築物夠牢。

Rather, the 40 seconds of rumbling from the earthquake turned out far different than expected, based on what was known from earlier earthquakes and the geology of the region. Instead of quick oscillations, one second or faster, which shake apart smaller buildings, the ground shook back and forth once every four or five seconds, a slower rate that is more dangerous to tall structures.
這起地震轟隆40秒的結果，和基於先前多起地震及地質情況所做預期遠遠不同。不同於以往一秒或更快搖晃震裂較小建築物，這回地面每4或5秒搖晃一次，較慢的速度對於高的建築物更危險。

That could explain why a 60-meter-tall watchtower, which had survived a stronger earthquake in 1934, fell while many rickety but shorter buildings remained standing.
那可解釋為何一座60公尺高的守望塔撐過了1934年更強烈的地震，卻在這次倒塌，而更矮的建築物卻安然挺立。

The findings raise concern that a similar earthquake could pose unexpected danger to cities with skyscrapers.
這項發現令人們擔心，一場類似的地震可能對有摩天大樓的城市造成危險。

In a paper in Nature Geoscience, Dr. Avouac described using seismological records and radar imagery to study the rupture in Nepal.
阿夫艾克博士在發表於「自然地質科學」期刊的報告中，描述他如何使用地震學紀錄和雷達影像去研究尼泊爾的地震。

India is colliding into the rest of Asia at a pace of up to 5 centimeters a year. But the tectonic plates have been stuck, not sliding smoothly. “Over the last 20 years, at the surface, the fault was completely locked,” Dr. Avouac said.
印度板塊以一年5公分的速度撞擊歐亞板塊，但是板塊已卡住，無法順暢滑動。阿夫艾克博士說：「過去20年來，斷層表面是完全鎖死的。」

When the strain becomes too great, the fault breaks, setting off an earthquake. On April 25, the earthquake started northwest of Kathmandu and spread east at 2.7 kilometers per second, rupturing 140 kilometers of the fault.
當壓力太大時，斷層破裂，造成地震。4月25日，這起地震在加德滿都西北方開始，以每秒2.7公里的速度往東擴散，撕裂140公里的斷層。

“This earthquake is actually a relatively small earthquake,” Dr. Avouac said. Stress remains in other parts of the fault, particularly in western Nepal, where it has been locked since a magnitude 8.5 earthquake struck in 1505. “I’m worried about that part,” he said.
阿夫艾克說：「這起地震事實上相對較小。」壓力仍存在於斷層中其他部分，尤其是尼泊爾西部，自1505年規模8.5的地震後便鎖死，「我擔心這個部分。」

In the second paper, published in the journal Science, Dr. Avouac and his colleagues, showed that the 19-kilometer-wide and 140-kilometer-long patch where the fault slipped broke smoothly, generating long-period waves.
在發表於「科學」期刊的另一篇報告中，阿夫艾克博士和同事指出，在斷層平順滑動的破裂處，有19公里寬、140公里長的痕跡，產生長期的波動。

Susan E. Hough, a seismologist with the United States Geological Survey said the new findings are “clearly part of the story,” but added that other factors played a role.
美國地質調查所的地震學家休伊說，這項新發現「很明確是原因的一部分」，但補充道其他因素也扮演一定角色。

The shape of the Kathmandu valley, partly filled with sediments, tends to amplify shaking with a period of one second, Dr. Hough said. For the magnitude 7.8 earthquake, the shaking was so strong that the behavior of the sediments changed.
加德滿都谷地有一部分充滿沖積物，它的形狀可能使周期一秒鐘的震動效應擴大。休伊說，以這起7.8的地震來說，震動太強烈以致沖積物行為改變了。

“We talk about a bowl of Jell-O as an analogy for how it shakes,” Dr. Hough said. “But it’s actually more like a big sandbox.”
休伊說：「我們談的是拿一盒果凍作為類比，觀察它如何搖動，但是事實上它更像一大盒沙子。」

For the strong shaking, “it doesn’t transmit the energy the same way it does for a small earthquake,” she said. “It’s basically behaving like a different material.”
她說，以強震而言，「它傳送能量的方式不會和小規模地震相同，基本上它的表現就像是一種不同的材料。」

What the scientists do not know is whether long-period shaking is common for earthquakes along the Himalayas, or whether similar faults elsewhere, usually quiet, could generate similar earthquakes.
科學家不知道的是，周期較長的震動在喜馬拉雅地區是否常見，以及在其他通常很安靜的斷層，是否也會產生相似的地震。

“There’s one in the Los Angeles area, for example,” Dr. Hough said. “Five-second energy like this earthquake released would be especially damaging to 50-story buildings.”
休伊博士說：「舉例來說，在洛杉磯若有一場地震，像是這起地震釋放的5秒鐘能量，就可能會損害50層樓高的建築物。」

原文參照：
http://www.nytimes.com/2015/08/07/science/studies-of-nepal-quake-raise-concern-about-skyscrapers.html

2015-08-25聯合報/G5版/UNITED DAILY NEWS 莊蕙嘉譯 原文參見紐時週報十版下