Scientists from the Institute of Geosphere Dynamics of the Russian Academy of Sciences and the Moscow Institute of Physics and Technology have for the first time been able to reproduce a slow earthquake with the generation of low-frequency oscillations in laboratory conditions. An analysis of the results showed that according to the recorded seismic data, it is possible to predict the processes in the faults of the earth’s crust, which are hypocenters of real earthquakes.
The work was published in the journal Scientific Reports. Science adds intermediate tones to the black-and-white perception of the surrounding world by man. In the case of earthquakes, it was previously believed that there are two extremes: either the earth’s crust is at rest, or an earthquake occurs that cannot be overlooked. In fact, everything is more complicated. An earthquake is a consequence of a shift of blocks of the earth’s crust relative to each other along a tectonic fault.
In addition to ordinary earthquakes, there are “slow” ones, in which the earth’s crust fluctuates so weakly that often people don’t feel them at all, although the energy released during such an event can be the same as during an ordinary earthquake. Such events can only be recorded instrumentally. Therefore, scientists need to understand what determines the formation of different types of earthquakes and whether there are general patterns between these processes.
“The potential task is to learn how to transform a regular earthquake into a slow one. Make sure that energy is released, and destruction due to elastic vibrations do not occur. But to date, there is still no deep understanding of the mechanics and nature of the different types of slip of the blocks of the earth’s crust along the fault, leading to different types of earthquakes. We are trying to understand these mechanics. And at the moment, we have already learned how to reproduce in the laboratory analogs of these events: slow and fast earthquakes, ”commented co-author Alexei Ostapchuk, a senior researcher at the Institute of Geosphere Dynamics of the Russian Academy of Sciences, associate professor of the Department of Theoretical and Experimental Physics of the Moscow Institute of Physics and Technology.
But in nature it is impossible to go down to the depth of the hypocenter of an earthquake and see what kind of material is there, to measure stresses. The only way to get at least some information about ongoing processes at depth is to analyze seismic vibrations coming from the fault zone, or vibrations that pass through the fault zone and can carry information about its state. The authors investigated the acoustic vibrations that occur during the nucleation of laboratory earthquakes in order to “read” the history of mechanical movements in the fault zone and predict the moment of the earthquake.
“We used granite blocks to model the tectonic fault, the space between which we fill with granular materials (sand, clay, granite chips) with different characteristics (particle size, humidity). In a real fault, the same fragmented environment, only the scale is much larger.
With different earthquakes, their characteristic waveforms are emitted: with fast (“ordinary”) earthquakes, we see an impulse with a sharp sudden start, and with slow events, the oscillations increase gradually, it is difficult to determine their beginning, so they were not detected in the laboratory before. We showed in this work that it is important to look at the waveform of the pulse along with the amplitude and energy, and that slow and fast types of earthquakes can be observed in laboratory experiments, ”explains Ostapchuk.
After processing a huge amount of seismic-acoustic data, the authors identified two fundamental classes of pulses. It turned out that by the shape of the acoustic pulses one can really judge what is happening with the fault, and fast and slow micro-events are responsible for different structural zones in this fault. Fast micro events – for the destruction of the power frame and the evolution of the stressed structure. Slow ones are associated with the mobility of individual unloaded elements of the fault zone. Thus, the revealed patterns suggest that an imminent earthquake can be predicted from the analysis of seismoacoustic data.
The next step, approximating the possibility of transforming fast earthquakes into slow ones, should be the study of technogenic-tectonic earthquakes, which are directly related to human engineering, namely, the extraction of mineral raw materials. Such events take place at depths where field development directly takes place. Knowing the structural features of faults and blocks in the field due to mechanical actions, it will be possible to predictably change the slip mode, which will allow working out methods for converting a fast earthquake into a slow one.
There are two different approaches. The first approach is to try to reduce the intensity of the earthquake due to external explosive impact, that is, exchange one large earthquake for many small ones. But this way often does not give any gain in safety for people. The second method is the injection of special liquids or fluids into the fault zone. Then, depending on the properties of the fluid, brittle fracture transforms into viscoplastic deformation mechanisms.
“In our laboratory experiments, we begin to understand what kind of fluid should be injected into the fault, what properties of this fluid should be, and what characteristic size of the impact zone should be. The next step is to identify the features of the structure of the faults based on seismoacoustic data and, thereby, determine the impact zone. This will be a new stage in our understanding of the nature of earthquakes, ”concludes Alexey Ostapchuk. This work was supported by the Russian Science Foundation.