Why do buildings collapse while others survive? What can poor, earthquake-prone countries like Nepal do to improve building safety? DW talked to Peggy Hellweg, a seismologist at the University of California, Berkeley. Dr. Peggy Hellweg is a physicist and seismologist with the Seismological Laboratory at the University of California in Berkeley

The earthquake in Nepal had a magnitude of 7.8. In recent photographs from Kathmandu we see buildings that have collapsed while others just next door are stil standing. How can that be explained?

Probably the most important factor in building collapses is the construction of the building, the structure. Like in Germany, like in the US and many cities in the East, there are a variety of structures, possibly new ones, possibly old ones next to each other. And they are of course built in very different ways. The other important factor is: how well is the building constructed? Have the people mixed the cement properly with proper amounts of the various components? If the cement is too sandy, for example, it doesn't serve the purpose of holding the building together.

But even buildings that had survived quakes before collapsed. What determines the stability of a building? Is it the frame or is it the flexibility of the material?

In general all buildings constructed in an old style, which is stone on stone; or stone, mortar and stone; are very, very susceptible to earthquakes. You can imagine a tall building like a whip: when the earthquake comes it shakes the bottom with the ground, but the back and the top shakes more strongly and that can make the building collapse. The structural elements that improve a building's performance include having wood buildings - or rather wood beams - and wood frames, including steel in the buildings. The problem is that we learn every time that our solutions are not always as perfect as we wish them to be.

Parts of Nepal have been moved several meters by the earthquake. What does that mean, when it comes to the safety of the people there? Will the ground remain stable for the buildings on it?

That depends on the underground. Buildings on rock tend to have better foundations and are shaken less. Buildings on ground fill or in valleys filled deeply with sediment shake a lot more. And they end up collapsing more frequently. The best example of that was the earthquake of 1985 on the southern coast of Mexico, which did incredible damage in Mexico City, which was several hundred kilometers away. Many people were killed simply because Mexico City is on a basin, it's on soft soil and it shook like a bowl of Jello – for Germans I should say it shook like a bowl of pudding.

In some parts of Europe – in Italy for example – you find deserted villages, abandoned after earthquakes hit the region and they were rebuilt somewhere else, in a less endangered area. Could that be the fate of some of Nepal's villages and towns?

Villages and towns can be rebuilt elsewhere. The problem in an earthquake-prone country like Nepal is, where is it really safe? That's the same problem in Italy. That the people may have moved to another location, which was safe in that earthquake, but may be affected strongly by the next earthquake, which happens in a slightly different location. Earthquake country is earthquake country!

What can poor nations like Nepal, situated in an earthquake region, do to prepare their countries for big earthquakes like the most recent one?

Having good building infrastructure is important. That is clearly a problem in developing countries. Haiti for example was a big disaster not so much because the earthquake was big. It was almost one magnitude unit smaller than this one, but simply because the buildings were all not built to any kind of code and were unsafe. It's important to have the buildings rebuilt - in this case – and not to allow immediate reconstruction with poor materials.

The other thing that a society can do is to prepare. Nepal knows that it's earthquake country and if the people are educated about what earthquakes can mean to them, then they have a better chance of taking an action which will save their lives. Having their earthquake kit prepared, whatever that means for Nepalese and being ready when the next earthquake comes.

Obviously poor countries don't have the funds for high-tech, earthquake-proof housing. Are there less costly alternatives to build safer houses or is there a way of making older buildings more stable?

Making older buildings more stable is a very costly effort regardless of what country you are in. There were experiments in Turkey, where the building infrastructure is also not terribly safe. They discovered that if people use the relatively old building style with rocks or concrete blocks and cement, but included boards as a kind of rebar in the building, then those buildings were less likely to collapse in earthquakes. So there are simple solutions that can help during rebuilding. The additional cost for earthquake-proofing during rebuilding is a small part of what it costs to build the building in the first place.