Earthquake damage is more than just shaking
Above ground, there are a lot more factors than just the geophysics, that determine the degree of damage that ensues in an earthquake. Wendy Bohon is in Washington DC where she’s part of the Incorporated Research Institutions for Seismology, and she spoke to Chris Smith about the different causes of earthquake damages...
Wendy - Not all buildings are created equal. So it depends on the type of material that the buildings are made out of and where the buildings are built. So there are certain types of soil that can be really bad during earthquakes. When seismic waves travel through rock, they go pretty quickly. But in areas that have soft sand and soil, like in valleys or on the edges of rivers, those areas actually trap the seismic waves. And so if you have a building that's built there, you'll shake harder. And for a longer period of time, which obviously is bad.
Additionally, certain types of building construction are more likely to fail during earthquakes. So the kinds of structures that have like car ports underneath, or parking lots underneath. Those can't take the side to side motion of the earthquakes. And often that soft first story will pancake causing complete destruction of the building. Additionally, wood frame structures are nice and flexible just like trees. So they're really good in earthquake shaking, but structures that may be more stiff that are made out of concrete or other types of materials like brick. Those can sometimes fail during earthquake shaking.
Chris - A lot to unpack there. Very interesting what you were saying about the location of where you put your building because what's underfoot really matters. Is it not just a question of - when you shake the ground, it's not just like the ground staying hard, can not the characteristics of the ground also change if you shake it up? I'm thinking we were talking about avalanches on the programme a year or so ago. And showing that actually snow, when you shake it, behaves quite differently than when it's just a snow pack.
Wendy - That's absolutely right. One of the things that we're concerned about in areas that have that soft sand and soil, but that also have a high water table, they can experience something called liquefaction. And this is where the ground turns almost into quicksand. So as the seismic waves pass through the ground, they push that water up towards the surface. This forces all those little pieces of sand and dirt apart, which means that the surface of the ground loses its strength. So anything that's built there or anything that may be standing there, like say a car, that will sort of sink down into the ground. Now it's not going to sit down and get totally swallowed, but you can imagine if your house is built on an area that experiences liquefaction and your whole house tips the side, even if your home doesn't fall down, that could still be a total loss.
Chris - Do we know roughly what fraction of the housing stock the world has are in those rather inopportune places? Because this is presumably something we've learned since. And we tended to live where it was nice to live rather than having done a survey of what was underfoot before we built a city there.
Wendy - Right! If only we had known what we know now! But we can improve those things moving forward. I'm not sure about the total amount of construction in the world that are built in seismic hazard zones. And there's lots of different areas that will experience landslides or liquefaction or ground shaking. But we can definitely make sure that we know what those hazards are. We can let the people that live in those areas know what they can do to help mitigate or solve those hazards. And we can build any new structures up to seismic standards. So we can take those older buildings also and do things called seismic retrofitting - things like strapping homes down to their foundation. So they don't slide off during earthquake shaking. Making sure that we put in shear walls, because when the earthquake passes through it doesn't just shake a building back and forth, it gives it an upward push, a sideways thrust, a downward drop, and then a sideways thrust in the other direction. So we need to have the building strong in all different directions. And we can do that even after the buildings have been built. So a combination of policy changes, making sure that we're putting money into the system so that we don't have to put money in after the earthquake happens to rebuild. And then also educating the people that live in those areas about what those hazards might be during the earthquake.
Chris - And are earthquakes made equal? As in, I know there can be stronger and weaker, but do they tend to all shake the ground the same way, or do they come in different varieties, which actually might shake the ground differently? And therefore it's all very well to say, well, we'll build our buildings to resist an earthquake like this, but because there are so many varieties, it could be really tricky therefore to defend against all those threats.
Wendy - The first thing is that we spend a lot of time talking about the big ones, the ones that affect, you know, really large areas, like say the San Andreas fault, all of California, something like that. But we're actually more likely to feel shaking or to have damage from smaller earthquakes because they happen so much more frequently. We need to think about the big ones and plan for those really, really big events. But we also really kind of need to think a lot about what's happening locally and plan for that. Luckily scientists can go and figure out exactly where the faults are moving, where that stress is building up. We can go and dig down into the fault zones to figure out when earthquakes happened before and how big those earthquakes were. So that we get an idea of what might happen and how hard the ground might shake in these places. That way we can go and work with engineers and architects and say, this is the type of shaking that's been felt here in the past. And this is we think we can experience in the future. So taking what's happened in the past and applying it to the here and now I think will make a big difference.
Chris - That's certainly true in countries that have the resources, the wherewithal, and they don't have the pressures on the environment, the climate, food supplies, and so on to do that where you can make a rational choice. But in many places, as population climbs we're finding that people are resorting to living in areas that were a second choice previously, and that often does push them into the path of danger, doesn't it?
Wendy - It really does. What we know for certain is that disasters do discriminate and the people that are the least able to prepare for those disasters are often the ones who are going to be the most impacted when they happen. And so we really need to take into account vulnerable populations and the equity of how we're doing our communication about earthquakes, our communication about what those hazards are and working within populations that may be more vulnerable. Working with countries that may not have the infrastructure or the resources to go in and do the types of improvements that will really help to save lives and property during these big events.
Chris - Well, what then must scientists bear in mind? Or what must be the bottom line in terms of what we need to communicate to people going forward about earthquake threats and how they're likely to impact on us in the years ahead?
Wendy - Earthquakes can happen anywhere. We need to be ready when they do happen. And know that scientists are working very hard on your behalf. This is our science and we love it. But we recognise that these kinds of events impact people's homes, their businesses, it affects their lives. And that matters to us. We are working as hard as we can to make the changes that we need to make in order to keep everyone safe.