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 today, 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 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, 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 much 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 along the fault, 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, a chain of earthquakes has broken 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, to the 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.”