Earth Shaking Advances in Plate Techtonics
Professor Jean-Philippe Avouac
Professor of Geology, Califonia Institute of Technology
E.A. Flinn Award 1993
The earth’s surface is thought to be composed of a dozen or so rigid plates. The sites where these plates come into contact are the sites of most of the interesting geological phenomena that exist, including earthquakes. The theory describing the action of these plates, termed plate tectonics, has been utilized for over 40 years, but may be limited in its utility.
Joining us today to discuss this issue is Prof. Jean-Philippe Avouac. Prof. Avouac is a Professor of Geology at the California Institute of Technology and was previously head of the Laboratoire de Télédétection et Risque Sismique and is a recipient of the prestigious E.A. Flinn Award from the American Geophysical Union.
Prof. Jean-Philippe Avouac (JPA) talks with Charles Lee (CL) about extending the theory of plate tectonics.
CL: This is certainly a very fascinating issue especially for those of us living in California. I’m curious if you can first explain what is the theory of plate tectonics.
JPA: Plate tectonics is the theory that describes motion at the ground surface at the global scale. So, the idea is that the outer shell of the earth can be divided into a number of plates, and these plates are rigid over the long term so that deformation over the long term actually occurs along the boundaries of these plates. So the idea is that would explain the seismicity over the global scale and volcanism. And this theory that was proposed in the 60s has done wonders for reconciling a number of geological and geophysical observations that have been made over the last decades.
CL: And, what types of issues did it resolve?
JPA: You may remember the theory of continental drift, this idea that continents used to be welded together and then split apart. This idea was proposed in the early 20th century by Alfred Wegener and it was based on the observation that you would find the same fossils on different continents, so suggesting that at some time these continents used to be together. Also, it was based on the observation of glaciations that spread over several continents that were far away, and in order to explain this pattern of glaciations you needed to have them together. At that time, this theory was not accepted by most people, because there was no explanation for how continents might move on the surface of the earth. And, plate tectonics came in the 60s with the idea that the oceanic floor would be created along the mid-oceanic ridges. So, actually you don’t need to have the continents plowing their way along the oceanic floor, but actually it’s the oceanic floor that is moving. It is created along the ridges and disappears along the mantle subduction zone. So, in particular, it could reconcile the old continental drift theory with a number of observations that were made after World War II.
CL: What is actually driving the motion of these plates?
JPA: The general idea today is that everything is driven by temperature. The earth is a huge thermal engine, and heat is transferred either by diffusion through the medium, or by advection, meaning that heat is transferred because the medium is moving. We know that in the earth, advection is the main mode of heat transfer. So, what happens is that there is a lot of heat from the initial formation of the earth and the decay of various radioactive elements, which is heating the earth. And, to cool down, heat is transferred outward by motion in the mantle, which we call convective motion. It’s just the same type of convective motion you would have in a pan of heated water. And, what happens is that convection may be driving plate tectonics. But actually, we don’t know the exact relationship between what we see at the earth’s surface and what is going on in the mantle. One way to address that is to do some physical modeling of this to get a better view of the entire earth from various techniques, and to get a better understanding of the kinematics of deformation at the earth’s surface.
CL: So, what are some of the challenges to better understanding plate tectonics?
JPA: So, as I mentioned, plate tectonics has been very successful in explaining a number of observations in geology, but at the same time it’s clear that it is only a first order approximation. According to the theory, all the deformation should occur along these very narrow plate boundaries. Now, if you look at the distribution of earthquakes on the earth’s active fault zones, you will see that it covers nearly 20% of the earth’s surface. So, most of the plate boundaries are actually relatively wide. We need to better understand what is governing the deformation.
Also, when we look at data gathered over the last 10 years with the global positioning system (GPS), we can now measure directly the motion of points on the earth’s surface. And, what we see is that to the first order it fits reasonably well the models that were based on geological data. But, we see differences, and we need to understand better what these differences mean. It may be that the deformation of the earth over the long term is not stationary, and also it has to do with what is going on along specific large plate boundaries. Actually, I refer here to the seismic cycle, which is another theory that we use a lot in earth sciences today.
The elastic rebound theory is that we can explain the return of large earthquakes on a given fault with a simple analogical model. Imagine that you have a slider on the table. You attach a spring to that slider, and then you pull the spring. What happens is that the spring will extend for a while until it transmits a force to the slider that exceeds the friction at the base of the slider, and then suddenly the slider will move a little bit then stop again. So, if you keep on pulling the spring at a constant velocity, the slider will move by sudden slippage that would be analogous to earthquakes. So, this is called the elastic rebound theory because each time the slider moves by a displacement that compensates the stretching of the spring since the previous slippage.
So, this is the model that is used today to address seismic activity on large faults. And, it is a very simplistic model, and we would need to understand a little better what is the physics behind the seismic cycle. We need to understand better what is controlling the location, the size, and the timing of large earthquakes on big faults. So, at the moment we have this sort of kinematics model, which is doing a good job of explaining geological data when we measure the deformation near a fault with GPS for example. We can explain well the observation with the elastic rebound model.
But, there’s no physics there that we can use to assess better on a given fault anywhere on earth what is the size and time of coming large earthquakes. That’s really a challenge in our field, bringing together some theory of the global deformation of the earth with what is going on locally at particular plate boundaries. In a way, I would say continental drift is embedded into plate tectonics. And, now we need to find some more general theory in which plate tectonics would be embedded that would bring into the model the relation between convection in the mantle, deformation at the earth’s surface, and hopefully some physical models to explain the details of seismic behavior along plate boundaries. That’s really the objective of this new center we are establishing at Caltech.
CL: So, it sounds like you are attempting a more specific theory of interaction at these fault zones.
JPA: So, we now have a number of techniques that were developed in a number of fields in earth sciences. We can use the GPS system, imagery from satellites, and new techniques from seismology to get a better image of the earth’s interior. We have new techniques in geochronology to date for example past earthquakes, or we can estimate rates of erosions. A lot of these techniques have brought a great number of observations about the earth’s dynamics. And we are probably at a point where we can try to bring together all of these pieces to fit them with a theory that encompasses plate tectonics. But, that needs to go beyond plate tectonics, because we want to go beyond a purely kinematic description of the earth. We want to obtain a more physical model, a more mechanical model, of the earth.
CL: What would this mean for predicting how earthquakes occur and when they occur?
JPA: Okay. That’s a difficult question. So, when we talk about earthquake prediction, there are several things we might mean. One thing that is already beneficial for society is to predict the location of future large earthquakes. If you are able to tell in a particular area that an earthquake is of a typical magnitude, it is already very useful information that can be used by the society, for example in building codes, and that’s probably the most important thing. For a practical impact on society, we need to address two questions. First, what is the size of the most probable earthquake in a given area? Also, what is the return period of these large earthquakes? And, those are the two critical things that we must work on establishing better than what we can today with present tools.
The question of predicting the exact timing of a coming earthquake is more or less a second order question, and probably the impact for the society would not be so great. Let’s say you have a technique that would allow you to predict within a few minutes the time of return of an earthquake. There’s little you can do, right? And, if you can predict a year before it occurs, you’re not going to evacuate the population of a big country just because there is the possibility of an earthquake occurring one year from now. So, in my view it seems most important to make the effort in assessing the type of ground motion we would obtain if a give earthquake were to happen, and build structure properly, rather than really try to predict the time of a given earthquake.
CL: Well, thank you for your time and for telling us about the evolving theories of earth motion.
JPA: Thank you.