Ancient Collision Made Nepal Earthquake Inevitable
By KENNETH CHANG

More than 25 million years ago, India, once a separate island on a quickly sliding piece of the Earth’s crust, crashed into Asia. The two land masses are still colliding, pushed together at a speed of 1.5 to 2 inches a year. The forces have pushed up the highest mountains in the world, in the Himalayas, and have set off devastating earthquakes.

Experts had warned of the danger to the people of Katmandu for decades. The death toll in Nepal on Saturday was practically inevitable given the tectonics, the local geology that made the shaking worse and the lax construction of buildings that could not withstand the shaking.

GeoHazards International, a nonprofit organization in Menlo Park, Calif., that tries to help poorer, more vulnerable regions like Nepal prepare for disasters, had noted that major earthquakes struck that region about every 75 years.

In 1934 — 81 years ago — more than 10,000 people died in a magnitude 8.1 earthquake in eastern Nepal, about six miles south of Mount Everest. A smaller quake in 1988 with a magnitude of 6.8 killed more than 1,000 people.

Brian Tucker, president and founder of GeoHazards, said that in the 1990s, his organization predicted that if the 1934 quake were to happen again, 40,000 people would die because of migration to the city where tall, flimsily built buildings would collapse.

In an update just this month, GeoHazards wrote, “With an annual population growth rate of 6.5 percent and one of the highest urban densities in the world, the 1.5 million people living in the Katmandu Valley were clearly facing a serious and growing earthquake risk.”

The organization helped set up a local nonprofit to continue preparations, including the reinforcement of schools and hospitals.

Saturday’s earthquake occurred to the northwest of Katmandu at a relatively shallow depth, about nine miles, which caused greater shaking at the surface, but at magnitude 7.8, it released less energy than the 1934 quake.

Roger Bilham, a professor of geological sciences at the University of Colorado who has studied the history of earthquakes in that region, said that the shaking lasted one to two minutes, and the fault slipped about 10 feet along the rupture zone, which stretched 75 miles, passing under Katmandu.

The earthquake “translated the whole city southward by 10 feet,” Dr. Bilham said.

Aftershocks as large as magnitude 6.6 have occurred mostly to the northeast of Katmandu.

It is possible that the Saturday quake is a preface to an even larger one, but Dr. Bilham said that was unlikely.

Katmandu and the surrounding valley sit on an ancient dried-up lake bed, which contributed to the devastation. “Very, very soft soil, and the soft soil amplifies seismic motion,” Dr. Tucker said.

Steep slopes in the area are also prone to avalanches like the one that the quake triggered on Mount Everest on Saturday.

Katmandu is not the only place where a deadly earthquake has been expected.

Dr. Tucker said Tehran; Haiti; Lima, Peru; and Padang, Indonesia, were similarly vulnerable. In those places, nearby tectonic faults are under strain, and building standards and disaster preparations are seen as inadequate.

But not everywhere has been complacent. Over the past 76 years, many earthquakes have occurred along a fault in northern Turkey, starting in the eastern part of the country and progressing west, toward Istanbul. An earthquake in 1999 killed more than 17,000 people, mostly in the city of Izmit, east of Istanbul. The expectation is that the epicenter of the next big earthquake will be in or around Istanbul.

“Istanbul is the place that has been most aggressive in enforcing building codes,” Dr. Tucker said. “I think Istanbul has been doing a good job.”

震歪了…加德滿都 南移了三米

地震學家早知尼泊爾會發生災情慘重的大地震，由於尼泊爾位於仍在互相撞擊的印度板塊和歐亞大陸板塊的交界處，專家預測此地區大約每七十五年會有一次強震，在1934年發生規模八點一強震，距今八十一年。

全球五十位地震與社會科學家一周前才在加德滿都集會，構思如何協助這個貧窮、擁擠、過度開發和偷工減料的地區，為即將來臨的強震做準備，未料這麼快就噩夢成真。

紐約時報報導，兩千五百多萬年前，原本為島嶼的印度板塊撞擊歐亞板塊，這兩個大陸板塊迄今仍以每年一點五到兩英寸的速度相撞，撞擊的力量已把喜馬拉雅山的珠穆朗瑪峰推高為世界最高峰，並引發大地震。

廿五日的地震是尼泊爾過去205年來發生的第五次強震，包括1934年規模八點一強震，造成一萬多人喪生。而這次屬淺層地震，釋放的能量不及1934年那次。

科羅拉多大學地質學教授畢爾漢說，加德滿都下方120公里長斷裂區的斷層滑動三公尺，「地震把加德滿都往南移三公尺」。

尼泊爾法律規定，房地產須由所有兒子均分，為了分配，房子都蓋成瘦瘦高高，甚至頂樓加蓋，極不耐震。

原文參照：
http://www.nytimes.com/2015/04/26/science/ancient-collision-made-nepal-earthquake-inevitable-epochs-later.html

紐約時報中文版翻譯：
http://cn.nytstyle.com/technology/20150426/t26seismic/zh-hant/

2015-04-27．聯合報．A3．焦點．編譯田思怡