Moscow. March 28th. INTERFAX - Cosmonautics and the use of space technology have become firmly embedded in our lives. Our special correspondent Vyacheslav Terekhov talks about one of these areas with corresponding member of the Russian Academy of Sciences in the department of Earth Science, head of the laboratory at the Otto Yulievich Schmidt Institute of Physics of the Earth Valentin Mikhailov.
To identify an earthquake from a satellite: yes, no, it is possible – what do we choose?
Correspondent:.If you look at articles on the possibility of predicting an earthquake from satellites, then an interesting picture develops: some say that it is impossible at all, others that it is possible using GPS, but the technology is still poorly developed for this, others believe that everything can be determined by the Earth's displacements from satellites, and even predict earthquakes, but not earlier than two hours. That is, almost three directions are different. Which one do you belong to?
Mikhailov: In principle, there is some progress in the issue of earthquake forecasting, but in fact, strictly speaking, no one predicts earthquakes reliably yet, and there is no such technology to do this.
What does it mean to predict an earthquake? To do this, you must answer three questions: where, when and with what force, what magnitude will be. That's what, in my opinion, no one can do.
Corr.: But is it possible to monitor the state of the Earth from satellites?
Mikhailov: It is not only possible, but also necessary. Our laboratory is engaged in such satellite monitoring.
In general, satellite methods are very widely used in geology and geophysics. It all started naturally with optical images. You can see a lot from them, and not only changes in landscapes, but, for example, identify modern and ancient faults.
However, optical images have one big disadvantage: they can only be obtained in daylight and in the absence of clouds. Then they decided not to passively register reflected rays, but to use radar to send electromagnetic radiation to the Earth, receive reflection from it and make monitoring continuous regardless of the time of day and the state of clouds.
Special radars were created, and scientific thought moved on. The phase of the satellite signal that bounced off the Earth and returned to the satellite characterizes the distance from the Earth's surface to the satellite. Analyzing the phase of the reflected signals in two images allows you to build a digital relief model not only of a certain area, but also of the entire Earth's surface as a whole. Previously, it took years to build relief maps using theodolite and leveling!
This is how global digital terrain models were built. But the idea went further: they took a repeat survey of the same area and removed the phase shift associated with the terrain. But the phases of the signals did not match again when shooting again. This is due to a change in the conditions of the signal passing through the atmosphere, and some other factors, but the main thing is that the reflecting object has shifted during the time between repeated shots. As a result, based on the phase difference of the reflected signals on successive satellite images, after appropriate corrections for relief, atmosphere, and other factors, it became possible to estimate small displacements of the Earth's surface over large areas. This is an extremely interesting and important result for further research. This method is called satellite radar interferometry.
Tectonic shifts – a view from space!
Corr.: Did I understand correctly that tectonic shifts can be determined using a satellite, from space? Then is this the prediction of a possible earthquake?
Mikhailov: No, not really. Displacement does not always cause an earthquake. Let's go from the opposite: you have already had an earthquake. If you take a satellite radar image before and after it, you can build a displacement map for the earthquake area. Many did this last year after the earthquake in Turkey. We used satellite images before and after the tragedy and obtained a displacement map in central Turkey, approximately 100x100 km, with an accuracy of up to a centimeter.
Reporter: But that's after the earthquake! And is it possible to register the beginning of the displacement along the fault?
Mikhailov: You have received a map that shows exactly how the faults went through. It shows which part of the faults has been opened and which has remained intact.
But not everything is so clear! My American colleagues and I had different opinions regarding forecasts for future events. This is due to the difference in the definition of ruptures: for colleagues, the rupture of the earthquake on February 6, 2023 did not reach the rupture of the earthquake that occurred north of January 24, 2020. And they came to the conclusion that in this place there is likely to be another earthquake with a magnitude of six points.
There was no gap on our map. We have a longer gap, it overlapped with the gap in 2020, and there is no need to wait for an earthquake here, respectively. And we said: there is no gap here, nothing will happen here. And now a year has passed - we were right.
Thus, to some extent, the forecast is possible, because we determine the surface of the rupture, the displacement on it, and thus we can assess how the stresses in the earth's crust have changed in the vicinity of the earthquake source, will they lead to an increase in stresses on neighboring faults or not? This is also one of the methods when it is possible to assess the prospects for the activation of faults by changing stresses.
