Question: What specific geological activity caused the recent earthquake in Chile?
Arthur Lerner-Lam: Okay. Well Chile is a particularly interesting earthquake from a seismological point of view. First of all of course, it was very large, a magnitude 8.8 and that’s what we call a great earthquake where we use great in terms of the wow factor if you will. It did kill 700 people thereabouts. It caused billions, tens of billions of dollars worth of damage, so the economic impact in Chile is going to be severe, but we don’t get a magnitude 8.8 very often. The only reason by the way there weren’t more casualties in Chile and more damage in Chile is that the earthquake was off the coast. It was in the subduction zone. It generated a tsunami as we saw, but it wasn’t a direct hit, but being anywhere close to an 8.8 is enough of a hit to be dangerous and that is what happened in Chile, but it was the result of a subduction zone. It’s part of what seismologists call the circum-Pacific Seismic Belt or Earthquake Belt. It’s what colloquially known as the Pacific Ring of Fire. That’s actually a great name because indeed the Pacific is rimmed, is ringed by these subduction zones. When… A subduction zone means that the heavier, denser crust of the Pacific Ocean basin is essentially diving under the lighter, less dense crust of the continents surrounding and as it dives underneath it, it sticks and slips and causes earthquakes. You know roughly 70 or 80% of the world’s biggest earthquakes occur in the Pacific Ring of Fire. You know it’s a gigantic source of tectonic energy, of earthquake energy. The other interesting thing about that by the way, and the reason it’s called the Ring of Fire is because as these plates dive down underneath the lighter continent pieces of them melt and that melt becomes magma that just drives up through what we call the mantle and the crust and creates volcanoes, so it’s kind of part of an ecological recycling mechanism going on. You’re taking old crust, throwing it out, sending it down the subduction zone, recycling it into new material just like you throw a paper bag into the garbage or in the recycling bin. So that is the basics of the tectonic activity.
These earthquakes tend to be very large because these are large plates. The continents present a significant barrier to subduction in many cases and it takes a long time for stress to build up. One interesting thing is that we knew about this problem in Chile of course for a long time and of course the Chileans, who have an extraordinarily advance and professional capacity to deal with earthquakes knew about this as well, but 20 or 30 years ago using very simple components of the plate tectonic theory this particular area was identified as a so-called seismic gap, a place where an earthquake was basically overdue. Couldn’t predict it, but we could forecast it and that is what in some sense, saved Chile.
Question: How did the Chile earthquake shift the earth’s axis?
Arthur Lerner-Lam: Yeah, that’s… You know, large earthquakes do strange things. Of course what an earthquake does is in addition to breaking the ground it actually moves big crustal blocks around and when it does that it shifts the distribution of mass of the earth. Now because this wasn’t an asteroid or a comet hitting the earth and causing all these vibrations the earth has to conserve its angular momentum and the way the earth conserves its angular momentum when crustal blocks shift like that is to change the rate of rotation and to slightly shift the axis of rotation. It can be calculated, but it really isn’t measurable. On the other hand, if you’re being paid by the microsecond maybe it’s something that you might worry about, but the interesting thing is that we can calculate it, not that is in fact something that we ought to worry about.
Question: What specific geological activity caused the recent earthquake in Haiti?
Arthur Lerner-Lam: Haiti is a different kind of an animal, but it pays to think about what is really happening around South America. The Chilean event, a subduction event was a subduction of a piece of the Pacific plate, in this case called the Nazca plate, but a piece, a smaller plate going underneath Chile, underneath the Pacific Ring of Fire, and so this is a case where the continent and the Pacific Ocean are sliding… are converging against each other in the Pacific, diving down because it’s denser, but if you look at the northern and southern parts of South America. Picture it in your mind. You’ve got the Caribbean to the north, of course, and you’ve got the Scotia Sea to the south between South America and Antarctica, and those are two very interesting plates. You know in some sense you could think about those plates as resulting from the squirt of material in geologic time going around South America, so if you can’t subduct this stuff, if you can’t get everything underneath South America through subduction that stuff has to go someplace and it goes north and south and sort of squirts around and the Caribbean plate is one of these plates got squirted through the gap between North and South America. Of course it’s a vast simplification, but it’s actually the way we teach it. It’s kind of a fun thing to think about. And so the Caribbean is actually itself a small plate. It is a significant part of plate tectonics. It was well recognized as a plate, but it’s basically squirting passed South America between North and South America going from west to east and it’s moving at a little less than an inch a year relative to North America.
You know, the unfortunate thing for Haiti is that for geologic reasons Haiti lies along that northern boundary and because this is a squirt boundary going from west to east that is what we call a strike slip fault. It’s a fault that moves sideways. There is some complexities to that, but that fault was also recognized as being capable of supporting a magnitude 7 or greater earthquake, actually a 7.2. In fact, that forecast was made in the professional literature in 2008 two years ago and we’ve known for about 20 years that that fault was dangerous. Anyway, that wasn’t a subduction earthquake. That was a sliding earthquake and sliding earthquakes or strike slip earthquakes are dangerous also for a number of reasons. First of all, they tend not to be as large as a subduction earthquake, but they can be large enough. They also in this particular case they occurred on land and unfortunately it was almost a direct hit on Port-au-Prince. The earthquake itself was shallow, which increases the amount of vibration that people on the surface feel and it was very close to major population centers, so if you will, it was sort of the perfect storm of bad affect. Even if Chile had good building codes and good construction I think we would have seen tens of thousands of casualties anyway. Very difficult to deal with an earthquake of that sort, but it was a different kind of an animal.
We’ve looked at the Chile earthquake in many ways and we continue to do so. In fact, there are a number of conferences coming up in the next few days to try to move to the next phase of Chile rebuilding and sort of essentially make the rebuilding of Chile more risk-conscious, not just for earthquakes, but for the storms coming up and the annual storm season, the annual hurricane season and concomitant landslides and other natural hazards, but you know trying to understand these earthquakes on land, these strikes of earthquakes is a very complex problem and one of the things that we’re very worried about in Haiti is that this earthquake may have stressed parts of the fault that did not rupture on January 12th.
You know when an earthquake happens it doesn’t zip open the whole fault. It only zips open, in this case, about a 40 kilometer segment of it and there is still stuff on either side that did not rupture. Well that stuff may have been overstressed. That piece of the fault may have been overstressed and so the potential for an earthquake on those segments may actually have increased. That again is a computer model. It’s something that we’re working on. It is hard to predict, but it is something that as a forecast ought to play into the way Haiti is being rebuilt. Chile we could do the same thing, but to be honest Chile apart from the economic damage and the economic recovery Chile has a good capacity to deal with future earthquakes and the outcomes of this earthquake. With Haiti I hope that the international community will work with the Haitians to build that capacity so that they can monitor the potential for these future earthquakes.