GWEN IFILL: And earlier this afternoon, I spoke via Skype to one of the many international aid workers trying to help in Nepal.

Andy Bastable is head of water and sanitation at Oxfam. He is in Kathmandu.

Andy, tell us, what are the immediate challenges you face on the ground?

ANDY BASTABLE, Oxfam: It's mainly around logistics.

So, you have got — it's a big — Kathmandu is a big city. And then it's got big, big suburbs. And then you have got bits of areas where buildings have fallen down and then you have got little groups of displaced people. So, at the moment, there's 16 designated official areas of displaced people, and then you have got a lot more unofficial sort of areas where people are.

So, to get round these kind of quite blocked roads, blocked with rubble or blocked with kind of traffic, to get around to each site, our job, as Oxfam, is supplying kind of water and we're doing sanitation of these sites. We have started doing water trucking. So it's mainly around the logistics of getting to each place quickly.

GWEN IFILL: It seems like the biggest challenge might be the scope. How do you even gauge what the scope of the need is?

ANDY BASTABLE: That's true, that we're trying to actually do two things at once, one, actually start work.

So, we're starting water trucking. We're starting building kind of Oxfam tanks at these displaced centers. And at the same time, we have got other teams out assessing kind of further areas, because, at the moment, it does seem that a lot of the efforts are in the immediate Kathmandu area, because it's easier to get to.

GWEN IFILL: Are the aftershocks presenting a challenge, a logistical challenge?

ANDY BASTABLE: Yes, but mostly in the fear of the people, the people who have been in their house and know family and friends who have been killed or their own house is going to collapse.

These aftershocks represent a huge traumatic event, and just it reminds them of all the trauma that they went through kind of on Saturday morning. So, it's more traumatic, I think. Most of the old buildings that have fallen down will have fallen down and now we have just maybe got small falls as the aftershocks occur.

GWEN IFILL: Is there any way to compare what we're seeing unfold here with other disasters of this type around the world?

ANDY BASTABLE: Yes.

From what I have seen so far, that this isn't on the scale of Haiti. There's not — the mass devastation of Katmandu is not the same as the mass devastation in Port-au-Prince in 2010. So it's not quite as big as that, but it's, we could say, covering a wider area. And, yes, I think some of the poverty levels when you get out of Kathmandu are more extreme.

GWEN IFILL: Thank you, Andy Bastable of Oxfam.

ANDY BASTABLE: Thank you very much.

GWEN IFILL: The 7.8-magnitude earthquake was the strongest to hit the Himalayan nation in more than 80 years. The country is at the junction of a major fault line between two tectonic plates, the Indian and Eurasian ones.

As you can see on this color-coded map, strong shaking was felt far away from the epicenter northeast of Kathmandu. Orange and yellow areas indicate strong to severe shaking. In the days since, there have been dozens of aftershocks.

To help us understand more, I'm joined by David Applegate, associate director for natural hazards with the U.S. Geological Survey.

So, give us the geological explanation, the layman's explanation for what actually happened here.

DAVID APPLEGATE, Associate Director for Natural Hazards, U.S. Geological Survey: So what we're looking at with this area is, it's a collision zone.

And most of the areas around the world where we have tectonic plates colliding, one against the other, we have oceanic plates going underneath continental ones. Well, this is one where you have a head-on collision, India slamming into Asia. It's been going on for the last 50 million years.

But while it's an inexorable process, the actual on-the-ground effect is, the faults are locked up. They gain stress. It builds up, builds up, and finally they break. And that's an earthquake.

GWEN IFILL: This is an area that was prone to these quakes? Was it inevitable that it was — did you see it coming?

DAVID APPLEGATE: Absolutely. It was inevitable when we think about the long-term hazard.

There's been significant earthquakes. Now, there haven't been a lot in the past, say, 50, 60 years. But if we look back deeper into time, we see a series of large quakes. The fact that we hadn't had them recently means that stress has been building up. And so it was — there was an inevitability, not the exact moment of when it would happen, but that it would happen.

GWEN IFILL: So, is what we saw the culmination of a slow buildup or is it a precursor to more?

DAVID APPLEGATE: Well, for this particular zone, it relieved the stress. You saw on the map that it starts in one place, that epicenter, but then it ruptured to the east.

GWEN IFILL: Which is northwest. I said northeast, but yes.

DAVID APPLEGATE: That's right, off to the northwest. And then it ruptured to the east past Kathmandu. So a whole segment of this large fault has ruptured.

Now, that relieves the stress there, but it does mean that it's — there may be additional stress on other sections of the fault.

GWEN IFILL: So, is it fair to say, if the last big quake in this region happened in 1934, or more than 80 years ago, that this was overdue?

DAVID APPLEGATE: Well, there have been — there have been other events, other sections of the plate, but for this particular zone, that 1934 quake didn't relieve the stress.

It was further off to the west. I think the epicenter was just south of Mount Everest. So it relieved the stress in that area, but it didn't relieve it in this area.

GWEN IFILL: Much discussion about aftershocks. We heard the aid worker talk about fear. But there's also some real other concerns as well.

DAVID APPLEGATE: Well, aftershocks are one of the things that are most difficult of dialing with a large disaster, an earthquake disaster.

With a hurricane, the weather comes through, the sun comes out, people are able…

GWEN IFILL: Then it's over.

DAVID APPLEGATE: It's over. But with the aftershocks, it's that constant drumbeat.

And as we heard from the — from Oxfam, the issue here is, it's also a mental one, in addition to the physical effects of it, that for every magnitude 5 — and we have had several — let's see — we have had over 50 magnitude 4 and 5 earthquakes and even a couple of magnitude 6s in the aftershock zone across that whole area that we described.

Well, so, in addition to that, that means you still have, say, another — hundreds of smaller 3s and 4s that are going to be affecting people just constantly. And so that's a huge challenge.

GWEN IFILL: Given what you know about the area's topology, is the death toll likely to rise?

DAVID APPLEGATE: There are still a lot of areas where — as we heard, a lot of the focus has been on Kathmandu, but particularly the epicentral region, it is very remote.

We know it's not just the issue of the earthquake shaking itself, but landslides are going to be — happen throughout that region. That can have further effects, damming rivers, potential for down the stream.

GWEN IFILL: David Applegate of the U.S. Geological Survey, thank you.

DAVID APPLEGATE: Thank you.