Earthquakes for dummies
Who do I think I am? Well… I’m a geologist. I know the problem.
A bit of a big headline. I’ll explain the earthquakes. Who do I think I am? Well… I’m a geologist. I know the problem. If you want to know about heart attacks, you ask a cardiologist, right? If your tap leaks you call the plumber, not a cardiologist. Or am I wrong? Geologists know about earthquakes. They have to. It’s a must. Even if they’re not going to deal with earthquakes in their career, they must be familiar with the phenomenon. So, by academic background geologists know very well that earthquakes are an entirely natural phenomenon over which man has no influence. Earthquakes happen because Earth’s lithosphere (the outermost rocky envelope of the planet) is divided into a series of plates and microplates; most of the earthquakes are distributed along plate margins because plates move one with respect to the other. And huge blocks of rock “rubbing” each other make a big mess. The “mess” are earthquakes: rock breaks, and the energy released at the moment of breaking propagates in all directions in the form of seismic waves, oscillations of the rocky body that of course involve the surface on which we live. They are waves completely similar to those generated by a rock thrown into the water (but they are not only those – it’s just to give an idea).
The plates into which our lithosphere is divided move during geological time. We’re talking about a few inches a year. The edges of the plates, the seismic zones, are complex interweaves of breaking surfaces of the local rock. Each of these surfaces is called a fault (in reality faults are two rock surfaces in contact with each other). They are not fractures: the difference is that there is a movement along the fault surfaces. But movement is not continuous. They are rocky surfaces, irregular and full of roughness that prevent them from sliding immediately. The movement generated by the migration of the plates is blocked by these asperities, by the very nature of the surfaces that mark their boundaries. And energy accumulates, accumulates, accumulates on and on. At a certain point it will have accumulated enough to allow the movement to take place, but it will be a sudden snap: the earthquake. This is the cause.
Take a look at the link above: by walking around mountain paths, geologists can observe a large number of faults. Geologists are able to recognize them, for example, by an evident shifting of the layers that do not match each other; by the contact between two geological formations of different ages; and so on… The faults observable on the outcrop are, except in special cases, presently inactive. They have already performed their task of deforming the geological sequences to accommodate the stresses in place. The movements we are talking about now take place elsewhere, usually at a certain depth and, in geological times, they stack entire sequences thickening them until they become mountain ranges. Other faults, on the other hand, behave in the opposite way, accommodating extensions that take place in a certain area. Still others, such as the famous San Andreas fault in California, allow lateral sliding, but in this case it is a real active plate margin observable on the surface! To put it more precisely, the three types of movement along a fault plane (see picture above) can all be found in the same deformed area. It is a matter of identifying the faults that are still active.
Imagine squashing a balloon; the balloon will deform under our pressure. The same happens to rocks, they deform under the enormous pressure of entire lithospheric plates that would like to move. If I insist on squeezing the balloon, at some point I will pop it. Even the balloon has its own resistance limit. If I keep squeezing it, sooner or later, I’m going to overrun that limit. Imagine you know there is something squashing the balloon, something that will certainly not stop. Even knowing it perfectly, it will be impossible to predict in milliseconds when the balloon will explode. The only certainty is that it will explode. The same thing happens with geological structures that we know to be seismogenetic, that is, capable of causing earthquakes. We know where they are and how they were generated, why they are there and what they are for. We can’t say when they will “snap”, but we know for sure that they will, because they have already done so in the past. And we know the “magnitude” of the earthquakes they can generate because it has already happened in the past. We can’t do any more than that.
We also know that the natural phenomenon that triggers these movements is immeasurably greater than we are. It is a movement on a planetary scale involving parts of the planet just below the lithosphere which is about 100 km thick on average. Semi-melted and warm material rises up in the middle of the oceans and solidifies into a new lithosphere (a plate). As it cools down, the material becomes denser and heavier and in certain areas it plunges back down in the depths of the planet causing the plates to move. In these areas earthquakes, volcanoes, mountain ranges are generated. It’s a bit simplified but I wanted to give an idea of the immensity of the phenomenon compared to human dimensions.
I have often had to deal with people, even of a certain level of culture, who wondered if the extraction of fluids from underground (water, gas, oil) could trigger earthquakes. Some people were convinced of it, no matter what I said. At first I laughed at how bizarre the idea might seem to any geologist. I explained it to myself with the extreme superficiality with which the science program in school is approached, especially when it comes to geology. And I am still convinced that this is a bigger problem than it may seem: more often than not, we find people in the places where important decisions have to be made for their country and they are not at all able to discriminate what the real reasons for certain natural phenomena are. Not everyone has to be an expert in geology, but if you were to be a little more careful in studying our planet at school, you probably wouldn’t have to resort to a geologist to understand that a water borehole or oil well literally tickle planet Earth in terms of depth and energy involved compared to the natural seismicity of an area.