You mentioned GPS. They are now in many seismically active areas, but it is very rare for them to see any displacements before earthquakes in advance. Very rarely.
What happens when tectonic plates shift?
Corr.: We talk about the movement of the plates all the time. But they are not made by a jeweler, they cannot have smooth surfaces! This means that they rub, politely speaking, with jagged sides against each other, and when one plate moves on top of another, even with a screech.
Mikhailov: Yes, the contact of the plates is uneven. Moreover, the plates are not even monolithic inside. For example, the famous San Andreas fault, which runs through the whole of California, lives its own very complicated life. It has sections that are tightly locked, interlocked with each other, as if they were fastened with a nail. And that's what happens: two adjacent plates are moving, but the adhesion is not the same at different parts of their contact. This means that where, as we said, the sections are sewn together with nails, stresses will accumulate there, which at some point will cause a rupture, and the sides of the fault will shift.
Thus, with the help of space geodesy (the science that studies the displacement of the Earth's surface from space - IF), you can determine where your stress accumulates, and where they partially or completely relax due to the displacements occurring, and the danger is removed. Thus, space geodesy discovered "quiet earthquakes" in the areas of "seismic gaps".
Reporter: Quiet earthquakes, seismic breaches?..
Mikhailov: I'll explain. Consider the areas where one slab sinks under another. These are the so-called subduction zones where the strongest earthquakes occur. Earthquakes often occur along the subduction zone at a fairly large distance, and between them there are areas where there have not been earthquakes for a long time. These areas are called seismic gaps. It is natural to expect an earthquake here. But sometimes, using space geodesy methods, relatively slow displacements are detected in such areas, which relieve stresses, but do not create Earth tremors. Such displacements do the same thing as earthquakes, but quietly, without noise and casualties.
The Japanese were among the first to discover this using space geodesy methods. When they installed a detailed network of GPS receivers, they found that there were no movements in some area for a long time, and then they start in creep mode. In Japan, there is a very frequent network of GPS stations that track everything, data is regularly transmitted to the Institute of Geography, where time shifts are analyzed.
Corr.: And this is evident from the results of work on the very surface of the Earth, or is it observations from space?
Mikhailov: Classical methods of land geodesy are used in relatively small areas. The global plate motion system was built thanks to the methods of space geodesy. The velocities of the plates and the relative displacements at their boundaries are determined. This is not an earthquake forecast, but important information for assessing seismic hazards.
Another danger is landslides!
Corr.: Destruction is sometimes caused not only by an earthquake, but also by landslides. Is it possible to do them from space?
Mikhailov: Yes, we study not only earthquakes, but also landslides. For example, our laboratory monitors landslide slopes in the Greater Sochi area. A lot of construction is underway there, including on landslide slopes. The result? More recently, in the village of Sergei-Pole, the cottage settlement "Gornaya Polyana" almost completely descended from the slope. It's good that there were no casualties. Experts knew that this was a landslide slope. And we recorded the movements of the slopes from satellite images before the landslide. We wrote an article about it.
We also monitor subsidence over potash mines, in particular, in the Perm Region in the city of Berezniki, where there are sinkholes. The city is now gradually "moving". Previously, residential buildings were built next to a factory or a mining department so that it was not far to work. But gradually the water penetrated into the underground space, and sinkholes formed.
We are tracking these movements from space because it is a socially important task. We also study displacements on volcanoes. Now, on the Shiveluch volcano, we have discovered that one of its slopes has risen by forty centimeters. We are trying to simulate and show that magma has probably penetrated there. It rose, but did not break the surface.
Correspondent.: Haven't you broken out yet? Magma can penetrate the crust, and this danger can be seen from space?
Mikhailov: Magma often bursts out of the volcano's mouth, but there are also fractured eruptions. This was the case on the Tolbachik volcano. There, an extended crack formed on the slope, and lava flowed out in a huge stream. But often magma does not reach the surface. Such processes can be tracked by satellite radar interferometry methods based on deformation of the slopes of the volcano.
Now the methods of satellite radar interferometry are used very widely in construction. In St. Petersburg, for example, when laying a new metro line, we were asked to see if residential or historical buildings would fall into a possible subsidence area.
So, as you can see, space technology is very, very necessary on Earth!