Unfortunately, communities today appear literally terrified at the idea of taking a small sting at the immense lithosphere. A borehole of that kind can go as deep as, in particular cases, a few thousand meters, but most of the time it is hundreds. The lithosphere is over 100 km thick. The upper part, called the Earth’s crust, reaches about 70 km only below the mountain ranges, otherwise it is about 50 km in the continents (much less in the oceans – where it is not useful to drill wells). A well could reach 1 km, or at most 4-5 km (don’t bring up the Kola well, which has long held the depth record, but it was a challenge, not what is usually done for commercial reasons). It’s true that faults can be shallow, but the physics of earthquakes teaches us that extracting fluids from the rock near a fault increases slip resistance lowers the chances of an earthquake (or rather, slows down its evolution). On the contrary, the injection of fluids decreases resistance along the fault. However, only very rarely has it been demonstrated that earthquakes are triggered by this type of operations. The cases are rare because the amount of fluids injected is really huge and much higher than what the permeable rocks of the subsoil can normally contain. In short, it has happened only in rare cases where it has really gone too far! And it has never been about devastating earthquakes, that is, dangerous ones. Perhaps it is worth pointing out that when we talk about the extraction or injection of fluids underground, we do not talk about doing it in caves or underground caverns that would act like tanks: the rock can be porous. If the pores communicate, it is also permeable. Usually there are fluids in the pores, often water, sometimes hydrocarbons (gas and/or oil). They are dispersed in the rock like in a soaked sponge. A gas or oil deposit, or an aquifer, are not huge cavities full of fluid. A gas field is not like a cylinder under pressure. It can’t explode! And if you empty it, it won’t collapse! The fluid part is a very low percentage of the volume of rock and you can extract a very low percentage of it in turn.
Gas storage sites are found in old, already exploited deposits, whose pores have already been emptied by extracting the gas. Nature would not ask for anything better than to restore the initial conditions by reintroducing gas back to where it was removed from. The important thing is not to exaggerate: do not exceed the original quantities by much. I stress that, in order to risk the triggering of earthquakes, it is necessary to exceed that quantity by a great deal and only in the case of gigantic deposits, of exceptionally large size in the world.
It is worth adding that if, absurdly enough, we could somehow manage to “unblock” a fault before it does so on its own (perhaps after more years, decades or centuries of energy accumulation), the earthquake generated, whatever its energy, would in any case be inferior to the one the fault would release in the future by its nature. In short, if by injecting fluids into the subsoil we could actually trigger earthquakes along seismogenetic structures, we would release their energy earlier and prevent its accumulation: we would lower the magnitude of earthquakes in that area and we would also know when they would occur! It would be almost desirable rather than to fear…
If you are afraid of geothermal energy, then the injection of fluids is ridiculous compared to gas storage. It’s ridiculous to think that it can influence the seismicity of an area (I drill a well and I affect the movements of the Eurasian and African plates? Come on…) But reality is made of blocked projects because people are afraid of wells; and you can accept it: “people” don’t have to be experts. The subsoil is unknown, we cannot see it, what we know about it we have extrapolated from indirect investigations. I can understand that someone is afraid of something they don’t know. The problem is when those who have to decide believe these nonsense and block strategic projects for a country. A little more science at school would have been enough…
If we really want to say it all, the earthquake is a seismic event: it is generated by movement, by sliding along a fault surface. Any tremor on the earth’s surface that can be recorded by a seismograph is not an earthquake of seismic origin. A seismogram of a seismic event is different from one that records a collapse in a quarry, a nuclear explosion, the passage of a truck, the movement of a magma underground, or a volcanic eruption. If the injection of fluids into the subsoil is made at “exaggerated” pressures, it can trigger fracturing. As the fractures open, small vibrations are generated, which is quite normal. If they reach a seismograph and are classified as earthquakes with Richter magnitude, it does not mean that the injection of fluids has generated earthquakes: in that case there were no movements along a fault, we should not speak technically of earthquakes but more generally of tremors. But nobody does…
It is different to hypothesize that the injection of fluids can trigger movement along a fault that is already accumulating energy. Theoretically it is possible, as long as the fault is shallow enough to be close to the injection area so to be affected by the change in pressure. However, the change in pressure in the rock must be particularly important and carried out in a very large area.